Handbook of air conditioning and refrigeration / Shan K

17.56 CHAPTER SEVENTEEN
2. Fan selection—selecting a fan that provides the optimum system pressure loss
3. System expansion— expanding the imaginary duct section into the original duct system before
condensing with the optimum distribution of total pressure loss between various duct
sections
During optimization, local loss coefficients are considered constant at various stages of iteration.
For details, refer to Tsal et al. (1988).
The T method can be used for sizing duct systems with certain total pressure losses in the
branch ducts. However, the local loss coefficients are actually varied at various stages of the iteration
and should be taken into consideration during optimization.
17.11 DUCT SYSTEMS WITH CERTAIN PRESSURE LOSSES
IN BRANCH TAKEOFFS
Design Characteristics
Cost Optimization
Supply, return, or exhaust duct systems with certain pressure losses in branch takeoffs have the following
characteristics:
● Duct is sized based on the optimization of the life-cycle cost of various duct sections of the duct
system as well as the space available in the building.
● System balancing is achieved mainly through pressure balancing of various duct paths by changing
of duct sizes and the use of various configurations of duct fittings and terminals instead of
dampers or other devices.
● Sound level will be checked and analyzed. Excess pressure at each inlet of VAV box at design
conditions is to be avoided. Sound attenuation arrangements are added if necessary.
● Local loss coefficients of the duct fittings and equipment along the critical path are minimized. It
may be beneficial to use the surplus pressure available in the branch takeoff to produce a higher
branch duct velocity and a smaller straight-through local loss coefficient C s,c.
● Supply volume flow rates are adjusted according to the duct heat gain. For VAV systems, diversity
factors are used to determine the volume flow rate of various duct sections along the critical path
so that the volume flow rate nearly matches the block load at the fan discharge.
For any duct section in an air duct system, the total life-cycle cost C to, as shown in Fig. 17.23, in
dollars, can be calculated as
C to � C e
In Eq. (17.81), CRF indicates the capital recovery factor and can be calculated as follows:
CRF �
1
CRF � C di
i(1 � i)n
(1 � i) n � 1
(17.81)
(17.82)
where i � interest rate and n � number of years under consideration.
The first-year energy cost C e, in dollars, can be calculated as the product of electric energy
consumed at the fan and the unit energy cost E r, in $/kWh, times the annual operating hours t an, in