Engineering Design Criteria for District Heating
The Board of Public Utilities (BPU) presently provides District Heating service to the foundation of the customer's building and shut-off valves to separate the building from the main distribution system. The building owner has the responsibility to install all necessary hydronic equipment to utility District Heating. This would include heat exchanger, expansion tank, pumps and control system. The Board of Public Utilities is responsible to install and monitor the British Thermal Unit (BTU) meter and the flow meter.
District Heating Valve
The District Heating valve for each exchanger should be located on the District-return side and meet the following conditions:
- Control valve to exceed heat exchanger pressure drop by 1 pound per square inch (psi), but not greater than 4 psi for building heating or 5 psi for District Heat water, pool, etc.
- Minimum inlet pressure and temperature for design of valve 232 pound per square inch gauge (psig)/250 degrees Fahrenheit
- Minimum turn down 30 to 1
- Temperature sensor located in District-return line shall have authority, working through temperature controller, over building cooling control valve to limit District-return water temperature to a maximum of 150 degrees Fahrenheit. A unit may not be required if load is small; please check with BPU.
- Valve should be capable of closing against a differential pressure of 100 psi
- Valve should fail in closed position for air, as is position for electricity
- Valve type - two-way, single seated, plug type with a maximum leakage factor of 0.1% of the rated flow coefficient (Cv).
District Heating Water Temperature
The District Heating water temperature leaving each exchanger should not be greater than 150 degrees Fahrenheit and should be reduced by a minimum of 40 degrees Fahrenheit from the incoming District Heating water at all times.
A balancing valve (B and G circuit setter or equal) should be installed for each control valve and if there is more than one control valve, another balancing valve shall be installed which is common to all control valves. The balancing valve shall be sized as small as possible using three feet of pressure drop across the readout ports at design flow. This will usually result in a balancing valve which is one size less than line size.
Y Type Strainer
A Y type strainer with monel screens with a mesh of 1/16 inch opening should be furnished and installed on the
District Heating supply. This is to protect the flow meter for the BTU meter and the exchanger, if used.
Plate & Frame Type
A plate and frame type unit should be used. The following should apply:
- Each heat exchanger should be designed for 350 psi/340 degrees Fahrenheit on the District Heating side, except as noted. Use a fouling factor = 1 for U tube type units.
- Exchange plates should be a minimum of 0.8 Millimeters (mm) thick.
- Gaskets should be peroxide-cured (peroxide vulcanized material); nothing else should be acceptable.
- Gaskets should be crush-proof at all temperatures within their operational range.
- Gaskets should not have more than fifty percent change in hardness when exposed to liquids at 212 degrees Fahrenheit or a change in volume more than ten percent when exposed at liquids at 212 degrees Fahrenheit.
- Plate heat exchanger shall be designed for 300 psi/350 degree Fahrenheit.
- Use a fouling factor = .0005 ftsqhrdegree F/BTU or 20% over dimension for building heating units. Do not use plate type units for domestic hot water applications unless a water conditioner is installed.
District Heating Additional Information
The following is useful contractor information for consulting engineers about BPU District Heating:
- Pressure drop through exchanger should not exceed 3 psig on the District side.
- The district heating piping and valves supplied by the owner should be designed for 232 psi / 250 degree Fahrenheit.
- Pressure gauges, valves and thermometers should be provided generally as shown on Service connection Detail Drawing.
- The District Heating piping and valves should be 1 & 1/2 inches insulation for pipes size 2 inches and less; 2 inches of insulation for pipe sizes over 2 inches.
- Building Owner’s engineer should submit to the BPU the maximum pressure drop that will occur in the Building Owner’s District supply and return piping prior to installing the system
- The BTU flow meter must be installed with straight pipe upstream and downstream of flow meter. The length of strength pipe needed will be provided by the BPU.
- Contractor should test District Cooling chilled water supply and return piping at 350 psig for a minimum of one hour and until all joints have been inspected.
- Contractor supplied pipe is to be schedule 40 black steel; above 2 inches to weld, up to 2 inches to weld or screw. Use schedule 80 pipe if using screwed joints.
- Contractor may have to supply 150 pound flanges for flow meter; the BPU will provide direction. Contractor to provide and install with a spool piece. Install BTU sensors in elbow facing into flow of water. Hydro prior to flushing lines.
- Owner/engineer to submit design calculations and proposed equipment cut sheets to the BPU for approval prior to hook up. It should be noted that designing the building heating heat exchanger for a 150 degrees Fahrenheit return water temperature will result in a saving on the heat exchanger cost.
- If cascaded heat exchangers are used, this item shall apply only to the last downstream heat exchanger.