168. The various systems of steam heating now in vogue may be divided into four classes, according to the pressure of the steam and the manner in which it is used in them.

The high-pressure system is operated with steam at any pressure above 10 pounds by the gauge, or thereabouts. The radiators require less heating surface, and the piping may, in some cases, be made a size smaller than for pressures of 2 to 5 pounds. The fall of pressure that may be permitted at the radiators is, however, no greater than in a low-pressure system, therefore the size of the piping cannot be reduced to any considerable extent. This system requires a better class of steam generators, which are more costly than low-pressure generators, and the radiators also require to be made extra strong. High-pressure heating is seldom used, and is not to be recommended for domestic work.

169. The low-pressure system is usually operated with pressures ranging from 2 to 5 pounds above the atmosphere. This system is most commonly employed, and is the one referred to in this section, unless otherwise stated.

170. The exhaust system is in every respect a low-pressure system, except that it is provided with special apparatus which adapts it to receive the exhaust steam from engines and pumps.

The exhaust system is used for the purpose of utilizing and saving the heat in exhaust steam which would otherwise go to waste. A pound of exhaust steam at 5 pounds gauge pressure, and a pound of live steam at 60 pounds pressure, will give up practically the same quantity of heat when condensed in the radiators. The practice of allowing exhaust steam to escape into the atmosphere in any situation where it can be used in heating apparatus, either for house warming or heating liquids, etc., is, therefore, inexcusably wasteful.

171. The general arrangement of apparatus for controlling the steam supply and drainage, in an exhaust system, is shown in Fig. 63. The steam-heating main a is connected to the exhaust pipe b, and also to a pipe c which supplies live steam from the boilers. This steam passes through a pressure-reducing valve e, and is lowered in pressure to the desired amount before entering the heating main. By this arrangement the heating system will be supplied with exhaust steam as long as the engines are in operation, but if for any reason the supply becomes insufficient to maintain the proper pressure, then live steam will enter through the reducing valve and make up the deficiency. If the supply of exhaust steam becomes excessive, so that the pressure rises unduly, the excess will escape by opening the back-pressure valve f and blowing into the atmosphere. When the engines are stopped, the steam in the heating apparatus is prevented from passing backwards and filling them up with water, by means of the check-valve g. This valve is similar to the valve f in construction, and is so nearly balanced by its counterweight that it will open very easily. The relief valve /"is usually adjusted to blow off at a pressure about 1 pound higher than that maintained by the reducing valve e.

The exhaust steam is passed through a separator d before entering the heating system, for the purpose of removing the entrained water, and especially the oil which accompanies it, from the engine.

The drainage from the heating apparatus is collected in the pipe h, and is returned to the boiler by means of a pump p, as shown. The returns have no direct connection with the boiler, consequently the water level in them may be maintained at any convenient height, as at i i. This is accomplished by means of the pump and its governor m.

Fig. 63.

The pump governor is merely a closed vessel containing a float u which rises and falls with the water level. The steam which drives the pump is taken from the high-pressure pipe c through the stop-valve n, and passes through a throttle valve l which is controlled by the float. When the water rises above the desired level, the float opens the throttle and starts the pump; and when it subsides, the float lowers and shuts off the steam. The exhaust from the pump is turned into the exhaust main through the pipe s. The pump governor is connected to the heating main a by a small pipe o, for the purpose of equalizing the pressure on top of. the water therein.

Valves are provided in the main pipes at k and v, for the purpose of shutting off the heating apparatus during the summer season. It will be noted that these valves are located so that they do not interfere with the supply of steam to the pump, nor with the exhaust therefrom. The returns are shut off from the pump by the valve r, and an independent water supply is attached at w. The pump delivers through the pipe t to the boiler.

Care must be taken to locate the valves f and g in proper relation to each other, as shown. If the check-valve is placed between the heating main a and the valve f, and the reducing valve e should get out of order, the pressure would rise in the heating system until it equaled that in the boiler. This would probably burst the radiators and do serious damage. The safety of the whole apparatus depends upon the good working condition of the relief valve f.

172. The vacuum system of steam heating differs from all others in one important particular, viz., a vacuum, more or less perfect, is constantly maintained in the returns. This permits the system to be operated with steam of any convenient pressure, high or low, and from any source, either exhaust or otherwise. The pressure and temperature throughout the whole system may be adjusted and maintained at any degree between full boiler pressure and a low vacuum, thus making the system adjustable to suit all conditions of weather and service.

Usually the system is operated with exhaust steam, the supply being arranged as shown in Fig. 63. The piping is usually arranged on the two-pipe system, and the returns are generally made independent, although it is not necessary to do so in all cases.

173. Fig. 64 shows the essential features of the system. The returns a are connected to a receiver b, which collects all of the air and water in the system. These are pumped out by means of the vacuum pump v, which thus maintains a constant vacuum of any degree desired in the returns.

Fig. 64.

By this arrangement any steam may be used in the radiators that is warm enough to operate the traps. It permits steam to be used at a pressure far below the atmosphere, and at any temperature down to about 140°, the limit being fixed only by the ability of the pump to keep up the vacuum in the returns.

The water and air which are drawn from the receiver by the vacuum pump are discharged into an open tank, from which the air readily escapes. The water is then pumped back into the boiler by any ordinary feed pump.

174. In some cases, the fresh, cold water which is otherwise required to feed the boilers, is injected into the receiver in a series of fine streams, through the pipe w, the object being to condense as much as possible of the steam which is present, and thus improve the vacuum. At the same time that the water becomes warmed it gives up the air accompanying it, thus increasing the amount to be removed by the pump. This air expands into the vacuum and partially neutralizes the effect of the condensation. Thus it will be seen that the introduction of the feedwater into the system at this point is of doubtful utility. If it is sent through an ordinary feedwater heater instead, it will become much hotter and the air will be eliminated without difficulty.

175. It will be understood that when the exhaust steam from an engine is turned into the ordinary low-pressure heating system, the back pressure is increased, and the efficiency of the engine is correspondingly decreased, sometimes to such an extent as to become very detrimental.

One of the principal advantages of the vacuum system is that a great part of the back pressure is taken off the engines, and the capacity of the engines to do useful work is thereby increased.

176. The size of the piping required for the vacuum system of steam heating is about the same as for the ordinary low-pressure systems. The radiators, however, must be larger than for any other system, in proportion as the temperature of the steam used is lower.

177. The district system of steam heating is practised in large towns and cities by means of steam mains which are laid underground through the streets. The arrangement of the connections from the street mains to the house pipes is shown in Fig. 65. The service pipe a is provided with a valve b inside the basement wall, so that the house system can be shut off when desired. The steam passes through a pressure-reducing valve c, and thence into the distributing pipe or house main e. The water that may enter from the service pipe is led away by the drain pipe d. The returns are all connected into the pipe f, which is submerged below the water level. The level of the water in the returns is fixed by the elevation given to the steam trap t; thus, in the figure, it is at the line g. The hot water from the trap should never be discharged directly into the house drains, because of its destructive effect upon the pipes, but should be cooled before escaping to the sewers, by first allowing it to flow through a coil of pipes. This coil is usually called a cooling coil. It should never deliver directly into the drainage system, but in all cases should deliver into a deep, sealed trap. This is to prevent drain air from entering the heating system or the building. The trap, or hotwell, should always deliver into the house sewer connection on the sewer side of the main-drain trap, to prevent hot vapors from passing up the iron drainage system in the building.

Fig. 65.