This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
In arriving at the surface required in the heater, it has first to be stated that steam is the heating medium, and this may be exhaust steam at about 216°, or may be live steam at any pressure and temperature up to, say, 80 lbs. per square inch. The heater is now invariably built up of wrought tube secured to suitable base sections, as shown in Fig. 78, each section carrying two or four rows of pipe. The ill effects of expansion and contraction are avoided by this means, while movement lengthwise is allowed for by the ball bearing. These heaters commonly have one section without communication with the remainder, this section being heated by the exhaust steam from the fan engine. This is supposing, of course, that an engine is devoted to the fan, which may not be the case in mills and factories. By utilising the exhaust steam from the fan engine there is, needless to say, a distinct fall in the ultimate cost of running the fan. When desired, these heaters can be further divided up so as to use exhaust steam from other engines in one part and live steam in the other.
The area of surface required in the heater, for given results, may be computed from any of the recognised tables, but the common custom is to allow one lineal foot of this pipe (1 inch) to a given number of cubic feet in the building. In factories and mills it is calculated that a foot of pipe can serve 100 cubic feet of space if the pipe is heated with exhaust or low-pressure steam, or 150 cubic feet for high-pressure steam. This is supposing that all the heated air goes to waste after once passing through the building, and that the air is changed three times per hour. In assembly rooms, schools, and buildings having the occupants closely congregated the changes of air have to be more frequent, with the temperature a little higher, and the allowance is then either 70 or 100 cubic feet to each foot of pipe, according to whether low or high-pressure steam is used.
In Fig. 79 is shown a fan and heater complete, a corner broken away from the heater case disclosing the arrangement of the pipes within. The engine operating the fan has its cylinder below the fan axle. It will be seen that the outfit is of the design that draws the air through the heater. The fan can be made to deliver at floor level or into underground ducts, or at a high level horizontally ; or again, a vertical delivery can be arranged if it best suits the conditions.
In Fig. 80 is illustrated a combined fan and heater, of the blow-through type, specially designed for what is known as the "double-duct" system. This system of work is finding favour for schoolwork, or similar buildings, in which the air supply has to be constant regardless of the outdoor temperature. This result can be obtained without double ducts, by simply allowing cold air to mix with the heated air, as already mentioned, but this latter plan does not admit of independent regulation of temperature to each and all of the rooms. In many cases independent regulation is not needed, but when it is recourse is usually had to twin ducts.
With the introduction of hot and cold-air ducts there came the necessity of devising a regulating device, easily operated, by which the quantity of cold or warm air, making up the whole volume delivered, could be regulated. This device has been named the "mixing-damper," and Fig. 81 will give a general idea of its operation. The ends of the twin ducts, where they terminate in the single-delivery duct, are fitted with a section of tube - a cylinder to act as a damper. The tube does not serve any purpose as a tube, but is a form of damper that operates with more certainty, and is more easily fitted, than a flat plate. Scarcely any description is needed, as it will be seen how readily the passage of air from either duct can be regulated by the weighted cord which appears in the room above.