TYPES OF SYSTEMS. PUMPS. WATER PURIFICATION Distribution of Heat Losses.—Various experiments on the subject of Diesel heat losses check very closely as to final results. The consensus of opinion is that the total heat evolved in the combustion of a charge of oil in the engine cylinder is absorbed in doing work and in various losses in the engine at the following percentages: Heat generated in the cylinder 100 Heat converted into work 30 Heat lost in engine friction 6 Heat lost in the exhaust gases 28 Heat absorbed by the cooling water 34 Heat lost by radiation, etc 2 Figure 164 covers the losses at various loads. These values are the result of a number of experiments on various engines. If the work done is 30 per cent. of the heat generated, then the engine will 'consume 8470 B.t.u. per b.h.p. The cooling water must be of a quantity sufficient to absorb 34 per cent. of the amount or 2879 B.t.u., in round numbers 3000 B.t.u. per hour.
Cooling Water Required.—The calculations necessary to em ploy in determining the amount of cooling water required for a Diesel are quite simple. As an example of the maximum amount that could be used, the intake water temperature at the jacket entrance can be taken as 90°. This is at least higher than normal, even with a cooling pond. The engine discharge water temperature can be taken as 140° Fahrenheit, which is 20° below the value most successfully used. Then the rise in temperature will be 50°; consequently each pound of water will absorb 50 B.t.u. The water per h.p. hour will be 3000 = 60 lbs.
50 This expressed in the form of an equation appears as XH W — 100 (ti — t2) where W = weight of water required per h.p. hour.
X = percentage of heat absorbed by the water. H = total heat supplied to the engine.
t1 = discharge water temperature deg. Fahr.
= intake water temperature deg. Fahr.
The inlet and discharge temperatures given may not check with those observed in any particular installation, nevertheless the temperature range is approximately correct; it is this factor that is important. Table I gives the temperatures and quantities of water passing through Diesel engines. This table is the result
of a test on three McIntosh & Seymour 500 h.p. Diesels in stalled by the Texas Light and Power Co. at Paris, Texas. Since these engines were developing close to 500 h.p. each, this value may well be assumed in computing the water rate per b.h.p. With this assumption the water per b.h.p. per minute was lbs. or 75 lbs. per b.h.p. per hour. This considerably ex ceeded the value computed above, which can be explained on the grounds that both the exhaust head and air compressor were maintained at a very low temperature. In ordinary operation these engines tested do not carry the exhaust header tempera ture lower than 130 to 135° Fahrenheit.
The cooling system should then be based on a pumping and cooling tower capacity of at least 60 lbs. per b.h.p. per hour of installed engine rating.
Types of Cooling Systems. Closed System.—Two designs of cooling systems are in quite general use. Figure 165outlines the closed system often found in small installations. With this design the water from the engine jacket is discharged through a distributing pipe D on a cooling tower C. The water drips down through the tower and is stored in the sump A, from which point it is drawn by the circulating pump B and forced through the engine jacket and out the discharge again. When this system is adapted to a horizontal engine, the discharge line should rise vertically from the engine until it is above the cooling tower dis tributing pipe. With such a layout it is necessary to place a vent pipe in the discharge line, immediately above the engine. This prevents the formation of steam or air pockets in the jacket with consequent overheating of the cylinder.
While the figure embodies a centrifugal circulating sump, this design of pump is one that should never be employed with a closed system. The objection to this pump is based on the liability of losing the suction. This is of frequent occurrence when the drive belt becomes dirty or oily, and in the best-kept plants a belt at times breaks.