For gravity-return systems with but little difference in level between the water-line in the boiler and the return connection of the heater, the pipe sizes should be as follows: PIPE SIZES FOR HEATERS WITH GRAVITY RETURN.

In proportioning heaters to condense the exhaust from main engines, not less than 75 lin. ft. of 1" pipe per H. P. should be provided. The supply connections should not be smaller than 3 in. up to 30 H. P., 4 in. for 30 to 50 H. P., 5 in. for 50 to 80 H. P., 6 in. for 80 to 120 H. P., 7 in. for 120 to 160 H. P., and 8 in. for 160 to 220 H. P.

DUCTS AND FLUES.

Theoretical Area of Ducts and Flues.-The size of the ducts from the fan or heater to vertical flues, as well as the size of the latter, depend on the velocity with which the air flows through them.

With the blow-through type of fan-heating system, the main air duct, directly on leaving the mouth or outlet of the heater, is expanded so as to make its area from 25 to 50 per cent. greater than that of the fan outlet, in order to reduce the velocity of the air and to insure a large volume with the least possible resistance.

DUCT AND FLUE SIZE FACTORS BASED ON

By using the above table, the area of ducts and flues, in square inches, neglecting friction, is found by multiplying the number of occupants by the tabulated factors corresponding to the amount of air to be supplied, per hour, at the desired velocity in feet per minute.

The factors given in the table were obtained by means of the formula

in which A = area of duct or flue, in square inches; S air supply, in cubic feet per hour per person; number of occupants;

N

The factors given are for conditions under which N = 1. For example, S = 1,800, N 1, V = 900; then,

A =

2.4 X 1,800 X 1

900

4.8, the tabulated factor.

The required coefficient for any velocity and air supply not given in the table may be found easily by applying the formula given.

When the air velocity in ducts and flues in feet per second, the number of air changes per hour, and the size of the room are given, the required duct or flue area in square inches, friction neglected, can be found by multiplying the factor corresponding to the air velocity and the number of air changes taken from the table "Duct and Flue Size Factors Based on Air Changes," by the contents of the room in cubic feet.

10

15

.0080 .0120 .0160 .0200
.0053 .0080 .0106 .0133 .0160

20

25

30

.0240 .0280 .0320 .0360 .0400 .0186 .0040 .0060 .0080 .0100 .0120 .0140 .0032.0048 .0064 .0080 .0096 .0112 .0026 .0040 .0053 .0066 .0080 .0093 .0106 35 .0023 .0034 .0046 .0057 .0068 .0080 .0091 .0020 .0030 .0040 .0050 .0060 .0070 .0080 45 .0018 .0026 .0035 .0044 .0053 .0062 .0071 50 .0017 .0024 .0032 .0040 .0048

.0213

.0240.0266

.0160

.0180.0200

The B. F. Sturtevant Company presents the preceding table for obtaining the flue area, in square inches, when the number of minutes required for one air change, the velocity of air desired in the flue in feet per minute, and the size of the room are given. To apply the table, divide the contents of the room, in cubic feet, by 1,000, and multiply the quotient by the factor in the table corresponding to the time of air change and the desired air velocity.

The number of round branch pipes that have the same air-carrying capacity as a round main blast pipe of a given diameter is given in the table "Number of Branches Supplied by Main Blast Pipes." In this table, allowance has been made for the friction of pipes smaller in diameter than the main blast pipe. Thus, sixteen 3" pipes will have the same crosssectional area as one 12" pipe; owing to the excessive frictional resistance of the small pipes, thirty-two pipes 3 in. in diameter will, in practice, as shown by the table, have the same carrying capacity as one 12" pipe. In some cases, the number representing the number of branch pipes equivalent to a given main pipe is a whole number and a fraction; thus, 5.7 two-inch pipes are given as equivalent to one 4" pipe. In practice, the nearest whole number of pipes must be selected, as a fractional number of pipes is an impossibility.

Allowance for Frictional Resistance.-To counteract the effect of drop in pressure and reduction in velocity due to frictional resistance when air is carried through piping of different lengths, it is necessary to increase the area of the pipe according to the distance to which the air is to be transmitted. In other words, to supply a given amount of air at a distance from the fan, the area of the pipe should be increased by multiplying the area of the outlet through which the given quantity of air would be discharged at the given initial velocity, by the factors given in the table "Factors for Duct and Flue Area Increase." For example, it is desired to supply 2,000 cu. ft. of air per min. at a distance of 360 ft. from the fan, the initial velocity at the latter being 3,670 ft. per min. at 1" pressure, corresponding to the delivery capacity of a 10" pipe. The area of a 10" pipe being 78.54 sq. in., and the area