As the evaporator cools the air it also takes out its moisture; every gram of water that condenses or freezes on the fins has been drawn from the product and the air. Most fruit and vegetables want 90–95% relative humidity; if the humidity falls to 80% the product loses a few per cent of its weight within weeks, shrivels and loses its value along with its kilos. A 5% water loss means visible wilting in most products. Humidification keeps the room at the desired humidity by giving back the water the evaporator takes; it is standard equipment in banana ripening, ripening, mushroom, flower and seedling rooms.
In an ultrasonic humidifier the piezo crystals at the bottom of the water tray vibrate about 1.7 million times a second and break the water into droplets 1–5 microns in diameter; this fog evaporates in the air without wetting anything. The devices are made in capacities delivering 1–20 kg of water per hour and are switched on and off by a humidity sensor (humidistat). The other option is the high-pressure fogging system, which sprays water through fine nozzles at a pressure of around 70 bar; it is economical in large stores. Steam humidifiers are not preferred in cold stores because they bring a heat load to the room.
Water quality is decisive. The lime in mains water settles as white dust on the product and the evaporator in an ultrasonic device and ruins the crystals in a short time; so reverse osmosis or demineralised water is used. Standing water breeds bacteria: the tray should drain automatically, the system should be cleaned regularly and, if possible, supported by an ultraviolet lamp. The fog is introduced not at the evaporator intake but into the discharge air, so that it is distributed through the room; water dripping onto the product or the cartons means decay and collapsing cardboard.
A humidifier is not the cure for a wrongly selected evaporator. First one looks at why the humidity is falling: if the evaporator is small and the temperature difference (TD) is 8–10 K, the system strips moisture continuously, the water supplied turns to ice on the fins and the number of defrosts rises. In a room that needs high humidity the evaporator is selected with a temperature difference of 4–6 K, and the humidifier only covers the remaining shortfall. Humidification is not carried out in rooms below zero. Above 95%, sensors lose their reliability and condensation begins on surfaces. The humidity requirements of products are given in the food storage conditions guide.