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PROOF
The effect of radiation shields around the air condenser and
compressor of a refrigerator on the temperature
distribution inside it
Clito Afonso
*
, Joaquim Matos
Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200 465 Porto, Portugal Received 12 January 2005; received in revised form 16 January 2006; accepted 30 January 2006
Abstract
In almost all domestic refrigeratorsefreezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the com-pressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigeratorefreezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigeratorefreezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radi-ation. The improvement is achieved by placing a radiation shield e a sheet of aluminium foil e over the surfaces close to the condenser and the compressor. For validating this technique a refrigeratorefreezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigeratorefreezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both sit-uations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out.
Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations. Ó 2006 Elsevier Ltd and IIR.
Keywords: Freezers; Refrigerators; Air temperature and air velocities inside freezers; Radiation shields
1. Introduction
Since long time ago all the components of commercial domestic refrigeratorsefreezers have been assembled in the same relative position. The four typical equipments of the vapor compression refrigeration systems, the evaporator, compressor, expansion valve and the condenser, are placed
almost in the same position in all the refrigeratorsefreezers. While the compressor is located in a recess at the bottom of the refrigerator, the natural air-cooled condenser is located in the rear wall of the refrigerator. Of course there have been differences in the refrigerators along the time mainly due to minor esthetic aspects and, of course, due to the use of the new zero depletion ozone refrigerants [1,2]. Thermodynamically speaking, all refrigerators are based on conventional vapor compression refrigeration cycles
[3,4]. Usually the refrigerant after adiabatic expansion in a capillary tube or some type of expansion valve[5]flows
* Corresponding author. Tel.:þ351 225081746. E-mail address:clito@fe.up.pt(C. Afonso). 0140-7007/$35.00Ó 2006 Elsevier Ltd and IIR. doi:10.1016/j.ijrefrig.2006.01.006
International Journal of Refrigeration xx (2006) 1e8
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through the freezer evaporator and then through the refrigerator evaporator. After heat gain in both spaces, the refrigerant effect, the refrigerant is compressed and then releases the accumulated energy in the condenser, usually of a natural air-cooled type, closing in that way the refriger-ation cycle.
Previous experimental work done[6,7]had shown that the outside surface temperature of the rear wall where the condenser is located as well as the walls close to the com-pressor have a much higher temperature than the outside air with which all freezererefrigerator surfaces exchange heat. This is due to the heat release from the condenser and from the compressor. This causes larger heat gains to the freezererefrigerator through those walls when compared with the others.
In order to evaluate the effect of the heat release of the condenser and the compressor on the thermal behavior of this kind of equipment, tests have been done in a new com-mercially available refrigeratorefreezer. Modifications in the rear wall and in the recess walls where the compressor is located in the refrigeratorefreezer have been carried out separately in order to evaluate the individual influence of each modification. In this work, the modifications simply consist in placing a radiation shield composed of a sheet of aluminium foil glued over to these walls. After the eval-uation of the influence of each modification of the surfaces on the internal air temperatures and energy consumption of the compressor a final test was done with the radiation shield placed simultaneously in the rear wall and in the recess walls of the compressor.
The refrigeratorefreezer was monitored with previously calibrated thermocouples and with an energy meter. All the measured values were recorded in a data logger, [8], for further analysis.
With the measured temperatures of the interior walls of the refrigerator cabinet as boundary conditions for a com-mercial available code, Fluent [9], simulation of all air temperature distributions and air velocities were obtained. The simulated temperatures were compared with the mea-sured ones, the overall differences being very small, which validate the simulation carried out.
Results have shown that application of radiation shields (aluminium foil or some reflective painting) to improve ther-mal insulation properties,[10,11], when used in refrigera-torsefreezers, is an effective way to decrease the internal air temperatures and only with minor additional costs. In the tests carried out differences around 2 K were recorded.
