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

HVAC Control in the New Millenniumport several semiconductor manufacturing processes with their associatedexhaust requirements.If the flow controller is set manually, it must be changed each timethe process step changes. The setting of the flow controller needs to bedone with an individual in-place calibration using a portable hot wireanemometer reference.Some of the available mixing box designs should not be used for thisapplication. Some of these mixing boxes are pressure dependent wherethe total flow change has some dependence on the air supply pressure.Pressure-independent types need to be used since both the cold and thehot air supply pressures to the mixing box may vary some. A flow meterwith a 3:1 rangeability and accuracy of ±5% of actual flow is needed.Some flow meters use a selector or override design. In these unitseither the flow or temperature is controlled on a selective basis. Theseoverride designs will periodically neglect the requirements of the temperaturecontroller and will produce upsets and temperature cycling.Zone OptimizationEach row of workstations makes up a zone. Each zone is served bya cold deck (CD), a hot deck (HD), and a return air subheader or finger.The operation of the controllers for the individual workstations orsubzones are dependent on the zone finger conditions.The subheader should be able to control the subzone pressure aslong as the pressure difference across the damper is high enough toremove all the return air without forcing the damper to full open. Aslong as the dampers are throttling and not fully open or completelyclosed, the pressure controller is in control mode.A nonlinear controller is used to control the vacuum in the returnair finger. The nonlinear controller has a dead band that protects thecold deck finger temperature from being changed until a substantialchange takes place in the return air pressure.The mixing box will be in a control mode as long as its damper isnot forced to a full open or closed position. If a damper is fully open, thecontrol loop is out of control. Damper position controllers are used toprevent the dampers from moving to full open.The relative humidity in the return air must also be controlledwithin acceptable limits. Figure 4-5 shows how this can be done.©2001 by The Fairmont Press, Inc. All rights reserved.