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

Advanced HVAC Control, Information Technology and Open Systemsadding another control loop for each workstation.A more economical solution would be a hand-operated bypassdamper which is manually set during the initial setup and balancing.This solution is workable, but the setting would need to be changed asthe manufacturing process changes.Temperature ControlThe temperature at each workstation is controlled by a thermostat.The temperature controller adjusts the ratio of cold air to hot air withinthe supply air mixing box to maintain the space temperature.The temperature gradients in the clean room area should be ±1°Fof 72°F (-0.6°C of 22°C). Conventional thermostats cannot generallymeet this requirement because of their measurement accuracy and controlquality. Even if individual calibration is used, the error in overallloop performance would still be about ±2 or ±3°F (±1°C or ±1.7°C).Part of the error will be due to the offset that cannot be eliminatedin proportional controllers, such as thermostats. In operation the thermostatmust move its output from the midscale value with 50% mixingof cold and hot air. An error in room temperature must exist which is thepermanent offset. The size of this offset error for the control of maximumcooling can be estimated by dividing the spring range of the coldair damper by twice the thermostat gain. Typical dampers have an 8 to13 PSIG (55 to 90 kPa) spring which gives a spring range of 5 PSI, or 34.5kPa. If the thermostat gain is 2.5 PSI/°F (31 kPa/°C), then the offseterror is 1°F (0.6°C).This means the space temperature must rise to 73°F (22.8°C) beforethe damper can be fully opened. The offset error will tend to increase asthe spring range increases or as the thermostat gain decreased.Control of the clean room temperature to ±1°F (±0.6°C) requires anRTD-sensor or a semiconductor temperature sensor with a proportionalplus-integralcontroller. This will eliminate the offset error and can best bedone with microprocessor-based controllers that communicate with thesensors over a pair of twisted pair wires that serve as a data highway.High accuracy temperature transmitters and sensors may cost alittle more initially, but can save time and money over the long term byimproving efficiency and reducing maintenance costs. If the accuracycan be extended to the entire system, the savings may be dramatic.©2001 by The Fairmont Press, Inc. All rights reserved.