Table XXIX is prepared especially for determining the size of mains for different conditions, and is used as follows: Example. Suppose that a heating system requires the circulation of 480 gallons of water per minute through a circuit main 600 feet in length. The pipe contains 12 long-turn elbows and 1 swing check-valve. What diameter of main should be used ? Assuming a velocity of 480 feet per minute as a trial velocity, we follow along the line corresponding to that velocity, and find that a 5-inch pipe will deliver the required volume of water under a head of 4.9 feet for each 100 feet length of run.
The actual length of the main, including the equivalent of the fittings as additional length, is 600 + (12 X 9) + 50 = 758 feet; hence the total head required is 4.9 X 7.58 = 37 feet. As both the assumed velocity and the necessary head come within practicable limits, this is the size of pipe which would probably be used. If it were desired to reduce the power for running the pump, the size of main could be increased. That is, Table XXIX shows that a 6-inch pipe would deliver the same volume of water with a friction head of only about 2 feet per 100 feet in length, or a total head of 2 X 7.58 = 15 feet.
The risers in the circuit system are usually made the same size as for gravity work. With double mains, as shown in Fig. 109, they may be somewhat smaller, a reduction of one size for diameters over 11 inches being common The branches connecting the risers with the mains may be pro portioned from the combined areas of the risers. When the branches are of considerable size, the diameter may be computed from the available head and volume of water to be moved.
Pumps. Centrifugal pumps are usually employed in connection with forced hot-water circulation, in preference to pumps of the piston or plunger type. They are simple in construction, having no valves, produce a continuous flow of water, and, for the low heads against which they are operated, have a good efficiency. A pump of this type, with a direct-connected engine, is shown in Fig. 111.
Under ordinary conditions the efficiency of a centrifugal pump falls off considerably for heads above 30 or 35 feet; but special high speed pumps are constructed which work with a good efficiency against 500 feet or more.
Under favorable conditions an efficiency of 60 to 70 per cent is often obtained; but for hot-water circulation it is more common to assume an efficiency of about 50 per cent for the average case.
The horse-power required for driving a pump is given by the following formula: Centrifugal pumps are made in many sizes and with varying proportions, to meet the different requirements of capacity and head.
Heaters. If the water is heated in a boiler, any good form may be used, the same as for gravity work. In case tubular boilers are used, the entire shell may be filled with tubes, as no steam space is required.
In order to prevent the water from passing in a direct line from the inlet to the outlet, a series of baffle-plates should be used to bring it in contact with all parts of the heating surface.
When steam is used for heating the water, it is customary to employ a closed feed-water heater with the steam on the inside of the tubes and the water on the outside.
Any good form of heater can be used for this purpose by providing it with steam connections of sufficient size. In the ordinary form of heater, the feed-water flows through the tubes, and the connections are therefore small, making it necessary to substitute special nozzles of large size when used in the manner here described.
When computing the required amount of heating surface in the tubes of a heater, it is customary to assume an efficiency of about 200 B. T. U. per square foot of surface per hour, per degree difference in temperature between the water and steam.
It is usual to circulate the water at a somewhat higher tempera ture in systems of this kind, and a maximum initial temperature of 200 degrees, with a drop of 40 degrees in the heating system, may be used in computing the size of heater. If exhaust steam is used at atmospheric pressure, there will be a difference of 212 — 180 = 32 degrees, between the average temperature of the water and the steam, giving an efficiency of 200 X 32 = 6,400 B. T. U. per square foot of heating surface.
From this it is evident that 6,400 _ 170 = 38 square feet of direct radiating surface, or 6,400 - 400 = 16 square feet of indirect, may be supplied from each square foot of tube surface in the heater.
Example. A building having 6,000 square feet of direct, and 2,000 square feet of indirect radiation, is to be warmed by hot water under forced circulation. Steam at atmospheric pressure is to be used for heating the water. How many square feet of heating surface should the heater contain ? 6,000 - 38 = 158; and 2,000 16 = 125; therefore, 158 + 125 = 283 square feet, the area of heating surface called for.
When the exhaust steam is not sufficient for the require ments, an auxiliary live steam heater is used in connection with it.
1. A building contains 10,000 square feet of direct radiation and 4,000 square feet of indirect radiation. H o w many gallons of water must be circulated through the mains per ute, allowing a drop in temperature of 40 degrees? Acs. 165 gal.
2. In the above example, what size of main should be used, assuming the circuit to be 300 feet in length and to contain ten long turn elbows? The friction head is not to exceed 10 ft., and the velocity of flow not to exceed 300 feet per minute. Arcs. 4-inch.
3. What horse-power will be required to drive a centrifugal pump delivering 400 gallons per minute against a friction head of 40 feet, assuming an efficiency of 50 per cent for the pump?