1、OR-05-1 6-2 Altered Bi-Phase Flow Regime in Supermarket Evaporative Coils: Laboratory and Field Experiences David A. Wiqhtman Bernard Wendrow, PhD, PE Associate Member ASHRAE Richard S. Sweetser Member ASHRAE ABSTRACT Examining a typical pressure-enthalpy chart, it is gener- ally believed that the e
2、vaporator benefits from a complete stream of liquid entering the coil. The conclusion from this is that maximum enthalpic capacity is achieved by subcooling or even super-subcooling the entering liquid and that anything less than a full column of liquid might limit the amount of heat transfer that t
3、he evaporator can accomplish. The evolution in refrigerantflow testing and mapping is now providing support for a different view. This paper will examine the net efSect ofan altered bi-phaseflow (ABF) regime in evaporative coils, with consideration of a high vapor frac- tion and turbulent refrigeran
4、tflow (HVFT). Furthermore, the pressure-enthalpy chart lends improving evaporator pres- sures, control during transient operation, refrigerant density at the compressor, and rate of heat transfer among other factors cun combine to reduce energy consumption. Two test and verification projects in supe
5、rmarket medium- and low-temperature display cases and storage applications provide significant support for examining old rules-of thumb. Thefirstproject involves a deli service case tested under labo- ratory conditions. This test demonstrates the efect of evapo- rator operation utilizing an ABF regi
6、me and compares this to the operation of a pulse-type electronic expansion valve system. The results demonstrated operation with the ABF regime permitted increased compressor suction pressure, improvedproduct temperature, provided more stable refriger- ant temperatures, and improved case humidity. T
7、he second test and verification project involves field retrofitting of existing direct expansion evaporators to the ABF regime and measur- ing the results. The resulting change in refrigerantflow regime demonstrated improved performance, resulting in consistent William M. Worek, PhD Member ASHRAE an
8、d reduced conditioned supply air andproduct temperatures, improved oil return, reduced compressor discharge tempera- tures, and increased evaporator pressure. Product quality and energy savings were also measurable. INTRODUCTION The industry has had a view that liquid at the entry of the evaporative
9、 coil is desirable and that superheated vapor at the exit of the evaporative coil is necessary. The common view has been that any improvement in Delta-h improves overall system capacity and is, therefore, the pinnacle. It is often commonly held that the heat transfer coefficient at the entry to the
10、coil cannot be improved enough to profoundly impact capacity at the entry of the evaporator or to provide increased evaporator capacity. The superheated passes at the exit of the evaporator have been viewed as necessary due to the conven- tional liquid flow pattern that advances and recedes in the d
11、irect expansion (DX) coil, which, if regularly extended toward the refrigerant outlet from the evaporator coil, might extend toward the compressor inlet and then encroach the compressor inlet during periods of abnormal refrigerant flow. Although counterintuitive to this conventional thought and whil
12、e enthalpic capacity is important, research in two-phase refrigerant flow regimes and heat transfer rates is supporting the position that improved flow regimes can make more dramatic system-wide performance increases. PRIOR STATE OF THE ART The supermarket industry is faced with challenges. Prod- uc
13、t quality, new Food and Drug Administration (FDA) product temperature mandates, product shelf-life and shrinkage, as well as product liability, all have brought new attention to David A. Wightman is CEO of XDX Innovative Refrigeration, LLC, Arlington Heights, Ill. Bernard Wendrow, a consultant in Hi
14、ghland Park, Ill., was director of the Undergraduate Chemical Engineering Laboratories, Northwestern University. Richard S. Sweetser is president of EXERGY Partners Corp., Herndon, Va. William M. Worek is director of the Energy Resources Center of Illinois at Chicago. 02005 ASHRAE. 1061 Figure 1 Imp
15、act of ambient temperature on display case performance. reffigeration performance and its effect upon product and the bottom line of a business. Target food holding temperatures have been reduced. Energy awareness is high. The call for a reduction in energy consumption with simultaneous reduction in
16、 case temperature is an oxymoron. As the supermarket industry has moved away from energy-saving programs that adversely affect product, we have seen a movement toward use of electronic controllers to stage compressors and monitor system performance and toward the empowerment of posi- tions in food s
17、afety or creation of positions with titles such as “Energy Czar.” The more traditional approaches to energy reduction are now merely controlled electronically, as the commercial supermarket industry continues its focus on head pressure reduction and increased subcooling as the primary targets for im
