mymechanical

Condenser

noreply@blogger.com (Satchamo) — Wed, 19 Mar 2008 02:19:00 +0000

Since refrigerant leaves compressor in high pressure vapor form, so it requires a way to change vapor into liquid again. This is the function of condensing unit to condense vapor into liquid, so it can reuse in cooling cycle.
When cooling vapor pumped into condenser by compressor, the temperature and the pressure increase, that high temperature facilitate the effective heat transfer from condenser surface to entire room. Some of heat which is transffered to room air is a latent heat that absosrbs refrigerant in evaporator. Heat releasing into room is enough to condense cooling vapor into liquid.
There are some condenser types, i.e. air cooled with free air circulation, force air cooled with fan, indirect air cooled through thermal contact with a cabinet.
This type is usually bigger phisically than force air cooled with fan condenser, and construsted from small finned or fin that surrounded by wire. Condenser is put on back side of refrigerator and continous the thermal expansion to surrounding air room to create a convection current at pipes.
The construction of forced draft condenser type is usually made by small fin. The fan of the condenser convective to circuit, so only work during compressor work.
This type put in compartment and requires air baffles to direct air at that small fins.
Warm wall condenser is other form of static condenser. At that condenser, condenser pipe is attached directly in thermal contact with inside of refrigeration. Heat is removed from refrigeration wall to surrounding air room. The outside wall of refrigeration will feel hot if this type condenser work.

Supercharging

noreply@blogger.com (Satchamo) — Wed, 19 Mar 2008 02:19:00 +0000

Supercharging meant to multiply the bunkering amounts into combustion chamber, where from the machine itself attached compressor to give the amount more airs, usually take place during intake step. The purpose of airs amount addition to piston chamber to enable the fuels amount more burning, so that will result in the effective pressure and the machine power is getting strength where machine weight only increasing but meaningless.
Supercharging provided with some forms compressor to boost up the pressure in intake channel. Machine for diesel motor at car utilized centrifugal compressor which is turned around by exhaust gas turbine.
Exhaust gas turbine alter the heat of exhaust gas and kinetic energy to rotary movement and there is no energy taken away from the machine to rotate compressor as according to pressure at rotator mechanic cover of compressor. The part of supercharging is consists of the exhaust gas turbine, bearing house and centrifugal compressor. Exhaust gas moves passing through the exhaust pipe direct to turbine side and give rotary movement at turbine impeller. Air intake flows through air filter and pipe to compressor side where cause pressure 0,2 – 1 kg/cm2 that resulted, pressure generation is depend on machine speed.Air pushed through pipe to intake channel and passes intake valve that opened into piston chamber. The shaft of impeller turbine and impeller compressor supported in bearing at bearing house and lubrication of machine lubrication system. The part of supercharging made with high precision, works above 72000 rpm and needs a good balance and bearing duty must very assure.

Electric Defrost System

noreply@blogger.com (Satchamo) — Sat, 15 Mar 2008 03:40:00 +0000

For diagnosing exactly about defrost, engineers have to understand about electric defrost system. During refrigerating cycle, evaporator collects humidity in frost form, that humidity heap must removed periodically to maintain refrigerating temperature. If the frost let continuous formed, it will close airflow in evaporator fins, so then heat transfer does not occur and freezer and food temperature will increase.
To defrost, refrigerator must be defrosted. This defrost procedure can be done automatically or by hand. If done by hand, the way is as follows.
1. Remove all frozen food from the freezer and the tray of cold storage; pack that frozen food with some lining of newspaper paper in order not to soften.

2. Direct the damper, so that the defrost water go into the tray of cold storage.

3. Turn the temperature control dial to OFF position.

4. Put a high vessel that contain hot water in freezer and then close the refrigerator door.
5. If the refrigerator overall cleaned when defrosting, food and freezer equipment and inner tray of cold storage can be removed and inside of refrigerator can be cleaned.

6. After the frost melt down, rub the freezer until clean.

7. Do not use sharp tools like knife or ice clamper to scrape the frost, use dull tool like plastic or rubber.

8. Return the tray of cold storage and damper to initial position.

9. Install temperature control dial at initial position.

10. Return the frozen food, inner equipment and food. Suggest to customer for defrosting the freezer if the frost heap reach ¼ inch.

Timed defrost with heater spool, so far the most popular automatic way to defrost refrigerator is timed defrost method that use heater spool.
This defrost system type usually consists of four main part i.e. as follows.

A. Defrost Timer

This automatic timer will active if the refrigerator enters defrost cycle. Generally, defrost timer consists of a motor timer, a gear train, a cam of I set contact, a terminal board, a wire spool, plate of prop and a container box. At some models, defrost cycle is activated by the composition of door switch, a cam at underside of trip of a counter mechanic. If the door has been opened and closed many times as much as amount that has been determined before, for example 20 times, and then defrost operation start works. If not use method above, timer defrost system will work with same pattern with timer automatic.

B. Sheath or Defrost Heater

Sheath or defrost heater, is an electric heater element that defrost (stop cooling) evaporator. Most defrost heater have the rod type that planted at fins or radiant type that placed near evaporator. That both types are easy to be changed.

C. Defrost Thermostat or Limiter

This thermostat is usually bimetal type, that function controlling the duration time of heater that given to evaporator. This thermostat installed at certain place to feel evaporator temperature, so that it can give indication if all frost has removed. Most defrost thermostat will turn off if evaporator temperature reaches 55° until 70°F, defrost heater can not be recharge until bimetal limiter switch getting cold again until 30°F.
Because of this reason, if defrost limiter switch shifted too close to defrost heater, it can reaches cut off temperature too quick, so that all evaporators defrosted. This unit will return to freezing cycle with spool that has not defrosted all. After some cycles, evaporator will be covered by ice. Air cannot flows through closed spool to cool freezer and the refrigerator.

