Pressure equalising valve
Also known as: pressure relief valve, pressure relief port, pressure equalizing valve
Short definitionA two-way flap vent fitted in the panel that evens out the air pressure difference between the inside and outside of the room by itself. In freezer rooms it is heated; it prevents panels from caving in and the door from locking shut.
A cold room is an almost airtight box. Warm air that enters through the door shrinks as it cools and a vacuum forms inside; during defrost the air warms and expands. In a fully airtight room a 10 K temperature change shifts the pressure by about 3,500 Pa, which means a load of 350 kg on every square metre. The flaps of the valve open at a small difference, let air through and close again once the difference is gone.
Without a valve the door will not open for minutes after closing, the gasket whistles and the ceiling panels bow. Selection is made from the manufacturer’s table by room volume and by how fast the temperature changes; blast freezing rooms and large freezer stores get more than one valve or a model with a larger cross-section. In a sub-zero room an unheated valve locks up with ice. It is mounted away from the evaporator air stream, where stacked goods cannot cover it.