178. The boilers used in hot-water heating systems are similar in all respects to the best forms of steam boilers, except that the spaces commonly reserved for steam room may be dispensed with, or be utilized for tubes and other heating surfaces. Thus, in a common tubular boiler the entire shell may be filled with tubes.

The circulation inside of a hot-water boiler is quite different, however, and in many parts is likely to be much slower than in a steam boiler. The cold water enters at the bottom, and when heated passes out at the top; the general movement is, therefore, upwards, and there is only a moderate amount of return, or local, circulation within the boiler.

The water should pass from the inlet, over the heating surfaces, to the outlet in the most direct manner and with the least possible resistance.

179. The area of heating surface required in a hot-water boiler, for any given transmission of heat, is the same as in a steam boiler working at the same temperatures of water and combustion. The required areas of grates and chimney are also about the same.

The numerous varieties of hot-water boilers now on the market differ greatly in the volume of water which they contain, although they may have equal heating power. A heating system which contains only a small amount of water can be heated quickly, but it will also cool quickly; while, if the volume of water is large, it will act as a reservoir of heat and will maintain a moderate temperature for a considerable time after the fire has failed.

180. High-pressure boilers are used for heating with hot water having a high temperature and correspondingly high pressure. They are usually constructed of tubing in the form of a box coil. The tubing is of wrought iron or soft steel about 7/32 inch thick, and the bore is usually 3/4 or 1 inch. All the joints on the boiler or heating coils are welded.

High-pressure heaters ought never to contain a large volume of water, because the danger from an explosion will be too great.

181. Draft Regulators

Draft Regulators. It is more difficult to automatically regulate the draft in a hot-water heater than in a steam boiler. In the latter case the regulator is operated by the pressure of the steam, and the pressure is nearly the same in all the pipes which lead from the boiler. In a hot-water apparatus, however, only the temperature varies to any considerable degree, and this change in temperature is employed to regulate the draft. Draft regulators are constructed to operate in many different ways, but the most efficient class is that which utilizes the expansive force of some volatile liquid, which is acted upon by the hot water passing through the apparatus. A regulator of this kind is shown in Fig. 66. It contains a diaphragm of ordinary construction, which operates the lever d in the usual manner.

Fig. 66.

A large corrugated brass cup, having an aperture at the bottom only, is attached to the under side of the diaphragm. The space between the cup and the bowl is filled with some liquid, as shown, which is more volatile than water, usually a mixture of water and alcohol. The cup is filled only with air or vapor. Hot water from the heater enters the chamber c through a and returns to the heater through b, as shown by arrows. This water heats the liquid and vapor in the bowl and cup. The pressure generated is proportional to the temperature of the water, and is sufficient to properly operate the diaphragm and the apparatus attached to it.