Proceedings of the 1992 International Refrigeration Conference / Energy Efficiency and New Refrigerants, Purdue, July 199

Bartolini, C. M.

Caresana, F.

English

BARTOLINI, Carlo Maria

CARESANA, FLAVIO

IRIS UniversitÃ  Politecnica delle Marche

Air-cooling  open cycle with peripheral machines

BARTOLINI, CARLO MARIA

Archivio istituzionale della ricerca - eCampus Università Telematica

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering1992Air-Cooling Open Cycle with Peripheral MachinesC. M. BartoliniUniversita di Ancona; ItalyF. CaresanaUniversita di Ancona; ItalyFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlBartolini, C. M. and Caresana, F., "Air-Cooling Open Cycle with Peripheral Machines" (1992). International Refrigeration and AirConditioning Conference. Paper 172.http://docs.lib.purdue.edu/iracc/172AIR-_cOOLING ~PEN CYa.E WITH PERIPHERAL MACHINES -Carlo M. BAIUO!fNI--F1avto CARESANA Dlpartlmento dl Energetlca - · Unlversita dl Ancona vta Breece Blanche~ 60100 Ancona- Ital!a -Tel. +39·71-2204772 Fax +39·71-894677 · ABSTRACT · - Air-conditioning plants With air as working fluid In open cycle have been wtdely employed and they provtde high performances In terms of compactness -and low weight. In the on-aboard systems Interesting developments have been obtained as well as In the land applications same good improvements can be expected. As a matter of fact the absolute lack of pollution problems and the compactne:ss can make this kind of plant suitable ·for all that applications in which reliabtltty, maintenance and easy service are fundamental qualifications. The paper analyses the possibWty to employ the peripheral machines as compressor and turboexpander In order to improve the compactness and to reduce costs. A complete theoretical analysts of the thermodynamtc cycle ls followed by the schell;latlc description of a small air- coollng experimental prototype. COP Cp Cv h k m ·11 11 0 Subscript c e t 0 1 2 3 4 NOMENCLATURE Coefficient of performance · spectfic heat at constant pressure spectfic heat at constant volume specific hentalpy cp/cv operating mas:s ratio compre:ssion ratio heat exchanger effectiveness effiCiency · · (k-1)/k compression/ compressor espanslon turbine ambient conditions compressor Inlet compressor outlet turbine Inlet turbine discharge 337 INTRODUCTION nte sclcntiftc and technical communities are looking for replacing the CFC's In the air conditioning plants, In order to solve the well known serious pollution problems. At the same ttme with the research of new refrigerant fluids, that Is provtdtng the ·first results. the study of alternative cycles or the close re-examination of refrigerating plants until now seldom employed because of their low performances is widely developing. This is the case of the air-cycle refrigeration systems that are more commonly used In the air conditioning of alrcrafts than In surface and stationary applications because the lightweight compact equipment typical of air-cycle systems usually offset -itS Inherently low emctency (1.2,3,4). The air-cycle refrigerating units have been deeply Investigated in the literature with both dry and wet air as working fluid; the thermodynamic analysis pointed out the Influence_ of the compressor, turbine and heat exchanger efficiencies on the global performance, and the influence of the expansion process on the final thermodynamic conditions of the atr' In the open cycle. An expansion that makes possible to maintain the steam in equtllbrtum with the dry air allows a lower temperature at the end of transformation expectally with htgh pressure ratios, affecting improvements. tn the coemcient of performance; -such expansion can be more Ukely obtained In a turbine rotating at low angular speed. A1r-cye~s air conditioning Is not economic In many appllcations because .. (lf the high power required and the solution of the pollution .problems cannot be the only· reason for their larger dtffusion. However they are used In specialized appllcatlons as the envtronmental control for remotely located temporary milltary bases, where emciency 1s not the prtmary-factor[3,4). Aa a matter of fact In the above mentioned applications the ratio cost/benefit Is accettable by vtrtue of particular conditions connected with the operative system; typical from this point of vtew Is for Instance the air conditioning into aircrafts, because the · compression of low-pressure air from out-side the airplain to nearly standard atmospheric pressure is in ~y case operated. All these reasons suggest to look for new ways to reduce the ratto cost/benefit maintaining