Carrier 37HS specifications Ð Variable Volume Controls Schematic

VAV COOLING Ð In VAV operation, the ®lter and volume controller perform the same functions as in CV operation. The unit-mounted thermostat modi®es control operation as described below.

Refer to Fig. 55. The air enters the low pressure chamber of the volume controller through a ®xed ori®ce. The low pres- sure chamber is connected through a stub ®tting and tube to the unit-mounted thermostat. The thermostat senses room tem- perature, closes its port as the temperature rises, and opens it as the temperature falls. This makes it possible to control the pressure in the low pressure chamber in response to the space temperature. With normal space temperatures, and the thermostat satis®ed, the low pressure chamber valve is open to the atmosphere through the thermostat. The rate at which the valve bleeds air from the low pressure chamber is high in relation to the rate at which air enters through the ori®ce, and the pressure decreases in relation to the high pressure. The high pressure force is stronger than the low pressure force and the bleed valve of the bellows pressure chamber of the controller is held closed. Since no air escapes, the bel- lows pressure becomes equal to the high pressure in the ple- num and the unit damper is closed, shutting off the unit.

As the room load increases, the space temperature in- creases. The thermostat senses this change and starts to close,

raising the pressure in the low pressure chamber of the con- troller. As the low pressure rises, it gradually overcomes the high pressure and opens the bellows pressure-chamber bleed valve, lowering the bellows pressure proportionally. This allows the unit damper to open and gradually increase the ¯ow of conditioned primary air into the space. See Fig. 56.

This process continues until the ¯ow of primary air is suf- ®cient to offset the load, or until the ¯ow level set point of the volume controller is reached (Fig. 57). At this point, the thermostat bleed is closed and the unit is actually operating in CV con®guration. As the load is reduced, the process is reversed and the unit ¯ow decreases proportionally until the unit is shut off when the thermostat is satis®ed (bleed fully open) (Fig. 58). In this manner, the unit normally delivers only the actual amount of primary air needed to offset the existing load.

When the wall-mounted DA thermostat is used in place of the unit-mounted thermostat, variable air volume control op- eration is the same as described above for the unit-mounted thermostat. The only difference is that the wall-mounted ther- mostat does not include the aspiration feature.

Fig. 55 Ð Variable Volume Controls Schematic

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