2. Methodology
The freezer,Fig. 1, works with a standard vapor compres-sion cycle with 0.13 kg of R-134a as a refrigerant. The re-frigerant after adiabatic expansion in the capillary tube enters the upper cabinet freezer (negative air temperatures) and then flows to the refrigerator evaporator (positive air temperatures). The cycle is closed with the refrigerant flow-ing through the hermetic compressor sited at the bottom of the refrigerator and after through the natural convection air-cooled condenser. The compressor has a nominal current of 0.77 A and a nominal voltage of 220 V. The net inside volume is 224 l.
The double door refrigeratorefreezer was monitored with previously calibrated thermocouples type T (copper, constantin) and located in several points of the internal
Fig. 1. View of the refrigeratorefreezer.
Nomenclature
~F external body forces r~g gravitational body force p static pressure Sm mass added ~v velocity Greek letters t time r density ~ G stress tensor 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224
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PROOF
surfaces, outside surfaces, inside air in both cabinets (at three different levels) and outside air, numbered as shown inFig. 2. This location of the thermocouples enables to mea-sure all the spatial inside surface temperatures as well as the inside air temperatures. For the test with the aluminium foil, three additional thermocouples no. 15, 18 and 25 (not shown inFig. 2) where placed on the inside surface of it. All ther-mocouples, which have an uncertainty of0.25C, were
connected to a data acquisition system that reads all temper-atures at 20 s intervals. As it was necessary to have the right spatial positions of all inside air thermocouples, they were
fixed on rulers to be possible to compare the measured values with the simulated ones from the Fluent.
At the same time the compressor power and energy consumption were measured and the results were kept in a conventional PC for later analysis.
The radiation shield used in the tests was of the type of aluminium foil with a thickness of 3m 5%. The emissivity (value given by the producer) is 0.07.
Four tests were carried out: the first one with the refrig-eratorefreezer as it comes from the factory (reference test), the second one with the radiation shield on the rear wall, a third one with the radiation shield in the recess of the compressor and finally a fourth one with the radiation shield on the rear wall and on the recess.
In all the tests the refrigeratorefreezer had no internal loads and at the beginning it was in thermal equilibrium with the ambient air (door open). All tests ended when steady state was reached, i.e., when the refrigeration cycles were similar. During the entire tests, the door of the freezer and of the refrigerator was permanently closed.
The measured temperatures of the internal walls were used as input in a commercial code,[9], as boundary condi-tions, to simulate all spatial and temporal surface tempera-tures, inside air temperatures as well as air velocities in one point or plane inside the freezer.
3. Results
3.1. Original refrigeratorefreezer
The temperature evolution measured with the thermo-couples mentioned in Section 2and located on the points shown inFig. 2is shown inFigs. 3 and 4for the refrigerator and the freezer, respectively.
Fig. 2. Location of the thermocouples inside the refrigeratore freezer.
Fig. 3. Temperature evolutions in the refrigerator. 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336
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The transient regime took approximately 51 min from the starting point in which the freezer was in thermal equi-librium with the outside environment until the minimum sur-face temperature of the evaporator was reached and the compressor stopped working. From Fig. 3 it can also be seen that the compressor works between a maximum sur-face evaporator temperature of 5.5C and a minimum of 24.5C, and stopped working in the reverse direction of
the so mentioned temperatures (thermocouple 4 e bottom curve of the graph). The entire cycle takes about 35 min, be-ing approximately 42% of the time spent with the compres-sor working and the other 58% when the comprescompres-sor stopped working (Table 1).
The measured outside air temperature in the laboratory was almost constant and equal to 23.8C with a standard de-viation of 0.2C (thermocouple 13) e horizontal fourth line from the top ofFig. 3.
Generally speaking, all the measured temperatures of the inside air follow the compressor working cycle (set of 10 lines almost superimposed in the middle of the figure, third line from the bottom of the figure). On the top of these lines are the ones that represent the measured wall temperatures. The first three are also superimposed and with a much lower
temperature amplitude when compared with the air temper-atures. Over these ones is the line that represents the bottom left wall temperature (thermocouple 28). On an average the temperature is about 4 K higher than the other ones and with much larger amplitude of temperature. This is due to the heat release of the compressor sited close to this surface.
