Water in tanks is sometimes heated by a steam coil immersed in the water. This method of heating has the advantage of requiring no care whatever, and saves the labor, expense and dirt of an extra fire. When exhaust steam is available the cost of heating water by the method is practically nothing.

A steam coil can be placed in either a vertical or in a horizontal tank, the only requirements being that the pipe used in the coil be large enough to take care of the water of condensation, and that it have a slight fall from the top connection where the steam enters to the bottom outlet towards which the water of condensation drains.

In a vertical tank, Fig. 122, the steam coil is spiral and placed near the bottom. This type of coil is used principally in connection with kitchen ranges. Large size hot water tanks are usually placed in a horizontal position. Fig. 123 shows a method of placing a coil for high pressure steam inside of a hot-water tank. In this coil steam or the condensation travels through the entire length of coil. When exhaust steam is used, however, a shorter course should be provided to minimize the back pressure on the engines. A heating coil for

Fig. 122 exhaust steam is shown in Fig. 124. Steam coils for tanks may be made of copper, brass or iron pipe. Copper and brass pipes last longer than iron and transmit more heat to the water per square foot of heating service. For these reasons, either copper or brass coils are preferable to iron pipe coils. The size of steam coil in square feet required to heat a certain quantity of water in a given time, can be found by the following rule :

Rule

Multiply the weight of water in pounds by the number of degrees temperature Fahr. the water is to be raised, and divide the product by the coefficient of transmission times the difference between the temperature of the steam and the average temperature of the water.

Expressed as a formula: s=w r c (T-t) In which s=surface of copper or iron pipe in square feet w=weight in pounds of water to be heated r=rise in temperature of water t=average temperature of the water in contact with coils T=temperature of steam c=coefficient of transmission

The value of c for copper is 300 B. T. U. and for iron 200 B. T. U. transmitted per hour per square foot of surface for each degree difference between the temperature of the steam and the average temperature of water.

Fig. 123

Fig. 124

In computing the heating surface of copper or iron pipe in steam coils, the inner circumference of the pipe must be taken, as that is the real heating surface to which heat is applied. The average temperature of the water in contact with the coil is taken as the temperature of the water.

Example

How many square feet of heating surface will be required in a copper coil to heat 300 gallons of water per hour from 50 degrees to 200 degrees Fahr. with steam 15 pounds pressure?

Solution

300x8.3=2490 pounds of water to be heated 200°-50° = 150=rise in temperature of water 150°/ 2+50° =125°=average temperature of water 250°=temperature of steam at 15 pounds gauge pressure (Table XLIX).

250°-125°=difference between temperature of steam and average temperature of water. Substituting these values in the formula: s = (2490 X 150)/ {300 X (250-125)}= 9.9 square feet of coil. Answer.

Some convenient constants for steam coils that pro-duce approximations sufficiently accurate for most purposes follow. The values will be found safe:

W=gallonw water to heat per hour

W/10=square feet iron pipe coil required for exhaust steam W/15=square feet copper coil required for exhaust steam WX.07=square feet iron pipe coil for 5 pounds pressure steam WX.045=square feet copper pipe coil for 5 pounds pressure steam WX .05=square feet iron pipe coil for 25 pounds steam pressure WX.035=square feet copper pipe coil for 25 pounds steam pressure WX.04=square feet iron pipe coil for 50 pounds steam pressure WX.025=square feet copper pipe coil for 50 pounds steam pressure WX.03=square feet iron pipe coil for 75 pounds steam pressure WX.02=square feet copper pipe coil for 75 pounds steam pressure

Taking the foregoing example for comparison, the nearest value to 15 pounds steam is 25 pounds, and the coefficient for copper pipe at this temperature is .035. Hence, 300X.035=10.5 square feet. Answer. Heating Water by Steam in Contact - The quickest and most economical way to heat water with steam is to bring the steam into direct contact with the water. This method is used extensively to heat water in swimming pools, vats for industrial purposes, dish washing, etc., and is usually accomplished by forcing steam through a perforated pipe or steam nozzle located near the bottom of the tank and submerged by the water. When perforated pipes are used for this purpose they should be of brass or copper to prevent corrosion, and the combined area of the perforations should be at least eight times the area of pipe to equal it in capacity. Exhaust steam from pumps, engines or other apparatus that is liable to contain oil or grease, is not suitable for this purpose.

When steam is brought in contact with water in an open vessel steam bubbles are formed, rise toward the surface and collapse with a report. For this reason water is heated by steam in direct contact through perforated pipes only when noise is not objectionable. Noiseless Water Heaters - A steam nozzle for noiselessly heating water by steam in direct contact is shown in Fig. 125. This apparatus consists of an outward and upward discharging steam nozzle covered by a shield which has numerous openings for the admission of water, so that the jet takes the form of an inverted cone, discharging upwards.

Air, admitted through a small pipe, is drawn in by the jet, and by mixing with the steam prevents the sudden collapse of bubbles and the consequent noise which is such a great objection to heating by direct steam in the old way. A valve or cock on this air pipe regulates the air to' the quantity most desirable.

If water is to be heated to a less temperature than 165 degrees Fahr. the air pipe is not used, as the heater will operate noiselessly without it. If, however, the temperature of the water is to be raised above 165 degrees Fahr. an air pipe must be used. A pressure of air is not required in the air pipe when the pressure of steam is sufficient to draw air in by inspiration. The pressure of steam required for this purpose is proportioned to the depth of water above the heater in the tank, and cannot be less than those given in Table L:

Fig. 125

Table L - Pressures Of Steam For Heads Of Water

If water is to be heated to a greater temperature than 165 degrees Fahr. with less steam pressure than is called for in the foregoing table, air must be supplied under pressure, and both the air pressure and the steam pressure must equal in pounds the height in feet of water above the heater.

Stock sizes of this type of heater, with the manufacturers' ratings in B. T. U. per minute under different steam pressures, can be found in Table LI.

Table LI - Capacity Of Noiseless Water Heaters

To find the size of heater required to heat a certain quantity of water in a given time, first find the number of

B. T. U. required per minute and the pressure of steam and the size will be found in the table.

Example

100 cubic feet of water shall be heated from 60 to 180 degrees, or 120 degrees increase, in 30 minutes, with steam of 80 lbs. pressure.

Weight of 1 cubic foot of water, 62.5 pounds.

(62.5X100X120)/ 30 = 750,000/ 30 = 25,0000 heat units per minute.

Comparing this with table indicates the 1-inch steam pipe is the size required.

Another Example

200 gallons of water shall be heated from 30 degrees to 90, or 60 degrees increase, in six minutes by steam of 10 pounds pressure. Weight of 1 gallon, 8.3 pounds.

(8.3 X 2 00 X 60)/ 6 = 99.600/ 6 = 16,600 heat units per minute.

Comparing this with the table indicates that 1 1/2-inch steam pipe is the size required.