© 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.                     o...
Until recently, the economizer cycle                                                                                     w...
system unloading strategy. (As known,                                                           Hence, it is not uncommon ...
1.05                                                                                                   1.05Capacity Ratio ...
for the R-410A economizer systems, while any conventional and a decision is made regarding incremental design augmenta-app...
but a cost parity point is quite different for R-410A and economized configurations are still limited, particularly forR-13...
Next, we will consider the economiz-                                                                 Concluding Remarkser-...
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Ciclos Refrigerantes Regenerativos 2005


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Ciclos Refrigerantes Regenerativos 2005

  1. 1. © 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae. org). Reprinted by permission from ASHRAE Journal, (Vol. 47, No. 7, July 2005). This article may not be copied nor distributed in either paper or digital form without ASHRAE’s permission.Is Economizer CycleJustified forAC ApplicationsBy Michael F. Taras, Member ASHRAER -410A is becoming increasingly popular in commercial air-con- as well as incremental improvements to existing performance augmentation tech- ditioning applications. Although several design enhancement niques. Consequently, new systems may incorporate novel design approaches,options are feasible, R-410A still exhibits a performance deficiency such as an economizer cycle, that in the past would not have been considered forat certain environmental conditions in comparison to R-22 that is similar applications. The choice of a particular systembeing phased out. These issues present new challenges to system design and configuration is defined by essential performance characteristics,designers and must be addressed in the most comprehensive and component reliability, applied cost and specific application considerations.cost-effective manner. This article evaluates the advantages offered by the economizer cycle for air- For instance, performance degradation didate for R-22 substitution. However, conditioning and heat pump applications,issues for the equipment containing R- equipment size concerns, addressed at the and suggests that such an approach to en-410A become more profound at relatively component level with respect to the com- hance system performance has significanthigh ambient temperatures, where the pressor pump, piping and heat exchanger potential. The benefits of the economizersystem performance is valued and needed design, become crucial considerations cycle are especially evident in the envi-the most. This alone can drive technology and major obstacles for the implementa- ronment in which conventional methodsinnovation and justify its application. tion of R-134a. On the other hand, a low-pressure These developments have prompted the About the Authorrefrigerant with a relatively high critical introduction of nonconventional methods Michael F. Taras is manager of technology and prin- cipal staff engineer at Carrier in Syracuse, N.Y.point, such as R-134a, is a viable can- to significantly boost system efficiency,38 ASHRAE Journal ashrae.org July 2005
  2. 2. Until recently, the economizer cycle was not considered a cost-effective solution for air-conditioning installa- tions where conventional technology was pushed to the limit. Oversized heat exchangers (condensers and evaporators) reached their effective- ness plateaus and compressor volu- metric and isentropic efficiencies ap- proached the respective maximums.reach a plateau of rapidly diminishing returns on investment respect to specific system configuration and implementationor become physically impossible. details, two basic design solutions exist and are shown in Figures 1 and 2.Economizer Cycle The economizer cycle is more complex than the conventional The economizer cycle is well-justified for high compression system, integrates additional components, and entails moreratio conditions, such as refrigeration applications, where the sophisticated control algorithms. When the economizer cycle30% increase in performance provides a substantial advantage. is in operation, a portion of liquid high-pressure refrigerant isUntil recently, the economizer cycle was not considered a diverted from the main circuit and rerouted through the econo-cost-effective solution for air-conditioning installations where mizer loop, where its temperature and pressure are reducedconventional technology was pushed to the limit. Oversized to some intermediate level in an auxiliary expansion device.heat exchangers (condensers and evaporators) reached their ef- Therefore, the colder economizer flow can be used, throughfectiveness plateaus and compressor volumetric and isentropic the heat transfer interaction in the economizer heat exchanger,efficiencies approached the respective maximums. to further boost subcooling of the main refrigerant. This extra Now, however, the implementation of environmentally sound subcooling allows for the enthalpy difference increase in therefrigerants