WeBBusterZ Engineering Articles

Heat Exchanger Suite

2022-11-21T13:32:00.000+00:00

Heat exchanger suite (HExS) is a collection of heat exchanger software that has been bundled together in one software under one license. Heat exchanger suite interface can access the following software products:

    Shell and tube heat exchanger design (S&THex)
    Shell and tube condenser design (CnD)
    Helical coil heat exchanger design (HCHex)
    Double pipe heat exchanger design (DHex)
    Vertical thermosiphon design (VTD)
    Gasketed plate heat exchanger design (PHex)
    Air cooled heat exchanger design (ACHex)
    Kettle reboiler design (KRD)

Visit webbusterz.com for more information on heat exchanger suite

Gasketed plate heat exchanger design (PHex) software v8.0

2019-12-16T10:02:00.001+00:00

Plate heat exchanger software (PHex) - v7.0

2017-01-22T16:47:00.001+00:00

Relief valve sizing calculator - Sizing for vaporizing liquids (wetted vessels)

2016-07-14T16:59:00.001+01:00

Relief valve sizing calculator

2016-07-14T16:51:00.001+01:00

Plate heat exchanger software

2015-10-11T19:19:00.001+01:00

Gasketed plate heat exchanger software - phex

2015-09-12T11:19:00.001+01:00

What is the difference between Shell passes and Tube passes

2013-11-26T10:47:00.000+00:00

Difference between Shell passes and Tube passes

In shell and tube heat exchangers the number of shell passes or the number of tube passes will always come across. The number of passes in general affects your pressure drops in shell and tube heat exchangers. This short article will show the difference between them for quick reference.

Tube Passes

The tubes are arranged within the shell and tube heat exchangers in passes. The passes can be One, Two, Four, Six or Eight.
This arrangement can be illustrated in the figure below:

Two tube passes

Four tube passes

Six tube passes

Eight tube passes

Shell passes

As shown below:

Shell passes

A Quick Note:
It’s always better to design within the general guideline as this would have an impact on cost. In other words, it’s easier to approach manufactures with common configurations to reduce you capital costs.

Problems affecting performance in Shell and tube heat exchangers

2012-11-16T20:34:00.000+00:00

The main problems affecting the performance are usually due to one of the following:
+ Fouling
+ Tube vibrations
+ Leakage
+ Dead Zones

Fouling

This can be generally defined as the precipitation of unwanted material within the heat exchanger over time which hamper the performance.
The principal types of fouling encountered in process heat exchangers include:
• Particulate fouling
• Corrosion fouling
• Biological fouling
• Crystallisation fouling
• Chemical reaction fouling
• Freezing fouling
In the case of corrosion, the surfaces of the heat exchanger can become corroded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop in connection with the fluid flowing on the inside of the exchanger.
To improve the performance of fouled heat exchangers requires that the tubes be cleaned periodically. Tube cleaning procedures for shell and tube heat exchangers are performed off-line, the most frequently chosen and fastest method being mechanical cleaning. Among other off-line methods is the use of very high pressure water but, since the jet can only be moved along the tube slowly, the time taken to clean a heat exchanger can become extended. Chemicals are also used for the off-line cleaning of heat exchanger tubes. Several mildly acidic products are available and will remove more deposit than most other methods; but it is expensive, takes longer for the operation to be completed, and the subsequent disposal of the chemicals, an environmental hazard, creates its own set of problems.

Tube vibrations

Another problem that often arises in connection with the use of heat exchangers is tube vibration damage. Tube vibration is most intense and damage is most likely to occur in cross flow implementations where fluids flow is perpendicular to the tubes, although tube vibration damage can also occur in non cross flow (i.e. axial) implementations in the case of very high fluid velocities. Vibration may be eliminated by reducing velocities, decreasing the unsupported span or, in some cases, by altering the method of fixing or pinning the ends of the unsupported span.
This problem can cause significant damage to the exchanger if within high limits.

Leakage

Sometimes the fluid of the tube side can leak to shell side or vice versa, This problem can cause huge production loss.
Leaks may develop at the tube to tube sheet joints of fixed tube sheet exchangers because differential thermal
expansion between the tubes and the shell causes overstressing of the rolled joints. Or, thermal cycling caused by frequent shutdowns or batch operation of the process may cause the tubes to loosen in the tube holes. Floating heads or U-bend exchangers would be considered first for this type of service. If a fixed tube sheet
unit is required, an expansion joint will be specified. An exchanger that will be thermally cycled two or three times a day will require superior mechanical construction such as the strength welding of tubes to the tube sheet, complete inspection of the shell and channel welds during fabrication. Welding the tubes to the tube sheets does not guarantee that a leak will not occur as sometimes weld failure due to porosity in the welds or just one poorly welded tube out of the hundreds of welds can cause a leakage. The use of double tube sheets to minimise
the chances of leakage between the tube side and shell side can be a good solution to the problem. Nevertheless, double tube sheet can cause considerable maintenance problems because the outboard and inboard tube sheets may be subjected to considerably different process temperatures and this can have
differential expansion between the tube sheets resulting in bending the tubes

Dead zones

Areas that have the flow to minimal or even non existent and usually produce poor heat transfer and can lead ultimately to excessive fouling.
Existing shell and tube heat exchangers suffer from the fact that they must typically use baffles to maintain the required heat transfer. This, however, results in "dead zones" within the heat exchanger where flow is minimal or even non existent. These dead zones generally lead to excessive fouling. Other types of heat exchangers may
or may not employ baffles. If they do, the same increased fouling problem exists. Further, in heat exchangers fitted with baffles, for example, the cross flow implementation results in the additional problem of potential damage to tubes as a result of flow induced vibration. In the case of such damage, processes must often be interrupted or shut down in order to perform costly and time consuming repairs to the device.

