Help better understand the physics of low refrigerant charge HVAC and coil freeze up.

[Bradley5]

I don't do daily work in this field but I am licensed and have good experience in the field. I have realized I really don't have the 'complex' answer to why an evaporator coil can freeze with low refrigerant. We can just assume a fixed orifice for simplicity. It is understood that we need to reference the P/T charts to ensure saturation temp stays above freezing. I am just trying to mentally resolve what is actually happening. I would explain my current understanding but believe it is misguided. I would ask another HVAC tech but I haven't heard one yet that can truly explain it.

I do understand that there is that 'sweet spot' in low refrigerant charge before no liquid is being delivered to the evaporator so things go the other way.

 

[Bradley5]

Nevermind, got it figured out. Having mental fog for a moment I guess.

 

[Snickster]

Like all liquids, the temperature that refrigerant liquid boils varies with the pressure. The higher the pressure in the surrounding atmosphere, the higher the boiling temperature. For instance water boils in a pot on the stove at 212 deg F in atmospheric pressure of 14.7 psia. The water will completely turn into gas/steam at this constant temperature until all the water boils. If you put the same water in a pressure cooker it will boil at a higher temperature say 300 deg F since it is under a higher pressure.

The pressure in the evaporator coil is controlled by a equilibrium between how much the compressor is pumping the evaporated gas refrigerant out versus how much is being boiled off due to heat transfer from the air to the coil. The compressor is a positive displacement machine that pumps out a given volume flow rate at a given fixed rpm. If the refrigerant charge is low then the compressor is pumping out more refrigerant than is boiling off in the evaporator which drops the pressure in the evaporator causing the boiling temperature of the refrigerant to get lower and lower. Eventually there is an equilibrium reached between the amount of fluid being pumped out by the compressor and how much is being evaporated but this occurs at a low pressure in the evaporator. This is because the lesser the volume of gas in the evaporator the lesser the pressure. So as the pressure drops so does the boiling temperature.

 

[Bradley5]

Like all liquids, the temperature that refrigerant liquid boils varies with the pressure. The higher the pressure in the surrounding atmosphere, the higher the boiling temperature. For instance water boils in a pot on the stove at 212 deg F in atmospheric pressure of 14.7 psia. The water will completely turn into gas/steam at this constant temperature until all the water boils. If you put the same water in a pressure cooker it will boil at a higher temperature say 300 deg F since it is under a higher pressure.

The pressure in the evaporator coil is controlled by a equilibrium between how much the compressor is pumping the evaporated gas refrigerant out versus how much is being boiled off due to heat transfer from the air to the coil. The compressor is a positive displacement machine that pumps out a given volume flow rate at a given fixed rpm. If the refrigerant charge is low then the compressor is pumping out more refrigerant than is boiling off in the evaporator which drops the pressure in the evaporator causing the boiling temperature of the refrigerant to get lower and lower. Eventually there is an equilibrium reached between the amount of fluid being pumped out by the compressor and how much is being evaporated but this occurs at a low pressure in the evaporator. This is because the lesser the volume of gas in the evaporator the lesser the pressure. So as the pressure drops so does the boiling temperature.

Thank you. I mean, I understood the gas laws, I was just not getting my head around it for some reason. I guess I decided today I would self educate. Where I am at in my own head is looking at the suction pressure as sort of 'back pressure', or basically dictating at what temp we 'allow' the refrigerant to boil. If we hold the pressure at say 38* sat, if the coil tries to go below that, it just won't boil. So we basically just set that threshold of temp by the pressure.

In my head, I was thinking of theoreticals where I could just change the suction pressure at will, in which I would both change where in the coil things would boil off, and at what temp.

I have no idea why I was thinking about compression ratios in all of this changing the flowrates of refrigerant.