**Do we have the expertise to optimize operation of our
refrigeration plant? (3)**

** **

Optimization of hot gas defrosting is a significant part
of the energy saving process in industrial refrigeration. It is very important
to choose the right criterion of this optimization.

Frost formed on the evaporator of the cold storage will
reduce the capacity of this evaporator. To keep evaporator capacity at the
required level, an additional evaporator should be operated for a certain period
of time. The fans of this evaporator will use energy and this energy will be
released in the cold room as parasitic refrigeration load. Additional capacity
of the refrigeration plant should be operated and additional energy should be
spent to remove this parasitic load. Total hourly losses related to the frost
and to the hot gas defrost should be a criterion of the defrosting optimization
for cold storage evaporators. The evaporator fan power is an important factor of
this optimization.

Evaporator fans of production (spiral, tunnel, blastâ€¦)
freezers should be operated continuously during cooling mode. This means that
the evaporator fan power is not a factor in optimization of hot gas defrosting
of these freezers.

**Example**

An
evaporator of the spiral freezer has a capacity of 40 TR. Due to frost built up,
after 10 hours of operation, capacity of this evaporator was reduced to 90% of
initial capacity. Average hourly evaporator capacity will be 95%. After 10 hours
of operation this evaporator will be defrosted and the length of defrosting will
be 60 min. The length of hot gas supply will be 40 min. After hot gas defrosting
this evaporator should be operated for 1 hour in cooling mode to remove the heat
of the defrosting. The length of one cycle is 12 hours.

During the initial 10 hours of operation total useful
refrigeration capacity of this evaporator will be 40 x 0.95 x 10 = 380
TR

Average hourly useful refrigeration capacity of
mentioned evaporator will be 380/12 = 31.66 TR/Hrs.

Different frequencies of hot gas defrosting will give us
different hourly useful refrigeration capacities. Optimum hot gas defrosting
will have maximum hourly useful refrigeration capacity.

Optimization of hot gas defrosting of the production
freezers requires 2 steps.

**Step
1**. Estimate optimum evaporator
capacity( 60%, 70%, 80%, 90%...) to initiate hot gas
defrosting

**Step
2**. Determine real capacity of the
frosted coil.

It
is not an easy task to do these steps. However, it should be done, because
optimized hot gas defrosting can significantly improve efficiencies of many
production refrigeration plants and a lot of energy can be saved.