182. The purpose of an expansion tank is to keep the pipes and other apparatus constantly full of water. The water in the heating system expands when heated, and if it fills the apparatus when cold it will overflow it when hot, and the expansion tank serves to receive this overflow.

The capacity of the tank required in any given case depends upon the volume of water in the heating system and the highest temperature which it will attain. In an ordinary system, where the tank is open to the atmosphere, commonly called the low-pressure, or open-tank, system, the temperature cannot be raised much above 212°. The capacity of the expansion tank, between high and low water marks, should be about 1/20 of the total contents of the apparatus to which it is attached.

183. When the water is to be heated to higher temperature, as in the so called high-pressure, or closed-tank, system, the expansion tank must be closed and its capacity must be greatly increased. Not only must space be provided for the expansion of the water, but there must be additional space above the water, when expanded to its utmost, sufficient to contain the air which originally filled the tank, without compressing it too much. The pressure of the air thus compressed should not exceed the pressure of steam corresponding to the maximum temperature of the water.

For example, if the water is heated to 350°, it will expand about .12, or 1/8 of its original volume, and the tank must have a capacity between high and low water marks equal to 1/8 of the total contents of the apparatus, including pipes, radiators, and boiler. The pressure of steam having a temperature of 350° is about 120 pounds, gauge. Air must be compressed to about 1/9 of its original volume to produce that pressure; therefore, the space for air, above high-water mark, in this case should be, at least, 1/9 of that allowed for expansion.

184. Safety Valves

Safety Valves. In practice, any closed hot-water system is liable to be neglected and overheated. The expansive force of the water is practically irresistible, and unless room be provided for the expansion, it will burst the apparatus. The only mode of securing safety is to provide the closed tank with a safety valve. This may be set to blow off at the pressure which steam would have at the maximum temperature desired in the apparatus. No closed tank should be allowed to operate without a safety valve.

A hot-water apparatus having an open tank is absolutely safe from accident by bursting so long as open communication exists between the tank and boiler. But if the tank is closed for any reason, or its connections are closed, it is thereby converted into a dangerous apparatus.

185. Freezing

Freezing. The connection of the tank to the heating apparatus must be carefully protected against frost. When this connection is frozen, the apparatus is deprived of the relief afforded by the tank, and a rupture is sure to occur in some weak part when the water is being heated.

186. Stop-Valves

Stop-Valves. Open communication between the expansion tank and the boiler must be maintained at all times. No stop-valve should ever be placed on this pipe, as such a valve is liable to be closed, and thus produce disaster.

187. This construction of an ordinary low-pressure expansion tank is shown in Fig. 67. The body and heads are made of wrought iron, and should be galvanized inside and out. A glass water gauge b is attached to show the height of the water inside. The surface of the water should be visible in the gauge both at its lowest and highest levels. The tank is connected to the heating apparatus by an expansion pipe c. A connection to the cold-water house-supply pipe may be made at e, for convenience in filling the tank. The top of the tank is always open to the atmosphere through the pipe d. This opening must never be closed.

188. An expansion tank suitable for high pressure differs from the form shown in Fig. 67 mainly in its proportions, being of smaller diameter in proportion to its length, and it is also made of much thicker materials. The outlet pipe is controlled by a safety valve. The height of the water is usually shown by means of gauge, or try, cocks screwed into the side. Glass water gauges are not suitable for high-pressure tanks, because they are very liable to crack and burst, and thus allow the water to escape and damage the building.

Fig. 67.