oil logged evaporator or bad txv?

zepfan

i am having a problem with a vertical self contained air cooled cool air ac unit.it started out that the unit was going off on low pressure.i tried to add some refrigerant and the low pressure did not rise.the body of the txv started to ice up.i thought it was a bad txv,i changed the txv,the unit operated the same.suction pressure was low (20 psi) and the high side was low (105 psi) this is an r22 unit with 5 lb. total charge.i thought that maybe the power element on the new txv was bad, i changed it,but the unit operated the same.both times the system was pressure tested,evacuated,and the charge was weighed in.the suction pressure is low and the unit goes off on low pressure.i have blown through the unit with nitrogen, and there appears to be good flow,however once the unit is running the pressure is low and the unit shuts off.i have ruled out air flow, and and have calculated the unit is moving 1900 cfm of air (five ton unit) i do not know what else this can be but the whole txv is bad (a doa) or the evaporator is clogged.there is no temperature drop across the drier,and every time the body of the txv,and distributor ice over.any assistance,or suggestions would be much needed.thank you

Eugene_Silberstein

A couple of more readings...

A couple of more readings would be helpful. What is the temperature of the liquid line at the outlet of the condenser coil? What is the temperature of the suction line at the outlet of the evaporator?

These two temperatures, teamed with your operating pressures will help determine if there is a refrigerant flow problem or a charge problem.

Also, please let us know exactly where the pressure readings were taken (for example, on the discharge line or the suction line)

If the system is so equipped, high side and low side readings at multiple points in the system (discharge line and liquid line) are often useful to help detect internal blockages in the system.

Please keep us posted.

furnacefigher15

I agree with

Eugene. Pressure readings alone are useless.



5 #'s sounds low for a 5 ton system with a TXV.



If the TXV, or the Evap were plugged, the head pressure would be higher.



We need Liquid line temp, Suction line temp.



TXV systems are charged by subcooling, sometimes this requires more refrigerant then what you may be used to adding to a system using a piston.



Is 5 pounds the labeled charge?

When you say self contained, do you mean packaged, as in the condensor and evap are all in one unit?



Is the unit water cooled?



What does the evap look like? Dirty?

zepfan

five pounds is the charge

thanks for the feedback.yes five pounds is the label charge which i thought was low as well for a five ton unit.i have talked to the manufacturer to make sure this was not a misprint,and they have confirmed that five pounds is the entire system charge.what i mean by self contained is that the evap. and condenser are all in the same package.the unit is air cooled and the evaporator coil is clean.the pressures were taken at the discharge port and the suction port,both within 6" of the compressor.there is no liquid line port on this unit,and there is less then 5' of suction piping.the superheat is high, about thirty degrees.they only place that i am noticing any icing or temperature change is right at the distributor,and the body of the txv.the liquid line is warm (warmer then ambient) right until it enters the txv.i will get a better subcooling reading on monday.if i had not talked to the manufacturer i would have thought five pounds was a misprint.thanks again

Eugene_Silberstein

Based on what you say...

Based on the information you provided, it looks like there is a problem with the TXV.

The thermostatic expansion valve has a one-line job description, which is to maintain constant evaporator superheat. If the superheat is high and the refrigerant charge is correct, it means that the valve is not feeding the correct amount of refrigerant to the evaporator coil.



While you are taking your subcooling readings, take a temperature reading at both the outlet of the condenser coil and the inlet of the TXV. SInce this is a self-contained unit, the temperature difference between these two points should be very low.



Now... There is another possibility that will be addressed when you get your subcooling reading. If there is physical blockage in the condenser coil, the TXV will not be able to feed the proper amount of refrigerant into the evaporator coil, even if it is fully open.



Is there a receiver on this unit, or is the bottom of the condenser coil serving the purpose?



Have fun!

zepfan

no receiver

thanks for your help.there is no receiver on this system.i am goingto try another txv and add a liquid line port to the unit monday.thanks again

zepfan

it was the txv

thank you for your help with this matter.i changed out the txv today,and that took care of the problem.the strange part of this was that the txv that i changed out would open when you push down on the pushrods.the valve i replaced it with was a five ton valve but had a larger valve body then the one the manufacturer sent.either way the evaporator behaved completely differant with the larger valve body.thanks again and take care.

don_9

txv

You don't say.I tell ya back in the old day it was very rare to have a bad txv, but now days it is a given.

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