CHAPTER IV A. The Compression System (Part 1)

CHAPTER IV A. The Compression System While it is impossible to show in a single illustration all the details entering into the mechanical construction of a complete refrigerating system, the diagrammatic representation, Fig. 17, is more complete than the elementary diagrams already shown, and Fig. 17. — General Arrangement of Compression Kefrigerating System will suffice for the explanation of the cycle of operation of the compression' system. Expansion Side The cycle begins with liquid anhydrous ammonia conducted to the expansion coils through a liquid line and regulated by appro- priate expansion valves. The source of the liquid ammonia may be either a liquid receiver, which, as already shown, is one of the essential members of the system in practical operation; or it may be an ammonia shipping-drum from which ammonia is introduced into the system by the process of charging. In either case the first function of the refrigerant is to enter the expansion coils in the compartment to be refrigerated and there to evaporate (boiling at a temperature dependent on the suction or "back pressure" Digitized by V^OOQIC THE COMPRESSION SYSTEM 45 within the coils) and absorb heat from the surrounding objects. If the back pressure be 46 pounds gauge, or less, the temperature of the boiling liquid will be 32° Fahrenheit, the freezing point of water,^ or below, and the pipes containing the refrigerating medium at these temperatures will soon be covered with a coating of frost. If, on the other hand, the back pressure in the coils is above 46 poimds gauge, the temperature of the boiling liquid will be above 32° Fahrenheit, and no frost will be formed. Refrigeration will be produced whenever the temperature of the expansion coils is lower than that of the air in the cooler, regardless of whether frost is formed or not and, according to the amount of atmospheric humid- ity, uncongealed moisture may or may not be precipitated on the pipes. Having vaporized in the expansion coils, the ammonia vapor enters the return header which conveys it back to the suction side of the compressor. This return line is usually fitted with a "scale trap" constructed similarly to a simple steam separator. The function of this trap is to prevent any scale from the inside of the pipes, or other foreign substance, from entering and damaging the compressor. From the scale trap the gas passes through the suc- tion valves into the compressor. Compression Side On leaving the compressor the hot, high-pressure ammonia gas passes through the discharge valves and out into the two legs of the main discharge pipe, which come together in a "T'' just below the large hand-operated discharge valve at the left-hand side of the compressor cylinder. Leaving the discharge valve, the gas passes into the side of the pressure-tank head, and is given a spiral motion as it descends into the tank. The centrifugal force produced by this spiral motion is intended to aid in precipitating any entrained oil which the gas may hold in suspension against the outside of the tank. The gas then passes up through the hot gas line, through a ' check valve" (sometimes omitted) located a little above the level of the top of the condenser, and down into the header connecting the bottom pipes of the several stands of condensers. The loop is placed in the hot-gas line to prevent con- densing liquid from running back down into the pressure tank when the compressor is shut down. • The gas entering the bottom pipe of the condenser passes up through successive pipes while the water distributed over the top Digitized by V^OOQIC 46 ELEMENTARY MECHANICAL REFRIGERATION pipe trickles down, producing the desirable countercurrent cooling efifect, in which the hottest water encounters the hottest gas at the bottom and the coolest water the coolest gas at the top of the condenser. As fast as the ammonia is condensed in the pipes of the con- denser it is conducted away through smaller liquid-drip pipes con- nected with a common liquid header. The outlet from this header also rises to form a short "goose-neck" which is intended to keep the header always full of liquid and prevents gas from being drawn down to the tank through the liquid line. In trying to ascend through the colimin of descending liquid in a small liquid line, bubbles of gas ofifer no inconsiderable resistance to the passage of the liquid. The obvious remedy for this diflEiculty is the installa- tion of lines of liberal diameter. If the gas is carried down into the liquid tank it can escape by going up the equalizer line into the top of the condenser. The ammonia previously liquefied in the condenser under a pressure of from 135 to 200 pounds, according to the temperature of the cooling water, is conveyed first to the receiver. This consists of an appropriate cylindrical containing vessel which acts as a storage tank in which the liquefied ammonia is collected and from which it passes as required into the expansion coils. The flow of this high-pressure liquid into the expansion coils is regulated by expan- sion valves, which are virtually nothing more than convenient mechanical devices for accurately varying the opening through which the liquid ammonia must pass on its way to the expansion coils. Refrigeration Available in Expansion As stated, the word "expansion" has been erroneously applied to these valves and coils, because of the idea, also erro- neous, that the liquid ammonia vaporizes or expands immediately when the pressure is relieved as it passes the regulating valve and enters the cooling coils. As a matter of fact, before it is pos- sible for a pound of ammonia to change from the liquid to the gaseous state, it must be supplied with about 573 B.t.u. of heat. In practice, not all of the heat-absorbing capacity, or negative heat, of a pound of anhydrous ammonia available at 0° Fahren- heit can be utilized for useful cooling work, on account of the cooling work which must first be expended on the ammonia ♦Evaporation assumed to be under atmospheric pressure. Digitized by V^OOQIC THE COMPRESSION SYSTEM . 