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Recent Developments in Plate Exchangers—Ammonia/Carbon Dioxide Cascade CondensersZahid Ayub a , M. Sultan Khan b , Amir Jokar c , Tariq S. Khan d & Niel Hayes ea Isotherm, Inc. , Arlington , Texas , USAb Department of Mechanical Engineering , Mohammad Ali Jinnah University , Islamabad ,Pakistanc Exponent, Inc. , Los Angeles , California , USAd GIK Institute of Engineering Sciences and Technology , Topi , Pakistane Hill Phoenix , Conyers , Georgia , USAAccepted author version posted online: 04 Sep 2012.Published online: 20 Nov 2012.

To cite this article: Zahid Ayub , M. Sultan Khan , Amir Jokar , Tariq S. Khan & Niel Hayes (2013) Recent Developmentsin Plate Exchangers—Ammonia/Carbon Dioxide Cascade Condensers, Heat Transfer Engineering, 34:5-6, 401-408, DOI:10.1080/01457632.2012.721312

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Heat Transfer Engineering, 34(5–6):401–408, 2013Copyright C©© Taylor and Francis Group, LLCISSN: 0145-7632 print / 1521-0537 onlineDOI: 10.1080/01457632.2012.721312

technology and development

Recent Developments in PlateExchangers—Ammonia/CarbonDioxide Cascade Condensers

ZAHID AYUB,1 M. SULTAN KHAN,2 AMIR JOKAR,3 TARIQ S. KHAN,4

and NIEL HAYES5

1Isotherm, Inc., Arlington, Texas, USA2Department of Mechanical Engineering, Mohammad Ali Jinnah University, Islamabad, Pakistan3Exponent, Inc., Los Angeles, California, USA4GIK Institute of Engineering Sciences and Technology, Topi, Pakistan5Hill Phoenix, Conyers, Georgia, USA

The use of carbon dioxide and ammonia in low temperature cascade systems is gaining momentum in the industrialrefrigeration market. The use of a plate exchanger as cascade condenser is a viable option due to the high thermalefficiency and smaller footprint characteristics of such exchangers. There is a lack of reliable data in the open literatureon condensation of carbon dioxide and evaporation of ammonia in such heat exchangers. This article presents the latestresearch on condensation of carbon dioxide and evaporation of ammonia in various corrugated plate exchangers at differentsaturation temperature and heat/mass flux. The data are reduced to generalized empirical correlations to be used asdesign tools by engineers. It also discusses the mechanical aspects of plate exchanges and their suitability in cascadesystems.

INTRODUCTION

The potential danger to our environment due to depletionof the ozone layer and global warming has prompted the air-conditioning and refrigeration industry to probe more deeplyinto this global concern. Currently there seems to be no de-lay but swift movement toward a complete ban on hydrochlo-roflourocarbons (HCFC) and hydroflourocarbons (HFC). Ac-cording to the European Regulation EC2037/2000, the sale ofvirgin HCFCs, which includes R-22, the workhorse of the heat-ing, ventilating, air-conditioning, and refrigeration (HVAC&R)business, was prohibited as of January 1, 2010. This ban isalso applicable to the storage of virgin HCFCs (excluding ma-chinery itself). Between January 1, 2010, and January 1, 2015,

Address correspondence to Dr. Zahid Ayub, Isotherm, Inc., 7401 Commer-cial Blvd. East, Arlington, TX 76001, USA. E-mail: zahid@iso-therm.com

operators will only be allowed to use recycled or reclaimedHCFCs. Thereafter, sale and/or storage of recycled or reclaimedHCFCs will be prohibited. Operators will be required to keepa strict record of inventory with equipment containing a chargeof 3 kg or more. The goal is a suitable refrigerant with neg-ligible ozone depletion potential (ODP) and a global warmingpotential (GWP) of less than 150. Most HFCs have low ODP;however, their GWP is much higher than 150. Currently, the au-tomotive industry after heated political debate has shelved theidea of carbon dioxide and is opting for an olefin-based refrig-erant HFO-1234yf (hydrofluoro-olefin) that is supposed to havea single-digit GWP. Since it is mildly flammable, it would beclassified under the American Society of Heating, Refrigeratingand Air-Conditioning Engineers, Inc. (ASHRAE), safety classA2L.

