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Winter Start Control— A2
1
⁄
2
-minute low-pressure switch
(LPS) bypass function in the timer prevents nuisance LPS
trips during start-up in low-ambient conditions.
High-Pressure Switch — This switch has nonadjust-
able settings. Figure 29 shows connection on a cylinder head.
See Table 18 for pressure switch settings.
NOTE: High-pressure switch must be removed from cylin-
der head before removing compressor from the unit.
TO CHECK — Slowly close the discharge shutoff valve un-
til the compressor shuts down. This should be at approxi-
mately 426 psig (2935 kPag). Slowly open the valve. When
the pressure drops to approximately 320 psig (2205 kPag),
the pressure switch resets. To reenergize the control circuit,
manually switch the fan circuit breaker off and then on. The
compressor starts again under Time Guard controls.
Low-Pressure Switch — The low-pressure switch (LPS)
has fixed nonadjustable settings. It is located at the pump
end of the compressor above the bearing head. See Table 18
for pressure switch settings.
TO CHECK — Slowly close the suction cut-off valve and
allow the compressor to shut down. This should occur at
approximately 27 psig (186 kPag). Slowly open the valve.
The compressor restarts under Time Guard control when the
pressure builds to approximately 67 psig (462 kPag).
Table 18 — Pressure Switch Settings,
Psig (kPag)
SWITCH CUTOUT CUT-IN
High
426 ± 7 320 ± 20
(2935 ± 48) (2205 ± 138)
Low
27 ± 4 67 ± 7
(186 ± 28) (462 ± 48)
Capacity Control
38AH044-084 DUAL-CIRCUIT UNITS — Capacity con-
trol is achieved by a pressure-actuated cylinder bank un-
loader on lead compressorA1. As the cooling load decreases
and the suction pressure drops, the unloader actuates at the
pre-set suction pressure and unloads the cylinder bank. (See
Fig. 44 and Table 19A.) The unloading of the compressor
cylinder bank has no effect on the operation of lag compres-
sor B2, which is controlled by TC2.
38AH044-084 SINGLE-CIRCUIT UNITS AND MOD-
ULES 124A, 124B, 134A, AND 134B — Capacity control
is achieved by a pressure-actuated cylinder bank unloader
on lead compressor A1. See Table 16. As the cooling load
decreases and the suction pressure drops, the unloader
actuates at the pre-set suction pressure and unloads the cyl-
inder bank. (See Fig. 44 and Table 19B and 20.) Lag com-
pressorA2is controlled by fixedsetting capacity control pressure
switches (CCPSs) as follows:
CUT-IN —
Psi (kPa)
CUTOUT —
Psi (kPa)
CCPS 1 83 (572) 63 (531)
CCPS 2 80 (551) 53 (365)
CCPS — Capacity Control Pressure Switch
If suction pressure continues to drop after lead compres-
sor A1 unloads and lag compressor A2 is operating, A2 will
stop operating when the suction pressure drops to the CCPS
cutout point.
38AH094,104 — Capacity control is achieved by a pressure-
actuated cylinder bank unloader on the lead compressor
(A1, B1) on each of the 2 refrigeration circuits. As the cool-
ing load decreases and the suction pressure drops, the un-
loader actuates at the pre-set suction pressure and unloads
the cylinder bank. (See Fig. 44 and Table 20.) The unloading
of the compressor cylinder bank has no effect on the opera-
tion of the circuit lag compressor (A2, B2 [except 38AH094,
circuit B]) which is controlled by fixed setting capacity con-
trol pressure switches (CCPSs) as follows:
CUT-IN —
Psi (kPa)
CUTOUT —
Psi (kPa)
CCPS 1 83 (572) 63 (531)
CCPS 2 80 (551) 53 (365)
CCPS — Capacity Control Pressure Switch
Pressure Relief — High-side pressure relief is pro-
vided by a fusible plug in the liquid line at the service valve.
For low-side pressure relief, a fusible plug is inserted in the
side of the accumulator (all units except 38AH044-084 dual-
circuit, constant-volume units). See Fig. 45. The 38AH044-
084 dual-circuit, constant-volume units have a fusible plug
in the suction tubing. A pressure relief valve installed on the
compressor relieves at 450 psig (3102 kPag) (see Fig. 1-6).
Unloader Settings
Unload, psig (kPag) 56 (386)
Load, psig (kPag) 76 (524)
*Unloader location.
Fig. 44 — Unloader Location and Settings
47