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View metadata, citation and similar papers at core.ac.ukARTICLE IN PRESS brought to you by CORE provided by Repositório Aberto da Universidade do Porto 1 57 2 58 3 59 4 International Journal of Refrigeration xx (2006) 1e8 60 5 www.elsevier.com/locate/ijrefrig 61 6 62 7 63 8 64 9 The effect of radiation shields around the air condenser and 65 10 compressor of a refrigerator on the temperature 66 11 67 12 distribution inside it 68 13 69 14 70 15 Clito Afonso*, Joaquim Matos 71 16 72 Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200 465 Porto, Portugal 17 73 18 Received 12 January 2005; received in revised form 16 January 2006; accepted 30 January 2006 74 19 PROOF 75 20 76 21 77 22 Abstract 78 23 79 In almost all domestic refrigeratorsefreezers all components are assembled in the same relative position since several years 24 80 ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the com- 25 81 pressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, 26 82 part of it is due to thermal radiation that causes an overheating of the refrigeratorefreezer surfaces adjacent to those equipments. 27 83 As a consequence there are more heat gains to the refrigeratorefreezer through these surfaces and hence higher air temperatures 28 84 inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radi- 29 85 ation. The improvement is achieved by placing a radiation shield e a sheet of aluminium foil e over the surfaces close to the 30 86 condenser and the compressor. For validating this technique a refrigeratorefreezer was monitored with thermocouples for the 31 87 measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this 32 88 practice the average inside air temperatures in the refrigeratorefreezer could decrease to about 2 K. An available commercial 33 89 code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both sit- 34 90 uations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which 35 91 validates the experiments carried out. 36 92 Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations. 37 93 Ó 2006 Elsevier Ltd and IIR. 38 94 39 95 Keywords: Freezers; Refrigerators; Air temperature and air velocities inside freezers; Radiation shields 40 96 41 97 42 98 43 99 1. Introduction almost in the same position in all the refrigeratorsefreezers. 44 100 While the compressor is located in a recess at the bottom of 45 101 Since long time ago all the components of commercial the refrigerator, the natural air-cooled condenser is located 46 102 domestic refrigeratorsefreezers have been assembled in in the rear wall of the refrigerator. Of course there have 47 103 the same relative position. The four typical equipments of been differences in the refrigerators along the time mainly 48 104 the vapor compression refrigerationUNCORRECTED systems, the evaporator, due to minor esthetic aspects and, of course, due to the 49 105 compressor, expansion valve and the condenser, are placed use of the new zero depletion ozone refrigerants [1,2]. 50 106 Thermodynamically speaking, all refrigerators are based 51 107 on conventional vapor compression refrigeration cycles 52 108 * Corresponding author. Tel.: 351 225081746. [3,4]. Usually the refrigerant after adiabatic expansion in 53 þ 109 E-mail address: [email protected] (C. Afonso). a capillary tube or some type of expansion valve [5] flows 54 110 55 0140-7007/$35.00 Ó 2006 Elsevier Ltd and IIR. 111 56 doi:10.1016/j.ijrefrig.2006.01.006 112 JIJR1130_proof 12 April 2006 1/8 ARTICLE IN PRESS 2 C. Afonso, J. Matos / International Journal of Refrigeration xx (2006) 1e8 113 Results have shown that application of radiation shields 169 114 Nomenclature (aluminium foil or some reflective painting) to improve ther- 170 115 mal insulation properties, [10,11], when used in refrigera- 171 ~F external body forces 116 torsefreezers, is an effective way to decrease the internal 172 r~g gravitational body force 117 air temperatures and only with minor additional costs. In 173 p static pressure 118 the tests carried out differences around 2 K were recorded. 