materials of this sort by providing each rircter with a sriiall box refrigerated by solid carbon dioxide. The relative novelty even yet of this material in the indust,rinl field prevtxits a more complete survey of its possiliilitics, many of whiclr readily occur to one who studies it5 propcrties. The commercial development of large-scale ttiariirfacture of solid carbon dioxide has progressed to the point of providing nation-wide distribution on the basis of its iisc in refrigeration of perisliable proiluats in transit. Methods of shipment liy accirstorried incans with low eraporation losses permit supplies to be provided on short notice tlirouglioiit the country. R ~ - c e i v r oApril 15, 1932
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~1~'RIGEKATlU as~ applied to air i!onditi(jrrii,~ presents a distinctly different set, of conditions from those met elsewhere in refrigeration practice. Because the product of the air-conditioning process must primarily be free from any impurities, every effort in design must be directed toward making evcrytlrini: connected with i t safe beyond any possible question. This imsideration has led to the invt!stiga.tion of refrigerating systcnrs and to the desnlopnient of what is at Icast a unique type of r?irigerant and machine adnptcd for use wit,li it. The refrigerants, dichiorocthylenc (I)ii!liiic) and diehloromethane (Carrene), differ from the usual conirriercial cooling media in being liquid at ordinary temperatures aiid pressures, and the machine in wliich dliey are used is unique in operating always at pressures below atmospheric. The entire unit and the engineering behind it are sufficiently different from the ordinary to justify a rather full discussion liere. Aitt CosnImmIxG Ttre ~xocessof conditioning air, although aside frmi the rxirrciual objects of this discussion, must be s k e t c h i d lriefly here to form a proper basis for the discussion to follow. The phases of air conditioning pertinent to this paper are cooling and dehumidifying, other phases RS heating, h u m i d i f y i n g , ctr., being i r r e l e v a n t . The process consists essentially in filtering a i r , w a s h i n g it w i t h w a t e r cooled to a point which will fix it,s dew point at the desired level, and subsequently 11r i 11gin g to the required temperature for delivery to the space to bc mnditioried. In ordinary practice, a r e l a t i v e ti u m i (1it y of 45 to 55 per cent, requiring a dew point, 15" to 25" P. b e l o w tlie ultimate desired dry-ldk temperature, is cnnsidered b e s t f o r s p a c e s where summer cooling for comfort
of persons is involved. To attain this condition, the dew point is fixed by a tliorougli washing of the air with water i:ooled to the desired temperature to iusiire its dehumidifieation in Puminer to tile desired nioisture content. This air may then be mixed with other air until it has the desired dry-bulb temperature for introduction into the conditioned pace. This temperature will vary from 10' to 25" below that being niaintained in the occupied spaccs, depetending on the mctliod of distribution and other local factors. It is of prinrary importance that the mechanical equipnient arid tlie media or element employed in such air-conditioning offer no hasards t o the occupants within the building or space heirig conditioned. n'ot only must the danger from direct accident and hazard he entirely eliminated, but the more serious out,come of a pa& initiated by the most trivial muse must be scrupulously avoided. I n industrial air conditioning, the purpose of sui+ a systeni is to supply pure air of urriforni characteristics, and altliough a minor failure mag not involve such great hazards 5s in public Iiuildings, rii:\wtlielcss the objective of the system is defeated by any possible contamination that might get into the air from the failure of any part of the equipment.
