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The University of North Carolina at Chapel Hill owns, maintains and operates a district cooling system comprised of 4 production plants and a thermal energy storage system, distribution system consisting of over 26 miles of underground piping, and building bridge systems consisting of over 150 bridges controlling chilled water in over 140 buildings or locations. The chilled water group also operates and maintains remote systems located outside the district cooling systems called standalone chillers. These are 16 chillers located at ten different sites.

The production plant capacities are:

Name Number of chillers Tons Capacity
North 10 17,150
South 7 13,500
Cobb 5 10,000
Marsico Hall (Imaging Research Building)* 1 600
Tomkins (Winter Operation) 2 Heat Exchangers 4,500
Thermal Energy Storage 40,000-ton hrs

* Cooling capacity is completely dependent upon available heat load

The remote systems include:

Name/Location Number of chillers Tons Capacity
Brooks Hall 1 50
Carolina Crossing 1 80
Family Physicians Center (Aycock Family Medicine) 1 165
Frank Porter Graham Child Development Center 1 80
General Administration 2 400
440 West Franklin 3 253
Friday Center 2 1,000
Facility Services Chilled Water Plant 2 394
Bingham Animal Facility 2 200
RDU (AHEC Hangar) 1 15

Chilled water is centrally produced and distributed throughout the campus, and this district cooling system shall be utilized wherever possible. The district cooling system is comprised of four major subsystems; the production system, the distribution system, the building bridge system, and the building cooling system. The Designer responsible for connecting to this system is primarily concerned with the last two subsystems.

NOTE: These specifications do not cover any of the requirements for the production facilities. Any specifications for these facilities are custom and handled on a per need basis.

The building system includes all chilled water piping in the building; the chilled water pump and all cooling coils, heat exchangers and other equipment using chilled water. The Designer must consider the following when designing the building chilled water systems.

The range of allowable elevation of chilled water piping in the building is a maximum of 565 feet above sea level and a minimum of 350 feet above sea level.

Designer must calculate chilled water static plus dynamic head for each project and determine if pressure limits of the chilled water system are exceeded. Buildings that require higher or lower elevations or higher heads must have plate and frame heat exchangers. Plate and frame heat exchangers must have the flow regulated on the primary (or supply) side of the heat exchanger by means of a properly sized control valve. The temperature sensor must be located on the secondary side of the heat exchanger in the leaving water line for controlling the chilled water supply temperature to the loads.

The cooling coils and heat exchangers must be designed for variable flow, constant temperature differential. At design conditions these units must have a return temperature of at least 59°F (60°F if a heat exchanger is used), but not more than 64°F, and not require a supply temperature of less than 45°F. The return temperature during low load conditions shall not drop below 55°F.

A bridge enable signal shall be provided from the BAS (Building Automation System).

Chilled water from this system shall not be used for any application where the temperature of the heat exchanger surface in contact with the chilled water exceeds 100°F.

The building pump must be selected for the building system head and flow requirements. A variable volume pump is recommended, particularly in buildings with large cooling loads.

The control valves and control systems on equipment served by the chilled water system must be capable of accurate low load control and close off across the building pump shutoff head.

Use of a separate bridge interface system for unusual or special cooling loads is required. An example of a special load is one that requires an elevated supply temperature, such as process equipment, or an essential load in a building with otherwise only non-essential AC loads, such as a computer room.

By definition, the primary/secondary bridge connections exist when the primary circuit (distribution mains) is connected to the secondary circuit (building system) by means of a low-pressure loss pipe common to both circuits. The correct operation of the district cooling system is dependent on the design and operation of the primary/secondary bridge.

Factors that affect the operation of the primary/secondary bridge are described below:

Flow head loss in distribution mains from production plant to point of connection. This value varies primarily with changes in distribution system load.

Flow head loss in branch lines between the bridge and the mains. This value varies primarily with changes in building system load. Generally, the branch piping should be designed with a velocity of 3 to 6 FPS depending on actual length. When determining the flow in the pipe, consider what future loads may be imposed upon it. Use the following schedule to determine branch piping size: (length = total equivalent feet of supply + return runs).

GPM Length (feet) Pipe Size
0-150 0-400 4”
150-250 0-200 4”
200-1000 6”
250-600 0-250 6”
250-1000 8”
600-1000 0-400 8”
400-1000 10”
1000-1500 0-500 10”
500-1000 12”
1500-2000 0-800 12”
800-1200 14”
2000-4000 0-500 14”
500-1000 16”