Steam is a very effective heating medium. Until recently, this property of steam has resulted in its being the most commonly used method of heating residential, commercial, and industrial buildings. Over the past 40 years or so, steam heating has been largely replaced in residences and small buildings by other heating systems that have often proven to be less expensive to install and operate or that operate at similar or greater levels of efficiency in small structures.
The basic operating principles of steam heating are relatively simple. A boiler is used to heat water until it turns to steam. When the steam forms, it rises through the pipes in the heating system to the heat-emitting units (radiators, convectors, etc.) located in the various rooms and spaces in the structure. The metal heat-emitting units, being cooler, cause the steam to condense and return to the boiler in the form of water (condensate, also called condensation) for reheating.
Classifying Steam Heating Systems
There are a number of different methods of classifying steam heating systems, but the most commonly used methods include one or more of the following features:
1. Pressure or vacuum conditions
2. Method of condensate flow to the boiler
3. Piping arrangement
4. Type of piping circuit
5. Location of condensate returns
Steam heating systems can be divided into low-pressure and high-pressure types, depending on the operating pressure of the steam used in the system. A low-pressure system commonly oper- ates at a pressure of 0 to 15 psig, whereas a high-pressure system uses operating pressures in excess of 15 psig.
Both vapor and vacuum steam heating systems operate at low pressures (0 to 15 psig) and under vacuum conditions. The latter system uses a vacuum pump to maintain the vacuum; the vapor system does not, relying instead on the condensation of the steam to form the vacuum.
The condensate from the heat-emitting units is returned to the boiler by either gravity or some mechanical means. When the former method is used, the system is referred to as a gravity return system. If mechanical means of returning the condensate are employed, the system is referred to as a mechanical return system. The three types of mechanical devices used to return the condensation in mechanical return systems are: (1) the vacuum return pump, (2) the condensate return pump, and (3) the alternating return trap. (Each of these devices is described in the appropriate sections of this chapter.)
Using the piping arrangement as a basis for classification, a steam heating system will be either a one-pipe or a two-pipe system. A one- pipe system is designed with a single main that carries the steam to the heat-emitting units and the condensate back to the boiler. In other words, it functions as both a supply and a return main. In a two-pipe system, there is both a supply main and a return main.
The piping circuit may best be described as the path taken by the steam to the riser (or risers). In a divided-circuit installation, two or more risers are provided for the steam supply. A one-pipe-circuit installation, on the other hand, employs a single riser from the boiler to carry the steam supply to the heat-emitting units. A loop- circuit installation is used when it is necessary to operate heat- emitting units at locations below the water level of the boiler.
A steam heating system may also be classified according to the direction of steam flow in the risers (i.e., supply mains). An upfeed system is designed so that the risers are below the heat-emitting units. In other words, the steam supply moves from the boiler up to the heat-emitting units in the rooms and spaces within the structure. An upfeed system is also sometimes referred to as an upflow, or underfeed, system. A downfeed system is one in which the steam supply flows down to the heat-emitting units. In this system, the supply main is located above the heat-emitting units.
Sometimes the location of the condensate return is used as a basis for classifying a steam heating system. If the condensate return is located below the water level in the boiler, it is referred to as a wet return. A dry return is a condensate return located above the water-level line.