D. Drip Through Heater

Generally, this heater is aluminum foil insulated blanket type and wrapped entire underside of drip through. This heater prevents defrost water freezing in drip through and possible to close drain and air flow to evaporator. Once again, the duration time of giving energy to this heater is determined by defrost thermostat. At model that use radiant heating element, no needs drip through heater. Radiant heating element located below evaporator and has function as evaporator heater and drip through heater.
To finish defrost operation, timer has to stop the fan motor of compressor and condenser to prevent hot air blowing to freezer area.

Reynolds Number

noreply@blogger.com (Satchamo) — Thu, 13 Mar 2008 01:42:00 +0000

Reynolds Number,
Re = (V.d1.ρ)/μ
Gives relation between density ρ, dynamic viscosity μ, and average velocity of fluid V in mass flow rates of fluid m, kg/s. From continuity equation, velocity V can be written:
v = (4m)/d1.d1.ρ.π
By substituting V in equation Re = (V.d1.ρ)/μ, can be obtained :
Re = (4m)/d1.μ.π

Cavitations

noreply@blogger.com (Satchamo) — Thu, 13 Mar 2008 01:42:00 +0000

If in a pump installation is used high suction head or if peripheral velocity at inlet side of impeller is too high, so it is possibly happened high enough local vacuum condition at inlet side of impeller. It may cause water vaporization, so that bubbles will appear. This phenomena is called cavitations. Cavitations often happen at area where atmospheric pressure decreased, for instance at mountains area, where water boils at lower temperature than lowland area. The bubbles that occurred will flow together with water flow. If at once upon a time, the bubbles reach a high pressure region, so the bubbles will collapse and disappear. If bubbles appearance and bubbles collapse occurs continuously, pump will noise and it may cause vibration. But, worse than that, cavitations may cause erosion and impeller damage. In worst condition, finally pump can not works anymore.
To prevent that trouble, low suction head should be used, suction pipe dimension to be determined in such a manner so that loss friction that happened can be made as low as possible, suction pressure is higher than vapor pressure of water pumped temperature.

GENERATOR

noreply@blogger.com (Satchamo) — Sun, 09 Mar 2008 23:46:00 +0000

Generator or referred also as a dynamo, is an appliance that change mechanic energy to electric energy. The work principle of generator is based on electro magnetic induction principality. If a wire is moved so that it cuts a magnetic field (the lines of force), hence at wire will be generated an electro motive force. Electro motive force will cause incidence of electric current that flows through the wire and generated electro motive force depend on magnetic field strength and wire speed to cut the lines of force. Increasing the density of the field or increasing the speed of the wire therefore increases the voltage.

The First Law of Thermodynamics

noreply@blogger.com (Satchamo) — Mon, 03 Mar 2008 01:57:00 +0000

The First Law of Thermodynamics state that energy can't be created and can't be destroyed either, but it can be changed from one form to another form. This law can be defined as Conservation Energy Law.
This law also can be stated with simple words i.e. during interaction between system and surronding, the amount of energy which is gained by system must be equal with the amount of energy which is discharged to surronding.

HYDRAULIC SYSTEM

noreply@blogger.com (Satchamo) — Sun, 02 Mar 2008 06:54:00 +0000

Hydraulic system is an energy transferring system by using fluid as a medium. This hydraulic system generally used to remove a component from one place to other place mechanically by using energy pressure that got from a hydraulic pump that driven mechanically (engine) that changes into pressure energy and kinetic energy in hydraulic system and it’s changed again into mechanical energy to work.

The History of Programmable Logic Controller (PLC)

noreply@blogger.com (Satchamo) — Sun, 02 Mar 2008 06:51:00 +0000

Programmable Logic Controller (PLC) has introduced first time in 1960’s. PLC made to decrease the cost of maintenance and replacement of the machine control system that use relay. Bedford Associates propose MODICON (Modular Digital Controller) for the company in America. MODICON 084 is the first PLC that used on commercially production. The production process requirement that is getting much cause the system has to be changed. If used system is mechanic relay, that thing will become a big problem indeed. Besides its useful period is limited, system also requires careful treatment. If damage occurred, hence will very difficult to find it. Therefore, controlling that is facilitating either in maintenance and or using is required.
In 1970’s, the dominant PLC technology is sequencer machine and CPU which is based on bit slice. Processor AMD 2901 and 2903 popular enough used in MODICON and PLC A-B. The ability of communication on PLC emerges in the early of 1973. The first system is Modbus dari MODICON.
In 1980’s the effort to standardize communications with protocol property of General Motor (MAP) has done. In 1990’s the reduction of new protocol and the modernization of physical layer of protocols that popular in the year of 1980 had done. The last standard was IEC 1131-3, it tried to combine the programming language PLC into one standard.

Physical Mechanism Of Natural Convection

noreply@blogger.com (Satchamo) — Wed, 23 Jan 2008 02:22:00 +0000

Many familiar heat transfer applications involve natural convection as the primary mechanism of heat transfer. Some examples are cooling of electronic equipment such as power transistors, TVs, and VCRs; heat transfer from electric baseboard heaters or steam radiators; heat transfer from the refrigeration coils and power transmission lines; and heat transfer from the bodies of animals and human beings. Natural convection in gases is usually accompanied by radiation of comparable magnitude except for low-emissivity surfaces.
We know that a hot boiled egg (or a hot backed potato) on a plate eventually cools to the surrounding air temperature. The egg is cooled by transferring heat by convection to the air and by radiation to the surrounding surfaces. Disregarding heat transfer by radiation, the physical mechanism of cooling a hot egg for any hot object in a cooler environment can be explained as follows.
As soon as the hot egg is exposed to cooler air, the temperature of the outer surface of the egg shell will rise as a result of heat conduction from the shell to the air. Consequently, the egg will soon be surrounded by a thin layer of outer layers of air. The cooling process in this case would be rather slow since the egg would always be blanketed by warm air, and it would have no direct contact with the cooler air farther away. We may not notice any air motion in the vicinity of the egg, but careful measurements indicate otherwise.
The temperature of the air adjacent to the egg is higher, and thus its density is lower, since at constant pressure the density of a gas is inversely proportional to its temperature. Thus, we have a situation in which some low-density or “light” gas is surrounded by a high-density or “heavy” gas, and in a vinegar and oil salad dressing rising to the top. This phenomenon is characterized incorrectly by the phrase “heat rises”, which is understood to mean heated air rises. The space vacated by the warmer air in the vicinity of the egg is replaced by the cooler air nearby, and the presence of cooler air in the vicinity of the egg speeds up the cooling process. The rise of warmer air and the flow of cooler air into its place continues until the egg is cooled to the temperature of the surrounding air. The motion that results from the continual replacement of the heated air in the vicinity of the egg by the cooler air nearby is called a natural convection current, and the heat transfer that is enhanced as a result of this natural convection current is called natural convection heat transfer. Note that in the absence of natural convection currents, heat transfer from the egg to the air surrounding it would be by conduction only, and the rate of heat transfer from the egg would be much lower.

