Air Cooled Liquid Chillers - Page 28

ELECTRICAL CONTROL SYSTEM

The controls used on CGA 10 and 15- ton units are classified either as "safety" controls or "operational" controls. Brief descriptions of the specific safety and operating controls used in the CGA control scheme are provided in the following paragraphs. Refer to the following control descriptions for control settings.

UNIT SAFETY CONTROLS

Low Pressure Cutout (LPC01, LPC02)

Mounted below the unit control box are two low pressure cutouts that open and stop compressor operation if the operating pressure drops below 38.5± 1 psig. The cutout automatically resets when the pressure reaches 44.5 ± 2 psig. The LPCO is a SPDT device. If it opens at low ambient start-up, it will energize ODF relay, stopping the outdoor fan(s) while the compressor remains energized through the LAST (Low Ambient Start Timer).

High Pressure Cutout (HPC01, HPC02)

These units have high pressure cutouts that open and stop compressor operation if the discharge pressure reaches 400 ± 10 psig. The cutout automatically resets when pressure drops to 250 + 15 psig.

Reset Relays (RRS, RR2)

If the unit is shut down by the low pressure cutout (or high pressure cutout), the reset relay locks out the compressor contactor (CCS, CC2). This prevents the system from recycling until the condition that caused the low (or high) pressure cutout to trip is determined and corrected.

CAUTION: To prevent unit damage, do not reset the control circuit until the cause of the safety lockout is identified and corrected.

To reset RR1 and RR2, open and reclose the unit disconnect switch.

Low Temperature Cutout (LTC)

The LTC is designed to disable the unit if the leaving water temperature falls too low. The LTC’s remote sensing bulb is mounted at the outlet end of the evaporator, where it monitors leaving water temperature.

During normal unit operation, if the low temperature cutout (LTC) senses a temperature falling to 36°F ± 3.0°F, the LTC will open to interrupt compressor operation. (Manual reset is required.)

Motor Overloads —These units have internal compressor and condenser fan motor overloads. These overloads protect the motors from overcurrent and overheating conditions and automatically reset as soon as they cool sufficiently.

UNIT OPERATIONAL CONTROLS

Water Temperature Thermostat (WTT)

System operation for 10 and 15-ton CGA units is governed by a two-stage water temperature thermostat (WTT). The remote sensing bulb of this device is factory-installed in a bulb well located on the evaporator water inlet; here, it monitors the temperature of the water returning to the evaporator.

Low Ambient Start Timer (LAST)

When one of the two timers energizes, the low pressure control is bypassed for four minutes, this allows time for suction pressure to build sufficiently for the low pressure cutout contacts to close.

Hot Gas Bypass Timer, Solenoid (HGBT, HGBS)

The hot gas bypass option is factory-installed only, and is used in a chilled water system to keep the first stage compressor on-line during short no-load or light-load conditions. When Water Temperature Thermostat (WTT) first stage opens, 24-volt power is supplied to the Compressor Contactor (CC1) through Hot Gas Bypass Timer (HGBT) pins one and four. Power is also applied from WTT-B to HGBT coil (fixed 30- minute time delay pick-up) and to the Hot Gas Bypass Solenoid (HGBS) through HGBT pins eight and five. If first stage cooling remains satisfied for 30 minutes, HGBT will energize and shut down the compressor. If there is a call for cooling during HGBP mode, the unit will return to cooling mode.

The adjustable hot gas bypass valve is factory set at 70 psig.

Note: Hot gas bypass is available only on the lead compressor circuit.

Anti-Short Cycle Timers (ASCT1, ASCT2)

An anti-short cycle timer is provided in each compressor control circuit to protect the compressors from starting too frequently. This can occur as a result of just over 0%, or just over 50% of the unit capacity, or because of sudden power outages of short duration. Whenever the contacts of the water temperature thermostat (WTT) open — or when there is a momentary power outage — the anti-short cycle timer will lock out compressor operation for three minutes.

Delay Between Compressors (DBC)

The delay between compressors prevents both compressors from starting at the same time by delaying compressor number two for 30 seconds.

Definition of Acronyms