Heat exchange efficiency

[tbedford]

I always understood that counter flow positioning of temperatures was optimal for heat exchange.

With this in mind, it seems to be intuitive that the return air in a rooftop package would first come in contact with the colder coil face. That is to say the side of the coil face where the metering device first introduces refrigerant into the evaporator.

I have recently reviewed various refrigeration schematics. The air entering the evaporator enters on the opposite side of the evaporator...that is to say the side where the majority of refrigerant is boiled off and is warmer.

Can someone explain this to me or provide a link to clear explanation?

Thank you,
Tom

 

[Compositepro]

"I have recently reviewed various refrigeration schematics. The air entering the evaporator enters on the opposite side of the evaporator...that is to say the side where the majority of refrigerant is boiled off and is warmer."

That is counter flow. The leaving refrigerant (already warmed-up) contacts the warm entering air so that the last bit of "cold" can be extracted. The coldest refrigerant is used to further cool air that has already been precooled as much as possible.

Another way of looking at it is that the entering air contacts the leaving refrigerant. Again, counter-flow.

 

[dynamo78]

@ Compositepro, Cant agree more.
Yes, Tbed, its like pre cooling the air for "free",utilizing the available heat, and then elevating the suction temp will lessen the compressor work. Remember those old school economizers?