has negatively affected system performance at certain evaporator and subsequent system performance augmentationenvironmental conditions, and industry standards and govern- (see the P-h diagram in Figure 3).ment regulations have raised the bar for the minimum efficiencies Downstream of the economizer heat exchanger, the auxiliaryof the air-conditioning equipment, a trend that will continue. flow is returned to the intermediate compressor port, usually This course of events has opened the door to various kinds of at the superheated state controlled by the auxiliary expansionalternate technologies, encouraging a more serious evaluation device. When the colder economizer flow is mixed with theof the economizer cycle. partially compressed suction vapor, temperature of the latter is reduced, benefitting the compression process. Furthermore, thePrinciple of Operation bypass loop between the economizer and suction compressor Although the economizer cycle schematics may differ with ports, also exhibited in Figures 1 and 2, can be used for theJuly 2005 ASHRAE Journal 39
  3. 3. system unloading strategy. (As known, Hence, it is not uncommon for the Outdoor Airthe flash tank, separating liquid and va- economizer heat exchanger effective-por phases and essentially representing 2 1 ness to reach 90%, promoting extremelya 100% effective heat exchanger, can be Economizer close temperature approaches of nearly Condenserused in place of the economizer.) HX 1°F (0.6°C). Although the economizer cycle ad- 7 The compressor injection scheme is of Economized high significance as well. Therefore, thevantages are obvious, the full benefits Compressorof the concept are slightly diminished intermediate compression pockets mustby three phenomena concurrently oc- 6 Bypass Valve have an optimal and restricted exposurecurring in the system. First, the evapo- to the economizer ports, limiting cyclic Indoor Airration temperature is slightly reduced, Economizer parasitic compression and expansion Expansionnegatively impacting refrigerant flow Device Evaporator losses associated with the pressure dif-rate at the compressor suction port and ference in the economizer loop and in the 3 4 5 Main aforementioned compression pockets.subsequently system capacity. This side Expansioneffect of extra subcooling is of second- Device For instance, it was observed that theorder significance and, since it equally Figure 1: Economizer cycle. economizer effectiveness reduction byaffects capacity and power, no change in 10% to 15% or use of the conventionalthe system efficiency is registered. Outdoor Air compressor injection scheme leads to a Second, the refrigerant flow rate in decline in the economizer cycle perfor- 2 1 mance gains by 30% to 50%, making thethe condenser is increased, promotingcondensation temperature elevation. approach extremely difficult to justify. CondenserSince the economizer flow is relatively 7small in air-conditioning applications, Performancethis concern can be easily addressed Economized ARI Standard Rating Conditions. Economizer Compressorthrough proper condenser design. HX As discussed earlier, the two strategies, Last, additional compressor power 6 Bypass Valve namely equal capacity approach andconsumption is expected, as extra Economizer Indoor Air equal efficiency tactic, are feasible. In Expansioneconomizer flow must be compressed Device each scenario, it is prudent to determine Evaporatorfrom the intermediate to the discharge 3 4 5 the performance enhancement minimumpressure. These three phenomena moder- Main for the economizer cycle, particularly atate, but do not completely overshadow, Expansion the ARI standard rating conditions* for Devicethe performance benefits achieved in commercial equipment (ARI Standardthe “ideal” economizer cycle, at most Figure 2: Economizer cycle (alternate). 340/360). A midsize packaged scrolloperating conditions. compressor system of premium The economizer cycle per- P Critical Point efficiency would serve as aformance diminishes with the logical contender for such a 3 2 1pressure ratio and subsequent study.decrease of the available thermal For the purpose of this analy-potential in the economizer heat sis, the condenser and evapo-exchanger, approaching the con- rator dimensions and circuit- 6 6 7 7ventional system characteristics ing were preserved, althoughat the point where the aforemen- 7 circuiting optimization couldtioned losses become equal to provide a performance en-the performance gains. 4 5 h hancement potential. The economizer cycle advan- The test results closely Figure 3: P-h diagram for economizer cycle.tages can be used in two ways. matched by the analyticalFirst, an equivalently sized economized compressor can be used model predictions are summarized in Table 1. The R-410Ato deliver more capacity to the system. In another approach, economizer cycle provides a performance boost of at least 4%the compressor size reduction by about 8% to 10% leads to a to 5% at the ARI standard rating point. Also, it becomes obvioussystem efficiency boost, while preserving capacity character- that the economizer cycle may not be required to match the R-22istics at the Air-Conditioning and Refrigeration Institute (ARI) performance for some premium efficiency systems.standard rating point. However, if further performance enhancement is desirable Managing all system losses, especially in the compressor or a number of systems of various capacities share an identicaland economizer heat exchanger, are crucial for the economizer *The ARI standard rating conditions are 80°F (26.7°C) dry bulb/67°F (19.4°C) wet-bulbapproach to be successful in air-conditioning applications. indoor air temperatures, and 95°F (35°C) outdoor air temperature.40 ASHRAE Journal ashrae.org July 2005
