HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Energy and Power Management, Distributed Control Trendsings that are unoccupied for any portion of the day. There may be someexceptions such as large computer facilities.Control SequencesMorning warm-up is used with VAV air handlers that have a heatingcapacity. Most VAV air handlers are cooling only, however, in somecases the design or the climate are such that heat is used in the air handler.The greatest building-wide demand for heat during the day is inthe warm-up mode prior to occupancy. Since the building temperaturemay be 10 to 15 degrees F below the setpoint, a great deal of energy isused to warm-up the building.Warm-up is done with the dampers full open and the setpoint inthe air handler is modified to a heat setpoint, which is usually above thestandard operating setpoint. This allows heat to be quickly driven to thezone level for building warm-up.A reset functions to change the control setpoint, temperature, orpressure and is tripped by a change in related conditions. The dischargeair temperature reset for a VAV air handler might depend on the spacetemperature and act to reset the temperature setpoint.The key is to identify a change in the demand due to reduced orincreased load and react accordingly. The energy and cost saving comesfrom modifying the setpoint under milder conditions and avoidingwasted energy. This can save energy associated with the fan horsepowerif the volume can be reduced due to a decrease in load. Reset is oftenimplemented on both supply air and mixed air sequences in a variableair volume air handler.Return Fan ControlWhen a return fan is installed in the VAV air handler, the controllermust provide some type of fan control. The simplest technique is tointerlock the fan starter with the supply fan so both always run at thesame time.A control sequence can be used to track the return fan to the supplyfan. If a variable frequency drive is used to control the motor speed,there can be an energy saving in return fan horsepower. This can be©2001 by The Fairmont Press, Inc. All rights reserved.