Therefore, in view of the current situation, it is apparentthat the HVAC&R industry is seriously looking into an ex-panded use of natural refrigerants. However, there are some

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impediments to this; most prominent are the flammability andtoxicity issues. To reduce the effects of these issues the industrymust strive for ways to reduce the refrigerant charge in a system.One way to achieve this would be to use compact heat exchang-ers as evaporators and condensers. Recently, there has been fairactivity in research and development related to two prime nat-ural refrigerant candidates: ammonia and carbon dioxide. Theimportance of natural refrigerants has also been realized offi-cially by ASHRAE [1] in a form of a recently published positiondocument. The following information is taken from pages 5–6of this report.

Ammonia is the most important of the natural refrigerantsbecause of its long-standing and widespread use in food andbeverage processing and preservation, and because of its grow-ing potential in HVAC chillers, thermal storage systems, processcooling and air conditioning, district cooling systems, supermar-kets, and convenience stores. Since the mid-19th century therehave been many changes in types of refrigerants, but ammoniais unique because it has seen continued use over this 150-yearperiod.

Ammonia has ODP and GWP equal to zero. It has inherentlyhigh refrigeration system energy performance, excellent ther-modynamic properties, and high heat transfer coefficients. In avapor state it is lighter than air. It is easily detected by smell,or by a variety of electrochemical and electronic sensors, andis readily available at a relatively low price. Less than 2% ofall ammonia commercially produced in the world is used as arefrigerant; however, ammonia enjoys low cost due to the largevolume of production for use as a fertilizer.

The primary disadvantage of ammonia is its toxic effect athigher concentrations (i.e., well above 300 ppm); however, thisrisk is somewhat mitigated by its pungent smell alerting humansto its presence, since even at lower concentrations (5 ppm) itis self-alarming in the event of a leak. Ammonia is classifiedas “moderately flammable” in air when its concentration rangesbetween 16% and 28% (by weight), and it is not compatible withcopper and copper alloys.

In some jurisdictions, ammonia refrigerating systems aresubject to legal regulations and standards because of person-nel safety considerations. These do not necessarily present ad-ditional barriers because legal regulations, proper maintenance,and training of personnel are required for other refrigerants aswell. Furthermore, the use of fluorocarbon refrigerants is dis-couraged in many countries with imposition of environmentallegislation and taxes, and uncertainty concerning the Kyoto Pro-tocol consideration. If the regulations and standards are appliedin practice, and if suitable training for maintenance personnel isprovided, then danger from ammonia use is no different fromthat of most other refrigerants.

Ammonia provides useful cooling across the range of tem-peratures, from air-conditioning to low-temperature applica-tions. Some air-conditioning systems with ammonia chillershave recently been installed in commercial and public buildings.These units are currently more expensive than fluorocarbon-based chillers, but the price difference is expected to reduce asproduction volumes increase.

Like ammonia, carbon dioxide was also used in the midto late 19th century, particularly on board ships and in shopsand theaters where the smell of ammonia was not acceptable.However, as ammonia system safety and efficiency improvedat the beginning of the 20th century, carbon dioxide systemsbecame less common. With the introduction of fluorocarbonsin the 1930s carbon dioxide fell out of use by the 1950s. Thelow toxicity, nonflammability, zero ozone depletion potential,and low global warming potential attracted the attention of sys-tem designers beginning in the early 1990s when alternatives tochlorofluorocarbons (CFC) were being sought. Since then, car-bon dioxide has found widespread acceptance in the full rangeof vapor-compression systems, from low-temperature freezersto high-temperature heat pumps. It has also been widely usedas a secondary refrigerant, offering significant improvementsin efficiency compared with traditional water, glycol, or brinesystems.

