DEMAND CONTROLLED ECONOMIZER CYCLES: A DIRECT DIGITAL CONTROL SCHEME FOR HEATING, VENTILATING, AND AIR CONDITIONING SYSTEMS
Abstract
Conventional economizers admit excess outside air into a Heating, Ventilating, and Air Conditioning (HVAC) system whenever this air is cool enough to reduce the air conditioning load. When these economizers are used with dual duct, multi-zone, variable air volume, or any other type of HVAC system which uses a common air supply for the heating and cooling coils, the admission of outdoor air has the undesirable side effect of increasing the heating load. The economizer control must therefore balance the increased heating cost against the decreased cooling cost when deciding whether or not to admit this cool air. Conventional economizers base this decision on the outdoor air temperature, but often this is not a reliable indicator of the actual building loads. In this experiment an improved economizer control was developed which based the control decision on the measured demand for hot and cold air. The control system was activated in January of 1984 and was operated through February, March, and April. Data taken during these months showed the experimental control system reduced the coil operating costs by over 20% during February and March, and reduced costs by over 30% during April. A computer simulation of this system predicted an annual savings of 22%, or approximately $2200 for the HVAC system being studied. The payback period for the demand controlled economizer ranged from 6 months (if added to an existing DDC system) to 2 years (if installed as a stand-alone system). In addition to studying the performance of the experimental control system, the instrumentation installed as part of this experiment allowed the entire HVAC system to be studied. Temperature sensors, coil stratification, and perimeter heating systems are among the topics discussed in this thesis. The calculation of the coil loads required the airflow through the coils be known, so flow sensors were installed in the ductwork. Heated thermistor sensors were used for this purpose, and since these sensors are not commonly found in HVAC systems particular attention was paid to their performance. It was found that they performed quite well, and provided invaluable data as to how the entire HVAC system was operating. The use of these sensors may prove to be valuable even in systems which do not use a demand based economizer.
Degree
Ph.D.
Subject Area
Mechanical engineering
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