Steam PlantPlant

General Overview

A low pressure steam plant takes input energy (e.g., fuel, gas, biomass) and uses a boiler to generate steam that is distributed throughout a facility or group of facilities to provide 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.

Steam Plant Systems and Components

Steam Boiler System

A steam boiler system is the largest energy-consuming system in the steam plant. 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. The primary components of a steam boiler system are shown in Figure 2.

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). The steam distribution system contains various valves and steam traps to regulate heating and manage condensate throughout a 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.

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). This increases the longevity of the system equipment, at a cost of the energy required to heat the feedwater, which is colder than the recirculated condensate. Feedwater systems may include a heat exchanger – commonly called an economizer – which recovers heat from the boiler fuel gases and uses it to pre-heat the feedwater.
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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, generates service hot water or other applications.

Evaluation of Energy Consumption

The total energy consumption of a steam plant is the sum of the energy consumption of the components of each system: 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. Table 1 provides a summary of system component measurements and values needed to quantify the annual energy consumption and operating characteristics of the steam plant.

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.