18、provement in system performance. Improving compressor COPS through floating head pressures does reduce overall power consumption. Figure 1 shows the effect that flooded condenser low-ambient controls have upon system power usage. The ambient temperature is shown drop- ping more than 11C (19.SF), and
19、 power consumption remains relatively constant because the compression ratio reduction is minimal. TEST AND VERIFICATION PROJECTS Two test and verification projects are presented. The first is a laboratory analysis that demonstrates evaporator effi- ciency increase in a controlled environment, resul
20、ting in energy reduction and product temperature improvement. The second project is a retrofit of an operational California super- market, which showed that a dramatic improvement in evap- orator efficiency can have a systemwide impact on energy and product quality improvement. ABF Regime Comparativ
21、e testing was performed between before and after systems designs. The first system setup uses an optimized conventional direct expansion refrigerant feed, operated following all manufacturer-specified recommendations. The laboratory project used a substantial number of data points in accordance with
22、 European and ASHRAE standards, with extensive baseline verification of peak system performance. The field verification project conducted a full “re-commis- sioning” of each of 134 evaporators followed by a baseline period. The second setup combines the DX refrigerant feed in conjunction with the AB
23、F regime and varies the vapor fraction of the refrigerant and creates turbulent flow through amechan- ically induced fluid process. This process uses a device that separates liquid and vapor during the chaotic exit from the expansion valve through use of a two-stage expansion. The liquid is then ent
24、rained in the vapor fraction at a mass velocity enabling rapid transition to intermittent or annular flow. Theo- retically, the reduced heat transfer of the slug, stratified, strat- ified-wavy, and wavy flow regimes are eliminated from the evaporator. Testing continues to validate the theoretical mo
25、dels. Each test and verification project is monitored to measure the effect that this ABF flow regime can have on cooling rates, compressor work, temperature differences, evaporator effi- ciency, control of superheat, and in other significant observa- tions. The hypothesis being tested is that enter
26、ing an evaporator coil with the ABF and a high vapor fraction enables a novel and highly efficient flow regime to be achieved throughout the evaporative coil. Furthermore, annular flow near the outlet and partial dry-out at the outlet of the evaporator coil in the ABF system communicates very effici
27、ently with the superheat- sensing bulb, whereas the conventional vapor barrier of super- heat in pre-ABF operation has extremely poor heat transfer and cannot communicate well. The ABF-equipped system has been operated throughout the study in multiple and single pass circuiting, aidventilated and gr
28、avity feed coils, and high, medium, and low temperature applications. Reduction in superheat exiting from the evapo- rator coil is accomplished with minimal liquid and is very tightly controlled with little fluctuation, as verified on the glass tube evaporator test stand. Lower superheat can mean gr
29、eater surface exposure to the refrigerant and result in higher evaporator pressures. Lower superheat allows for a more dense refrigerant, boosting compressor capacity and lowering compressor superheat. Energy is reduced in each case. Uniformity of evaporator temperatures allows for frost to build mo
30、re uniformly across the coil and can therefore reduce defrost frequency or duration by not causing a restriction in air-side velocity. Laboratory Test of One Deli Case An independent laboratory in the United Kingdom conducted a performance analysis of a ventilated deli service case using electronic
31、pulse-type valve technology to meter the refrigerant and compared that to performance of operation with the ABF flow regime in series relationship with the elec- 1062 ASHRAE Transactions: Symposia tronic pulse-type valve. While previous work had been done to determine a performance increase using th
32、e pulse-type tech- nology in comparison to a conventional mechanical thermal expansion valve (TXV) in this refrigerated case, this test found results showing little change in system performance and that the ABF flow regime could not be sustained using the pulse- type valve technology. However, the t
33、est went on to compare the conventional electronic pulse operation with that of the mechanical valve in operation with the ABF flow regime. The laboratory was equipped with a monitored and fully operational psychometric chamber and complied with ASHRAE method of test standards. The ambient condition
