Chemical Engineering

The Principle of Increase of Entropy

2011-09-01T00:29:00.000-07:00

The entropy of the system never reduces. In the ideal process it can remain remain constant, but in actual process the entropy of system and universe always increases. Let us why the entropy of the universe always increases and its relation to second law of thermodynamics.

Introduction

In the previous article on what is entropy, we saw the causes of increase in entropy of the sysem. Let us repeat them here once again.

In actual practice the reversible isentropic process never really occurs, it is only an ideal process. In actual practice whenever there is change in the state of the system the entropy of the system increases. Here are the various causes of the increase in entropy of the closed system are:

1) Due to external interaction: In closed system the mass of the system remains constant but it can exchange the heat with surroundings. Any change in the heat content of the system leads to disturbance in the system, which tends to increase the entropy of the system.

2) Internal changes in the system: Due to internal changes in the movements of the molecules of the system there is further disturbance inside the system. This causes irreversiblities inside the system and an increase in its entropy.

The Principle of Increase of Entropy

The entropy of the isolated system is the measure of the irreversibility undergone by the system. More is the irreversibility more increase is the entropy of the system. As such the reversible process is an ideal process and it never really occurs. This means the certain amount of the irreversibility is always there in the system, this also means that the entropy of the isolated system always goes on increasing, it never reduces. Here let us keep in mind that isolated system can always be formed by including any system and surroundings within a single boundary.

In his book Engineering Thermodynamics, the author P K Nag says, “An irreversible process always tends to take the isolated system to a state of greater disorder. An isolated system always tends to a state of greater entropy. So there is link between entropy and disorder. It may be roughly said that the entropy of a system is a measure of degree of molecular disorder existing in the system. When the heat is imparted to a system, the disorderly motion of the molecules increases and so the entropy of the system increases. The reverse occurs when the heat is removed from the system.”

Summarizing the first and second law of thermodynamics, Clausius made two statements:

1) The energy of the world (universe) is constant.

2) The entropy of the world tends towards a maximum.

Thus the entropy of the isolated system tends to go on increasing and reaches maximum value at the state of equilibrium. When the system reaches equilibrium the increase in entropy becomes zero.

Entropy and Second Law of Thermodynamics

As per second law of thermodynamics, all the spontaneous processes occur in the nature from higher to lower potential. It requires external work to carry out the process against the nature that is from lower to higher potential. Thus all the spontaneous processes are irreversible and they lead to increase in entropy of the universe. Further, since the entropy of the isolated system always tends to increase, it implies that in nature only those processes are possible that would lead to the increase in entropy of the universe, which comprises of the system and the surroundings.

If the potential gradient between the two states of the system is infinitesimally small (almost equal to zero) the process is said to be isentropic process, which means the entropy change during this process is zero. The change in entropy tends to zero when the potential gradient becomes zero. The value of entropy becomes maximum when system reaches equilibrium position.

Thermodynamic Equilibrium

2011-09-01T00:21:00.000-07:00

The two systems are said to be in thermodynamic equilibrium with each other when they are in mechanical, chemical and thermal equilibrium with each other. Here are various types of equilibrium and the conditions for thermodynamic equilibrium of the system.

Thermodynamic Equilibrium Defined

Let us suppose that there are two bodies at different temperatures, one hot and one cold. When these two bodies are brought in physical contact with each other, temperature of both the bodies will change. The hot body will tend to become colder while the cold body will tend to become hotter. Eventually both the bodies will achieve the same temperatures and they are said to be in thermodynamic equilibrium with each other. In an isolated system when there is no change in the macroscopic property of the system like entropy, internal energy etc, it is said to be in thermodynamic equilibrium. The state of the system which is in thermodynamic equilibrium is determined by intensive properties such as temperature, pressure, volume etc.

Whenever the system is in thermodynamic equilibrium, it tends to remain in this state infinitely and will not change spontaneously. Thus when the system is in thermodynamic equilibrium there won’t be any spontaneous change in its macroscopic properties.

Conditions for Thermodynamic Equilibrium

The system is said to be in thermodynamic equilibrium if the conditions for following three equilibrium is satisfied:

1) Mechanical equilibrium

2) Chemical equilibrium

3) Thermal equilibrium

1) Mechanical equilibrium: When there are no unbalanced forces within the system and between the system and the surrounding, the system is said to be under mechanical equilibrium. The system is also said to be in mechanical equilibrium when the pressure throughout the system and between the system and surrounding is same. Whenever some unbalance forces exist within the system, they will get neutralized to attain the condition of equilibrium. Two systems are said to be in mechanical equilibrium with each other when their pressures are same.

2) Chemical equilibrium: The system is said to be in chemical equilibrium when there are no chemical reactions going on within the system or there is no transfer of matter from one part of the system to other due to diffusion. Two systems are said to be in chemical equilibrium with each other when their chemical potentials are same.

3) Thermal equilibrium: When the system is in mechanical and chemical equilibrium and there is no spontaneous change in any of its properties, the system is said to be in thermal equilibrium. When the temperature of the system is uniform and not changing throughout the system and also in the surroundings, the system is said to be thermal equilibrium. Two systems are said to be thermal equilibrium with each other if their temperatures are same.

For the system to be thermodynamic equilibrium it is necessary that it should be under mechanical, chemical and thermal equilibrium. If any one of the above condition are not fulfilled, the system is said to be in non-equilibrium.

Air Standard Cycle: Part – 1

2011-09-01T00:16:00.000-07:00

To carry out the analysis of the heat engines, the concept of air standard cycles was conceived. In these cycles certain mass of air is considered to be working in the thermodynamic cycle.The two most commonly used air standard cycles are Otto cycle and Diesel cycle.

Introduction

The thermodynamic cycles like Carnot cycle, Stirling cycle, Brayton cycle are closed cycles as the working fluid keeps on circulating inside the cycle. In these cycles the addition of heat takes places externally, hence they also called external combustion engines. The most commonly used practical engines used in our day-to-day life are internal combustion engines for our vehicles, motorcycles, construction machineries etc.

The engines in which combustion of fuel takes place inside the engine are called as internal combustion engines or IC engines. The IC engines comprises of the piston and cylinder arrangement with suction and exhaust valves.

In the beginning of the internal combustion cycle, the air-fuel mixture is inducted inside the cylinder. When the fuel is combusted lot of heat is released which makes the engine cylinder very hot. To avoid the deterioration of the engine, it is very important to cool the engine either with water or with external air. After combustion of fuel the pressure of gases becomes very high which pushes the piston producing the work. Thereafter the burnt gases are released to the atmosphere as exhaust gases and the fresh air-fuel mixture is inducted inside the cylinder. Since the burnt air-fuel mixture does not moves into the cycle again and fresh air-fuel mixture is inducted at the beginning of each cycle, the internal combustion engines are said to be working in non-cyclic process.

The Concept of Air Standard Cycles

No matter whether the engine works in cyclic or non-cyclic process, its analysis is very important to find out the amount of fuel consumed and work produced by it. To carry out the analysis of the heat engines, the concept of air standard cycles was conceived. In these cycles certain mass of air is considered to be working in the thermodynamic cycle. The addition and rejection of heat is considered to take place with the external reservoir and ideally all the processes are reversible.

The internal combustion engines are considered to be working on the principle of air standard cycles. The two most commonly used air standard cycles are Otto cycle and Diesel cycle. The Otto cycle corresponds to four stroke gasoline or petrol engines also called Spark Ignition (SI) engines. The Diesel cycle corresponds to four stroke Diesel engines also called as Compression Ignition (CI) engines.

DIESEL CYCLE

2011-09-01T00:12:00.000-07:00

Diesel Cycle

Processes of Ideal Diesel Cycle

The ideal Otto cycle comprises of two isentropic, one constant volume, and one constant pressure processes. The Diesel cycle is an open cycle or non-cyclic process since the fresh air and fuel is inducted inside the engine during each cycle and the burnt mixture is released to the atmosphere To understand these processes let us consider piston and cylinder engine with air and diesel as the working fluid. Refer the P-V diagram given at the bottom.

1) Air intake process 1-2: During this process the inlet valve of the engine is open, the piston moves towards the bottom position inducting air at constant pressure.

2) Isentropic compression process 2-3: During this process the inlet and exhaust valves of the engine remain close and the air, which has been inducted inside the cylinder, is compressed to the minimum volume. The compression ratio of air is between15 to 20. High compression ratios are very important in the Diesel cycle as we it increases the efficiency of the cycle also allows uniform burning of fuel.

