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Domestic refrigerators with absorption-diffusion units and heat-transfer panels
ELSEVIER
PIE S0140-7007(96)00039-4
Int J. Refrig. Vol. 19, No. 8, pp. 517- 521, 1996 Copyright © 1996 ElsevierScienceLtd and IIR Printed in Great Britain. All rights reserved 0140-7007/96/$15.00
Domestic refrigerators with absorption-diffusion units and heat-transfer panels G. F. Smirnov, M. A. Bukraba, T. Fattuh and B. Nabulsi O d e s s a Institute o f L o w - T e m p e r a t u r e T e c h n i c s a n d Energetics, 1/3 P e o t r Veliky Street, Odessa 270100, U k r a i n e Received 22 March 1994; revised 22 February 1996
The possibility of essential increasing of absorption-diffusion cooler set refrigerating power by means of separating cold transmission and distribution processes in the inner refrigerator volumes from cold generation processes has been shown. The problem is solved by using heat-transfer panels on the base of heat tubes and vaporization thermosiphons. The construction developed on the 'Kristall' type domestic cooler basis has attained a low temperature of -15°C in the 155-1 volume at the ambient temperature of 20°C and at the ambient temperature of 43°C it was allowed to reach - I ° C temperature without using additional heat insulation. Copyright © 1996 Elsevier Science Ltd and IIR (Keywords:refrigeratedcabinet;absorption-diffusionrefrigeratingunit;heat-transferpanel;heat tube;evaporatingthermal siphon)
Refrigerateurs menagers a groupes absorbeurs-diffuseurs et a panneaux echangeurs de chaleur La possibilitO est montr~e d'une augmentation considOrable du rendement frigorifique des rOfrig~rateurs absorbeurs-diffuseurs grhce ~ la s~paration des processus de gOnkration et distribution du from dans l'espace interne du r~frig~rateur. Le probl~me est r~solu par utilisation de panneaux Ochangeurs de chaleur sur la base de tubes thermiques et de thermosyphons ~vaporateurs. Les modifications constructives correspondantes d'un rOfrig~rateur m~nager du type 'Cristall" ont permis d'obtenir une temperature de - 15°C dans un volume de 155 l avec une temperature du milieu ambiant de 20°C et, avec une temp&ature du milieu ambiant de 43°C, une tempOrature de - 1 ° C sans calorifugeage suppl~mentaire. Copyright © 1996 Elsevier Science Ltd and IIR
(Mots cl+s:armoire frigorifique;groupe frigorifiqueabsorbeur diffuseur;panneau 6changeurde chaleur;tube thermique" thermosyphon 6vaporateur)
The absorption-diffusion refrigerating units (ADRU) have a number of advantages over compression ones: noiseless operation, absence of moving parts, easy attendance, possibility to use various heat sources, such as electric current, kerosene, etc. Construction of A D R U s comprises ramified systems of interconnected loops of natural circulation. Under the conditions, where the driving force of circulation is provided by density differential, a complex hydraulic system with phase transition processes in each element of A D R U restricts the rate of heat-and-mass exchange processes, which can affect adversely the level of refrigerating capacity of ADRUs. Simplified diagram of an A D R U is presented in Figure 1. The unit is made from carbon steel and charged with water-ammonia mixture containing 35% ammonia. Present within the unit is inert gas hydrogen and operating pressure in the unit is 1.7-2.0 MPa. Principle of operation of A D R U is described below. Nominal heat load amounting to 112 W is applied to the generator 1 and w a t e r - a m m o n i a solution boils. Ammonia vapour ascends by vapour line 2 to the condenser 3
where it is condensed and liquid ammonia enters the evaporator 5 by the line 4. In the evaporator, ammonia is evaporated and diffuses into hydrogen-ammonia mix of low ammonia content. Owing to these processes refrigerating effect in A D R U is achieved. Cooled vapour-gas hydrogen-ammonia mix of high ammonia content enters the receiver 6 and then ascends by coiled absorber 7 to the evaporator 5. Flow of vapourgas hydrogen-ammonia mix is induced by buyoancy force due to density difference in connection with varied concentration of ammonia. Brought to the top part of the absorber from the generator 1 by the line 8 due to action of thermocompressor operating according to vapourlift pattern is a weak water-ammonia solution which flows down through coiled absorber 7 and absorbs ammonia from the vapour-gas hydrogen-ammonia mix which ascends opposite to solution flow. As a result, water-ammonia solution of high ammonia concentration is formed and brought to the generator 1. Outer tubular surfaces of the evaporator 5 and absorber 7 included in the investigated unit have no fins. Overall length of the absorber is 6.5m, outside
517
G.F. Smirnov et al.
