Handbook of air conditioning and refrigeration / Shan K

17.8 CHAPTER SEVENTEEN
Theoretically, the velocity pressure p vt should be calculated as
pvt � (17.21)
Its value is slightly different from pv calculated from Eq. (17.9), which is based on mean velocity v.
For a fully developed turbulent flow, pvt � 1.06pv. Since most experimental results of pressure loss
(indicated in terms of velocity pressure) are calculated by pv � �v2/(2gc), for the sake of simplicity
pv is used here instead of pvt. �[�v2 / (2gc)] vdA
�v dA
17.2 CHARACTERISTICS OF AIR FLOW IN DUCTS
Types of Air Duct
Air ducts can be classified into four types according to their transporting functions:
1. Supply duct. Conditioned air is supplied to the conditioned space.
2. Return duct. Space air is returned (1) to the fan room where the air-handling unit is installed or
(2) to the packaged unit.
3. Outdoor air duct. Outdoor air is transported to the air-handling unit, to the fan room, or to the
space directly.
4. Exhaust duct. Space air or contaminated air is exhausted from the space, equipment, fan room,
or localized area.
Each of these four types of duct may also subdivide into headers, main ducts, and branch ducts or
runouts. A header is that part of a duct that connects directly to the supply or exhaust fan before air
is supplied to the main ducts in a large duct system. Main ducts have comparatively greater flow
rates and size, serve a greater conditioned area, and, therefore, allow higher air velocities. Branch
ducts are usually connected to the terminals, hoods, supply outlets, return grilles, and exhaust
hoods. A vertical duct is called a riser. Sometimes, a header or a main duct is also called a trunk.
Pressure Characteristics of the Airflow
During the analysis of the pressure characteristics of airflow in a fan duct system such as the one in
Fig. 17.3, it is assumed that the static pressure of the space air is equal to the static pressure of the
atmospheric air, and the velocity pressure of the space air is equal to zero. Also, for convenient
measurements and presentation, as previously mentioned, the pressure of the atmospheric air is
taken as the datum, that is, p at � 0, and pressure is expressed as gauge pressure. When p � p at, p is
positive; and if p � p at, then p is negative. In a fan duct system, a fan or fans are connected to a
ductwork and equipment.
At cross section R 1, as the recirculating air enters the return grille, both the total pressure p t
and static pressure p s decrease as the result of the total pressure loss of the inlet. The velocity pressure
p v, indicated by the shaded section in Fig. 17.3, will gradually increase until it is equal to the
velocity of the branch duct. Both p t and p s are negative so that air will flow from the conditioned
space at a datum of 0 to a negative pressure. Because velocity pressure p v is always positive in the
direction of flow, from Eq. (17.11) p t � p s � p v, so p s is then smaller, or more negative, than p t.
When the recirculating air flows through the branch duct segment R 1-1 1, elbow 1 1-1 2, branch
duct segment 1 2-1 3, diffuser 1 3-1 4, and branch duct segment 1 4-1 5, both p t and p s drop because of the
pressure losses. Velocity pressure p v remains the same between cross sections R 1 and 1 3. It gradually
decreases because of the diffuser 1 3-1 4 and remains the same between 1 4 and 1 5.
As the recirculating air flows through the converging tee 1 5-1, this straight-through stream meets
with another branch stream of recirculating air from duct section R 2-1, at node or junction 1. The