Refrigeration machinery - a review

Refrigeration machinery - a review

Refrigeration machinery - a review A. L. Stolk Le rapport donne un aperqu de quelques developpements dans le domaine du froid, qui sont particuli...

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Refrigeration

machinery

- a review

A. L. Stolk

Le rapport donne un aperqu de quelques developpements dans le domaine du froid, qui

sont particulierement du domaine de la Commission B2 Les sujets suivants sont etudies: machines a compression; machines a absorption; machines a detente; refroidissement thermoelectrique; tubes vortex, pompes a chaleur; mediums refrigerants," systemes de propulsion; et, materiel frigorifique refroidi.

The paper gives a survey of several developments in the field of refrigeration which are of specific interest to Commission B2 of the IIR. The report is divided into the following sections: compression

machines; absorption machines; expansion machines; thermo-electric cooling; vortex-tubes; heat pumps; refrigeration media; driving systems; and air cooled equipment.

Compression machines

Netherlands 1 the hybrid method is used with good results also with the extensions built on the simulation model. All these models are important tools for optimization of valve designs towards lower energy consumption and longer valve life.

Machines re vue

frigorifiques-

The development of compression systems is mainly directed towards better efficiency, lower noise, better control systems and increased reliability. Progress in these four aspects can only be achieved if there is a good understanding of the dynamic behaviour of components and systems. There is a lot of work done in this field, but investigations are time-consuming and progress is, in general, rather slow. A very important factor, influencing the energy consumption and reliability of reciprocating compressors, is the compressor valve design 1. To make optimal valves it is necessary to know how valves behave under different conditions and how sensitive they are to variation of the relevant parameters2, 3. During the Moscovv'Congress of the IIR in 1975 a number of papers4,~, 6 on the development and application of simulation models for compressor valves were presented. Most of these models have been developed further and adapted for use as real design tools. In the USA a model is available and is in use for reed-valves and bending ring valves, giving an optical picture of the movements of the valves inclusive of the deformation. Also the tension in the material on different spots can be displayed simultaneously. Other models are ready to be used with multiple cylinders, which interfere with one another, and on one computer model two-stage compressors with intercoolers can be simulated. Models of gas pulsation in pipelines are being incorporated with the compressor valve models, which again will be an important step towards better accuracy and reliability of the computer models. In The

Energy consumption is very important for small compressors used in household refrigerators and home freezers. A low noise level is also a vital aspect in this sector. Recently in Denmark a new compressor type has been presented, with an efficiendy 30% better than that of existing models and with extremely low levels of noise and vibration. Noise reduction is also becoming an important factor for bigger compressors. Valve and gas pulsation are the main noise generators and the above mentioned simulation models are usable tools in this type of research. Valves are avoided in rotary compressors. The monoscrewcompressor, presented at the Moscow Congress, has made its mark and quite a number of these machines are used in refrigeration systems with satisfactory results. However, the energy consumption of screw compressors is rather high and there is an urgent need to find ways of reducing it. In the field of turbo-compressors 7, the normal process of solving minor problems and refining details goes on. Penetration into the sector of lower ca pacities has co me to a stand still.

Absorption machines

The author is at Laboratorium voDr Koudetechnik, Technische Hogeschool, Delft, The Netherlands. Full biographical details appear on page 74, Vol. 1 No. 2 of the journal. Paper received February 1978

Due to the difficult energy position in the world, there is a renewed interest in absorption systems. The possibility of using rejected heat as the driving force for refrigeration systems may be realized in a simple way via the absorption machine. In many cases the heat is available at rather low temperatures and a lot of investigations are under way to find systems able to create low evaporating temperatures with relatively low generating temperaturesS, 9. This need arises with

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solar driven absorption machines connected to flat-plate collectors whose working temperature is not higher than 85-90°C. Possible methods employ multi-stage absorption or resorption. There is a search for new combinations of refrigerants and absorbents, but until now not with much success. In Denmark some promising progress ~° has been made with solid absorbers such as calcium chloride and strontium chloride. R22 is also mentioned 11 as a refrigerant for absorption, together with, for instance, iso-butane and iso-butylene. Another research direction is the use of zeolites as an absorbent. This work is mainly done in the USA. In Japan experiments are done with combinations such as ethanol-lithium bromide and methanol-lithium iodide and zinc bromide. The above mentioned solar driven absorption systems are in the stage of demonstration plants for air conditioning purposes only. About 400 to 500 units are recorded in the USA. Units for lower temperatures are only running on a laboratory scale, for instance in Denmark, Thailand and The Netherlands. The efficiency is reasonable compared to the small absorption units for household refrigerators.