3) Injection of fuel and its combustion at constant pressure 3-4: During this process the fuel valve opens and fuel is injected inside the cylinder consisting of hot air at high pressure. After injection the combustion of fuel takes place instantly. Since the combustion of fuel occurs due to high pressure, the diesel engine is also called Compression Ignition engine. Due to combustion fuel high pressure and temperature is generated.

4) Isentropic expansion process 4-5: Due to extremely high pressure, the piston is pushed again towards the bottom most position of the cylinder. It is during this process that the actual work is produced from the engine.

5) Constant volume heat rejection process 5-6: During this process the exhaust valve opens and all the exhaust gases are ready to be released to the atmosphere. The pressure inside the cylinder falls drastically.

6) Exhaust process 6-1: During this process the exhaust valve is open and the piston moves upwards and removes all the exhaust gases inside the cylinder at constant pressure.

Thereafter the exhaust valve closes, the piston starts moving in downward direction, the inlet valve opens and fresh air-fuel mixture is inducted. The whole cycle is completed in four strokes of engine, hence it is called four-stroke engine.

Thermodynamic Diesel Cycle: Air Standard Cycle

2011-09-01T00:10:00.000-07:00

Thermodynamic Diesel cycle corresponds to the four stroke internal combsution diesel engines used by us in our vehicles and machines. In these engines the air is compressed to high pressures and then combustion of fuel takes place, hence they are also called as Compression Ignition (CI) engines.

Introduction

Apart from Otto cycle, thermodynamic Diesel cycle is one of the most popularly used cycles. The internal combustion diesel engines that are fitted in your cars and others machines run on the principle of Diesel cycle. Like petrol engine running on the principle of Otto cycle, the Diesel engines also comprise of four strokes: suction of fuel, compression of fuel, combustion and expansion of fuel and exhaust of burnt fuel. The four stroke engine has been described in details in article: How Four-Stroke Compression Ignition Engines Work

In diesel engine initially only air is inducted inside the cylinder and it is compresses to very high pressures. Thereafter the diesel fuel is injected inside the cylinder and its combustion takes place to high temperature of the compressed air. Since the combustion of fuel takes place due to compression of air, the diesel engine is also called Compression Ignition (CI) engine.

Four stroke Diesel cycle was invented by Rudolf Diesel in the year 1897. The principle of Diesel cycle is used in the compression ignition (CI) type of internal combustion engines using diesel as the fuel. Originally when Rudolf Diesel discovered this engine, it was meant to run on the coal dust.

The Diesel Engine

2011-09-01T00:07:00.000-07:00

The diesel internal combustion engine differs from the gasoline powered Otto cycleby using a higher compression of the fuel to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition").

Air standard diesel engine cycle

In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature to the ignition temperature of the fuel mixture which is formed by injecting fuel once the air is compressed.
The ideal air-standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and anisovolumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes a-e-a on the diagram.

Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats:

It is convenient to express this efficiency in terms of the compression ratio r_C = V₁/V₂ and the expansion ratio r_E = V₁/V₃. The efficiency can be written

and this can be rearranged to the form

Show

For an air standard engine with g = 1.4 , compression ratio r_C = 15 and expansion ratio r_E = 5, this gives an ideal diesel efficiency of 56%.

The diesel cycle depends upon this temperature being high enough to ignite the fuel when it is injected.

*psig is pounds per square inch gauge pressure. Common pressure gauges in the U.S. measure pounds per square inch excess over atmospheric pressure.

INTERNAL COMBUSTION ENGINE

2011-09-01T00:00:00.000-07:00

The Wright brothers used a gasoline powered, four-stroke, internal combustion engine to power their aircraft. In an internal combustion engine, fuel and air are ignited inside a cylinder. The hot exhaust gas pushes a piston in the cylinder which is connected to a crankshaft to produce power. The burning of fuel is not a continuous process, but occurs very quickly at regular time intervals. Between ignitions, the engine parts move in a repeated sequence called a cycle. The engine is called a four-stroke engine because there are four movements (strokes) of the piston during one cycle. The brothers' design was based on early automobile engine designs which used the Otto cycle, developed by the German, Dr. N. A. Otto, in 1876.

The brothers' design is very simple by today's standards, so it is a good engine for students to study to learn the fundamentals of engine operation. There are two main parts to engine operation: the mechanical operation of the engine parts, and the thermodynamics through which the engine produces work and power. On this page we will discuss the basic thermodynamic principles and on a separate page we present the thermodynamic analysis that allows you to design and predict engine performance.

Thermodynamics is a branch of physics which deals with the energy and work of a system. It was born in the 19th century as scientists were first discovering how to build and operate steam engines. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. The basic ideas of thermodynamics are taught in high school physics classes, so the Wright brothers knew and used these concepts, particularly in their engine design.

We have broken the Otto cycle into six numbered stages based on the mechanical operation of the engine. At each stage, we show a cut through the cylinder to reveal the movement of the piston and the amount of the gas volume created by the head of the piston and the cylinder to the right of the piston head. On the figure we show a plot of pressure versus gasvolume throughout one cycle. The cycle begins at the lower left with Stage 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure and the gas volume is at a minimum with the piston far to the right in the cylinder. Between Stage 1 and Stage 2 the piston is moved to the left, the pressure remains constant, and the gas volume increases as fuel/air mixture is drawn into the cylinder through the intake valve (red). Stage 2 begins thecompression stroke of the engine with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back to the right, the gas volume decreases, and the pressure increases because work is done on the gas by the piston. Stage 3 is the beginning of the combustion of the fuel/air mixture. The combustion occurs very quickly and the volume remains constant. Heat is released during combustion which increases both the temperature and the pressure, according to theequation of state. Stage 4 begins the power stroke of the engine. Between Stage 4 and Stage 5, the piston moves back to the left, the volume in increased, and the pressure falls as work is done by the gas on the piston. At Stage 5 the exhaust valve (blue) is opened and the residual heat in the gas is exchanged with the surroundings. The volume remains constant and the pressure adjusts back to atmospheric conditions. Stage 6 begins the exhaust stroke of the engine during which the piston moves back to the right, the volume decreases and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to Stage 1 and the process repeats itself.

During the cycle, work is done on the gas by the piston between stages 2 and 3. Work is done by the gas on the piston between stages 4 and 5. The difference between the work done by the gas and the work done on the gas is shown in yellow and is the work produced by the cycle. The work times the rate of the cycle (cycles per second) is equal to the powerproduced by the engine. The area enclosed by the cycle on a p-V diagram is proportional to the work produced by the cycle. On this page we have shown an ideal Otto cycle in which there is no heat entering (or leaving) the gas during the compression and power strokes, no friction losses, and instantaneous burning occurring at constant volume. In reality, the ideal cycle does not occur and there are many losses associated with each process. These losses are normally accounted for by efficiency factors which multiply and modify the ideal result. For a real cycle, the shape of the p-V diagram is similar to the ideal, but the area (work) is always less than the ideal value.

Chinese Scientists Plan to Pull an Asteroid into Orbit Around Earth

2011-08-31T23:56:00.000-07:00

Pictured: Humans Tempting God to Smite Them Hexi Baoyin, Yang Chen, Junfeng Li via arXiv

Last week Chinese scientists wanted to divert an asteroid away from Earth. This week, they want to pull one into orbit around the Earth. What’s possible objections could anyone have to this idea?

The notion stems from a phenomenon the researchers from Tsinghua University in Beijing noticed from time to time with Jupiter. Every now and then our solar systems biggest planet pulls in an object from space, which orbits the planet for a time before jetting off into interplanetary space again.

Technology, Clay Dillow, asteroid mining,asteroids, earth orbit, energy, jupiter, satellites,SpaceWe could do something similar with a number of near earth objects (NEOs) that will pass near Earth in the coming years and decades. None of these objects will pass close enough to be naturally captured by Earth’s gravity, but a few will come so close that a small nudge in the right direction would put them in orbit--likely a temporary orbit--around Earth.

The idea isn’t simply to flirt with cataclysmic danger, but to bring a small object (they suggest a 10-meter object called 2008EA9 that will pass nearby in 2049) into a loop around the Earth so we can study it closely for a few years. If we can get the art of capturing asteroids orbitally down to a science, we could use it to temporarily make asteroids into Earth-bound satellites (orbiting at about twice the distance of the moon), mine them for minerals, and then send them on their ways.