CONDUCTION

noreply@blogger.com (Satchamo) — Thu, 10 Jan 2008 04:51:00 +0000

Conduction is the mode of heat transfer in which energy exchange takes place from the region of high temperature to that of low temperature by the kinetic motion or direct impact of molecules, as in the case of fluid at rest, and by the drift of electrons, as in the case of metals. In a solid which is a good electric conductor, a large number of free electrons move about in the lattice; hence materials that are good electric conductors are generally good heat conductors (i.e., copper, silver, etc.).
The empirical law of heat conduction based on experimental observations originates from Biot but is generally named after the French mathematical physicist Joseph Fourier who used it in his analytic theory of heat. This law states that the rate of heat flow by conduction in a given direction is proportional to the area normal to the direction of heat flow and to the gradient of temperature in that direction. For heat flow in the x direction, for example, the Fourier law is given as
Qx = -kA (dT/dx) Watt (1)
or
qx = Qx/A = -k(dT/dx) W/m2 (2)

where Qx is the rate of heat flow through area A in the positive x direction and qx is called the heat flux in the positive x direction. The proportionality constant k is called the thermal conductivity of the material and is a positive quantity. If temperature decreases in the positive x direction, then dT/dx is negative; hence qx (or Qx) becomes a positive quantity because of the presence of the negative sign in Eqs. (1) and (2). Therefore, the minus sign is included in Eqs. (1) ans (2) to ensure that qx (or Qx) is a positive quantity when the heat flow is in the positive x direction. Conversely, when he right-hand side of Eqs. (1) and (2) is negative, the heat flow is in the negative x direction.
The thermal conductivity k in Eqs. (1) and (2) must have the dimensions W/(m.C) or J/(m.s.C) if the equations are dimensionally correct. There is a wide difference in the range of thermal conductinities of various engineering materials. Beetwen gaes and highly conducting metals, such as copper or silver, k varies by a factor of about 10000. The highest value is for highly conducting pure metals, and the lowest value is for gases and vapors, excluding the evacuated insulating systems. The nonmetallic solids and liquids have thermal conductivities that lie between them. Metallic single crystals are exceptions, which may have vary high thermal conductivities; for example, with copper crystals, value of 8000 W/(m.C) and even higher are possible.
Thermal conductivity also varies with temperature. This variation, for some materials over certain temperature ranges, is small enough to be neglected; but for many cases the variation of k with temperature is signifficant. Especially at very low temperatures k varies rapidly with temperature; for example, the thermal conductivities of copper, aluminum, or silver reach values 50 to 100 times those that occur at room temperature.

Film Condensation Theory

noreply@blogger.com (Satchamo) — Thu, 10 Jan 2008 03:59:00 +0000

When the temperature of a vapor is reduced below its saturation temperature, the vapor condensed. In engineering applications, the vapor is condensed by bringing it into contact with a cold surface. The steam condensers for power plants are typical examples of the application of condensing of steam. If the liquid wets the surface, the condensation occurs in the form of a smooth film, which flows down the surface under the action of the gravity. The presence of a liquid film over the surface constitutes a thermal resistance to heat flow. Therefore, numerous experimental and theoritical investigations have been conducted to determine the heat transfer coefficient for film condensation over surface. The first fundamental analysis leading to the determination of the heat transfer coefficient during filmwise condensation of pure vapors (i.e., without the presence of non-condensable gas) on a flat plate and a circular tube was given by Nusselt in 1916. Over the years, improvements have been made on Nusselt's theory of film condensation. But with the exception of the condensation of liquid metals, Nusselt's original theory has been succesful and still is widely used.

Introduction of Pump

noreply@blogger.com (Satchamo) — Wed, 26 Dec 2007 01:53:00 +0000

A. Introduction
Pump is a machine or mechanic equipment which is required to lift liquid from low level to high level or to flow liquid from low pressure area to high pressure area and also as a debit booster in a piping network system. This is reached by making a low pressure at suction side of pump and a high pressure at discharge side of pump.
Principally, pump converts mechanic energy of motor into fluid flow energy. Energy which is received by fluid will be used to lift pressure and to bridge over resistances which are exists in the line that passed.
Pump also can be used in process that requires a high hydraulic pressure. This can be seen in heavy duty equipments. In the operation, heavy duty equipment requires a high discharge pressure and a low suction pressure. Due to low pressure at suction side of pump, so fluid will lift from certain depth, whereas due to high pressure at discharge side of pump, so it will push fluid to lift until reach desired height.

