Frosting question

Im newish to the trade so be easy on me.

Found a Toshiba DX unit short of refrigerant (R410a), electronic sniffer going off at flare.
Gauges showing 70psi suction and 290psi Discharge.

I was lead to believe that when a unit is short, the refrigerant bubbles off early causing to much superheat. so frosting early in the evap coil and expansion line, and a warm suction pipe and hot discharge pipe.

But this dx unit had a sweating expansion line and a frosting suction line through the suction line accumulator and all the way back to the compressor. Airflow was good over the evap coil.

Just wondered why this is happening. Sorry for the noob question, im still learning.

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If you lower the pressure you lower the temperature.
In some instances the refrigerant inside the evaporator,
evaporates at a lower temperature due to the lower pressures
because of the loss of refrigerant.

This results in the evaporator frosting over and the frost acts as
an insulator, that in turn causes the refrigerant to continue to
evaporate out through the suction and back to the compressor.

It is a quirk of the AC system, when they lose gas they tend to freeze
the evaporator up and the suction back to the compressor.

Rob

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Thanks for your reply rob

I understand that when the evaporating pressure/ temperature drops due to being short and can ice up the evaporator.
Just didn't think that could travel all the way back to the compressor without adding any superheat.
I this case the evaporator wasn't iced up as I only turn the unit on minutes before, it could of been frosting tho but hard to see as it was in a ahu.

Remember that R410a AC units have the expansion device located inside the condenser.

Therefore the condenser outlet/discharge pipe is part of the evaporator circuit, that line will always be cool/cold but will sweat more as the leak progresses and the evaporating temperature drops. 70psi / -9.6°C

Quote:

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Originally Posted by Greedygoose

Thanks for your reply rob

I understand that when the evaporating pressure/ temperature drops due to being short and can ice up the evaporator.
Just didn't think that could travel all the way back to the compressor without adding any superheat.
I this case the evaporator wasn't iced up as I only turn the unit on minutes before, it could of been frosting tho but hard to see as it was in a ahu.

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If the suction line began frosting shortly after start-up, either the coil was already frosted or the airflow isn't good.

Carrying Rob's explanation a little further, the low charge condition results in initial frosting at the evaproator inlet. As the frost accumulates that portion of the coil restricts reducing heat transfer so the liquid has to travel further down the evaporator circuit "looking" for the necessary heat of vaporization. And as it travels without any increase in pressure or saturated temperature, the frosting continues, further restricting the coil.

Eventually, the entire coil is restricted, the liquid hasn't fully vaporized and continues to boil off in the suction line creating more frost. There is some reduction in capacity (fixed meteriing) but not enough to eliminate liquid exiting the evaporator.

So, what starts out as a low charge problem becomes an airflow problem...

The thing about most evaporators fully iced up is that a substantial amount of time is required to fully defrost them without the evaporator blower running. The evaporator is essentially a solid block of ice which is slow to melt without any external forces to help it along. If the unit was turned off shortly before you arrived, it was likely still iced.