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For summer cooling, the daily 24-h operating cycle can be pided into three periods: off-peak, direct cooling, and on-peak. Off-Peak. This is the period from 8 p.m. until the air-handling units start the next morning. During this period, the primary operating mode is ice making, at a maximum capacity of 650 tons (2285 kW). The chillers also provide direct cooling at a capacity less than 200 tons for refrigeration loads that operate 24 h.
In this operating mode, the ice-on-coil, internal-melt storage tanks are charged. At the same time, a 34°F (1.1°C) ethylene glycol solution is supplied to the air-handling units for nighttime cooling. The DDC controller controls the ice storage system in the following operating sequence, as shown in Fig. 31.2:
1. Open control valves CV-1, CV-2, CV-5, CV-6, CV-8, and CV-11; and close control valves CV-3, CV-4, CV-6, CV-7, CV-9, and CV-10.
2. Reset the temperature of the glycol solution leaving the chiller Tel to 22°F ( 5.6°C).
3. Reset the load limit of both chillers to 100 percent.
4. Start the chiller and condenser pumps. Chiller pumps operate at high speeds during ice making to provide a higher flow rate and a high rate of heat transfer in the storage tanks as well as a greater head to overcome the pressure drop for both the evaporator and the coils in the ice storage tanks. Chiller pumps operate at low speeds in direct cooling mode.
5. Start chillers 1 and 2 following the lead / lag sequence.
6. After chillers are started, open control valves CV-3 and CV-7.
7. Start the building chilled water circulating pumps in sequence.
8. Modulate control valves 4 and 5, and maintain a 34°F (1.1°C) glycol solution supply temperature to the air-handling units.
If the ice-on-coil, internal-melt ice storage tanks are all 100 percent charged, the ice storage capacity is 7500 ton h (26,370 kWh). When the sensors detect that the ice storage tanks are 100 percent charged, the ice-making mode is terminated. If nighttime after-hours cooling is not needed, the ice storage system shuts down. If the ice inventory (the amount of stored ice in the tanks) falls below 90 percent, ice making starts again.
There are two additional operating modes during this period: ice making without direct cooling for after-hours use and ice burning for after-hours use with all chillers shut off.
Direct Cooling. Direct-cooling operation lasts from the start of the air-handling units until noon on weekdays. This period has two operating modes:
Direct cooling mode. In this operating mode, chillers are operating and are reset to 34°F (1.1°C).
Direct cooling with ice-burning or ice-melting mode. In this mode, both chillers are turned on.
When the required refrigeration load exceeds both chillers’ capacity, some ice storage will be discharged to supplement the chillers.
On-Peak. On-peak hours are from noon until 8 p.m. weekdays. Ice-burning mode, with or without chiller operation, is used in this period. In ice-burning or ice-melting mode, one chiller is operated at the demand limit. This is the primary operating mode during summer cooling. The operating sequence is as follows:
1. Open control valves CV-3, CV-6, CV-7, and CV-10; and close CV-8, CV-9, and CV-11.
2. Open control valve CV-1 and close CV-2 if chiller 1 is required to operate. Open CV-2 and close CV-1 if chiller 2 is required to operate.
3. Modulate control valves 4 and 5 at normal open positions.
4. Reset chilled water temperature leaving the chiller to 32°F (0°C).
5. Set the load limit of the operating chiller to 400 kW.
6. Start one condenser pump.
7. Start chiller pumps 1 and 2 at low speed. Both pumps will operate during ice burning.
8. Start one chiller according to the lead / lag sequence.
9. Modulate control valves 4, 5, 6, and 7 to maintain a 34°F (1.1°C) chilled water supply temperature to the air-handling units.