Understanding Thermal Systems: Steam Systems
Steam systems are commonly used in industrial, commercial, institutional, power, and process applications to supply heat or to turn turbines. The development of steam systems dates back to the eighteenth century. In 1781, James Watt patented a steam engine that produced continuous rotary motion and developed a 10-horsepower engine that could power a wide range of manufacturing machinery. The stationary steam engine was a key component of the industrial revolution. Today, steam turbines have replaced reciprocating steam engines and are common in the power and process industries. In industrial, commercial, and institutional (ICI) applications, electric motors have largely replaced steam for providing motive power. Steam systems in ICI applications are typically used for supplying heat for HVAC and process requirements. Insulation is used extensively on steam systems to limit heat losses.
Steam systems offer several advantages. Steam is a gas and it quickly expands to fill the available space. It therefore flows through the system unaided by external energy sources such as pumps. Since most of the heat content in steam is stored as latent heat, large quantities of heat can be distributed throughout a system with little change in temperature. Furthermore, its low density means that it can be used in tall buildings without excessive pressures. Terminal units may be added easily to existing systems without making basic changes to the design. A schematic showing the basic elements of typical steam systems is shown in Figure 1 . Steam systems consist of (1) a steam source (steam generation), (2) a distribution system, and (3) terminal or end-use equipment.
Steam is normally generated in boilers using oil, natural gas, coal, or nuclear energy as the fuel. Steam can also be generated using solar, biofuels, municipal waste, or geothermal as the heat source. Indirect sources for steam include recovered heat from processes or equipment like diesel or gas engines. Steam may be generated by a facility’s on-site sources or purchased from a utility serving the geographic area.
There are a wide variety of sizes and designs of steam boilers. Boilers are classified based on working pressure, fuel used, materials of construction, draft (natural or mechanical), shape and configuration, and whether they are condensing or non-condensing. Low-pressure boilers operate at pressures of 15 pounds per square inch gage (psig) or below. High-pressure boilers operate at pressures above 15 psig. Gage pressure is the pressure that is read from a pressure gauge, which is relative to the local atmospheric pressure. Absolute pressure (psia) is the gage pressure plus the atmospheric pressure (normally assumed to be 14.7 psia). Referring to the saturated steam table on page 28 (Figure 2) we see that a low-pressure boiler operating at 15 psig produces saturated steam with a temperature of 250°F.
Boilers contain a combustion section (where the fuel/air mixture burns) and a heat-transfer section (where heat is transferred to water). Boilers may be either fire tube (where hot combustion products pass through tubes surrounded by water) or water tube (where water passes through tubes that are exposed to hot combustion products). Smaller boilers for commercial heating systems will typically be low- pressure fire-tube designs while large boilers for process and power-plant applications will typically be water tube.
Steam boilers are rated by their output in either pounds of steam per hour, Btu/hour (h), or boiler horsepower (bhp); 1 bhp is equal to 33,475 Btu/h. Package boilers are factory made to a range of standard designs and are available off-the-shelf. Package boilers come in standard sizes ranging from 60,000 Btu/h to 100,000,000 Btu/h (2–3,000 bhp). By contrast, a coal-fired steam boiler serving a large electric power plant may generate up to 8,000,000,000 Btu/h (240,000 bhp). Package boilers are normally factory insulated to minimize heat losses. Larger site fabricated boilers are insulated in the field.
Steam piping distributes steam to the terminal units, returns the condensate, and removes air and non- condensable gases. Since supply-line temperatures remain high even at periods of low load, insulation of the steam piping is critical to maintain overall system efficiency. In applications where the loads are remotely located from the steam source, distribution losses from uninsulated piping can be substantial. Supply and condensate return lines should be insulated, as well as drip legs, valves, strainers, condensate receiver tanks, and pumps. Insulation is also an important health and safety feature of well-engineered steam systems as the potential for burns to personnel is always present when steam systems are operating.
For commercial applications, building codes require minimum insulation levels depending on the size and location of the piping. The 2015 International Energy Conservation Code (IECC) requires 2 ½ or 3 inches of insulation (depending on pipe size) for low-pressure steam piping (≤250°F). Requirements for high-pressure piping (≤250° F) range from 3″ to 5″.
