CEMP-E
TI 810-11
30 November 1998
(10) The control damper of the VAV box shall modulate in response to the signal from a flow
sensing element at the discharge or inlet of the VAV box to a microprocessor based VAV box velocity
controller. The velocity controller shall control the box damper from the minimum flow position to the full
flow position from the signal of a space temperature sensing element located as shown. When the space
temperature decreases, the damper shall gradually close to the minimum flow position to maintain the
cooling setpoint as shown. When the space temperature calls for heating after the minimum flow
position is reached, control shall then pass through a temperature dead band as shown. When the space
temperature has dropped through the dead band, the duct heater coil shall be gradually controlled to
maintain the heating setpoint as shown.
22. VARIABLE AIR VOLUME (VAV) HVAC CONTROL SYSTEM WITH RETURN FAN.
a. Description of the HVAC system. This is an air handling system with inlet vane equipped supply
and return fans, economizer dampers, filters, and cooling coil. Variable air volume (VAV) boxes, some
of which have heating coils, are located downstream in the ductwork near the areas served by the
system. Figures 4-18a through 4-18f show the control system design for this type of air handling system
using a SLDC control panel. Figures 4-18g through 4-18j show the control system design for this type of
air handling system using DDC controls. A variation of this system which uses variable speed drives
instead of inlet guide vanes is shown in the following chapter. The return fan volume control loop is
costly, complex, and, in some applications, difficult to control with stability. A return fan should be used
in a VAV system only when absolutely necessary. The need for a return fan can be reduced by keeping
return duct losses to a minimum using a plenum or making the return duct as short and as large as
possible.
Figure 4-18a. Control system schematic for VAV HVAC system with return fan.
Figure 4-18b. Control system ladder diagram for VAV HVAC system with return fan.
Figure 4-18c. Control system equipment for VAV HVAC system with return fan.
Figure 4-18d. Control panel interior door layout for VAV HVAC system with return fan.
Figure 4-18e. Control panel back panel layout for VAV HVAC system with return fan.
Figure 4-18f. Control panel terminal block layout for VAV HVAC system with return fan.
Figure 4-18g. DDC control system schematic for VAV HVAC system with return fan.
Figure 4-18h. DDC control system ladder diagram for VAV HVAC system with return fan.
Figure 4-18i. DDC control system equipment for VAV HVAC system with return fan.
Figure 4-18j. DDC control system I/O table and data terminal strip layout for VAV HVAC system with
return fan.
b. General sequence of operation.
4-46
Previous Page
