APPENDIX C (Continued)
"two-position." It is better to pay some fan energy penalty and
have a system that provides more stable control. Properly sized
control dampers will be smaller than fan casings and have greater
maximum flow pressure drops than those usually used.
b) If PI terminal units are used, sizing becomes even
more critical. Oversizing the boxes to reduce maximum flow
pressure drop results in low neck velocities, which in turn
create problems for PI flow measuring devices. To minimize
problems with sensing minimum flows in PI VAV boxes, size the box
such that at minimum flow, the neck velocity will be at least 500
Give special attention to linkages (VAV terminal unit
damper, fan inlet guide vanes, and controllable pitch vanes).
Feedback: Problems due to poorly designed/constructed linkages
are very common in Naval VAV systems. Either specifically
describe linkage requirements (because manufacturers are so
different, this may be difficult for VAV terminal units) or at
least require detailed shop drawings of linkages and pay close
attention to them. Finally, linkages should be included in the
specifications as a specific item to be performance checked
during the acceptance testing of the system.
Pay close attention to Contractor's responsibility of
coordinating which of his subcontractors provides VAV terminal
units/controls/actuators. Feedback: Contractors' lack of
coordination of the VAV terminal unit and controls often leads to
problems which go undetected. Even though DOD specification
policy recognizes only the Contractor as the single entity, and
not the subcontractors (thus not stating who does what), careful
attention to submittals in this area will avoid many problems.
To help with this, the VAV boxes, thermostats, and associated
equipment should be specified as a system and the Contractor be
required to make his submittal for all items at the same time.
Whichever of the available methods of selecting supply
air outlets is used, consider the full range of flow rates
expected. Recognize the effects of room geometry on satisfactory
performance (e.g., the Coanda effect depends upon having a flat
ceiling, without interruptions, etc.).
a) It is recommended that VAV supply diffusers be of
the linear slot type capable of supplying air horizontally along
the ceiling utilizing the Coanda effect to provide good air
distribution. In cooling only applications, the diffusers should
be located centrally in the space and blow in all directions. In