question on delta T values in air conditioning work

[beachfronttech]

Hi a/c fellows. I mostly work on refrigeration systems, but rarely I need to do some comfort air conditioning job. That´s why I would like to clarify my knowlegde on these systems.
In refrigeration, it´s normally assumed that the refrigerant inside the heat exchanger (evaporator) is about 5 deg C (10 deg F) colder than the surface temperature of the heat exchanger itself. Furthermore, the ambient temperature of the refrigerated space is suppose to be about 5 deg C (10 deg F) warmer than the surface temperature of the heat exchanger. Therefore, the total delta T between the ambient air and saturated refrigerant is about 10 deg C (20 deg F). Are these delta T assumptions valid for comfort air conditioning work?

[Abby Normal]

focus on superheat and subcooling rather than just the temperatures.

Its a lot more to do with the wet bulb of the enetering air and the ambient dry bulb temperature than the dry bulb of the entering air.

Not going to be a big range of entering dry bulbs, unless of course system is down for 2 days during hot humid weather.

[Gary]

Hi a/c fellows. I mostly work on refrigeration systems, but rarely I need to do some comfort air conditioning job. That´s why I would like to clarify my knowlegde on these systems.
In refrigeration, it´s normally assumed that the refrigerant inside the heat exchanger (evaporator) is about 5 deg C (10 deg F) colder than the surface temperature of the heat exchanger itself. Furthermore, the ambient temperature of the refrigerated space is suppose to be about 5 deg C (10 deg F) warmer than the surface temperature of the heat exchanger. Therefore, the total delta T between the ambient air and saturated refrigerant is about 10 deg C (20 deg F).

Let's start with some clarifications:

A delta-T is the change in temperature of a single substance or flow of substance. The evaporator delta-T is the temperature difference between the air entering the coil and the air leaving the coil.

A TD is the temperature difference between two different substances or flows of substances. The evaporator TD is the temperature difference between the air entering the coil and the saturated refrigerant.


Are these delta T assumptions valid for comfort air conditioning work?

No, they are not. Normal evaporator TD would be about 35-40F (20-22K) on an A/C system.

As to coil surface temps, I generally assume (for both refrigeration and A/C) that it is a degree or two below leaving air temp, because of the coil bypass factor (a small portion of the air makes it through the coil without actually touching anything).