Air Systems

Energy Efficiency and Air Systems

The HVAC Air System can be considered the "lungs" of the facility. Many major energy-using components are necessary to provide the desired environment. As in the cases of chillers and boilers, energy savings can be realized in air-handling by modularizing these systems. We examine energy efficiency by reviewing major system requirements for laboratory and cleanroom environments. [Charneux, 2001]

Laboratory-type facilities benefit especially from VAV systems. VAV systems reduce both operating energy costs and capital costs. By continuously adjusting to match the environmental conditioning required by the facility, VAV systems save operating energy. When diversity or varying loads are taken into account, the additional first cost of VAV systems can have life-cycle paybacks, including operational energy savings, in less than six months. [Atwell and McGeddy, 1989; Neuman and Guven, 1994; Parker et al., 1993] [Basso, 1997]

When air-handling equipment is operated at low air-flow rates, the reduction of the pressure loss and the higher degree of efficiency of the heat exchangers can more than compensate for the higher purchase costs of the VAV system. Outside working hours, the air-flow rate can be reduced to 50 percent of the design value. The resulting energy consumption of the VAV system for conveying the air decreases to less than 25 percent of peak load. [Schicht, 1991]

One of the largest subsystem energy users in a laboratory's space conditioning system is the make-up air-handling system. Make-up air units can use tremendous amounts of energy unnecessarily in part because of basic design decisions regarding the temperature and humidity tolerance allowed in the laboratory or cleanroom. The energy requirements to heat, cool, dehumidify, or humidify the make-up air are considerable and can represent 30 percent to 65 percent of the total energy required to maintain the laboratory or cleanroom environment. Charneux (2001) describes an interesting laboratory design in which classroom and office area airflow is combined with supply make-up air for the facility's lab spaces, resulting in an overall outside air demand reduction of 30 percent. As noted by Lacey (1997), an innovative "focused" make-up ventilation system is used in an animal anatomy lab to provide "spot" ventilation. This system, which also uses air-to-air energy recovery, consumes 10 percent of a conventional bulk ventilation system. [Charneux, 2001] [Lacey, 1997] [Kruse, 1991; Naughton, 1990a; Brown, 1990]

Cleanrooms of class 1000 and cleaner have air change rates of 600 to 900 per hour. Large amounts of energy are necessary to transport these huge quantities of cleanroom air and remove fan heat. Recirculation air systems for cleanroom designs can maximize energy savings by reducing both the unidirectional air-flow rate and the pressure drop in the air recirculation loop. Significant energy savings are also possible when high-efficiency components are used for circulating these large quantities of air. [Naughton, "HVAC Systems Part 1, 1990]

In cleanrooms, air flow is a generally fixed parameter based on the air velocity desired. Required fan horsepower can be reduced by one-third if the clean room is provided with a mixed HEPA filter air velocity and only the product and the production equipment are covered with 90 fpm (0.457 m/s) air flow while the remainder of the cleanroom operates at a lower velocity of 60 fpm (0.305 m/s). [Naughton, "HVAC Systems Part 1, 1990]

Major energy savings can be achieved by lowering system static pressure and improving fan efficiency. The energy required to overcome the system static pressure rises at a cubed rate, thus increasing energy requirements exponentially. [Ciborowski and Pluemer, 1991]

More:

VAV systems

Make-up air systems

Air recirculation systems


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