28
7.11 - Head pressure control
For air-cooled units, the condensing pressure of each circuit
is generated by 10 fans maximum. As an option, a speed
variator can be used to control up to four fans so that the
speed of the fans is adjusted to maintain the head pressure
setpoint. The condensing pressure is independently controlled
in each circuit based on the saturated condensing temperature.
The control permanently adjusts its setpoint to guarantee
optimal performance and ensure anti-short-cycle protection
of the fans.
For water-cooled units, condensing pressure control is assured
if the three-way valve option is selected. The saturated
condensing temperature is controlled based on a user-
configurable.fixed.setpoint.(SETPOINT.menu)..The.three-
way.valve.control.can.be.configured.only.by.Carrier.service.
7.12 - Circuit lead/lag selection (multi-circuit units)
This function determines the lead and lag circuit on dual-
circuit or triple-circuit units. It controls the start/stop
sequence of the refrigeration circuits called circuit A, circuit
B.or.circuit.C..The.circuit.authorised.to.start.first.is.the.lead.
circuit..Lead.circuit.is.used.first.for.capacity.increases.and.at.
the same time should be decreased last when decreasing
capacity. The lead/lag circuits can be selected manually or
automatically. according. to. the. unit. configuration. (GEN_
CONF.–.General.Configuration).
• Automatic lead/lag circuit determination:
The control
system determines the lead circuit to equalise the operating
time of each circuit (value weighted by the number of
start-ups of each circuit). As a result, the circuit with the
lowest.number.of.operating.hours.always.starts.first.
•
Manual lead/lag circuit determination:
Circuit A, B or C
selected as the lead circuit. The selected circuit is always
the.leader..It.is.the.first.to.start.and.the.last.to.stop.
7.13 - Compressor loading sequence (multi-circuit
units)
This function determines in which order the circuit capacity
is changed. Compressor loading is managed by starting/
stopping the compressors and controlling the position of the
slide valve. Two types of sequencing are available and can be
configured. by. the. user. via. the.Touch. Pilot. user. interface.
(GEN_CONF.–.General.Configuration).
Balanced loading sequence
: The control maintains equal
capacity between all circuits as the machine loads and
unloads.
Staged loading sequence:
The control loads the lead circuit
completely before the lag circuits are started. When the load
is.decreasing,.the.lag.circuits.are.unloaded.first.
Staged loading sequence is incorporated under the following
conditions:
•
One of the circuits is shut down due to its failure
•
One of the circuits is in capacity override mode
•
Remaining circuits are shut down or fully charge
Loading sequence (%)
Unloading sequence (%)
Lead circuit
Lag circuit
Lead circuit
Lag circuit
0
0
100
100
30 (15)
0
100
95
35
0
95
95
40
0
95
90
45
0
90
90
50
0
90
85
55
0
85
85
60
0
85
80
65
0
80
80
70
0
80
75
70
30 (15)
75
75
70
35
75
70
70
40
70
70
70
45
70
65
70
50
65
65
70
55
65
60
70
65
60
60
70
70
60
55
75
70
55
55
75
75
55
50
80
75
50
50
80
80
50
45
85
80
45
45
85
85
45
40
90
85
40
40
90
90
40
35
95
90
40
30 (15)
95
95
40
0
100
95
35
0
100
100
30 (15)
0
100
100
0
0
Loading sequence (%)
Unloading sequence (%)
Lead circuit
Lag circuit
Lead circuit
Lag circuit
0
0
100
100
30 (15)
0
100
95
35
0
100
90
40
0
100
85
45
0
100
80
50
0
100
75
55
0
100
70
60
0
100
65
65
0
100
60
70
0
100
55
75
0
100
50
80
0
100
45
85
0
100
40
90
0
100
35
95
0
100
30 (15)
100
0
95
30 (15)
100
30 (15)
90
30 (15)
100
35
85
30 (15)
100
40
80
30 (15)
100
45
75
30 (15)
100
50
70
30 (15)
100
55
70
0
100
60
65
0
100
65
60
0
100
70
55
0
100
75
50
0
100
80
45
0
100
85
40
0
100
90
35
0
100
95
30 (15)
0
100
100
0
0
7.14 - Circuit capacity loading sequence
7.14.1 - Dual circuit – balanced capacity loading
7.14.2 - Dual circuit – priority given to one circuit
Note: (15) minimum capacity for standard water-cooled units (without the
option for high condensing temperature).