B. Classification of Pump
Pump can be classified into two categories, i.e.:
1. Positive displacement pump
2. Dynamic pump

B.1. Positive displacement pump
In positive displacement pump, energy is periodically added by application of force to one or more movable boundaries of any desired number of enclosed, fluid-containing volumes, resulting in direct increase in pressure up to the value required to moves the fluid through vales or ports into the discharge line. Positive displacement pump divided into three:
a. Reciprocating pump
b. Rotary pump
c. Diaphragm pump

B.1.1. Reciprocating pump
Reciprocating pump is a pump where mechanic energy of pump drive is converted into fluid flow energy which is transferred by using an element that moves reciprocating in a cylinder. Fluid enters through suction valve and leaves through discharge valve with high pressure. This pump delivery liquid in limited volume with output debit depends on rotation and the stroke of the piston. Liquid volume which is transferred during one piston stroke will be equal to multiplication of piston area and piston stroke.
B.1.2. Rotary pump
Rotary pump is a pump which transfers energy from drive to liquid by using an element that moves rotate in casing. Fluid is sucked from reservoir through suction side and then pushed away through closed casing to discharge side at high pressure. How much fluid pressure that will leave the pump depends on the pressure and the resistance of flow system, whereas output debit depends on rotation speed of rotary element, the rotary element is usually called rotor.
B.1.3 Diaphragm pump
Diaphragm pump is a pump that transfers energy from drive to liquid through drive rod that moves reciprocating to move diaphragm, so then suction and discharge appear by rotation between suction valve and discharge valve. The advantage of this diaphragm pump is just on the diaphragm that touches transferred fluid, so then reducing the contamination with other part especially driver part.

B.2. Dynamic pump
Dynamic pump is consist of one or more impeller which is completed by blades, which is installed on moving shafts and receive energy from pump motor and it is covered by a casing. Energize fluid enters impeller axially, and then fluid leaves impeller with relative high speed and collected in volute or diffuser, after fluid collected in volute or diffuser, the conversion of velocity head to pressure head occur, which is followed by velocity decreasing. After this conversion process has done and then fluid out of pump through discharge valve. Dynamic pump can be divided into some types:
a. Centrifugal pump
According to flow direction n the impeller, centrifugal pump divided into:
- Radial flow
- Axial flow
- Mixed flow
b. Special effect pump
- Jet pump
- Gas lift pump
- Hydraulic ram

B.2.1. Centrifugal pump
This pump is moved by motor. The power of the motor is given to pump shaft to rotate impeller which is installed on the shaft. Resulting in impeller rotation that cause centrifugal force, so liquid will flow from the center of the impeller and exit through line between blades and leaves impeller with high velocity.
The liquid that leaves impeller with high velocity, it through the line that the area is getting wide which is called volute, so then the conversion of velocity head to pressure head will occur, so total head of the liquid out of the discharge flange of the pump will get high. Whereas suction process occurs because after the liquid delivered by impeller, space between blades become vacuum, so then the liquid will be sucked into the impeller.
The total head of the liquid difference between discharge flange and suction flange is called pump total head, so then it can be said that centrifugal pump has function to convert motor mechanic energy into fluid flow energy. This is the energy that causes pressure head, potential head and velocity head addition continuously.
The advantage of the centrifugal pump compared with other type pump:
a. At the same head and capacity, centrifugal pump usage is the cheapest.
b. Has the cheapest operational.
c. Has smooth and uniform flow.
d. Reliable in operation.
e. Has low maintenance cost.
Centrifugal pump can be classified into some types:
a. According to capacity:
- Low capacity (<> 60 m3/hour)
b. According to output pressure:
- Low pressure (<> 50 kg/cm2)
c. According to specific speed:
- Low speed
- Medium speed
- High speed
- Mixed flow pump
- Axial flow pump
d. According to the number of the impeller with the stage:
- Single stage pump
- Multi stage pump
e. According to suction side of impeller:
- Single suction
- Double suction
f. According to casing design:
- Volute
- Diffuser
g. According to shaft position:
- Horizontal shaft pump
- Vertical shaft pump
h. According to drive system:
- Direct coupled to drive unit
- Through some various types of transmission

COMPRESSOR

noreply@blogger.com (Satchamo) — Wed, 26 Dec 2007 01:53:00 +0000

A. Definition of Compressor
Machines for repeated compression of gases are termed compressor. Since compression of the aspirated gas requires energy from some external source, these devices are working machine. By their action, compressors convert mechanical work into heat and therefore belong to the class of thermal machines.

