Low Pressure Steam Plant
Understand how the low pressure steam plant works. Explore system overviews, measurement steps, and additional resources.
Overview
A low-pressure steam plant uses input energy (e.g., fuel, gas, or biomass) to generate steam in a boiler, which is then distributed throughout a facility or group of facilities for heating. Low pressure steam plants are generally operated at 15 psig or less (steam temperature below ~250 F). Low pressure steam can be used directly as a heating medium and distributed to the terminal equipment, or it can be used indirectly by being piped to heat exchangers to heat water to use at terminal equipment and/or domestic hot water (DHW) systems. A low pressure steam plant and associated systems are shown in Figure 1. A steam plant consists of a steam boiler, steam distribution system, condensate recovery system, blowdown system and economizer.
Systems
Steam Boiler
A steam boiler generates steam by transferring heat from the combustion gases in the combustion chamber to the water. The steam produced is then transported out to the facility to meet the heating loads.
Steam Distribution System
A low-pressure steam distribution system supplies the steam to the facility or facilities. Steam distribution systems are generally closed-loop, where the steam condensate is returned to the steam boiler to be re-heated (see Condensate Recovery System below).
Feedwater System
The feedwater system provides fresh or, at times, treated water to the steam boiler system and allows for removal of water that contains pollutants (i.e., unwanted minerals and/or sediment).
Hot Water Loop
The hot water loop system encompasses the distribution system responsible for moving hot water throughout the building.
Condensate Recovery System
A condensate recovery system complements the steam distribution system to maintain the efficiency of the steam plant. The condensate usually returns to the boiler, minimizing the feedwater and fuel supply to the boiler.
Blowdown System
The blowdown system removes suspended solids and sludge from the boiler water in order to preserve the operation and efficiency of the steam plant components. An integrated blowdown system reduces unnecessary blowdown by monitoring water quality and allows for heat recovery from the blowdown water to pre-heat the feedwater, generate service hot water or other applications.
How do I…
Assess Plant Energy Usage
The total energy consumption of a steam plant is the sum of the fuel usage consumed by the boiler to satisfy the heating load of the building, and the electricity required by the burner fan motor, condensate pump, and feedwater pump.
Quantify Fuel Usage (Btu)
Determine the amount of fuel used by the boiler based on the outdoor air temperature.
Equipment to Measure
What to Measure:
- Average hourly fuel usage by the boiler (gal/h for fuel oil or propane, ccf/h for natural gas).
- Average hourly outdoor air temperature (°F).
Measurement Locations
Quantify Electricity Usage (kWh)
Determine the amount of electricity used to operate all motors in the plant.
Equipment to Measure
What to Measure:
- Average hourly true RMS power draw (kW) of the burner fan motor.
- Average hourly outdoor air temperature (°F).
What to Measure:
- Average hourly true RMS power draw (kW) of the condensate pump motor.
- Average hourly outdoor air temperature (°F).
What to Measure:
- Average hourly true RMS power draw (kW) of the feedwater pump.
- Average hourly outdoor air temperature (°F).
Measurement Locations
Further Reading
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 11. STEAM SYSTEMS. I-P Edition.
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 32. BOILERS. I-P Edition.
ASHRAE (2019). “ASHRAE Handbook: HVAC Applications,” Chapter 50, Section 2.5. WATER TREATEMENT: DEPOSITION, CORROSION, AND BIOLOGICAL CONTROL. I-P Edition.
Spirax Sarco (2022). “Learn More About Steam” Steam Insights. https://www.spiraxsarco.com/learn-about-steam.