Document Jb8ZoqG9k5nO07w2kXVrVKka

836 CHAPTER 37 ,, 1954. Guide, do nottepen until the piston has completed.part of its stroke. Hence, the volume of fresh vapor introduced into, the .compressor per stroke is less than the;volume, sweph by the piston. The-ratio of actual volume of fresh gas to swept Volume is, by definition, the clearance volumetric eflk ciency,1 CVE. In equation form, i ... (CVE) - 100 - (8) where ' .OJVE'=.clearance volumetric efficiency:' : >-- - Vo clearance, percent of volume swept by piston, .which is contained in .spaces " `.at endof cylinder when piston is at end of stroke (clearance includes valve spaces, etc.). -U:- inn. . m: :. r = specific volume of gas at compressor inlet.' n ; : d specific volume of'gas at compressor discharge:n .. . ; Values of v, and vd can be obtained directly or by calculation from the tables of properties of refrigerants. ` In addition to clearance, there are several other factors which tend to reduce the volumetric efficiency. ,, The suction gases from the evaporator are heated and expanded upon contact with the hot cylinder walls during the suction stroke. Thisvresults in a reduction of the actual charge drawn into the cylinder. Wire-drawing through the suction and discharge valves reduces the suction pressure in the cylinder below that in the evaporator, and increases the discharge pressure above that in the condenser. Leakage of gases around the pistons also decreases the volumetric efficiency. The total volumetric efficiency (TVE). includes all of these factors and is reliably obtained only by .laboratory measurements.,. It, is too difficult to predict the effects of these factors to any degree of accuracy comparable to actual tests. ; ... .,i . . .. ,,.| Complex Refrigeration Cycles r, , , . The, preceding, sections, have , dealt only with, refrigeration systems in which .there is,but ;one evaporator; compression is, accomplished through but,a single stage, and expansion proceeds thro.ugh a single,expansion valve., In large systems .or in low temperature systems in which the compression ratio is' highj the compression process can be carried .out' in stages, with the refrigerant' passing through several, cylinders arranged for operation. 'in series. The therinodynafnic advantage of such, compound compressiop arises'from the fact that intercoolers. can.be placed between the stages of; compression to extract heat from the vapor, and. thereby ckfidd the overall, compression process to approach more closely the ideal condition of iso thermal compression. Essentially, suchiinlercopiers.serye the. same .pur^ pose as a cooling jacket, but with greater effectiveness because of the more satisfactory heat transfer conditions. , . i In the simple'saturation cycle the saturated liquid entering"the ejspansion valve commences to vaporize as soon as: its pressure starts'to drop.. The vapor produced during the expansion process has ri'o further use. iri temtf of refrigerating effect, since it has already picked up'its latent heat of vaporization as a result of heat which it has extracted from the iinvaporizw residue. Thus the instant such vapor forms, its usefulness is at an end,' and to allow such material to undergo a further drop in pressure is uneconomical- With `compound compression,' there is at least one intermediate pressure Refrigeration 837 ;at. which flash vapor can be .extracted. In such .cases! several expansion valves can be utilized with all ,of the refrigerant from .the condenser passed through a first expansion valve to the higher suction pressure,and-the flash vapor, then extracted and returned to the -condenser, through the: high compression stage, , The remaining, .refrigerant pan . then pass through a second expansion.valvewhere the pressure is dropped-to that corresponding to the low-pressure evaporator; ..The.number of, expansion valves, is limited, by the number of stages of compression., , i , - i: . Further cycle.complications may arise if, more than,one. evaporator is to be, operated with a single compressor, and particularly; if the pressures in these evaporators are to differ. The, most common solution, is, to operate the compressor at the suction - pressure of the lowest pressure, evaporator, and to equip all,other evaporators, with, back-pressure regulating valves or throttling devices between the evaporator and the compressor, suction,:,,,.; The Air Cycle System Fundamentally, the air cycle1 is essentially the same as the vapor cycle. Compression is accomplished by a reciprocating qr; centrifugal Compressor, and, since there is no change of phase of the refrigerant upon expansion, an air cooler replaces the condenser", and-a refrigerator, the evaporator. Although some cooling >yould result from the expansion of the gas through an ordinary expansion valve, a much greater drop in air. temperature is accomplished if the expansion is controlled to approach the isentropic by replacing the valve with an expansion engine or turbine. Furthermore, the work recovered by suefia-fi expansion engine Can be utilized- to supply part 'of the work of compression or-to drive other devices. The Steam Jet System '' - .- The steam jet system, under certain circuihstances, is desirable for use in air conditioning.*. Steam , supplies directly , the power used for com pressing the refrigerant, thus eliminating the losses connected with other methods , of supplying energy. As``the compression, ratio- tietween the evaporator and condenser under normal circumstances is large, the mechan ical efficiency of the equipment is somewhat lower than that of the positive mechanical type compressor. The condensing water requirements are considerably greater, as both the refrigerant and the impelling steam miist be condensed. .... . .. .. The,steam jet system functions, on ;the -principle that-waiter .under high vacuum, will, .vaporize at low temperatures.. Steam jet boosters- qr;com pressors of the type, commonly used in power plants for various processes, will produce the necessary low absolute pressure to cause evaporation of the water. .... A diagrammatic representation of a typical steam ejector water cooling system is shown in Fig. 6. The ..figures, correspond tO;an average repre sentative system. The water,,tq.-be cooled enters the .eyaporatqr,and,*is cooled to a temperature corresponding ; to,-the. vacuum maintained.: . Be- cause of the high vacuum, a small amount of the water introduced in,the evaporator is. flashed, into, steam. As this -requires heat, - andpthe only source-of heat is the'rest of the-svaterin the .evaporator tank, .thispother water is almost instantly cooled to a temperature corresponding; to,the -toiling point determined by, the. vacuum maintained.-,, The; amount of ^Jater flashed into steam is a-small percentage, of the. total water, circulated. "Tough the evaporator, amounting to approximately 11 IB per (hr) (ton)