Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATION CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
276
TOTAL PRESSURE
VELOCITY
PRESSURE
DIRECTION
OF AIRFLOW
AIR DUCT
OPEN END
OF DUCT
C2646
FRICTION LOSS
STATIC
PRESSURE
PRESSURE
An actual duct system (Fig. 10) encounters a phenomenon
called pressure loss or friction loss. Pressure loss is caused by
friction between the air and the duct wall. Dynamic losses also
occur due to air turbulence caused by duct transitions, elbows,
tees, and other fittings. At the open end of the duct in Figure
10, the static pressure becomes zero while the velocity pressure
depends solely on the duct size. The pressure loss due to friction
appears to be a static pressure loss. However, in reality the total
pressure decreases because the pressure loss due to friction also
indirectly affects the air velocity in the duct. When the duct
inlet and outlet sizes are identical, the velocity pressures at both
places are equal and the difference in static pressure readings
actually represents the pressure loss due to friction.
Fig. 10. Actual Changes in Pressure with
Changes in Duct Area.
In most applications, the duct outlet is larger than the duct
inlet (velocity is lower at the outlet than at the inlet). When the
duct size increases, a small part of the initial velocity pressure
is converted into static pressure and lost as friction loss (Fig.
11). This concept is called static regain. Similar to water flow
through a pipe, a larger airflow through a given duct size causes
a larger pressure loss due to friction. This pressure drop or
friction loss cannot be regained or changed to static or velocity
pressure.
The size of a duct required to transport a given quantity of
air depends on the air pressure available to overcome the friction
loss. If a small total pressure is available from the fan, the duct
must be large enough to avoid wasting this pressure as friction
loss. If a large total pressure is available from the fan, the ducts
can be smaller with higher velocities and higher friction losses.
Reducing the duct size in half increases the velocity and the
friction loss increases.
In most low pressure airflow systems, the velocity component
of the total pressure may be ignored because of its relative size.
For example, if a supply fan delivers 10,000 cfm at 2 in. wc
static pressure in a supply duct that is 3 ft x 4 ft (or 12 ft
2
), the
Velocity (V = Q ÷ A) is 10,000 cfm ÷ 12 ft
2
or 833 fpm. The
Velocity Pressure [VP = (V ÷ 4005)
2
] is (833 ÷ 4005)
2
= 0.043
in. wc. The velocity pressure is 2.2 percent of the static pressure
at the fan [(0.043
÷ 2.0) x 100 = 2.2%].
In most high pressure airflow systems, the velocity pressure
does become a factor. For example, if a supply fan delivers
10,000 cfm at 6 in. wc static pressure in a round supply duct
that is 24 in. in diameter or 3.14 ft
2
, the Velocity (V = Q
÷
A)
is 10,000 cfm ÷ 3.14 ft
2
or 3183 fpm. The Velocity Pressure
[VP = (V ÷ 4005)
2
] is (3183 ÷ 4005)
2
or 0.632 in. wc. The
velocity pressure is 10.5 percent of the static pressure at the
fan [(0.632
÷ 6.0) x 100 = 10.5%].
EFFECTS OF FITTINGS
Ducts are equipped with various fittings such as elbows,
branch takeoffs, and transitions to and from equipment which
must be designed correctly to prevent pressure losses.
In elbows, the air on the outside radius tends to deflect around
the turn. The air on the inside radius tends to follow a straight
path and bump into the air on the outer edge. This causes eddies
in the air stream and results in excessive friction losses unless
prevented. Turning vanes are often used in elbows to reduce
the friction loss. In addition, they provide more uniform and
parallel flow at the outlet of the elbow.
In transitions to and from equipment an attempt is made to
spread the air evenly across the face of the equipment. If the
diverging section into the equipment has too great an angle,
splitters are often used. The splitters distribute the air evenly
and reduce friction losses caused by the air being unable to
expand as quickly as the sides diverge. In converging sections
friction losses are much smaller, reducing the requirement for
splitters.
TOTAL
PRESSURE
STATIC
PRESSURE
TOTAL PRESSURE
VELOCITY
PRESSURE
VELOCITY
PRESSURE
ATMOSPHERIC
PRESSURE
DIRECTION
OF AIRFLOW
AIR DUCT
OPEN END
OF DUCT
C2645
FRICTION LOSS
Fig. 11. Pressure Changes in a Duct with
Outlet Larger than Inlet.