With this fresh-water system it is essential that a riser pipe be installed with a small tank or barrel at its top and with a check valve between the barrel and the pump, as well as a cock in the engine line. The barrel must be placed at a height sufficient to prevent it from overflowing. This storage will provide water for running the engine until the pump begins to pick up its suc tion when started up at the commencement of the day's run. In a pumping plant, the engine jacket can be connected to the pump discharge by a by-pass line. By placing a gate valve in the main pump discharge, between the engine jacket connec tions, the flow of water through the engine can be adjusted to suit conditions. The amount of water that flows through the jacket is a small percentage of the pump's capacity; consequently the rise in temperature of the water discharge is not noticeable and forms no objection to the use of the water for drinking or general waterworks purposes. If the discharge head on the pump exceeds 75 feet, this method of handling the cooling water is open to objection since too great a pressure is placed on the thin jacket walls with a likelihood of fracturing the jacket. In in valve in this line and to allow the water flowing through the jacket to run into the sewer or drainage ditch, or back into the well if water be scarce.
Tank and Tower System.—The ideal method of coping with the circulating water problem, where the water is not heavy with mineral, is by means of the tank and tower plan. In low-pressure engines, since they are usually but moderate in size, the storage tank need not be as large as with a Diesel engine. A tank 6 feet in diameter with a 5 foot stave is ample for a 50 h.p. engine while a 100 h.p. engine should be supplied with an 8X8 ft. tank. The tank should be mounted on a wooden tower at least 20 feet high; the height depends on the surrounding buildings. On top of the tank are placed the cooling screens. The cooling apparatus may be made of wooden slats in the form of a pyramid. Any ordinary mechanic can build this, and it is as efficient as the more expensive designs. Conical sheet pans superimposed on each other, quite like in steam condenser prac tice, are used in many plants. The objection to this cooling tower is the rapid corrosion of the galvanized sheet-steel pans.
The tank and tower system includes a sump to which the engine discharge water flows. Since a positive supply of water is obtained by the overhead storage, the use of a centrifugal pump to lift the water from the sump to the tower is permissible. This pump can be placed close to the engine and belted direct from the shaft. If the sump can be located so that the water level is within 2 feet of the engine room floor, the pump can be placed in a small pit by the engine and the intake line will then be under a slight head at all times. This eliminates the trouble of losing the pump suction. The pressure line from the tower to the en gine should be equipped with a regulating cock, while the tank should have a float with a bell-ringing attachment to guard against the danger of the supply giving out without warning to the operator. In this system the discharge line from the cylinder
jacket is usually provided with a funnel so that the flow of water can be in plain view at all times.
The Closed Circulating this design the water is forced, by a pump, through the engine jacket and out over a cooling tower. From the cooling tower it drops into a sump underneath, fiona which the pump draws it again. When the engine is 50 h.p. or less, a tower such as that outlined in Fig. 338 is cheap to build, satisfactory in operation and has a fair life. The cooling water is brought from the engine and dis charged at a height sufficient to allow it to flow over the top of the tower. The tower supports a number of pipes branch ing from the main 3-inch discharge header, and each branch has a series of h-inch holes drilled along its top surface. A rectangular sheet of 5-mesh wire netting is suspended from each pipe. The water issues from the holes in the pipes and runs down the vertical netting, being cooled to the temperature of the surrounding air before it strikes the sump. This sump, or storage tank, may be either a concrete pit or a cypress tank above ground. The use of a pit allows the tower to be lower and the engine jacket pres sure head less.
Figure 332 outlines a very excellent cooling system. In this design the circulating water pump is of the plunger type and is an integral part of the engine. One very important detail is included—a drain valve on the suction line. In localities where r , there is danger of frost, the lines should be drained every cold night. One of the greatest items of expense in engine operation in the Northern climates is the replacement of fractured parts due to freezing. This type of tower is quite suitable for instal lations of 150 h.p. or less. In the event a plant possesses more than this capacity, the cooling systems discussed in Chapter XIII will be found adaptable for low-pressure engines as well.
Because of the danger resulting from pump failure the over head tank system is by far the best. In case the first cost is considered too high to permit of its use, and the closed system is adopted, the circulating pump should be driven direct from the engine either by a silent chain or a linkage. In no case is a belted pump good practice where the closed system is used. The plunger type pump is more suitable for a circulating system than is a centrifugal pump even though hundreds of the latter are in such service. The centrifugal pump must be primed on starting. Frequently the suction is temporarily stopped by leaves or the like. If extra care is not exercised, the pump may lose its suc tion and fail to maintain the flow of cooling water.
In operation it is quite often a question as to the temperature the discharge water should have. It is self-evident that the engine's efficiency will improve with an increase in cooling water temperature. With heavy oils 160° Fahrenheit is not too hot, while with high-gravity distillate or kerosene 125° is as high as can be used successfully.