system leak symptoms

[Mild Bill]

I have a co-worker telling me that a leak in a refrigeration system will frost the evaporator coil and the frost will travel back to the compressor suction side.
I believe that a leak will frost only where you have a liquid and probably start at the inlet to the metering section of the system. Once the refrigerant has changed to a gas, there is no more capacity for absorbing heat to the point of frosting. I have seen expansion valves frosted and believe that this is the extent of frosting that occurs from a low charge.

My co-worker claims that low refrigerant produces an even lower suction pressure, thereby causing the suction line to frost. I can't see how a forty degree evaporator, low on gas, can continue/start to produce frost unless the load is diminished somehow.

Are we both neglecting some aspect of the cycle?

[mikeref]

A low refrigerant charge( due to gas leak) cannot supply the evaporator correctly, therefore making it impossible for frost to appear on the suction line. Depending on how much of the charge is left, and there is no EPR in line then the evaporator will be much colder for only a portion of the tubes after the distributer

[lana]

Hi there,

I think what you co-worker is saying is this :
If there is lack of refrigerant then suction pressure goes done and if corresponding evaporating temperature goes below 0°C then moisture in the surrounding air could become frost of the suction pipe. BUT this does not mean liquid flood back because there is not enough refrigerant to flood back. If he means this then it is correct.

Cheers

[R. skiffington]

If the evaporator ices up due to a shortage of refrigerant this will cause liquid to flood back to the compressor, the Low pressure switch should be set to realise a shortage of refrigerant and cycle the compressor off but more often than not engineers think the LP switch is just to stoip a system pulling a vacuum. If the evaporator ices up the refrigerant can no longer absorb heat from the air so will not evaporate.

[Mild Bill]

The system I pictured, as my co-worker and I discussed the point, was a refrigerator with a cap tube metering the refrigerant. However, as I drew out the diagram for the refrigeration cycle, I drew an expansion valve with a sensing bulb at the outlet of the evaporator, as you could find in an AC unit.

R.Skiffington,

I agree that, should the coil become blocked and unable to react to the load, liquid feedback is a possibility, depending on the location of the compressor relative to the evaporator coil.

And, yes the LP switch should protect the compressor.

However, the load applied to the evaporator in a normally sealed refrigerator box, IMHO, should climb as the lack of refrigerant is unable to carry heat away from the box. On an air conditioner there is the RH factor of the load to contend with and therein lies the possibility of freezing the coil. In a book my co-worker provided today, the principle cause of a frosted coil is indeed a loss of applied load. The next possible cause stated a low charge. At this stage of the discussion I have to tell you that my co-worker is upset with me because I have not yielded to his logic or replied to the book statement.

I studied the chapter in his book, regarding charging, and my interest was drawn to the portion discussing the subcooling of refrigerant in the condenser coil. Am I correct in thinking that, should there be a leak in such a refrigerant system, the subcooling portion of the charge would be the first to disappear?

It has been my experience that whenever mechanics see frost, they rush to judgement with the proclamation that the charge is low and more gas is needed. I recall my time in the Navy where we were instructed to charge a unit until the frost was 18" from the compressor. That was in the 1970's. Oh Oh, the frost is too close, just release gas until the frost moves back toward the evaporator. Life was simple then.

I have a really hard time accepting the frosting of an evaporator because of a gas leak, as a common occurance. If so, there have to be other ambient conditions also affecting the system to allow that to happen.

What do you think?

[tbirdtbird]

Re-read Lana. This all goes back to the P-T tables. If the refrigerant is low enough, then the suction pressure is low, and if the corresponding evaporating temperature (per the table) goes below 0°C then moisture in the surrounding air could become frost on the suction pipe. This does not mean any meaningful refrigeration "work" is being done, just a low pressure.

[Mild Bill]

tbirdtbird,

I re-read Lana and my question is, How does the 40F box get to zero(32F)? Are you saying that at forty degrees there is not enough load to boil the gas in the evaporator coil?

I would think that a refrigerant leak would first deplete any subcooled liquid on the high side and then starve the evaporator, allowing the box temp to rise.

"and if the corresponding evaporating temperature (per the table) goes below 0°C " How do you suppose this happens in a closed box?

Which way do you suppose the condensing temperature goes as the discharge pressure decreases?

[tbirdtbird]

maybe another poster can explain this better than Lana or myself

[Mild Bill]

"and if the corresponding evaporating temperature (per the table) goes below 0°C "

The "if" is hypothetical, what really happens in the field?

[monkey spanners]

Usually with a coldroom system if its short of refrigerant you will just get frost over a portion of the coil until all the liquid has boiled off, the rest of the coil just superheats the vapour.

But just this week i got called to a high humidity cheese store room that had gone down over the weekend. System was frosting back on to the compressor. Found the evap frozen up with 1.2"/2" of frost over the entire coil. Once defrosted i found the system had lost about 70% of its charge due to a leak on the txv outlet.

I suspect it started out just frosting the start of the coil and once the air flow stopped in this part, the frost line moved along till it reached the compressor.
This was the first time i've seen this on a coldroom.

The more time goes by the less i realise i know for sure!

Jon :D