76
RT-SVX063E-EN
C
Co
oo
oll E
Ecco
on
no
om
miizze
err +
+ D
DX
X —
—Cool – Econ DX is
reported as the Unit sub-mode of operation when both
economizer and DX cooling are active. If actively
economizing, outdoor air damper is 100% and supply
fan speed reaches 100% then DX cooling will be added
if the unit is not satisfying space cooling requirements.
If DX cooling is active and economizer cooling enables,
the control will transition to increase economizer
damper above minimum position (or flow setpoint) to
100% to satisfy space cooling while decreasing DX
cooling. DX cooling will steadily be removed as long as
economizer cooling is able to satisfy the cooling load.
The supply fan operates to the lowest speed possible
during the transition.
Single Zone VAV – Heating
The supply fan operates at minimum speed (or will
cycle off) when the space has no heating demand.
When space temperature control calculates an
increasing demand for heat in the space, the calculated
discharge air temperature setpoint will increase. When
heating capacity initiates, the supply fan will operate at
the required minimum speed for the heat type
installed. The minimum fan speed can be 100%. See
Heat Types for more information.
As the Discharge Air Temperature Setpoint increases,
the heat capacity increases stages and/or modulates.
The control manages heating capacity to deliver a
discharge air temperature to the Discharge Air
Temperature Setpoint within the control deadband. If
the unit discharge air temperature reaches the
Discharge Air Temperature Maximum Heat Limit,
heating capacity will hold and the supply fan will
further increase (if possible) to satisfy the space
heating demand.
Constant Volume — Space Temperature
Control
Constant Volume space temperature control units
provide heating and cooling in the same sequence of
operation as Single Zone VAV except the supply fan,
when On, will operate at its maximum speed. Heating
and cooling have the same constant volume, maximum
speed.
Compressor Staging and Timers
There are timers associated with compressor staging
control to optimize refrigeration system performance:
•
Minimum OFF Time
•
Minimum ON Time
•
Inter-Stage Time
M
Miin
niim
mu
um
m O
OF
FF
F T
Tiim
me
e — For each compressor (fixed or
variable speed), when the compressor has been turned
OFF for any reason, a timer will be started for that
compressor(s). The compressor(s) will not be allowed
to be turned back ON until their timer has expired.
M
Miin
niim
mu
um
m O
ON
N T
Tiim
me
e — Once a compressor has been
turned ON a timer will be started for that compressor.
The compressor will not be allowed to be turned back
OFF until its timer has expired. A normal stop request
(Front Panel Stop, normal shutdown diagnostic, Off
mode, etc...) will honor the Minimum ON Time and
keep the compressor on until the timer expires
IIm
mp
po
orrtta
an
ntt:: Any Immediate stop request or Immediate
Shutdown diagnostic will override any
minimum on time and shut the compressor
off immediately.
IIn
ntte
err--S
Stta
ag
ge
e T
Tiim
me
e — For normal unit operation the
timing between stages will be maintained at a
minimum of a fixed Inter-stage Time. The compressor
control algorithm may request staging to occur beyond
3 minutes but the time span from one stage turning ON
(OFF) until the next compressor stage turning ON(OFF)
will be no less than the 3 minute inter-stage time. Rapid
Restart and manual compressor control can override
this time.
S
Su
up
pp
plly
y F
Fa
an
n D
De
ella
ay
y
A 60 second supply fan off delay is enforced after the
last compressor de-energizes.
General Compressor Staging
Compressor staging will occur based on the unit’s
algorithms cooling capacity request and the specific
Compressor Staging Table defined per the unit’s
configuration. When the request is ADD, the next
highest stage sequence will identify which
compressors will be turned ON, or OFF if a ‘Double
Swap’ is required. When the request is SUBTRACT, the
next lowest stage sequence will identify which
compressors will be turned OFF, or ON if a ‘Double
Swap’ is required. Capacity Control limits may also
affect staging sequence.
D
Do
ou
ub
blle
e S
Sw
wa
ap
p
To meet efficiency or optimized capacity requirements,
it will be necessary on some units, when staging up or
down, to turn ON one compressor and, for the same
stage, to turn OFF another compressor.
C
Co
om
mp
prre
esssso
orr P
Prro
ov
viin
ng
g
•
Fixed Speed Compressors: The compressor
contactors have an auxiliary contact that is wired to
a compressor proving input.
•
Variable Speed Compressors: The variable speed
compressor VFD has an integrated Run Inhibit
binary input wired to a second HPC (High Pressure
Cutout) switch. When this HPC switch opens the
VFD will be stopped and the run proving status will
be communicated back to the controller.
V
Va
arriia
ab
blle
e S
Sp
pe
ee
ed
d C
Co
om
mp
prre
esssso
orr O
Op
pttiim
ma
all S
Stta
arrtt
This circuit-level feature applies to the variable speed
compressor and enforces a maximum speed limit for a
fixed time period following startup. During normal
operation the speed range limits are determined based
S
Se
eq
qu
ue
en
ncce
e o
off O
Op
pe
erra
attiio
on
n