A variety of insulation materials are used on steam piping. Fiber glass insulation with factory adhered all- service jacket is frequently used on low-pressure systems in ICI applications. Mineral wool, calcium silicate, perlite, and cellular glass are more common on higher pressure systems.
Steam piping systems are complex. Pressure-relief valves, isolation valves, pressure-reducing valves, blow-down valves, strainers, and traps are all important elements of the distribution piping system. Piping must be adequately sized, configured, supported, and insulated. Piping must be equipped with an adequate number of drip legs to drain condensate, and must be pitched properly to promote the drainage of condensate to these drip legs. Steam traps are an important component of steam systems. A steam trap is an automatic valve that can pass condensate to the return piping but prevents the passage of steam. Traps must perform 3 important functions:
- Traps must open to vent air and other non-condensable gases. On start-up, air and other non-condensable gases must be vented from the system since steam cannot fill the system until gases are vented.
- Traps must close to prevent the loss of steam. Steam leaking trough a trap is wasteful.
- Traps must open to drain condensate.
Condensate forms as steam gives up heat and must be removed, or else the terminal device will fill with condensate, which will prevent heat transfer. In addition, condensate buildup in mains and supply piping can result in water hammer (a surge of pressure caused when a fluid is forced to stop or change direction suddenly).
Traps are categorized as either (1) thermostatic traps, (2) mechanical traps, or (3) thermodynamic traps. Selected examples of each are shown in Figure 3.
Thermostatic traps use a temperature-sensitive element (either a fluid-filled bellows or a bimetallic element) to operate a valve. On start- up, the trap is cool and the element contracts, opening the valve so that air is vented. When steam reaches the trap, the element expands, closing the valve to prevent steam loss. As condensate reaches the trap and cools, the element opens and condensate flows through the trap until steam again reaches the trap. It is important to note that the condensate must cool before the thermal element can open the trap. For cooling to occur, thermostatic traps must have a “cooling leg”upstream of the trap. For this reason, thermostatic traps are not insulated.
Mechanical traps use a float apparatus to sense the presence of condensate. The float and thermostatic (F&T) trap is a common mechanical trap. The F&T trap is actually a combination of a mechanical trap (the float) and a thermostatic trap. The float operates a linkage to modulate the discharge orifice in the presence of condensate. The thermostatic element opens to discharge gases.
Thermodynamic traps (or disk traps) depend on differences in the flow characteristics of the steam and condensate. They use a disk on a flat seat. On start-up, air or condensate lifts the disk off its seat and is discharged. When steam enters the trap, the increased velocity of the flow reduces the pressure on the underside of the disk, causing it to snap shut with an audible popping sound. Thermodynamic traps should not be insulated.
A wide variety of terminal units are used in steam heating systems, and they may be broadly classified as either (1) natural convection units, or (2) forced convection units. Natural convection units include the familiar “cast-iron radiators,” convectors, and cabinet units. These devices actually supply heat by a combination of convection and radiation, but they function without the use of fans.
Forced convection units include unit heaters, unit ventilators, and fan-coil units. Unit heaters are fan- powered devices commonly used to heat spaces such as warehouses, garages, and factories. They are also used to heat corridors, vestibules, and similar spaces in all sorts of buildings where ventilation air is not required or is provided by a separate system. Unit ventilators provide both heat and ventilation to a space. Terminal units for steam systems also include a wide variety of steam-to-air or steam-to- water heat exchangers such as heating coils for air-handling units, reheat coils for variable air volume (VAV) systems, steam-fired hot water heaters, and a myriad of similar devices.
- U.S. Department of Energy, Improving Steam System Performance: A Sourcebook for Industry, 2nd Edition, 2012, http://tinyurl.com/o7x46xd.
- A boiler horsepower was historically the amount of steam required to serve a one horsepower steam engine.
- 2012 ASHRAE Handbook—HVAC Systems and Equipment. ASHRAE, 1791 Tullie Circle, Atlanta, Georgia.