  4. 4. 1.05 1.05Capacity Ratio (Relative to R-22) R-134a EER Ratio (Relative to R-22) Conventional ARI Rating Point 1 1 R-134a ARI Rating Point Conventional 0.95 0.95 R-410A Economized R-410A 0.9 R-410A 0.9 Economized R-410A Conventional Conventional 0.85 0.85 75 80 85 90 95 100 105 110 115 120 125 130 75 80 85 90 95 100 105 110 115 120 125 130 Ambient Temperature, °F Ambient Temperature, °FFigure 4: Minimum capacity degradation. Figure 5: Minimum EER degradation.chassis size, the economizer cycle becomes a useful technique The packaged commercial unit of premium efficiency thatto achieve a desired target. In such cases, twice as much en- we used in this study serves here as well to determine mini-hancement (in comparison to the numbers exhibited in Table mum performance degradation for the R-410A systems at high1) can be obtained from the economizer approach, while the ambient temperatures.conventional technology will experience rapidly diminishing Table 2 displays performance characteristics of the R-410Areturns on investment or become physically impossible. systems as compared to the equivalent R-22 systems operating One of the side benefits of the economizer cycle is its aug- at the performance parity at the ARI standard rating condi-mented dehumidification capability, due to lower evaporation tions. The 125°F (51.7°C) ambient temperature limit reflectstemperatures, that potentially can be used in hot and humid the absolute minimum demanded by a majority of commercialenvironments to replace the reheat coil methodology. applications today. High Ambient. When a life-cycle cost analysis of any air-con- The R-410A economizer cycle reveals superior performanceditioning equipment is performed, system operation at a wide and is lagging the R-22 system by only 5%, while the conven-spectrum of environmental conditions must be examined. tional R-410A system exhibits twice as much performance It is not unusual that a high ambient temperature region at- degradation. Obviously, the gap between the R-410A and R-22tracts particular attention, since at those conditions, the system conventional systems (as well as the R-410A conventional andperformance is most often demanded and valued by end custom- economizer systems) becomes much deeper for less efficienters. Any refrigerant system suffers performance degradation in equipment and at higher ambient temperatures. As a result, thehigh ambient environments, particularly nowadays when the equipment will accumulate more hours at less efficient opera-operation requirements are stretched to temperatures as high tion to satisfy external load demands at those conditions.as 135°F (57.2°C). For R-134a, similar critical temperatures and somewhat The downward trend for the equipment containing the R- analogous shapes of the two-phase region in the vicinity of the410A refrigerant is more profound than the respective decline critical point suggest that both R-134a and R-22 refrigerantsfor the R-22 systems. In some cases, the performance parity exhibit comparable performance degradation at high ambientat the full-load ARI standard rating conditions (and at the temperature conditions. Furthermore, the R-134a refrigerant80°F/67°F-80°F [26.7°C/19.4°C-26.7°C] part-load conditions) has a lower compressor discharge temperature (by 25°F to 30°Fis not a sufficient criterion to obtain identical life-cycle cost [14°C to 17°C]), promoting higher reliability and translatingcharacteristics over the entire system operation envelope. into a wider compressor operation envelope. Distinct design points of the equivalent R-410A performance The relative performance results for all three refrigerants aremust be chosen to adequately represent a region of high ambient presented in Figures 4 and 5, with the R-134a indeed revealingtemperatures for various classes of applications. Some R-410A less performance reduction than the R-410A at high ambientsystems must have superior performance at the ARI conditions temperatures.as compared to the equivalent R-22 equipment, to have parity Last, we can conclude that it requires significantly less effortat high ambient temperatures. and investment to reach performance parity at high ambient Design Identical Reduced Design 95°F (35°C) 125°F (51.7°C) Option Compressor Size Compressor Size Option Outdoor Temp. Outdoor Temp. Capacity EER Capacity EER Capacity EER Capacity EER Design Basis (R-22) 100% 100% 100% 100% Design Basis (R-22) 100% 100% 100% 100% R-410A Economized 105% 100% 100% 104% R-410A Economized 100% 104% 95% 95% R-410A Conventional 100% 100% 95% 104% R-410A Conventional 100% 100% 91% 88%Table 1 (left): Minimum economizer cycle benefits at ARI standard rating conditions. Table 2 (right): Minimum performance degradationat a high ambient temperature of 125°F (51.7°C).July 2005 ASHRAE Journal 41
