Unit Controls
RETURN CHILLED LIQUID CONTROL
(DUAL SYSTEM 4 AND 6 COMP UNITS ONLY)
Return chilled liquid control is based on staging the com- pressors to match the cooling load. The chiller will be fully loaded when the return water temperature is equal to the Cooling Setpoint plus the Range Setpoint. The chiller will be totally unloaded (all compressors off) when the return water temperature is equal to the Cooling Setpoint. At return water temperatures between the Cooling Setpoint, and Cooling Setpoint plus Range Setpoint, compressor loading and unloading will be de- termined by the formulas in Table 37 or Table 38.
Normal loading will occur at intervals of 60 seconds ac- cording to the temperatures determined by the formu- las. Unloading will occur at a rate of 30 seconds ac- cording 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
As an example of compressor staging (refer to Table 36 and Table 37), 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 37, 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 Setpoint is the point all compressors are on. Specifi- cally, 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 evapora- tor.
If the return water temperature drops to 53.3°F (11.8°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 in- creased, the compressors would stage on every 1.7°F (.94°C).
Also notice that Tables 37 and 38 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 ca- pacity control scheme that can be viewed under the OPER DATA key. Please refer to the section on the DIS- PLAY/PRINT keys for specific information on the OPER DATA key.
88 | YORK INTERNATIONAL |
