[colnago]

Okay, was doing some pms and keep coming across AC units with low boiling points in evaporator 27 degrees with high SH 35. Added R22 got SH down to target using WB return condenser entering air dry bulb chart but boiling point stayed at 27/28 degrees. I thought from my schooling the saturation temp should be around 40 degrees ideal. My trainer said it was because of air flow issues etc(not enough heat load on evap). I was thinking about this and thought the fixed orifice would cause the pressure drop so its temp would be 40 degrees without any air acting on evap coil. The heat load varies the superset but would have very little effect on boiling point temp because most is latent heat of vaporization then SH is added which might increase pressure a little but not a lot. This unit is a fixed orifice. I am figuring either it is a restriction in orifice (to much pressure drop), not sized right, etc. Also, high side pressure was 188psig and outside temp was only 76 degrees. Any nuggets of wisdom will be really appreciated. Starting to get charging down on systems that run probably now just slowing learning the thousands of variables that effect those important SH and SC numbers.

Thanks,

 

[jtrammel]

Low suction with high superheat is usually a liquid line restriction. What was your subcooling before and after adding charge?

Low airflow (low heat load on evap) would have low suction pressure with low superheat (usually close to or at 0* superheat). With fixed orifice metering.

 

[colnago]

The subcooling went from 12 to around 20.

 

[jtrammel]

The subcooling went from 12 to around 20.

Sounds like you possibly have a LL restriction. Is there a drier in the LL?

 

[m singer]

The heat load varies the superset but would have very little effect on boiling point temp

WRONG. If that were the case a low airflow situation would not cause a evap to freeze up. You have reached your target super heat, but are still evaporating below freezing. You have a low load on the evaporator. You need to solve that.

 

[chrisjc]

Typically fixed metering devices will always have higher sc compared to expansion valves. No superheat results in not all of the refrigerant boiling of possibly causing liquid refrigerant to flood back to the compressor. Have to be carefully not to overcharge a fixed as the refrigerant will stay in the condenser longer and have a longer time to cool as it enters the evap at this much lower temp the heat load in the house may not be enough to boil it off causing liquid to flood back. A expansion valve maintains superheat. Lack of air flow will also cause not enough heat to be picked up by the evap causing it to freeze.

 

[colnago]

Yes there is a filter drier. (WRONG. If that were the case a low airflow situation would not cause a evap to freeze up. You have reached your target super heat, but are still evaporating below freezing. You have a low load on the evaporator. You need to solve that.) Very well explained makes more sense, so heat load and pressure drop of metering device work hand and hand to determine saturation temperature of vapor. Definitely a lot to think about doing AC but the more I keep doing it it should sink in an become second nature. Thanks everyone for the replies really helped! Is there a general rule for what is acceptable for boiling temp?

 

[CircusEnvy]

Thanks everyone for the replies really helped! Is there a general rule for what is acceptable for boiling temp?

Not really, not for pistons at least. Ideally, you want the saturation temperature above freezing on any system, but sometimes that just won't happen due to low indoor/outdoor temperature, low airflow, or mechanical problems. Don't slam a system with refrigerant just to get the saturation temperature up. If your superheat and temperature drop is where it should be, leave it alone.

TXVs will generally stay at a 40-45 degree saturation temperature. If you're below 40 degrees, make sure your superheat and subcooling are on point. I had one yesterday that was running around 35 degrees, superheat was 30 degrees (25 degrees subcooling if anyone is wondering), which is too high for any TXV especially when it's 80 degrees inside. Changed it out and it ran at a more normal 50 degree saturation temperature with 12 degrees superheat.