One major difference between carbon dioxide and other re-frigerants is in its pressure/temperature characteristic becausethe pressures experienced are approximately 10 times higherthan those in ammonia or R-404a systems. This high pressurerequires special equipment designs, but it also offers many ad-vantages over other refrigerants. The high pressure results inhigh gas density, which allows a far greater refrigerating effectto be achieved from a given compressor. It also produces verysmall reductions in saturation temperature for a given pressuredrop, allowing higher mass flux in evaporators and suction pipeswithout efficiency penalties. This effect is particularly noticeableat low temperatures (–30 to –50◦C), which is why carbon dioxidesystems perform so well under these conditions. Exceptionallygood system performance has been noted in low-temperatureplate freezers and multichamber blast freezers where improve-ments in efficiency and reductions in freezing time have beenreported.

AMMONIA/CARBON DIOXIDE CASCADE SYSTEM

In large industrial systems where there is a need for lowtemperatures (–30 to –50◦C), ammonia and carbon dioxidehave been successfully used in cascade refrigerating systems, asshown in Figure 1. Various manufacturers use different types ofheat exchangers as cascade condensers. Lately, there has been atrend to use all welded plate type exchangers as shown in Fig-ure 2; however, there are no reliable data especially for carbondioxide condensation in plate exchangers. In order to addressthis important aspect ASHRAE undertook two important stud-ies on the subject. The first ASHRAE project involved the studyof carbon dioxide condensation in plate exchangers [2], and thesecond ASHRAE project investigated boiling of ammonia withand without miscible oil in plate exchangers [3]. The goal ofthis two-pronged study was to collect reliable data to be used inthe design of cascade condensers involving these two importantnatural refrigerants.

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Figure 1 Typical schematic of an ammonia/carbon dioxide cascade refrigera-tion system. (Color figure available online.)

CARBON DIOXIDE CONDENSATION IN BRAZEDPLATE EXCHANGERS

In order to use carbon dioxide in refrigeration cycles, itis important to understand the heat transfer and mechanicalintegrity aspects of equipment that would be able to with-stand high condensing pressures. Therefore, it is importantto undertake comprehensive studies on the subject that wouldidentify exchanger(s) with high mechanical integrity and en-hanced heat transfer characteristics. This requirement couldbe fulfilled by undertaking an experimental program on abrazed plate exchanger. These types of exchangers offer highersurface-area-to-volume ratios along with enhanced heat transferand high pressure ratings. It is apparent that 1◦C reduction inapproach temperature on average results in 2–3% improvementin coefficient of performance (COP).

This study was sponsored by ASHRAE under project num-ber 1394-RP to help understand the physics of condensing phe-nomenon of carbon dioxide in complex geometries such as plateheat exchangers (PHE) and also to help in optimizing such ex-

Figure 2 Typical shell-plate exchanger. (Color figure available online.)

changers for industrial applications. The main objective was toperform condensing heat transfer tests on plate-type heat ex-changers at various temperature and pressure conditions withinthe subcritical region. The tests were not manufacturer specificbut rather geometry specific, so that universal correlations and/orcharts could be developed.

Plate Configuration, Test Setup, and Data Reduction

In this study, tests were conducted on three different stainlesssteel plate geometries (Figure 3) as follows:

• Soft (low) profile (L plate), β = 30◦/30◦.• Hard (high) profile (H plate), β = 60◦/60◦.• Medium (mix of low/high) profile (M plate), β = 45◦

(30◦/60◦).• Corrugation pitch, Pc = 6 mm.• Corrugation depth, b = 2 mm.

Test data were taken on each of the plate configurations justdescribed operating at the following conditions:

• Saturated CO2 temperature range: –17.8◦C to –34.4◦C.• Heat flux range: 2.5 kW/m2 to 15.7 kW/m2.• Inlet condition: superheated gas to saturated vapor.• Exit condition: saturated liquid to subcooled liquid.• Approach temperature: not to exceed 5.6◦C.

