Hey guys. 3rd year commercial/residential mechanic here. I'm getting conflicting info from different sources regarding subcooling, so I thought I'd ask here to see what you guys thought. TXV system...As my head pressure falls (getting colder outside) my condenser is going to run cooler.
As the temperature/pressure falls in the condenser...more liquid will start to back up in it. As more liquid backs up, subcooling increases... True or false.
If ambient temp is the only thing that changes...false. You will get less subcooling as the ambient drops.
Hey Jayguy. Thanks for the reply...
Ok. I don't understand how that's possible. I do understand that when charging, as I add more refrigerant to the system my head pressure increases as well as my subcooling. Fine. Add gas to increase subcooling.
However, sometime later, if the outside ambient temperature drops, it allows the condenser to run cooler. A cooler condenser means more liquid. More liquid means more time for it to subcool(no?). I would think that as the outdoor ambient drops, subcooling increases. Cooler condenser, more liquid, more time to subcool...
Don't think about "cooling." That's where intuition is misleading you.
This value is based on DIFFERENCE of temperatures.
When it is hotter outside, and head pressure rises (with the higher condensing temperature) there is a greater difference between the temperature of the liquid line and condensing temperature.
More difference = more subcooling.
[Avatar photo from a Florida training accident. Everyone walked away.] 2 Tim 3:16-17
On a TXV system you should charge it based on superheat. With a piston you wanna pay more attention to SC.
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You have these mixed up Paul, reverse it and you are good.
But in reality, both should always be taken into consideration, when "troubleshooting", regardless of txv/piston.
Loss of subcooling with pressure drop... similar to loss of subcool on long vertical rise due to pressure drop, then run it (liquid line) uninsulated through hot attic and watch it flash (zero subcool) at the valve!
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On a TXV system you should charge it based on superheat. With a piston you wanna pay more attention to SC.
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Hey Paul. I think you have it backwards, as that's exactly the opposite of what I was taught in school. Fixed orifice is charged by superheat. TX is charged by subcooling, however this isn't about charging, it's about what happens to subcooling with changing outdoor ambients.
Hey Paul. I think you have it backwards, as that's exactly the opposite of what I was taught in school. Fixed orifice is charged by superheat. TX is charged by subcooling, however this isn't about charging, it's about what happens to subcooling with changing outdoor ambients.
Don't think about "cooling." That's where intuition is misleading you.
This value is based on DIFFERENCE of temperatures.
When it is hotter outside, and head pressure rises (with the higher condensing temperature) there is a greater difference between the temperature of the liquid line and condensing temperature.
More difference = more subcooling.
Neither of the above. On some systems SC will rise, on others it will drop. On others it will first drop and then rise as ambient drops further. See the attached SC chart (cooling mode) in the center of the attached image. Subcooling depends upon several variables.
I can't see the chart with sufficient definition to read it.
However, I have found that with all of the other variables being equal, it is still a difference value that tends to rise with condensing temperature, and decrease with a decrease in condensing temperature.
If other variables also change, than the outcome will change.
[Avatar photo from a Florida training accident. Everyone walked away.] 2 Tim 3:16-17
It is also important to note that subcooling is just a temperature difference number. It is not a linear measure of how much liquid is in the condensing unit. It is a number we use to give us an idea about the amount of liquid that is available to leave the condenser via the liquid line.
[Avatar photo from a Florida training accident. Everyone walked away.] 2 Tim 3:16-17
It is also important to note that subcooling is just a temperature difference number. It is not a linear measure of how much liquid is in the condensing unit. It is a number we use to give us an idea about the amount of liquid that is available to leave the condenser via the liquid line.
Hey Timebuilder. I'm following you but I'm not.
Can you expand a little more on the second half of your statement. Cheers.
Neither of the above. On some systems SC will rise, on others it will drop. On others it will first drop and then rise as ambient drops further. See the attached SC chart (cooling mode) in the center of the attached image. Subcooling depends upon several variables.
while you are correct that subcooling depends upon several variables, the chart shows "subcooling required" not necessarily what the sub cooling will be at various temperatures...i can not say if the subcooling at 1 point on this chart will be the subcooling at a different point on the chart without the addition of more or less refrigerant...i don't know this company or equipment or what the chart is used for.
"Right" is not the same as "Wise".
Don't step on my favorite part of the Constitution just to point out your favorite part.
Just because you can measure it, doesn't mean it is important. Just because you can't measure it, doesn't mean it isn't important.
while you are correct that subcooling depends upon several variables, the chart shows "subcooling required" not necessarily what the sub cooling will be at various temperatures...i can not say if the subcooling at 1 point on this chart will be the subcooling at a different point on the chart without the addition of more or less refrigerant...i don't know this company or equipment or what the chart is used for.
You could say the same about any SH chart. What would be the point in plotting the required SH or SC at different conditions if each entry corresponded to a different total charge?
Last edited by hvacrmedic; 07-15-2016 at 09:59 PM.
You could say the same about any SH chart. What would be the point in plotting the required SH or SC at different conditions if each entry corresponded to a different total charge?
again, i don't know what this chart is used for...is it a charging chart or is it an engineering chart? does anything else change as the temperature drops on the units this chart is associated with? such as fan cycling or some other head pressure control? does the chart indicate a constant load or constant indoor WB/DB situation or are they assuming there are some changes?
i don't know.
"Right" is not the same as "Wise".
Don't step on my favorite part of the Constitution just to point out your favorite part.
Just because you can measure it, doesn't mean it is important. Just because you can't measure it, doesn't mean it isn't important.
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I think you have it backwards, as that's exactly the opposite of what I was taught in school. Fixed orifice is charged by superheat. TX is charged by subcooling, however this isn't about charging, it's about what happens to subcooling with changing outdoor ambients.
