Calculate Heat Exchanger Outlet Temperatures

[miclwl]

Hi all,

I would like to ask is there any method to calculate the outlet temperatures of the two fluids from a PLATE heat exchanger, provided that I know their inlet temperatures and mass flow rate? Please specify what information of the heat exchanger I need to know, and the equation needed for the calculation.

Thanks a lot

Regards,
Michael

 

[25362]

I can refer you to the article in the ChE magazine of August 11, 1980 by K.S.N.Raju and Jagdish Chand, both of the Panjab University, named "Consider the plate heat exchanger".

 

[JOM]

miclwl,

There are many factors that determine the heat transfer rate with a plate heat exchanger.

. number of plates & area of plates
. plate arrangement - no. of passes on each side
. plate material
. plate thickness
. plate profile
. viscosities & densities & heat capacities of process streams
. fouling factors

Manufacturers of PHEs have proprietary software that accurately calculates heat transfer performance for the heat exchangers they manufacture.

Cheers,
John.

 

[srfish]

In addition you need the overall heat transfer coefficient, total surface and the specific heat of both streams. The outlet temperatures can be calculated by trial and error but it is easier to calculate the thermal efficiency. The charts for this can be found in most heat transfer text books. Some book indexes have the charts listed under NTU method.

From the thermal efficiency, you can calculate t2. Since you already know t1, you calculate Q, then you have all the variables neede to calculate T2.

Have Fun
Dale

 

[miclwl]

srfish,

Could you please specify some books which have the thermal efficiency charts that you have mentioned?

Thanks!

Michael

 

[srfish]

Two references for thermal effectiveness are:
Fraas,Heat Exchanger Design,1989,2nd edition,P.459
TEMA,8th Edition,p.147

An Away you go
Dale

 

[miclwl]

srfish,

Thanks for telling me the reference book. I've found the method.

However, I found that the method needs to use the heat capacity of both fluids. Since the hot fluid in my case is a mixture of gases, and, as you know, the heat capacity of gases change according to the temperature. So how can I obtain the heat capacity of a mixture of gases, and at what temperature should it be assumed, in order to solve the problem?

Thanks a lot!

Michael

 

[srfish]

miclwl,

It depends on the operating pressure. If the gas has an operating pressure lower than 100 psi, you can use weight fractions of the composition to calculate the specific heat. Above 100 psi, I suggest finding someone who has access to process software.

As a starting point on temperatures to use:
Hot stream t = T1 -(T1 -t1)/4.
Cold stream t = t1 +(T1 -t1)/4.

Dale

 

[davefitz]

I would recommend the book by Kays & London" compact heat exchangers", or any text that describes the e-NTU method.

If you already have a heat exchanger and know at least one desing case ( from the vendor data sheet), you should be able to accurately prorate to other cases, but you will need to know :
a) flowrates of each fluid, W1 and W2
b)heat capacities of both fluids, Cp1 and Cp2
c) design inlet and outlet temp of both fluids, T1h, T1c, T2h, T2c
d) knowledge of heatexchanger basic configuration: counter flow or parrellel flow.

From the design case, you back calculate the value of the psuedo factor "NTU" ( number of transfer units). Once this is known , you can prorate to any other off design case.

 

[MJCronin]

miclwl,

In my opinion, this is another case where I would abuse a plate-frame technical rep.

Present the design info to your local Alfa-laval rep and have him "quote" you a new unit........

Yeah, I know it is not right,....but he has the software,........... nobody else can use it........it's not a simple task......and .........um..........what do they expect ?

MJC