34、s surrounding the refrigerated case were maintained throughout the test. Monitored points included, but were not limited to, the following: Wet-bulb and dry-bulb temperature and CFM inside the chamber Wet-bulb and dry-bulb temperature of chamber supply and return air Instantaneous and totalized refn
35、gerant mass flow (CH- Multiple product simulators in accordance with ASHRAE standards Multiple temperature and pressure sensors throughout the system. Outdoor DB and WB temperature Product surface temperature probe (CH-25,27,28, 30) Evaporator supply air temperature (CH-32), wet bulb and dry bulb Ev
36、aporator return air temperature, wet bulb and dry bulb Compressor suction pressure and temperature Liquid line pressure (CH-54) and temperature Evaporator inlet pressure (CH-53) and temperature Evaporator outlet pressure (CH-52) and temperature Evaporator tube surface temperature (CH-33) Evaporator
37、tube fin temperature Additional product weight tests were conducted Defrost frequency and duration Suction pressure was regulated by means of an electronic 51) pressure regulating valve, and this was the only setpoint changed during the test in effort to maintain similar product temperatures. Evalua
38、tion of the test results between the pulse-type elec- tronic expansion valve (EEV) equipped evaporator and the evaporator in operation with the ABF flow regime demon- strated that the heat transfer of the ABF flow regime equipped evaporator was significantly better than the conventionally equipped e
39、vaporator. The average mass flow of the conven- . Temperature and pressure data were sampled at a minimum of once per minute. Comparisons were made over a 24-hour period. All auxiliary thermocouples and sensors were manufactured in accordance with the guidelines set forth by IS0 9001 and were recent
40、ly calibrated. Table 1. Equipment for Deli Case Test Uneven parallel compressor rack system R-404a European design, ventilated service deli IBtu load of approximately 7,800 Btu per hour I I Electronic smart controller I Electronic evaporator pressure regulating valve Remote condenser, air-cooled Fig
41、ure 2 Baseline EEV equipped evaporator mass flow rate. Figure 3 ABFJlow regime equipped evaporator mass flow rate. tional system was 27.59 kglh (60.82 lbhr) and average mass flow of the ABF flow regime device equipped system was 27.83 kgh (61.35 lbhbsee Figure 2 as compared to Figure 3. However, the
42、 display case air temperatures as seen for the conventional pulse-type EEV system (shown in Figure 4) exhib- ited an average air supply temperature of -237C (26.83“F) ASHRAE Transactions: Symposia 1063 Figure 4 Baseline EEV equipped evaporator supply air (air OB temperature. with an average evaporat
43、or coil temperature of -53C (22.01“F). The average supply air temperature of the evaporator utilizing the ABF flow regime was reduced, as shown in Figure 5, with an average air supply temperature of -3.69“C (25.36“F) and an average evaporator coil temperature of -5.38“C (22.32“F). Figure 6 shows the
44、 operation of the evaporator using the conventional pulse-type EEV and the effect that display case air temperature has upon the product temperature in that they are slow to drop below 2C (35.6“F) and are unable to drop below 0C (32F) before the next required defrost period. Note also that there is
45、approximately 2C (3.6“F) between the high- est and lowest product temperature. Figure 7 shows the effect that the ABF flow regime equipped evaporator has on product temperature, demonstrating an average product temperature more than 1C (1.8“F) lower than the operation of the evapora- tor using the c
46、onventional pulse-type EEV. Comparing Figure 7 to Figure 6 also reveals a more rapid drop in product tempera- ture with the ABF flow regime equipped evaporator, with prod- uct temperatures dropping below the 0C (32F) mark. Figure 8 and Figure 9 compare the system evaporator pressures with the ABF fl
47、ow regime, enabling evaporator pressure operation at an average of 18 kPa (2.61 psig) above the EEV equipped evapora- tor. Results from this initial test demonstrated improved heat transfer using the ABF flow regime equipped evaporator. The mass flow rate was similar, yet the display case air temper
48、a- tures were improved, the product temperatures were reduced, product pull down was significantly faster, and suction pres- sure was increased dramatically. Field Test and Verification of a Complete Supermarket Two supermarkets were studied. While similar results were found in each store, this pape
49、r focuses on the store in Fresno, California, which had the higher data integrity, as the second store had repeated irregularities concerning power meter disruption and modem access interruption. Figure 5 ABFflow regime equipped evaporator supply air (air 08 temperature. standard Cal. oprmmn 1 Figure 6 Baseline system pulse-type EE V equipped evaporator product temperatures. The supermarket in Fresno, California, has an installed compressor capacity of 257 kW (344.5 HP) distributed unevenly across five parallel compressor systems. This test program consisted of monitori
