4 replies to this topic

[abbaschemical]

Dear All,

             I have observed in my operational experience that , when the system pressure of a heat exchanger(Ammonia Condenser) decreases ( from 18 bar to 14 bar) the performance of the exchanger detoriates i.e the exchanger does not takes the heat load.  There are two exchangers in parallel and this happens in the first exchanger where the velocity is higher.

            My question is that,  is this phenomena is due to decrease in the pressure drop and correspondingly increase in velocity across the exchanger due change in system pressure? Please not that this problem is only in the first exchanger in parallel. And this first exchanger performs well at higher pressures(around its design pressure - 18 bar) ....

Does anyone experienced the same phenomena ?

[fseipel]

Can you provide additional information?  i.e. is this a direct expansion condenser with vaporizing liquid ammonia on the service side, or is the ammonia a vapor on the process side, which you are trying to condense?  If you have two exchangers in parallel how is flow balanced?  Or are tube lengths/diameters the same?  What type of H/X's are being used (shell/tube, plate/frame, etc)?  What is the media on process & service sides, temperatures/pressures/flow rates?

[Propacket]

abbaschemical
1.First you need to correct your thoughts on the issue. When velocity increases, pressure drop also increases not decreases as you say.
2.The increase in velocity upon decrease in pressure is due to decrease in density and increase in volumetric flow.
3."Heat exchanger does not take heat load". It means you are not getting required outlet temperatures for the same inlet temperatures? Right?
4.In condensers, high velocity/pressure drop means more liquid back up which steals the exchanger surface area. This is the reason you are not getting the required load.
Not sure why it's not happening in the second exchanger.
Thanks
Haseeb Ali

[staffel]

I agree with fseipel,

"There are two exchangers in parallel and this happens in the first exchanger where the velocity is higher"

assuming they have similar characteristics (by design)
if they are properly designed and connected they should have similar conditions inside,
from your report it would seem that one works in different conditions
and this may depend from several reasons,
for example in a plant I have had an ammonia float regulator not working properly,
too liquid and oil in condenser and the compressor not working properly.

Give us some more information.

Edited by staffel, 28 November 2013 - 06:37 AM.

[DB Shah]

"There are two exchangers in parallel and this happens in the first exchanger"

In parallel configuration there is no first or second, ideally both gets 50%-50% of flow with same inlet conditions. There is a possibility that due to configuration the distribution is not 50-50.

 

Let us take a hypothetical case where distribution in A & B is 60% and 40% at 18 Barg operation. Pressure drop in both A & B circuit will be same, let us assume 0.5 Bar. In this case wrt design load of 50% in each, A is operating at 60/50 = 120% load where as "B" is operating at 40/50 = 80% of design load.

 

Now if the pressure is reduced, flow will increase linearly. .

The volumetric flow will increase by 19/15 = ~ 127%. The distribution will be 1.27*0.6=76% in "A" and 51% in "B" (total 127%)

The pressure drop will increase to 0.5 *(1.27)^2 = 0.8 bar for both the exchangers.

 

Kindly note that now A is operating at 76/50 = 152% load where as "B" is operating at 51/50 = 101% of design load.

 

Thus you can now see a significant difference of heat load, exchanger designed may work at 120% utilizing all excess area as in case of 18 Barg pressure, but at 14 Barg "A" exchanger is subjected to 152% of design load and hence pose limitation, whereas exchanger "B" is operating at 80% to 101% of design load and can cater your requirement.

 

Kindly check the flow distribution in your existing configuration.

 

Other point-

 

NH3 vap pressure data-

@ 48°C = 18 Barg, @38°C = 14 Barg. If the pressure is reduced, condensing temperature will reduce hence reducing LMTD. This will reduce heat transfer in both the exchangers.