518
Nomenclature F k S
Inner surface of refrigeration cabinet (m 2) Heat-transfer coefficient (W m-2 K - 1) Heat-transfer panel area (m 2) Environmental temperature (°C) Temperature in planes A, B, C (°C)
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diameter of the tube is 18mm. As evidenced by experimental investigations, such absorber ensures reliable operation of A D R U at ambient temperatures up to 45°C. Evaporator tube is elliptic in cross-section. Overall length of the evaporator is 1.7 m and its area is 0.14 m 2. It is known that fins are required for effective performance of an ordinary-designed evaporator. However, finning gives rise to temperature variations over the inside space of A D R U and usable space is blocked up. Application of heat-transfer panel allows better solution of this problem provided that reliable thermal contact between surfaces of panel and evaporator is achieved. The condenser 3 is designed with fins of overall heat exchanging area of 0.30 m 2. Since the low refrigerating power of domestic ADRUs raises the problem of judicious use of energy resources in each element of the unit, the A D R U construction is provided with gas and liquid heat exchangers, finned constructions of condenser and absorber, which significantly complicates the units manufacturing process. Taking into consideration the aforesaid features of ADRUs it seems necessary to investigate the possibility of raising the rate of processes in ADRUs and increasing their refrigerating capacity materially by separating the processes of the transfer of refrigeration and distribution in refrigerator's internal spaces from refrigeration generating processes. An effective approach to solution of
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Figure 1 Simplified diagram of ADRU: 1 - generator; 2 - vapourc a r r y i n g line; 3 - c o n d e n s e r ; 4 - l i q u i d a m m o n i a line; 5 - e v a p o r a t o r ; 6 - r e c e i v e r t a n k ; 7 - a b s o r b e r ; 8 - w e a k s o l u t i o n line; 9 - r e f r i g e r a t i o n cabinet; l0 - points of installation of temperature transducers, (1)-(6) - nos of temperature transducers F i g u r e 1 S c h e m a simplifid de G F A D : 1 - g~ndrateur; 2 - conduite de vapeur; 3 - condenseur; 4 - conduite d'ammoniaque liquide; 5 dvaporateur; 6 - cuve du collecteur; 7 - absorbeur; 8 - conduite de solution f a i b l e ; 9 - armoire frigorifique; 10 - endroits d'installation des transformateurs thermiques; (1) . . . . . (6)-num~ros des transformateurs thermiques
tx
h a,b,c,d,e T
Average temperature inside the refrigeration cabinet (°C) Altitude of the refrigeration cabinet (m) Geometry dimension (m) Time of work (h)
this problem is concerned with application of heattransfer panels based on heat tubes or evaporating thermal siphons, which involved the problem of heatengineering designing of refrigeration cabinets with heattransfer panels and ADRUs.
Survey of the subject The application of heat-transfer panels based on heat tubes and thermal siphons, which allow judicious redistribution of heat fluxes, needs substantiation. Thus, in ref. 1 the refrigerating chamber evaporator is thermally connected with a metal sheet made in the form of radiator. In the later design of ice generator 2 use is made of a heat tube, which runs from the freezing chamber to the cooled fresh products chamber. In ref. 3 connected thermally to A D R U branches are heattransfer devices, where phase transition processes take place, while the heat-transfer devices themselves form the walls of the refrigeration cabinet. A set of experimental investigations into application of heat-transfer panels connected with evaporators of ADRUs has been carried out in the Scientific and Production Association of Applied Mechanics 4. The results of those investigations have been adopted in production and domestic refrigerators type 'Kristall' with ADRU and heat-transfer panels brought to a fullscale production level at one of the refrigeratorproducing factories. Along with the problem of judicious use of refrigeration, there exists a problem of simultaneously increasing the usable space and refrigerating effect. Under conditions of large-scale production of refrigeration cabinets of a certain standard size with increased internal usable space, these problems can be improved by using two or three ADRUs instead of one. Depending on the standard sizes of refrigeration cabinets and ADRUs this approach can be implemented by different designing methods. A good design for refrigeration cabinets of small standard size is provided by mounting the added A D R U behind the main unit in parallel to the latter. The depth of the cabinet, in this case, will be increased by 0.10m maximum. Configuration of both ADRUs must allow effective implementation of thermal insulation, as shown for example in Figure 2a. The use of heat-transfer panels makes it possible to newly solve the problem of ADRU's application in the construction of chest-type refrigeration cabinets. If the refrigeration cabinet is of the chest type, it is expedient to arrange the units in the plane parallel to the rear wall of the chest, as shown in Figure 2b. Such arrangement of the units promotes better distribution of evaporator surfaces over the refrigeration cabinet space and facilitates designing of a common heat-transfer panel. In such a case the panel may be designed both with
Absorption-diffusion units and heat-transfer panels (a)
(b)
(c)
Figure 2 Different design versions of refrigerated cabinets with absorption-diffusion refrigerating units: a - echelon arrangement of ADRU; b one-plane horizontal arrangement of A D R U ; c oneplane vertical arrangement of A D R U ; 1 - A D R U ; 2 - refrigerated cabinet Figure 2 Variantes de r~alisation d'armoires frigorifiques avec G F A D : a - arrangement de G F A D en dchelons; b - arrangement horizontal de G F A D dans le plan; e - arrangement vertical dans le plan de G F A D ; 1 G F A D ; 2 armoire frigor!fique
refrigerant-carrying piping for individual A D R U and with a common refrigerant-carrying piping depending on specific problems of refrigerator engineering. The thermal insulation of the evaporators, in this case, must be arranged so that the heat-transfer panels should not interfere with the process of placing products into the chest and taking them out. In a vertical version of refrigeration cabinet the A D R U may be arranged vertically in the same plane one above the other (see Figure 2c). Use of doubled or tripled units in one refrigeration cabinet as per the layout shown in Figure lc (without heat-transfer panels) is ineffective because it leads to irregularity of the temperature field and reduces the usable space of the refrigeration chamber. To decrease such irregularity it is advisable to use heat-transfer panels in the form of heat tubes or two-phase thermal siphons built into metal panels. The operating conditions of domestic refrigeration cabinets predetermine the location of these heat-transfer panels along the side and rear walls of the cabinets. It is easy to see that the total area of the evaporator can be substantially (by one and a half or two fold) increased as compared with that of the known evaporator design. Therefore, in spite of the added heattransfer panel, which creates an additional thermal resistance, the total heat flux removed will be increased. It is a crucial problem to reduce the contact's thermal resistance. The maximum area of contact between the A D R U ' s evaporator and the heat-transfer panel can be obtained by using the evaporator made in the form of a flat coil oriented in the vertical plane. This paper presents the experimental results which corroborate the possibility in principle to make a freezerrefrigerator with two A D R U s where the evaporators will provide service for the common heat-transfer panel with built-in heat tubes.
