Steam–Heating Systems

Steam-heating systems may be classified according to any one of, or a combination of, the following heating features: (A) by the piping arrangement; (B) by the pressure or vacuum conditions obtained in operation; (C) by the method of returning condensate to the heating boiler.
A. By the Piping Arrangement:

1. A steam-heating system is known as a one-pipe heating system when a single main serves the dual purpose of supplying steam to the heating heating and conveying the condensate from it. Ordinarily there is but one connection between the main and the heating unit which must serve as both the supply and the return, although separate supply and return connection may be used.
2. A steam-heating system is known as a two-pipe heating system when steam and condensate flow to and from the heating unit in separate mains and branches.

3. The heating systems may also be described as up-flow and down-flow, depending on the direction of steam flow in the risers, and as a return heating system or a wet-return system, depending on whether the condensate mains are above the water line in the heating boiler or condensate receiver.

B. By Pressure or Vacuum Conditions:

1. Steam-heating systems may also be classified as high-pressure, low-pressure, vapor, and vacuum systems, depending on the pressure conditions under which the heating system is designed to operate.
2. A heating system is known as a high-pressure system when the operating pressures employed are above 15 psi, as a low-pressure system when pressures range from 0 to 15 psi: as a vapor heating system when the system operates under both vacuum and low-pressure conditions without the use of a heating vacuum pump: and as a vacuum system when the heating system operates under low-pressure and vacuum conditions with the use of a vacuum pump.

3. When heating automatic controls are employed to vary the pressure conditions in the system in accordance with outside weather conditions, the system may be known as a sub-atmospheric, differential, or synchronized heating system. These latter classifications are proprietary designations.
4. When heating orifices are employed on the inlets to the heating units, the heating system may be known as an orifice system.

C. By Method of Returning Condensate:

1. When condensation can be returned to the heating boiler by gravity, the system is known as a gravity-return system. All heating units in a gravity-return system must be located sufficiently above the water line of the boiler so that the condensation can flow freely to the boiler. The elevation of heating units above the water line must be sufficient to overcome pressure drop due to flow as well as pressure differences between supply and return piping due to operating conditions.
2. When conditions are such that condensation cannot be returned to the boiler the action of gravity, either traps or pumps must be employed to return the condensation to the boiler. and the system is known as a mechanical-return system. There are three general types of mechanical- return devices in common use ~kith mechanical-return systems today. These are the alternating-return trap, the condensate-return pump. and the vacuum-return pump.




3. In systems where pressure conditions in the system vary between that of a gravity-return and a forced-return system, a boiler return trap or alternating receiver is employed. and the system may be known as an alternating-return system.
4. When condensate is pumped to the boiler under atmospheric pressure or above the stem is known as a condensate-return pump system.
5. When condensate is pumped to the boiler under vacuum conditions, the system is known as a vacuum heating pump-return system. In either the condensate or vacuum pump-return systems, it is highly desirable to arrange for gravity flow to a receiver and to the pump. The pump then forces the condensate into the boiler against boiler pressure.