In laundries, hotels, etc., where a large amount of hot water is used, it is necessary to have a larger storage tank and a heater with more heating surface than can be obtained in the ordinary range water-back. Fig. 18 shows an arrangement for this purpose.

Fig. 18.

The boiler may be of wrought iron or steel of any size desired, and is usually suspended from the ceiling by means of heavy strap iron. The heaters used are similar to those employed for hot water warming. The method of making the connections is indicated in the illustration.

The capacity of the heater and tank depends entirely upon the amount of water used. In some cases a large storage tank and a comparatively small heater are preferable; and in others the reverse is more desirable.

The required grate surface of the heater may be computed as follows: first determine or assume the number of gallons to be heated per hour, and the required rise in temperature. Reduce gallons to pounds by multiplying by 8.3, and multiply the result by the rise in temperature to obtain the number of thermal units. Assuming a combustion of five pounds of coal per square foot of grate, and an efficiency of 8,000 thermal units per pound of coal, we have

Fig. 19.

Grate Surface in sq. ft. = gal. per hour X 8.3 X rise in temp. 5 X 8,000

Example. - How many square feet of grate surface will be required to raise the temperature of 200 gallons of water per hour from 40 degrees to 180 degrees?

200 X 8.3 X (180-40) + 5.8 square feet

5 X 8000 In computing the amount of water required for bathtubs it is customary to allow from 20 to 30 gallons per tub, and to consider that the tub may be used three or four times per hour as a maximum during the morning. This will vary a good deal, depending upon the character of the building. The above figures are based on apartment hotel practice.