Description of test bench and investigation procedure Used as a subject of investigation was a refrigeration cabinet type 'Kristall-401-1' of 1551 capacity with one or two A D R U s whose evaporators were tested in thermal contact with heat-transfer panels of different designs. In the first set of experiments the evaporator of A D R U was thermally connected with a U-shape heat tube connected to the panel which formed a freezing chamber.
51 9
The temperature inside the refrigeration cabinet and the environmental temperature were constantly monitored. The temperature transducers in the refrigeration cabinet were mounted in three equally spaced planes parallel to the bottom. The temperature measurement error was not more than + / - 0 . 5 ° C . All measurements were taken in steady-state conditions. Used as a measuring instrument was an electronic measuring system which allowed for taking measurements from 22 temperature transducers within 30 s. As a result of the measurements (at the ambient air temperature of 20°C) it was found that the average temperature in the freezing chamber was -18°C. No appreciable difference of temperatures was observed in the refrigeration chamber between the central and lower planes. These temperatures were within minus 1 to minus 2°C. The evaporator construction of three-dimensional version with separately located freezing and refrigerating sections, as well as the small capacity of the freezing chamber tangibly impair the consumer qualities of that construction. The aforesaid disadvantages can be avoided by using stock-produced A D R U s with evaporators of flat construction, attached to which are heat tubes with developed fins. Use of various forms of fin on the heat tubes makes it possible to design a refrigeration cabinet with increased freezing chamber or to create a single volume of freezing cabinet with uniform temperature field. However, as can be seen from the calculations and experimental data, at the ambient air temperature of 20°C the average temperature in the refrigeration cabinet of that construction will not drop below minus 5°C, i.e. the refrigeration cabinet cannot function as a freezer. Under the conditions of efficient full-scale production of refrigeration cabinets and A D R U s of certain standard sizes the problem of increasing the usable space of a refrigeration cabinet and at the same time decreasing the temperature levels in the working chambers of the refrigeration cabinet, can be settled by increasing the number of units operating for one common volume. Such an engineering solution under temperate climatic conditions allows for obtaining a freezing chamber instead of refrigerator, or a corresponding refrigerator with an A D R U , if it takes place under tropical conditions. Installation of two units, particularly for the refrigeration cabinet type 'Kristall' of 155-1 capacity, can be affected without alterations, if one of the units has a flat-type evaporator while in the other unit it is of three-dimensional and flat constructions. The experience of designing this type of freezer-refrigerators exists in our Institute. Another set of experiments was carried out to determine the effectiveness of the refrigeration cabinet with two ADRUs whose evaporators are connected to heat-transfer panels. Performance of individual units of A D R U was checked against results of temperature measurements made on flat evaporator unit. Arrangement of temperature transducers is shown in Figure 1. Temperature transducer (a) was installed at the outlet of the condenser while (b), (c) and (d) were mounted to the evaporator in 0.45 m intervals starting from the inlet hole and (e) and (f) were placed to the top and bottom of the absorber, respectively. Results of temperature measurements are summarized in Table 1. Experiments show that A D R U integrated with heat-transfer panel operates satisfactorily over the whole examined range of ambient temperatures. It is known that the temperature of A D R U evaporator
G.F. Smirnov et al.
520
surface contrary to the temperature of evaporators of vapour-compression refrigerating machines varies continuously from the minimum value to the maximum one when no refrigerating effect is observed. Therefore, calculation of COP value for ADRU is arbitrary by its nature. On the other hand, no techniques for calculation of COP for ADRU are currently available and experimental data obtained for determination of this parameter are rather particular. If guided by determination of refrigerating capacity of ADRU from outside heat fluxes using experimental values of kF obtained by the technique in ref. 5, COP value for investigated ADRUs at temperatures in the refrigeration cabinet between 0 and -15°C will be 0.35. Used as a specimen was the refrigeration cabinet of refrigerator 'Kristall-404-1' of 155-1 capacity with the main ADRU having a three-dimensional and flat type evaporator which was in contact with the freezing chamber where a U-shape heat tube was mounted. The freezing chamber was formed by the plates of the heat tube finning. Mounted behind the main unit was the additional unit with evaporator of fiat construction. A system of L-shape and U-shape heat tubes with developed fins was attached to the evaporator located inside the refrigeration cabinet in parallel to its rear wall. The layout of the temperature transducers in the refrigeration cabinet under test are illustrated in Figure 3. Heat-transfer panels of used versions constructed on the basis of heat tubes are shown in Figure 4. The heattransfer panels are made from aluminium and ammonia is used as a refrigerating medium. Other designs of heat-transfer panels more judicious than investigated in this paper may also be used. Areas of heat-transfer surfaces of panels shown in Figure 4 are, respectively, Sa = 0.64m"2, Sb = 0.13m 2, Sc = 0.45m 2. The refrigeration cabinet was enclosed in a controlled confined space where the ambient temperature was preset and maintained within the range of 20 to 45°C
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Table 1 ADRU
Results o f t e m p e r a t u r e m e a s u r e m e n t s o v e r the s p a c e o f
Tableau 1 GFAD
Rdsultats des mesures de temperatures dans le circuit de
Temperature transducer
20
! 2 3 4 5 6
32 -27 -17 - 8 38 41
Ambient temperature,~'C 32 36 -24 -12 - 3 43 49
43 46 -22 - 8 1 48 59
with variations not exceeding + / - I ° C . In the course of the experiments the temperature field in the refrigeration cabinet and thermostatted space was continuously monitored. The time to reach steady-state conditions was from 5 to 7 h. The temperature variations within each plane did not exceed +/-0.5°C. The temperature distribution with the height of the refrigeration cabinet in three configurations A, B and C is illustrated in Figure 5. Within the entire range of the environmental temperature the variations of the temperature with the height of the refrigeration cabinet did not exceed +/-0.8°C, therefore to characterize the temperature field inside the refrigeration cabinet it is possible to use the arithmetic mean value of the readings received from all the temperature transducers mounted inside the cabinet. The relationship of the average temperature inside the refrigeration cabinet to the environmental temperature is shown in Figure 6. It can be seen that with the rise of the environmental temperature from 20 to 32°C the average temperature in the refrigeration cabinet is increased from -15 to -10°C, i.e. within that range of the environmental temperatures the construction under investigation proves to be a good refrigerator and provides a certain potential for designing a freezer on that basis.
