Calculating Exit Temperature of a Heat Exchanger 2

[Mechwill]

Hi all, recently join the eng-tips forum here

I have a question regarding to the outlet temperature of a heat exchanger.

A standard set up of a fan and a radiator like in a car system. The fan is behind the radiator to draw the cold air to the radiator to heat up a room. As far as I know, there are two types of basic analysis on a heat exchanger: LMTD method and NTU method.

However, the information that I know is really limited to both analysis. Here is the information i know:

Heat Exchange Fluids: Air/Water
The radiator generates 20,000 BTU/h
The air flow rate of the fan is 800 cfm
Ambient temperature is around 10 ºC

What i am looking for is the exit temperature of the fan (actually use the fan to heat up a container)

Therefore, i cannot use the LMTD method.

The problem that I have right now is the following:

1. Inlet temperature of the radiator is unknown
2. Heat transfer surface is unknown
3. Flow rate inside the radiator is unknown

Not sure if i am using the right method or calculation set to solve this problem. If anyone has any experience with this, i appreciate all the suggestion there is.


Thanks

 

[IRstuff]

Seems like you're making it overly complicated to me. You have the volumetric mass flow and the heat generated. The temperature of the exiting air must support the heat flow. Seems more like an algebra problem: power/(mass_flow*specific_heat) Assuming that this is a steady state and real problem, that's all there is to it.

TTFN
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[Mechwill]

I think i am making it overly complicated to myself as well. Sorry about that

 

[chicopee]

Well, you have a cross flow heat exchanger, so do research which is the best way to learn. Web sites and libraries are terrific instruments for research. Subjects such as the type you describe are pretty well covered in heat transfer classes; H.T. textbooks and M.E or CE handbooks have sections relating to heat exchangers.

 

[MintJulep]

Since you have I-P heat and I-P flow

Q = 1.1 * CFM * [Δ]T

Rearrange and solve for [Δ]T = 20,000 / (1.1 * 800) = 22.7[°]F = 12.6[°]C

So your leaving air temperature is 22.6[°]C

 

[Mechwill]

Thanks MintJulep. I guess I did overlook the problem. Somehow i think of the system as a heat exchanger problem. Thanks again

 

[Mechwill]

Another question regarding to the exit temperature of the air. Is it the temperature at the outlet? Does that mean the temperature will decrease after a distance from the outlet?

 

[IRstuff]

If there's a temperature difference...

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[Brown23]

If that's the exit temp of the air, then I'm pretty sure it will not remain at that temp unless it's contained in some sort of insulated vent, otherwise it will mix with the surroundings and just average out with the room temp.