SCIENCE INNOVATIONS

2011-08-31T23:51:00.001-07:00

Needs More Clouds A rendering of what an alien habitable planet might look like if we could ever get close enough to see it. Illustration courtesy L. Calçada, ESO

Scientists have tracked down another goldilocks planet 31 light-years from Earth, and according to astronomers it has some strong points in its favor when it comes to the possibility of harboring the ingredients for life. HD85512b orbits an orange dwarf in the constellation Vela, and it’s just the right distance from the sun--and just the right mass--to rank among the most Earth-like planets ever discovered.

And by “among,” we mean really one of just two (or three, depending on how you feel aboutGliese 581g). Of the hundreds of exoplanets astronomers have recently discovered orbiting distant stars, only one--Gliese 581d--has been of the proper mass and distance from its star to be considered a strong candidate for habitability. Nearby Gliese 581g was once thought to be even more Earth-like than 581d, until some scientists asserted that 581g doesn’t even exist--a point that is still under debate.

Technology, Clay Dillow, eso, exoplanets,harps, HD85512b, life in the universe, SpaceHD85512b was discovered by the ESO’s High Accuracy Radial velocity Planet Searcher, or HARPS, in Chile (it’s the same instrument that found Gliese 581d. The data show that HD85512b is roughly three-and-a-half times the mass of Earth and rings its planet on the inner fringe of the so-called “goldilocks zone” that is not to distant and not too close to harbor liquid water. It’s size is also indicative of an Earth-like atmosphere of oxygen and nitrogen rather than the hydrogen and helium that dominate the atmospheres of larger worlds.

That alone makes it a potential candidate for life, but HD85512b has a couple of other characteristics working for it. For one, its orbit is almost perfectly circular and stable, so any climate on the planet wouldn’t swing wildly as it orbits. The planetary system is older than our own--a full one billion years older--so clearly it’s had enough time for life to potentially have developed there. in the same vein, its star is also more mature than our sun so it is less prone to violent solar activity that could destabilize the planet’s atmosphere.

Of course, there’s no way to tell if it actually has an atmosphere with modern instruments, and atmosphere is a critical ingredient here. Since HD85512b is orbiting on the inner portion of the goldilocks zone, it is more akin to Venus than to Earth in the amount of solar energy it’s taking on. But scientists speculate that cloud cover of fifty percent or more could offset that proximity enough to allow life to thrive--albeit a kind of life more suited to a balmy, hot environment (relative to Earth’s).

On average, Earth boasts 60 percent cloud cover so the idea of HD85512b having 50 percent isn’t so far-fetched. In fact, it’s probably more likely than the idea of humans building a light-speed spacecraft and then making the 31-year journey to go in for a closer look at the weather.

SCIENCE INNOVATIONS

2011-08-31T23:49:00.000-07:00

Needs More Clouds A rendering of what an alien habitable planet might look like if we could ever get close enough to see it. Illustration courtesy L. Calçada, ESO

Coke Oven Gas Purification and Cooling Process

2011-07-08T09:57:00.000-07:00

The gas collected at gas collecting main from coke oven is cooled by spraying circulating Ammonical liquor. The cooled coke oven gas along with excess liquor enters into separator. Here the gas and liquor get separated. The coke oven gas from separator comes to the Primary Gas Coolers where the temperature of gas is reduced from 80 degC to 30 degC by means of cooling water.

PRIMARY GAS COOLER:

The coke oven gas from the separator is fed to the PGC from the top. The cooler consists of three zones. This is a shell and tube heat exchanger in which the CO gas exchanges with service water in the top two zones and with chilled water in the bottom zone the tubes are inclined in all the zones. The inclination is maintained at 15°c with the horizontal. This provides any naphthalene condensate to drain easily. The main purpose of PGC is to cool the gas from 90°c to 30°c.

Final Coke Oven Gas Scrubber

During this cooling process the naphthalene and traces of tar present in the gas condense and this is collected at the bottom of the PGC. The liquid collected at the bottom is sent to the seal pot by gravity. The level in the seal pot should be maintained constant as this acts as a seal to the gas in the cooler.

Tar at a temperature of 90°c is flushed from the top of the cooler to remove the condensed naphthalene on the tubes. The tar is then collected at the bottom in the seal pot. Tar and naphthalene from the seal pot is fed to the storage tank of the CPH. The CO gas from the bottom of the PGC is fed to the electrostatic precipitator.

• Gas temperature before PGC 80-95°c

• Gas temperature after PGC 27-30°c

• Service water inlet temperature 32-33°c

• Service water outlet temperature 43°c

• Gas condensate flow to each PGC 10-15 Nm3/hr

• Chilled water inlet temperature 11-12°c

Electrostatic precipitator:

The CO gas enters the ESP from the bottom. Electrostatic precipitators are cylindrical vessels with a conical bottom. Each ESP is provided with a seal pot. A round disk having electrodes in suspended state is present inside the precipitator. Electrodes are nothing but SS metal rods. Three are present at the top of precipitator, which supplies the necessary power to each ESP. high voltage of about 50,000 volts is supplied to each electrode. Due to high voltage the fine and foggy target slicked to the walls of the electrodes and they fall down due to gravity. The liquid thus collected at the bottom is fed to the seal pot. Each electrode is covered to prevent the connection between any two electrodes.

Capacity 30,000 Nm3/hr
Voltage 70KV
Number of electrodes 148
ESP insulator boxes temperature 80°C

FINAL GAS COOLING PLANTS PROCESS:

The coke oven gas from the asp plant contains crude benzol and naphthalene and these should be removed from the gas to prevent damage to the pipelines.
The gas enters the cooler at nearly the middle, it meets the down coming water and the naphthalene present in the gas is almost carried away with it to the bottom.
The gas along with crude benzol is now cooled to 28-30 degC in a chiller.It then enters
The scrubber where the benzol in the gas in absorbed into a (solvent) SOLAR OIL
The solar can be recovered completely by removal of the crude benzol in it.
The naphthalene which is carried along with water to the bottom section of the cooler is now treated with TAR so that the naphthalene in the water is absorbed into TAR and both naphthalene and tar can be separated by further treatment

FINAL GAS COOLERS PROCESS:

Final Gas Cooling and scrubbing system

From the saturator the co-gas enters the acid trap where the acid vapors that might be carried along with the co-gas are separated and sent to saturator .the acid free vapor of co-gas enters the final gas cooler. There are two final gas coolers at diameter 5m & 45.5m height each cooler has got two sections in the top sections co-gas is sent from the bottom at the top sections and cooled service water is sprayed from the top there is parallel perforated plate aluminum packing of each scrubber which provides the necessary contact time for the water co-gas.

The co-gas enters the final gas coolers min at 55degC cooling of the co-gas is done by direct water spray through a closed cycle system in this way naphthalene that is present in co-gas is scrubbed by water & this enters the bottom of the section. The bottom section naphthalene washer having tar will come in contact with warm water from top sections lighter than tar the water to the top leaves the cooler. During this process whatever naphthalene present in the water will get dissolved in tar because the solubility of naphthalene in tar &many items higher than water than this tar is taken out & stored in tanks and sent to the tar distillation plant for processing.

In this scrubber solar oil is used, as solvent for scrubbing of benzol from co-gas benzol is dissolves in solar oil from co-gas. Benzol is dissolves in solar oil from benzolised oil it will be sent to the benzol distillation column where solar oil and benzol are separated the separated solar oil will be recycled again to scrubber in this way it is closed system where 1 ton 70-75kgs of oil is consumed because losses.

DOUBLE-Pipe Heat Exchanger Experiment

2011-07-08T09:52:00.000-07:00

Aim: To determine,

(i) LMTD for parallel flow arrangement
(ii) LMTD for counter flow arrangement
(iii) Effectiveness (ε) for parallel flow arrangement
(iv) Effectiveness (ε) for counter flow arrangement

Learning Objectives: To,

1. Study about various heat exchanging equipments
2. Study fundamental methods of designing heat exchanging equipments and comparing them
3. Study LMTD and its necessity
4. Know the effectiveness of heat exchanger
5. Know the temperature profiles of parallel and counter flows

Apparatus: Experimental test rig, stop watch, thermometers.

Theory:

A heat exchanger is a device used for the process of heat exchange between two fluids that are at different temperatures. Heat exchangers are useful in many engineering processes like those in refrigerating and air conditioning systems, power systems, Food processing systems,chemical reactors and space or Aeronautical applications. Heat exchangers are of basically of three types.

