© Copyright 2009 Humidity Control Systems Ltd        Tel: 0044 (1) 522 753722 Desiccant Dehumidifiers in Cold Stores Health & Safety Ice, frost, snow, fog and condensation can all occur whenever warm humid air comes into contact with a colder surface or environment. When cold store doors are open, a turbulent exchange of air occurs across the door space. Moisture from outside air condenses on the cold store floor and other surfaces producing ice, frost and snow. This phenomenon creates a slippery surface; a serious hazard to staff and fork lift operators. Ice forming on ceilings will gradually increase in thickness and unless build-up is removed regularly, there is a serious risk of it collapsing, potentially causing injury to personnel. When humid air meets cold store air around doors and traffic routes, moisture vapour can condense to create low level fog that reduces visibility; another potential safety issue, especially with fork lift drivers. Refrigeration Plant Efficiency As cold store air is circulated through the evaporator coil, airborne moisture vapour condenses and then freezes to form frost. Moisture originates from the product and personnel, but by far the greatest source is from outside air infiltrating into the store through doors. Frost build up on the evaporator coils reduces the overall heat transfer coefficient and impedes air circulation. Both cause deterioration in the performance of the evaporator and refrigeration plant. A decrease in coil efficiency causes the compressor to work for longer periods and can reduce the refrigerant evaporating temperature. Both factors increase energy consumption. Regular defrosting of the evaporator is necessary because equipment performance deteriorates with increasing thickness of ice. This is an expensive operation which adds to the heat load of the cold store. Nevertheless, as equipment performance deteriorates with increasing ice thickness it is necessary to defrost periodically. A desiccant dehumidifier operates by extracting moisture in its vapour state, effectively removing moisture without producing condensation. In most cases, a dry air supply of minus 20°Cdp or lower is easily attainable. The dry air can be ducted into distribution boxes above the cold store doors. The resulting curtain of dry air provides an effective barrier against moisture infiltrating into the cold store. Alternatively, an airlock arrangement can be used. The dehumidifier continuously removes moisture in the airlock by recirculating the process air. The dry air outlet duct is positioned above the cold store door and directs the driest air down across the door face. The airlock and curtain effect together create a ‘buffer’ zone which considerably reduces moisture ingress into the cold store.