# Regarding Heat Transfer and Heat Exchangers

Hi folks,
Just a few general questions I have here.

1) The situation: A cubic block is taken from an oven and suspended so that both its top and bottom surfaces are exposed to the room air.

Would the rate of heat transfer from the top surface be greater, equal to, or less than the rate of transfer from the bottom surface? Why?

2) Is it possible for the exit temperature of the cold fluid to be greater than the exit temperature of the hot fluid in a heat exchanger? (simple scenario, let's assume it's a 1-1, and that both fluids are single phase fluids)

For 1) it's stagnant air, there is no movement of air. Of course, the only thing I could think of was that the top surface > bottom surface, due to natural convection. I'm just not sure if the same principle would apply on the bottom surface.

For 2) I didn't think it was possible if it was a cocurrent heat exchanger, since the log mean temperature difference would be undefinable. It could be applicable in a countercurrent heat exchanger though.

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anorlunda
Staff Emeritus
Would the rate of heat transfer from the top surface be greater, equal to, or less than the rate of transfer from the bottom surface? Why?
The mechanisms are radiation, convection, and conduction. It sounds like conduction is zero. Radiation should depend on the temperatures of what the block is facing. The floor, ceiling and so on. Convection is complicated. You say stagnant air, but the block itself will heat the air and cause air currents. Bottom line, I don't know.

2) Is it possible for the exit temperature of the cold fluid to be greater than the exit temperature of the hot fluid in a heat exchanger?
Yes, in a counter flow heat exchanger. LMTD can be calculated but you must be careful to subtract temperatures at the left/right sides, not the in/out temps.