REFRIGERANTS ClfoSEN FOB SYSTEM Xearing these points in mind, it was necessary to consider first the safety of t,he system and its ability to function properly inspite of possible errors on the part of operators. This led to the idea of a refrigerant of a relatively high boiling point operating u n d e r a s u b atmospheric p r e s s u r e , for under these coiidit.ions any leakage would necessarily be inward instead of outward. This required the development of a refrigerating machine to handle such a. substance under the prescribed conditions, which necessarily involved a relatively large-capacity compressor to handle the at-
June, 1932
I N 11 U S T 13 I A L A h D E N G I N E E 1%I N G C €3 E h1I S T R Y
61i
Unlikemost otherre tenuated vapor f r i g e r a t i o n applicathrough the reirigerattions, air conditioning ing cycle satisfactorily. is c h a r a c t e r i z e d by The r e f r i g e r a n t s sp. great variations in the lected as heing m o s t load factor, amounting suitable are dichloroto as much as from 100 ethylene and dichloroper cent to zero load n i e t h a n c , aiid the in a matter of a few standard m a c h i n e in hours with an ordiiiary which they are used i s (.harige iii iwatlier. I n built aroiirid a ccritriiuaddition to the ordigal or turbo-coiiiprrsnary elianges i n outside 'or OS higli c:ip:tcity. \\-est,ht.rconditions, the 1lie eliarrtcteristies of variation iri internal as these ti& thein ailiiiirdistinguisiied froin exali:. to the prolileins t cr t i i i I , or w c s t herof air t~xiiiditioi~ing aiid nirintiuii I I ~ rcpre, wggest other possibilisciitcd tis the dcprture ties where their unique OS a l a r g e a u d i e n c e Eratiires may well he from n t h e a t e r , may of importaric.c. Iring about aii eqiislly l h e preSerrt:d relarge variation in the Srigeraiit is oric wliicli work to bc done by the vaporizes and conrefrigerating plant. denses at ttie upper ' h w vari:it,imis iiiusi, be quickly arrd aeciirat,ely followed iiiid lover ttw3perature pliases of eilcli c words, ttie heat transfer is t h i s isutherwi!, and the more by the iiischiiic if (:fficiont operation is to result, and eondit at tile optimum. K O mnniial operator i:ffcient the refrigerant the more complete the tr:irisScr. t,hc alertness and dexterity necessary for Air lias iiee,ri used as a refrigerant on acvoiint of its safety but, siiice it does not eondcnae dining the SIICII wnt,rol. Autrrinatie control is tlierefore highly desirniric. ?'he :dilit,v i,o r i i o ~ tt h e roiiditions is iiilrrrent in a inal efficiency is low. Water vapor at higl is admirably suited to the poaer cycle, but at r dri'hperatixig teiiiperatures its characteristics are unsuited t.o the orrliiiary types of compressors. The ideal refrigerant r i ~ ~ u l d of course be thenriod~~nnmicallyperieet and follow Carriot's cycle esactly, for that! is the standan! by wliicli perioriiiance is properly judged. The rcfrigtxarit i m s be ad;iptihle to some practicable nieims of eoloprt most lie clrcmically stable and inert under all prae i,oiiditions operations, arid ririist bc relatively cheap. h s ~~rl~evious~y cnlphasiacd, safety is an iiiiportmt considcration in all refrigeration, bot more particularly so in airconditioning yrnctice.
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ceiitrifugal compressor througli its full flexibility and ability to float with the load dernarids. Let us consider the characteristics of the two refrigerants in use in centrifugal-compression systems. Their ptiysical and thermodynamic properties arc?given in Table I. Obviously these two refrigerants are satisfactory in possessing high thennodynamic efficiencies and capable of operat.ion witliin tlie range required. Furthermore, at least
618
I N D U S T R I A L A N D E N G I N E E R I N G C H E 31 I S T R Y
dichloromethane meets all safety requirements. However, it is evident that a positive displacement compressor for use with either is out of the question from the standpoint of practicability, since the volumes of vapor to be handled are
Vol. 24, KO.6
