Examining fan performance, energy use, and control strategies in real installations.

Recently I asked some friends for suggestions of ways to improve freezer performance. One response (thanks, Bruce!) really caught my attention.

I was told that some operators had seen great benefits in blast freezers by arranging for the fans to reverse direction partway through the freezing cycle in order to hit the product with the coldest air from the opposite side. This would be most beneficial in a racked blast freezer where pallets of unfrozen product, sometimes well-insulated inside cartons, might be stacked 10 rows deep so that the air reaching the last row could be substantially warmed at the beginning of the freezing process by the heat from the pallets it had already passed over.

I was told that switching the airflow like this could reduce the freezing time by as much as 20%. If the freezer works on a daily cycle this might not be such an advantage, but if the next batch is loaded as soon as the space is available, it would be like building a fifth chamber for every four that you already had.

This could, in theory, be applied to existing systems, although not all fans are capable of running in reverse. Some equipment replacement might be required and the performance of the evaporator might be affected by switching the airflow, especially if it requires superheat to control the expansion valve. It would also affect the way in which frost would build up on the coil, although that could also be an advantage in extending the runtime between defrosts and further improving productivity.

Of course this wouldn’t work for some types of freezer, such as a fluidized bed freezers. Blasting the peas or raspberries or French fries from above would not be helpful. However, in addition to blast chambers, it could also be applied to some types of tunnel and spiral freezers and would be particularly beneficial for larger and denser products such as tubs of ice cream or blocks of cheese, as well as for boxed meat.

Despite all these possible advantages it reminded me of an emergency call I received from a biscuit factory many years ago. They produced a popular type of wafer biscuit which was topped with roasted puffed rice and then coated in chocolate. Immediately after the coating process the biscuits went into a chill tunnel to set the chocolate before the handling process disturbed the unstable pile of rice on top of the wafers.

The caller complained vehemently that our chiller had suddenly gone haywire, blasting the biscuits at high velocity, and the chilling tunnel was knee-deep in a gooey mess of chocolate coated rice.

Inspection of the tunnel showed that the cold air, instead of wafting over the biscuits from the far side of the chamber towards the air cooler, was being jetted horizontally across the conveyor belt, picking up the sticky rice mess and splatting it against the far wall.

We asked if they had been doing any electrical work in the factory and learned that one of the site electricians had been doing something to the local distribution board. He had accidentally switched two of the phases of the supply. Everything else on the system, remarkably including the small reciprocating compressor on the refrigeration system, was still working fine, but the fans were in reverse. After a quick revisit to the distribution board, normal service was resumed.