Water Circulating Systems.—Every oil engine demands some form of water circulating system. The particular system most applicable to any plant depends upon many factors. The systems most used may be classified as: Thermo-syphon, Fresh Water, Closed Cooling Tower, and Tank and Tower Systems.
Thermo-syphon System.—The thermo-syphon principle of circulating water is generally adopted on small oil engines of 25 h.p. or less. It consists of one or more galvanized steel tanks that are connected to the engine water jacket by an inlet and discharge pipe. The water in the cylinder jacket be comes warm, and the weight of the heavier cold water causes it to raise up the pipe A and to flow into the cooling tank where its temperature is lowered by radiation and convection; see Fig. 336. The cold water in the bottom of the cooling tank flows through the inlet pipe B to displace the warm water. The dis charge pipe A must be provided with a riser, as shown, in order to prevent any air pockets and to allow all accumulations of steam or water vapor to escape. The advantage of this system lies in its simplicity and freedoth from trouble. Two precautions must be observed. First, the water level in the cooling tanks should be maintained at least 4 inches above the outlet of the discharge or hot-water pipe. If it falls below this outlet, the circulation will cease. Second, if the locality is subject to cold weather, the jacket should be drained. This calls for a three-way cock in the cold-water connection.
The average purchaser, since he has paid so much for his oil engine, is usually loathe to invest any great amount of money in his steel cooling tanks. The result is a cooling system much too small for the engine's requirements. It is customary to find tanks with a total capacity of around 20 gallons of water per engine horsepower, while the smallest value that should be con sidered is 50 gallons per engine horse-power. The warm water loses its heat by surface radiation and evaporation; little heat is lost through the tank walls. It then follows that the larger the diameter of the cooling tank the smaller need be the volume of water. This factor often dictates the use of a cypress tank with
a 3-foot stave. A tank of this kind costs less than the steel tanks of equal cooling capacity and will last much longer. The cypress tank can be mounted on a platform to bring its top above the engine's water discharge pipe. It can safely be from 5 to 10 feet above the ground level.
Fresh-water System.—When water can be secured within 30 or 40 feet of the ground surface, the most economical way to cool the engine is by the fresh-water system. A two-stroke cycle low-pressure engine loses more heat to the jacket than does the Diesel, and so a greater amount of water must be used. A fair value upon which to estimate the water requirements is 70 lbs. of water per b.h.p. per hour or 1 lb. per b.h.p. per minute. The power required to handle this water from a depth of 30 feet and through the jacket will not exceed one-half of 1 per cent. of the engine's rating.
If the water is over 16 feet from the surface, some form of power pumping head with an extended pump cylinder should be installed. The pump cylinder is best placed below the surface of the water. This arrangement keeps the pump primed at all times and also prevents the plunger rings from drying out when the engine is idle. The best cylinder is one fitted with brass poppet valves and with the pump plunger of brass with at least four leather rings, while the cylinder itself should be brass, or brass-lined. The working head can be belted and of almost any design. The well is best placed close to the engine in order to allow the pump to be belted direct from the engine shaft. The head need have no idle pulley, for frequently the belt will run on to this pulley and the pump will stop, Fig. 337.
If the water is within 16 feet, a belted horizontal force pump is by far the best that can be used. A centrifugal pump for this work is liable to lose its suction and, when starting, fre quently must be primed due to a leaky valve. The centrifugal pump for plants using low-pressure engines has no attraction other than its low price. No matter what pump is used a foot valve should be included.