Over these lines there are two more that describes also the wall temperatures and shows an exponential decay until steady state is reached and then a constant temperature is at-tained (fifth and sixth line from the top of the figure). These temperatures were measured inside the existing bottom box for vegetables in the left wall (thermocouple 24) and on the bottom (thermocouple 26). While the first one is due to the existing bottom box for vegetables that somehow isolates this part of the refrigerator from the upper part, the second one is due to the proximity of the compressor and also due to the mentioned box. The measured temperature of this sur-face when steady state is reached is about 2C higher than the left one.
The temperature of the wall that supports the evaporator was also measured (thermocouple 22 e second line from the bottom of the graph). As can be seen it is the wall that has the lowest temperature. The maximum temperature picks are similar to the evaporator surface while the minimum mea-sured temperature is around4 C.
The outside surface temperature of the rear wall was also measured by three thermocouples (15, 18 and 25 as already mentioned) e first three lines from the top of the figure. While the first two are in front of the air condenser and are affected in a very similar way by the heat release from it, the last one is located above the others not being so af-fected by the air condenser. So, while in the first two the measured temperatures varied on average from 27.0C to 35.3C, in the last one the variation lies between 23.0C and 29.0C.
The inside air temperatures of the refrigerator are in some points higher than the recommended one for this kind of refrigerator, which should be between 5 and 7C,
[12e14]. Table 2 shows the maximum and minimum air temperatures measured by the three thermocouples located on four different levels (see Fig. 2). These temperatures were recorded in the vicinity time of the start and stop of
Fig. 4. Freezer surface wall and air temperatures.
Table 1
Compressor cycle operation for different locations of the radiation shield
Radiation shield Cycle time (min) (%) Stopped (%) Running Time to reach steady state (min) Without 33.3 61.0 39.0 51 Rear wall 35.3 61.3 38.6 42 Comp. recess 34.3 62.1 37.8 42 Rear wallþ comp.
recess
36.3 62.4 37.6 52
Table 2
Minimum and maximum inside air temperature at different heights in the refrigerator (C) for different locations of the radiation shield (rs)
Height (cm)
Without rs Rear wall with rs Recess with rs Real wall þ recess with rs 83.5 4.5/6.6 4.3/6.9 4.1/5.9 3.9/6.3 58 3.8/7.0 2.6/6.9 3.7/6.3 2.5/5.2 53 2.9/7.4 1.8/6.3 2.5/6.1 1.1/5.1 35 3.1/6.5 2.3/5.7 2.2/5.3 1.8/5.6 Taver. 3.6/6.9 2.8/6.5 3.1/5.9 2.3/5.5 s 0.6/0.35 0.93/0.49 0.79/0.37 1.0/0.47 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448
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PROOF
the compressor during steady state. As can be seen, in the medium and bottom levels of the refrigerator, the maximum air temperature exceeds the recommended ones. It is also possible to conclude that the inside air temperatures de-crease on average from top to bottom when the compressor is running (minimum air temperatures). This is due to the ef-fect of the evaporator. In an opposite way, when the com-pressor is stopped the inside air temperatures increase from top to bottom of the refrigerator (maximum air temper-atures). This is due to the effect of the dividing top wall of the refrigeratorefreezer that is much colder than the other surfaces. However, the average air temperatures in the me-dium height of the refrigerator are higher when compared with the other ones and the recommended ones. This is due to a stagnant air zone in that region already verified with thermal simulations carried out[8].
The results obtained in the freezer are shown inFig. 4. As it can be seen, the surface temperatures of the top, bottom, left and right walls are coincident (thermocouples 6, 10, 16 and 20 e superimposed bottom lines and strongly oscil-lating). This is due to the evaporator that is located on those surfaces. After steady state, their measured minimum tem-perature is about27C and maximum is about15C.
