4 replies to this topic

[Chem01]

Hello all,
I am working for relocation of an Ammonia plant from a cold to hot climate region. Due to hot climate cooling water (CW) temperature will be higher so we need more flow for the same exchanger duty. For most of the cooling duty CW exchangers are in a pair form, i.e two exchangers connected in series on both CW and process side. This means CW first pass through the 1st exchanger then same CW (now relatively hot) pass to the 2nd exchanger.
I want to know if we connect these exchangers in parallel, i.e freh CW to both exchangers, then CW requirement will increase , decrease or remain same?

[muchlis nugroho]

Hi,

The CW requirement are flowrate and DT. If your CW's delta temperature is lower than design, it may be necessary to increase the flow and to increase the heat transfer surface are (i.e. by adding new exchanger in series or parallel, and additional CW tower).

For DT impact, I think you should make a composite curve of "DT vs Heat Duty" and make Pinch Analysis of your CW exchangers. For flowrate impact, you have to make hydrolic analysis. Then you will know what will be necessary to be implemented.


Rgds,
MNG

[djack77494]

Chem01,
Looking at your problem in simple terms, your cooling water supply temperature has increased. I understand that you'd like to keep all your process conditions essentially the same as they had been. Since the DT in your cooler(s) has been decreased, the expected duty will also decrease.

My quick take on the problem is that: 1) You will need additional cooling water because your inlet temperature is higher. If you hold the outlet temperature and the duty, then the heat balance says more cooling water is needed. 2) You will almost surely need to do other things to increase your heat transfer. You may need more shells or some means of enhancing the heat transfer from what you've already got.
Good luck,
Doug

[Chem01]

thanks for replying,

Muchlis
ur first point is valid,,,well what r the techniques for pinch analysis i've not done that so far?

djack77..
its true, for the higher inlet CW temperature and same outlet, i need to increase the flow rate.

Should i conclude that if two exchangers r connected in series or parallel on CW side then flow requirement is same if DT across the exchanger is same?

[djack77494]

Chem01,
I find it difficult to supply a simple answer to your question. I must better appreciate what your design parameters are in order to be accurate in a response. Better describe these two heat exchangers that you considering for parallel or series connections. Will you want to maintain the process duty and temperature in and out conditions?

Without more definitive data, I can only make some general suggestions. If you run in parallel, you can pass more process fluid and cooling water through the combined 2 exchangers before running into hydraulic limitations. However, your heat transfer coefficients will drop due to decreased velocities in the exchangers. (One or both sides depends on whether you'll hold or double your total cooling water flow.) You're in an area where it's not intuitive or obvious what the net effect will be.

For simplicity sake, if you assume a constant "U" and you consume twice the amount of cooling water for parallel operation, then you would get a lower process outlet temperature/higher duty for parallel flow. If you don't increase your cooling water flow, heat removal will be degraded in the parallel flow configuration. I'm sorry if I'm missing the point, but I don't fully appreciate the situation you're inquiring about.
Doug