Heat pumps can be applied to provide the environmental control in the paint spray booths extensively used in the automotive industry. In one
installation, the heat source is the spraybooth exhaust air during the heating mode. Heat exchangers are located
in each exhaust stack. In the cooling mode, these same heat exchangers become
the heat sinks, dissipating the condenser heat to the exhaust air.
The spraybooths are designed to operate at 80°F and 60 percent relative humidity in the summer and at 70°F and 50 percent relative humidity during the winter. Because of this, the central heat pump installation is made up of three centrifugal liquid chillers with a dual operation control. In summer, the condenser leaving temperature is set to 104°F with an evaporator outlet at 50% Rh, and in winter, this is lowered to 86°F leaving the condenser and 28°F leaving the evaporator.
The heat transfer liquid used throughout is a 30 percent solution of ethylene glycol in water, with both heating and cooling systems arranged in interlinked circuits. Diurnal variation in ambient conditions can be considerable, with daytime temperatures in the upper 60's, followed by 30 to 50°F lower temperatures at night. The heating and cooling circuits are valved to enable automatic changeover from a heating to a cooling function as the needs arise. The heat pumps operate with capacity control governed by the returning chilled liquid temperature and by sequential control to operate one, two, or three machines governed in accordance with demand. Capacity control for both heating and cooling within the air replacement plants is carried out in the hydraulic distribution system .
With all six spraybooths in operation, a maximum of 371,000 cfm of outdoor air is conditioned in the air replacement units, with a maximum cooling capacity of 17 million Btu/hr and a heat output of up to 23 million Btu/hr. During winter operation at the lower temperature settings, the system output remains at 23 million BTUH, but the necessary power input is increased by about 8 percent.
