Supply Fan Control with IGV Ð In most VAV units, the supply fan static pressure is controlled by inlet guide vanes. The inlet guide vanes operate independently from the micro- processor. The supply static pressure is controlled by a dif- ferential pressure switch. If the unit is equipped with a re- turn fan, building pressure is controlled by another differential pressure switch.
For example, assume that set point on supply fan differ- ential switch is 1.9 in. wg. If pressure in supply duct goes above 1.9 in. wg, switch will make to the normally open contact and energize inlet guide vane motor to drive inlet guide vanes to a more closed position, thus reducing air¯ow and lowereing duct pressure. Once set point pressure is reached, switch will open and deenergize inlet guide vane motor. If pressure in supply duct is below 1.9 in. wg, the switch will make to the normally closed contact and energize inlet guide vane motor to drive inlet guide vane to a more open posi- tion; increasing air¯ow and raising duct pressure. Once again, once desired pressure has been reached, switch will open and deenergize inlet guide vane motor. How far above or below the set point setting the switch goes before energizing de- pends on setting of null span (null span is pressure change that can be made without contacts opening or closing). If null span is at maximum position, pressure will vary from
0.17in. wg to 0.31 in. wg depending on set point (if set point is at minimum setting, null span will be 0.17 in. wg, while if it is at maximum position, the null span will be
0.31in. wg) before switch acts. If null span is adjusted to a minimum setting, duct pressure will vary from 0.06 in. wg to 0.11 in. wg (again depending on switch set point) before switch acts. Setting null span to minimum position will re- sult in a smaller pressure ¯uctuation than if it is set at maxi- mum position.
Supply Fan Control with VFD Ð When equipped with the VFD option, the supply fan static pressure is con- trolled by modulating the fan wheel speed. The VFD oper- ates independently from the microprocessor. A duct pressure transducer monitors duct static pressure. The transducer out- put (4 to 20 mA) is directed into the VFD. The VFD adjusts supply fan motor speed (which changes wheel speed) as mea- sured duct pressure varies from set point as established at the VFD. The VFD will modulate fan speed until the duct pressure set point is achieved.
NOTE: The VFD will always provide the proper phase se- quence to the supply fan motor. This motor will operate in proper rotation regardless of the phase sequence to the unit. If, upon
Modulating Power Exhaust (Option or Acces- sory except FY,JY units) Ð The power exhaust as- sembly consists of two parallel and independent
Fan no. 1 is equipped with a variable position discharge damper located in the outlet of the fan housing. This damper is controlled by an actuator (PEDM), based on signals from the building pressure differential pressure switch (DPS). Avail- able range on the DPS is −0.50 to +0.50 in. wg, adjustable. Building pressure is sensed by a
Operation of the modulating power exhaust is a combi- nation modulating/staged control, with fan no. 1 providing modulating control from 0 to 100%, and fan no. 2 being staged On/Off according to damper position on fan no. 1.
As the economizer actuator opens past 17% open, auxil- iary switch DMS1 closes, energizing fan contactor PEC1. Fan motor no. 1 starts and runs.
Capacity of fan no. 1 is controlled by the position of the outlet damper. As building pressure increases above set point, the DPS will close its contact and drive the power exhaust damper motor (PEDM) open until set point is achieved. DPS then opens its control contacts and PEDM maintains current position.
When space demand moves PEDM to 90% of
If space pressure decreases until PEDM position is re- duced to 10% of open position, PEDMS will open, deener- gizing fan contactor PEC2 and shutting off fan no. 2.
Unit Staging Ð Compressor loading and unloading se- quences are shown in Table 19.
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