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Thread: Causes of Negative Subcooling
07-23-2011, 04:01 PM #40
The refrigerant gas returning to the compressor either absorbs the heat produced by the compressor, or it will be to super-heated to absorb that heat, thus not helping cool the compressor. In short, the refrigerant removes the heat created in the compressor, (ie: Heat of compression), or the compressor will most likely overheat. (Assuming it does not have some other external means of cooling the compressor).
07-23-2011, 04:14 PM #41
07-23-2011, 04:26 PM #42
07-23-2011, 04:48 PM #43
The temperature corresponding to your gauge pressure, should be higher than that indicated by the thermometer on the suction line in order to have sub-cooling.
If the sub-cooling is indicating a negative sub-cool. Then that sounds like your line temperature is above the temperature indicated by your gauges.
I am guessing it has an undercharge. The refrigerant should be in a liquid state well before leaving the condenser. I am guessing it is not. It is still a supper-heated gas, or liguid gas mixture.
Or as the others stated, lack of air flow.
07-24-2011, 01:27 AM #44Professional Member
- Join Date
- Dec 2010
- Waco, Kentucky
I was on a new install a few days back. Complaint was the it was not cooling well. Residential 2 ton condenser, 2.5 ton coil and 70k BTU 80% GF with a 3 ton drive. Not positive it was 70k Btu but defiantly a 3 ton drive. It had a clean filter, clean coils, the correct piston and fan speed matched accordingly to the 2 ton condenser. I didn't check wet bulb but I know the dry bulb return at the coil was 78 and supply 63 with a SH or around 15. It was 98 outside and the condenser was in direct sunlight. My concern was that I had no SC. Actually the liquid line had 3 deg SH. I've seen this same issue with several units lately. So my questions are this: What could be the problem? and How do I fix it?
07-24-2011, 08:52 AM #45
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Last edited by beenthere; 07-24-2011 at 08:58 AM. Reason: No *
07-24-2011, 09:14 AM #46
We also could use an amp-draw number; that has to be compared to the average readings U got with similar conditions. At 78F RA a WB of near 70F could result in total amps 'around' 9.9-amps.
The lack of Subcooling could indicate an under charge. If the latent load is normal or low that could point to a low charge.
At an outdoor 98F & mid-range indoor humidity the superheat target is 5F; U had 15F, too high - low charge.
Is that a single speed condenser fan motor?
Is the fan motor delivering the Rated condenser CFM?
Check the condenser discharge air temp-rise. What was the head pressure?
We need to determine the level of the BTUH load on the condenser; we need to know if there is an excessive evaporator heatload. Or, could something be starving the evaporator coil?
Last edited by udarrell; 07-24-2011 at 09:20 AM. Reason: Is something starving evap...?
07-24-2011, 10:57 AM #47Professional Member
- Join Date
- Dec 2010
- Waco, Kentucky
udarrell: I only have two years in the trade and I am still acquiring the necessary tools. Psychrometer is still on my to purchase list. A brand suggestion would be helpful. I have considered the undercharged idea but i have to get my SH down so low that I'm afraid it may bring back liquid under a lesser load. Yes it is a single speed cond. fan mtr. I did not check if it was delivering the rated CFM or temp rise. Head was 100 deg sat so around 325 psi. By starving do you mean refrigerant or air flow? Is there a chart or calculation available that includes all important measurements and information that will result in the precise SH and SC measurement for the given situation? Every time I talk with an experienced tech I get to relearn how little I know and how much more I have left to learn about this trade. Thanks for all the help.
07-24-2011, 11:05 AM #48Every time I talk with an experienced tech I get to relearn how little I know and how much more I have left to learn about this trade.
As soon as you get your post count to 15, apply for pro membership. So you have access to the pro forums, where we can go into detail, and help you learn. Can 't go into too much in the open forums. And this thread is about at the limit of what can be said.Contractor locator map
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