Page 206 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 191

stands for variable air volume. The space control ther- produce a differential measurement that indicates air-
mostats are used to regulate the flow of conditioned air flow. This permits a quick comparison of actual airflow
into the spaces to maintain the space dry bulb tempera- to the design airflow shown on the drawings. Before
ture. Varying the airflow, however, can create difficul- long I imagine the low-cost differential pressure sensors
ties with space ventilation. Passing of small clouds that will permit you to check the airflow in any zone at a
block sunlight to one or two rooms of a zone served by remote terminal. You’ll need to use some judgment in
the same VAV box can produce detectable temperature monitoring the airflow because the loads served by the
swings in those rooms just like systems on reheat coils. VAV box are seldom at design conditions. Most of the
A VAV box maintains temperature control in the room time however, you’ll be looking at a problem during
or zone by varying the quantity of conditioned air sup- periods of maximum load where the readings will be
plied to the room or zone. All the VAV boxes served by meaningful.
one piece of apparatus are supplied conditioned air to A VAV box can be a simple device consisting of
serve the largest load of the group so, theoretically, only nothing more than an enclosure with a damper con-
one or two VAV box dampers will be wide open at any trolled by a modulating motor to open or close the damp-
time. As long as the loads served by that air handling er depending on the temperature in the space served by
unit change uniformly no room supplied by a VAV box the air passing through the VAV box. One VAV box can
will have its damper throttled so much that the air regis- serve several offices along one exposure of one floor of
ters cannot mix the conditioned and room air enough to an office building. Because of multiple exposures (sun-
prevent poor ventilation and drafts. Despite that many light being the major load in most air conditioning sys-
systems are now installed with separate ventilation air tems) corner offices typically require an independent
supplies. VAV box. That’s not always the case when the box serves
Some VAV boxes are fan powered to provide the all offices along one building wall and, since the higher
differential pressure required for proper distribution of up gets the corner office, the thermostat is in the corner
conditioned air into the space. Electrical savings are re- office. When space utilization or heating or cooling load
alized when those fans are shut down during periods of varies such as interior versus exterior exposures, office
no occupancy. The fans in the VAV boxes do not vary air- space versus conference room, operating room in a hos-
flow because the airflow has to be maintained to provide pital compared to a patient’s room, separate VAV boxes
proper mixing of supply and room air at the registers. are normally provided. Changes in use of a space, in-
So, the fan takes a suction on the inlet of the supply air stallation of partitions and the like can make space tem-
after the VAV box damper and a return air connection perature control impossible. So, despite the advantages
from the conditioned space. of VAV boxes satisfying everyone isn’t always possible.
VAV boxes serving rooms or zones at the sides of The variable speed drive of the fan is controlled to
the buildings, or under the roof, are typically fan pow- maintain a constant pressure at one or more points in
ered and include a heating coil to absorb the load of heat the distribution system. Cooling and fan motor horse-
losses to cold outside air. These are normally fitted with a power are saved by these systems while simultaneously
filter in their return air connection to prevent accumula- providing more precise cooling and heating in the con-
tion of dust and airborne debris in the fins of the heating ditioned spaces.
coil and on the fan (the conditioned air should be clean).
Most of these maintain temperature control during the Air Registers and Diffusers
winter and on cold days by throttling the flow of the Registers, typically in walls and floors, and diffus-
heating medium, usually hot water, through the heating ers, usually devices installed in the ceilings of the rooms,
coil. Since those filters are always in service monitoring are designed to mix conditioned supply air with room
of their condition until an optimal replacement period is air to absorb most of the rising heat load when cool-
established is an added predictive maintenance require- ing. The intent of the design is to mix the air to within
ment for a new system. Other than situations where a couple of degrees of the room air temperature before
the air in the zone can be contaminated inconsistently it strikes a wall or occupants to produce sensations of
during regular activity a filter replacement program can draft. There is little need for concern with the delivery of
be established with longer periods between replace- heated air because very few occupants object to warmer
ments than those recommended by the manufacturers. temperatures of delivered air in the heating season.
Almost all VAV boxes are fitted with airflow sen- Temperature controlled registers that vary the de-
sors that normally permit connection of a manometer to livery of supply air by throttling the flow of air through