Expansion machines Expansion machines have been used mainly for temperatures lower than -100°C. Some exceptions are aeroplane air conditioning and automobile cooling. The principle is very interesting, because a number of the disadvantages of compression and absorption machines are avoided. During the Moscow Congress interesting developments in the USSR TM were described. Air expansion machines for food freezing are already being built in quantity in the USSR. In the USA 13 a commercial unit is also available. The efficiency of the system, however, is rather low and freezing costs are high. An important factor is the efficiency of the heat exchangers. The development of liquid to liquid heat exchangers as built in the USSR may result in an improvement of the system. Lorentzen has proposed an expansion machine without a heat exchanger at all, but it has not been built so far. The Stirling machine is still going strong and new models for specific purposes are being added to the existing range.

Thermo-electric cooling Research in thermo-electric refrigeration was stimulated by space technology. It does not seem likely that in the next few years thermo-electric cooling will achieve a much higher efficiency or broad practical use. Refining and optimisation of existing methods and components is the most we can expect ~5.

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Vortex-tube The vortex-tube has a very low refrigeration efficiency, which is a great disadvantage in an era when energy is becoming scarce. A Russian paper ~6 mentioned an improvement of 50% by the application of hot end cooling to a non-adiabatic vortex-tube.

Heat pumps Nowadays, the heat pump is only economical in a rather small number of applications. If the heat, extracted from a refrigerated object, can be used at a reasonable temperature level in the immediate neighbourhood, an attractive heat pump results, tn many cases the amount of refrigeration does not balance with the required heat, or there is a discrepancy in time. Heat storage is possible, but gives an inevitable loss of energy. Space-heating17,~8,19 with electrically-driven heat pumps is only economic in a few specific areas of the world. The efficiency of heat pumps may be raised by developing better heat exchangers or by driving the compressor with an internal combustion engine 2o. The heat of the exhaust gas and the cylinder cooling water can be used to improve the overall efficiency. A heat pump with an absorption system can be driven by heat from gas, oil or even coal-burners or by heat rejected from another process. The overall coefficient of performance depends on the circumstances and the adaption of the absorption unit in a complete system. Investigations are under way, using computer models, to see whether these systems can be useful and economic. There are a number of proposals for combinations of solar collectors and heat pumps for space heating, but the subject needs further study before an economic judgement can be given.

Refrigeration media During recent years few new refrigerants have come on the market. Studies have been made, in a number of countries, of the use of non-azeotropic m i x t u r e s 2 1 , 22 in simple vapour compression installations. Further studies of topics such as system stability, controls and oil-refrigerant solubilities are needed before fully reliable machines can be built. The renewed interest in absorption refrigeration machines has triggered a search for better combinations of absorbents and refrigerants, as already mentioned. It seems difficult, however, to match the conventional combinations of ammonia-water and water-lithium bromide.

Driving systems Newly developed electronic controls have made it possible to run alternating current motors with a

International Journal of Refrigeration

c o n t i n u o u s l y v a r i a b l e speed. ] h e system is not cheap, but f u r t h e r i m p r o v e m e n t s can make it m o r e e c o n o m i c in f u t u r e In places w h e r e the t r a n s p o r t of fuel is easier than the d i s t r i b u t i o n of electric energy, the internal c o m b u s t i o n engine is a good alternative. A t t e n t i o n has to be paid to the c o u p l i n g design, e s p e c i a l l y w h e n two r e c i p r o c a t i n g m a c h i n e s are interc o n n e c t e d . Gas t u r b i n e s are used for very big r e f r i g e r a t i o n c o m p r e s s o r s in the c h e m i c a l and p r o c e s s i n g industries, such as natural gas liquefaction. The linear electric m o t o r used as a driver for r e c i p r o c a t i n g m a c h i n e s is an interesting possibility. If it is possible to control the stroke a c c u r a t e l y and reliably, this motor could p r o v i d e a g o o d s o l u t i o n for r e c i p r o c a t i n g c o m p r e s s o r s with c o n t i n u o u s v a r i a b l e c a p a c i t y control. Condensing

