A – The evaporator consists of an arrangement of tubes into a “coil” through which the refrigerant passes. There is a meshwork of thin metal fins that act to extend the refrigerating surface of the coil of tubes. The amount of refrigerating surface in the coil determines the refrigeration capacity (and purchase cost) of the coil. The larger the surface, the less temperature difference between the refrigerant and the air will be required to do the work of refrigeration. Why is this important? To achieve a -30F air temperature, you want to have a -40F refrigerant temperature; hence, a 10F TD (temperature difference). A cheap blast freezer evaporator that has only half of the refrigeration surface, will require -50F refrigeration temperature in order to achieve a 30F air temperature – thus a 20F TD. This makes the refrigeration equipment (the compressor) work much harder. Furthermore, a very high temperature difference increases product dehydration. So the rule of thumb: the larger the evaporator, the better! Look for an evaporator that has a face area very close to the cross sectional area in which the product is arranged.

Another very important feature of a blast freezer evaporator is the spacing of the fins through which the refrigerant coils pass. As I mentioned earlier, the purpose of these “fins” is to extend the surface area of the coil. Fin spacing in an evaporator coil is extremely important due to frost formation. Blast freezing will pick up a fair amount of moisture from the surface of the fish, as we discussed earlier. If the fin spacing is too narrow, the evaporator will very quickly become plugged with frost, blocking airflow, stopping the blast freezing process. Look for fin spacing that gives no more than three fins per inch. If you want to view a good comparison, look at the fin spacing in a cold storage evaporator – it will be much denser – up to seven fins per inch. When you are looking at fin spacing, make sure to compare both sides of the coil! Often blast freezer evaporator manufactures will provide wide fin spacing at the air inlet of the evaporator to catch moisture, and narrow fin spacing at the outlet of the freezer to give better evaporator performance.

The fans are as critical to the operation of the blast freezer as refrigerant coil surface. The greater the volume of air passing through the coil and the product, the faster the freeze time will be. Look for fans that fill the face area as much as possible. If you are purchasing new evaporators, ask what the fan static pressure beyond the evaporator will be – well sized fans should be able to develop 1/2″ to 1″ water column pressure (a way of measuring fan static pressure) external to the unit discharge in order to do the job of blast freezing. In most cases, high-powered fans will require heavy-duty fan assemblies of cast aluminum, whereas cold storage duty fans are quite often constructed of stamped sheet metal.

Finally, the blast freezer evaporator must have a means of defrosting, either electric or “hot gas”. Electric defrost depends on electric elements that pass through the coil, supply heat during the defrost cycle to melt the frost. Hot gas defrosted coils, during the defrost cycle, use compressor discharge gas to pass through the same tubes that liquid refrigerant passes during the freezing cycle. During the defrost cycle, the compressor discharge gas will condense into liquid refrigerant, and thereby give up its heat in order to melt frost (the reverse of refrigeration!). Not all coils defrost well, so it is very important to keep an eye on the success of the defrost – to make sure that the coil is completely clear of frost. If not, then the subsequent freeze cycle will be impeded by blocked airflow. Repeated freeze cycles after incomplete defrost cycles can eventually ruin the coil due to ice damage.