Carrier 37HS specifications Control Operating Sequences, System-PoweredControls

CONTROL OPERATING SEQUENCES

System-Powered Controls

CV COOLING Ð See Fig. 54. Air from above the distri- bution baffle (high pressure) enters the ®lter through the up- per port of the control block, while the lower port receives air from below the baffle (low pressure). These air streams pass through separate ®lter chambers where particulate con- taminants are removed.

The low pressure air stream enters the low pressure cham- ber of the volume controller from the top port of the ®lter, while the bottom port of the ®lter feeds the high pressure stream into the controller's high pressure chamber.

Air from the high pressure chamber feeds into the bellows pressure chamber of the controller through the ®xed ori®ce. Pressure in the bellows chamber of the volume controller is determined by the relationship between the entering ¯ow re- sistance of the ®xed ori®ce and the leaving ¯ow resistance of the control valve variable ori®ce.

As the control valve opens, its resistance decreases and relatively more air is allowed to bleed, lowering the pressure in the bellows chamber. As the control valve closes, the ef- fect is reversed, increasing the bellows pressure. This is the principle of control by matched ori®ces.

The control valve is positioned by 2 diaphragms, one in the low pressure chamber and one in the high pressure cham- ber, and an adjustable spring. Pressure above the low pres- sure diaphragm tends to open the control valve while the high pressure diaphragm tends to hold it closed. The spring tension balances these forces as the pressures vary and de- termines the valve position. Rotating the adjusting dial on the volume controller changes the spring tension and pro- vides a means of establishing a ¯ow set point.

The bellows pressure determined by the action of the con- trol valve is communicated to the bellows. As the plenum pressure changes, the control bellows valve is constantly re- set to maintain a corresponding bellows pressure. The bel- lows pressure chamber of the volume controller feeds air back through the middle port and chamber of the ®lter (also called a bellows pressure chamber), to the bellows connection on the unit end panel, and into the bellows. The bellows pres- sure re-sets the bellows on the unit air valve to provide a constant air¯ow through the unit at the value selected on the volume controller dial.

CV HEATING Ð Control arrangements and operation are the same for CV heating as for CV cooling. The controls and their operation are not affected by the temperature of the sup- ply air.

Fig. 54 Ð Constant Volume Controls Ð Controller Bleeding, Unit Supplying Air

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