Exhaust steam turned into a heating system creates a back pressure on the engine, which must be avoided as much as possible by using large steam-distributing pipes. A direct connection to the boilers through a pressure-reducing valve must be employed, to automatically furnish steam when the exhaust fails; and a relief valve should be placed upon the system so that surplus exhaust steam may escape. Before the steam enters the system, the 20 or 25 per cent, of water and oil it usually contains should be removed.

To proportion an exhaust-heating system, it is necessary to know the weight of steam discharged from the engine, in order to determine how many square feet of radiating surface are required to properly condense the steam.

Weight Of Water Used Per Hour Per Indicated Horsepower

Class of Non-Condensing Engine.

Weight of Water. Pounds.

Compound automatic................................

25

Simple Corliss............................................

30

Simple automatic......................................

35

Simple throttling.....................................

40

From these figures about 10 per cent, must be deducted for cylinder condensation, etc., in order to obtain the real available weight of steam for heating purposes.

Ratio Between Cubic Contents And Radiator Surface For Exhaust Heating

Class of Building.

Direct Radiation.

Indirect Radiation.

Blower System.

S. Ft.

C.Ft.

S.Ft.

C.Ft.

S.Ft.

C.Ft.

Dwellings......................

1 to

50

1 to

40

1 to

300

Offices.............................

1 to

70

1 to

60

1 to

365

Stores and shops.............

1 to

100

1 to

80

1 to

500

Churches, etc.................

1 to

200

1 to

150

1 to

900

The figures on page 307 are reasonable averages, and allowances must be made for exposure, etc.

Each square foot of direct radiating surface gives off to the air around it about 1± Thermal Units per hour per degree of difference between the temperature of the steam and that of the surrounding air. This is equivalent to about 1/4 lb. of steam per hour; or, from 4 to 4 1/4 sq. ft. of surface to each pound of steam to be condensed.