The rear wall is the one that shows higher temperatures when compared to the others (thermocouple 7 e top line of the figure). This is due to the presence of the hot air con-denser close to this surface that gives rise to higher heat gains through this surface and hence higher inside wall tem-perature. The maximum measured inside wall temperature after steady state is 13.8C and the is minimum
17.5C. The inside surface door temperature
(thermocou-ple 19) lies between the others. The two measured inside air temperatures have the same values (thermocouples 9 and 17) and so their representative lines are superimposed in the fig-ure (second line from the bottom of the figfig-ure). After steady state the maximum and minimum air temperatures are 16C and22.2C, respectively. The air temperatures
inside the freezer (thermocouples 6 and 9) varied between 16C and21C,Table 3, which match the regulations.
3.2. Radiation shield only on the rear wall, only on the compressor recess and on both surfaces of the refrigeratorefreezer
Energy efficiency is improved by the application of a ra-diation shield. Several possibilities were investigated in the
same way as for the refrigeratorefreezer system without ra-diation shield. Tests were carried out independently, one by one. The aluminium foil was glued first on the rear wall of the refrigeratorefreezer (the three thermocouples 15, 18 and 25 where then covered by it), then on the compressor re-cess and finally on both the surfaces. The going of the tem-perature evolutions in the three tests are identical toFigs. 3 and 4, respectively for the refrigerator and freezer, except for the temperature limits and cycle times. The main results are shown inTables 1e3.
As it can be seen in these three cases the cycle time is lon-ger when compared with the refrilon-geratorefreezer in its orig-inal situation due to the increased time when the compressor does not work. It can also be seen that the compressor run-ning time is lower. Regarding the time to reach the steady state no conclusion can be drawn from the tests as there is a significant reduction in time when the radiation shield is located only in the rear wall and only in the compressor re-cess (9 min less than the original situation) and there is an increase in time when the radiation shield is located over both the surfaces (1 min more than the original situation).
Regarding the air temperatures inside the refrigerator it can be seen that the minimum air temperatures decrease from the top to the bottom of the refrigerator, except for the lower level (located at a height of 35 cm) that is on average slightly higher than the immediate upper level (height of 53 cm). As these air temperatures are reached when the compressor did not work, this can be explained by the cooling effect of the much colder evaporator. It can also be seen that the minimum inside air tem-peratures are reached when the radiation shield is mounted on the rear wall and on the compressor recess. Thus, it can be con-cluded that the radiation shield has a positive effect on the air temperatures inside the freezer system.
Regarding the maximum inside air temperatures inside the refrigerator (when the compressor is about to start run-ning) it can be seen that on average they increase from bot-tom to the top of the refrigerator. This is expected due to the heat gain through the walls.
Again, the radiation shields located in the rear wall of the refrigerator and in the compressor recess have an effect of lowering on average the inside air temperatures.
Concerning the freezer, there are no significant variations in the air temperatures due to the location of the radiation shields,Table 3.
3.3. Thermal simulations of the refrigeratorefreezer
For the 3D thermal simulations the standardke3 turbu-lent flow model was used in a commercial available code, the Fluent. This program based on a finite volume method,
[15], solves the conservation equations for mass and mo-mentum, respectively, Eqs.(1) and (2):
vr
vtþ Vðr~vÞ ¼ Sm ð1Þ
Table 3
Minimum and maximum inside air temperature in the freezer (C) for different locations of the radiation shield
Radiation shield Air temperatures Without 22.2/16 Rear wall 21.4/16 Comp. recess 20/17 Rear wallþ comp. recess 22/16 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560
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PROOF
v
vtðr~vÞ þ Vðr~v~vÞ ¼ Vp þ V~Gþ r~gþ ~F ð2Þ
These equations were integrated over each small control volume around discrete points in the flow domain. The grid resolution has 432 000 control volumes. A non-uniform grid was used to reduce the volumes and was refined close to the walls. The minimal and maximal control volumes were, respectively, 1.101 108m3 and 7.155 106m3. The convergence criteria were 102for the mass conservation equation and superior to 105for other equations.
