Air washers make use of water sprays or cooling coils for evaporative and sensible cooling of mine air. Their use has largely been limited to shallow coal mines in the United States, where it is desirable to reduce the dry-bulb temperature of the intake air during the hot summer months to prevent slaking of the roof due to excessive expansion. An air washer is essentially a heat exchanger and is similar to the type of unit employed for heat transfer with refrigeration or evaporative-cooling systems.