1、Volume 15, Number 2, March 2009An International Journal of Heating, Ventilating,Air-Conditioning and Refrigerating ResearchAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.Volume 15,Number2,March 2009HVAC accepted October 3, 2008This paper is based on findings resulting
2、 from ASHRAE Research Project RP-1352.Plate heat exchangers have been widely used in dairy, food processing, paper/pulp, heating,ventilating, and other related industry. While single-phase flow in plate heat exchangers hasbeen studied extensively, the industry lacks basic information on fluid flow e
3、vaporating in plateheat exchangers. This paper provides a review of heat transfer and pressure drop correlationsfor fluid flow evaporating in plate heat exchangers. Emphasis is placed on the application of aplate heat exchanger as an ammonia evaporator in a refrigeration system. It is shown via thor
4、-ough research of related thermalhydraulic phenomena that heat transfer and pressure dropcorrelations are needed for liquid ammonia flow evaporating in plate heat exchangers. Theeffects of plate geometry, plate material, oil/lubricant concentration, and several operatingparameters on heat transfer c
5、oefficient and pressure drop for plate heat exchangers also need tobe quantified.INTRODUCTIONPlate heat exchangers are designed to achieve high heat transfer capacity in a small volume.Due to their compact size, plate heat exchangers have clear advantages over shell-and-tube heatexchangers and are r
6、apidly replacing conventional shell-and-tube evaporators. Several types ofplate heat exchangers are currently used in industry, including conventional gasketplate-and-frame, compact brazed, semiwelded plate-and-frame, and shell-and-plate (Ayub2003). The disadvantage of conventional gasket heat excha
7、ngers is leakage due to failure of gas-ket material. Brazed heat exchangers were initially designed for cooling oil and liquid-to-liquidapplications. They are also used as evaporators and condensers in the refrigeration industry.When used as evaporators, brazed heat exchangers showed poor performanc
8、e at high loadcapacities, and failures were reported for low temperature applications (Ayub 2003).Shell-and-plate is the newest design in the plate exchanger technology. It has high mechanicalintegrity and superior thermal characteristics (Ayub 2003).Some main geometric features of a heat exchanger
9、plate are discussed below and are shown inFigure 1.Tariq S. Khan is a doctoral student, Mohammad S. Khan is an assistant professor, and Javed A. Chattha is a professorand dean of the Faculty of Mechanical Engineering, GIK Institute of Engineering Sciences and Technology, Topi, Paki-stan. Ming-C. Chy
10、u is a professor in the Department of Mechanical Engineering, Texas Tech University, Lubbock, TX.Zahid H. Ayub is a researcher at ISOTHERM, Inc., Arlington, TX. 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC therefore, the fl
11、ow regime is relatively simple, and phase separation is not asevere issue even at low mass fluxes. Ayub (2003) proposed the following two-phase heat trans-fer coefficient correlation for the evaporation of ammonia and R-22 in direct expansion (DX)and flooded evaporators. The correlation was develope
12、d based on field data, and no detailedexperimental work was performed in a laboratory environment:(1)htpCklDe-Rel2hfgLp-0.4124ppcr-0.1265 ()0.35=172 HVAC Re and Pr are Reynolds and Prandtl numbers,respectively; and is dynamic viscosity. Subscripts m and wall correspond to bulk and wall con-ditions,
13、respectively. The evaporation heat transfer data of R-134a flow in the plate heatexchanger were correlated with an average deviation of 8.3% by the following equation:(6a)where Bo is Boiling number. The equivalent values for mass flux G, Reynolds number, and boil-ing number are as follows:(6b)where
14、is heat flux, and is latent heat of vaporization. The coefficient Cx, a function ofmean vapor quality (xm) and the liquid-to-vapor-density ratio, is given as follows:(6c)where land gare liquid and vapor densities, respectively. Correlations of Fanning friction fac-tor, Cf for evaluation of pressure
15、drop of evaporating R-134a flow, with an average deviation of7%, are as follows:(7)(8)UQwh,A LMTD-=Nusp0.2121Re0.78Pr13mwall-0.14=NutpPrl13Rel0.5Boeq0.31.926Reeqfor 2000 Reeq10 000,16,000, +8%, field dataSterner and Sunden (2006) = 59 and 65, 50 Refo 225Used five plate heat exchangers3.6 mm dh 5.6 m
16、m, 0.095 m2 A 0.124 m212 kW q 185 kW, 0.5kg/m2s Gref 0.9 kg/m2s20 kg/m2s Gwater 630 kg/m2s0.05 x 1.0, 6C Tsat 3C Correlations for Other RefrigerantsYan and Lin (1999)R-134a, = 30, dh= 5.8 mm 2,000 Re 10,000, 11 kW/m2 q 15 kW/m250 Gref 70 kg/m2s, 0.05 xm 0.90.675 MPa P 0.8 MPa, +8.3%Ouazia (2001) R-1