The experimental setup is shown in Figure 4. The test datawere reduced to heat transfer coefficients under specified steady-state conditions. Brine-side heat transfer coefficients were es-tablished first, and for validation purposes, they were comparedto existing single-phase correlations for plate exchangers in theopen literature, as presented in detail by Hayes and Jokar [4].Next, a thorough two-phase study with uncertainty analysis wasconducted on the condensation of carbon dioxide in the threebrazed plate heat exchangers (BPHE). The condensing heattransfer performance of the high-profile plate was better thanfor the mixed- and low- profile plates under similar flow condi-tions. The low-profile plate exhibited heat transfer behavior asexpected in the near laminar to transitional flow regime, whereasthe medium- and high-profile plates seem to have been mostlikely in the turbulent regime. The final data were presentedin the form of correlations. The details of this experimentalwork with uncertainty analysis as well as a comprehensive lit-erature review on the condensation of refrigerants in PHEs aregiven in Hayes et al. [5, 6] and in the ASHRAE 1394-RP finalreport [2].

Approximately 30 two-phase data points for each plate wererecorded and analyzed. In two-phase condensation, there aretwo heat transfer factors that contribute to the phase change,namely, forced convection and film condensation. Componentsof these two factors work together in concert with one anotherto condense a vapor and can be modeled in the dimensional

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Figure 3 Typical corrugated plate geometry.

analysis formula, as given in Eq. (1):

Nutp = 0.53Re0.691 Pr0.35

1

[G2/ρ2

1cp,1�T]1.05 [

ρ21i′fg/G2]0.85

× [ρ1σ1/μ1G]0.08 [ρ1/σ1 − ρv]0.7

× [(90 − β)π/180]0.06 (1)

where 67 < Rel < 1276; 0.22 < htp < 4.95 (kW/m2-K); and2.5 < q′′ < 20 (kW/m2).

The preceding empirical correlation fit the experimental datavery well, as presented in Figure 5 where β is in degrees, and

Figure 4 Carbon dioxide condensation experimental setup. (Color figure avail-able online.)

was found to have an average standard deviation and uncertaintyof less than 10% and 8%, respectively.

AMMONIA EVAPORATION IN PLATE HEATEXCHANGERS

To complete the design cycle of a cascade plate exchangerit is equally important to have reliable data and design tools toestablish appropriate evaporating heat transfer coefficient. Asmentioned earlier, to satisfy this requirement ASHRAE undertook an experimental research study on boiling of ammonia withand without miscible oils in plate exchangers [3]. The extensiveliterature search performed by Khan et al. [7] showed a lack ofinformation on plate heat exchangers with natural refrigerants.No previous study has been reported that quantifies effects of

0

30

60

90

120

150

0 30 60 90 120 150

L PLATE

M PLATE

H PLATE

+10%

-10%

Nutp(Experiment)

Nu t

p(Co

rrel

a�on

)

Figure 5 Experimental and correlated Nusselt numbers for the three BPHEs.(Color figure available online.)

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Figure 6 Ammonia evaporation experimental setup. (Color figure available online.)

miscible oil mixed with ammonia evaporating in a plate heatexchanger.

Experiments were performed to investigate steady-state heattransfer in a plate heat exchanger with two symmetric 30◦/30◦,60◦/60◦ configurations and mixed 30◦/60◦ chevron plates(Figure 3). The main objective of this work was to developNusselt number correlation for evaporation of ammonia in acommercial plate heat exchanger with and without miscibleoil. For the specific plate configuration, experiments were per-formed in such a way that effects of heat flux, vapor quality,saturation pressure, and mass flux could be studied. Experi-ments were carried out for selected values of mass flux over arange of heat flux and vapor quality. Five levels of saturationtemperature were considered for experimentation. The refriger-ant properties were evaluated at the saturation temperature. Thesingle-phase heat transfer coefficient on the hot fluid side wasestimated by a single-phase heat transfer correlation developedby Khan et al. [8] on the same plate heat exchanger.

Scope, Plate Configuration, and Results

The scope of experimental work was as follows:

• Saturation temperature range: –2◦C to –25◦C.• Exit vapor quality: 0.5 to 0.9.• Equivalent Reynolds number: 1225 to 3000.• Heat flux: 21 to 44 kW/m2.