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Figure 3 Scheme of arrangement of heat-transfer panels, evaporators and points of installation of temperature transducers: 1 - main ADRU e v a p o r a t o r ; 2 - a d d i t i o n a l A D R U e v a p o r a t o r ; 3 - h e a t tubes; 4 - fin t y p e f i b b i n g h e a t - t r a n s f e r panels; 5 - t e m p e r a t u r e t r a n s d u c e r s ; 6 - A D R U ; 7 r e f r i g e r a t e d c a b i n e t casing; 8 - r e f r i g e r a t e d c a b i n e t d o o r ; A , B , C z o n e s o f i n s t a l l a t i o n o f t e m p e r a t u r e t r a n s d u c e r s ; h = 1.0 m; a = 0.28 m; b = 0.35 m; c = 0.35 m; d = 0.38 m; e = 0.03 m ; f = 0.41 m F i g u r e 3 SchOna d'arrangement d'dvaporateurs avec p a n n e a u x de transmission de chaleur et endroits d'installation des thermocouples." I - ~vaporateur principal de G F A D ; 2 - dvaporateur additionnel de G F A D ; 3 - caloducs; 4 p a n n e a u x de transmission de chaleur ~ nervures-nageoires; 5 - thermocouple; 6 - GFAD; 7 corps de l'arreoire frigorifique; 8 - porte de l'armoire frigorifique; A , B , C des zones d'installation des thermocouples: h = 1,0 re," a = 0, 28 re," b = 0, 35 re; c = 0, 35re," d = 0 , 3 8 m ; e = O, 0 3 r e , ' f = 0 , 4 1 re
Absorption-diffusion units and heat-transfer panels
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Figure 4 Investigated versions of heat-transfer panels based on heat tubes: a - heat-transfer panel with U-shaped heat tube; b - heat-transfer tube with three L-shaped heat tubes; c - heat-transfer panel with two L-shaped heat tubes; 1 - heat tube; 2 - finning
Figure 4 Variantes ~tudi~es de panneaux de transmission de chaleur sur la base de tubes thermiques: a - panneau de transmission de chaleur avec tube thermique en II; b - panneau de transmission de chaleur avec trois tubes thermiques en F; c panneau de transmission de chaleur avec deux tubes thermiques en F; 1 - tube thermique; 2 - nervures-nageoires +
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T (H) Figure 5 Temperature distribution over the height of refrigerated cabinet at different ambient temperatures as a function of work time: l - generalizing lines; A,B,C - see Figure 3 Figure 5 R@artition de la temperature sur la hauteur de l'armoire frigorifique en fonction de la durde de fonctionnement pour diverses temperatures ambiantes: 1 - courbes de gdn#ralisation ; A,B,C - voir la l#gende de la Figure 3
When tropical conditions were simulated in the temperature controlled space, the average temperature in the refrigeration cabinet was 0°C, i.e. the construction under investigation demonstrated stable performance in the capacity of a refrigerator under tropical climatic conditions.
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t x (°C) Figure 6 Dependence of the average temperature in refrigerated cabinet from ambient temperature: l - generalization of the experimental data
Figure 6 Relation entre la temp#rature moyenne clans l'armoire frigorifique et la temp#rature ambiante; 1 - g#n~ralisation des donn~es expdrimentales
substantiate the field of effective application of ADRUs in designing freezing chambers and in creating domestic refrigerators with those units intended for service under tropical climatic conditions, as well as domestic refrigerators with ADRUs with usable volume of 300 to 4001.
