Computer simulated studies for energy-efficient operation of HVAC equipment

The International Business Machine Corporation (IBM) is currently studying the possible advantages of retrofitting heating, ventilating, and air conditions (HVAC) control systems at their Kingston, New York facilities. Steady-state computer simulations of a single-duct air handler, dual-duct air handler and perimeter radiation heating system are developed to compare existing control strategies with proposed improvements. These simulations apply mass and energy balance principles, along with appropriate semi-empirical and empirical models to describe the system fans, heating coils, cooling coils, heat exchangers and load calculations.

Studies of the single-duct air handler show that it is advantageous to apply year—round continuous operating control strategies and therefore eliminate the need for operator assistance in control and set point change summer/winter transition periods. An investigation of dehumidification operation shows that reheat set points should be placed as high as possible for optimum energy usage.

The dual-duct simulation demonstrates the large energy savings that can be accomplished by applying zone temperature feedback rather than feedforward control to determine duct temperature set points. Further savings are incurred by using outside air for free cooling whenever possible. Sensitivity studies are made to show the effect of varying the air handler's mixing box temperature set points and the Icoal zone thermostatic temperature set points. A comparison is also made to show that the zone-temperature feedback method is better than a literature suggested louvre position feedback method to determine duct temperature set points.

Simulation of the perimeter system demonstrates the advantages of a feedback method over a feedforward approach based on outside air temperature to determine the feed water temperature to the perimeter face pipe. The feedback method uses just enough energy to give the desired wall air temperature whenever physically possible. The feedforward strategy either uses too little energy and gives a cold wall air temperature or too much energy and gives a hot wall air temperature.