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INDIVIDUAL ROOM CONTROL APPLICATIONS
In the "dr aw-thr ough" unit v entila tor (F ig. 21) the fan dr aws
filter ed outdoor and r etur n air acr oss the coil and b lows the
conditioned air into the space.
WALL
HEATING
COIL
OUTDOOR
MIXING
AIR
DAMPER
Fig. 21. Draw-Through Unit Ventilator.
Some unit v entila tor s use separ ate hea ting and cooling coils
or a combina tion hot-w ater/c hilled-water coil. F igur e 22 shows
a typical air conditioning unit v entila tor with tw o separ ate coils.
The hea ting medium ma y be hot w ater, steam, or electr ic
resistance elements, and the cooling medium may be chilled
water or DX r efrigerant. If hea ting and cooling sour ces ar e both
water, they ar e sometimes combined in a single coil b y pr oviding
separ ate sections of the coil f or eac h function. With DX cooling ,
the condensing unit ma y be an inte gral par t of the unit v entila tor
or may be remotely located.
WALL
HEATING
COIL
DRAIN PAN
OUTDOOR
AIR
Fig. 22. Unit Ventilator with Separate
Heating and Cooling Coils.
DISCHARGE
AIR
FAN
FACE AND
BYPASS
DAMPER
FILTER
RETURN
AIR
C3036
DISCHARGE
AIR
FAN
COOLING
COIL
FILTER
MIXING
DAMPERS
RETURN
AIR
C3035
Face and b ypass damper s ar e frequentl y found on unit
ventila tor s wher ein the r oom contr oller modula tes the damper s,
and the coil v alve closes after the f ace damper is c losed.
CONTROL
Unit ventila tor contr ol regulates the amount of outdoor air
introduced into a space and the amount of heating or cooling
medium required to heat or cool the room. Day/night systems
can lo wer the setpoint and c ycle the f an dur ing unoccupied
hour s to maintain minim um temper atur es and sa ve ener gy.
ASHRAE Contr ol Cyc les
ASHRAE c lassifies the contr ol of unit v entila tor s as follows:
— Standby/Warmup Stage (can be used with any of the
following cycles)
— Cycle I–Fixed Maximum Percentage of Outdoor Air
— Cycle II–Fixed Minimum Percentage of Outdoor Air
— Cycle III–Variable Outdoor Air
These classes, defined around the 1950's, simplified the
specifications for unit ventilators, unit ventilator controls and
sequences, and the factory mounting of control components.
These cycle definitions are for heating only, although ASHRAE
recognized that cooling coils may also be required. Cycles I,
II, and III differ in the sequence of damper action in response
to a rise in space temperature and in the amount of outdoor air
admitted at various temperatures. ASHRAE control cycles may
be implemented by pneumatic, electric, electronic, or digital
control.
STANDBY/WARMUP STAGE
Dur ing cold r oom per iods, all thr ee cycles position the v alves
and damper s the same . Figur e 23 shows the standb y/war mup
stage in which the unit v entila tor f an is shut do wn (man ually or
by time clock). The outdoor air damper is c losed, and the r etur n
air damper is full open. The hea ting coil valve is open. The coil
acts as a con vector as air cir cula tes b y convection acr oss the
coil. The r oom ther mosta t modula tes the v alve, closing the v alve
as space temper atur e r ises above setpoint. The fan does not
oper ate in the standb y stage. Dur ing the w ar mup sta ge, the fan
ener gizes and the unit r ecirculates space air f or a r apid r ise in
space temper atur e. The ther mosta t signal oper ates the v alve to
control the heat.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
412
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