Johnson Controls YORK YCAL0043E Series, Installation Operation & Maintenance

Page: 94 / 188

JOHNSON CONTROLS
94
FORM 150.62-NM9 (709)
UNIT CONTROLS
As an example of compressor staging (refer to Table 18),
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 19, 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 compres
-
sors running. The compressors would continue to cycle
off approximately every 1.7°F (.94°C), with the excep-
tion 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 compres-
sor 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 18, 19 and 20 not only provide
the formulas for the loading (ON POINT) and unload-
ing (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. Please refer to the section on the
DISPLAY/PRINT keys for specific information on the
OPER DATA key.
TABLE 18 – COMPRESSOR STAGING FOR RETURN WATER CONTROL
*Unloading only
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 46.25°F 46.7°F 48.3°F 50.0°F 51.7°F 53.4°F 55.0°F
(7.2°C) (7.9°C) (8.2°C) (9.1°C) (10.0°C) (11.0°C) (11.9°C) (12.8°C)
TABLE 19 – 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
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.
* STEP can be viewed using the OPER DATA key and scrolling to COOLING DEMAND.
** 5-Compressor Chillers stop at 7 steps