trane rooftop-cap tube

[hvac17011]

discharge pressure 250 suction 55 subcooling 20 superheat 27. ll drier no real temp difference across.what will I see if cap tubes are partially plugged.

[sonc]

frost

[cde72]

sonc: frost
If you have a restriction in any MD, you would be starving the evap. creating a low charge situation, however with the low suction pressure you would have an increased head pressure. With a cap tube it can be little more subtle because the restriction could only be blocking 1 distribution tube, but you would still see frost in at least part of the coil.

[hvac17011]

I need to clean both coils but with that kind of subcooling my superheat should be lower. or dont you even consider subcooling when charging cap tube(i know with fixed md you charge by superheat).I see no frost

[cde72]

Sub-cooling is "in layman terms" the amount of liquid refrigerant you have going to the metering device... you don't really use it for cap tubes as they are "fixed". I might try adding a bit of refrigerant since you don't see any frost... might just be low.

[hvac17011]

I did 3#.I always have both hooked up superheat and subcooling.l

[cde72]

Lemme clarify...
A dirty condensor will drive up your high side pressure and cause a higher sub-cooling... so clean your coils before adding. Once you do that... your head pressure should come down, and if your suction pressure is still low with a high superheat, you still need to add. What was your pressures/temps before adding? In regards to superheat-subcooling...
superheat is intended to let you know how super saturated the vapor is after leaving the evaporator... the higher the temperature the less refrigerant or higher the load you have...
subcooling is intended to let you know how much liquid refrigerant you are supplying the md with...
superheat will help diagnose all types of md, but you should use superheat to charge only fixed devices.
subcooling is intended for charging hp's/txv's.

[jeffcolledge]

A plugged md will not drive up the head pressure, it's just like pumping down the system into the condenser.
Also, a dirty condenser will not increase your subcooling, if anything it will decrease it as it is less efficient, taking longer to condense to a liquid.
If your md became restricted enough, it may stack the ref. in your condenser, appearing as though you are properly charged. I would first clean both coils, then open the system, blow it out from suction and head side, install a hi capacity drier, evacuate and recharge. If you are still having problems, you may need to replace the cap tubes.

[Edited by jeffcolledge on 07-19-2004 at 10:42 PM]

[Paul Pippin]

The easiest way to check out the metering device on a rtu like that is to stop the indoor blower and watch the frost pattern. It will tell you in a few min wheter the metering device is stopped up.

[sonc]

Hey Jeff, you are right again. How you get so smart living in Windsor?

[jeffcolledge]

Originally posted by sonc
Hey Jeff, you are right again. How you get so smart living in Windsor?

I went to trade school in the GTA! (Actually Hamilton, but it's close enough)

[seaboard]

[QUOTE]Originally posted by jeffcolledge
[B]A plugged md will not drive up the head pressure, it's just like pumping down the system into the condenser.
Also, a dirty condenser will not increase your subcooling, if anything it will decrease it as it is less efficient, taking longer to condense to a liquid.
If your md became restricted enough, it may stack the ref. in your condenser, appearing as though you are properly charged. I would first clean both coils, then open the system, blow it out from suction and head side, install a hi capacity drier, evacuate and recharge. If you are still having problems, you may need to replace the cap tubes.

This was one of the first things I had to "unlearn" out of school. On a small system like a refrigerator for instance a partially plugged metering device may cause the head pressure to be high and eventually the compressor will overheat and trip on thermal protection. However, on most equipment, as the compressor "starves" to get vapor back, the head pressure falls from a restriction. A momentary high head on start up may not be unusual, but as it starves the head and suction will fall and subcooling will go through the roof as all that refrigerant gets stacked up in the condenser. I wonder how many schools out there are still teaching this?

[jeffcolledge]

[QUOTE]Originally posted by seaboard
[B]

Originally posted by jeffcolledge
A plugged md will not drive up the head pressure, it's just like pumping down the system into the condenser.
Also, a dirty condenser will not increase your subcooling, if anything it will decrease it as it is less efficient, taking longer to condense to a liquid.
If your md became restricted enough, it may stack the ref. in your condenser, appearing as though you are properly charged. I would first clean both coils, then open the system, blow it out from suction and head side, install a hi capacity drier, evacuate and recharge. If you are still having problems, you may need to replace the cap tubes.

This was one of the first things I had to "unlearn" out of school. On a small system like a refrigerator for instance a partially plugged metering device may cause the head pressure to be high and eventually the compressor will overheat and trip on thermal protection. However, on most equipment, as the compressor "starves" to get vapor back, the head pressure falls from a restriction. A momentary high head on start up may not be unusual, but as it starves the head and suction will fall and subcooling will go through the roof as all that refrigerant gets stacked up in the condenser. I wonder how many schools out there are still teaching this?

I agree...I said a dirty condenser wouldn't raise the subcooling, and that the md would stack the ref, increasing the subcooling.