References 1
Conclusions
2
By using heat-transfer panels the process of transfer and distribution of refrigeration in the internal spaces of the refrigeration cabinet with A D R U can be separated from refrigeration generating process. This allows better cooling within the refrigeration cabinet space. On the basis of the stock-produced refrigeration cabinet, ADRUs and heat transfer panels it is possible to
3 4
5
Swedish Patent no 177203, 125B15/10, appl. no. 10650/53 of Nov. 28 (1953) U.S. Patent no. 4003214, 125C1/08, appl. no. 645627 of Dec. 31 (1975) Eur. Patent no. 0326881, 125B15/10, 25/00, BI no. 32 (1989) Smirnov-Vasiliev, K. G., Chernyshov, V. F., Dvirny, V. V. et al. Application of heat tubes and thermal siphons in domestic refrigerators, thesis of paper for Sc. and Eng. Conf. Refrigeration for National Economy, Leningrad (1991) Chooklin, S. G., Chumak, L G. Laboratory manual for the course Refrigerating Machines Food Industry Publishers, Moscow (1974), 287
PIE S0140-7007(96)00039-4
Int J. Refrig. Vol. 19, No. 8, pp. 517- 521, 1996 Copyright © 1996 ElsevierScienceLtd and IIR Printed in Great Britain. All rights reserved 0140-7007/96/$15.00
Domestic refrigerators with absorption-diffusion units and heat-transfer panels G. F. Smirnov, M. A. Bukraba, T. Fattuh and B. Nabulsi O d e s s a Institute o f L o w - T e m p e r a t u r e T e c h n i c s a n d Energetics, 1/3 P e o t r Veliky Street, Odessa 270100, U k r a i n e Received 22 March 1994; revised 22 February 1996
The possibility of essential increasing of absorption-diffusion cooler set refrigerating power by means of separating cold transmission and distribution processes in the inner refrigerator volumes from cold generation processes has been shown. The problem is solved by using heat-transfer panels on the base of heat tubes and vaporization thermosiphons. The construction developed on the 'Kristall' type domestic cooler basis has attained a low temperature of -15°C in the 155-1 volume at the ambient temperature of 20°C and at the ambient temperature of 43°C it was allowed to reach - I ° C temperature without using additional heat insulation. Copyright © 1996 Elsevier Science Ltd and IIR (Keywords:refrigeratedcabinet;absorption-diffusionrefrigeratingunit;heat-transferpanel;heat tube;evaporatingthermal siphon)
Refrigerateurs menagers a groupes absorbeurs-diffuseurs et a panneaux echangeurs de chaleur La possibilitO est montr~e d'une augmentation considOrable du rendement frigorifique des rOfrig~rateurs absorbeurs-diffuseurs grhce ~ la s~paration des processus de gOnkration et distribution du from dans l'espace interne du r~frig~rateur. Le probl~me est r~solu par utilisation de panneaux Ochangeurs de chaleur sur la base de tubes thermiques et de thermosyphons ~vaporateurs. Les modifications constructives correspondantes d'un rOfrig~rateur m~nager du type 'Cristall" ont permis d'obtenir une temperature de - 15°C dans un volume de 155 l avec une temperature du milieu ambiant de 20°C et, avec une temp&ature du milieu ambiant de 43°C, une tempOrature de - 1 ° C sans calorifugeage suppl~mentaire. Copyright © 1996 Elsevier Science Ltd and IIR
(Mots cl+s:armoire frigorifique;groupe frigorifiqueabsorbeur diffuseur;panneau 6changeurde chaleur;tube thermique" thermosyphon 6vaporateur)
The absorption-diffusion refrigerating units (ADRU) have a number of advantages over compression ones: noiseless operation, absence of moving parts, easy attendance, possibility to use various heat sources, such as electric current, kerosene, etc. Construction of A D R U s comprises ramified systems of interconnected loops of natural circulation. Under the conditions, where the driving force of circulation is provided by density differential, a complex hydraulic system with phase transition processes in each element of A D R U restricts the rate of heat-and-mass exchange processes, which can affect adversely the level of refrigerating capacity of ADRUs. Simplified diagram of an A D R U is presented in Figure 1. The unit is made from carbon steel and charged with water-ammonia mixture containing 35% ammonia. Present within the unit is inert gas hydrogen and operating pressure in the unit is 1.7-2.0 MPa. Principle of operation of A D R U is described below. Nominal heat load amounting to 112 W is applied to the generator 1 and w a t e r - a m m o n i a solution boils. Ammonia vapour ascends by vapour line 2 to the condenser 3
where it is condensed and liquid ammonia enters the evaporator 5 by the line 4. In the evaporator, ammonia is evaporated and diffuses into hydrogen-ammonia mix of low ammonia content. Owing to these processes refrigerating effect in A D R U is achieved. Cooled vapour-gas hydrogen-ammonia mix of high ammonia content enters the receiver 6 and then ascends by coiled absorber 7 to the evaporator 5. Flow of vapourgas hydrogen-ammonia mix is induced by buyoancy force due to density difference in connection with varied concentration of ammonia. Brought to the top part of the absorber from the generator 1 by the line 8 due to action of thermocompressor operating according to vapourlift pattern is a weak water-ammonia solution which flows down through coiled absorber 7 and absorbs ammonia from the vapour-gas hydrogen-ammonia mix which ascends opposite to solution flow. As a result, water-ammonia solution of high ammonia concentration is formed and brought to the generator 1. Outer tubular surfaces of the evaporator 5 and absorber 7 included in the investigated unit have no fins. Overall length of the absorber is 6.5m, outside
517
G.F. Smirnov et al.