The plate geometric parameters are as follows:

• Plate width, Lw (mm), 185.• Vertical distance between port centers, Lv (mm), 565.• Port diameter, Dp (mm), 43.• Horizontal distance between centers, Lh (mm), 125.

• Mean channel spacing, b (mm), 2.2; 2.9; and 3.6, for β =60◦/60◦, 30◦/60◦ and 30◦/30◦, respectively.

• Plate thickness, t (mm), 0.5.• Effective plate area, A (m2), 0.095.• Corrugation pitch, Pc (mm), 13.25 and 6.25.• Surface enlargement factor, ϕ, 1.117.

The experimental setup as shown in Figure 6 consists of arefrigerant loop, a plate heat exchanger test loop, a water/glycolsolution loop, instrumentation, and a data acquisition system.A cooling tower was also installed to serve the cooling re-quirements of the condenser and the compressor. Ammonia wascirculated in the refrigeration and test loop. Primary measure-ments in the experiments were flow rates, temperature, systempressure, and differential pressure drop for both hot side andrefrigerant side, across the plate heat exchanger. The two-phaseheat transfer coefficient on the refrigerant side was estimatedfrom the overall heat transfer coefficient. Details can be foundin the final ASHRAE report [3].

The plate geometry, saturation temperature, and other op-erational conditions strongly influenced the heat transfer. Heattransfer coefficient was found to increase with chevron anglefor the entire range of heat flux and exit vapor quality. It alsoincreased with saturation temperature. This effect was foundto be more evident for soft plate configuration (β = 30◦/30◦).The experimental results of this study were compared with two-phase evaporation data available in the open literature [9–12] asshown in Figure 7. Uncertainty analysis showed maximum errorof about ±10% in the experimental Nusselt number data. Theheat transfer coefficient increased with an increase in miscibleoil concentration up to 3%, but decreased with further increasein oil concentration, indicating a reverse trend as compared toprevious single-tube and bundle studies such as Zheng et al. [13]and Chyu et al. [14], respectively.

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Figure 7 Comparison of heat transfer results for 60◦/60◦ plate with previousstudies.

A generalized empirical correlation for two-phase Nusselt num-ber (Nutp) for all chevron angle plate configurations was devel-oped in terms of reduced pressure (P∗), equivalent Reynoldsnumber (Reeq), and equivalent Boiling number (Boeq) as fol-lows:

Nutp = (−173.52β∗ + 257.12) P∗(0.624β∗−0.8218)[ReeqBoeq]m

(2)

where 1225 < Reeq < 3000; 0.000178 < Boeq < 0.00049; 21 <

q′′ < 44 (kW/m2); and m = –0.0847 for β = 60◦/60◦, m =–0.0787 for β = 30◦/60◦, m = –0.0370 for β = 30◦/30◦.

This correlation overpredicts symmetric plate configurationdata by 6% and underpredicts—onfiguration data by 10%. Theterm β∗ is defined as β/βmax, where βmax is the maximum chevronangle of 60◦.

MECHANICAL INTEGRITY

Mechanical integrity of any system is very important forsafe operation. In a CO2/NH3 cascade system one of the mostimportant components is the cascade condenser. Any potentialleak would have a devastating effect on the operation of thesystem. It is a well-known fact that ammonia and carbon diox-ide mixing results in an ammonium carbomate powder. Thisresults in contamination of the entire system and could result in

Figure 8 All-welded plate pack of a shell-plate exchanger. (Color figure avail-able online.)

