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 CHAPTER 2.
MECHANICAL VENTILATION SYSTEMS
2.1 GENERAL. Mechanical ventilation systems at Navy installations supply
fresh air and remove heat, dust, toxic gases, fumes, and odors. They
usually consist of an opening for air intake, and one or more outlets for
discharging unsuitable air. Air intakes and outlets are louvered for
controlling air during cold weather, and screened to keep out birds,
insects, and rodents. Mechanical systems are usually provided with ducts
for distribution of fresh air and for exhausting air from rooms or other
spaces, or they may be roof- or wall-type ventilators without ducts. Fans
may be located at any point or points in the system, depending on
accessibility and economy of space. Sometimes they are placed near the
fresh air entry, and at other times, they are located in attics or in roof
monitors to pull undesirable air from the building.
This chapter describes the movement of air through duct systems. It
will review the types of fans used, the application of each type, ductwork,
the relationship between fans and ducts, and room air distribution.
2.2 FAN AND DUCT SYSTEMS. Figure 2-l shows a simple fan and duct system,
and the pressures that might be found within various parts of that system.
The room, at point A, is at atmospheric or neutral pressure. The fan
creates a suction in the return air duct which draws the air out of the
room. The lowest suction pressure exists at point B, just at the inlet to
the fan.
As the air passes through the fan, the fan imparts velocity and pressure
to the air. At point C (the fan discharge), the air has its highest total
pressure of any place in the system. Because the pressure is higher at
point C than in the room, the air flows from the fan discharge into the
room, However, on its way, the air is constantly losing some energy due to
friction against the duct walls, This results in a constantly falling
static pressure.
The amount of air delivered through the duct system depends upon the
performance characteristics of the fan used, Conversely, the amount of air
delivered by a given fan will depend upon the size and the configuration of
the duct system and its components, The following sections will describe
these relationships.
2.3 FAN CURVE. The fan curve is a graph that describes how the performance
of a fan will vary when the fan pushes air through any system. The
apparatus shown in Figure 2-2 may be used by the fan manufacturer to create
a fan curve for any fan.
With the damper in the duct wide open, the fan will deliver its maximum
cubic feet per minute (cfm). The static pressure at the fan outlet will be
quite low due to the increase in the velocity flow of the air. The
manufacturer measures the airflow and the increase in static pressure being
produced across the fan. This will be used as one data point on the fan
curve.
2-l
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