Low Refrigerant

[sailoday280]

Can someone provide the theoretical and practical reasons that low refrigerant will cause icing on the outside of the evaporator?
Thanks

 

[MintJulep]

Because the compressor is sucking refrigerant out of the evaporator faster than it can enter, so the suction pressure drops below the freezing point of water.

 

[Compositepro]

I would phrase it differently, because flow in always equals flow out. It also depends on how the flow in the loop is controlled. Not all systems will react that way. But Mint is absolutely correct that it is due to lower pressure in the evaporator, which causes the refrigerant to boil at a lower temperature.

 

[sailoday280]

Compositepro and mint julep

With flow in =flow out,won't the expansion valve open further to Carry the heat load? If so, how does this lower pressure in the evaporator?
Thanks so far for your prompt replies.

 

[Compositepro]

See, that is why I said it depends. You have an expansion valve. That's a detail you did not tell us before. How is the valve controlled? Pressure? Temperature? Where is the temperature sensor? In any case, if your coil is freezing, the pressure in the evaporator went down (or an independent cause is you don't have enough air flow over the evaporator, perhaps due to a dirty air filter).

 

[MintJulep]

Expansion values only work properly if there is 100% liquid at the valve inlet.

Because the charge is low, you don't have enough refrigerant in the system to fill the liquid line with liquid.

For any given pressure drop across the expansion valve the mass flow rate of a mixture of gas and liquid will be less than for pure liquid.

So the valve opens. It opens as far as it can, then it can't open any farther.

Mass flow reduces and suction pressure falls.