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FIXED QUANTITY OF OUTDOOR AIR CONTROL
Functional Description
RA
OA
2
OPEN
CONTROL
PROGRAM
Item
No.
Function
1
Control system energizes when fan is
turned on.
2
Damper opens on fan startup, closes on fan
shutdown.
FEATURES
1. A fixed quantity of OA is admitted when the fan is
operating.
2. This system is composed of a minimum of ventilation
and control equipment.
CONDITIONS FOR SUCCESSFUL OPERATION
1. The system provides the desired proportions of OA and
RA (a manual RA damper, shown, is usually required).
2. The system is designed so that coils are not subject to
freezing.
LIMITATIONS
The MA temperature varies as a function of OA temperature.
The equation is:
OA (m 3 /s)
MAT =
(OAT - RAT)
Total (m 3 /s)
or:
MAT = (RAT x %RA) + (OAT x %OA)
ENGINEERING MANUAL OF AUTOMATIC CONTROL
MA
SA
ON
1
M10445
AIR HANDLING SYSTEM CONTROL APPLICATIONS
Example, calculate the mixed air temperature of a 4.720 m
fan with 25% OA at –15 C. RA is 24 C.
1.180
MAT = 24 +
[(-15) - 24] = 14.25 C
4.720
or:
MAT = (24 x 0.75) + (-15x 0.25) = 14.25 C
SPECIFICATIONS
The operation of the OA damper shall be two-position. The
OA damper shall open when the fan is on and close when the
fan is off.
PSYCHROMETRIC ASPECTS
The proportions of OA and RA admitted are constant at all
OA temperatures.
In the following chart it is assumed that:
1. One-third OA and two-thirds RA are admitted.
2. RA condition is 26 C DB and 17 C WB.
3. OA condition is 4 C DB and 1.5 C WB.
4. MA = (2/3 x 26) + (1/3 x 4) = 18.3 C.
MA
1
OA
4 C DB, 1.5 C WB
The following results are obtained:
Item
No.
Explanation
1
MA condition at 4 C DB OA condition.
2
As OA temperature rises, the MA temperature
moves to the right of the initial MA
temperature.
211
18.3 C DB
RA 26 C DB, 17 C WB
2
C3235
3
/s
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