B. Classification of Compressor
The pressure of a gas or vapor may be increased either by decreasing its volume (as in the case of positive displacement compressor) or by imparting to it a high kinetic energy which is converted into pressure energy in diffuser (dynamic compressor).
Positive displacement compressor is further sub divided into piston compressor in which the gas volume changes due to the action of one or two reciprocating pistons moving axially in a cylinder, membrane compressors, in which the volume variations are effected by deflection of an elastic partition, and rotary compressors of various types.
The dynamic principle is utilized in multi-blade compressors which are further sub divided into centrifugal and axial compressors, and in jet type devices.
In centrifugal compressors, a high peripheral velocity is imparted to the gas being compressed by the blades of the impeller which rotates at a speed of 3000 – 27000 rpm (in special cases speed in excess of 100000 rpm are reached). The centrifugal force developed in this way partially compresses the gas. Further compression occurs after the gas has left the impeller by reducing its velocity in the diffuser. The flow of gas here is approximately radial. In the other type of multi-blade compressors the gas streams mainly in the direction of the axis (hence the name axial compressors). The gas flow is effected by impellers having aerodynamically shaped blades and appropriately directed at the outlet from each set of blades by guide vanes, which act as diffusers increasing the pressure of the gas at the expense of its kinetic energy.
The essential parts of a jet compressor are the Laval nozzle in which the driving vapor attains a high supersonic velocity, the mixing chamber where the vapor is mixed with the gas or vapor to be compressed, and the diffuser into which the mixture enters at high velocity. Here kinetic energy is converted into pressure energy.
In other case, water or air is used as a driving agent in jet compressors. For water a convergent nozzle is used.
With piston compressors may be grouped hydraulic compressors in which the task of the piston is carried out by a water column falling through the compression tube.
Another criterion for the classification of compressors is the suction or delivery pressure. The ratio of delivery pressure to suction pressure is termed the compression ratio (or pressure ratio). This must be distinguished from the compression ratio as used in the design of internal combustion engines, where it denotes the ratio of the total cylinder volume including clearance volume to the clearance volume alone.
Machines operating with suction below atmospheric pressure and compressing to atmospheric or slightly higher pressure are termed vacuum pumps (multi-blade machines are usually term exhausters, jet type devices working under similar conditions are called ejectors).
Multi-blade machines overcoming only the resistance which arise in the flow of gases (up to 1200 mm of water column) are termed fans.
Machines with suction at approximately atmospheric pressure and having a compression ratio less than 3 may be termed blowers. Only machines for a final pressure of 2 to 500 atg are compressors in the narrower sense of the word. They may be further sub-divided into a) low pressure, b) medium pressure, and c) high pressure compressors. Up to the present, the interpretation of these terms has been rather inconsistent. For this reason, the limits of their application have been recently standardized on the basis of their final pressures as under:
1. Low pressure compressors for a final pressure of up to 25 atg
2. Medium pressure compressors for a final pressure of from 25 to 100 atg
3. High pressure compressors for a final pressure of from 100 to 500 atg
A classification of compressors according to size, based on the quantity of free air delivered is as follows:
1. Small – piston compressors handling up to 160 m3/hr
2. Medium – compressors handling 160 to 4000 m3/hr
3. Large – compressors handling more than 4000 m3/hr
For compressors having a high compression ratio the compression is divided into or more stages. This makes for improved safety and can also produce a reduction in the power input. Depending upon the fluid pumped and the size of the machine the maximum pressure ratio for single-stage compression may be from 5 to 8.
Multistage reciprocating machines for delivery pressures exceeding 500 atg are termed hypercompressors. They have up to 7 stages.
For compressors with a high suction pressure (e.g. 270 atg) and low compression ratio (e.g. 1,1) the term circulation pump or circulator has been adopted.
A further criterion of compressor classification, important from the point view of the designer, is the medium to be compressed. This may be air, oxygen, hydrogen, nitrogen, acetylene, lighting gas, and vapors and gases used in refrigeration cycles such as methyl chloride, carbon dioxide, the fluorocarbons, ammonia, etc. Their physical and chemical properties exert a considerable influence upon the design of compressors.
Further differentiation may depend upon type of drive, method of cooling, arrangement of cylinders, etc.
Each type of compressor has its own operation characteristics which limit its efficient application.
In multi-blade compressors leakage losses are significant. For this reason these compressors are suitable for low compression ratios and the compression of large gas volume. In the case of positive displacement machines, rotary compressors are suitable for low compression ratios and small or medium gas volumes, while reciprocating compressors are used for medium and high compression ratio irrespective of the volume to be compressed. Membrane compressors, in which the absence of leakage and of oil contamination are the main assets, are used only for small volumes of gas.
Jet compressors are capable of operating at low pressure and can handle the largest volumes of vapors or gases to be compressed.

Design of Centrifugal Pump for Sea Water Circulation

noreply@blogger.com (Satchamo) — Mon, 17 Dec 2007 00:49:00 +0000

Industries that use equipments or machines in its production process certain need cooling for the machines. Cooling is used to maintain the performance of the machines and make them long lived. Usually, cooling is done by water as a coolant. Production quality of industry is depend on its production machines performance, while production machines performance is determined by cooling system of the machine.
PT. Kaltim Parna Industry use sea water as coolant in its production process because of the location that near the sea. Besides PT. Kaltim Parna Industry, other industries circumjacent it also use sea water in their production process, so then sea water temperature increase. Therefore, PT. Kaltim Parna Industry use Sea Water Cooling Tower in its system in order to prevent efficiency descent as a result of increasing temperature of sea water as a coolant.
In this research, a centrifugal pump which is used for circulating sea water in Sea Water Cooling Tower is designed, so highly expected that the efficiency of the system will not decrease because sea water which is used as a coolant is cold sea water after circulated at Sea Water Cooling Tower.
This designed sea water circulation pump is a double suction centrifugal pump which has capacity 2,5 lt/s and total head 47 m. This pump use impeller which outside diameter (d2) is 1019 mm, impeller eye diameter (d1) is 633 mm and it has 9 blades. Specific speed (nsq) of pump impeller is 36,748. Prime mover of this pump is an induction motor with 590 rpm rotation and power 2000 HP. From this design is obtained total pump efficiency 89% and pump characteristic is good enough and filled the bill.
Keyword : pump, circulation, sea water

REFRIGERATION MACHINE

noreply@blogger.com (Satchamo) — Mon, 17 Dec 2007 00:49:00 +0000

A. Definition of Refrigeration Machine
Refrigeration is the production and maintenance of temperature degree of a material or chamber at a degree which is lower than surrounding temperature by material or chamber’s heat absorption. In other hand, refrigeration is a process of heat transfer from a material or chamber to other material or chamber.
According to definition explanation above, we can say that refrigeration machine is a device that applied refrigeration principle.

B. Principle of Refrigeration Machine
The principle of refrigeration machine is so simple i.e. absorbing heat of an insulated chamber and then carried it over surrounding.
The principle of refrigeration machine also can be explained broader i.e.:
1. Managing heat in order to create low temperature. The purpose is creating a lower temperature degree than surrounding temperature at a material or chamber, by getting heat absorption from a material or insulated chamber and then carried it over surrounding.
2. Employing the change of energy form. In heat absorption, refrigerant changes from liquid to vapor. Afterwards refrigerant vapor is compressed in order to increase its pressure and temperature and then refrigerant is condensed again as a liquid, so heat condensation which is absorbed before is released and gone with the coolant (air or water). At whole system, heat only flow from a hotter material to a colder material. The change of heat and other energy form (electricity energy, mechanic energy, etc) is involved in all process and cycle.
3. Employing engineering, thermodynamic and physic process. In the change of refrigerant form process, the change of pressure and temperature refrigerant is involved. In the flow of heat from a hotter material to a colder material, with refrigerant flow regulation in the system, low pressure side and high pressure side will be created. Temperature difference will be created if there is a pressure difference.
4. Employing thermal and physic characteristics of various type of material. The exchange of heat which is going on all cycle is employing thermodynamic characteristics of materials included refrigerant, material that has high thermal conductivity (ex. steel, aluminum, copper) and material that has low thermal conductivity (insulation material such as polyurethane).
5. Restraining and controlling process. All process and cycle of refrigeration system must be restrained and controlled in order to make heat transfer and pressure and temperature change run smoothly.

