8 replies to this topic

[daraj]

Anyoen who has used aspen plus or other software to design heat exchangers would have an output called TEMA sheet which gives the details of the exchanger design.

 

In that sheet the heat transfter coefficients are listed for 3 cases labelled as clean, service and dirty.

 

I would appreciate if someone explains

 

1.The PHYSICAL meaning of these 3 coefficients and not just the mathematical formula associated with it. I want to know the physical meaning of thes3 terms in order to have a feel

 

2. I want to know a broad approach of software like aspen plus in calculating these 3 coefficients. That is the sequence of steps that aspen plus follows to arrive at these 3 values. Assume user inputs= inlet/outlet properties of cold/hot streams and/or heat duty required.

 

I would think the sequence of steps is as follows but I could be wrong, pease correct:

 

1. Software gets inlet/outlet stream properties from user along with type of exchanger(whether BEM or BEU etc.) and M.O.C of shell/tube

2. Aspen calculates the missing parameter-either outlet property of oen of the streams or the heat load

3. software calculates U from first principles(using pressure drop limits set by user, flow limits in shell/tube side set by user, bulk properties of process fluids etc.)

 

This is clean.

4. Software applies user supplied fouling factors and calculates revised U-this is dirty

 

5. software uses dirty U tocalculate required area from Q=U.A.deltaT.

 

6. software from mechanical design(how?), calculates maximum available area, which excludes tubes extending outside tubesheets and so on. This is theoretical max. area available. Software plugs this area in Q=U.A.deltaT. and calculates U, which is the service U, or the U that corresponds to when thje heat exchanger is able to just meet the requirements of the duty, under maximum fouling conditions.

 

Please revise thanks.

[daraj]

Hi, I would greatly appreciate if any experiences professionals would take a shot at this. Thanks

[daraj]

Art, and other experienced professionals, can you pl help out here? Thanks

[DB Shah]

You are right.

Based on property, configuration, velocity etc, software (or you can manually also) calculate U clean.

With the help of dirt factor (Based on service, predefined by process engg) Udirty is calculated (1/ Ud = 1/Uc + dirt factor)

Based on Ud, Area required is calculated Q = Ud.A.LMTD => A required = Q / (Ud.LMTD)

A margin of 15~ 20% is provided and configuration of hex is fixed, this is area provided. Back calculate U based on the provided area. The U arrived is U service.

Thus Ratio of U dirty and U service is the excess area provided (margin)

[daraj]

Hi DB Shah, thanks. Does the software just add 15-20% as excess area and backcalculate U service? I thought there was some formula to that. U-service I believe is defined as heat transfer coefficient correponding to condition where maximum fouling happens to the point where required heat transfer just happens.

THat is, given the user inputs on configuration of type of heat exchanger, max. allowable pressure drop etc. the maximum possible physical area is calculated. This area is then plugged into Q=UA.LMTD. to give U service which will have lowest value since the fouling is so bad that you njeed the entire possible area to complete the heat transfer. I thought this was what it meant conceptually. Please correct.

 

Others, your inputs are also welcome.

[DB Shah]

Software does not add any area, based on your input it suggests various design ( you run in design mode), select a design based on configuration, pressure drop and other constraint, export the design to "Rating" mode of simulator. Rating mode does rigorous run and the results may vary slightly compared to design mode. In rating mode you can vary dimensions to fine tune configuration. Thus with this changes you may end up with a result different than design mode.

 

By the way, when you mention Aspen, I understand you are referring to Aspen Tasc which is a module for heat exchanger simulation (Aspen Tasc was earlier HTFS later on taken over by Aspen)

Edited by DB Shah, 30 July 2013 - 05:55 AM.

[daraj]

sHAH, i WAS REFERRING TO THIS COMMENT OF TYOURS

"A margin of 15~ 20% is provided and configuration of hex is fixed, this is area provided. Back calculate U based on the provided area. The U arrived is U service."

 

for calculating U service you need  area provided.how does aspen tasc+ calculate area providedafter calculating U dirty and area required, is my question.

[DB Shah]

Software will not always give you heat exchanger with area ratio =1. When you export & run your selected design in "Rating" mode, you may not get area ratio as per your desired margin, then you can increase/decrease the area either by varying tube length or no of tubes & run in rating mode (If you vary your tube nos, take care of clearances like shell/OTL etc and confirm the layout in output drawing). Thus now you have the available area which is higher or lower as per your changes.

 

Uservice = Q / (Area available*LMTD)

 

Ud is the U calculated by software based on flow/properties/configuration/dirt factor

 

Ud/Us = Area available/Area required = Area ratio margin

[daraj]

Thanks Shah. So it looks like area required and area available are independent calculations, but if the ratio is not greater than 1 then the user can manually adjust configration to make this ratio greater than 1