Honeywell AUTOMATIC CONTROL Engineering Manual page 215

e. Locate humidifier leaving air humidity sensors
no less than eight and no more than thirty feet
downstream of the humidifier.
9. CONSIDER THE PHYSICAL ARRANGEMENT OF
HUMIDITY SYSTEM COMPONENTS.
a. Locate humidifiers downstream from a source of
heat.
b. Locate reheat coils downstream from cooling coils.
c. Provide unlined ductwork downstream of
humidifiers, and straight for a minimum of ten feet.
10. PROPERLY SIZE AND SELECT THE CONTROL
VALVES.
a. Do not oversize modulating control valves.
Refer to the Valve Selection and Sizing section.
b. Select control valves that position properly upon
HVAC shutdown and upon loss of motive force.
(Refer to Table 1.)
11. PROVIDE THE AIR HANDLING SYSTEM WITH
LOW TEMPERATURE PROTECTION WHERE
FREEZING TEMPERATURES ARE POSSIBLE.
a. For steam coils, give consideration to:
1) Providing vertical tubes.
2) Pitching coils properly to the trap.
3) Providing vacuum breakers.
4) Providing traps of proper type, size, and
location.
5) Providing adequate drip and cooling legs.
6) Locating steam valve at high point.
7) Providing face and bypass type coils.
b. For hot and chilled water coils, give
consideration to:
1) Providing coil pumps to assure flow through
coils during periods of subfreezing
temperature.
2) Using antifreeze solutions.
3) Operating all water pumps when OA is
below 35°F.
4) Draining idle coils and lines.
c. For control applications, give consideration to:
1) Providing low temperature limit controllers
for all systems to enable one or a
combination of the following:
NOTE: Ensure that temperature sensing
elements are exposed to coldest
portion of airstream.
a) Opening valves to provide full flow to
coils.
b) Starting pumps.
c) Closing OA dampers.
d) Starting fan to circulate RA.
e) Stopping fan if 100 percent OA system.
AIR HANDLING SYSTEM CONTROL APPLICATIONS
f) Initiating low temperature alarms.
g) Stopping fan if steam is not present.
2) Providing failure alarms for pump, coils, and
other heating systems components.
12. ALLOW AIR HANDLING AND CONTROL SYSTEM
DESIGN TO PROVIDE ENERGY CONSERVATION.
a. Use space sensors, rather than OA sensors, to
determine reset schedules. For example, use the
damper signal from space PI control loops to
reset multizone unit hot and cold deck
temperature controller setpoints.
b. Do not permit air handlers to introduce OA to a
building area which is unoccupied or during the
warm-up period unless required for night purge
or IAQ.
c. Use PID control where elimination of control
offset conserves energy or increases comfort.
13. PROVIDE HVAC VENTILATION SEQUENCES THAT
COMPLY WITH CURRENT IAQ CODES AND
STANDARDS.
14. NETWORK DIGITAL CONTROLS FOR BUILDING-
WIDE ENERGY AND COST PERFORMANCE.
a. Share points such as OA temperature among
controllers.
b. Have chiller strategies address fan system
demands.
c. Have pumping system strategies address control
valve demands.
d. Have fan system strategies address space
terminal unit demands.
15. SEE THAT CONTROL SYSTEM DESIGNERS FULLY
UNDERSTAND THE COMPLETE BUILDING HVAC
SYSTEM.
Refer to HVAC system components
manufacturers recommendations for application
requirements and guidelines.
16. HARD-WIRE SAFETIES IF HAND-OFF-AUTO
SWITCHES ARE PROVIDED
a. Hard-wire all temperature low limit, fire, and
pressure safeties if the system can be easily
operated manually. In cases where a PC operator
monitoring station is provided, the safeties are
also usually monitored by the local digital
controller.
b. If override switches are not provided, and
system operation is always dependent upon the
digital control system, safeties may be wired to
the digital controller for control and monitoring,
thus saving duplicate wiring.
c. The real value of the safeties is achieved by
proper mounting, testing, and maintenance of
such devices.
205 ENGINEERING CMANUAL OF AUTOMATION CONTROL