For the mathematical model, the refrigerator cabinet was considered as a parallelepiped with internal dimensions of (1.05 0.48 0.45) m3
and the air inside was the default fluid. The measured temperatures of all internal surfaces (two in each vertical surface and one in each horizontal sur-face) were used as boundary conditions in the code. It can be found in the literature of similar simulations, but the bound-ary conditions were constant and calculated, not measured
[16]. The maximum air temperature difference between the measured and simulated ones was about 1 K, which is a quite good result[17,18].
Figs. 5 and 6show the results obtained with this code for the simulated refrigerator as it come from the factory and with the radiation shield on the rear wall and compressor re-cess, respectively. The results of the thermal simulations were obtained for the minimum evaporator temperature. As can be seen in the central plane (plane perpendicular to the door and on the center of the refrigerator) as well as in the lateral plane (plane parallel to the door and in the center of the refrigerator) e first two sketches ofFigs. 5 and 6e there are lower air temperatures inside the refrigerator when the radiation shield is placed. As can also be seen in the velocities’ profiles (shown as vectors) on three different levels e bottom sketch ofFigs. 5 and 6ethere are two main air loops inside the refrigerator: one clockwise with a leg of descending cold air close to the evaporator and an ascending warmer air leg close to the door (see central plane); a second one, not so clear, but somehow also clockwise. It is also possible to conclude that in the central part of the refrigera-tor there is an almost stagnant air zone (very low air move-ments) as already noticed in the measurements (seeTable 2). This effect is most pronounced for the system without radi-ation shield. These conclusions can also be seen in the graphs of temperature and velocities’ distributions in three different horizontal levels inside the refrigerator.
3.4. Energy consumption
In spite of a slight decrease in the percentage of the run-ning time of the compressor with the location of the radia-tion shield in all the rear sides of the refrigeratorefreezer, the energy consumption of the compressor is almost con-stant, about 0.031 kWh per cycle in all tests. This may be due to the control thermostat that is located in the mid-height
of the refrigerator where the temperatures are higher than in the bottom of the refrigerator. Thus, changing the placement of the thermostat could contribute to a decrease in the energy consumption of the compressor.
4. Conclusions
In this work it was shown that with minor modifications in the available commercial refrigeratorefreezers it is
Fig. 5. Results of the thermal simulations with the original refrigeratorefreezer. 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672
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PROOF
possible to have a decrease in the inside air temperatures. These modifications simply consist in covering the external rear wall of the refrigerator and the compressor recess wall with a sheet of aluminium foil avoiding in that way the radi-ation heat transfer from the condenser and the compressor to the external surfaces of the refrigerator.
Results have shown that when those surfaces are covered with a sheet of aluminium foil there is a significant decrease in the inside air temperatures of the refrigerator and of the freezer, about 2 K. This is enough to ensure that the inside air temperatures stay within the limits of the recommended
ones. With a proper location of the refrigerator thermostat this can contribute to a lower energy consumption of the compressor.
Thermal simulations of the refrigerator were also done with a commercial code, Fluent, and the results are in full agreement with the temperature measurements carried out (the maximum air temperature difference between the mea-sured and simulated ones was about 1 K) which suggests that it is a powerful tool to simulate refrigerators. From the ther-mal simulations carried out, it is possible to conclude that there are two air flows inside the refrigerator that creates in the middle of the refrigerator, a zone where the air is al-most stagnant. Thus perishable products should not be kept in that zone.
Acknowledgements
The authors are grateful to the support given to this work by Instituto de Engenharia Mecaˆnica, Polo FEUP e IDMEC e Porto, Portugal.
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