Logic behind Evaporator freeze up?

[janekyu]

I hope I am posting in right place. I am studying for 313D and also doing some troubleshooting for friend.

Now I know that whenever evaporator is freezing up, it comes down to some of the following issue.

1) Evaporator is dirty
2) Evaporator fan is not upto the required speed / filter is dirty (this is a refrigerator, so no filter)
3) Refrigerant pressure is low
4) TXV is defective

I would like to know why would evaporator freeze in above condition.



English is my second language, so I guess some of the info in books go over my head. I was wondering if someone can give me simple explanation.

[Admin]

If a dirty filter or evaporator coil were restricting the airflow across the evaporator, it would cause a low suction pressure which in turn would lower the saturation temperature of the refrigerant.  Once the satuturation temperature of the refrigerant drops below 32F the coil will start to freeze.

Is this a residential refrigerator?  Have you checked the defrost thermostat as possibly being the cause of the ice?

[janekyu]

Thanks for the reply, It is a commercial cooler. Thanks for the tip regarding Defrost thermostat. I will be going there tomorrow to check the evaporator compartment. I will definitely check defrost thermostat. What reading am I looking when checking defrost thermostat.

Thanks

If a dirty filter or evaporator coil were restricting the airflow across the evaporator, it would cause a low suction pressure which in turn would lower the saturation temperature of the refrigerant.  Once the satuturation temperature of the refrigerant drops below 32F the coil will start to freeze.

Is this a residential refrigerator?  Have you checked the defrost thermostat as possibly being the cause of the ice?

[Admin]

Set your multimeter to the ohms setting. When your thermostat is cold, it should produce a reading of zero on your multimeter. If it is warm (anywhere from 40F to 90F), then this test should produce a reading of infinity.

[airtrackinc]

Lower-than-normal pressure readings may indicate a dirty coil.

[airtrackinc]

Insufficient air flow across your air conditioner's evaporator coil can cause it to freeze up.  Your air conditioner is designed to drop the temperature of your house by taking heat from it.  If your air conditioner is not properly blowing your inside air over the evaporator coil, then this heat exchange cannot take place and the Freon that should be removing heat from your home is not.  Over time, its temperature will continue to drop and your air conditioner can freeze up.