Frosting Evap

[Collie]

Hi guys, can someone explain to me, scientifically, why when a system is low on refrigerant, the evaporator can ifrost up. For example an r22 DX coil on an AHU with full fresh air. thanks

[cool runings]

Hi guys, can someone explain to me, scientifically, why when a system is low on refrigerant, the evaporator can ifrost up. For example an r22 DX coil on an AHU with full fresh air. thanks

It is simple realy.

Refrigerants are temperature / pressure related.

If you lower the pressure you lower the temp at which the refrigerant boils.

So if your system is short of gas, the expansion valve opens fully because the the superheat at the exit of the evap is too high. Because the system is short of gas the pressure inside the evap is lower than it would normaly be.
This lower pressure results in a lower temp at the first part of the evap and it causes the water too freeze and then that causes a reduced air flow over the evap which then compounds the problem and the evap runs even colder.

coolrunnings

.

[moideen]

Even if any refrigerant, If evaporator temperature is going to below 32F or 0c, it will make ice on coil. So all refrigerant should boil at a temperature of 4.4c (40F) inside the evaporator (for high temp.applications). To achieve this in evaporator should maintain proper refrigerant pressure related to its temperature.

Moideen-dubai

[bwd111]

Even if any refrigerant, If evaporator temperature is going to below 32F or 0c, it will make ice on coil. So all refrigerant should boil at a temperature of 4.4c (40F) inside the evaporator (for high temp.applications). To achieve this in evaporator should maintain proper refrigerant pressure related to its temperature.

Moideen-dubai

True. But I like 45/80 pressure/temp relation..