A cold room is an almost airtight box, and the volume of the air inside it changes with temperature. When the door has been opened, warm air has entered and the door is closed again, this air cools quickly and contracts; a vacuum forms inside. During defrost, on the other hand, the air warms and expands and the pressure rises. If the air temperature changes by only 10 K, the pressure in a completely tight room changes by about 3.5%, that is, 3,500 Pa; this means a load of 350 kilograms on every square metre of wall. The panels and ceiling are not designed for this force.
Without a valve the symptoms are familiar: the door cannot be opened for a few minutes after it has closed, the gasket whistles, the ceiling panels bow inwards, the joints open up; in the extreme case the ceiling collapses. The pressure equalising valve has two flaps that open at a set differential pressure (typically a few tens of pascals) and let air pass in either direction; when the difference has gone it closes by itself. In rooms below zero there is a low-power heater around the flaps; without it the humid air coming in locks the flaps with ice.
Selection is based on the room volume and on how quickly the temperature will change. Manufacturers give, for each valve model, the air flow it can pass and the room volume it covers in table form; one is enough for a small room, while large frozen stores and blast freezers that cool rapidly are fitted with more than one or with a large-section model. In rooms with hot gas defrost and doors opened frequently the pressure changes fast, so a margin is allowed. In controlled atmosphere stores special gas-tight types with a water or oil seal are used.
In installation the valve is fitted in a wall where the evaporator’s discharge air stream does not strike it directly, at a height where it will not be blocked by stacked goods; its outer opening should face an enclosed, dry space. Its maintenance is brief but important: it is checked at set intervals that the heater is working, that the flaps move freely and that no ice has built up around them. In a frozen room whose door is hard to open this is the first place to look. The valve is not an air leak; when closed it is tight, so it does not appear as a separate item in the cooling load calculation.