Carrier 09DC - Manual (Page 4)

General — Efficient operation of evaporator expansion
valves requires a 90 F minimum condensing temperature for
100% compressor capacity, 80 F for 75% capacity, and 70 F
for 50% and 25% capacity. The capacity of an air-cooled
condenser increases with increased temperature difference
(cond temp minus entering air temp) and decreases with
decreased temperature difference. Therefore, a drop in en­
tering air temperature lowers the condensing temperature
for a given heat rejection. When outdoor air temperatures
drop below prescribed minimums, some form of head pres­
sure control is required. (See Head Pressure Control Data
table for minimum outdoor temperatures and specific head
pressure controls for 9AB and 09DC units.)
HEAD PRESSURE CONTROL
9AB, 09DC Head Pressure Controls — Head pressure on
9AB and 09DC condensers may be controlled by the Motor-
master Electronic Control, by fan cycling, or by fan cycling
supplemented by the Motormaster or a modulating damper.
Motormaster — When outdoor temperatures are low enough
to cause low condensing pressures, the Motormaster modu­
lates the motor speed of one condenser fan from full to zero
rpm to maintain a constant saturated condensing tempera­
ture. The Motormaster, available only with single-phase
motors (on unit sizes thru 09DC016), may be the sole con­
trol on single fan units or may be used in conjunction with
fan cycling control on multiple fan units for full year-round
head pressure control.
Fan Cycling — The fan cycling control, used primarily
during intermediate seasons, cycles one fan or two fans.
On two-module condensers (09DC034, 064, 024), the fan
cycling control cycles four of the six condenser fans (two
of three in each module) from a two-step thermostat respond­
ing to variations in ambient temperature. The first step of
the thermostat cuts out two corresponding fan motors, one in
each module; the second step cuts out a second pair of cor­
responding fan motors. The two condenser modules are
controlled from a single, completely wired fan cycling con­
trol box consisting of one two-stage thermostat, one power
and one control terminal block, and two quick-connect re­
ceptacles (one at the end of a 10-foot flexible conduit).
Head Pressure Control Damper — An accessory head
pressure control damper is available to supplement the
fan cycling control on the 09DC024 and larger units. The
09DC054, 064, and 084 require two damper packages in­
stalled for parallel throttling of air to the two uncontrolled
fans. Each damper assembly has a pressure-operated actu­
ator to operate the damper blades which modulate inlet air
flow to the uncontrolled fan in response to head pressure
variations. The pressure sensing tubing (capillary) for both
pressure actuators is connected to the same point in the
liquid line serving the last refrigerant circuit to shut down.
Each capillary is provided with a 1/4-inch flare connection.
A single shut-off valve at the liquid line is recommended,
for service.
Multiple Circuit Condensers — Multiple circuit conden­
sers applied to separate refrigeration cycles present special
problems when head pressure is controlled from a single
control point. For such applications, more positive system
control may be ensured by using individual condensers and
head pressure controls.
HEAD PRESSURE CONTROL DATA
UNIT
SIZES
HEAD
PRESSURE
CONTROL TD
COMPR CAP. (%)*
100 75 50 25
Min Outdoor Temp (F)t
All None
30
25
20
60
65
70
57
61
65
55
57
60
62
64
65
9AB6 Motormaster
30
25
20
-20 -20 -20 -20
^ABS,Ï2
Far\
Ort i'<>>
3Q
2S
20 : 1 S3 i
43
52
38
45
52
42
4?
5i
'S3
9AB8,12
Fan Cycling,
One Fan with
Motormaster
30
25
20
-20 -20 -20 -20
024j,028,
OH 044i
С/с''в^ j,
Tw«^ FiTK,
: 30
: 25
ГС
3
30
26
! m
1 4'i
43
S2
-
SS
09DC016
Fan Cycl ing,
Two Fans with
Motormaster
30
25
20
-20 -20 -20 -20
09DC024,
028, 034,
044t
Fan Cyc 1 ing,
Two Fans
With Mod Damper
30
25
20
-20 -20 -20
-2
13
25
Ш
Winter Season
TD - Temperature Diff (cond temp minus ent air temp)
* Interpolation permitted.
t Based on 90 F minimum cond temp at 100% capacity, 80 F at 75%
and 70 F at 50% and 25%. (Motormaster holds cond temp to 100 ±
10 F at all loads and at temperatures down to -20 F.) On some ap­
plications with multiple DX coils, the compressor may be unloaded
while individual coil(s) are still fully loaded. In this case, expan­
sion valve selection must be based on condensing temperatures
listed, or the minimum outdoor air temperature must be raised by
the difference between listed cond temp and cond temp used for
valve selection.
t Applied also to each module of 09DC054, 064 and 084.
SCHEMATIC WIRING DIAGRAM
— CONDENSER CONTROL
(Single Refrigerant Circuit, Single Phase)
LP — Low Pressure Switch ^
HP — High Pressure Switch
NOTES:
1. For three-phase service, holding coils are wired in parallel.
2. Multicircuit units may be similarly controlled by wiring fan motor
holding coil(s) in series with thermostat, compressor overloads,
and low pressurestat of each refrigerant circuit
3. Two or more condensers, piped in parallel on a single refrig­
erant circuit, must be of equal size and wired in parallel.