  5. 5. for the R-410A economizer systems, while any conventional and a decision is made regarding incremental design augmenta-approach may become thermodynamically prohibitive. tions and the appropriate methodology to be employed. This Part-Load Operation. A conventional multicircuit system decision is based on a cost performance tradeoff.unloads by turning circuits on and switching them off to satisfy The economizer cycle offers significant advantages in sys-external load demands and to control temperature and humidity tem operation and performance, but it becomes particularlywithin the conditioned space. difficult to justify this concept from a cost standpoint for the For the economizer system, multiple additional steps of un- commercial systems below 5 tons (17.6 kW) in capacity, sinceloading can be accomplished naturally, reducing life-cycle cost this equipment is extremely cost sensitive. Consequently, itof equipment and augmenting part-load performance charac- is expected that the conventional techniques of performanceteristics. More particularly, the economizer cycle capacity can enhancement will prevail in the market segment of small com-be decreased by bypassing a portion of the refrigerant between mercial packaged equipment while alternate technology willthe economizer and suction compressor ports (Figures 1 and become affordable for the larger systems. (2) as well as by operating the system in the economizer and Obviously, it is especially difficult to establish a hard uniformconventional modes. cost basis across various platforms and product lines, due to This approach offers four operation modes for each inde- variability in refrigerant-oil and material prices, inconsistencypendent circuit within the system: 1) conventional mode, 2) in chassis sizes and compressor frames, disparity in coil manu-economizer mode, 3) conventional mode with bypass, and 4) facturing capability and dependence on particular manufacturereconomizer mode with bypass. labor and burden rates. The numbers presented in this section These four modes offer should be viewed only as ra- 30many opportunities for the tional estimates. Cost Premium, Percentunloading strategy. Further- 25 The first step is to assess the 20 Ton Systemmore, adjustable flow control costs associated with convert- 20devices can be used for the ing larger systems (above 5economizer expansion and 15 tons [17.6 kW] in capacity)bypass valves, offering an to the HFC refrigerants whileinfinite number of unloading 10 preserving the efficiency lev- Standard 90.1-1999steps through modulation or 5 15 Ton System els. To reach a general conclu-pulsation techniques and extra sion independent of systemflexibility in control of vari- 0 size and configuration specif-ous operational parameters. 9 9.5 10 10.5 11 11.5 12 12.5 ics, the analysis was conducted Such parameters may in- Energy Efficiency Ratio, Btu/h · W on a relative scale with theclude (but are not limited to) Figure 6: Efficiency cost premium (R-410A systems). normalization to the R-22system capacity, compressor standard system cost.discharge temperature and power, and conditioned space It was observed that for the R-410A systems this incrementaltemperature and humidity. For instance, for abnormally high cost is between 5% and 10%, and results primarily from the pre-ambient temperatures, a sequence of small unloading steps can miums paid for R-410A components that can achieve efficiencylimit compressor power while preserving system performance, levels comparable to the existing systems. The lower values areextending the operation envelope, and preventing nuisance associated with the smaller equipment sizes (5 to 15 tons [17.6shutdowns. to 52.7 kW]), since these systems usually have relatively large Thus, continually changing load demands are satisfied with cabinet structure and, hence, operate at lower condensationgreater precision, keeping the conditioned space parameters temperatures (further away from the critical point).within the comfort zone, eliminating temperature and humid- The R-134a equipment exhibits 12% to 16% cost increaseity variations, and enhancing system reliability and efficiency with significantly smaller variation with respect to the equip-through a reduction of start-stop cycles. ment size, which can be explained by substantially higher Manufacturing Cost. Cost is one of the most critical ingredi- critical point for this refrigerant and lower sensitivity to theents in validation and justification of nontraditional technology, condensation process.especially at the initial stages of its application. The assump- Extra costs associated with the economizer cycle are relatedtion is that presently manufactured equipment containing HFC to the complexity of manufacturing an economized compressor,refrigerants is produced and sold at a premium. (This trend an economizer heat exchanger, an auxiliary expansion device,may change with the refrigerant and oil price correction due and economizer loop piping, plus the cost of the extra refriger-to a boost in production volumes of the HFC refrigerants and ant charge. Obviously, cost becomes less of an issue when it isphaseout of R-22.) On the other hand, the newly introduced physically impossible for the traditional technology to achievesystems must operate at least at the existing efficiency levels to the desired efficiency levels.adequately satisfy customer expectations and perception. There- For the reasons outlined earlier, the economizer cyclefore, a lower boundary for the system performance is established becomes more advantageous for the larger equipment,42 ASHRAE Journal ashrae.org July 2005
