14 replies to this topic

[mal]

hi can you please help me i am trying to calculate the outlet(exit) temperature for the hot and cold fluid in shel and tube heat exchanger (parallel flow) (basically it's tube inside tube and the two fluids flowing in the same direction)

Here is the knowns

mass flow for Cold fluid =3621.197 Lbm/hr Variable simple (mc) c is subscribt for cold
mass flow for Hot fluid=2218.512 Lbm/hr Variable simple (mh) h is subscribt for hot
Specific Heat flow for Cold fluid = 1.00 BTU/Lbm/F Variable simple (Cc) c is subscribt for cold
Specific Heat flow for Hot fluid = 0.527 BTU/Lbm/F Variable simple (Ch) h is subscribt for cold
Overall heat transfer cofficient = 15.385 BTU/hr.ft^2.F Variable simple is (Uo)
Temperature for (inlet) entering Cold fluid = 60 F
Temperature for (inlet) entering Hot fluid = 200 F

Temp. for Outlet Cold fluid = ??
Temp. for Outlet Hot fluid = ??


thanks a lot for your help

P.s. there is a picture of the heat exchanger attached

Attached Files

•  Heat_Exchanger.jpg   29.69KB   44 downloads

[Allen]

It seems to me you have insufficient information here - 2 missing exit temperatures. It isn't possible to calculate the heat load from this data - a key piece of information. If you had an exit temperature for the process fluid then you could calculate the heat load and then, using some assumed exhanger geometry and size and using some assumed pressure drop criteria, determine the heat transfer area and converge to a solution. Try reading that excellent book by Kern - it's full of examples to help you.

[Zauberberg]

Basically, the outlet temperatures could be calculated by using exchanger thermal design software. In the "simulation" mode, inlet temperatures and exchanger geometry/type are specified, and the software calculates (predicts) the final outlet temperatures for both fluids. It is done by calculating heat transfer rates for each differential step, which is - I believe - impossible to do by hand calculation methods.

[Allen]

Basically, the outlet temperatures could be calculated by using exchanger thermal design software. In the "simulation" mode, inlet temperatures and exchanger geometry/type are specified, and the software calculates (predicts) the final outlet temperatures for both fluids. It is done by calculating heat transfer rates for each differential step, which is - I believe - impossible to do by hand calculation methods.

I'm old school - never used simulation software - hope thats not considered to be heresy!

[JoeWong]

I still think the problem still open with 1 degree of freedom.

Either the geometry is missing or the minimum approach is missing.

From heat balance, an equation with Tho and Tco.
Tho being hot side outlet temperature
Tco being cold side outlet temperature

If we assume minimum approach Th-Tc is 9 degF, then the problem is closed.

[Zauberberg]

I'm old school - never used simulation software - hope thats not considered to be heresy!

Actually it's a special bonus and few extra points for you, in my opinion. Nowadays, engineers are simply jumping from their universities to engineering design offices and getting themselves totally immersed with simulation software - without knowing the basic concepts of equipment operation in the field.

I know for sure HETRAN can do such calculation for you, HTRI should be able to perform it as well. By knowing exact exchanger type and geometry (dimensions), heat transfer between the two streams is simulated and you'll get an estimates of both outlet temperatures.

[djack77494]

Sorry, mal, no can do. You have not provided sufficient information for this calculation. Missing is the surface area available for heat transfer.

Nonetheless, I can bracket your answer for you. With a very small available area, there will be little heat transfer, and
T cold out ~ T cold in and
T hot out ~ T hot in.
(Yes, I know this is a trivial result.)

The second limit arises if you have lots of available surface area. In the limit, infinite surface area,
T cold out = T hot out
Once you know this, you can find the outlet temperature by energy balance. The energy lost by the hot fluid = energy gained by the cold fluid. Both are obtained using
Q = m * Cp * (T2 - T1)

As previously mentioned, you still have an unspecified degree of freedom (the area) and can achieve any answer between the limits based on the area available. The problem without surface area is not mathematically complex, and does not require the use of a simulator. Even if the hardware is known, your case is quite ideal and should be easy to solve.

BTW, pleeese don't beg for help. WE can put 2 & 2 together and realize that you're posting because you want help.

[Zauberberg]

If the exchanger is fully specifiied - by type and geometry/dimensions, it can be simulated. Full specification assumes knowing the surface area; otherwise it would be meaningles to try to calculate the final heat transfer result.

[mal]

Here is the additional information and I have attached a picture of the heat exchanger

L = Length
Di = inside tube diameter
Do = outside tube diameter
D2 = Inside diameter of shell

Di Do D2 L Ai Ao
ft ft ft ft ft2 ft2
0.25 0.292 0.5 30 0.049 0.130

Attached Files

•  HE.jpg   39.68KB   17 downloads

[Zauberberg]

I'm still of opinion that one will need thermal design software in order to predict heat transfer rates throughout the exchanger, and final fluid temperatures as well - especially if the subject is process-related and not being the academic exercise.

[fallah]

I'm still of opinion that one will need thermal design software in order to predict heat transfer rates throughout the exchanger, and final fluid temperatures as well - especially if the subject is process-related and not being the academic exercise.

Would you please exactly clarify the main intention of doing themal design.

[fallah]

I'm still of opinion that one will need thermal design software in order to predict heat transfer rates throughout the exchanger, and final fluid temperatures as well - especially if the subject is process-related and not being the academic exercise.

Would you please exactly clarify the main intention of doing themal design (rating) by a software such as HTRI.

[Zauberberg]

Hello Fallah,

In this particular case we would actually have to simulate the heat exchanger, since only inlet conditions are known: flowrates and temperatures.

Exchanger geometry and dimensions are also known, and based on these parameters software will continuously calculate film coefficients on both sides, and continuously update the temperatures of both fluids at actual (differential) heat transfer rates across the exchanger. Therefore it will be possible to predict the final temperatures for both fluids. I think it's impossible to do it by hand, it would be very rough estimation of exchanger performance.

Because the exchanger duty is not known, we cannot say we are performing design or rating calculations - we are limited to "simulating" (predicting) exchanger performance, based on its geometry and inlet conditions of both fluids.

I hope this answers your query.
Best regards,

[fallah]

Dear Zauberberg,
Thanks a lot.If we have inlet and outlet conditions : flowrates and temperatures,and also duty.Can the software such as HTRI give us the detail of geometry?If so, what name can we title this operation? modelling,.....
Regards

[Zauberberg]

That would be a standard design procedure: both fluids and their conditions (inlet/outlet) are known, along with the duty which has to be accomplished in the exchanger. From heat transfer point of view, only surface area is missing. HTRI and HETRAN can be used to find the optimum exchanger design: the type, size, and the number of units which will have the total heat exchanged (duty) set by user.