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Bridge Controls

A chilled water bridge system shall fall into one of two categories, depending on the kind of building loads that are served. The building category will be designated when the designer/engineer have reviewed the building loads with the Chilled Water Engineer/Director. These categories are: (1) critical loads, (2) non-critical loads. The “non-critical loads” category generally includes comfort-cooling applications. The “critical loads” category includes research facilities, their auxiliary equipment, medical facilities with operating rooms, and computer facilities.

Bridge enable signal shall be a dry contact from BAS. Contact shall close when there is a demand for chilled water and open when there is no demand.

When the building control system is used to provide this signal, outdoor air temperature, cooling coil valve output, or other parameters may be used to initiate bridge operation/shutdown. Designer shall specify parameter to be used. Bridge modes of operation will be controlled by chilled water based on the bridge enable signal from the BAS.

In a failure of the bridge controls or pump failure, the position of the TCV-A will be determined by the type of bridge. In the event of a failure, a non-critical valve will fail in the closed position and a critical valve will fail in the open position. The Chilled Water Engineer shall determine whether the building is critical or non-critical.
The Chilled Water Bridge devices below will be provided by UNC Chilled Water. Contact the Chilled Water Engineer for assistance with placing these devices. Chilled Water personnel will mark the appropriate location on the piping if desired.

  • Chilled Water Pump VFD(s) (Pumps to be provided by contractor)
  • TCV-A(B) Valve(s) including actuator
  • RTDs for Supply and Return Temperature
  • Thermowells for RTDs
  • Flow Meter
  • Differential Pressure Transmitter (Utility – if needed; connection points for this DPT are required)
  • Differential Pressure Transmitter (End of Line)
  • Chilled Water Bridge Controller
  • Control Panel for Bridge Controller
The contractor shall install ALL components (with exception of the Chilled Water Bridge Controller) and shall be responsible for providing all components not listed above. Contact Chilled Water Engineer to determine if the Utility DPT is required.

The Bridge Enclosure Cabinet is supplied by UNC – Chilled Water Department at a time when requested by the contractor through UNC Construction Management. The panel is mounted in a place agreed upon by contractor and UNC Chilled Water Engineer. The panel shall be mounted and conduits and wiring to the field instruments installed before the back plane is requested and delivered. ALL control wiring in bridge panel, VFDs and all Chilled Water instruments will be terminated by UNC – Chilled Water Department personnel.

INSTALLATION NOTE:

  1. No penetrations are allowed in the top of the panel box. All penetrations must be made with liquid tight connectors.
  2. The 120VAC power to the panel must be on its own designated breaker and have nothing else tapped off it. No junction boxes are allowed in the conduit run to ensure that no other circuits can be tapped off it. A pulling “C” may be used instead of a junction box to assist in wire pulling or if maximum number of bends in conduit has been reached.

All control functions for this system are performed by a multi-loop controller. This controller will be purchased, programmed and installed in a control panel by Chilled Water Department. Before the installation of the back plane, the instruments and transmitters shall be checked for communication and operational capability by UNC – Chilled Water personnel. To perform this testing all associated equipment for the operation must be completed, including compressed air lines and any other required equipment. Once this testing is completed, the responsible Chilled Water technician will bring the back plane, complete installation and termination of all control wiring and test the operation and communication of the bridge panel. The controller shall be provided with two Ethernet connections to the campus network.

Designer Note: The VFD must not be powered up or operated until it has been certified by the VFD supplier’s personnel and verified by Chilled Water Personnel.

Unless otherwise as required by the VFD manufacturer, the motor drive output wiring from the VFD shall be properly sized XHHW-2 run in grounded metallic conduit. The use of specific “VFD Cable” is generally not required. No top entry allowed.

Installation Note: Separate conduit shall be used for input power wiring, motor wiring, control and communications wiring and if supplied, brake unit wiring.

UNC Chilled Water shall provide a lug style or flanged, Class 150 butterfly valve for installation by the contractor. The valve shall include the actuator. The actuator will be electric and shall use 120V power. Backup power to insure the proper failure position will be from a UPS, which will be provided and installed by UNC Chilled Water.

Preferred valve orientation is with the shaft in the horizontal plane. When mounted in the horizontal plane, the actuator assembly must not be located at the bottom of the pipe. The position indicator must be visible from the ME room floor. There must have sufficient clearance to remove the actuator assembly from the valve. Slip-on flanges shall not be used for control valve installation and can only be used if approved for installation by UNC Chilled Water Director.