(1) Transfer type, in which both fluids pass through the exchanger and heat gets transferred through the separating walls between the fluids,

(2) Storage type- in this, firstly the hot fluid passes through a medium having high heat capacity and then cold fluid is passed through the medium to collect the heat. Thus hot and cold fluids are alternately passed through the medium,

(3) Direct contact type- in this type, the fluids are not separated but they mix with each other and heat passes directly from one fluid to the other.

Heat exchangers may be classified in several ways. One classification is according to the fluid flow arrangement or the relative direction of the hot and cold fluids. The fluids may be separated by a plane wall but more commonly by a concentric tube (double pipe) arrangement shown in fig. If both the fluids move in the same direction, the arrangement is called a parallel flow type. In the counter flow arrangement the fluids move in parallel but opposite directions. In a double pipe heat exchanger, either the hot or cold fluid occupies the annular space and the other fluid moves through the inner pipe.

The method of solving the problem using logarithmic mean temperature difference is typical and more iteration must be done. So it takes more time for the problem to solve. Therefore another method is practiced for solving this type of problems. This method is known as Effectiveness and Number of Transfer Units or simply ε-NTU method.“Effectiveness of heat exchangers is defined as actual heat transfer rate by maximum possible heat transfer rate”.The LMTD method may be applied to design problems for which the fluid flow rates and inlet temperatures, as well as a desired outlet temperature, are prescribed.If the LMTD method is used in performance calculations for which both outlet temperatures must be determined from knowledge of the inlet temperatures, the solution procedure is iterative. For both design and performance calculations, the effectiveness-NTU method may be used

without iteration.

Experimental setup

The experimental setup consists of two concentric tubes in which fluids pass. The hot fluid is hot water, which is obtained from an electric geyser. Hot water flows through the inner tube, in one direction. Cold fluid is cold water, which flows through the annulus. Control valves are provided so that direction of cold water can be kept parallel or opposite to that of hot water. Thus, the heat exchanger can be operated either as paralle1 or counter flow heat exchanger. The temperatures are measured with thermometer. Thus, the heat transfer rate, heat transfer coefficient, LMTD and effectiveness of heat exchanger can be calculated for both parallel and counter flow.

Specifications

1. Heat exchanger -

(a) Inner tube - 12 mm OD and11 mm ID copper tube.
(b) Outer tube - 25 mm G. I. Pipe.
(c) Length of Heat exchanger is 1 m.

(2) Electric heater - 3 KW Capacity to supply hot water.

(3) Valves for flow and direction control- 5 No’s.

(4) Thermometers to measure temperatures - 10 to 110°C - 4 No's.

(5) Measuring flask and stop clock for flow measurement.

Experimental Procedure:

1. Start the water supply. Adjust the water supply on hot and cold sides.
2. Keep the valves V2 & V3 closed and V1 & V4 opened so that arrangement is
parallel flow.
3. Switch ON the geyser. Temperature of water will start rising. After temperatures
become steady, note down the readings in the observation table.
4. Repeat the experiment by changing the flow. Now open the valves V2 & V3 and
then close the valves V1 & V4. The arrangement is now counter flow. Wait until
the steady state is reached and note down the readings.

Observation Table:

Calculations:

1. Mass flow rate of hot water, mh = Density x Volumetric flow rate of hot water (kg/s)
2. Specific heat of hot water at bulk temperature, Cph = J/kg-0C
3. Mass flow rate of cold water, mc = Density x Volumetric flow rate of cold water (kg/s)
4. Specific heat of cold water at bulk temperature, Cpc = J/kg-0C
5. (mCp)h =
6. (mCp)c =

Precautions:

1. Never switch ON the geyser unless there is water supply through it.
2. If the red indicator on geyser goes off during operation, increase the water supply,
because it indicates that water temperature exceeds the set limit.
3. Ensure steady water flow rate and temperatures before noting down the readings, as
fluctuating water supply can give erratic results.

Result:

Hence,
(i) LMTD for parallel flow arrangement = deg C
(ii) LMTD for counter flow arrangement = deg C
(iii) Effectiveness (ε) for parallel flow arrangement =
(iv) Effectiveness (ε) for counter flow arrangement =

Level Measuring Methods

2011-07-08T09:48:00.000-07:00

Direct methods used for level measurement in a tank or storage vessel are

1.Based on direct visual observation on the indicators like

   a.Gauge stick
   b.Hook gauge
   c.Gauge glass

2.Based on position of a detectable material

   a.Float gauge

3.Buoyancy methods:

   a.Float gauge
   b.displacer gauge

Indirect methods for measuring the liquid level in a storage containers
1.Utilizing the hydrostatic head developed by the liquid

   a.Pressure gauge
   b.Air- trap
   c.Diaphragm box method
   d.Air or Gas purge method
   e.Differential Pressure method

2.By using radio active radiation

3.Electrical properties like capacitance, conductance and resistivity can be use used for level measurement

4.By using the property of reflection of waves from the liquid surface.

The above class of methods used for measuring the level of fluids which are stored in storage vessel and container in chemical industries, their even advanced method practiced for accuracy and precision. The methods discussed are easy and cheaper to use at service. In a evaporator the liquid level should be maintained at certain depth in the heat region so that the rate of heat transfer which it was designed will be achieved, if the case liquid level drop the coil which used for heating purpose may fail out. Even controlling the pumping operation a level control system will help to start and stop the pumping of fluid into the storage tank. A simple level control can be made easily with minimum accessories.

Flow Meters Comparison

2011-07-08T09:46:00.001-07:00

A comparison table of commonly used flow meters based on the viscosity of fluids that they can handle and for which type of fluid they are suitable, working conditions , accuracy , range are given below

Sr no	Type	Viscous liquid	Slurry	Gas	Output	Pressure loss	Accuracy% full scale	Full range
1	Orifice	Limited	Poor	Good	Square root characteristic	High	±0.5to ±2%	10^-3to 5.5×10³m³/hr
2	Venturi	Limited	Limited	Good	--do--	Minimal	±0.5to ±3%	1to 5.5×10³m³/hr
3	Flow nozzle	Limited	Limited	Good	--do--	Minimal	±0.5to ±3%	1to 5.5×10³m³/hr
4	Pitot tube	Poor	Poor	Good	--do--	Poor	±5to ±10%	10to 10⁴m³/hr
5	Electromagnetic	Good	Good	Good	Linear	Minimal	±2to ±5%	5.5×10⁴to 4.95×10³m³/hr
6	Variable area meter	Limited	Limited	Good	Linear	Average	±0.5%	10^-7to 5.5×10³m³/hr
7	Positive displacement	Good	Good	Good	Linear	Minimal	±0.5to ±1%	10^-5to 5×10²m³/hr
8	Turbine	Limited	poor	Poor	Linear	Minimal	±2to ±5%	05 to 4.95×10²m³/hr

Thermocouple Types and Comparison

2011-07-08T09:44:00.001-07:00

Thermocouple types and probes and thermocouple cables used for measurement of temperature, mode of furnace thermocouple working and high temperature thermocouples working procedure, functioning of termistor. Calibration of temperature sensors, material of construction of various thermocouple based on the utilisation, mechanism of fitting the thermocouple to the process equipments and working principle of thermocouple testing. B,E,J,K,R,S,T types of thermocouple comparison

Sr no	Type	Positive wire	Negative wire	Temperature range ⁰C	Linearity
1	B	Pt-70—Rh-30	Pt-94—Rh-6	0 to 1860	Good at high temperature
2	E	Chromel	Constantan	-196 to 999	Good
3	J	Iron	Constantan	-196 to 760	Nearly linear between 149 to 438
4	K	Chromel	Alumel	-190 to 1371	Most linear
5	R	Pt-87—Rh-13	Platinum	-18 to 1704	Good at high temperature
6	S	Pt-90—Rh-10	Platinum	-18 to 1760	Good at high temperature
7	T	Copper	Constantan	-190 to 399	Same as J
8		Tungsten W	W 74 –Re 26	-18 to 2316	Good at high temperature
9		W 94 –Re 6	W 74 –Re 26	-18 to 2316	Good at high temperature
10		Copper	Gold – Cobalt	-268 to -18	Linear above 60k
11		Ir 40- Rh 60	Ir	-18 to 2093	Good at high temperature

Pt- Platinum
Rh- Rhodium
W- Tungsten
Re- Rhenium
Ir-Iridium

At various corrosive environments the instrument should function and measure the temperature, for that purpose a list of thermocouple will help to determine the right suitable temperature measuring instrument.