the compactness and the reUabUlty. THE PERIPHERAL MACHINES In the environmental control 'of small rooms low flow_ rates are generally required so that the volumetric machines can be regarded as more suitable. However several problems are Introduced by their- operating charatertstics. first of all the Internal lubrication, when the working air ts used directly to cool the space requiring refrigeration and therefore it cannot be contamtned. In the air· cycle refrigeration units until now bullt centrifugal compressors and radial turbines have found wide applications: the low flow rates involve small dimensions of the Impellers that, In tum, require high angular speeds In order to get the necessary vaiue of Up velocity. Peripheral machines are typical reaction machines with parttalized admission and have a field of appllcatlon limited to extremely low specific speeds so that they are directly competitive with certain types of volumetric machines. The low efficiency of the pertpheral machines however causes the lack of interest in them. even though there are a number of construction features, above all In the case of low power. that make them economically conventent .. Contrary to total -admission turbomachtnes, in terms of construction. the layout of this type of partlalized machine has a non cyltndrtcal flow. The geometry of the machine consists of an open rotor with generally radial, blade-to-blade action, tn front of which there Is a channel In which the gas flows freely In radial direction; the flow Is therefore within a toroidal space, so that the fluid Is -repeatedly tn contact with the blades before being discharged. The case has suction and delivery ports 338 which connect the outside with the channel. divtded by dynamically sealed strippers. The earliest recorded studies on these machines date back to the thirties and started tn Oermany [5) and in Japan [61 at the same time, after them several theoretical and experimental researches have been presented "in ltterature. Prevtous papers by the Author [7 ,8,91 describe the early phases of the research on peripheral machines started by the Dipartimento di Energetlca of Ancona University. As discussed in several papers. the low values of specific speed and ratio between wheel speed and spouting velocity, favor the application of the peripheral turbines in several cases: especially where comparatively small amounts of power are to be extlacted at low rotative speeds. The low values of speed of revolution In these cases are particularly advantageous since usually gearing between the turbine and load can be eliminated thus offering sizable contructive simpllftcations and increased reliability [10.111. Safety also benefits considerably since the runaway· speed of peripheral turbines ts extremely low. usually within the safe operating ltm1ts of bearings and turbine disk. so that a loss of the turbine load will not lead to disastrous failures. even when no overspeed trips are provtded. · An other advantage of peripheral turbines looks therefore eVident if one compares the areas of the distribution channel cross-sections: the straight cross-section area of the channel-should in fact be compared with that: of the nozzle of conventional turbines With parttallzed adml:Sston. It ts evtdent that. even at partlalb:ed admtssion, · the cascade of a conventional turbine need very close working tolerances.· consequently increasing construction problems and costs; It Is with regard to costs therefore that the peripheral turbtne can have many possibilities of application, because 9f the more limited technologtcal. production l!IJld construction materlal requirements. Other advantages have been considered In the case of the atr-cycle cooling applJcations: the no internal lubrication requirements and the application of gas-bearings allow to introduce the air directly in the space requtrtng refrigeration ·wfth_out Its contamination: moreover the typl<;a.I low angular speed permit a slow expanston process with decreasing of the final temperature. ' ' · · The proposal of employing peripheral machines in the air-cycle refrigeration plants has been realiZed tn· the· prototype that ts now under experimental testing: to choose the best values of the _parameters the follow thermodynamic analysts has been developed. _ · · : . THERMODYNAMIC ANALYSIS A simple open atr cycle system is shown schematically tn Fig.l while Its corrtspondent ·thermodynamic cycle is displayed on the temperature-entropy diagram in Fig.2. Refrigeration ts obtained by three basic steps. Ambient a1r ts firstly compressed and then the heat produeed by the compression is removed· by means of the ambient atr Itself; next atr is ~oolc:d by extracting work from it as it expands through a turbine. So air leaves the turbine· and enters the conditioned space at a low temperatur!! T4. Neglecting the work done to drive the fan which pulls air over the heat exchanger. the coefficient ·or performance of the cycle can be computed as follows: . , _ . · ( 1) Neglecting pressure drops in the heat exchanger and considering dry atr. the temperatures of a1r in the charatertstic points of the cycle can be computed from 339 the foJiowtng equations : (2) (3) T = T . [1 -1'1 . (p ere- 1]] . 