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Subcooling

mjohnson2981 on Wed June 18, 2008 4:29 PM User is offline

Year: 1979
Make: New Holland
Model: 1069
Engine Size: 460
Refrigerant Type: R134a
Ambient Temp: 75
Pressure Low: 8
Pressure High: 125
Country of Origin: United States

Working on a piece of farm equipment...

Small Cab
York/CCI Compressor
Lengthy Hoses - Probably 20 feet each side.
Modine Serpentine condenser
High volume (non clutch) engine fan for cooling the condenser - Vehicle movement doesn't affect air movement.
TXV System


Ambient Temp: 75
Humidity: Approximately 40%
Low Side (Fitting on compressor): 8 psi
High Side (Fitting on compressor): 125 psi
Condenser Temp IN: 152
Condenser Temp OUT: 85
Compressor High Side Temp: 155 (Non Contact Temp Gun)
Compressor Low Side Temp: 58 (Non Contact Temp Gun)

I have charged it with 3.25 lb of R134a

The spread across the compressor is more than the 25-35 degree that it is supposed to be to charge by subcooling, as well as the compressor discharge temperature is higher than a 45 degree rise.

This is a new condenser and have not had a compressor problem. Compressor has a little bit of flutter on the high side ( + or - 15 psi)

Is there a condenser blockage? Is it under charged (by gauges) or overcharged (by temperatures)?

Also - It seemed to make the delivery air colder (About 42) by blocking airflow across the condenser and allowing the pressures to increase.

Thanks for any insight.

I have another piece of equipment set up similar giving me similar readings...




bearing01 on Wed June 18, 2008 6:41 PM User is offline

Phigh = 125psi (=100'F sat temp) and condenser exhaust at 85'F suggests 15'F subcooling. That would suggest the system has a good bit of refrigerant backed up in the condesner. You got 20' of lines but that amount of subcooling should be able to handle that pressure drop... I would think... unless you got really small lines.

Plow = 8psi (=3'F sat temp) and compressor suction inlet at 58'F suggests 55'F superheat at your compressor inlet. Assuming there's some warming of the suction line, the evaporator is still clearly being starved of refrigerant. 8psi is too low a pressure for the low side so sounds like you got a restriction somewhere between the condenser and compressor inlet. Perhaps in the evaporator or TXV? A restriction would let the refrigerant back up in the condenser and give the subcooling, even if your low on charge.

Also you say it seems to cool better when the air is restricted to the condenser and head pressure rises. Perhaps the expansion valve is behaving badly because the evaporator pressure is way too low... and it is liking higher pressure drop across it to force more refrigerant through. Do you have access to the TXV? Is it frozen up? Maybe put a wet towel on it to see if it thaws out. Does the system blow cold before the TXV freezes up?

Any liquid lines have frost? Is there a temperature difference across the receiver-drier?



Edited: Wed June 18, 2008 at 6:44 PM by bearing01

mk378 on Wed June 18, 2008 6:57 PM User is offline

Is this a conversion? You may have a drier that's incompatible with R-134a. These will blow the desiccant out and clog up the TXV.

mjohnson2981 on Wed June 18, 2008 8:24 PM User is offline

Thanks for the quick replies.

I felt with my hands the receiver/drier and did not notice a temperature drop from the inlet/outlet lines OR from top/bottom of the drier. The system was originally a R-12 system but has had a new compressor, TXV, receiver/drier, and hoses at the retrofit. The condensor was added a few years later when it developed a leak.

I personally did not do the conversion, but am sure that it was flushed at the time of conversion. I can get to the front of the evaporator fairly easily, and should be able to see the TXV as well.

No frost on any of the lines that I had seen. The lines that connect that carry the distance are fairly good sized... I don't know what sizes are common for AC, but I'd guess roughly 1/2"

I will dig further tomorrow if my schedule allows me to.

I am sort of new to the understanding of A/C... I've been working with it for years without understanding it. I have the mechanics of it figured out. Is there a website or book that you would recommend to me, because I'm having a hard time grasping the chemical side of it.
I've read through a Snap-On book that was printed just after R134a came about because it has R12 and R134a in it.

Thank you.

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