The backs and sides of flues in exposed walls should be covered with non-conducting material.

Flue Velocities. The flue velocities will be somewhat lower than with steam heating, because of the lower temperature of the air. Reasonable allowance would be 250, 350, 400, and 450 feet per minute for the first, second, third, and fourth floors respectively.

Heating Water. The size of heater or steam coil necessary to heat water may be very readily determined on the heat-unit basis, if one knows the volume of water to be heated, the number of degrees its temperature is to be raised, and the time during which the heating must be done.

For example, what size of heater would be required to heat 300 gallons of water in 6 hours from 60° to 160°?

In one hour 50 gals. would be heated 100° F.; and since one gal. weighs 83 lbs., 50 × 8 × 100 = 41,667 heat units would be required.

Small heaters may be counted on to transmit to the water about 7000 heat units per pound of coal burned. The rate of combustion should be assumed to be from 3 to 6 pounds per square foot of grate per hour, according to the amount of attendance it is convenient to give.

With a 4-pound rate, 28,000 heat units would be furnished per square foot of grate surface per hour for heating the water. Therefore the heat units per hour necessary to raise the temperature of the water— viz., 41,667— divided by 28,000, gives the number of square feet of grate surface required, which is equal to about 11⁄2 corresponding to a diameter of 16 inches.

To determine the size of steam boiler and coil required to heat a large volume of water in a tank, proceed as follows: Take, for example, a 24,000-gallon tank, the water in which is to be heated from 45° to 75° in 10 hours. Now 24,000 gals. X 83 pounds X 30° rise in temperature = 6,000,000 heat units, or 600,000 heat units per hour.

Assuming 8000 heat units to be utilized per pound of coal burned at, say, a 7-pound rate, one square foot of grate will supply 60,000 heat units per hour; hence, 10 square feet of grate surface will be required.

There will, however, be a certain loss of heat from the tank by radiation, conduction, and evaporation; therefore, not less than, say, 12 square feet should be used in order to provide a reasonable margin. As to the size of steam coil required, a square foot of pipe surface

surrounded by circulating water may be assumed to transmit to the water not far from 100 heat units per degree difference in temperature between the steam and the water in contact with the pipe.

Assume the steam temperature to be 230°, corresponding to a *rifle more than 5 pounds gauge pressure. When the water in the tank s cold, the condensation of steam in the coil will be much more rapid than when the surrounding water becomes warmer. The average temperature of the water during the 10-hour period is 60°; but the water leaving the pipe and in contact with the upper half of its surface is at a considerably higher temperature than the main body of water in the tank; therefore, with natural circulation, it is well to make ample allowance for the effect of this skin of warm water surrounding the steam coils, and to assume that they will not give off more than as much heat as that corresponding to the difference in temperature between the steam and the water in the tank, based on 100 heat units per degree difference as stated above.

In other words, allow only 663-or, in round numbers, 70-heat units per hour per degree difference in temperature between the steam and the water in the tank.

=

If the difference in temperature is 230°-70° – 160°, on the basis stated, one square foot of coil would give off 70 × 160 = 11,200 heat units per square foot per hour; and since 600,000 heat units must be supplied to the water, a 53-square foot coil or slightly larger would be required, equal to about 122 ft. of 14-inch pipe.

SANITARY APPLIANCES

TRAPS

Object of Traps. There are endless varieties of traps marketed, and each shape and type may have some particular advantage for the purpose it is used.

Broadly, we can define a trap as a device holding a quantity of water and having an upper portion which dips into the water and forms the seal.

The seal should be at least 11⁄2 inches in depth for traps up to 2 inches in diameter and may be 2 inches deep for water closets and the large traps.

The object of the trap is to cut off from the interior of the building, by means of the water seal, the foul gases which are ever present in soil and waste pipes. To accomplish this object satisfactorily, a trap must be placed immediately under every, sanitary fixture and the water seal of such trap must be kept intact, otherwise the occupants of a house may be placed in a position of false security, thinking no foul gases can pass the trap, yet the seal may be broken and dense volumes of obnoxious gases enter into and pollute the entire building.

Characteristics of Good Trap. A good trap embraces the following features: It should be free from all sharp angles and corners in which foul matter may collect or, in other words, it must be perfectly self-cleansing. It must have a good depth of water seal, yet contain only a minimum quantity of water. It must be constructed of a material which will not be acted upon by sewage or foul gases in the trap and that can be joined readily to both the sanitary fixture and the waste pipe.

Methods by Which Water Seal May Be Broken. The seal of a trap may be broken by five distinct methods, namely, evaporation, momentum, capillary attraction, leakage, and siphonage.

Evaporation. The trap seal is broken by evaporation by the drying-up of the water seal in the trap, due to the passing away of the water in the form of invisible vapor. This process