174 Sm mass added 119 175 ~v velocity 120 2. Methodology 176 121 Greek letters 177 122 t time The freezer, Fig. 1, works with a standard vapor compres- 178 123 r density sion cycle with 0.13 kg of R-134a as a refrigerant. The re- 179 124 ~G stress tensor frigerant after adiabatic expansion in the capillary tube 180 125 enters the upper cabinet freezer (negative air temperatures) 181 126 and then flows to the refrigerator evaporator (positive air 182 127 temperatures). The cycle is closed with the refrigerant flow- 183 128 through the freezer evaporator and then through the ing through the hermetic compressor sited at the bottom of 184 129 refrigerator evaporator. After heat gain in both spaces, the the refrigerator and after through the natural convection 185 130 refrigerant effect, the refrigerant is compressed and then air-cooled condenser. The compressor has a nominal current 186 131 releases the accumulated energy in the condenser, usually of 0.77PROOF A and a nominal voltage of 220 V. The net inside 187 132 of a natural air-cooled type, closing in that way the refriger- volume is 224 l. 188 133 ation cycle. The double door refrigeratorefreezer was monitored 189 134 Previous experimental work done [6,7] had shown that with previously calibrated thermocouples type T (copper, 190 135 the outside surface temperature of the rear wall where the constantin) and located in several points of the internal 191 136 condenser is located as well as the walls close to the com- 192 137 pressor have a much higher temperature than the outside 193 138 air with which all freezererefrigerator surfaces exchange 194 139 heat. This is due to the heat release from the condenser 195 140 and from the compressor. This causes larger heat gains to 196 141 the freezererefrigerator through those walls when compared 197 142 with the others. 198 143 In order to evaluate the effect of the heat release of the 199 144 condenser and the compressor on the thermal behavior of 200 145 this kind of equipment, tests have been done in a new com- 201 146 mercially available refrigeratorefreezer. Modifications in 202 147 the rear wall and in the recess walls where the compressor 203 148 is located in the refrigeratorefreezer have been carried out 204 149 separately in order to evaluate the individual influence of 205 150 each modification. In this work, the modifications simply 206 151 consist in placing a radiation shield composed of a sheet 207 152 of aluminium foil glued over to these walls. After the eval- 208 153 uation of the influence of each modification of the surfaces 209 154 on the internal air temperatures and energy consumption 210 155 of the compressor a final test was done with the radiation 211 156 shield placed simultaneously in the rear wall and in the 212 157 recess walls of the compressor. 213 158 The refrigeratorefreezer was monitored with previously 214 159 calibrated thermocouples and with an energy meter. All the 215 160 measured values were recordedUNCORRECTED in a data logger, [8], for 216 161 further analysis. 217 162 With the measured temperatures of the interior walls of 218 163 the refrigerator cabinet as boundary conditions for a com- 219 164 mercial available code, Fluent [9], simulation of all air 220 165 temperature distributions and air velocities were obtained. 221 166 The simulated temperatures were compared with the mea- 222 167 sured ones, the overall differences being very small, which 223 168 validate the simulation carried out. Fig. 1. View of the refrigeratorefreezer. 224 JIJR1130_proof 12 April 2006 2/8 ARTICLE IN PRESS C. Afonso, J. Matos / International Journal of Refrigeration xx (2006) 1e8 3 225 fixed on rulers to be possible to compare the measured 281 226 values with the simulated ones from the Fluent. 282 227 At the same time the compressor power and energy 283 228 consumption were measured and the results were kept in 284 229 a conventional PC for later analysis. 285 230 The radiation shield used in the tests was of the type of 286 231 aluminium foil with a thickness of 3m Æ 5%. The emissivity 287 232 (value given by the producer) is 0.07. 288 233 Four tests were carried out: the first one with the refrig- 289 234 eratorefreezer as it comes from the factory (reference 290 235 test), the second one with the radiation shield on the rear 291 236 wall, a third one with the radiation shield in the recess of 292 237 the compressor and finally a fourth one with the radiation 293 238 shield on the rear wall and on the recess. 294 239 In all the tests the refrigeratorefreezer had no internal 295 240 loads and at the beginning it was in thermal equilibrium 296 241 with the ambient air (door open). All tests ended when steady 297 242 state was reached, i.e., when the refrigeration cycles were 298 243 similar.PROOF During the entire tests, the door of the freezer and 299 244 of the refrigerator was permanently closed.

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