Evac & recharge increases temperature split?

[alexb]

Any science behind this?

I've had techs say that the charge in my system (AC only, no heatpump) was A-OK. Then last year, I decided to have a full evacuation & recharge done. I can tell the return air is much colder now - the tech said the split was 17degrees, but we didn't measure before the evac.

Any reason why the temp. split is much more favorable, after an evacuation & recharge? Pressures measured OK before the evac.

[hvacjamie]

too many factors. outdoor / indoor temps. airflow. pressure wasn't right as thought before? humidity. all factors that can change readings regularly. best measurment is superheat/ subcooling. why did you request evac in the first place? should never be necessary unless repaired and line is open and exposed to atmosphere.

[Wild Leg]

The science behind it involves parts of Boyles' Law, Charles' Law and the Law of Perfect Gases.

If it was done properly the first time, and had not modified, then there would have been no difference.
The techs that said it was "A-OK" were apparently wrong.
Pressures do not tell the whole story.
There was something wrong with your refrigerant.
Now the refrigerant is cleaner, and dryer.
It can work now.

Temperature split (Delta T) can be misleading.
A dirty filter will give a greater delta T, but less net cooling effect.
How can you tell the return air is colder now?
If return air is colder, that just means the room is colder.
Now, if the supply air was colder, with the same return temp, then you would have a greater Delta T.
Many factors must be considered before declaring a system
"A-OK".



[Edited by bwal2 on 02-05-2005 at 12:39 PM]

[Shophound]

If your system was originally installed like so many are, it's quite likely a proper evacuation and charge was not done. Overcharged systems with non-condensables are common on residential split system installations (and light commercial, for that matter).

You didn't hurt anything by having this done. There's no way for the homeowner to really know if a proper evac was done at installation or not, but having a clean system now certainly works in your favor.

To directly address your question, the science involved is as bwal2 alluded to; the law of partial pressures (the total pressure within a vessel is the sum of all partial pressures of each gas within the vessel). Your ideal refrigerant circuit contains only refrigerant and oil. No moisture, no atmospheric air, no other gasses unrelated to the refrigeration effect. These gasses are known as non-condensables and, in essence, take up room within the system that the refrigerant should have to do its work.

Non-condensables raise the operating pressure within the condenser, forcing the compressor to work harder. Raised condenser pressures also raise the evaporator pressure (especially on piston metering device systems...TXV's will compensate to maintain superheat - to an extent), which in turn raises the saturated vapor temperature, resulting in a warmer coil overall (concurrent with non essential gasses within the evaporator that cannot produce any refrigerating effect).
Moisture in the system reacts with the compressor oil, breaking it down over time and causing it to become acidic. This in turn eats away at the compressor motor winding insulation, which could lead to a shorted or grounded compressor motor.