17、34a, = 0, 30 and 60Hsieh et al. (2002)R-134a, subcooled flow boiling, = 30, G = 50, 100 and 200 kg/m2sTsat= 21.6C and 26.7C, dh= 5.8 mm 8.5 kW/m2 q 30 kW/m2, +12.8%Hsieh and Lin (2002)R-410A, = 30, dh= 5.8 mm G = 50, 75,100 and 125 kg/m2s, 5 kW/m2 q 35 kW/m2Tsat= 10C, 15C and 20C, +20%1For nomenclat
18、ure and further details, refer to the main body of this paper.htpCklDe-Rel2hfgLp-0.4124ppcr-0.1265 ()0.35=3065Nu CRefomJanCop=NutpPrl13Rel0.5Boeq0.31.926Reeq= 2000 Reeq10,000Cftp,Re0.56.947 105Reeq1.109=Req6000Cftp,Re0.531.21Reeq0.04557eeq6000htpaklDh-G 1 x()Dhl-bPrl13lwall-0.171 Cl1Xtt-C2+=hrsub,hr
19、l,1.2Fr0.7513.5Bo13/Ja14/+()=dp g-0.93 lg()1.23Re0.35Ja 165 lg()1.23+Bo0.487Re1.58-=hrsat,hrl,=88Bo0.5()ftp61,000 Reeq1.25()=VOLUME15,NUMBER2,MARCH2009187Table 1. Summary of Correlations for Heat Transfer and Pressure Drop for Evaporation of Fluid Flow in Plate Heat Exchangers1 (Continued)Reference
20、Correlations Test Conditions, Accuracy, and Other Information Correlations for Other Refrigerants (cont.)Hsieh and Lin (2003)R-410A, = 30, dh= 5.8 mm, 2000 Re 12,000, 0.1 x 0.8, 50 kg/m2s Gr 100 kg/m2s10 kW/m2 q 20 kW/m2, +25%Park and Kim (2003) R-134a, plate and shell heat exchanger, = 45, 0.1 xm 0
21、.8, Tsat= 10C, 15C, and 20C, 45 kg/m2s Gr 55 kg/m2s, 4 kW/m2 q 8 kW/m2, +15%Han et al. (2003)R-410A and R-22, = 45, 35, 2013 kg/m2s Geq 34 kg/m2s2.5 kW/m2 q 8.5 kW/m2Tsat= 5C, 10C, 15C, 0.15 xm 0.9, +25%Jokar et al. (2006)R-134a, = 60A = 0.026 m2, b = 2 mm 200 kPa Pevap 600 kPa 900 kPa Pcond 2100 kP
22、a0.01 kg/s mref 0.06 kg/s0.13 kg/s mliq 0.45 kg/s450 Reeq 3400, 70 Rel 440, +25%Longo and Gasparella (2007a)R-134a, herringbone plate heat exchanger = 25, 9.7 Tsat 20.1C, 0.78 xo 1.00 11.8 Gref 36.7 kg/m2s, 4.5 q 18.9 kW/m21For nomenclature and further details, refer to the main body of this paper.h
23、tpEhlShpool+=ftp23,820 Reeq1.12=NuPr13/- Re0.5Boeq0.3532.2Reeq0.3237=NutpGelReeqGe2Boeq0.3Pr0.4=ftpGe3ReeqGe4=Nutp evap,0.603Rel0.5Prl0.1x2G2l2Cpl,T-0.1 l2hfgG2-0.05=llG-1.1llv-2Nutp cond,3.371Rel0.55Prl0.3G2l2Cpl,T-1.3 l2hfgG2-1.05=llG-0.05llv-2Cftp,3.521 104Rel1.35Cx1=pf1.425KEV-=188HVACaccepted S
24、eptember 7, 2008The cosorption characteristics of water and toluene vapors in various concentrations of trieth-ylene glycol (TEG) solution flowing through a packed-bed dehumidifier are investigated inthis paper. A multi-component model was constructed using the reported equilibrium relation-ships of
25、 toluene and water vapors in TEG solutions together with the Krishna-Standartmulti-component mass transfer correlation. The effects of liquid-to-air ratios, TEG inlet tem-peratures, air inlet temperatures were reported on the moisture and toluene removal rate aswell as the moisture and toluene remov
26、al efficiency of the packed dehumidifier. Running thepacked dehumidifier in a higher liquid-to-gas flow ratio generally increased the removal ratesand efficiencies of both water vapor and toluene vapor from the airstream. Increasing inlettemperatures of the TEG solution led to a decrease in the remo
27、val rate of water vapor whenrunning the packed dehumidifier at a high liquid-to-gas flow ratio. However, there was no sig-nificant change in the toluene vapor removal rate or toluene removal efficiency when the flowrate of the inlet TEG solution was increased. INTRODUCTIONThe environmental awareness
28、 that occurred in the 1980s has led to the public demand for healthyoutdoor and indoor environments. Today, the common approach adopted for cleaning contami-nated indoor air is diluting indoor air with clean outdoor air in order to achieve acceptableindoor air quality. Air cleaning may be an attract
29、ive alternative when the outdoor air quality ispoor or when there is a desire to reduce energy costs associated with high outdoor air exchangerates. Accordingly, substantial research efforts has been carried out to identify effective methods toclean indoor air while minimizing energy consumption.Alt
30、hough water vapor is not considered to be an air contaminant, it has a significant impact onboth comfort and health of human occupants. Unlike other contaminants in the air, water vaporcannot be completely removed from the airstream. Extremely low relative humidity can lead toeye irritation, mucous
31、dryness, and other health problems (Arundel et al. 1992). On the otherhand, high indoor relative humidity can lead to condensation on cold faces, such as ducts, dry-ers, windows and various building materials, and can also promote the growth of various micro-organisms. Therefore, the control of the
32、indoor relative humidity is an important designparameter for HVAC industry practitioners and researchers.In conventional vapor-compression systems, the process of dehumidification and humiditycontrol is brought about by cooling the air below its dew point and reheating the air to thedesired relative