Figure 9 Left: good weld; right: failed weld.

a replacement of not just the condenser itself but the compres-sor and other auxiliary components such as piping and controlvalves. Plate exchangers are efficient and compact equipment;however, they are vulnerable to material and/or weld failures.Compact brazed exchangers are prone to intermedia mixing ifa flaw develops in the plate material itself. Most of the timechances are trivial for such a scenario; however, it is possible.On the other hand, brazed plate exchangers have size limita-tions. Semi-welded plate exchangers also fall within the samecategory; however, their usage as cascade condensers is not ad-visable since one side of the exchanger is sealed by gaskets andtherefore could result in leakage problems. Currently the com-mon type of plate exchanger in cascade systems is the all weldedshell-plate exchanger as shown in Figure 2. Figure 8 shows theplate pack before insertion in a shell and a close-up view ofthe peripheral welds between the adjacent plates that isolate thetwo streams. This type of exchanger possesses favorable ther-mal and momentum characteristics with the exception that inthe case of a small pin-hole leak in a seal weld a catastrophicfailure could occur. The welds could especially be vulnerable tofatigue stress if the system undergoes abnormal cyclic patterns.Figure 9 shows a good weld and a bad weld at the peripherybetween the adjoining plates of such heat exchanger. A weakjoint such as shown in the right photo in Figure 9 may not leak atthe time of the initial pressure test; however, after several cyclicoperations the crack may propagate as shown in Figure 9 andtherefore cause havoc in the system.

CONCLUSIONS

This article presents a timely discussion on the use of carbondioxide and ammonia in a cascade system with emphasis on theoptimization of the cascade condenser to reduce the footprint aswell as reduction of refrigerant inventory. Plate-type exchangersare being used as CO2/NH3 cascade condensers; however, thereare no reliable experimental data available in the open litera-ture especially for carbon dioxide condensation and ammoniaevaporation in plate exchangers.

Three brazed plate heat exchangers with differing chevronangle, each consisting of three channels, were experimentallytested for carbon dioxide condensation. The condensing heattransfer performance of the high-profile plate was better thanfor the mixed- and low-profile plates under similar flow con-ditions. The low-profile plate exhibited heat transfer behavior

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near laminar to transitional flow regime, whereas the medium-and high-profile plates seem to have been most likely in theturbulent regime.

Experiments were also performed to investigate steady-stateboiling heat transfer of ammonia in a plate heat exchanger withthree different symmetric plate configurations. The effects ofmass flux, heat flux, saturation temperature, and equivalentReynolds number on heat transfer characteristics of the plateheat exchanger were investigated. The plate geometry, satura-tion temperature, and other operational conditions strongly in-fluenced the heat transfer. Heat transfer coefficient was found toincrease with saturation temperature and mass flux. The experi-mental data were compared with limited data on evaporation ofammonia.

The data were reduced to generalized empirical correlationsfor carbon dioxide condensation and ammonia evaporation. Thisarticle also discussed the mechanical integrity aspects of plateexchangers for use in cascade systems. The results from thisstudy will enhance the literature on the subject and the resultingcorrelation will help practicing design engineers.

NOMENCLATURE

A effective plate area, m2

b corrugation depth, mmBo boiling number, q”/Gi’fg

cp specific heat, J/kg-KD diameter, 2b/ϕ, mmDp port diameter, mmG mass flux, kg/s-m2

h heat transfer coefficient, W/m2-Ki’fg latent heat, J/kgk thermal conductivity, W/m-KLh horizontal distance between centers, mmLv vertical distance between port centers, mmLw PLATE width, mmNu Nusselt number, hDeq/kP pressure, MPaPc effective corrugation pitch, mmPcr critical pressure, MPaP∗ reduced pressure, P/Pcr

Pr Prandtl number, μcp/kq′′ heat flux, kW/m2

Re Reynolds number, GDeq/μt plate thickness, mmT temperature, ◦CTs saturation temperature, ◦C�T temperature difference, ◦Cx quality

Greek Symbols

β chevron angle, degreesβmax maximum chevron angle of 60◦

β∗ β/βmax

μ viscosity, N-s/m2

ρ density, kg/m3

σ surface tension, N/mϕ surface enlargement factor

Subscripts

eq equivalentl liquid phasetp two phasev vapor phase

REFERENCES

[1] ASHRAE, Natural Refrigerants, A Position Document,ASHRAE, Atlanta, GA, 2009.

[2] Study of Carbon Dioxide Condensation in Chevron PlateExchangers, Project Report 1394-RP, ASHRAE, Atlanta,GA, 2010.