C. Vapor Compression Cycle
Processes that run in refrigeration system i.e.:
1. Evaporation. In this process, liquid refrigerant that exist in evaporator metal pipe boils and evaporate at constant temperature, although it has absorbed much heat from surrounding i.e. fluid and food in close insulated chamber. Heat absorption during boiling and evaporation that runs without refrigerant temperature increase, shows how refrigerant act as material or freezing and refrigeration agent effectively. Absorbed heat is called evaporation latent heat.
Component of refrigeration system where evaporation process runs, is called evaporator. Vapor which is formed leave evaporator in saturated state and because it absorbs heat that comes from outside suction pipe or heat exchanger device, so its state becomes superheated but the pressure doesn’t change when sucked into compressor.
2. Compression. Low pressure and temperature refrigeration superheated vapor that comes from evaporation process is compressed by compressor becomes high pressure and temperature vapor in order to condensed easily then, vapor becomes liquid again in condenser.
Compression process runs at high velocity (in short time) and the difference of temperature between refrigerant vapor and compressor wall is very small, so then heat flow between both of them can be neglected. Therefore, vapor compression process in compressor is assumed runs adiabatically, it means that there is no heat which is added or eliminated, or more specific called isentropically i.e. adiabatic without friction. As a consequence, gas energy increase accord with amount of heat which its value equal to work which is done to gas to compress it (compression heat). It means that vapor enthalpy and temperature proportional to energy or compression heat which is provided by compressor. Theoretically, required power to move compressor also can be determined directly from compression heat.
Vapor temperature which is discharged by compressor always bigger than saturation temperature that appropriate to vapor pressure. That vapor is in superheated state which its pressure and temperature some times over pressure and temperature inlet compressor.
3. Condensation. Condensation process is a rejection or heat transfer process from high pressure and temperature vapor refrigerant which is result of compression compressor to condensate outside condenser. Condensate is air, water or air – water combined. All latent heat which is absorbed by refrigerant during evaporation process is discharged to outside refrigeration system. Heat which is discharged per vapor refrigerant weight unit to be condensed is called condensation latent heat, whereas the numeric value is equal to evaporation latent heat numeric value.
In order to maintain the directness of refrigeration effect, so, refrigerant temperature must be maintained below refrigerated product or chamber temperature degree. In order to proportion, heat that discharged with condensate in condenser must be at the same velocity like heat that absorbed by system in evaporator, suction pipe and compressor.
Heat transfer rate through condenser is depend on temperature difference between refrigerant and condensate, because in every condenser, condensation surface area and conductance coefficient of condenser pipe wall has been already a constant factor when condenser made. Condensation temperature is equal to condensate temperature added temperature difference between condensed refrigerant and condensate. That condensation temperature is varied to heat transfer rate which is required by condenser. Then, liquid refrigerant that formed during condensation process at high pressure and temperature flows into refrigerant controller device (expansion valve).
4. Expansion. Expansion is an opportunity regulation process for liquid refrigerant to expansion in order to able to boils and evaporates in evaporator then. Liquid pressure is reduced to evaporator pressure, so then refrigerant saturation temperature below refrigerated chamber temperature. Refrigeration system needs refrigerant regulator in order to make compression and suction process in compressor can create and maintain a pressure difference which is required to change refrigerant state from liquid to vapor. This task is done by refrigerant controller device, expansion valve or other type.

