power and horse-power hour?- Answer, horse-power is the capacity of the engine or the work it can do if it runs a specified time. The horse-power of an engine remains the same, no matter how long it runs. One horse-power hour is the output of a 1 horsepower engine running 1 hour. The output of an engine in horse-power hours depends both on its horsepower and on the length of time it runs. Power is measured in horse-power. Output or work done may be measured in horse-power hours.

Q. What is the difference between a kilowatt and a kilowatt hour? Answer, a kilowatt is 13 horsepower. A kilowatt is a measure of power. A kilowatt hour is the energy delivered or work done by 1 kilowatt running for 1 hour.

Q. If power at the rate 1 kilowatt is delivered for 10 hours, how many kilowatt hours of energy are delivered? Answer, 10 kilowatt hours.

Q. If 10 kilowatts are delivered for 1 hour, how many kilowatt hours are delivered? Answer, 10 kilowatt hours; 10 kilowatts for 1 hour will do the same amount of work as 1 kilowatt for 10 hours.

Q. How can a 50 K. W. machine be made to have an output of 500 K. W. H.? Answer, simply run it 10 hours.

Q. Why do you pay for electricity in kilowatt hours and not in kilowatts? Answer, because if we paid for a kilowatt of power we could use that power for any length of time. In that case we would pay the same, no matter how long we used the power. One man might get a hundred times as much electrical energy as another man for the same money. If you hire a team to do a job of work you pay for the length of time the team works.

Q. Why do we speak of the capacity of a plant in K. W. and not in K. W. H.? Answer, simply because

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Standard Rule for Figuring Steam-heating Systems.

Q. 1. By what standard rule can a low-pressure, direct steam-heating system be figured? Such as to the amount of square feet of radiation? Size of steam mains and users? Referring particularly to a one-pipe gravity-return system, such as is being put in most of the combined office and manufacturing buildings of to-day?

Q. 2. Has there ever been any special rule established as a standard by the American Society of Mechanical Engineers?

Q. 3. If any, please state what rule is generally held as a standard.

Answers.

A. 1. In designing a system of steam heating, there is such a variety of conditions to be met that it is difficult to reduce the necessary calculations to simple rules or formulas which will give reliable results in all cases.

For that reason the American Society of Heating and Ventilating Engineers has never adopted any rule or formula as standard for such calculations. However, there are certain well-established rules and formulas which give results that agree closely with standard American practice, and which are widely used by heating and ventilating engineers.

When a system of steam heating is to be designed for a given building the first, and perhaps the most important, calculation is the determination of the amount of heat which the building will lose per hour. Heat is lost by radiation from exposed wall and glass surfaces, and by the escape of warm air from the building through doors, windows and ventilating ducts. When this heat loss has been determined, a heating system must be designed to supply that amount of heat.

Professor Carpenter's Rule.

A. 2. Carpenter gives the following formula for calculating this heat loss, which gives variable results: the

=

H (G+.25 W + .02 n C) tx, where H total heat loss from the building in B. t. u. per hour; G the number of square feet of exposed glass surface; W the number of square feet of exposed wall surface; n = the number of times per hour that the air will be changed by leakage at doors and windows and by ventilation; C the volume of air in the building in cubic feet; tx desired difference in temperature between building and outside air in F.°, usually taken at 70°.

This formula gives reliable results for a room or building having a south exposure. For an east exposure add 10 per cent to the area of exposed wall and glass; for a west exposure add 20 per cent, and for a north exposure add 30 per cent. The formula is based on a wall thickness of 16 inches, but experience shows that a wall somewhat thicker or thinner than 16 inches will differ but slightly in the rate of heat transmission.

Values of n may be taken as follows: Stores, first floor, n 2 to 3; second floor, n = 11⁄2 to 2. Offices, first floor, n 2 to 22; second floor and above, n 11⁄2 to 2.

Public assembly rooms, n = 34 to 2. Large rooms with small exposure, n 1⁄2 to 1. Having calculated the total heat loss per hour from the building, the next step is to determine the number of square feet of radiation required to supply this amount of heat.

In a low-pressure system of steam heating (2 to 5 pounds gauge) the temperature of the steam in the radiator is assumed as 220° F. An ordinary cast-iron radiator will give off heat at the rate of 1.7 B. t. u. per