[3] Evaporation in Flooded Corrugated Plate Heat ExchangersWith Ammonia and Ammonia/Miscible Oil, Project 1352-RP, ASHRAE, Atlanta, GA, 2010.

[4] Hayes, N., and Jokar, A., Dynalene/Water Correlations tobe used for Condensation of CO2 in Brazed Plate HeatExchangers, ASHRAE Trans., vol. 115, no. 2, pp. 1–17,2009.

[5] Hayes, N., Jokar, A., and Ayub, Z., Study of CarbonDioxide Condensation in Chevron Plate Exchangers; HeatTransfer Analysis, International Journal of Heat and MassTransfer, vol. 54, pp. 1121–1131. 2011.

[6] Hayes, N., Jokar, A., and Ayub, Z., Study of Carbon Diox-ide Condensation in Chevron Plate Exchangers; PressureDrop Analysis, International Journal of Heat and MassTransfer, vol. 55, pp. 2916–2925. 2012.

[7] Khan, T. S., Khan, M. S., Chyu, M.-C., Ayub, Z., andChattha, J., Review of Heat Transfer and Pressure DropCorrelations for Evaporation of Fluid Flow in Plate HeatExchangers (RP-1352), International Journal of Heating,Ventilating, Air-Conditioning and Refrigerating Research,vol. 15, no. 2, pp. 169–188, 2009.

[8] Khan, T. S., Khan, M. S., Chyu, M.-C., and Ayub, Z. H., Ex-perimental Investigation of Single Phase Convective HeatTransfer Coefficient in a Corrugated Plate Heat Exchangerfor Multiple Plate Configurations, Applied Thermal Engi-neering, vol. 30, pp. 1058–1065, 2010.

[9] Ayub, Z. H., Plate Heat Exchanger Literature Surveyand New Heat Transfer and Pressure Drop Correlationsfor Refrigerant Evaporators, Heat Transfer Engineering,vol. 24, no. 5, pp. 3–16, 2003.

[10] Djordjevic, E., and Kabelac, S., Flow Boiling of R134a andAmmonia in a Plate Heat Exchanger, International Journalof Heat and Mass Transfer, vol. 51, pp. 6235–6242, 2008.

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[11] Arima, H., Kim, H.J., Okamoto, A., and Ikegami, Y., LocalHeat Transfer Characteristics of Ammonia in a VerticalPlate Evaporator, International Journal of Refrigeration,vol. 33, pp. 359–370, 2010.

[12] Yan, Y. Y., and Lin, T. F., Evaporation Heat Transfer andPressure Drop of Refrigerant R-134a in a Plate Heat Ex-changer, Journal of Heat Transfer, vol. 12 (1), pp. 118–127,1999.

[13] Zheng, J. X., Jin, G. P., Chyu, M.-C., and Ayub, Z. H.,Flooded Boiling of Ammonia With Miscible Oil Outsidea Horizontal Plain Tube, International Journal of Heating,Ventilating, Air-Conditioning and Refrigerating Research,vol. 7, no. 2, pp. 185–204, 2001.

[14] Chyu, M.-C., Zheng, J., and Ayub, Z., Bundle Effect ofAmmonia/Lubricant Mixture Boiling on a Horizontal Bun-dle With Enhanced Tubing and Inlet Quality, InternationalJournal of Refrigeration, vol. 32, no. 8, pp. 1876–1885,2009.

Zahid Ayub holds Ph.D. in mechanical engineer-ing from Iowa State University. He is President ofIsotherm, Inc., a manufacturer of heat transfer equip-ment in Arlington, TX. He has extensive experiencein the area of applied heat transfer and has success-fully designed, fabricated, and installed several thou-sand heat exchangers, pressure vessels, and refriger-ation/heat transfer systems worldwide. He is recog-nized as one of the pioneers in the field of enhancedheat transfer for ammonia applications and is actively

