# Theory Question: Pressure drop across evaporator

• 08-12-2008, 11:45 AM
pwn01
Theory Question: Pressure drop across evaporator
Is the pressure drop across the evaporator enough in a normally operating system that the temperature correlating to the pressure read at the outlet of the evaporator will be lower than the coil temperature at the inlet? Does this account for being able to observe liquid condensate on the inlet while the suction pressure indicates a temperature below freezing?
• 08-12-2008, 01:01 PM
beenthere
Your question is confusing. Atleast to me.

Yes, evap coils have a PD between the metering device and the outlet of the evap.

But, as the refrigerant absorbs heat, it expands. So teh coil temp at the outlet should not be below that of the metering device end.
If it is, you have other troubles.
• 08-12-2008, 10:27 PM
pwn01
Let me give the scenario to help clarify the question. I am adding charge to an R-22 unit that is way undercharged (Suction pressure 25 psi). The evaporator is frosting because at 25 psig the temperature of the coil is a little above 0*F. In the process of adding refrigerant the pressure passes through 55 psig which is about 30*F. At this point, the frost begins to melt. By the time the pressure rises above 58 psig (32*F) all of the frost has long since melted.

At some point during this process, the pressure indicates that the coil temperature should be below freezing, but the frost has melted already.

Would this be because the gauge was out of calibration, or is it possible for the pressure at the outlet of the evaporator to be a few points below the inlet pressure causing the frost to melt while the outlet pressure indicates below freezing coil temperature?
• 08-12-2008, 10:38 PM
ar_hvac_man
Quote:

Originally Posted by pwn01
Let me give the scenario to help clarify the question. I am adding charge to an R-22 unit that is way undercharged (Suction pressure 25 psi). The evaporator is frosting because at 25 psig the temperature of the coil is a little above 0*F. In the process of adding refrigerant the pressure passes through 55 psig which is about 30*F. At this point, the frost begins to melt. By the time the pressure rises above 58 psig (32*F) all of the frost has long since melted.

At some point during this process, the pressure indicates that the coil temperature should be below freezing, but the frost has melted already.

Would this be because the gauge was out of calibration, or is it possible for the pressure at the outlet of the evaporator to be a few points below the inlet pressure causing the frost to melt while the outlet pressure indicates below freezing coil temperature?

Warm air.
• 08-12-2008, 10:41 PM
beenthere
The refrigerant temp may have been under 32.
But the actual coil temp was either at, or just slightly above 32.
Heat transfer through the coil isn't 100&#37; efficienct.
Plus, the moisture in the air, won't freeze at 32&#176; instantly.

Remember, water freezes to ice at 32, and ice melts to water at 32&#176;.
You have to remove 144 BTUs of latent heat to freeze 1 pound of water.
Or, 9 BTUs latent per oz.
• 08-12-2008, 10:59 PM
pwn01
I think I see. The refrigerant may have been at or even slightly below freezing, but it didn't have enough "umph" to win the heat of fusion fight with the ambient air to keep the condensate frozen.
• 08-12-2008, 11:13 PM
beenthere
You got it.
• 08-12-2008, 11:21 PM
pwn01
Thanks.