APPENDIX C (Continued)
pressure. This analysis will be similar to that used for
specifying the supply fan. Knowing the range of total static
pressure expected in normal operation of the supply fan, estimate
the expected range of mixing box pressures. Select the fan to
deliver design minimum ventilation at an average system operating
point (typically less than design peak). With this method of
selection, the fan will deliver slightly more than design under
peak flow conditions and slightly less than design under minimum
flow conditions. The fan should be scheduled/specified to
satisfactorily meet flow requirements at each of these three
g) Submit detailed computerized design calculations
(this is mandated by the static regain method for sizing) which
indicate the amount of diversity used for sizing ductwork and
where the diversity was applied.
h) It is recommended that the system be designed for 8
to 12 air changes per hour with a minimum supply airflow of 4 air
changes per hour.
Design supply ductwork using the static regain method.
This will require computerized ductwork design analysis. Design
return ductwork using the equal friction method. The static
regain method keeps the static pressure in the supply system more
nearly constant throughout. This enhances the inherent control
stability of the system. It also greatly assists in naturally
balancing airflow through the system minimizing any advantage for
using PI terminal boxes. Using the static regain method requires
that more attention be given to the design of the duct system but
this is effort well spent.
To control humidity and for simplicity, design for a
constant cooling supply air temperature. The leaving air
temperature should be controlled using a chilled water valve
modulated to maintain supply air temperature as sensed by a
leaving air sensor. Resetting the supply air temperature upwards
increases the sensible heat ratio of the coil and leads to high
space relative humidity and poor indoor air quality. The
potential to save refrigeration energy by raising the cooling
supply air temperature is more than offset by the increased fan
energy needed to move more air. In addition, changes in supply
air temperature can lead to condensation on and around diffusers.
Exception: There are cases where, to prevent overcooling at
minimum flow or to minimize reheating, resetting the cooling