Gage Pressure—Absolute Pressure.

—In the practice of engineering among English speaking people, pressures are stated in pounds per square inch, above the atmosphere. This is termed gage pressure. It is that indicated by the gages of boilers, tanks, etc., subjected to internal pressure. Under ordinary conditions the term pressure is understood to mean gage pressure, the 0 point being that of the pressure of the atmosphere. This system requires pressures below that of the atmosphere to be expressed as a partial vacuum, a complete vacuum being 14.7 pounds below the normal atmospheric pressure.

In order to measure positively all pressures above a vacuum, the normal atmosphere is 14.7 pounds; all pressures above that point are continued on the same scale, thus:

Gage pressure 0 = 14.7 absolute
Gage pressure 10 = 10 + 14.7 = 24.7 absolute
Gage pressure 20 = 20 + 14.7 = 34.7 absolute

Absolute pressures are, therefore, those of the gage plus the additional amount due to the atmosphere. All references to pressure in this work are intended to indicate gage pressure unless specifically mentioned as absolute pressure.

Steam heating as applied to buildings may be considered under two general methods: the pressure system in which steam under pressure above the atmosphere is utilized to procure circulation; and the vacuum system in which the steam is used at a pressure below that of the atmosphere. Each of these systems is used under a great variety of conditions, and to some is applied specific names but the principle of operation is very much the same in all of a single class.

Steam heating plants are now seldom installed in the average home but they are very much employed in apartment houses and the larger residences. In large buildings and in groups of buildings heated from a central point, steam is used for heating almost exclusively. The type of plant employed for any given condition will depend on the architecture of the buildings and their surroundings. In very large buildings and in groups of buildings, the vacuum system is very generally employed. This system has, as a special field of heating, the elaborate plants required in large units.

The low-pressure gravity system of heating is used in buildings of moderate size, large residences, schools, churches, apartment houses, and the like. Under this form of steam heating is to be included vapor heating systems. This is the same as the low-pressure plant except that it operates under pressure only slightly above the atmosphere and possesses features that frequently recommend its use over any other form of steam heating. The term vapor heating is used to distinguish it from the low-pressure system.

The low-pressure gravity system, with which we are most concerned, takes its name from the conditions under which it works. The low pressure refers to the pressure of the steam in the boiler, which is generally 3 or 4 pounds; and since the water of condensation flows back to the boiler by reason of gravity, it is a gravity system.

The placing of the pipes which are to carry the steam to the radiators and return the water of condensation to the boiler may consist of one or both of two standard arrangements. They are known as the single-pipe system and the two-pipe system.

Fig. 1 shows a diagram of a single-pipe system in its simplest form. In the figure the pipe marked supply and return, connects the boiler with the radiators. From the vertical pipe called a riser, the steam is taken to the radiators through branch pipes that all slope toward the riser, so that the water of condensation may readily flow back into the boiler. The water of condensation, returning to the boiler, must under this condition, flow in a direction contrary to the course of the steam supplying the radiators. In Fig. 2 is given a simple application of this system. A single pipe from the top of the boiler, in the basement, marked supply and return pipe, connects with one radiator on the floor above. The radiator and all of the connecting pipes are set to drain the water of condensation into the boiler.

When the valve is opened to admit steam to the radiator, the air vent must also be opened to allow the escape of the contained air. The steam will not diffuse with the air in the radiator and unless the air is allowed to escape, the steam will not enter. As the steam enters the cold radiator, it is rapidly condensed, and collects on the walls in the form of dew, at the same time giving up its latent heat. The heat is liberated as condensation takes place, and as the dew forms on the radiator walls the heat is conducted directly to the iron. The water runs to the bottom of the radiator and then through the pipes; back to the boiler. The water occupies but relatively a little space and may return through the same pipe, while more steam is entering the radiator. As the steam condenses in the radiator, its reduction in volume tends to reduce the pressure and thus aids additional steam from the boiler to enter. In this manner a constant supply of heat enters the radiator in the form of steam which when condensed goes back to the boiler at a temperature very near the boiling point to be revaporized. It should be kept in mind that it is the heat of vaporization, not the temperature of the steam that is utilized in the radiator, and that the heat of vaporization is the vehicle of transfer. The water returning to the boiler may be at the boiling point and the steam supplying the heat to the radiators may be at the same temperature.

Fig. 3 is a slightly different arrangement of the same boiler as that shown in Fig. 2, connected with two radiators on different floors. The same riser supplies both radiators with steam and takes the water of condensation back to the boiler.

Fig. 4 is an example of the single-pipe system applied to a small house. In the drawing, the boiler in the basement is shown connected with four radiators on the first floor and three on the second floor. The pipes connecting with the more distant radiators are only extensions of the pipes connecting the radiators near the boiler. As in Figs. 1, 2 and 3, all of the pipes and radiators are set to drain back into the boiler. If at any place the pipe is so graded that a part of the water is retained, poor circulation will result, because of the restricted area of the pipe, and the radiators will not be properly heated. This lack of drainage is also a common cause of hammering and pounding in steam systems, known as water-hammer. The formation of water-hammer is caused by steam flowing through a water-restricted area, into a cold part of the system, where condensation takes place very rapidly. The condensation of the steam is so rapid and complete that the resulting vacuum draws the trapped water into the space with the force of a hammer stroke. The hammering will continue so long as the conditions exist. The pipes in the basement are suspended from the floor joists by hangers as shown in the drawing. In practice the pipes in the basement are covered with some form of insulating material to prevent loss of heat.

As stated above, the single-pipe system may be successfully used in all house-heating plants except those of large size. It requires the least amount of pipe and labor for installation of the circulating system and when well constructed performs very satisfactorily all of the functions required in a small heating plant.

One of the commonest causes of trouble in a single-pipe system is due to the radiator connections. The single radiator connection requires the entering steam and escaping water of condensation to pass through the same opening. Under ordinary conditions this double office of the radiator valve is accomplished with satisfaction but occasionally it is the cause of considerable noise. At any time the valve is left only partly open the steam will enter and condense because of the lower pressure inside the radiator but the condensed water will not be able to escape. The water has only the force of gravity to carry it out of the radiators and if it meets no opposition will flow back through the pipe to the boiler; but if it is required to pass a small opening through which steam is flowing in a contrary direction, the water will be retained in the radiators. Single-pipe radiators, therefore, work satisfactorily only under conditions which will permit the steam to enter and the water to leave as fast as it is formed. In ordinary use the valve at any time is apt to be left slightly open and this produces undesirable working conditions.

In larger buildings, where greater distances require longer runs of pipe and more complicated connections, and where the volume of condensed steam is too great to be taken care of in a single pipe, this system does not work satisfactorily.