47 itself in order to reduce its temperature from that of the condenser to that of the cooler. This may be illustrated by a similar process in which water is the medium. The amount of heat that must be abstracted from one pound of water at 32° Fahrenheit in order to freeze it is 144 B.t.u. On this basis, a ton of ice would represent 288,000 B.t.u. of negative heat. In practice, the expenditure of this amount of cooling will not freeze a ton of water, because it must first be reduced from its natural temperature, or, in crystal- ice systems, from the temperature of the distilled water tank to 32° Fahrenheit. This involves a further expenditure of one nega- tive B.t.u. per pound per degree cooled. If the 573 B.t.u. were absorbed at the expansion valve, which its immediate vaporization assumes, there would be no further heat-absorbing capacity in the ammonia, and its introduction into the expansion coils would be useless. Besides the principal pipe circuit just described, the compressor is provided with a set of bypass connections for reversing its oper- ation so as to draw the ammonia from the condenser and discharge it into the expansion coils, as well as other so called "pump-out" lines through which ammonia may be pumped out of other parts of the system in case it becomes necessary, as when making repairs. Direct Expansion Cylinder Cooling In Fig. 17 is shown a small liquid line running from the liquid tank to the compressor cylinder. This line is provided with an ex- pansion valve through which ammonia may be admitted to the compressor, to prevent abnormal heating of the piston and pack- ing when starting up, or at any other time when the ammonia returning to the compressor is not suflBciently cold to insure satis- factory operating of the compressor. Another small pipe line connects the lubricating system on the compressor with the pres- sure tank. Through this line oil passing over with the discharged ammonia gas and separated out in the pressure tank may be blown back into the lubricating system. On entering this line the oil passes first through a small strainer which intercepts any scale or foreign substances that might otherwise return to the compressor and cut the valves or cylinder walls. Details of construction of the various parts of a compression system are too numerous to warrant an attempt to fully describe them here. Since the ammonia compressor is so important a mem- Digitized by V^OOQIC 48 ELEMENTARY MECHANICAL REFRIGERATION ber of the refrigerating system, however, a brief description setting forth its several characteristics will be in order. Types of Ammonia Compressors Ammonia compressors are divided into two principal classes, double-acting and single-acting. The former type is most com- monly horizontal, although frequently of vertical construction. The single-acting type is almost exclusively a vertical machine. Each type has its own followers among builders, and under certain conditions possesses some advantages over the other. While there is much variation in details of design among the various builders, Fig. 18. — Vertical Single-acting Ammonia Compressor — Section and Typical Elevation. the accompanying illustrations. Figs. 18 and 19, are the most characteristic of the general types. Fig. 20 shows a modification of the vertical single-acting machine which may be said to be typical of the compressor of small capacity. Vertical Single-acting Compressors The accompanying illustration. Fig. 18, giving a lateral ele- vation in section of a characteristic vertical single-acting ammonia Digitized by V^OOQIC THE COMPRESSION SYSTEM 49 compressor, shows the relative arrangement of compressor and engine cylinders as' well as the principal details of design. In this type of compressor the vaporized refrigerant enters the compressor near the bottom, passes up through the suction valve, located in the compressor piston, during the downstroke, and is compressed and discharged through the discharge valve, located in the safety head, during the upstroke of the piston. The compressor may be water jacketed or not. Popular preference, however, is in favor of the water jacket, and most machines are so built. Vertical compressors possess the advantage of requiring less floor space than horizontal machines and the disadvantage of being less accessible for repairs. The inaccessibility of suction valves located in compressor pistons is offset by the immistakable advantage offered by this type of machine in that these suction valves can be made of generous area, and the inertia of the valve tends to hasten its closing promptly as the piston reverses its direction of travel at the lower end of its stroke. This largely prevents gas from escaping from the cylinder during the time re- quired for the acting of the ordinary stationary valves unless they be heavily spring loaded, a condition which tends to prevent the back pressure in the cylinder from quite reaching the height of that in the suction line from the coolers. Inertia also tends to open the valve immediately as soon as the piston begins its downward stroke, giving full opportunity for the cylinder to fill. The spring below the suction valve should be of such strength as to aljnost balance the weight of the valve, so that the inertia of the valve may act promptly at each end of the stroke. Vertical single-acting compressors are usually provided with a "safety head ' which is normally held securely to its seat by strong springs, but which, in the event of abnormal quantities of liquid ammonia or broken parts entering the cylinder above the piston, is pushed back, compressing the springs and thereby saving the machine from the strains that would otherwise occur. One of the principal advantages claimed by the advocates of the single-acting compressor is that the use of the safety head allows the compressor pistons to be operated with less clearance than would be practicable in the case of double-acting machines, a condition which insures a more complete expulsion of the gas. Digitized by V^OOQIC 50 