ASHRAE and other professional groups have published guidelines on the amount of air needed in different spaces. The quantities are generally provided in cubic feet per minute per occupant, or in air changes per hour. Older buildings may have HVAC systems sized to satisfy less strict requirements. Newer buildings may not comply with the present standards either, if space use or occupancy have changed. It therefore behooves the engineering staff servicing any commercial building to perform a ventilation audit periodically.
The measurement of airflow into a space is a relatively simple operation. A hood is placed over each diffuser, and the volume of air escaping is measured. The total volume of the room divided by the volume of air entering the room gives the air changes per hour. The total cubic feet per minute of outside air supplied to a room divided by the usual number of occupants gives the cubic feet per minute per person.
In some instances, the air supplied to a specific zone will be in excess of the applicable codes and standards. If the total volume that an air handler must provide to its zones is less than the present value, the system can be rebalanced to the new cubic feet per minute. The fan motor can be modified to provide less air by replacing the pulley. This saves energy because the motor’s brake horsepower is reduced. Also, less heat is emitted into the conditioned space in the cooling season if the motor is situated in the conditioned area.
Another, more subtle bonus is that the quantity of air flowing through the ductwork after sheaving the system is decreased, so the fan has to overcome less static pressure. Consequently, the motor does not have to work as hard to supply the necessary air. It may even be possible to reduce the size of the fan motor; using a high-effciency model will garner further energy savings.
There’s another option, for spaces that have two distinct and separate occupancy rates. One example is a gymnasium that has twenty people during class hours and up to several hundred people for assemblies or athletic contests a couple of times each week. The air quantities for each circumstance are different enough that a two-speed motor may be capable of supplying the two airflows, at great savings when on the lower speed (and a higher dehumidification rate, too).
Each of these projects should be planned with care and implemented with caution. Whenever the air flowing across the air-handler coils is modified, the performance of the equipment is changed globally. This phenomenon can affect the dehumidification potential of the air handler, i.e., its ability to handle its design latent load. The performance under the new flow regime can be estimated with accuracy by a competent engineer. A prudent facilities manager should endeavor to get such an expert opinion before implementing any noteworthy airflow modifications to the air handler.
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Excerpt from Energy Conservation Projects.