THE EFFECT OF REDUCED AIR FAN SPEED ON FREEZING TIMES AND ENERGY CONSUMPTION IN A FREEZING TUNNEL

摘要

Large ammonia refrigeration systems are commonly used by Norway's fish freezing industry. While the energy efficiency of these systems varies, it has improved in recent years but can continue to benefit from improvements. Air blast freezing tunnels are used to freeze large numbers of fish. Typically, a batch tunnel is filled, the refrigeration system and air fans are started and run uninterrupted until the freezing tunnel is unloaded after 20 h. The refrigeration load from the product falls as the product temperature drops, but the fans often run at full speed during the entire freezing period, which results in a constant heat load to the freezing tunnel. This paper examines the efficiencies that result from reducing the fan speed after a certain time. A simulation program was developed to test different operation modes. It calculated the product temperatures in the tunnel and the electricity demand by the refrigeration system and the air fans. The air velocity field inside the freezing tunnel varied with location, which resulted in different product freezing times. The results show a reduction of 73% in total energy consumption compared to the baseline case when the freezing time was extended to 29.2 h and the fan speed was reduced to 33% after 8 h. Reductions in air temperatures (2°C and 4°C) during the second part of the freezing period could decrease the total freezing time, but the net effect on energy consumption depends on the rest of the refrigeration system.