Sr no	Type	Positive wire	Negative wire	Suitable atmosphere
1	B	Pt-70—Rh-30	Pt-94—Rh-6	Inert or slow oxidizing
2	E	Chromel	Constantan	Oxidizing
3	J	Iron	Constantan	Reducing
4	K	Chromel	Alumel	Oxidizing
5	R	Pt-87—Rh-13	Platinum	Oxidizing (fast response)
6	S	Pt-90—Rh-10	Platinum	Oxidizing
7	T	Copper	Constantan	Oxidizing or reducing(good corrosion resistance)
8	Y	Iron	Constantan	Reducing
9	--	Tungsten W	W 74 –Re 26	Inert or vacuum(high temperature)

Urea Production Process from ammonia and carbon dioxide

2011-07-08T09:42:00.001-07:00

Urea is manufactured by reacting ammonia and carbon dioxide in autoclave to form ammonium carbamate. This is heated at 135⁰C at about 35 atm pressure to produce Urea. This is endothermic reaction and ammonia is maintained in excess to shift the equilibrium towards urea formation. Urea production consists of main two reactions.

1. Formation of ammonium carbamate

2. Dehydration of ammonium carbamate to produce molten urea

Urea Production from NH₃ and CO₂

Types of Pressure gauges

2011-07-08T09:39:00.000-07:00

To measure absolute pressure, gage pressure, vacuum or draft pressure and differential pressure three physical methods can be used based on the working condition

1. Using a liquid column where density and height of liquid are important to measure pressure.

2. Using a pressure sensing elements

3. By electronic methods

Let’s see pressure gauges of different types and pressure range they can work.

DIAPHRAGM PRESSURE GAUGE:

This pressure gauge is widely used to measure pressure of a reactor or closed container.

Principle:

When pressure is applied on either sides of the tight diaphragm, by which it gets deflected. This deflection of the diaphragm is proportional to the differential pressure across it. Thus differential pressure across the diaphragm can be measured in terms of its deflection.

Construction and working:

Diaphragm gauge uses metallic or non metallic diaphragms along with the pointer mechanism.

1) Metallic diaphragms: These elements are flexible circular discs, either flat or corrugated. These elements convert pressure signal to pointer deflection.

The metal is heat treated before forming a diaphragm to produce maximum elastic limit. After forming, the diaphragms are heat treated to relive internal stresses. A diaphragm is usually designed so that the deflections versus pressure characteristics are as linear as possible over a specified pressure range.Metals used are brass, beryllium-copper, stainless steel and phosphor-bronze.

2) Non metallic diaphragms: very bendable diaphragms are used and so they can be utilized to measure vacuum and low pressures available.Non metallic materials used are synthetic rubber, neoprene, leather, Teflon, nylon or decron.

Pressure range:

For non-metallic diaphragm 0 to 10 inch water
Minimum= 0 to 2 inch of water
Maximum=0 to 400 psi

Advantages:

1. Small in size and cheaper
2. By using corrosion resistant material it has long life
3. We can obtain perfect linearity functions

Limitations:

1. Cannot be used for high pressure
2. Vibration will affect the reading

MATLAB Simulation Experiment from Basics to Process Dynamic Simulation Using Simulink

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Openloop simulation:

(1) open simulink by double clicking simulink from the menu of matlab command window.

(2) This opens the simulink window and from the menu bar select new file.

(3) A new file of simulink is opend and it is named (default) as untitled.mdl. All simulink files have mdl extension.

(4) From the simulink library open ‘continuous’ icon. Transfer function is given in the continuous library as 1/(s+1) and transport delay is also given in continuous library. Drag the transfer function and drop in the new simulink file. Similary drag and drop transport delay also.

(5) Now open simulink library icon ‘sources’. Drag step function from it and drop in new file.

(6) Now open simulink library icon ‘sinks’. From ‘sinks’ drag and drop ‘scope’ and ‘untitled mat’ in to the simulink file.

(7) Now connect all these blocks by using left click of the mouse.. If the existing line between two blocks is to be joined to another block, then use right click of the mouse.

(8) Once the block diagram is over, change the output file name from untitled mat to your pet name by double clicking it. The default output variable is ans in that file and you can change it according to your problem. Set the sample time as 0.01 which means the data is stored for every 0.01 sec. press ok.

(9) Double click the transfer function. Numerator and denominator are to be given according to your problem in the decreasing order of s. click ok

(10) Double click step icon and give the time at which the stepchange is to be given to the process and initial and final value for the step are to be given and sample time is same as 0.01.

(11) Double click the transport delay and set the delay of the process a sper the problem at hand and press ok

(12) Save this file with a name.mdl

(13) To simulate this block diagram, open simulation in the menu bar and select simulation parameters in it. Give the stop according to your problemand press ok.

(14) To simulate the block diagram press solid forward arrow symbol in the menu bar.

(15) Once the simulation is over over, open matlab command window and load the output file using the command load.

(16) Example: >>load out.mat

(17) To plot the step response use the command >>plot(y(1,:),y(2,:))[y is the output variable name which is used instead of default ans). This means the first row of y which is a time vector is plotted against the response vector (2 row of y). Then label the axis using xlabel and ylabel commands.

The openloop block diagram is given below

Open loop

Closed loop simulation:

For closed loop simulation, additional blocks of PID controller and a summation block are required. Additional step function is required for regulatory or load problem,

PID controller is available in simulink extras of simulink library. Double clicking simulink extras, select additional linear. PID controller is available in that. Drag and drop PID in to an untitled simulink file along with transfer function, step, to file, scope.

Drag and drop sum block from Math of simulink library and we need two of them for the simulation of servo (setpoint change) and regulatory problem (load change).

Connect the blocks as shown in the figure given below.

Closed loop

To give PID values, double click PID block. In Proportional give kc value, integral give kc/I and derivative give kc*I value. Click ok.

For example consider a transfer function model of a process as . Conduct closed loop simulation for servo and regulatory problem for unit set change in setpoint at time=0 and unit step change in load at time=100.

(a) P controller with kc=1.9

(b) PI controller with kc=1.7 and I=14.3s

To simulate for both servo and regulatory simultaneously, double click step (setpoint input) and time is set equal to zero and initial value is zero and final value is 1. sample time is 0.01. Now double click step (load input) and time is set equal to 100 (it is the time at which servo response reaches to 1) and initial value zero and final value 1. The sample time is 0.01. i.e. Load is initiated after 100s.

Simulation time is fixed by selecting simulation parameters in the menu bar. The simulation time should be around 200 to stabilize the regulatory response as the load step is initiated at 100.

Once the simulation is over, load the result file in the matlab command window.>> load result.mat>> figure,plot(y(1,:),y(2,:),'k')>> xlabel 'time'>> ylabel 'response'Here the output file name is result.mat and the variable name is y. First row of y stores time and second row of y stores response. The figures generated areP controllerIn PID block substitute P parameters as kc, 0I and 0 for proportional, integral and derivative time locations.Simulate and then load the file in matlab command window and plot as shown below.>> load result.mat>> figure,plot(y(1,:),y(2,:))>> xlabel 'time'>> ylabel 'response'

P controller response

PI controllerIn PID block substitute PI parameters as kc, kc/I and 0 for proportional, integral and derivative time locations.Simulate and then load the file in matlab command window and plot as shown below.>> load result.mat>> figure,plot(y(1,:),y(2,:),'k')>> xlabel 'time'>> ylabel 'response'

PI controller response

PID controllerIn PID block substitute PID parameters as kc, kc/I and kc*D for proportional, integral and derivative time locations.Simulate and then load the file in matlab command window and plot as shown below.>> load result.mat>> figure,plot(y(1,:),y(2,:),'k')>> xlabel 'time'>> ylabel 'response'

PID controller response

Pressure measuring instruments:

2011-07-08T09:31:00.000-07:00

Classification of pressure measuring instruments based on construction and working principles

Liquid column elements:

1. Barometer2. Manometer –u tube, enlarge leg well inclined legElastic element gauge:1. Bourdon tube2. Bellows3. Diaphragm4. CapsuleElectrical transducers:1. Resistance and inductance typeForce –balanced devices:1. Dead weight gauge2. Ring gauge3. Bell gauge

BAROMETER:

Barometer is used to measure atmospheric pressure. Atmospheric pressure is the pressure exerted by the air surrounding the earth that goes on decreasing away from the earth surface.