33、 humidity. This method of humidity control is extremely energy intensive. InC.K. Chau is an associate professor in the Department of Building Services Engineering, The Hong Kong PolytechnicUniversity, Hung Hom, Hong Kong. W.M. Worek is professor and head of the Department of Mechanical and Industria
34、lEngineering, The University of Illinois at Chicago, Chicago, IL. 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC (b) spraying the liquid in a fine dispersion in an upward airstream, as in a spray tower;(c) spraying the liquid
35、 over a bank of cooling tubes past which air is blown; or (d) passing the airand liquid streams through a packed bed. In this study, we focus on a packed bed, since it usu-ally offers lower pressure drop but has the greatest interfacial area between the air and the liquidstreams. This leads to high
36、interchange rates of heat and mass between the liquid desiccant andthe airstream.Apart from studies on the heat and mass transfer characteristics of desiccants, desiccantpotential for removing indoor air contaminants has also begun to be recognized. Hines andGhosh (1993) revealed the capabilities of
37、 solid desiccants such as silica gel and a molecularsieve for removing contaminants from the air. Activated carbon fiber has been shown to have astrong capability of removing volatile organic compounds (VOCs) (Das et al. 2004). Hydrpoho-bic zeolites, in membrane form, have also been found have an ab
38、ility to selectively remove oneor more organic pollutants from humid airstreams (Chitawar and Greene 1997; Aguado et al.2004). Consequently, solid absorbents have been employed in many industrial applications inthe US and Asia for independently controlling humidity and VOCs.On the other hand, Moscha
39、ndreas and Relwani (1990) demonstrated that a liquid desiccant-based, gas-fired dehumidification system using lithium chloride solution (LiCl) had the capacityto remove indoor pollutants. However, no systematic study has been conducted to assess the fulladsorption potential of LiCl solution. It was
40、not until the work done by Hines et al. (1992), acomprehensive study conducted to determine the removal capabilities of both organic and inor-ganic liquid absorbents, that organic compounds such as TEG solution were found to have astronger capability to remove the organic contaminants from the airst
41、reams than inorganic saltssuch as lithium chloride. The researchers attributed this phenomenon to the fact that the removalcapacity of the LiCl solutions depend to a great extent on the solubility of the particular pollut-ant in the water portion of the solution while the pollutant removal capacitie
42、s of the TEG solu-tion depends on the solubility of the pollutants in the absorbent portion of the solution. Although research on the absorption properties of TEG for water and various VOCs havebeen ongoing for some time (Peng and Howell 1981; Ng et al. 1983; Grasso et al. 1994), there isno publishe
43、d experimental or numerical data on the simultaneous absorption of water vapor andair contaminants by TEG. Chau and Worek (2007) developed a numerical model to simulate thecosorptive characteristics of TEG solutions for both water vapor and VOC air contaminantsunder various operating conditions thro
44、ugh packed-bed absorbers. Toluene, which has widelybeen recommended as a reasonable surrogate and representative for indoor total VOCs in desic-cant adsorption studies (Liu 1990, 1993), has been selected as another component in the air-VOLUME 15, NUMBER 2, MARCH 2009 191stream. In this paper, the in
45、fluence of various operating conditions on the cosorptiveperformance of packed dehumidifiers is examined. Also, the optimal operating conditions inwhich TEG solutions can absorb the maximum amount of toluene in the presence of water vaporare presented. MATHEMATICAL MODELIn order to model the cosorpt
46、ion processes that occur in the dehumidifier, the dehumidifier isdivided into a sequence of stages. Figure 1 shows a schematic representation of a packed dehu-midifier with a typical stage enlarged. The following assumptions were made when deriving themodel:1. no accumulation of mass and energy flux
47、es at the interface of each stage;2. plug flow of each phase (i.e., absence of radial gradients of velocity, temperature, and com-position for the bulk flow);3. the pressure drop across the packed column is small compared to the absolute pressure,which is assumed to be close to atmospheric pressure;4. uniform and constant concentrations at the inlet streams;5. ideal gas behavior for the vapor phase;6. constant specific heats for both the liquid and vapor phases;7. equal heat and mass transfer areas;8. finite and constant mass and heat transfer coefficients;9. negl
