Walk-in freezer retrofit

Question: My question pertains to a retrofit job I cam across. It was an 8-foot-by-10-foot walk-in freezer in which the customer replaced the condensing unit only and kept the original DX evaporator and TXV. The original refrigerant was R-502 and the new replacement condensing unit is of an R-404A/R-507 design. The superheat is 7 ° F and the desired holding temperature is 0 ° F. The choice of refrigerant was R-507.

All the information I have concerning R-507 does not cover this situation, that is, using 75 to 85 percent of the original charge by weight, etc., which is a retrofit procedure. Also, everything I have read indicates that R-507 is like R-502; it has no glide and is an azeotrope.

Since the new condensing unit (with receiver) is designed for the new refrigerant, I believe that the sight glass should be free of bubbles when the unit is operating within its design temperature range (not a “hot box pull down”). Is this true?

-Dennis Boxerman, CM, Auburn, Ga

Answer: While your question is about retrofitting a system with R-507, the answer applies to all refrigerants and also new installations. Sight glasses have been used for years to determine if refrigeration systems were fully charged. They also have caused a lot of systems to be overcharged by trying to clear the bubbles from them.

Before trying to clear bubbles, the technician should consider additional things such as excessive pressure drop through the receiver service valve, partially plugged drier, or other restrictions that will cause bubbles. Another common cause of bubbles is the erratic operation of a thermostatic expansion valve.

While the sight glass can be a valuable tool in determining if a system has the correct charge, checking the operating conditions is the only true way. This should include evaporator superheat, liquid subcooling, suction and discharge pressure, and compressor amperage.

-Hugh Redmon, application engineer, Alco Controls