  6. 6. but a cost parity point is quite different for R-410A and economized configurations are still limited, particularly forR-134a refrigerants. air-conditioning applications. Once again, to eliminate relevance to a particular equipment This shortage of the economized compressors for air-type, the study was carried out on a percentage basis with respect conditioning applications can be attributed mainly to theirto a traditional R-410A system cost. For the R-410A systems, the design requirements and the extreme difficulty of manag-economizer cycle cost becomes 3% to 5% lower than the conven- ing the losses associated with the economizer ports. Thus,tional unit cost for the equipment capacity range of 18 tons (63.2 original equipment manufacturers have started to search forkW) and above, comparable for the 10 to 15 tonnage (35.1 to 52.7 non-conventional thermodynamically similar arrangementskW) range and somewhat higher below 10 tons (35.1 kW). For using traditional compression technology.the R-134a refrigerant systems in the 15 to 20 tonnage (52.7 to Such options may include (but are not limited to) natural70.3 kW) range, the economizer cycle cost is already 6% lower refrigerants, secondary circuits for subcooling enhance-than the conventional unit, alluding to the fact that this concept ment, cascade systems, entropy recovery expansion devices,has a wider spectrum of applications with R-134a refrigerant. minichannel (brazed aluminum) heat exchangers as con-This phenomenon can be explained simply by the fact that the densers and evaporators, liquid-suction heat exchangers,pressure ratios for the R-134a refrigerant are higher in general, and economizer-like designs. The last two options havefurther promoting the economizer cycle approach. close resemblance to the economizer cycle and are worth Equipment cost increases exponentially with the efficiency further consideration.level, therefore, it is essential to evaluate the R-410A system Designs incorporating a liquid-suction heat exchanger arecost trends. It is logical to an-ticipate that the normalized cost Outdoor Aircurves for the units of various Outdoor Airsizes would collapse into a singletrend line, but the analysis suggests Condenser Auxiliary Condenserjust the opposite. For instance, two Compressor Expansionrepresentative cost curves for Devicethe systems of 15 and 20 tons Auxiliary HX Compressor Liquid-Suction HX(52.7 to 70.3 kW) in capacityare exhibited in Figure 6, wherethe least expensive method for Evaporatoreach incremental efficiency step Evaporatoris plotted. Although both costs Expansion Device Indoor Airare normalized by the respective Expansionsystem cost values, the growth rate Device Indoor Airfor the 20 ton (70.3 kW) system issignificantly higher than for the 15 Figure 7 (left): Cycle with liquid-suction heat exchanger. Figure 8 (right): Cycle with auxiliary heat exchanger (Configuration 1).ton (52.7 kW) system. The explanation is related to two phenomena. First, there is a well known in the refrigeration industry and provide the mostsomewhat disproportional cost increase for the system compo- advantages for the refrigerants with highnents, such as compressors and coils, relative to the increase inthe equipment size. This disproportionate cost increase is dueto limited manufacturing capability and inadequate competi-tion in the market. characteristics. This concept, shown in Figure 7, works reason- Fi Second, the larger equipment tends to operate closer to ably well for the refrigeration applications, where a thermala critical point because of a relatively small cabinet struc- potential between the refrigerant vapor exiting the evapora-ture, and therefore, requires an extra step for performance tor and liquid leaving the condenser is significant enough toaugmentation. provide adequate heat transfer interaction between these two The results described previously assume efficient manufac- refrigerant streams. As a result, an extra subcooling of liquidturing operations and can be extended to a wider spectrum of refrigerant entering an expansion device appreciably enhancesequipment sizes, although it is expected that the difference in evaporator performance. With air-conditioning equipment,cost will be smaller for the lower capacity equipment and just the aforementioned temperature difference is substantial-the opposite will be true for the larger systems. ly reduced. This smaller temperature difference, along with a lowerOther Performance-Boosting Contenders vapor density entering the compressor suction port, makes Although the compressors with integrated liquid injec- the performance benefits and this system concept practicalitytion ports are becoming readily available on the market, the questionable.July 2005 ASHRAE Journal 43