tary and reciprocating machines. This nizkes it simple to maintain and operate and, to a considerable extent, fool-proof, The fundamental design of the centrifugal compressor is identical with that of the modern centrifugal pump for handling water and other liquids (see Figures 1 and 2). The principal departures from this design are entirely mechanical, and are such that they tend to make the centrifugal compressor a more rugged machine than the centrifugal pump. The fluid handled is less dense than that handled by water pumps, permitting the use of lighter weight impellers. The mass of the rotor is concentrated in a heavy shaft, a feature which results in much lower centrifugal stresses than are possible with turbines which have an appreciable mass concentration in the blades a t the periphery. Thin metal labyrinths are used to prevent leakage between stages, as in the steam turbine. These prevent excessive leakage which would affect the efficiency, but at the same time do not actually introduce contact with the moving parts. Because of the very low pressures between stages and the large gas volumes in circulation, the clearance of these labyrinths is not critical. The centrifugal gas compressor is not subject to erosion or wear a t the points of clearance as i. the centrifugal pump. For this reason its life is several times that of n centrifugal pump even under the most favor'A able conditions of operation. In fact. the life of a centrifugal compressor is greater than that of any other type of com60 70 80 90 /oo pressor or pump, or of the steam turbine which, of course, Rd+em+hy €W is subject to slight erosion due to condensation. The only FIGURE4. CENTRIFUGAL COMPRESSOR PERFORVANCE parts subject to wear are the two. outside bearings, and these, AT VARIABLE SPEED with a proper system of lubrication and a suitable method of drive, will wear as long as the best motor bearings. When relatively large. These characteristics led to their adoption supplied with clean oil in a forced oiling system, these bearfor use with centrifugal-compression machines. -4s the ings and journals will retain their original dimensions for characteristics of the centrifugal compressor were found, many years, a result which is not obtainable with reciproupon analysis, to be ideal for air-conditioning loads, the cating machinery. availability of these chemicals as refrigerants made possible The centrifugal compressor has a high volumetric flexia refrigerating system which is unique in its field. bility and temperature stability which ideally suits it to variable demand a t a uniform temperature. The operation CENTRIFUGAL COMPRESSOR of a centrifugal compressor is like a centrifugal water pump Although designed upon different principles from a steam or electric generator. The physical head, voltage, or temturbine, the centrifugal compressor, or centrifugal pump, is perature head is established by the duty to be performed capable of giving equal or better efficiencies. Centrifugal and maintained by the machine. Changes in demand up machines can be designed to give an efficiency well over 80 to the capacity of the unit are automatically compensatpd per cent, but commercially they are usually designed to give for by the characteristics of the machine. In other words, the centrifugal compressor floats on the line or load, like over-all efficiencies between 70 and 80 per cent. The centrifugal pump has all the advantages of a steam an electric generator. The counterpart of t u r b i n e as r e g a r d s the centrifugal comfreedom from w e a r , pressor in the power maintained efficiency, plant is the multiand s i m p l i c i t y of stage s t e a m turbine operation. It differs running condensing, from the rotary comwhere large volumes pressor in that there a t low pressures are are no internal conhandled by the most tacts or wearing parts. efficient t u r b i n e s . There are no valves, The term rotary is packing, or stuffing frequently c o n f u s e d boxes. T h e drive with centrifugal, but end of the r o t a t i n g there is no similarity shaft is sealed simply beyond a r o t a t i n g b y a n oil b a t h , shaft. T h e r o t a r y thereby eliminating compressor has a fixed the necessity of any di s pl a c em e n t and contact between stafunctions in principle tionary and moving like a p i s t o n comparts. KO small mepressor. chanical clearances ARRANGEMENT OF SYSTEM SHOWISG FIGURE5. DIAGRAMMATIC Figure 3 shows the REFRIGERANT CYCLE are required as in ro-
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Jurw, 1932