How to calculate the heat duty for heat exchangers?

2012-09-30T00:19:00.001+01:00

How to calculate the heat duty for heat exchangers?

Lets first define the term “Heat duty” to understand what understand what exactly we are calculating. The Heat duty can be defined as the amount of heat needed to transfer from a hot side to the cold side over a unit of time.

The calculation is very important to all engineers and it’s one of the common ones that you need to know in your career if you are a process engineer. The equation to calculate the heat duty is normally written in two ways.

One that can be used for sensible heat transferred, this means that the fluid undergoes no phase change.

The other can be used for latent heat transferred, this means that the fluid undergoes a phase change. i.e. condenses.

Heat Duty (Sensible heat – No phase change)

Q = M * Cp * ∆T

Where;

Q – is the heat duty or the total heat transferred. Btu/hr or W

M – is the Mass flow rate for the fluid undergoing the temperature change. lb/hr or kg/s

Cp – is the heat capacity of the fluid undergoing the temperature change. Btu/lb.°F or J/kg.°K

∆T – is the temperature change in fluid normally calculated as the difference between outlet and inlet temperatures. °F or °C

I have put two different measurement units for each of the variables above, the first being in English / US Units and the second is in SI Units. There is many other variations but the above are common and I use all the time. The most important thing is to make sure your units of measurement is correct and consistent with the calculations.

Heat Duty (Latent heat – Phase change)

Q = M * λ

Where;

Q - is the heat duty or the total heat transferred. Btu/hr or kW

M – is the Mass flow rate for the fluid undergoing the temperature change. lb/hr or kg/s

λ – is the latent heat. Btu/lb or kJ/kg

For the hot side this is the Latent heat of Condensation of the vapor that is changing phase.

For the cold side this is the Latent heat of Vaporization of the liquid that is changing phase.

The equation can also be written in terms of the enthalpy change by replacing the λ with (H2 – H1) referring to the change in enthalpies of the fluid undergoing temperature change and this is also expressed in Btu/lb or kJ/kg

_____________________________________________________________________
Here is a free heat duty calculator that I built - you can read more about it from the webbusterz engineering software using the link below:

Heat Duty Calculator - Sensible and Latent heat

Download this calculator - windows version

If you have a smart phone with android operating system you can get this calculator from google apps by visiting this link

How to Calculate the Log Mean Temperature Difference (LMTD)

2012-02-01T18:22:00.001+00:00

This short article aims to show you how to calculate the LMTD for Counter current flow and Co-current flow also called (Parallel flow). The calculation has been referenced by many text books. You can also download the free calculator attached with this article.

Why do we need to calculate it?
The log mean temperature difference represents the driving force for heat exchanger operation. It will allow us to make a decision on the flow arrangement type and making the right choice here will also improve your final design and mainly the heat transfer.

Flow arrangements
There is mainly two different flow arrangement in heat exchangers,
Counter Current flow:
See the image below

: Counter current flow arrangement

Where:

T1 - Inlet temperature of hot stream

T2 - Outlet temperature of hot stream

t1 - Inlet temperature of cold stream

t2 - Outlet temperature of cold stream

Co-current flow (Parallel flow):

See the image below

Parallel flow arrangement

How do we calculate it?
The log mean temperature difference is normally calculated from the terminal temperature differences.

For Counter current flow:

For Co-current flow

Reference:
D. Q. Kern, Process Heat Transfer, McGraw-Hill, International Edition, 1950

Free LMTD Calculator

: Log mean temperature difference calculator

Download from webbusterz

How to determine stream allocation in heat exchangers

2012-01-15T11:31:00.003+00:00

Stream allocation is an important decision in heat exchanger design. This decision can impact the heat exchanger life, can also make the difference between higher and lower maintenance costs. Can also have a high impact on the thermal performance the heat exchanger. This short article aims to discuss the process of determining the fluid allocations in shell and tube heat exchangers. The decision is made based on the criteria below:
1- Fouling:
Check fouling factors for each fluid.
The fluid/stream with the high fouling factor should be in tubes – this makes it easy to clean and prolongs the exchanger life. Placing the fouling fuild in the tubes also allows better velocity control as increased velocities tend to reduce fouling.
2- Pressure:
Higher pressure fluid/stream is placed in Tube side. This hasan effect on the shell thickess as placing the high pressure fluid in the tubes would mean that the shell thickness can be reduced.
3- Corrosion rate:
Higher rate fluid/stream is placed in Tube side. In general fewer corrosion resistant alloys are needed if the corrosive fluid is placed on the tube side
4- Viscosity
Fluid/Stream with higher viscosity is placed in the Shell Side as higher heat transfer rates are generally obtained using this practice.
5- Phase change
The stream with phase change is assigned to Shell Side
in steam heated vaporizers/reboilers the condensing steam is placed in Tube Side
I hope the above provide a simple guidline for anyone not sure about this issue.
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