ELEMENTARY MECHANICAL REFRIGERATION The Horizontal Double-acting Machine Fig. 19 represents a horizontal half section of a characteristic horizontal double-acting ammonia compressor. The right-hand portion of the cut shows the exterior of the head end of the com- pressor-cylinder valve housings, suction and discharge connections and valves. The remaining portion of the cut shows the details of construction of the compressor cylinder, water jacket, piston, Fig 19. — Horizontal Double-acting Ammonia Compressor — Section and Typical Elevation. suction and discharge valves, double stuflSng box and means of lubricating the piston rod. The outer wall of the water jacket is formed by the main frame casting, which is bored and fitted with a working cylinder liner, consisting of a straight sleeve forced into place by hydraulic pressure and bored to the required size. The valves in this type of compressor are arranged radially to the hemi- spherical cyhnder heads. The piston rod is provided with a pri- mary stuffing box where it enters the compression cylinder. The packing in this box is tightened by a primary packing nut which carries a long sleeve, the other end of which. is provided with a Digitized by V^OOQIC THE COMPRESSION SYSTEM 51 secondary stuflSng box and packing nut. The main stufl^g box, containing the bulk of the packing, withstands the high pressure of the ammonia in the compressor cylinder. The smalt stuflSng box at the end of the sleeve is provided with suflScient packing to Ca Fig. 20. — Enclosed Crank Case Ammonia Compressor — Elevation in Section withstand the pressure of the oil circulated by the oil pump through the hollow sleeve surroimding the piston rod, in order to insure constant lubrication of and to maintain an oil seal on the main stuflBng box. The general appearance of the horizontal double-acting com- pressor just described is shown in the longitudinal elevation, just above the sectional view of the compressor cylinder. Inclosed Crank-case Compressors In addition to the two principal types of compressors previous- ly described, the inclosed-crank type is deserving of mention be- Digitized by V^OOQIC 62 ELEMENTARY MECHANICAL REFRIGERATION cause of the great number of such machiner of small capacity now being installed. Details of design of this type of compressor are even more varied than those of the machines already described, and it is difficult to point out a single design that can be said to be more characteristic of the type than another. In the illustration, Fig. 20, the refrigerant vapor enters the compressor cylinders through suction valves located in the cylinder head. Valves so located cannot be made of so liberal dimensions as those located in the compressor piston, and the assistance which inertia offers in the way of opening and closing suction valves located in the piston cannot be realized. To offset this disadvan- tage, oil from the crank case is much less likely to be carried over into the condenser and low-pressure side of the system. Machines of the inclosed type are especially adapted to use where little, or only inefficient, attendance is available. Less attention is possible in this type of machine, principally because of the design of the stuffing box and the main-bearing lubri- cation. The crank case being filled with oil to the center of the crank shaft, and the outboard bearing being usually ring oiling or provided with a compression grease cup, little attention to lubrication is necessary. There are no reciprocating piston rods to pack, the only stuffing box required being on the crank shaft, where it is always well lubricated and not subject to such extremes of temperature as are the pistons in other types of ma- chines. This type of compressor is peculiarly well adapted for use in connection with automatic systems. B. The Absorption Refrigerating System It has already been pointed out under the subject of "The Development of Mechanical Refrigeration'.' that, while Carr6 invented the absorption machine, the way was paved by the earlier experiments of Faraday, who discovered that silver chloride pos- sessed the property of absorbing ammonia. Faraday is said to have experimented with silver chloride saturated with ammonia in a closed glass tube, one end of which was immersed in a freezing mixture of ice and salt, while the other end was heated. The ammonia gas driven off from the silver chloride in the hot end of the tube was liquefied in the cold end of the tube under the pres- sure generated by the heat. See Fig. 21. Faraday discovered that if he reversed the tube so that the Digitized by V^OOQIC THE ABSORPTION SYSTEM 53 end containing the silver chloride from which the ammonia had been driven oflf was immersed in the freezing mixture, the liquefied Lioaefled Ammoida' Fig. 21. — ^Faraday's Elementary Absorption Machine ammonia in the other end of the tube would boil, producing a remarkably low temperature. The cold vaporized ammonia was absorbed by the silver chloride, so that if the tube were occasion- ally reversed the device would be made to traverse the cycle of an elementary, intermittent absorption machine. A modem commercial absorption machine consists primarily of five parts, three of which are also present in the compression machine. This is shown in Fig. 22, which is a diagrammatic rep- resentation of an elementary absorption and compression machine having their condensers and complete expansion sides in common. In the compression system it has been shown how the low- pressure cold gas returning from the expansion coils enters the suction end of the compressor cylinder, and how, after it has been transferred to the compression end of the cylinder, it is compressed and passed over into the condenser. Reference to the figure will show that in the absorption machine the compressor cylinder is replaced by an absorber, a liquid pmnp and a generator. In the absorption system the gas, returning from the expansion coils, enters the absorber (corresponding to the suction end of the com- pressor), is transferred to the generator (corresponding