Fig. 142.
Fig. 143.
The manner in which a temperature too low to start rapid combustion in wood in steam pipes, operates in originating a fire is by first reducing the oxide of iron (rust) to a metallic condition. This is possible only under certain external conditions, among them a dry atmosphere. Just as soon as the air is recharged with moisture, the reduced iron is liable to regain, at a bound, its lost oxygen, and in doing so become red hot. This is the heat that sets the already tindered wood or paper ablaze.
Where there is no rust there is no danger from fire with a less than scorching temperature in the pipe or flue. Hence the necessity of keeping steam or hot water fittings in good order.
The indirect system of heating is the most expensive to put in; as to the cost of providing nearly double the heating surface in the coils must be added the cost of suitable air boxes, pipes and registers. For a large installation, this is a serious matter, although for office warming the advantages gained on the score of healthfulness and greater efficiency of employees much more than counterbalance the extra expense.
One horse power of boiler will approximately heat 6,000 to 10,000 cubic feet in shops, mills and factories—dwellings require only one horse power for from 10,000 to 20,000 cubic feet.
From seven to ten square feet of radiating surface can be heated from one square foot of boiler surface, i.e., the heating surface of the boiler and each horse power of boiler will heat 240 to 360 feet of 1-inch pipe.
The profession most nearly related to that of steam engineers is the working steam fitters’ occupation. Strictly speaking, the engineer should produce the steam, and it is the steam fitters’ place to fix up all the steam pipes and make all the necessary connections: but where the steam plants are small, the engineer may be steam fitter also: hence the introduction in this work of these “Points” which are necessary to be known for the proper care and management of any system of steam or hot water heating.