145
JOHNSON CONTROLS
5276178-JIM-C-0118
5
FLEXSYS BYPASS DAMPER OPERATION
On some FlexSys installations, a bypass damper may
be installed to temper the air going to the underfloor or
overhead delivery system. The bypass damper mixes
return air with the air leaving the evaporator coil in order
to meet the programmed
“MX SUPPLY AIR TEMP
SETPOINT”
going to the conditioned space.
To determine the
“RETURN AIR BYPASS ACTIVE SP”,
the controller uses the
“MX SUPPLY AIR TEMP”,
“FLEX EVAP TEMP ACTIVE SETPOINT”,
and the
“RETURN AIR TEMP”
to calculate the
“RETURN AIR
BYPASS ACTIVE SP”
using formula 1.
If the
“RETURN AIR BYPASS ACTIVE SP”
is calculated
to be greater than the
“MAXIMUM BYPASS”
, the
“RETURN AIR BYPASS ACTIVE SP”
shall be equal to
the
“MAXIMUM BYPASS”
.
The following logic is used in the control of the Bypass
Damper:
•
If the
“FLEX EVAP TEMP”
is more than the
“MX SUPPLY AIR TEMP”
set point for 10
minutes the control will set the bypass damper
to 0%.
•
If the
“FLEX EVAP TEMP”
is less than the
“MX
SUPPLY AIR TEMP”
set point and a compressor
has been started or stopped in the last 7 minutes
the bypass damper is held at its current posi-
tion.
•
If the
“FLEX EVAP TEMP”
is less than the
“MX
SUPPLY AIR TEMP”
set point and the differ
-
ence between the
“FLEX EVAP TEMP”
and the
“FLEX EVAP TEMP ACTIVE SP”
is less than or
equal to the
“COOLING CONTROL OFFSET”
divided by 2 closing and opening of the bypass
damper is allowed.
“RETURN AIR BYPASS ACTIVE SP” =
“MX SUPPLY AIR TEMP”- “FLEX EVAP TEMP ACTIVE SP”
“RETURN AIR TEMP” - “FLEX EVAP TEMP ACTIVE SP”
x 100
FORMULA 1
x 100
“RETURN AIR BYPASS CURRENT” =
“
MX SUPPLY AIR TEMP”- “FLEX EVAP TEMP”
“RETURN AIR TEMP” - “FLEX EVAP TEMP”
FORMULA 2
•
If the
“FLEX EVAP TEMP”
is less than the
“MX
SUPPLY AIR TEMP”
set point and the differ
-
ence between the
“FLEX EVAP TEMP”
and the
“FLEX EVAP TEMP ACTIVE SP”
is more than
the
“COOLING CONTROL OFFSET”
divided
by 2 and the
“FLEX EVAP TEMP”
is more than
“FLEX EVAP TEMP ACTIVE SP”
opening of
the of the bypass damper is allowed.
•
If the
“FLEX EVAP TEMP”
is more than the
“MX SUPPLY AIR TEMP”
set point and the dif-
ference between the
“FLEX EVAP TEMP”
and
the
“FLEX EVAP TEMP ACTIVE SP”
is more
than the
“COOLING CONTROL OFFSET”
di-
vided by 2 and the
“FLEX EVAP TEMP”
is less
than or equal to
“FLEX EVAP TEMP ACTIVE
SP”
closing of the bypass damper is allowed.
•
If opening is allowed and the
“RETURN AIR
BYPASS CURRENT”
is 2.5% more than the
“RETURN AIR BYPASS ACTIVE SP”
the damper
will open.
•
If closing is allowed and the
“RETURN AIR
BYPASS CURRENT”
is 2.5% less than the
“RE-
TURN AIR BYPASS ACTIVE SP”
the damper will
close.
•
If the
“RETURN AIR BYPASS”
is within plus
or minus 2.5% of the
“RETURN AIR BYPASS
ACTIVE SP”
the damper will remain in the hold
position.
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