The Chilled Water Department will purchase this equipment with project funds. The mechanical contractor shall install this flow element in the piping system as specified by the designer. The contractor shall furnish and install flanges for flow meter. Designer shall clearly show the orientation and mounting of the flow meter on the construction drawings. The preferred meter installation is in horizontal pipe runs. For a meter being installed in the same size bridge piping, there shall be a minimum of 5 pipe diameters before the meter and 2 pipe diameters after the meter. If installing smaller meter than the bridge piping, have four pipe diameters between the meter flanges and the reducers on both sides of the meter, the same requirement shall be used for any fittings used adjacent to the meter. For horizontal installation, this meter must be installed in either the three o’clock or nine o’clock positions or a maximum of 45 degrees below these positions. The meter must not be installed downstream from a control valve. The meter must not be installed in the highest point of a pipe system.

  • RTU (Return Temperature Utility) must be installed a minimum of 7 pipe diameters downstream of the last connected tee. The thermowell must be installed in the same plane or above the tee and upstream of the control valve to avoid cold trap.
  • STU (Supply Temperature Utility) must be installed a minimum of 3 pipe diameters from pipe fittings.
Installation Note: Install with enough length in liquid-tite metallic conduit and leads to allow removal of the RTD for calibration without disconnecting wiring or liquid-tite metallic conduit. Ensure the thermowell is installed on the side of the pipe. See Appendices for proper installation.

Select devices for highest pressures and temperatures existing in respective systems in accordance with ANSI specifications.

Glass thermometer: Thermometer shall be an industrial glass thermometer with cast aluminum body and have a 9” scale. The scale will be from 30º to 120ºF degree scale with 2ºF degree division. The thermometer shall have a 3 ½” stem and variable degree angle adjustment and union connection.

Solar thermometer: Thermometer shall be an industrial solar powered thermometer capable of reading a temperature range 30º to 120ºF with an accuracy of +/- 2%.

The thermowell shall have ¾” MNPT process mount, with 1 1/8” instrument mounting, and 3 ½” length. The thermowell shall be compatible with the specified thermometer and be constructed of brass.

Taps for differential pressure transmitters shall be ½” Thread-o-lets® with a ½” ball valve attached. Downstream of the ball valve contractor shall run 3/8” copper or stainless-steel tubing to connect to the transmitter.

The End of Line Differential Pressure Transmitter (PDT-1) shall be installed at the worst location in the system from a pressure standpoint. This is typically near the most remote air handler, or the air handler located on the highest floor of the building. Contact the Chilled Water Engineer for assistance in determining the appropriate location.

The Utility Differential Pressure Transmitter (PDT-2) shall be located just inside the bridge isolation valves, as close as possible to the location that the chilled water pipes enter the building. The Chilled Water Engineer can assist with this placement as well.

Installation Note: Verify taps for PDTs are mounted on the side of horizontal runs in piping, not on top or bottom. PDTs shall be mounted with connection taps on top of unit and tubing run up to connections. Tubing must be run so air is not trapped in lines.

Control cable type: Charlotte Wire and Cable no. CW09305 (or equivalent approved by Chilled Water), 2-conductor, stranded, twisted, 18-gauge, foil shield with drain wire, Stranded, Tinned Copper, PVC jacket, 300Volt rating.

RTD Temperature Sensors: Charlotte Wire and Cable no. CW09306 (or equivalent approved by Chilled Water), 3-conductor, stranded, twisted, 18-gauge, foil shield with drain wire, Stranded, Tinned Copper, PVC jacket, 300Volt rating.

No bridge wiring conduits will be shared with any other system.

Power wiring shall be in one dedicated conduit with no junction boxes. Pulling “C” and/or “LB” only.

Installation Note: The cables shall be installed in one continuous run with all shield drain wires grounded at the control panel.

All new bridge installations will use an electric A-valve. For all bridge retrofit applications where there is a pneumatic A-valve, and air will continue to be available, all pneumatic lines for chilled water valves must be run from an air dryer and supplied with a filter regulator set. All lines must be stainless steel or soldered copper; plastic air lines are not permitted.

All conduit for wiring can be EMT conduit. Conduit will be run for all bridge panel wiring. Each conduit must have similar type wiring. Do not mix shielded cable with AC power.

All wiring in conduits shall be continuous wire runs with no splices. No junction boxes are allowed. Only pulling “C” and “LB”.

If there are any transitions from EMT to flexible conduit, that flexible conduit MUST be liquid-tight metallic conduit.

Installation Note: Conduit for PDT-1 will be run from the bridge panel to the transmitter with only the wire for the transmitter in the conduit.
Proceed to Appendices