Optimizing Hot Gas Defrost

Several methods are commonly used to remove accumulated frost from air cooling evaporators which operate below freezing. They include; water, electric, and hot gas defrost. If designed and operated properly, hot gas defrost offers the refrigeration system operator a method which is:

  • Effective
  • Automatic
  • Reliable, and
  • Safe

Why is using hot gas an effective method of defrosting evaporators?

  1. The evaporator becomes a condenser. During the hot gas defrost process, high-pressure hot gas from the discharge side of the compressor is introduced into the evaporator in a controlled fashion where it condenses back to its liquid state.
  2. The latent heat of the refrigerant is used. The process of condensing releases a large amount of energy, equal to the mass flow rate of the hot gas entering the evaporator times the latent heat of vaporization of the refrigerant. The heat released during condensing is called “latent” heat since there is no change in temperature during the condensing process (the term latent means “hidden”). If the condensing pressure is held constant, the condensing process will take place at a constant temperature. The amount of heat released during the condensing process is much greater than the amount of heat released when superheated gas is cooled without condensing (called “sensible” cooling).
  3. The condensed liquid is “recycled” and sent directly back to other evaporators.   The condensed liquid from the defrosting evaporator is expanded into the wet suction line and returned to the Low Pressure Receiver (LPR) or Intermediate Pressure Receiver (MPR) where it is “recycled” and pumped directly back out to evaporators.
  4. Hot gas defrosts acts like a heat pump to “move” heat. A heat pump moves heat “uphill” by gathering energy at a low-temperature level in the evaporator, compressing the evaporated refrigerant to a higher pressure, then releasing the energy at a higher temperature level during the condensing process. This process is 7 to 8 times more energy efficient than burning fossil fuel or electricity directly to produce the same heating effect. In the same way, the heat used for hot gas defrosting has actually been gathered from the refrigerated space by the operating evaporators, then “moved” to the defrosting evaporators by the compression process at a refrigerant pressure and temperature high enough to melt the frost. Hot gas defrosting is very energy efficient! (Source: Colmac coil manufacturing Inc.)
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