Steam Distribution System
Understand how the steam distribution system works. Explore component overviews, measurements, and additional resources.
Overview
A steam distribution system is responsible for distributing the steam produced by a steam boiler system as well as handling any condensate generated in the process. Common configurations for steam distribution systems include one-pipe and two-pipe systems. The main focus is on two-pipe systems, as they are common in larger facilities. In a two-pipe system, the condensate is handled by a dedicated return line and is kept separate from any steam. The condensate return line can be treated as its own system.
Components
Vacuum Pump
A vacuum pump, typically located after the steam traps, lowers the pressure in the piping network. This increases the rate at which the piping network and terminal units fill with steam and may allow the boiler to operate at a lower temperature, as placing the system under a vacuum reduces the boiling point of water.
Heat Exchanger
Steam distribution systems sometimes have steam-to-water heat exchangers that transfer heat to a heating hot water loop or domestic hot water systems. Converters and heated storage tanks can also be classified as steam-to-water heat exchangers.
Steam Header and Piping Network
A steam header acts as a heat reservoir for the distribution loops throughout the building. This reservoir allows for the boilers to run at a constant rate while the heating load of the building fluctuates. The steam piping network is equipped with valves, fittings, and connections to properly maintain the pressure differential in the system. It is covered with piping insulation that minimizes heat losses within the environment.
Steam Traps
Steam traps separate the condensate water from the steam distribution system, ensuring proper distribution and system efficiency. Steam traps can be mechanical or thermostatic. Mechanical traps rely on the density difference between condensate and steam. Thermostatic traps rely on the temperature difference between the condensate and steam.
Fan-coil Units
A fan-coil unit is a terminal unit that circulates and conditions air for either heating or cooling. The coil circulates an energy transfer medium, such as hot water, cold water, steam or refrigerant. The fan blows air over the coil to heat or cool it. Fan coil units can be connected to ducts, which transport outside air or mixed air, or they can be un-ducted in which case they condition the air in the room.
Radiators
Radiators are a common terminal unit used to distribute heat to a space. When routing steam through the radiator, the heat transfers to the piping, where it relies on radiation as well as free convection to warm a space.
Convectors
Convectors are similar to radiators in how energy is received, but rely almost exclusively on convection to distribute heat to a space. This leads to a cooler temperature to the touch, as well as a smaller terminal unit.
Unit Heaters
A unit heater is a standalone terminal unit containing a fan which blows over a small heat exchanger containing steam or some other heating source, to provide heat to a space.
How do I…
Assess System Energy Usage
The steam distribution system distributes heat energy, with losses to the non-conditioned environment along the way, but does not inherently consume energy. Generally, the energy usage is quantified at the low pressure steam plant, rather than at the steam distribution system. The exceptions are systems with a vacuum pump, which consumes electricity.
Quantify Electricity Usage (kWh)
Determine the total amount of electricity used by the vacuum pump motor.
Equipment to Measure
Vacuum Pump
What to Measure
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Further Reading
ASHRAE (2014). “ASHRAE Guideline 14-2014 – Measurement of Energy, Demand, and Water Savings.” Annex A.
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 11. STEAM SYSTEMS. I-P Edition.