the fish core should be at least -18 °C before the tunnel is unloaded. In order to keep an adequate working
schedule, unloading should be done within 24 hours. The construction of the tunnel and the arrangement of
the products are vital for the freezing time. The cold air should pass by every fish box in the tunnel before
returning to the fan, which means no air shortcuts and no stagnant air inside the tunnel. This paper presents
an analysis of how different configurations give different air distribution and also different product freezing
times. Common tunnel construction is normally not optimal and several improvements can be suggested
from simulations before building a new tunnel. An existing ammonia refrigeration system with several
freezing tunnels was used as a model plant.