Subcooling measurement

[Arc]

When measuring the temperature of the liquid line of a split system, must it be taken near the metering device or is it ok off the outdoor unit for ease?
Also when a port is only available on the discharge line and not the liquid line for pressure what allowances should be made?

[kvas]

When you measure subcooling temp. must be taken at the suction side of compressor as near as you can .. because there you take pressure and you can compare with the theoretical nums. when you have port at the discharge line (heating mode) you can see the superheat only. if anyone think that im wrong please correct me to be better.

[lana]

Hi there,

Sub-cooling is measured at the outlet of the condenser. You measure the liquid temperature out of the condenser (t1), then read the discharge pressure and by temp-press chart or table, convert the pressure to temperature. This temperature is the condensing temperature (tc). Now the sub-cooling is tc-t1=SC

Hope this helps.
Cheers

[kvas]

Of cource Lana is right, subcooling not superheat. stuck in my mind. exactly the opposite of what I said. Sorry.

[Arc]

Thanks for the response. Your logic makes sense, it seems there are different theories on these issues. Such as pressure should be taken from liquid line/temp should be taken near to metering device.

[Tradewinds]

Hi Arc,
I always measure it at the point leaving the condenser and look for 4-8K.

However, having recently read a Carrier manual they look for a subcooling value of 12K. I was wondering why this was so high and it turns out they measure the subcooling at the inlet of the expansion valve. As these were chillers with a fixed liquid line length they know how much subcooling they get in the liquid line. So they would be adding approximately 6K of liquid line subcooling to the 6K they get at the condenser.

From a theoretical point of view, if you look at a PH Chart, subcooling occurs the minute the temperature of the liquid is lowered below the saturated liquid line (approx 2/3 thru condenser coil) right up to the inlet of the TXV.

Hope that makes sense.

Also, the pressure is the same in the discharge and liquid lines so if you want to read pressure, it doesnt matter which one you use. Its the temperature and state of the refrigerant that is different.

[Gideon Beddows]

personally, i'd measure the liquid line pressure and at or as near to that point, measure the temperature.

That will give you the subcooling just there.

Unless is is a huge piperun, i doubt it will chanhe THAT much anywhere in the system

Hope that helps?