D. Refrigeration Machine Component
Refrigeration machine has 3 types component i.e.:
1. Main Component
Main component of refrigeration machine consist of:
a. Compressor
The function of compressor is moving refrigeration system in order to maintain the difference of pressure between low pressure side and high pressure side of the system. In implementing this function, compressor:
i. Creating low pressure side. Compressor sucks vapor refrigerant from evaporator, creating low pressure that makes possible liquid refrigerant to boil and evaporate at low temperature in evaporator. By this evaporating, heat is absorbed from food or product which is cooled or frozen in evaporator.
ii. Creating high pressure side. Compressor compress vapor refrigerant which is absorbed from evaporator, increase that vapor temperature and pressure then flow it to condenser in order to makes possible to be condensed becomes liquid by air or cooling water in condenser, so heat in refrigerant gone with air or cooling water.
So compressor has function as a pump that creates condition which is required to heat transfer in evaporator and condenser.
Compressor has many types i.e.:
- Reciprocating compressor
- Rotary compressor
- Screw compressor
- Centrifugal compressor
b. Condenser
Condenser is a part of refrigeration machine that receives high pressure superheated vapor refrigerant from compressor then reject condensation heat by cooling high pressure superheated vapor refrigerant to its dew point by rejecting its sensible heat. Next latent heat rejecting caused that vapor condense becomes liquid.
Types of condenser:
- Finned static natural convection
- Finned force convection
- Air cooled condenser
o Base mounted air cooled condenser
o Remote air cooled condenser
- Water cooled condenser
o Shell and tube condenser
o Shell and coil water cooled condenser
o Tube and tube condenser
o Evaporative condenser
c. Expansion valve
Expansion valve is used to expansion adiabatically low pressure and temperature liquid refrigerant until reach low pressure and temperature state; so, implementing throttle process or constant enthalpy expansion process. Besides that, expansion valve regulate refrigerant intake according to cooling load which have to be served by evaporator.
So, expansion valve regulate in order that evaporator always run well until obtained maximum efficiency of refrigeration cycle.
Expansion valve has many types i.e.:
i. Thermostatic automatic expansion valve
ii. Manual expansion valve
iii. Constant pressure expansion valve
iv. Capillary tube
d. Evaporator
Evaporator is a heat exchanger that plays the most important a part in refrigeration cycle, i.e. cools product.
There are some evaporator types, according to the purpose of use, the type can be different. It is caused media that will be cooled can be gas, liquid or solid. So evaporator can be divided into some group, according to refrigerant state which in evaporator, i.e.: dry expansion type, semi-wet type, wet type, and liquid pump type.
i. Dry expansion type
In dry expansion type, liquid refrigerant which is expanded through expansion valve, when go into evaporator it has already in air water mixture state, so it leaves evaporator in dry vapor state.
Because a considerable part of evaporator filled by vapor refrigerant, so heat transfer which is occurred is not so much, if compared with state where evaporator is filled by liquid refrigerant. But, dry expansion type evaporator doesn’t need refrigerant in large amount. Besides that, the amount of lubrication oil that left in evaporator is very small.
The amount of refrigerant that go into evaporator can be regulated by expansion valve so that all refrigerant leave evaporator in saturated vapor state, and even in superheated state.
ii. Semi-wet type evaporator
Semi-wet type evaporator is an evaporator with refrigerant state is between dry expansion type evaporator and wet type evaporator. In this type, there is always liquid refrigerant in its evaporation pipe. Therefore, heat transfer rate in semi-wet type evaporator is higher than dry expansion type, but lower than wet type.
In dry expansion type, refrigerant enter from up section of coil; whereas in semi-wet type evaporator, refrigerant is entered from down section of evaporator coil.
iii. Wet type evaporator
In wet type evaporator, considerable part of evaporator is filled by refrigerant liquid. The evaporation process runs like at boiler. Refrigerant bubble which is formed due to heating will raise, collapse on liquid surface or apart from its surface. Then some of refrigerant flow into accumulator that separate vapor from liquid. So, only refrigerant which in vapor form that flow into compressor. Liquid refrigerant which is separated in accumulator will go back into evaporator, together with refrigerant (liquid) which come from condenser.
So, evaporator tube is filled by liquid refrigerant. Refrigerant liquid absorbs heat from fluid which will be cooled (brine), which flows in pipe. Refrigerant vapor which formed is collected at up section of evaporator before enter compressor.
Refrigerant liquid surface height that is in evaporator is regulated by float valve; usually a little bit higher than half of tube height. The amount of refrigerant that is sent to evaporator tube is adjusted with cooling load which have to be served.
2. Auxiliary Component
Auxiliary component of refrigeration machine consist of:
a. Oil Separator
Oil separator is a device for separating oil from refrigerant vapor after leaving compressor and returning it to compressor. A little oil that brought to refrigerant system won’t harm, but if much oil that brought to refrigerant system will disrupt condenser, refrigerant controller, evaporator and filter.
Lubricant oil which is separated will be collected at down section of oil separator. If that oil surface has already reached a certain height, lubricant oil will flow to compressor crank-case automatically, i.e. if it float reach a certain position.
Because refrigerant can mix with lubricant oil, so oil presence in refrigerant system can’t be avoided completely. Thereby, in some refrigerant machine, lubricant oil re-enter to compressor over and over after trough condenser and evaporator, without using lubricant oil separator.
Lubricant oil separator must be set near compressor and must be kept in order to untouched by low temperature from condenser and receiver, when refrigeration machine not run.
b. Receiver
Receiver is used to temporary receive refrigerant which is condensed in condenser, before enter expansion valve.
Receiver volume must be able to serve refrigerant need according to load change. Besides that, receiver also has function to receive refrigerant from refrigeration machine, when the machine repaired or stop running for a long time. In this case, condenser also can be used to collect refrigerant too.
In Freon refrigeration machine with low capacity, receiver is oftentimes unused, because water-cooled condenser which is used can be function as receiver. But, in Freon refrigeration machine with large capacity, usually receiver is used, because condenser can’t receive all refrigerant from the machine. It is caused by condenser volume which is too small, moreover if finned cooling pipe is used. So it is if air-cooled condenser is used, with same reason above, receiver is always needed. In this case, refrigerant that already condensed in water-cooled condenser flows to (down) receiver without any left in condenser.
Therefore, receiver must be set lower than condenser and refrigerant temperature in receiver must lower than refrigerant temperature in condenser. The flow of refrigerant from condenser to receiver occurs because of gravitation.
But, if coolant temperature in condenser is relative low, and machine chamber temperature where receiver set is relative high, sometimes refrigerant liquid that occurs in condenser can’t flow easily. In this case, up section of condenser must be connected to up section of receiver by pressure equalizer.
c. Filter Drier
Filter drier is a device that besides evict vapor, also use to strain dirt, metal and clips from liquid refrigerant before flow to controller valve of a refrigeration system. Filter drier usually put in high pressure side liquid line, but there is a refrigeration system that puts filter drier at suction line (low pressure) to compressor.
In order to operate efficiently, a system must be clean. Refrigerant that allow to circulate in the system is only pure refrigerant which clean and dry (without water vapor) and at part of system, clean and dry oil (without water vapor, and free acid too.
Too much water vapor in the system will makes water vapor freeze and form ice in capillary tube and cause clogging at refrigerant controller valve, instead it can happens at whole system that operate at low temperature. Water vapor can fasten corrosion process. Acid formation also happens if lubricant oil expansion in system runs.
