System Specifications
Biomass Boiler
Manufacturer: Messersmith Manufacturing
Rated Output: 5.0 mmBtu/hr
Maximum Water Temperature: 230 °F
Maximum Water Pressure: 60 psig
Allowable Fuel Moisture Content (wet basis): 8%-45%
Stack Height: 40'
Stack Diameter (OD): 2'
Biomass Fuel Storage
Capacity (volume): 308 cubic yards
Capacity (weight): 75 tons of woodchips (based on 18 lb/ft3)
Loading Doors: 2
Depth Below Grade: 10'
Propane Boilers
Propane Boiler #1 Rated Output: 4.6 mmBtu/hr
Propane Boiler #2 Rated Output: 2.0 mmBtu/hr
Thermal Storage Tank
Capacity: 4,000 gallons
Design Pressure: 125 psig
Design Temperature: 250°F
Insulation: 3" fiberglass with 22 ga. steel jacket
Temperature Sensors: 3 (upper, middle, and lower)
Main Boiler Loop Circulator Pumps
(Pumps 1 and 2)
Power: 3 HP
Speed: Fixed
Rated Flow and Head: 500 GPM at 15 feet of head
Propane Boiler #1 Circulator Pumps
(Pumps 5 and 6)
Power: 2 HP
Speed: Fixed
Rated Flow and Head: 462 GPM at 10 feet of head
Propane Boiler #2 Circulator Pumps
(Pumps 3 and 4)
Power: 1 HP
Speed: Fixed
Rated Flow and Head: 199 GPM at 10 feet of head
District System Pumps
(Pumps 7 and 8)
Power: 20 HP
Speed: Variable
Rated Flow and Head: 472 GPM at 120 feet of head
Sequence of Operations
The fire in the biomass boiler will be manually ignited by plant staff at the start of the heating season. The biomass boiler will add fuel per Messersmith's control system as required to maintain 210°F leaving water temperature (adjustable).
The three way diverting valve on the boiler side of the thermal storage will divert hot water back to the boilers whenever the boiler return water temperature drops below 140°F (adjustable). The biomass boiler will cycle off to prevent condensation, and the LP gas fired boilers will be used to bring the boiler loop back to operating temperature. Upon reaching the setpoint of 210°F (adjustable), the three way valve will reposition to feed hot water into the thermal storage tank.
The 4.6 mmBtu/hr LP gas fired boiler (B-1) will be enabled when the outside air temperature is below 32°F (adjustable) and the hot water loop temperature falls 30˚F (adjustable) below the set point of 210˚F (adjustable).
The 2.0 mmBtu/hr LP gas fired boiler (B-2) will be enabled when the outside air temperature is above 32°F (adjustable) and the hot water loop temperature falls 30˚F (adjustable) below the set point of 210˚F (adjustable).
The temperature of water sent to the district system is controlled by the valve on the district side of the thermal storage. The water temperature is based on an outside reset schedule. When the outdoor temperature is 0°F or lower, the district supply temperature will be set to 210°F (adjustable). When the outdoor temperature is 60°F or higher, the district suppply temperature will be set to 180°F (adjustable). When outdoor temperatures are within the range of 0°F to 60°F, the district supply temperature will be set according to a linear interpolation.
The district pumps P-7 and P-8 operate on a lead/lag schedule, and vary their speed to maintain a constant differential pressure from supply to return of 33 psi (adjustable). Depending on configuration, this pressure can either be measured in the biomass plant, or in Gray, which is the building farthest away in the system.
Each building is isolated from the district system by a heat exchanger. A control valve on the district side of each heat exchanger modulates the water flow in order to maintain a setpoint of water leaving the heat exchanger in the building side supply. The setpoint is adjusted based on an ourdoor reset schedule. The flow meter and temperature sensors on the district side of each heat exchanger are used to measure the heat delivered to each building.