systems

The a v a i l a b i l i t y of c o o l i n g w a t e r for the removal of c o n d e n s e r heat is b e c o m i n g more and more d i f f i c u l t , e s p e c i a l l y for inland i n s t a l l a t i o n s For this reason there is a strong m o v e m e n t 23 t o w a r d s air cooled condensers, p a r t i c u l a r l y for very big i n s t a l l a t i o n s This change will, however, n o r m a l l y result in a higher c o n d e n s e r pressure and an e n e r g y c o n s u m p t i o n at the c o m p r e s s o r tt is t h e r e f o r e i m p o r t a n t that the overall heat transfer c o e f f i c i e n t should be as large as p o s s i b l e The standard air cooled c o n d e n s e r also c o n s u m e s a s i g n i f i c a n t a m o u n t of energy to drive the fan, and there is a lot of fan n o i s e These two d i s a d v a n t a g e s are avoided with natural d r a u g h t air cooled c o n d e n s e r s but a careful a p p r o a c h is needed to prevent high energy c o n s u m p t i o n due to high c o n d e n s a t i o n t e m p e r a t u r e s Evaporators

One of the p r o b l e m s with air coolers is the d e f i n i t i o n of the net c o o l i n g capacity, e s p e c i a l l y in l o w t e m p e r a t u r e a p p l i c a t i o n s when f r o s t i n g o c c u r s and d e f r o s t i n g is necessary during normal r u n n i n g hours of the plant. The Netherlands' Institute of S t a n d a r d i z a t i o n has issued a standard

Volume 1 Number 4 November 1978

test p r o c e d u r e and presentation of data for frosted and defrosted air coolers. ISO has instituted a w o r k i n g party to prepare a s i m i l a r i n t e r n a t i o n a l standard. Frosting and d e f r o s t i n g are c o m p l i c a t e d p h e n o m e n a and f u r t h e r study about this point is needed.

R eferences

1 Touber, S.A contribution to the improvement of valve design, Thesis for PhD degree Delft University of Technology(1976) 2 Varma, H.K., Singhal, J.P., Gupta, C.P. 5th National Symposium Refrigeration and Air Conditioning. Madras, India (December 1976) paper RAC-21 -76, 151 -157 3 Touber, S. Koe/technJek70(1977)8, 111-115 4 MacLaren, J.F.T., Kerr, S.V., Crawford, R.A. 14th International Congress of Refrigeration, Moscow (1975) paper B2.10 5 Moavecic, M., Hamilton, J.F., Cohen, R. 14th International Congress of Refrigeration, Moscow (1975) paper 82,55 6 MacLaren, J.F.T.,Tromschek, A.B. 14th International Congress of Refrigeration, Moscow (1975) paper B2 11 7 Sykora, M. Lult-undKaltetechnik12(1976)3, 133-136 8

Chinappa, J.O.V. So/arEnergylS(1976)4,337-342

9 8chmidt, R.N.Energytechnologyconference proceedings, Washington USA(1976) 163-171 10 Van derJagt, M.F.G. Koe/techmek70 (1977) 12 11 Holdorf, G.D~eKa/te(1977)12,497-507

12 Berglaz, G. Revue Polytechnigue, (1976) 1355, 1093-1095 13 Linhardt, H.D., Carter, T.A., Kirk, A. Air cycle food freezing system and method, US patent 3868827 (March 4, 1975) 14 Dubinsky, M.G., Drui, M.G., Gavrikov, V.Po 14th International Congress of Refrigeration, Moscow (1975) paper B265 15 ASHRAE Handbook, EquipmentVolume(1975) Section V, chapter 44 16 Alekseev, V.P. 14th International Congress of Refrigeration. Moscow(1975) paperB241 17 Norb~k, K. Scandmawan Refrigeration (1976) 4, 121 -123 18 Aureille, R. Revuegenera/e therrmque ( 1976) 179, 983-988 19 Lorentzen, G. Die Wirtschaftlichkeit der elektrisch angetnebenen Warmepumpe for die Gebaudeheizung, Essener Warmepumpentagung, Deutsche Kaltetechnisch Verein (1977) Forder_gemeinschaft Klima und Haus der Technik 20 Pohle, J., Rostek, H., Wilmers, G. Gas, Warme Internat~ona/ (1977)4, 163-168 21 Emblik, E. DJeKalte(September 1977) 22 Kruse, H., Jakobs, R. K/sma + Katemgenieur ( 1977 ) 7 -8, 253-260 23 Hotani, S. On the air cooler and air cooled condenser, both finned with a kind of corrugated plate. Meeting Commissions B1, B2 and E1 of IIR November 1977 Belgrade (to be published)

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