The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California
Hot Water Suply
24. Heating Surface of Steam-Coil Heaters.—Steam coils in tanks for heating water often use exhaust steam from engines; a live-steam connection is then usually provided, by which live steam can be turned on in case the engines are shut down. The amount of coil heating surface used in large tanks is about 1 square foot for every 15 gallons of capacity of the tank; or, what is equivalent, 1 linear foot of 1-inch pipe for every 5 gallons in the tank.
The approximate theoretical heating surface required in a steam coil to heat a certain quantity of water in a given time can be found by dividing the total B. t. u. required to heat the water by the B. t. u. given off in a given time, per square foot of heating surface, in the coil. It has been found in practice that 1 square foot of heating surface in a steam coil made of pipe about 1/8 inch thick will transmit in 1 hour 300 B. t. u. if the pipe is of copper, and 200 B. t. u. if the pipe is of wrought iron, for each degree of temperature difference between that of the steam and the surrounding water. From these considerations the following rule has been derived, in which an allowance of 25 per cent has been made for contingencies:
Rule.—To find the heating surface required to heat a given weight of water per hour by a steam coil, multiply 1.25 times the weight of water, in pounds per hour, by the required rise in temperature of the water, in degrees Fahrenheit. Divide the product by the difference between the temperature of the steam and the average temperature of the heated water multiplied by 300 far copper pipe and by 200 for wrought-iron pipe. The quotient will be the required heating surface, in square feet.
where S = heating surface of steam coil, in square feet;
W = weight of water, in pounds per hour;
t = temperature rise of water;
f = 300 for copper pipe, and 200 for wrought-iron pipe;
T= temperature of the steam;
ta= average temperature of the water.
It should be noted that by the term average temperature of the water is meant one-half the sum of the temperatures at entering and leaving the heater.
EXAMPLE.—How many square feet of heating surface will be required in a copper steam coil to heat 200 United States gallons of water per hour from 50° to 190° F., the steam having a temperature of 240° F. ?