518
Nomenclature F k S
Inner surface of refrigeration cabinet (m 2) Heat-transfer coefficient (W m-2 K - 1) Heat-transfer panel area (m 2) Environmental temperature (°C) Temperature in planes A, B, C (°C)
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diameter of the tube is 18mm. As evidenced by experimental investigations, such absorber ensures reliable operation of A D R U at ambient temperatures up to 45°C. Evaporator tube is elliptic in cross-section. Overall length of the evaporator is 1.7 m and its area is 0.14 m 2. It is known that fins are required for effective performance of an ordinary-designed evaporator. However, finning gives rise to temperature variations over the inside space of A D R U and usable space is blocked up. Application of heat-transfer panel allows better solution of this problem provided that reliable thermal contact between surfaces of panel and evaporator is achieved. The condenser 3 is designed with fins of overall heat exchanging area of 0.30 m 2. Since the low refrigerating power of domestic ADRUs raises the problem of judicious use of energy resources in each element of the unit, the A D R U construction is provided with gas and liquid heat exchangers, finned constructions of condenser and absorber, which significantly complicates the units manufacturing process. Taking into consideration the aforesaid features of ADRUs it seems necessary to investigate the possibility of raising the rate of processes in ADRUs and increasing their refrigerating capacity materially by separating the processes of the transfer of refrigeration and distribution in refrigerator's internal spaces from refrigeration generating processes. An effective approach to solution of
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Figure 1 Simplified diagram of ADRU: 1 - generator; 2 - vapourc a r r y i n g line; 3 - c o n d e n s e r ; 4 - l i q u i d a m m o n i a line; 5 - e v a p o r a t o r ; 6 - r e c e i v e r t a n k ; 7 - a b s o r b e r ; 8 - w e a k s o l u t i o n line; 9 - r e f r i g e r a t i o n cabinet; l0 - points of installation of temperature transducers, (1)-(6) - nos of temperature transducers F i g u r e 1 S c h e m a simplifid de G F A D : 1 - g~ndrateur; 2 - conduite de vapeur; 3 - condenseur; 4 - conduite d'ammoniaque liquide; 5 dvaporateur; 6 - cuve du collecteur; 7 - absorbeur; 8 - conduite de solution f a i b l e ; 9 - armoire frigorifique; 10 - endroits d'installation des transformateurs thermiques; (1) . . . . . (6)-num~ros des transformateurs thermiques
tx
h a,b,c,d,e T
Average temperature inside the refrigeration cabinet (°C) Altitude of the refrigeration cabinet (m) Geometry dimension (m) Time of work (h)
this problem is concerned with application of heattransfer panels based on heat tubes or evaporating thermal siphons, which involved the problem of heatengineering designing of refrigeration cabinets with heattransfer panels and ADRUs.
Survey of the subject The application of heat-transfer panels based on heat tubes and thermal siphons, which allow judicious redistribution of heat fluxes, needs substantiation. Thus, in ref. 1 the refrigerating chamber evaporator is thermally connected with a metal sheet made in the form of radiator. In the later design of ice generator 2 use is made of a heat tube, which runs from the freezing chamber to the cooled fresh products chamber. In ref. 3 connected thermally to A D R U branches are heattransfer devices, where phase transition processes take place, while the heat-transfer devices themselves form the walls of the refrigeration cabinet. A set of experimental investigations into application of heat-transfer panels connected with evaporators of ADRUs has been carried out in the Scientific and Production Association of Applied Mechanics 4. The results of those investigations have been adopted in production and domestic refrigerators type 'Kristall' with ADRU and heat-transfer panels brought to a fullscale production level at one of the refrigeratorproducing factories. Along with the problem of judicious use of refrigeration, there exists a problem of simultaneously increasing the usable space and refrigerating effect. Under conditions of large-scale production of refrigeration cabinets of a certain standard size with increased internal usable space, these problems can be improved by using two or three ADRUs instead of one. Depending on the standard sizes of refrigeration cabinets and ADRUs this approach can be implemented by different designing methods. A good design for refrigeration cabinets of small standard size is provided by mounting the added A D R U behind the main unit in parallel to the latter. The depth of the cabinet, in this case, will be increased by 0.10m maximum. Configuration of both ADRUs must allow effective implementation of thermal insulation, as shown for example in Figure 2a. The use of heat-transfer panels makes it possible to newly solve the problem of ADRU's application in the construction of chest-type refrigeration cabinets. If the refrigeration cabinet is of the chest type, it is expedient to arrange the units in the plane parallel to the rear wall of the chest, as shown in Figure 2b. Such arrangement of the units promotes better distribution of evaporator surfaces over the refrigeration cabinet space and facilitates designing of a common heat-transfer panel. In such a case the panel may be designed both with
Absorption-diffusion units and heat-transfer panels (a)
(b)
(c)
Figure 2 Different design versions of refrigerated cabinets with absorption-diffusion refrigerating units: a - echelon arrangement of ADRU; b one-plane horizontal arrangement of A D R U ; c oneplane vertical arrangement of A D R U ; 1 - A D R U ; 2 - refrigerated cabinet Figure 2 Variantes de r~alisation d'armoires frigorifiques avec G F A D : a - arrangement de G F A D en dchelons; b - arrangement horizontal de G F A D dans le plan; e - arrangement vertical dans le plan de G F A D ; 1 G F A D ; 2 armoire frigor!fique
refrigerant-carrying piping for individual A D R U and with a common refrigerant-carrying piping depending on specific problems of refrigerator engineering. The thermal insulation of the evaporators, in this case, must be arranged so that the heat-transfer panels should not interfere with the process of placing products into the chest and taking them out. In a vertical version of refrigeration cabinet the A D R U may be arranged vertically in the same plane one above the other (see Figure 2c). Use of doubled or tripled units in one refrigeration cabinet as per the layout shown in Figure lc (without heat-transfer panels) is ineffective because it leads to irregularity of the temperature field and reduces the usable space of the refrigeration chamber. To decrease such irregularity it is advisable to use heat-transfer panels in the form of heat tubes or two-phase thermal siphons built into metal panels. The operating conditions of domestic refrigeration cabinets predetermine the location of these heat-transfer panels along the side and rear walls of the cabinets. It is easy to see that the total area of the evaporator can be substantially (by one and a half or two fold) increased as compared with that of the known evaporator design. Therefore, in spite of the added heattransfer panel, which creates an additional thermal resistance, the total heat flux removed will be increased. It is a crucial problem to reduce the contact's thermal resistance. The maximum area of contact between the A D R U ' s evaporator and the heat-transfer panel can be obtained by using the evaporator made in the form of a flat coil oriented in the vertical plane. This paper presents the experimental results which corroborate the possibility in principle to make a freezerrefrigerator with two A D R U s where the evaporators will provide service for the common heat-transfer panel with built-in heat tubes.