4 3 t CJe . p (4) Assuming 11t~c.0.6, stnce these are the actual effictenctes of the pertpheral maeh!nes so far designed, and assuming temperature of ambient air To=T1'•308 K. the Fig.3 and Flg.4 charts can be obtained. where the coefficient of performance of the cycle (Fig.3) and the turbine discharge temperature (FtgAl are p1ottec;t against the heat exehanger effectiveness for different values of the compression ratto. In order to improve the performance of the cycle a refrigeration of the compreSISQr channel has been considered. lowertng both the compression work and the heat exchanger burden. lb.ts p~ win be attained by provtdlng the compressor channel with ftns and pul)Jng· ambient atr against It by means of a fan driven by the turbine Itself. At this step, to stmpllfy the calculations, the compression phase has been considered isoentroplc In every respect, wbile to compute the work done durtng an lntercooled compression It should be necessary to evaluate, besides the enthalpy variation of the working t1uid, the amount of heat removed durtng the compression. Taking ·Into account the pressure drop In the heat exchanger, as a function of Its effectiveness. the coefficient of per(omiance and the turbine discharge temperature have been obtained and they have been displayed versus compression ratio tn Figs 5 and 6. The COP Improvements are remarkable. compared with the previous system, ~ctally for those compression ratios that maxtrntze the COP. Furthermore It must be remembered that ln ·the ftrst case the pressure drops · have been neglected. As one can see from the diagrams, the maximum values of the COP can be attained at low compresston ratios and this tums to a machines design advantage. Nevertheless the turbtne-dtscharge temperature ls still too high for conditioning purposes, particularly at low heat exchanger effectlvenesses, so··that the use of a so c_alled "regenerative system" could be required. In the regenerative system. some of the a1r discharged from the turbine pasies back to cool the atr coming out from the heat exchanger before It enters the turbine. In this way the temperature of atr entertng the turbine can be lower than would be possible by cooling wtth ambient atr only. The regenerative system and Its thermodynamic cycle. displayed on a T -s diagram. are shown ln Fig. 7 and Flg.B. The coeffiCient of perfonnance of such a cycle can be computed as: (5) where "m" Is the operating mass ratio, that Is the mass flow ratio between air· entering conditioned space and atr compressed. Assunung T1=To=308 K and r...0.7, curves of Ftg.9 and Ftg.lO can be dertved. They represent the coefficient of performance and the turbine discharge 340 temperature as function of "m" for some; values of the compression ratio. Taking m=l one can read the COP and T4 for an air cycle without regeneration. As shown 1n Ftg.9 the COP reaches a maximum for a particular value of m which, for the compression rati_os considered. 'ranges from 0,6 to 0,7. Ftg.ll shows the maximum COP reachable adopting pressure ratios fr~IJl 1.1 to 3, for some values of the heat· exchangers. etrectiveness. " The experimental prototype IS' schematically showed 1n Ftg.l2; the two cross sections show both the aJr·cooled and the ambient air ways and the compact arrangement of the unit. CONCLUSIONS The proposal of the peripheral machines to compress and to expand air in an open refrigerating air-cycle appears trustworthy even if the COP values have not been Improved as regards those obtainable wtth radial turbomachlnes. The compactness and the low speed of revolution allow to reduce. the size problems for the system allocation and to rely on low level sound emiSSions that instead can be troublesome adopting the traditional machines. The first phase of the experimental program Will provide, interesting data tn this field: instead the economic evaluation of the construction costs has been already developed 1n comparison wtth the tradttlonal air condtttonlng plants at the same refrigeration power. The proposed group is characterized by the considerable reduction 1n the manufacturing cost and by very low maintenance costs wbtch the htgh running cost consequent up on low value of the COP 1s 1n contrast wtth. Watting for the experimental results. above all as for the hypothesis of the heat exchange during the compression process, the high rellablllty and the no pollution property at low cost can be considered Interesting pecullarlties for some particular llne of market. BIBUOGRAFY [1] M. KAUFFELD, H. KO~NIG, H. KRUSE,- "Theoretical and Experimental Evaluation of the Pmemial of Air Cyde.