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Optimized hot gas defrosting (1)

The most widely used method of defrosting industrial refrigeration coils is hot gas defrosting. The elements required for rapid and complete defrosting include the following:

1.  An adequate supply of hot gas.

This means that sufficient evaporators must continue cooling to evaporate adequate gas to be pumped by the compressor to the defrosting evaporators, which are then acting as condensers for hot gas. Usually, we have to run at least 2 evaporators in cooling mode to defrost 1 evaporator of the same size.

2.  Keeping a certain pressure inside the coil.

During defrosting, pressure inside the coil should be kept at least 60 – 80 psig. The colder the temperature in the refrigerated space, the higher the pressure should be in the coil.

There are 2 ways to control this pressure:

  • coil back pressure regulator (BPR).
  • hot gas line outlet pressure regulator. This regulator is used for the coils equipped with liquid drainers.

3.  In the evaporator, there must be proper routing and distribution of the entering hot gas and proper removal of the condensate.

Hot gas enters the coil, contacts the cold coil walls, releases the heat of condensation and

condensates. To give opportunity for next portion of hot gas to contact the coil walls, we have to remove the condensate from this coil, otherwise it will block the access of the hot gas to these walls.

We can have a look on defrosting of bottom-feed top-hot gas liquid recirculation coil equipped with BPR. During defrosting, hot gas enters the coil though hot gas balancing valve, distributes for each circuit, moving along the circuits from top to bottom, condensates, moving through liquid supply orifices to BPR and through BPR to suction line.

Example.  Head pressure is 120 psig, coil BPR is set to 70 psig.

 In the middle of defrosting we have the following pressures in our refrigeration system:

  • from compressor discharge to the coil hot gas balancing valve – 120 psig
  • from hot gas balancing valve to liquid supply orifice – 100 psig
  • from liquid supply orifice to BPR – 70 psig

Majority of operating engineers believe that defrosting pressure in the coil is equal to BPR setting.

However, for our coil this pressure is intermediate between head pressure and BPR setting, due to major flow restriction in the liquid supply orifices.

Pressure difference, delta P = 30 psig, between coil pressure 100 psig and BPR setting 70 psig, is moving hot gas and condensate through the coil circuits. The diameters of the orifices are greater for upper circuits to achieve uniform distribution of liquid refrigerant during the cooling mode. During defrosting, these upper circuits will defrost faster than lower circuits, because they have smaller flow restriction.

Sometimes, we have a poor defrosting of the lower circuits, because existing delta P is not sufficient to remove the condensate from these circuits due to small diameters of the liquid supply orifices.

Following measures can increase delta P and will eliminate a poor defrosting of the lower circuits:

  • increase head pressure
  • open hot gas balancing valve
  • lower setting of BPR
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