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typiml pcrfuriiialii.e of a ceiitrifugal-coinpression rcfrigeraiilig rnacbine riinriiiig at c o n s t a n t speed arid iiiaintniiiirig R c o l i stant evaporntor or refrigerator teniperat,ure. Thc ciiange in condenser temperature is v e r y s m a l l , This change may be e f f e c t e d , as in the fibyire, by control of tlie condenscr-water flov, the thermostatic element being respoiisive to the ovaportitor temperature. The eomlenser-water flow is tiius under constant automatic coiltrol and limitation. For esaiiii~le,if tlie refrigerating efiect is 100 a n d c o n d e n s e r temperature 96.5' F., tlie power required is 92 and the flow of condenser cooling water is 2 gallons per iiiiiiute per t,o~i. i f the refrigerating requircmerits suddenly drop to 70, the conden.ser-water flow is automatically rmhiced t u 1.I gdlons per miiiutr per ton, thus raising the eondenser temperature to 106" F. and niaiiitaining the same evaporator or refrigerator tcmperature as before. Tlie power of the rcduced capacity eomdition would he 80.5, a reduction in p o w r in ndditiori to a decrease in condeiiser water. These reductions arc1 charaeterirtics of the centrifugal syst,em, and thus follow tlie changes in demand regardless of frequency or duration. For conditions where there is no appr economy in the regulstion of the coiidenser-u,ater flow, otlier methods of mainta.ining a constant or desired evaporator temperature are employcd. Figure 4 shows the results of speed regulation through a narrow range with a constant flow of condenser cooliiig water as from 0 cooling ton-er. Since speed reducdion of alteriiatiiig current motors iiivolves slightly increased motor losses, tlie poner curve is bascd on relative a. c. motor input. If at the peak condition the refrigerating effect is 100, condenser temperature 95, condeliser-water flow 300, and variable speed a. c. motor input 79, tlieii at 70 per cent refrigeratiiig effect, 300 gallons per minute condenser miter, the condenser tf~niperatureis 90.5 and motor inpiit 5G. 'Fire tonrpcrature stability of the centrifugal niaclline iiia1;es it elis)- to maintain eitiicr a constant rdrigerant temperature i n tlit: evaporator, or a erinstniit, I old-water temperttt,ure to ur Sruni the evaporator. I t is t h i s entirely practical to cool fresh water to 34" li., uiidcr varying load conditions, iii a s l r d and tubo eoo1or wit,liout danger OS inwing and ljursting the tubcs. This is eiitirdy inipraetical
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Sur w positive displacement compressor, altliougli it rould tlieoretically be iiccoiriplisherl by a gradiiated ai:tion restriction in the compressor suction and a b y p a s s r e t u r ii i ii g soII1c or tilt: pressed viporu to tire e o m p r i m o r i i i le t , maiirt,ainiiig a fixed volun~cdispiaceini:nt. figure 5 sliowa the d i a g r ani m a t i c arrangcnicnt of tlie ceiitrifugal refrigerating system arid the cycle of the refrigerant.. Tlic refrigerant charge is c o n t a i n e d i n t h e hottom of the cooler. To obtain refrigerating effect, the liquid is pumped in a shower over the tube surfaces
results in a very efficient heat transfer. without a p p ble Erict,ioii or superheat loss in tlie cooler. The cooler and condciiser tubes are of brass. They do iiot scale and are easily cleaned on the water side. 'l'lie refrigerant side always remains clean, as there is 110 oil in t,lic refrigerant space8 of the system. The refrigerant is also effective in keeping these surfaces clean, resulting in a high maiiitaincd efficiency throughout the life of the equipment,. Tlie whole assembly of tlie system forms a compact unit. The suction and discharge eonnectioris of the compressor are short, with little opportunity for losses. The vapor formed in tlie eoolcr enters the compressor in a dry saturated condition. The compressor, functioning like LL centrifugal water piimp, discliarges the compressed vapor directly into the condenser. The condensate is trapped back to the reservoir in the hottom of t,he cooler through a float damper as rapidly as formcd. There is no erpmsion valve. The float trap serves to seal the condi:iiser pressure from the coolrr. Tlie trap does not Iiave tlie customary valve seat, Gut merely plain strrfaces closiiig sufficiently to retain a liquid seal on the condenser xlien the machine is operating a t high capacities. Tlic normal pressure difference between cooler and condenser is about 10 poimds per square inch when using dichiommrtliaiie. All prerwrcs equalize wheri the compressor stops, but t,he refrigerant !iquid is ret,ained mainly in the cooler. Kornial operating conditions arc established ininiedia,tely after the system is placed in operation. There ie no "pimipin ' of nny part of the system ly of a complete, totnlly required. Figure (i is aii i~iclosed,centrifugal refrigerating qsteni of 30 tons capacity, R ~ c s w April ~ o 19, 1932.
Ncu; drvelopmenls in the chemistry of foods are conslunlly sirnplplifying &e problems of feeding millions and are postponing with a slrong hand lhe possible corning of the world famine prophesied by :Wallhns a century ago.