d. Accumulator
In compression system, especially that use reciprocating compressor, refrigerant that flows from evaporator should be in vapor form before entering compressor. If the refrigerant is still in liquid form, the compressor could be damage.
In order to avoid the entry of liquid refrigerant into suction pipe in compressor, some ways and equipments have already devised i.e. (a) installing an accumulator in suction pipe (b) using heat exchanger to heat up suction pipe (c) using electric heater to heat up suction pipe, and (d) using hot gas by-pass valve to push up hot gas in suction pipe in order to be able to evaporate every liquid, etc.
e. Heat exchanger
The duty of the refrigerant system is transferring heat. In processes and during refrigeration cycle, heat transfer process is developed i.e. the process that gets and loses heat. This thing can be balanced by heat transfer, in order to create efficiency of refrigeration system.
This heat exchange, in refrigerant system, runs in some methods:
i. Heat transfer that occurs by doing contact between suction pipe (low pressure) and liquid (high pressure) by attaching each other.
ii. By using heat exchanger.
iii. Heat transfer process also occurs at condensation process in condenser.
3. Control component
In order to work efficiently and economically, refrigeration machine must stop running when not needed. Refrigeration machine is not designed to work all day or year. So, it must use refrigeration machine control device to regulate compressor or electric motor, in order to run only when needed. The capacity of the refrigeration machine is designed at maximum load, whereas maximum load condition only occurs some hours in a day and some weeks in a year.
The control device of the refrigeration machine has to be able to regulate refrigeration machine to maintain required temperature and relative humidity as accurate as possible. That control device can be used to regulate the work: heating, cooling, drying, humidifying, etc. Besides that, control device is designed to do a little work but it can make cooling system provides maximum capacity and gives safety to component, material and the man whose near it.
The control device of the refrigeration machine only works mechanically, electrically or mechanic – electric combination.
The control device of the refrigeration machine can be divided into three:
a. Refrigeration flow control
b. Electric control
c. Combination control
3.1. Refrigerant flow control
Refrigerant flow control can control pressure and volume of refrigerant that flows in refrigeration system.
This control device is an obstacle, such as a valve that puts at one of cooling system section.
Compressor compress low pressure refrigerant becomes high temperature. Refrigerant flows back from high pressure to low pressure through refrigerant flow control or expansion valve. That control device can control volume and pressure of refrigerant from high pressure side to low pressure side. High pressure liquid refrigerant is decreased to low pressure in exact amount, in order to evaporate easily in evaporator at maximum efficiency, but doesn’t make the compressor gets over load.
Refrigerant flow control runs according to:
a. Pressure change
b. Temperature change
c. Volume change
d. Combination of changing above.
Refrigerant flow control can run manually or automatically, because of pressure difference or outside effect. In the past, refrigeration system use refrigerant flow control that its work is manually regulated by needle valve. In the present, with experience, advance design and maintenance, more efficient, economic and automatic refrigerant flow control has already created.
Refrigerant flow control can be used to control fluid (liquid and gas). This control device has a lot of types and each type has its own function. Generally, this control device is put at liquid line near evaporator, but there is a control device which is put at suction line or discharge line.
Refrigerant flow control consists of:
a. Automatic valve
b. Thermostatic expansion valve
c. Low pressure float valve
d. High pressure float valve
e. Capillary tube
f. Evaporator pressure regulator
g. Suction pressure regulator
h. Discharge pressure regulator
i. Check valve
3.2. Electric control
Electric control device is a device which runs use electric power and able to regulate electric current. Electric current that flows to coil can makes magnet and generate mechanic motion, so that can open or close: electric contact, valve hole, spin rotor of motor, etc.
Electric control device can control electric current that flows to electric motor and other control device of refrigeration system. This control device also can changing compressor capacity, automatically melting down ice in evaporator, transferring refrigerant from one section to another section of refrigeration section, starting, stopping, regulating and protecting refrigeration system and its component.
Electric control device consists of: switch, solenoid, contactor, electric motor protection, signal lamp, transformer and many more.
3.3. Combination control
Control device is a device which can start or stop, regulate and protect refrigeration system.
Refrigeration control device such as expansion valve has very important function as main control device of refrigeration system. Whereas other control device has function for adjusting if load changing occurred, giving safety and increasing work efficiency of refrigeration system.
Combination control has many types. Mechanic control device can makes straight motion that can move electric switch open or close. That motion can stop or start compressor and other control device. Electric control switch can directly work if changing of temperature, pressure or humidity happened.
Electric energy usually used to transfer controlling result or changing of controlled condition to other section of the system and to change measurement result becomes work at actuating control device. Using electric power has many advantages because electric current can be transferred trough electric cable which is simple and easily installed.
Combination control consists of:
a. Operating or primary control
b. Actuating or secondary control
c. Limiting and safety control
3.3.1. Operating or primary control
Operating control runs as electric switch to start or stop refrigeration system directly or indirectly. This control device must be able to make quick and accurate change in the limit which is determined.
Operating control device must be sensitive in order to be able to change quickly in required condition, if change of temperature, pressure or humidity happened. Operating control receives that changing and change it to mechanic motion, to open or close electric switch contact spot.
Some operating control devices such as:
a. Temperature control or thermostat
b. Pressure control or pressurestat
c. Humidity control or humidistat
Pressure control is used to regulate pressure limit of refrigeration system. Temperature control and humidity control is used to regulate temperature and humidity in a chamber which being regulated.
3.3.2. Actuating or secondary control
Actuating control device regulate and/or protect refrigeration system due to condition in system or required by operating control to do like that.
Actuating control device can’t directly control and improve refrigeration system output. This control device has assignment to do what commanded by operating control to control some sections of refrigeration system.
Some actuating control devices such as:
a. Relay starting or contactor
b. Solenoid valve
c. Flow reverse valve
d. Water regulating valve
e. Evaporator pressure regulating valve
f. Suction pressure regulating valve
3.3.3. Limiting and safety control
Refrigeration system requires certain pressure or temperature limit to protect compressor and other component from damage due to run out of designed limit. This is limiting control device duty to open its contact and stop refrigeration system activity before determined limit is crossed over.
Electric control device can makes secure condition and automatic too. Mechanic control device automatically controls system to run and stop, regulate work of refrigeration system and signal condition. This control device controls liquid or air flow and connected to safety device.
Limiting and safety control can makes temperature and pressure of refrigeration system still in determined limit. If pressure and temperature of refrigeration system over safety limit, limiting and safety control will run, so electric motor can’t start and which is running will stop.
Some limiting and safety control devices such as:
a. High pressure circuit breaker
b. Low pressure circuit breaker
c. Oil pressure control switch
d. Flow switch
e. Pressure relief valve
f. Overload motor protector
g. Fuse
h. Fusible plug