Heat exchangers selection 1

[yoel280206]

I need your help.
I am selecting a heat exchanger for my ammonia application.
Here are the known data :
Hot Fluid : water, Tin (inlet temperature) and flow rate.
Cold fluid : ammonia, Tin (inlet temperature) and flow rate.
By selecting a heat exchanger from market, how to calculate the outlet temperature (hot and cold fluid) and drop pressure?
Hopefully these parameters will approach the real condition.
In real, does the flow rate change?
Thank you very much..

 

[vpl]

You also need to know either how much heat you need to remove or an upper bound on either the hot or cold temperature. With that information in hand, I'd suggest talking to a couple different manufacturers and let them figure out the remaining information.

Q= ?c_p(T₁-T₂) and Q_h=Q_c such that ?_hc_p(h)(T_1(h)-T_2(h)) = ?_cc_p(c)(T_1(c)-T_2(c)).

Patricia Lougheed

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[Twoballcane]

The vender should have an application engineer who can help you out.

Tobalcane
"If you avoid failure, you also avoid success."

 

[IRstuff]

Seems like you're going at it backwards. You should determine what outlet temperatures and heat removal rates your application needs. THEN, you look for an exchanger that meets the requirement. If you go to an application engineer, that'll probably be the first thing he asks you.

TTFN

FAQ731-376

 

[yoel280206]

I agree with you. In my opinion, if we knows the heat removal, i will know the temperature outlet. However how i know the best heat exchanger for my application. I thought that every exchanger has typical performance parameters such as cp, CMTD etc. So by selecting a certain heat exchanger and knowing the heat removal,flow rate and inlet temperature, the engineer will know the outlet temperature.
Q= ?cp(T1-T2)
By the way, would the manufacturer give the detail performance? In their website, they seldom put it in the spesification sheet. Do you think any other way to predict the outlet temperature? Am i right?

 

[IRstuff]

If you know the heat rate and inlet flow, then you can calculate the temperature rise using the specific heat.

TTFN

FAQ731-376

 

[rmw]

Q=m cp (dt) is a thermodynamic book keeping equation that keeps up with the heat flow. I use it all the time.

Q = UA(LMTD) will tell you what the size of your equipment will be and what the actual outlet temperatures will be. Once you know the actual T1 - T2, you can calculate the first equation and see how much heat transfer you have for the approach temperatures that result from your equipment selection.

Lots of intricate factors go into that simple little "U" value of the equation.

rmw

How did you do that little m(dot) Yoel?

 

[pmover]

yoel280206,

typically, the responsible process design engineer defines the process conditions (i.e. inlet/outlet P, T, density, viscosity, Cp, flow rates, duty, etc.) for both streams. This information can then be used to properly thermally, hydraulically, and mechanically design a heat exchanger. There are heat transfer specialists that can assist you with the design/rating of the exchanger or you may directly deal with the manufacturer. unless you are skilled with heat transfer methodologies and designs, i highly recommend using the specialists as ammonia is a caustic & a hazardous fluid. the specialists can also specify materials of construction.

good luck!
-pmover

 

[yoel280206]

Thanks all..
Dear rmw, in the second equation, how do you calculate the actual temperature if you don't know U?
Thanks.

 

[dcasto]

you use generalized tables for U to get the range. Then you see just how big your exchanger is going to be. You have two choices, see if you can get a higher U or increase the delta T which means a larger appoach and lose of Q. Now this is where engineering comes in, efficency versus cost.

 

[yoel280206]

@dcasto
It is interesting.
By the way, how about corected LMTD?
Does it include in the spesification of heat exchangers?

http://www.deltathx.com/uploadsDocs/Approach Temperatures.pdf

Is it right?
Deltathx said that but i don't know others.
Thanks so much..

 

[dcasto]

Yes, you use LMTD and there correction factors to that also based on arrangements. I just assumed that people know LMTD is used in Q=UAdT.

the report you attached suggestes different exchangers that have better U values for each case. that is why you need to try several exchanger types/designs to see what works best for the cost.