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