AIR TO LIQUID vs LIQUID TO AIR

[DECKERBILT]

We are currently using a heat exchanger that is designed & sold to cool oil in a hydraulic system but we are passing a cool liquid (water) through it to try to bring down the air temperature that we are passing over it. Would any body know if there are fundamental differences in the design of a heat exchanger when the cooling medium is not the air but is the liquid? Our air velocity is high and I suspect resonance time is a factor that we currently are not taking advantage of. I'm looking for qualatative answers, not quantitive.

 

[zdas04]

The heat-transfer arithmetic is exactly the same. You have to pay a lot of attention to the signs in your sums, but that sort of care should always be expected.

I would use the same heat-transfer coefficient as you would use with the vessel set up as a radiator. Solve the shell-side math first (since those are the BTU's you are trying to impact) and then solve for a volume flow rate on the tube side to get the delta T where you want it.

This should be a pretty straight-forward solution.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[sailoday28]

My only concern would be the basic pressure difference between the cooling medium and the oil.

Will the air be at a much lower pressure than the water?
With any tube leakage what will the consequences be?

 

[25362]

Some more qualitative aspects:

1. Switching from oil to water may double the overall heat transfer coefficient, HTC, as long as fouling doesn't interfere.

2. Too high air temperatures may oblige using soft water to preclude the deposition of salts.

3. Temperature differences affect heat transfer not only by changing the "driving force". The convection HTC for air is proportional to ([Δ]T)^1/3.

4. When you speak of air high velocities and low residence times, do you mean high as in high-speed supersonic aircraft ? Too high velocities involve considerations such as the conversion of kinetic energy to heat when air impacts the radiator's surface, etc.

5. Air pressure p, kPa, affects the convection HTC by a factor of (p/101.32)^2/3 in turbulent flow.

Good luck.