  7. 7. Next, we will consider the economiz- Concluding Remarkser-like schematics. Four arrangements Outdoor Air It is apparent that the economizerare available based on the location of cycle offers many benefits for air-con-the splitting and mixing points of the Condenser ditioning applications and stands outauxiliary and main refrigerant flows. among other potential contenders, but Compressor Figure 8 shows the splitting point up- also introduces several design, cost,stream of the auxiliary heat exchanger, and application challenges that must Auxiliary HXwhile the mixing point is positioned at be properly addressed. In particular, thethe compressor suction. An alternate economizer cycle provides a competi-arrangement (not shown) would locate tive advantage, where other methodolo-the splitting point downstream of the Auxiliary Indoor Air gies fail to be economically viable or Expansion become physically impossible.auxiliary heat exchanger. Device Evaporator Figure 9 exhibits the splitting point A typical performance augmentationdownstream of the auxiliary heat Expansion for the economizer system is expected Deviceexchanger, while the mixing point is to be approximately 8%. Such an im-placed downstream of the evaporator. Figure 9: Cycle with auxiliary heat exchanger provement can easily compensate forAn alternate arrangement (not shown) (Configuration 2). the R-410A performance deficiency atwould locate the splitting point upstream of the auxiliary heat the ARI standard rating point for commercial equipment.exchanger. Furthermore, the economizer cycle demonstrates its superior- The proposed technique is based on traditional compression ity at high ambient operation, where it outperforms conventionaltechnology and an addition of the auxiliary heat exchanger systems by 10% on average. Although this is a significant(preferably in the counterflow arrangement) to the system improvement, issues associated with R-410A performanceschematic. This design approach provides extra subcooling and degradation in such environments must be addressed further,overall system performance boost by establishing an optimal potentially by a combination of the economizer cycle and tra-refrigerant flow fraction that bypasses the evaporator to achieve ditional technology.extra subcooling in the auxiliary heat exchanger. Other benefits of the economizer cycle are related to flexibility Although various design configurations are feasible, the in the unloading strategy, improved dehumidification capabilityrefrigerant flow entering the evaporator is also proportionally and superior operation in the heat pump installations.reduced in all such schematics. As a result, the economizer-like The economizer concept can be used in a multicircuit systemschematics offer only a fraction of performance enhancement, configuration as well, where economizer heat exchangers as-in comparison to the economizer cycle. sociated with each circuit can be combined into a single unit, Consequently, the economizer cycle still stands out as the offering a reduction in the heat exchanger cost by 25%.most superior and feasible option in the category. As mentioned earlier, all the economizer cycle advantages can be realized only if special attention is paid to the econo-Heat Pump Applications mized compressor injection scheme and the economizer heat Heat pumps simultaneously serve cooling and heating markets, exchanger effectiveness, each of which may easily reducecovering broader applications and growing in importance. Such a foreseeable benefits by a factor of two.trend offers a tremendous opportunity for the economizer cycle, Additionally, it should be expected that the economizer sys-since the heat pump equipment usually operates at the elevated tem control logic will be more complex, the number of compres-pressure ratios (in comparison to the traditional air-conditioning in- sor suppliers will be limited, and the development cycle will bestallations), while having sufficiently high performance targets. longer, at least until sufficient experience is gained. The economizer cycle has only recently entered the air-con- Although, in principle, any system can use the economizerditioning arena, and most likely will penetrate the heat pump cycle, today the concept most likely is not economically viablemarket in the near future. Heat pump designs incorporating an for systems below 10 tons (35.1 kW) in capacity. However, foreconomizer loop are more complex than the traditional systems the larger systems, the economizer cycle cannot be matched byand, respectively, include an additional four-way valve, an ad- any other realistic contenders at a comparable cost.ditional economizer heat exchanger or an extra main expansion Although strong arguments can be presented, based on perfor-device–check valve assembly. Obviously, many variations of mance, for R-134a to be the refrigerant of choice for packagedthese basic designs (primarily related to the economizer loop commercial equipment, the cost concerns develop into a majorconnections and insertion of additional components such as an obstacle, even for the economizer cycle.accumulator) are foreseeable. Although extra system complexity, additional hardware, and Bibliographyincreased cost are to be justified by the benefits obtained from the Lemmon, E.W., et al. 2002. NIST Reference Fluid Thermody-economizer cycle performance, a vacuum in potential contenders namic and Transport Properties – REFPROP 7.0. National Institutewill further promote the technology. of Standards and Technology at www.nist.gov/srd/nist23.htm44 ASHRAE Journal ashrae.org July 2005