Description of test bench and investigation procedure Used as a subject of investigation was a refrigeration cabinet type 'Kristall-401-1' of 1551 capacity with one or two A D R U s whose evaporators were tested in thermal contact with heat-transfer panels of different designs. In the first set of experiments the evaporator of A D R U was thermally connected with a U-shape heat tube connected to the panel which formed a freezing chamber.
51 9
The temperature inside the refrigeration cabinet and the environmental temperature were constantly monitored. The temperature transducers in the refrigeration cabinet were mounted in three equally spaced planes parallel to the bottom. The temperature measurement error was not more than + / - 0 . 5 ° C . All measurements were taken in steady-state conditions. Used as a measuring instrument was an electronic measuring system which allowed for taking measurements from 22 temperature transducers within 30 s. As a result of the measurements (at the ambient air temperature of 20°C) it was found that the average temperature in the freezing chamber was -18°C. No appreciable difference of temperatures was observed in the refrigeration chamber between the central and lower planes. These temperatures were within minus 1 to minus 2°C. The evaporator construction of three-dimensional version with separately located freezing and refrigerating sections, as well as the small capacity of the freezing chamber tangibly impair the consumer qualities of that construction. The aforesaid disadvantages can be avoided by using stock-produced A D R U s with evaporators of flat construction, attached to which are heat tubes with developed fins. Use of various forms of fin on the heat tubes makes it possible to design a refrigeration cabinet with increased freezing chamber or to create a single volume of freezing cabinet with uniform temperature field. However, as can be seen from the calculations and experimental data, at the ambient air temperature of 20°C the average temperature in the refrigeration cabinet of that construction will not drop below minus 5°C, i.e. the refrigeration cabinet cannot function as a freezer. Under the conditions of efficient full-scale production of refrigeration cabinets and A D R U s of certain standard sizes the problem of increasing the usable space of a refrigeration cabinet and at the same time decreasing the temperature levels in the working chambers of the refrigeration cabinet, can be settled by increasing the number of units operating for one common volume. Such an engineering solution under temperate climatic conditions allows for obtaining a freezing chamber instead of refrigerator, or a corresponding refrigerator with an A D R U , if it takes place under tropical conditions. Installation of two units, particularly for the refrigeration cabinet type 'Kristall' of 155-1 capacity, can be affected without alterations, if one of the units has a flat-type evaporator while in the other unit it is of three-dimensional and flat constructions. The experience of designing this type of freezer-refrigerators exists in our Institute. Another set of experiments was carried out to determine the effectiveness of the refrigeration cabinet with two ADRUs whose evaporators are connected to heat-transfer panels. Performance of individual units of A D R U was checked against results of temperature measurements made on flat evaporator unit. Arrangement of temperature transducers is shown in Figure 1. Temperature transducer (a) was installed at the outlet of the condenser while (b), (c) and (d) were mounted to the evaporator in 0.45 m intervals starting from the inlet hole and (e) and (f) were placed to the top and bottom of the absorber, respectively. Results of temperature measurements are summarized in Table 1. Experiments show that A D R U integrated with heat-transfer panel operates satisfactorily over the whole examined range of ambient temperatures. It is known that the temperature of A D R U evaporator
G.F. Smirnov et al.
520
surface contrary to the temperature of evaporators of vapour-compression refrigerating machines varies continuously from the minimum value to the maximum one when no refrigerating effect is observed. Therefore, calculation of COP value for ADRU is arbitrary by its nature. On the other hand, no techniques for calculation of COP for ADRU are currently available and experimental data obtained for determination of this parameter are rather particular. If guided by determination of refrigerating capacity of ADRU from outside heat fluxes using experimental values of kF obtained by the technique in ref. 5, COP value for investigated ADRUs at temperatures in the refrigeration cabinet between 0 and -15°C will be 0.35. Used as a specimen was the refrigeration cabinet of refrigerator 'Kristall-404-1' of 155-1 capacity with the main ADRU having a three-dimensional and flat type evaporator which was in contact with the freezing chamber where a U-shape heat tube was mounted. The freezing chamber was formed by the plates of the heat tube finning. Mounted behind the main unit was the additional unit with evaporator of fiat construction. A system of L-shape and U-shape heat tubes with developed fins was attached to the evaporator located inside the refrigeration cabinet in parallel to its rear wall. The layout of the temperature transducers in the refrigeration cabinet under test are illustrated in Figure 3. Heat-transfer panels of used versions constructed on the basis of heat tubes are shown in Figure 4. The heattransfer panels are made from aluminium and ammonia is used as a refrigerating medium. Other designs of heat-transfer panels more judicious than investigated in this paper may also be used. Areas of heat-transfer surfaces of panels shown in Figure 4 are, respectively, Sa = 0.64m"2, Sb = 0.13m 2, Sc = 0.45m 2. The refrigeration cabinet was enclosed in a controlled confined space where the ambient temperature was preset and maintained within the range of 20 to 45°C
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7
Table 1 ADRU
Results o f t e m p e r a t u r e m e a s u r e m e n t s o v e r the s p a c e o f
Tableau 1 GFAD
Rdsultats des mesures de temperatures dans le circuit de
Temperature transducer
20
! 2 3 4 5 6
32 -27 -17 - 8 38 41
Ambient temperature,~'C 32 36 -24 -12 - 3 43 49
43 46 -22 - 8 1 48 59
with variations not exceeding + / - I ° C . In the course of the experiments the temperature field in the refrigeration cabinet and thermostatted space was continuously monitored. The time to reach steady-state conditions was from 5 to 7 h. The temperature variations within each plane did not exceed +/-0.5°C. The temperature distribution with the height of the refrigeration cabinet in three configurations A, B and C is illustrated in Figure 5. Within the entire range of the environmental temperature the variations of the temperature with the height of the refrigeration cabinet did not exceed +/-0.8°C, therefore to characterize the temperature field inside the refrigeration cabinet it is possible to use the arithmetic mean value of the readings received from all the temperature transducers mounted inside the cabinet. The relationship of the average temperature inside the refrigeration cabinet to the environmental temperature is shown in Figure 6. It can be seen that with the rise of the environmental temperature from 20 to 32°C the average temperature in the refrigeration cabinet is increased from -15 to -10°C, i.e. within that range of the environmental temperatures the construction under investigation proves to be a good refrigerator and provides a certain potential for designing a freezer on that basis.
