Specifications

48
Head pressure control
General
The microprocessor controls the condenser fans (30GX) or
analogue water valve (30HXC) to maintain the saturated
condensing temperature to a configurable setpoint. The fans will
be staged (30GX) or water valve controlled (30HXC) based on
each circuit's saturated condensing temperature and compressor
status. Water-cooled units (30HXC) operating at less than 20°C
for entering condenser water require the use of head pressure
control.
The chiller must be field configured for the options appearing
below:
CONFIGURATION DESCRIPTION HSIO FACTORY
OPTION LOCATION CONFIGURATION
Fan staging select A/C staging method <1> <SRVC> Yes
Motormaster select Applies to A/C only <2> <SRVC> Yes. 0 = none
Set to 1 to enable
(Motormaster only)
Water valve type Applies to W/C only <2> <SRVC> Yes. 0 = none
Set to 1 = 4-20 mA
2 = 0-10 V, 3 = 20-4 mA
4 = 10-0 V
Fan staging settings for A/C units
UNIT MODEL DESCRIPTION FAN STAGE FAN STAGING
NUMBER ON HSIO SELECT
NUMBER
30GX 080-105 1
st
stage compressor status FAN-5 16
2
nd
and 3rd stage common FAN-1 and 2
control based on highest SCT
30GX 115-135 1
st
stage compressor status FAN-5 14
2
nd
and 3
rd
stage common FAN-1 and 2
control based on highest SCT
30GX 150-160 1
st
stage compressor status FAN-5 16
2
nd
and 3
rd
stage common FAN-1 and 2
control based on highest SCT
30GX 175-265 1
st
stage each circuit, FAN-5 and 6 8
compressor status
2
nd
and 3
rd
stage circuit A FAN-1 and 2
independent
2
nd
stage circuit B independent FAN-3
30GX 280 1
st
stage each circuit, FAN-5 and 6 4
compressor status
2
nd
and 3
rd
stage circuit A FAN-1 and 2
independent
2
nd
and 3
rd
stage circuit B FAN-3 and 4
independent
30GX 295 1
st
stage each circuit, FAN-5 and 6 10
compressor status
2
nd
and 3
rd
stage circuit A FAN-1 and 2
independent
2
nd
and 3
rd
stage circuit B FAN-3 and 4
independent
30GX 325-350 1
st
stage each circuit, FAN-5 and 6 6
compressor status
2
nd
and 3
rd
stage circuit A FAN-1 and 2
independent
2
nd
and 3
rd
stage circuit B FAN-3 and 4
independent
Air-cooled units head pressure control
The fans are controlled based on Saturated Condensing
Temperature. The first fan stage for each circuit will be turned
on whenever the compressor is turned on and SCT greater than
35°C. A fan stage will be added when the Saturated Condensing
Temperature (SCT) exceeds the Head Pressure Setpoint. Once a
fan stage has been added, the software will temporarily modify
the head pressure setpoint by adding 8.3°C for 35 seconds. A
fan stage will be removed when the Saturated Condensing
Temperature has been less than the Head Pressure Setpoint -
19.4°C for two minutes. The control will use the higher of the `
Read circuit
saturated
condensing
temperature
andd current
fan stage
Is SCT greater
than head
pressure
setpoint ?
Increase
current fan
stage by one
Has SCT been
19.4 °C less than
head pressure
setpoint for 2
minutes ?
no
yes
Add 8.3°C to
head pressure
setpoint for
next 35
seconds
Decrease
current fan
stage by one
yes
no
Water-cooled head pressure control
The 30HXC chillers can be configured to control direct or reverse
acting water valves that are controlled by a 4-20 mA signal. A
2-10 VDC signal can be used by installing a 500 ohm resistor
across the two output terminals of the 4-20 mA signal. This control
scheme reads the Condenser Leaving Water temperature and uses
a PID loop to control the head pressure. Proportional, Integral
and Derivative gain parameters for water-cooled controls are
adjustable and can be found in the Service subfunction. Checkout
and adjustment of the PID loop should only be performed by
certified Carrier Comfort Network technicians.
two Saturated Condensing Temperature values for 30GX 080-
160 units. For the 30GX 175 to 350 units, each circuit's fan stage
is independently controlled based on the circuit Saturated
Condensing Temperature.
30GX head pressure control flow charts