Chilled Water Systems
Chilled water systems include chillers, chilled water and condenser water pumps, cooling towers, piping and piping specialties.
The chilled water systems shall have a 10°C (50°F) temperature differential in the central system, at the central plant, with a design supply water temperature between 4°C and 7°C (40°F and 45°F). In climates with low relative humidity, an 8°C (46°F) may be used. The chilled water system shall have a 6°C (43°F) temperature differential in the secondary systems, at the terminal points of use, such as coils with a design supply water temperature between 4°C and 7°C (40°F and 45°F).
District chilled water, if available, shall be used for cooling only if determined to be economical and reliable through a life cycle cost analysis.
Propylene glycol manufactured specifically for HVAC Systems is used for freeze protection, primarily in low temperature chilled water systems (less than 4°C) (less than 40°F). The concentration of antifreeze should be kept to a practical minimum because of its adverse effect on heat exchange efficiency and pump life. The water make-up line for glycol systems shall be provided with an in-line water meter to monitor and maintain the proper percentage of glycol in the system. All coils exposed to outside airflow (at some time) shall be provided with freeze protection thermostats and control cycles. Provisions shall be made for drain down, storage and re-injection of the glycol into the system.
All water-cooled condensers must be connected to a recirculating heat-rejecting loop. The heat rejection loop system shall be designed for a 6°C (43°F) temperature differential and a minimum of 4°C (40°F) wet bulb approach between the outside air temperature and the temperature of the water leaving the heat rejection equipment. Heat tracing shall be provided for piping exposed to weather and for piping down to 3 feet below grade.
Special Cooling Systems
Waterside Economizer Cycle.
In certain climate conditions cooling towers are capable of producing condenser water cold enough to cool the chilled water system without chiller operation. This option shall be considered in life cycle cost comparisons of water cooled chillers. Waterside economizer cycles are particularly cost effective in the low humidity climates of the western United States. In the eastern United States, enthalpy airside economizer cycles tend to produce lower operating costs. However, where used, any airside economizer shall be set so that no air with a dew point above 10°C (50°F) is allowed into the building. Waterside economizer systems shall be used only in areas where the outside air temperature will be below 4.4°C (40°F) wet bulb. Waterside economizers shall utilize a plate heat exchanger piped in parallel arrangement with its respective chiller.
For high occupancy applications where moisture removal is required, solid desiccant with silica gel may be used in combination with mechanical cooling. Heat recovery wheels may be used prior to the mechanical cooling process. Desiccant cooling units shall be equipped with airflow-setting devices for both process and reactivation air flows, and shall be equipped with gauges or digital displays to report those air flows continuously. The desiccant cooling system shall have self-contained microprocessor controls capable of connecting to and interoperating with a direct digital control (DDC) Building Automation system. Natural gas or condenser waste heat shall be used as fuel for reactivation of the desiccant. Lithium chloride liquid desiccants are not permitted.