CHAPTER XI.

HEATING WITH EXHAUST STEAM. NON-GRAVITY

RETURN-SYSTEMS.

130. General Remarks.—Steam after being employed in an engine contains the greater portion of its heat, and if not condensed or utilized for other purposes it can usually be employed for heating without materially affecting the power of the engine. The systems of steam-heating which have been described are those in which the water of condensation flows directly into the boiler by gravity. In other systems in use high-pressure steam is carried in the boilers, high- or lowpressure steam in the heating-mains and radiators, and the return-water of condensation is received by a trap and delivered either into a tank from which it is pumped into the the boiler or in some instances wasted. The exhaust steam may need to be supplemented by live steam taken directly from the boiler, which may be reduced in pressure either by passing, through a valve partly open, or a reducing-valve, as described in Article 137.

It will often be found that little attempt is made to utilize the heat escaping in the exhaust steam from non-condensing engines, and consequently a good opportunity exists for construction of systems which will save annually many times their first cost.

131. Systems of Exhaust Heating.-The exhaust steam discharged from non-condensing engines contains from 20 to 30 per cent of water, and considerable oil or greasy matter which has been employed in lubricating. When the engine is freely exhausting into the air, the pressure in the exhaust-pipe is, or should be, but slightly in excess of that due to the atmos.

phere. The effect of passing exhaust steam through heatingpipes is likely to increase the resistance and cause back pressure which will reduce the effective work of the engine. The engine delivers steam discontinuously, but at regular intervals at the end of each stroke. The amount is likely to vary with the work done by the engine, since the engine-governor is always adjusted to admit steam in such amount as is required to preserve uniform speed; if the work is light very little steam will be admitted to the engine. For this reason the supply available for heating varies within wide limits.

The general requirements for a successful system of exhauststeam heating must be, first, the arrangement of a system of piping having such proportions as will make little or no increase in back pressure on the engine and will provide for using an intermittent supply of steam; second, provision for removing the oil from the exhaust, since this will interfere materially with the heating capacity of the radiating surfaces; third, provision against accidents by use of a safety or backpressure valve so arranged as to prevent damage to the engine by sudden increase in back pressure.

These requirements can be met in various ways. To prevent sudden change in back pressure due to irregular supply of steam the exhaust-pipe from the engine should be carried directly to a closed tank whose cubic contents should be at least 30 times that of the engine and as much larger as practicable. This tank can be provided with diaphragms or baffleplates arranged so as to throw all or nearly all the grease and oil in the steam into a drip-pipe, from which it is removed by means of a steam-trap, as described in Article 98, page 164. To this tank may be connected a relief-pipe leading to the backpressure valve, and also a supplementary pipe for supplying live steam. The supply of steam for heating should be drawn from the top of the tank.

Any system of piping may be adopted, but extreme care should be taken that as little resistance as possible is introduced at bends or fittings. The radiating surface employed should be such as will give the freest possible circulation. In general, that system will be preferable in which the main steam-pipe is carried directly to the top of the building, the distributing

pipes run from that point, and the radiating surface is supplied by the down-flowing current of steam (Fig. 173). It is desirable to have a closed tank at the highest point of the system, from which the distributing-pipes are taken, and provided with drips leading to a trap so as to remove, before it can reach the radiating surface, any water of condensation or oil which has been carried to the top of the building.

132. Proportions of Radiating Surface and Main Pipes Required in Exhaust Heating. The size of exhaust pipe required for an engine of given power, in order that the back pressure shall not exceed a certain amount, may be computed, the only data required in addition to that already given for heating with live steam, being that relating to the steam required by engines. The amount of steam used by engines will depend upon the workmanship and class to which they belong, but we can assume with little error that non-condensing engines will require the following weights of steam per horse-power per hour: simple with throttling-governor 40 pounds, with automatic governor 35 pounds, with Corliss valves 30 pounds; compound using high-pressure steam 25 pounds. In order that the pipes may be sufficiently large it is better to proportion the systems for the more uneconomical type.

In the following discussion the dimensions of piping are computed for an engine using 40 pounds of steam per horse-power per hour ( pound per minute), and exhausting

against a back pressure above or below atmosphere as stated.* The preceding table gives properties of steam, also radiating surface supplied per horse-power by engines of various classes.

The computation of the size of exhaust-pipes can be made by the following algebraic process:

Let equal velocity of the steam in feet per second; v, velocity in feet per minute; 7, length of pipe in feet; D, diameter of pipe in feet; d, diameter in inches; A, area of pipe in square feet; Q, cubic feet of steam discharged per minute; h, back pressure above atmosphere expressed in feet of steam; p, back pressure expressed in pounds per square inch; HP, horse power of engine; c, number of cubic feet in one pound of steam.

From the formulæ, page 218, we have, for velocity in feet per second

from which by reduction the velocity in feet per minute

Q = Av = 3000 4√ √ | D = 4.723√ Hď...

1723√ 7ď. . . . . (3)

Since pound of steam is used per horse-power per minute,

In case the back pressure is equal to one foot of water column (0.433 pound per square inch) above atmosphere, h = 1598, c = 25.7, and we have

It is advisable to make the diameter one inch greater to overcome additional resistances. (See table.)

*Radiating surface 25 per cent less. See Article 121, page 218.

RADIATING SURFACE AND HORSE-POWER OF ENGINE FOR A GIVEN DIAMETER OF EXHAUST-PIPE.

The foregoing table is computed for steam having a pressure. of 0.43 pound above the atmosphere. For other pressures of exhaust multiply the results given in the table by the following factors (for other distances multiply by 0.17):

As an example; find the size of exhaust-pipe and amount of radiating surface supplied by the exhaust of a 50 horsepower engine of the automatic type, working against a back pressure of 0.43 pound. For this condition, the exhaust from one horse-power will supply 25 per cent less than 131 square feet of radiation (see table page 249), or 4900 square feet. From the table at top of page we see that a 6-inch pipe will be somewhat larger than required, but should be used. The amount of radiating surface needed to warm a given building will depend on pressure of the steam, exposure, and class of building, as explained on page 55.

133. Systems of Exhaust-heating with Less than Atmospheric Pressure. If a system of exhaust-heating discharge the water of condensation directly into the atmosphere, the pressure must be slightly above atmospheric; but systems