Rack Refrigeration Defrost
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The simplest form of defrost happens when we shut off the refrigeration for a set amount of time. This method works best on medium-temperature refrigeration. We rarely run medium-temp evaporators below freezing, so we can just let air move across the coil and get the ice off.
Electric defrost is also common; we use electric heat outside the coil to melt the ice off in medium-temp or low-temp refrigerators.
We can also use gas to defrost a refrigeration system, which is most common in low-temp refrigerators. The rack in this video uses gas defrost, and it has a solenoid valve for a defrost valve. We can either use hot discharge gas or with warm liquid, which is a Kool gas defrost. In either case, you’re bringing a warm fluid into the coil and melting ice from the inside out.
During a gas defrost, you’ll shut off the suction line to the suction header. Then, you’ll open the solenoid and allow the warm fluid to flow into the suction line. The gas then comes back via the liquid line and into the liquid header of the running rack. You can trace the header back to determine if a rack uses hot gas or Kool gas defrost. In this case, we can tell that this rack uses hot gas defrost because we can trace the header right back to the discharge line. (It ties in right past the oil separator.)
We can create flow back to the liquid header with a differential valve. That differential valve can be on the liquid line for hot gas or Kool gas defrost, and it may be on the discharge line for hot gas defrost only. The valve creates a pressure differential between the receiver side of the system and the liquid header side of the system. So, the liquid header will be at a lower pressure than the receiver, so the discharge line will be at a higher pressure than the liquid header. We can establish flow back from the high side of the system to our not-quite-high side of the system (the liquid header).
Typically, the pressure differential will be around 20 PSI, but that number can vary with customer expectations and manufacturer specs.
The hot gas defrost allows us to get some flow for our defrost while the system is still cooling.