Why Refrigerator Suction Line Frosts Up When Condenser Airflow Blocked
Would someone explain step by step why the suction line frosts up when the condenser is blocked as explained below?
We are developing a small refrigerator slightly smaller than a previous model and using all the same components. It performs properly. However as a test when blocking 50% or more of the airflow across the condenser, the suction tube at the compressor begins to frost up quickly (with in a minute or so), even to the point that part of the compressor shell had some frost leading down in a short path below the suction port (almost as if liquid refrigerant (or cold gas) was pouring into the compressor shell) I did not get superheat and subcooling during this experiment.
Prior to partially blocking the condenser airflow the superheat on the evap was about 3 to 4 deg F, the subcooling at the condenser outlet was 3 to 4 deg F. This is a cap tube system with a short 1 1/2 foot long suction-cap tube heat exchanger. The condensing unit airflow is not finalized, eventually the condenser airflow will be directed past the compressor which will help with its cooling.
My understanding: reducing the condenser airflow will reduce subcooling, the refrigerant entering the cap tube will be lower quality/less subcooling, the cap tube flow refrigerant mass flow rate will be reduced, the evaporator will have less refrigerant to boil, so the refrigerant should boil off sooner in the evaporator and the low side pressure should drop. Due to the earlier boil off, I would expect super heat to be greater at the end of the evaporator and in the suction tube. I would expect the suction tube at the compressor to be warmer with the refrigerant boiling off at the begining of the evaporator and the heat load due to the air moving across the evaporator being the same. I would think with less mass of refrigerant and the same quantity of heat to remove that the refrigerant exiting the evaporator would have to be of higher quality (warmer). Instead the suction tube at the compressor is frosting up more.
Please help me understand. Thanks.