·Refrigeration and Air Conditioning" -xvmth International Congress of Refrigeration- Montreal 1991. . · . [2] H. KRUSE, M. KAUFFELD- "Cold Air Refrigeration with the Joule-Process"- Ki 18, 5 (1990), . pg. 206-221. .. [3] Ashrae Handbook American Society of Heating·, Refrigerating and Air-Conditioning Inc. 86 [4] W. 1'. STOECKER· Refrigeration and Air Conditioning -McGraw Hill Book Company -1958 !51 C. PFLEIDERER-"Die Kreiselpumpen fur Flussingkelten und Gase" Springer-Verlag, 1955. !6] Y. SENOO -"Researches on the peripheral pump'', Report of the Research Institute for Applied Mechanics, Kyushu University , val. 3 , 1954 [7] C.M. BARTOLINI, G.ROMANI - "Theoretical Investigation and Performance Charactc· ristics of the Peripheral Pumps" Proceedings of meetings of International Institute of Refrigeration - Purdue Uni'o'ersity 1988. . [8) C.M. BARTOLINI, G.ROMANI ,- "Microturl::line a Reazione a Basso Numero Specifico" Congreso Intenacional Energla, Ambiente e InnovaciOn TecnoiOgica" Caracas, Venezuela'89 [9] C.M. BARTOliNI, G.ROMANI • "Peripheral Turbines for Industrial Applications" Proceedings of International Conference· RACIOENERGIA '91 -Bratislava, CSFR, Oct.91. [10] O.E. BALJE "Drag turbine performance•, A.S.M.E. Paper 56-AV- Aug. 1957 [11] J.D. BURTON -"The prediction and improvement of regenerative turbo-machine performance", B.H.R.A. Conference, Cranfield 1967. [12] W. A. WILSON , M. A. SANTALO , ].' A. OELRICH -"A theory of the fluid-dynamic mechanism of regenerative pumps",Transaction of the ASME, Nov.1955 [13] P.S. CATES·"Peripheral-compressor performan~;e on gases with molecular weights of 4 co 400", A.S.M.E. Paper 64 WA/FE-25 - 1964 . [14] E.B. COlYER, R.L. GESSENER ·"Miniature cryogenic refrigerator turbomachinery", Advances in Criogenic Engineering, val 13 Plenum, New York, 1968 p.484 341 sz -.. 1 4 Hot • iD Cold • o.tt P1g.l- Symple .open aJr cyele diagram kJ . s(kg K) F1g.2-1bermodynam1c air cycle T-s dlagn.m tl Iii 0,25 ! ~ 0,20 & - 0,15 Q ' = 0,10 ~ e 0,05 8 (,) 0,00 0,65 0,70 0.75 0,80 Heat exchanger effectiveness F1g.3 - Coeft'lctent of performance vs heat exchanger effectiveness 342 g 320 j 315 310 t 305 ::! ! 300 295 15 290 ~ g 285 li 280 .. ~ 0.65 0,70 0,75 0.80 Heat exchanger effectiveness Ftg.4 • Turbtne discharge temperature vs heat exchanger effectiveness 0.6 C> ...., ; 0,5 ~ '! 0,4 .. ~ 0,3 '-= ... ~ 0.2 e 0,1 .. 8 o.o 1.0 1.5 2,0 2.5- 3,0 . Compression ratio _ Flg.S- Coeftlclent of perfonnance vs compression ratio g 310 !:! ~ 300 & e ~ ~ 290 ..c ....,_ 'II 280 .:a_ l 270 1.0 1.5 2.0 2,5 3.0 Compression ratio F1g.6- Turbtne discharge temperature vs compression ratio 343 Motor Hoteirln Heat exchanger 1......---14 1\lrbme Cold e1r.011t Ftg. 7- Regenerative air cycle dtagram 2 kJ s (kg K) Ftg.a ~ Regenerative air cycle T-s dtagram ., <> ; ~ 0,3 ~ ., =- 0,2 '0 c .. !l 0,1 ~ u 0.0 0,0 0,2 0,4 0,6 0,8 1.0 m Ftg.9 - CoeffiCient of performance vs operating mass ratio for different values of the pressure ratio, In the regenerative cycle 344 g 320 1!:: .a f! 300 ~ B "' 280 ~ ..c:: <.> !l 260 "0 "' = :e 240 ~ o.o . 0,2 0.4 0.6 0.8 1.0 . m Ftg.lO • Turbtne·dlscharge te~perature ''1'4" vs operating mass ratio for different values· of the pressure ratio, in the- -regenerative cycle · 1;1 -~ e ~ "' Po ..... Q = "' OJs "' Q u Flg.ll· 0,5 0,4 0,3 0,2 0,1 0,0 1,0 1,5 2,0 '. 2.5 3,0 Compression raUo MaJdmum coefficient of performance vs pressure ratio for different values of the heat exchanger effectiveness, In the regenerative cycle. 345 CROSS SEC170N B-B CROSS SECT10N A-A Ftg.l2- Scheme of experl.mental prototype of air-cooling regenerauve untt The circled numbers refer to the diagram of Fig. 7. 346 

Air-Cooling Open Cycle with Peripheral Machines

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Air-Cooling Open Cycle with Peripheral Machines

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IRIS UniversitÃ Politecnica delle Marche

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