involved in “Green Refrigeration.” He holds six U.S. patents with several pend-ing. Dr. Ayub is the author of more than 70 international journal and conferencepapers. He is in the process of publishing a book on heat exchanger design forindustrial refrigeration. He has served as the technology editor for the Inter-national Journal of Enhanced Heat Transfer and is currently executive editorof the Journal of Heat Transfer Engineering and ASME Journal of ThermalScience and Engineering Applications. He also serves as a reviewer for sev-eral other heat transfer journals. Among the honors Dr. Ayub has received arethe Michigan New Product Award, ASHRAE Distinguished Service Award, andASHRAE Research Service Award. He is also a fellow of ASME and ASHRAE.He currently serves as a member of the Scientific Council–InternationalCenter for Heat and Mass Transfer representing the United States. He is alsoan active member of ASME, ASHRAE, IIAR, AIChE, IoR (UK), IIR, RETA,Eurammon, and the Sigma Xi Honor Research Society. He served as chair ofthe ASHRAE Standing Committee on Refrigeration and as a member of theASHRAE Research Advisory Panel for Strategic Policy 2010–2015, TC1.3 andTC8.5, and the ASME K-10 Committee. He is a founder and director of NaturalFluids Refrigeration Center (NFRC), GIK Institute of Engineering and Technol-ogy, Pakistan. He is regularly invited to international conferences and institutesof advanced learning to deliver keynote lectures in his area of expertise. Dr.Ayub has taught courses on thermodynamics, heat exchanger design, and heat-ing, ventilation, and air conditioning as an adjunct professor at University ofTexas–Arlington Mechanical and Aerospace Engineering Department since fall2000.

M. Sultan Khan is a professor in the Faculty ofMechanical Engineering at Jinnah University, Islam-abad, Pakistan. He is also a co-founder of NaturalFluids Refrigeration Center at GIK Institute of Engi-neering and Technology, Pakistan. He completed hisPh.D. in mechanical engineering from the Universityof British Columbia, Vancouver, Canada, in 2005. Heis an active member of ASME and ASHRAE.

Amir Jokar holds a Ph.D. in mechanical engineeringfrom Kansas State University (2004). He has a back-ground in thermal systems design, refrigeration, andtwo-phase flow. He has extensive experience in bothacademia and industry. He has conducted researchon industry-sponsored projects and won competitivegrants as the principal investigator, including an ex-perimental study of carbon dioxide condensation inchevron plate exchangers (ASHRAE 1394-RP). Hehas been the recipient of the ASHRAE New Inves-

tigator Award to study heat transfer and fluid flow of nanofluids in micro-/minichannel heat exchangers. He developed several courses and laboratoryprocedures at Washington State University–Vancouver on thermofluids, includ-ing advanced courses on thermal systems and electronics cooling. He is aregistered professional engineer in the state of California, and he is currentlypracticing thermal engineering as a consultant at Exponent, Inc.

Tariq S. Khan received his B.S. (1997) and M.S.(1999) in mechanical engineering from EasternMediterranean University, Turkish Republic of NorthCyprus, and a Ph.D. in mechanical engineeringfrom GIK Institute of Engineering Sciences andTechnology, Pakistan, in 2010. His Ph.D. disserta-tion was on evaporation in flooded corrugated plateheat exchanger with NH3 and NH3/miscible oil, anASHRAE-sponsored project. He is currently workingas an assistant professor at the Faculty of Mechanical

Engineering, GIK Institute. He is also the lead coordinator at Natural FluidsRefrigeration Center (NFRC) at GIK Institute. His diversified research workin thermofluids has been published in international journals and presented atvarious international conferences.

Niel Hayes holds a B.S. degree in mechanical engi-neering from Brigham Young University and an M.S.degree in mechanical engineering from WashingtonState University–Vancouver, USA. The ASHRAE-sponsored research project that he worked on atWashington State University for his thesis dealt withstudying how an environmentally friendly refriger-ant behaved thermodynamically in compact heat ex-changers. Upon completing the project, the findingshave been published in several international journals

and shared in conferences all around the world. He was awarded the best researchaward on natural fluids by Eurammon in 2011 for this work.

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