York YCAL0014E Series, Installation Operation & Maintenance

Page: 115 / 212

JOHNSON CONTROLS
115
FORM 150.62-NM8 (1020)
3
Normal loading will occur at intervals of 60 seconds
according to the temperatures determined by the formulas.
Unloading will occur at a rate of 30 seconds according to
the temperatures determined in the formulas.
The return chilled liquid setpoint is programmable from
40°F to 70°F (4.4°C to 21.1°C) in water chilling mode
and from 10°F to 70°F (-12.2°C to 21.1°C) in glycol
chilling mode. In both modes, the cooling range can be
from 4°F to 20°F (2.2° to 11.1°C).
As an example of compressor staging
(refer to Table 24),
a chiller with six compressors using a Cooling Setpoint
programmed for 45°F (7.20°C) and a Range Setpoint
of 10°F (5.56°C). Using the formulas in Table 25, the
control range will be split up into six (seven including
hot gas) segments, with the Control Range determining
the separation between segments. Note also that the
Cooling Setpoint is the point at which all compressors
are OFF, and Cooling Setpoint plus range is the point
all compressors are ON . Specifically, if the return water
temperature is 55°F (12.8°C), then all compressors will
be ON, providing full capacity. At nominal gpm, this
would provide approximately 45°F (7.2°C) leaving
water temperature out of the evaporator.
If the return water temperature drops to 53.4°F (11.9°C),
one compressor would cycle off leaving five compressors
running. The compressors would continue to cycle off
approximately every 1.7°F (.94°C), with the exception
of hot gas bypass. Notice that the Hot Gas Bypass would
be available when the return water temperature dropped
to 46.25°F (7.9°C). At this point one compressor would
be running.
Should the return water temperature rise from this
point to 46.7°F (8.2°C), the Hot Gas Bypass would
shut off, still leaving one compressor running. As the
load increased, the compressors would stage ON every
1.7°F (.94°C).
Also notice that Tables 24, 25 and 26 not only provide
the formulas for the loading (ON POINT) and unloading
(OFF POINT) of the system, the “STEP” is also shown
in the tables. The “STEP” is that sequence in the capacity
control scheme that can be viewed under the OPER
DATA key. Refer to the section on the DISPLAY/PRINT
keys for specific information on the OPER DATA key.
TABLE 25 – RETURN CHILLED LIQUID CONTROL FOR 5 & 6 COMPRESSORS (7 & 8 STEPS)
*STEP COMPRESSOR COMPRESSOR ON POINT COMPRESSOR OFF POINT
0 0 SETPOINT SETPOINT
1
1 W/HGB SP + CR/8 (Note 1) SETPOINT
2
1 NO HGB SP + CR/6 SETPOINT
3 2 SP + 2*CR/6 (Note 2) SP + CR/6
4 2 SP + 2*CR/6 SP + CR/6 (Note 3)
5 3 SP + 3*CR/6 SP + 2*CR/6
6 4 SP + 4*CR/6 SP + 3*CR/6
7** 5 SP + 5*CR/6 SP + 4*CR/6
8 6 SP + CR SP + 5*CR/6
* STEP can be viewed using the OPER DATA key and scrolling to COOLING DEMAND.
** 5-Compressor Chillers stop at 7 steps
TABLE 24 – COMPRESSOR STAGING FOR RETURN WATER CONTROL
COMPRESSOR STAGING FOR RETURN WATER CONTROL
6 COMPRESSORS
COOLING SETPOINT = 45°F (7.2°C) RANGE = 10°F (5.6°C)
# OF COMP ON 0 *1+HG 1 2 3 4 5 6
RWT
45°F
(7.2°C)
46.25°F
(7.9°C)
46.7°F
(8.2°C)
48.3°F
(9.1°C)
50.0°F
(10.0°C)
51.7°F
(11.0°C)
53.4°F
(11.9°C)
55.0°F
(12.8°C)
* Unloading Only
Notes:
1. Step 1 is Hot Gas Bypass and is skipped when loading occurs. Hot Gas Bypass operation is inhibited during pumpdown.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.