A-___A
!
i~
d
vl
e
Figure 3 Scheme of arrangement of heat-transfer panels, evaporators and points of installation of temperature transducers: 1 - main ADRU e v a p o r a t o r ; 2 - a d d i t i o n a l A D R U e v a p o r a t o r ; 3 - h e a t tubes; 4 - fin t y p e f i b b i n g h e a t - t r a n s f e r panels; 5 - t e m p e r a t u r e t r a n s d u c e r s ; 6 - A D R U ; 7 r e f r i g e r a t e d c a b i n e t casing; 8 - r e f r i g e r a t e d c a b i n e t d o o r ; A , B , C z o n e s o f i n s t a l l a t i o n o f t e m p e r a t u r e t r a n s d u c e r s ; h = 1.0 m; a = 0.28 m; b = 0.35 m; c = 0.35 m; d = 0.38 m; e = 0.03 m ; f = 0.41 m F i g u r e 3 SchOna d'arrangement d'dvaporateurs avec p a n n e a u x de transmission de chaleur et endroits d'installation des thermocouples." I - ~vaporateur principal de G F A D ; 2 - dvaporateur additionnel de G F A D ; 3 - caloducs; 4 p a n n e a u x de transmission de chaleur ~ nervures-nageoires; 5 - thermocouple; 6 - GFAD; 7 corps de l'arreoire frigorifique; 8 - porte de l'armoire frigorifique; A , B , C des zones d'installation des thermocouples: h = 1,0 re," a = 0, 28 re," b = 0, 35 re; c = 0, 35re," d = 0 , 3 8 m ; e = O, 0 3 r e , ' f = 0 , 4 1 re
Absorption-diffusion units and heat-transfer panels
(a)
521
(c)
(b)
r -
i~
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_
_-
_
_
-~
380
-.
Figure 4 Investigated versions of heat-transfer panels based on heat tubes: a - heat-transfer panel with U-shaped heat tube; b - heat-transfer tube with three L-shaped heat tubes; c - heat-transfer panel with two L-shaped heat tubes; 1 - heat tube; 2 - finning
Figure 4 Variantes ~tudi~es de panneaux de transmission de chaleur sur la base de tubes thermiques: a - panneau de transmission de chaleur avec tube thermique en II; b - panneau de transmission de chaleur avec trois tubes thermiques en F; c panneau de transmission de chaleur avec deux tubes thermiques en F; 1 - tube thermique; 2 - nervures-nageoires +
I
+
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-o- B + C
toc = 43°C
40
-5 0
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6" o
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)oc = 32°C
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o .1=
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/ ot c = 20°C
o,
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I
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20
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T (H) Figure 5 Temperature distribution over the height of refrigerated cabinet at different ambient temperatures as a function of work time: l - generalizing lines; A,B,C - see Figure 3 Figure 5 R@artition de la temperature sur la hauteur de l'armoire frigorifique en fonction de la durde de fonctionnement pour diverses temperatures ambiantes: 1 - courbes de gdn#ralisation ; A,B,C - voir la l#gende de la Figure 3
When tropical conditions were simulated in the temperature controlled space, the average temperature in the refrigeration cabinet was 0°C, i.e. the construction under investigation demonstrated stable performance in the capacity of a refrigerator under tropical climatic conditions.
-8
0
t x (°C) Figure 6 Dependence of the average temperature in refrigerated cabinet from ambient temperature: l - generalization of the experimental data
Figure 6 Relation entre la temp#rature moyenne clans l'armoire frigorifique et la temp#rature ambiante; 1 - g#n~ralisation des donn~es expdrimentales
substantiate the field of effective application of ADRUs in designing freezing chambers and in creating domestic refrigerators with those units intended for service under tropical climatic conditions, as well as domestic refrigerators with ADRUs with usable volume of 300 to 4001.
References 1
Conclusions
2
By using heat-transfer panels the process of transfer and distribution of refrigeration in the internal spaces of the refrigeration cabinet with A D R U can be separated from refrigeration generating process. This allows better cooling within the refrigeration cabinet space. On the basis of the stock-produced refrigeration cabinet, ADRUs and heat transfer panels it is possible to
3 4
5
Swedish Patent no 177203, 125B15/10, appl. no. 10650/53 of Nov. 28 (1953) U.S. Patent no. 4003214, 125C1/08, appl. no. 645627 of Dec. 31 (1975) Eur. Patent no. 0326881, 125B15/10, 25/00, BI no. 32 (1989) Smirnov-Vasiliev, K. G., Chernyshov, V. F., Dvirny, V. V. et al. Application of heat tubes and thermal siphons in domestic refrigerators, thesis of paper for Sc. and Eng. Conf. Refrigeration for National Economy, Leningrad (1991) Chooklin, S. G., Chumak, L G. Laboratory manual for the course Refrigerating Machines Food Industry Publishers, Moscow (1974), 287