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, blast…) 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.