3 replies to this topic

[Said Salim]

Hello,

 

In an attempt to design a shell & tube heat exchanger using Aspen EDR, I have designed a BHM exchanger to cool syngas from 46 DegC to 18.35 DegC using chilled water at 7 degC with excess area value of 0 %. However, when I used the same exchanger geometry generated by design mode in simulation mode for same flows and inlet temperatures of Syngas & Chilled Water  as design case the resultant heat duty is different than that of design mode.

 

Anybody can highlight what is the reason behind this difference of duty 

 

Said 

Attached Files

•  E-313 Design.pdf   247.35KB   695 downloads
•  E-313 Simulation.pdf   247.33KB   487 downloads

[Pilesar]

There should be three modes available. Design mode includes a 'safety factor' so that the equipment performs to meet the specifed conditions of fouling and has extra capacity just in case the calcs aren't quite accurate. The result should be based on 10% overdesign or some other overdesign figure the user specifies. In practice, you will not shorten the tubes by a small fraction just to precisely meet the required performance. Simulation mode takes that exchanger exactly as it is built and uses the feed streams to calculate the predicted output. You should see in the simulation mode results that you will have a different service heat transfer rate than the dirty heat transfer rate because there is a little extra surface area available. For a new exchanger, you want to end with the rating mode so you do make others assume they will get the results from using up the safety factor. Rating mode may give very close to design mode results but fixes the exchanger design so it is not recalculated. The included safety factor keeps them from pointing fingers at you saying 'you promised 100.3 degrees and I only got 100.1 degrees.' Simulation mode is for the purpose of modelling an existing exchanger to help analyze its real performance and is not intended for exchanger design.

[Said Salim]

Actually I have tried to maintain overdesign % to 0 by letting highest area ratio to be considered = 1 in optimization under design option
Also minimum excess % surface area required =0%

[Pilesar]

I've always found it good practice to have some safety factor in equipment designs. The economic consequences of 'just not quite large enough' can be much greater than the value of possible savings from 'this is the absolute cheapest equipment possible.' But in any case, the differences in the calculation mode answers you see are a result of which parameters are fixed and which are varied internally in the software calculations.