Working principle:

Barometric liquid balances the atmospheric pressure against vacuum and pressure head reading is obtained in the absolute units.

Construction and working:

Barometer has a glass tube closed at one end and opened at the other; the length of the tube must be greater than 76.2 cm. the tube is first completely filled by mercury and open end is temporarily plugged. Then the tube is inverted so that plugged end is immersed in a mercury pan. When the plug is removed, the mercury in the tube drops b y a certain amount, creating a vacuum at the top of the tube and then reading ‘h’ is noted. The reading ‘h’ is proportional to atmospheric pressure acting on mercury in the pan. Note that this atmospheric pressure reading is in absolute units.

We have stated that vacuum is present the top of the tube above mercury, but actually there is vapor pressure of mercury acting on mercury pressure ’P’ kg/cm2 is given by

P= 6.66 X 10^-3h

MANOMETER:

The device used to know about the pressure difference in the pipeline, it is a simple in construction, basic law of physics are applied for calculation of the pressure drop. It is a glass or metal tube with a 'U' bend providing with two legs. Manometeric fluids as mercury or carbon tetrachloride etc., where the density should be higher than the fluid which flow through the pipe, manometeric fluid will be filled in the tube for the value, the two legs are connected to the points on which we are interested to calculate the differential pressure, when this done the fluid which flow in the pipe or tube will enter into both the legs, the pressure on the leg will differ showing the deflection of height in the manomertic fluid.

Principle: all manometers work on the effect of hydrostatic pressure exerted by a liquid column. In manometer unknown pressure is determined by balancing it against some known pressure or vacuum.

Construction and working:

The U-tube manometer consists of glass U-tube partially filled with a suitable liquid like water, mercury etc. one of the arms or legs of the manometer, is connected to unknown pressure tap to be measured while other is connected to other pressure tap or it is left open to atmosphere.

When there is difference of pressure between two arms of the manometer, liquid levels in the two arms of the manometer, liquid levels in the two arms do not match. This level difference in the two arms of the manometer represents differential pressure (P1-P2). The static balance equation is

P2-P1=h ρ gh=height differenceρ=mass density of manometer liquidIf the fluid over manometer liquid has appreciable density, then static balance equation can be written as:P2-P1= h (ρm - ρl) gh= height differenceρm = mass density of manometric liquidρl = mass density of fluid over manometric liquid

INCLINED –LEG MANOMETER:

The construction is very similar to enlarged leg manometer except that small diameter tube is inclined to the vertical axis.When pressure P1 and P2 are applied then liquid rises in the tube,the level of manometer ic liquid inside the tube is measured from zero level along the inclined tube which represents the differential pressure (P1 – P2) the static balance equation can be written as P2-P1= ρd sinα[1+(A1/A2)]α=anlge of inclination of the inclined leg d= height difference measured ADVANTAGES:Due to inclined leg, the manometer reading gets amplified. Hence it can be used for measurement of low pressures of which cannot be measured by other manometers.By reducing angle α, the scale length and hence the sensitivity can be increased.

MANOMETRIC LIQUIDS:

Desirable properties of good manometric liquid should have

Lowfreezing point
High boiling point
Non wetting characteristics
Low surface tension
Chemically inert
Clear visible interface
Ability to maintain density at various temperatures

MANOMERTIC FLUIDS USED IN PRACTICE ARE:

Mercury: mercury has low freezing point(-38F) and high boiling point (675F) but it corrodes many metals and it is poisonous and expensive.Water with colouring agents: colour agents reduce surface tension of pure water, that reduce the capillarity effect in manometer.Benzene, Kerosene, CCl4,toluene etc.. to make CCl4 visible a small amount of iodine crystals can be added.

CALIBRATION:

Manometer is subjected to known differential pressure and the corresponding height difference is noted. The calibration curve can be prepared by plotting height difference versus differential pressure. This curve can be used to get the differential pressure for certain height difference.

SOURCES OF ERROR:

Temperature effect: rise in temperature causes decrease in manometric liquid density that affects the calibration which leads to an error.

Capillary rise: to avoide capillary rise effect, the tube diameter should be over 10mm otherwise capillary rise results in error in pressure reading.

Meniscus shape: For water the free surface is concave while for mercury free surface is convex. The level of manometric liquid should be noted at the center of the meniscus.

ADVANTAGES AND LIMITATIONS:

Advantages: Simple inexpensive construction

High accuracy and sensitivity
Can be used for low pressure measurements
Desired span can be obtained just by using suitable manometric liquids
Pressure range of manometers is 3 to 100KPa.

Limitations:

No over range protection
Requires larege space
Non portable
Leveling is required
Condensation of test liquid affects the reading.

ELASTIC PRESSURE TRANSDUCERS

Transducers are a device that converts one form of energy into some other form. These pressure gauges have an elastic element that converts pressure signal into proportional mechanical displacement. In this article we study Bourdanon gauge, bellow gauge, diaphragm gauge and capsule gauge.

BOURDON PRESSURE GAUGE:

Principle: E.Bourdon introduced Bourdon tube in 1852 as a curved or twisted tube having non circular transverse section, according to Bourdon theory a tube having internal cross section that is not a perfect circle if bent or distorted has the property of changing its shape with internal pressure variation, this cause the free end deflection of the tube which can be taken as the measurement of change in pressures inside it.

Construction: Bourdon pressure gauges use different types of Bourdon springs as C-shaped Bourdon tube which if formed by winding the tube to form a segment of a circle having arc- length of about 270 degrees. In spiral type, number of turns is wound in the shape of a spiral about a common axis. In helix type number of turns is wound in helix form. In these figures ‘P’ indicates direction of application of pressure, while ‘T’ indicates tip travel for rise in pressure

We study C-shaped Bourdon tube gauge as it consists of a C-shaped Bourdon tube, tip, adjustable link, segments lever, sector, pinion, spring, and pointer. A C-shaped Bourdon tube is a thin walled tube having a non circular or nearly elliptical transverse section as one end of the tube soldered or welded to a socket at the base through which pressure is fed inside the tube while the other end is sealed by a tip. Adjustable link, segment lever sector or pinion are connected to the tip, that convert linear motion of the tip into proportional rotary motion which is given to the pointer that moves on the scale calibrated in terms of pressure. A hair spring is connected to the spindle on which sector is mounted, that provides the necessary tension for meshing sector and pinion thus eliminating any backlash.

Under range protection is particularly required for gauges having partial ranges (like 20to 50psi).

Bourdon tube material: a bourdon spring can be made of any metal or alloy that exhibits satisfactory elastic properties. Material used are- brass, phosphor bronze, monel, beryllium, copper, stainless steel etc.

Pressure range:

C-shaped tube	0 to 1,00,000 psi
Gauge pressure	0 to 12,000psig
Absolute pressure	0 to 100 psia
Vacuum	0 to 30’’ Hg

Working:

When fluid under pressure to be measured enters the bourdon tube, its cross section tries to become more and more circular that caused straightening of the tube. Since one end of the tube is fixed straightening cause the free end to deflect that is called a tip travel. The amount of tip travel for given rise in pressure is a function of tube length wall thickness cross section geometry and elastic module of the tube material. This linear tip travel is guided and amplified by adjustable link and segment lever and then it is given to sector and pinion arrangement. Sector and pinion convert the amplified tip travel into proportional rotary motion of the pointer connected to the pinion. The pointer defection can be read on the scale calibrated in terms of pressure.

Helical and spiral types bourdon tubes have many numbers of turns hence the tops movements for give change in pressure is more than that for single turn C- shaped tube.

Gauge pressure measurement: when unknown pressure is fed inside the bourdon tube and its out side is exposed to atmosphere, the reading would be in gauge units.
Absolute pressure measurement: when unknown pressure is fed inside the tube and its outside(instrument case)is evacuated, then reading would be in absolute units.

Vacuum measurement: procedure is similar to gauge pressure measurement. Bourdon vacuum gauge have poor accuracy.

Calibration:

Bourdon gauge are calibrated using dead weight tester or by comparison calibration.
Advantages, limitations, Applications:

Advantages:

1.low cost and simple construction
2.wide pressure range
3. high accuracy in relation to low cost

Limitations:

1.Low spring gradient
2.Susceptibility to shock and vibration
3.Bordon tube material possess some hysteresis in a pressure cycle, hysteresis can be kept minimum by proper heat treatment and by using proper materials.

