A\C suction pipe freezing

[cretan]

Worked on a couple of older units yesterday.There was no charging chart on the units,so I used a "universal" one that I have.I charged them according to the chart.After they were running for a few hours I noticed that about a foot of the suction line was frosting over at the compressor.The suction line close to the evaporators were sweating.What is this a sign of?Why is there frost by the compressor and not the evaporator?There are no restrictions,filters are clean.

[wambat]

Worked on a couple of older units yesterday.There was no charging chart on the units,so I used a "universal" one that I have.I charged them according to the chart.After they were running for a few hours I noticed that about a foot of the suction line was frosting over at the compressor.The suction line close to the evaporators were sweating.What is this a sign of?Why is there frost by the compressor and not the evaporator?There are no restrictions,filters are clean.

The presence of frost can mean that the suction line or compressor is below freezing, and the moisture in the air has reached its dewpoint temperature and condensed. This condensed moisture can be frozen to ice because the temperature is below 32°F, However compressor superheat is the assurance that there is no liquid refrigerant present at the compressor, and that the saturated vapor in the evaporator has gained the required amount of sensible heat before reaching the compressor


If the heat load on the evaporator coil is reduced, ( like reduced airflow) not enough refrigerant will vaporize. The 100% saturated vapor point in the evaporator will crawl down past the end of the evaporator and the metering device loses control. Compressors can slug and/or flood in these situations like a cold compressor crankcase: if the compressor (total) superheat is low, sometime during the on cycle the compressor will flood or slug. Liquid refrigerant in the compressor’s crankcase will boil off. This can flash the oil and cause compressor damage.

It is the boiling of refrigerant in the crankcase that makes the crankcase cold to the touch. The crankcase may even sweat or frost if conditions are right. Or you may have a condition where wet vapor along with a higher then normal suction line pressure drop will create frost at the compressor suction bell. check the superheat at the compressor suction inlet. It should be about 60*F
Some symptoms for reduced air over evaporator coil include:

Low discharge temperatures;

Low head pressures;

Low condenser splits;

Low evaporator (suction) pressures;

Low superheats;

Cold compressor crankcase.

[coolments]

Hi Cretan

Is the expansion device cappilary, if so I think the unit maybe over charged a little, when i was an aprentice most units i worked on were either IMI or qualitair and they had no charge instructions, so charging was done by the skill of the engineer.
As they had no receivers or accumilators the charge was critical, especially if it worked with a cappilary, I was tought that if you put in little too much refrigerant the liquid would slug back and sit in the comp like as wambat says, we used to then vent a little out until the frost line crept back, then you would feel the top of the comp and (were talking hermetic comps like leHermetique) it should have felt the same temp roughly as your hand if all was well, a little cave manish but thats the way things were then and i am sure a few old school engineers will know what I am talking about.
My advice is connect a reclaim cylinder onto the liquid line and take small amounts of the charge out untill the frost creeps back and goes away leaving the suction sweating nicely back to the comp, if you take to much out then you can simply put it back in. this is of course assuming there is no air flow restrictions or any other problems with the unit, just have a play with the charge and forget scales and charts for this unit.

[cretan]

Thanks for the reply guy's.I figured it may be a little overcharged but what threw me off was the frost on the suction only by the compressor and nowhere else.No frost by the evaporator.It started to frost-up after a few hours.I am an hour and a half away from the unit.I will check superhaet again next week when i am back in town.Thanks again!!

[cretan]

Is there a rule of thumb for subcooling on an A\C unit?If so what should I look for?

[nike123]

Is there a rule of thumb for subcooling on an A\C unit?If so what should I look for?

http://wwwebworks.com/ht/fyi/Jim_Wheeler_Charging/Jim_Wheeler_Charging1p.htm

[cretan]

Thank you!!

[AcidSlasher]

http://wwwebworks.com/ht/fyi/Jim_Wheeler_Charging/Jim_Wheeler_Charging1p.htm


wow that was a great read. i learnt alot from that!!

got any more good pages similar to that one?

[nike123]

wow that was a great read. i learnt alot from that!!

got any more good pages similar to that one?

Of course, web is so vast these days!
http://www.refrigeration-engineer.com/forums/showthread.php?t=14449

[cretan]

Checked the units yesterday.Superheat was low.took a total of 2 pounds 5 ounces out of both A/C units.The previous guy had obviously overcharged them.Using a SH charging chart got them working great.It's been a while since I worked in the field,so I was a little rusty.It's all coming back to me now.Thanks for the help!!

[Arc]

Could you send me one of these charging charts. Haven't seen them.

[cretan]

The first one is a good read.The other two have a more accurate charging chart.Temperatures are taken with a dry and wet bulb thermometer.

[Dilip sahasrabu]

What about frosting coming and going even in capillary systems. We have a typical system of butyl tube cooling where we need the frost to be present and we are having a long copper tube as evaporator,we have 10 such units which are running for last 3yrs without much trouble. But for past few days two of the units showing the coming and melting of frost, is it the symptom of capillary choking?

[Arc]

Thanks cretan

[nike123]

What about frosting coming and going even in capillary systems. We have a typical system of butyl tube cooling where we need the frost to be present and we are having a long copper tube as evaporator,we have 10 such units which are running for last 3yrs without much trouble. But for past few days two of the units showing the coming and melting of frost, is it the symptom of capillary choking?

Do you have steady flow and entering temperature of that cooled media in these two circuit? Do you have condensation control of condenser.
With capillary tube you have principle that what comes in also comes out. If you have oscillation in condensing pressure you also have oscillation in evaporation pressure.

[Dilip sahasrabu]

well,it is not very sofisticated system but the problem there is that the condenser is aircooled which chokes too often since a kind of talcum powder is used in butyl tubes and wax too. Though this problem we have seen in our workshop also, even when condenser is very clean and temperature of surrounding is also lower than that in plant.

[kazmi]

pls. inform split Ac defect code it is godrej AC - showing F6, H6