Methods for Composition Analysis and Measuring Instruments

2011-07-08T09:24:00.001-07:00

A) Methods based on atomic or molecular or crystalline characteristics of matter:

Absorption Spectroscopy
Emission Spectroscopy
Mass Spectroscopy
X-Ray Diffraction Method

B) Methods based on measurement of following physical characteristics of matter:

Thermal Conductivity
Electrolytic Conductivity
Combustibility
Magnetic Susceptibility(Oxygen analyser)
Physical Absorption Characteristics(Chromatography)
Refractive Index
Density or Specific Gravity
Viscosity
Acidity or Alkalinity(pH)

Forced circulation evaporators:

2011-07-08T09:22:00.000-07:00

In natural circulation evaporators the liquid enters with velocity 0.3 to 1 m/s and generally the heat transfer coefficients are very low and particularly with viscous liquids. So, whenever we are dealing with concentration of highly viscous and scale forming solutions there is no alternative but to use forced circulation evaporators. By increasing the velocity of the liquid flow (generally 2 to 6 m/s) through the tubes heat transfer coefficients increases enormously and high liquid velocity which is resulting from the pumps prevents the scale formation on heating surfaces.

These are two types. They are,

(i) Forced circulation evaporator with horizontal heating element

(ii) Forced circulation evaporator with vertical heating element

Construction:

They consist of a centrifugal pump, a 1-2 shell & tube heat exchanger on the case of horizontal heating element and 1-1 shell & tube heat exchanger in the case of vertical heating element, an evaporator body with vapor outlet at the top, deflector plate, and outlet for product discharge at the bottom.

Working:

Centrifugal pump forces the liquid through the tubes at high velocity and gets heated by condensing steam on shell side. Solution becomes superheated and flashes into mixture of vapor and liquid in the evaporator body. Deflector plate separates the vapor and liquid in the vapor space, and vapors are removed at the top, concentrated liquid comes put at the bottom.

Advantages:

1. High transfer coefficients obtained even with viscous solutions.

2. Whenever we are dealing with concentration of highly viscous and scale forming solutions forced circulation evaporators prevents the scale formation on heating surfaces

3. Residence times are low so that heat sensitive viscous materials also can be used.

Disadvantage:

The main disadvantage of forced circulation evaporators is high pumping cost.

Applications:

These types of evaporators are widely used in industries for salting, viscous and scale forming solutions. And used in crystallizing operations in which it is necessary for the solids to be in suspension.

Evaporation

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Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. Evaporation from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces.

As shown in the accompanying interactive animation, the rate of evaporation depends upon:

Wind speed: the higher the wind speed, the more evaporation
Temperature: the higher the temperature, the more evaporation
Humidity: the lower the humidity, the more evaporation

Alt | Win

Evaporation and Types of Evaporation Equipments

Objective: “To concentrate a dilute solution consisting of non volatile solute and volatile solvent”

In this operation, the solvent to be evaporated is generally water and concentrated solution is a product.

The vapour generated usually has no value, it is condensed and discarded.

Properties of evaporating liquids that influence the process of evaporation:

1. Concentration: As the concentration increases, the viscosity and density increases thereby the boiling point of solution increases. The increase in boiling point and viscosity reduces the rates of heat transfer in evaporators.

2. Foaming: Solutions like organic compounds tend to foam during vaporization .The foam is carried away along with vapor leading to heavy entrainment.

3. Scale: Solutions deposit scales on the heating surface .overall heat transfer coefficient drastically decreases and leads to shut down of the evaporators for cleaning of the tubes.

4. Temperature sensitivity: Pharmaceuticals products, fine chemicals and foods are damaged when heated to moderate temperatures for relatively short times. So special techniques are employed to reduce temperature of the liquid and time of heating.

5. Material of construction: Evaporators are made of some kind of steel. however many sols attack ferrous metals and are contaminated by them . Copper, nickel, stainless steels can also be used.

Single effect evaporation:

The solution to be concentrated flows inside the tubes.The heating medium is steam condensing on metal tubes.Usually the steam enters at 3 atm abs. and boiling liquid is under moderate vacuum.This increases the temperature difference between the steam and boiling liquid.When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. This is called single effect evaporation.It is simple but utilizes steam ineffectively.To evaporate 1 kg of water from the solution we require 1-1.3 kg of steam.

Multiple effect evaporation:

Increasing the evaporation per kg of steam by using a series of evaporators between the steam supply and condenser is called multiple effect evaporation

Classification of evaporators:

Evaporation is a process of vaporization of a volatile solvent from a solution in order to increase the concentration of the solute.

(1) Natural circulation evaporators

(a) Long tube vertical falling film evaporator

(b) Long tube vertical rising film evaporator
These are used generally for simple evaporation operations either as single effect or multiple effects.

(2) Forced circulation evaporators

(a) Forced circulation evaporator with horizontal heating element

(b) Forced circulation evaporator with vertical heating element

These are used generally for salting, viscous and scale forming solutions.

(3) Agitated film evaporator

The main advantage of this type is to give high heat transfer coefficients even with very high viscous liquids. This is a modified falling film evaporator. Mechanical agitation is used to reduce the viscosity of very high viscous solutions.

Industry applications:

Long tube vertical rising film evaporator:

These types of evaporators are widely used in industries for handling of foaming, frothy liquors,and used for the production of condensed milk

Long tube vertical falling film evaporator:

These types of evaporators are widely used in industries for concentrating highly heat-sensitive materials such as orange juice, food materials etc. which require short residence times. And used for the production of condensed milk.

Forced circulation evaporators:

These types of evaporators are widely used in industries for salting, viscous and scale forming solutions,and used in crystallizing operations in which it is necessary for the solids to be in suspension.

Agitated film evaporator:

The main advantage of this type is to give high heat transfer coefficients even with very high viscous and heat sensitive liquids such as gelatin, rubber latex, antibiotics and fruit juices. But these are very costly and will be having smaller capacities.

Long vertical tube evaporator

These are two types.
(i) Long vertical tube falling film evaporator
(ii) Long vertical tube rising film evaporator

(i)Long tube vertical (climbing film) rising film evaporator:

Construction:

This consists of a long tubular heating element incorporating the tubes 25 mm to 50 mm and 3 to 10 meters in length. And also it contain at the top a separator or vapor space for removing entrained liquid from the vapor. Deflector is provided just above the tube sheet.

Working:

The feed at near boiling is fed to the bottom of the evaporator. It is then pumped inside the tubes. Steam is provided on the shell side. Liquid and vapor are separated in the vapor separator at the top. Deflector acts as a both primary separator and foam breaker. Multiple effects are used to achieve higher steam economy.

Applications:

These types of evaporators are widely used in industries for handling of foaming, frothy liquors,and used for the production of condensed milk.

(ii)Long tube vertical falling film evaporator:

Construction:

This consists of a long tubular heating element incorporating the tubes 50 mm to 250 mm and 3 to 10 meters in length.

Working:

The feed to the evaporator is fed to the top of the evaporator through efficient liquid distributors. Steam is given on the shell side. The concentrate is collected at the bottom. The same operation is repeated in multiple effects to achieve steam economy.
The main problem in this type of evaporator is that of distributing the liquid uniformly as a film inside the tubes. This is done by a set of perforated metal plates above a carefully leveled tube sheet.

Applications:

These types of evaporators are mostly used in industries for concentrating highly heat-sensitive materials such as orange juice, food materials etc. which require short residence times.

Residual Stresses - Definition, Properties, Effect on Behavior of Tension Members

2011-07-05T10:03:00.000-07:00

Residual Stresses

Tension members response to load is much similar to that of tensile-test coupon .
Member behavior may differ from coupon behavior because of:
Slip in bolted & riveted connections
Non-linear behavior of connections
Residual stresses in member
Residual stresses result principally from non-uniform cooling of hot rolled or welded shapes and from cold straightening of bent members.

They have more surface exposure per unit volume, the flange tips tend to cool faster than flange-to-web junctures.
Similarly, Central portion of web tends to cool faster than junctures
As a result, metal at junctures continues to contract as it cools after flange tips and web interior have cooled to temperature of surroundings.
This contraction is partially restrained by cooler metal.
Developing tensile stresses in regions of junctures & compressive stresses in remainder.
These are called residual Stresses
Magnitude and distribution of thermal residual stresses are influenced to considerable degree by geometry of x-section
20 W’s were investigated
An investigation on W’s Revealed that flange tip stress varied from 4.1 to 18.7 Ksi
Residual stresses in web varied from 41Ksi comp; to 18.2Ksi tension.
Showing some W’s develop residual tension over entire web, instead the pattern shown.
Only one out of 20 sections was thicker than 1 in.
Residual stresses tend to increase in magnitude with increase in thickness
Because of high concentration of heat, tensile residual stresses at the weld in welded members usually equal the yield strength of the weld metal itself.
Residual stresses in welded shapes are determined by section geometry and method of preparation of components.
Example H may be fabricated from Universal-mill plates or from plates flame-cut to width.
Because they are quenched and tempered, A514 rolled steel shapes are partially stress-relieved, so residual stresses are small
Quenching is the act of rapidly cooling the hot steel to harden the steel.
Quenched steel is hard and brittle. Often it is just too brittle and must be made more malleable, This is achieved by a process known as tempering.
The quenched steel is heated again but this time to a temperature between 200 °C and 300 °C.
When the metal reaches the tempering temperature, it is quenched again in cold water or oil. The result is a steel that is still hard but is more malleable and ductile
Fabricating operations such as cambering and straightening by cold bending also induce residual stresses.
These stresses are superimposed on the thermal residual stresses.
If member is straightened by rotorizing, which is a continuous procedure, the residual stress distribution will be changed along entire length of member.

UTM - Universal Testing Machine

HASEEB JAMAL

ENGINEERING MECHANICS

A machine used to test specimens for tensile strength, compressive strength, shear strength and to perform bend test along other important laboratory tests. The primary use of the testing machine is to create the stress strain diagram.

Once the diagram is generated, a pencil and straight edge or computer algorithm can be used to calculate yield strength, Young’s Modulus, tensile strength or total elongation.

Components of UTM:

It consists of two main parts, called:

Loading Unit
Control Unit

Loading unit

In this unit actual loading of the specimen takes place - consists of three cross heads namely upper head, middle head and lower head. Using appropriate cross heads tensile, compressive, shear, bending load with the help of different attachment can be applied. Loading unit of a UTM consists of:

Upper cross head – To clamp testing specimen from top
Lower cross head – To clamp testing specimen from below
Table – to place the specimen, used for compression test

Control Unit:

The load is applied and recorded by this unit. The load is applied with control valve and released by release valve. The load is applied with the help of hydraulic pressure.

Extensometer

An instrument used to measure elongation in the material

Tests UTM can perform

Tensile Tests
Adhesion Tests
Cycle tests with momentary stops
Pull-Out Tests
Creep Tests
Hysteresis Tests

Displays Test Traces and Values

Test Traces: An ongoing test can be displayed as either:
Load/Displacement
Load/Position
Load/Time
Position/Time
Displacement/Time

Digital Indicating Windows: The following are displayed:

Maximum Load (peak hold)
Current Load (during a test)
Cross head Position
Displacement (from the start of a test)
Applications of Universal Testing Machine

Use of Universal Testing Machine:

Universal Testing Machine can be used and applied to perform tests on the following samples.

Rope
Steel Rope
Winches
Steel Wire
Electrical Wire
Webbing
Spring
Slings
Cable
Nylon Rope
Links
Chain
Steel Chain

Plastic Analysis

2011-07-05T09:55:00.000-07:00

Definition:

Plastic analysis is defined as the analysis in which the criterion for the design of structures is the ultimate load. We can define it as the analysis inelastic material is studied beyond the elastic limit (which can be observed in stress strain diagram). Plastic analysis derives from a simple mode failure in which plastic hinges form.

Actually the ultimate load is found from the strength of steel in plastic range. This method of analysis is quite rapid and has rational approach for analysis of structure. It controls the economy regarding to weight of steel since the sections required by this method are smaller than those required by the method of elastic analysis. Plastic analysis has its application in the analysis and design of indeterminate structures.

Basics of Plastic analysis:

Plastic analysis is usually based on the idealization of stress strain curve as perfectly plastic. In this analysis it is assumed that width thickness ratio of plate elements is small so the local buckling does not occur. Broadly speaking the section will be declared as perfectly plastic. Keeping in mind these assumptions, it can be said that section will reach its plastic moment capacity and after that will be subjected to considerable moment at applied moments.

Principles of Plastic analysis:

There are following conditions for plastic analysis

Mechanism condition
Equilibrium condition
Plastic moment condition

Mechanism condition:

When the ultimate load is reached collapse mechanism usually formed.

Equilibrium condition:

∑ FX=0,     ∑ FY=0,     ∑ Mxy=0

Plastic moment condition:

The bending moment at any section in the structure should not be more than the full plastic moment (moment at which plastic hinges form and structure moves to failure) of the section.

Plastic moment:

If we consider the case of simply supported beam, when the load is gradually applied on it, bending moment and stresses increases. As the load is increased, the stresses in fibers of beam reach to yield stress. At this stage the moment which has converted the stresses into the yield stress is said to be as Plastic moment. it is usually denoted by Mp.at this stage the beam member cannot take up any additional moment but may maintain this moment for some amount of rotation and acts like a plastic hinge(hinge means having no capacity to resist moment).Plastic hinge behaves like an ordinary hinge allowing free rotation about itself. The yield moment and plastic moment has relationship which can be described by help of following relation:

My = 2/3 Mp

In calculation of plastic moments the term shape factor has its own importance. Shape factor can be defined as the ratio of plastic moment to yield moment is said to be as the shape factor. Shape factor depend usually on shape of the cross section.

For rectangular cross section the plastic moment can be calculated as:

Yield stress x   (bh2/4)

When the load is applied on the body which is elastic (return to its shape after the load is removed), it will show resistance against deformation, such a body is called to be as structure. On the other hand if no resistance is shown against the body, then it is known as mechanism. when plastic hinges equal to n+1 form in the structure, then the structure will collapse(where n is degree of indeterminacy of structure).It means if the plastic hinges in structures increases in number than the their degree of indeterminacy, structures move towards collapse.

Plastic hinge and degree of indeterminacy:

Whenever plastic hinge forms in the structure, equilibrium is obtained. As the result the degree of static indeterminacy reduces by one with the formation of one plastic hinge. We can say that if the structure has ‘n’ number of degree of indeterminacy, its degree of indeterminacy reduces and it becomes determinate structure if ‘n’ number of plastic hinges forms in it.

Keywords: Elastic Plastic Analysis, Elasto-Plastic Analysis of structures, Design of Steel Structures

Engineering Stress and Strain - Curve

2011-07-05T09:43:00.000-07:00

Engineering Stress

These quantities are defined relative to the original area and length of the specimen. The engineering stress (øe) at any point is defined as the ratio of the instantaneous load or force (F) and the original area (Ao).

Engineering Stress
Engineering Strain

Engineering Strain:

The engine

True Stress and Strain

Also see Engineering Stress and Strain

True Stress

The true stress (ø) uses the instantaneous or actual area of the specimen at any given point, as opposed to the original area used in the engineering values.

TRUE STRESS FORMULA

True Strain

The true strain (e) is defined as the instantaneous elongation per unit length of the specimen.

TRUE STRAIN

Relation between True Stress and True Strain

The relationship between true stress and true strain i.e. the flow curve can be expressed using the power law:

TRUE STRESS & TRUE STRAIN RELATIONSHIP

where K is called the strength coefficient and n the Strain Hardening exponent.ering strain (e) is defined as the ratio of the change in length (L-Lo) and the original length (Lo).

Engineering Strain.

Engineering Stress And Strain Curve:

Engineering stress-strain curve is obtained from the load-elongation curve. The Yield point, called the yield strength signifies the start of the plastic region.

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UTM - Universal Testing Machine

Components of UTM:

Loading unit

Control Unit:

Extensometer

Displays Test Traces and Values

Digital Indicating Windows: The following are displayed:

Use of Universal Testing Machine:

Plastic Analysis

Definition:

Basics of Plastic analysis:

Principles of Plastic analysis:

Mechanism condition:

Equilibrium condition:

Plastic moment condition:

Plastic moment:

Plastic hinge and degree of indeterminacy:

Engineering Stress and Strain - Curve

Engineering Stress

Engineering Strain:

True Stress and Strain

True Stress

True Strain

Relation between True Stress and True Strain

Engineering Stress And Strain Curve: