R245fa Evaporation Heat Transfer and Pressure Drop in a Brazed Plate Heat Exchanger for Organic Rankine Cycle (ORC)

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1 Proceedins World Geotheral Conress 2015 Melbourne, Australia, April 2015 R245fa Evaporation Heat Transfer and Pressure Drop in a Brazed Plate Heat Exchaner for Oranic Rankine Cycle (ORC) Kaiyon Hu a, Jialin Zhu a, Tailu Li b, Wei Zhan a a Key Laboratory of Efficient Utilization of Low and Mediu Grade Enery, MOE, Tianjin University, Tianjin , China b School of Enery and Safety Enineerin, Tianjin Chenjian University, Tianjin , China hky422@tju.edu.cn Key words: Heat transfer; Brazed plate heat exchaner; Heat transfer coefficient; Pressure drop; ORC ABSTRACT Evaporator which is of reat iportance in Oranic Rankine cycle (ORC) receives less attention copared with the ORC syste itself. An evaporator selected correctly not only can iprove the theral efficiency, but also can save refrierant. It is crucial to deterine the heat transfer features of refrierant in an evaporator. In paper presents an experiental study on the heat transfer and pressure drop of R245fa in a brazed plate heat exchaner (BPHE) for oranic Rankine cycle (ORC) was carried out. The effects in ters of heat flux, refrierant ass flux, saturation teperature, and superheat deree at the outlet of the BPHE were investiated. Experients were conducted for refrierant ass flux ranin fro 120 to 210 k/h at saturation teperatures ranin fro 78 to 92. The heat flux was varied between 20 kw -2 and 29kw -2. The current study indicated that the heat transfer coefficients show reat sensitivity to heat flux and refrierant ass flux. However, the correspondin pressure drop increased with increasin the ass flux and heat flux. 1. INTRODUCTION Traditional plate heat exchaners (PHE) have been wildly used in processin, heat and power, air-conditionin and refrieration, heat recovery, and other anufacturin industries for any years (Wan,2007). Copared with other heat exchaners, traditional PHE have soe obvious advantaes such as flexible theral sizin, easy cleanin for sustainin extree hyienic conditions, close approach teperature, and enhanced heat transfer perforance. In the interest of enery conservation and space savin, PHE are bein used in oranic Rankine cycle (ORC) syste. As refrierants used in ORC are corrosive and hih-pressure substance, traditional PHE are not proper. So another kind of PHE, the brazed plate heat exchaner (BPHE) in which stainless steel plates are vacuu brazed toether usin copper as the brazin aterial are eployed. In the open literature, it is possible to find soe investiations on refrierant vaporization inside PHE or BPHE. But the refrierants used are ainly low-teperature and hih-pressure refrierant, such as R-134a, R-410A and aonia. Besides, ost experients are conducted at air-conditionin condition. There are rather liited data available on evaporation features of refrierants which are hih-teperature and low-pressure in BPHE for ORC. In the followin the relevant literature on the present work are briefly reviewed. Yan and Lin (1999) experientally investiated evaporation heat transfer and pressure drop of refrierant R-134a inside BPHE in ters of ean vapor quality, ass flux, heat flux and saturation pressure. They showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. In addition, epirical correlations for the evaporation heat transfer coefficient and pressure drop were proposed. With the sae experiental facility used by Yan and Lin (1999), Hsieh and Lin (2003) extended their work by doin research on characteristics of heat transfer and pressure drop for R-410A flowin in a BPHE. Fro the easured data, raisin the iposed heat flux is found to sinificantly iprove the heat transfer coefficient for the entire rane of the ean vapor quality, while the pressure drop is insensitive to the iposed heat flux and refrierant pressure. Lee et al. (2013) reported experiental data on flow boilin heat transfer of R-134A in PHE at low ass flux condition in ters of vapor quality, heat flux, evaporation pressure, and ass flux. It indicated that heat transfer was ainly doinated by nucleate boilin heat transfer and the ass flux had a ild effect on the boilin heat transfer at a low fluid ass flux. Han et al. (2003) presented experiental data on evaporative heat transfer and pressure drop easured durin R-410A and R-22 vaporization inside a BPHE, respectively. Attention was payed on the effect of ass flux, saturation teperature, vapor quality, heat flux and plate eoetry (inclination anle of the corruation). Furtherore, epirical correlations for the heat transfer and pressure drop were proposed. Djordjevic and Kabelac (2008) studied heat transfer coefficients of R-134a and aonia in a PHE, respectively. They stated that the parallel flow case had a better heat transfer perforance than the counterflow case, and the low chevron anle corruations could iprove the evaporation heat transfer. Lono et al. (2007a,2007b,2007c) reported experiental data on vaporization heat transfer coefficients and pressure data on R-134a, R-410A, and R-236fa inside a BPHE, respectively. Data related to heat flux, refrierant ass flux, saturation teperature, outlet conditions and fluid properties were obtained. Taboas et al. (2010) experientally analyzed the flow boilin heat transfer and pressure drop of aonia/water ixture in a PHE and found that for the selected operatin conditions the boilin heat transfer coefficient was hihly dependent on the ass flux, while the pressure drop increased with increasin ass flux and quality. Khan et al.(2012a) and Khan et al.(2012b) investiated heat transfer and pressure drop characteristics of aonia in 60 and 30 chevron PHE, respectively. They reported that the heat transfer coefficient increased with an increase in saturation teperature, ass flux and the exit vapor quality, while the friction factor decreased with an increase in exit vapor quality and equivalent Reynolds nuber. They also proposed correlations for two phase Nusselt nuber and friction factor. 1

2 Many studies have been conducted for refrierants vaporization heat transfer characteristics in PHE or BPHE with air-conditionin conditions. However, experients on refrierants evaporation in BPHE with ORC conditions are relatively scarce. The objective of the work presented here is to contribute to the developent of BPHE as evaporator in ORC syste. For the purpose an apparatus was set up to easure the heat transfer coefficients and pressure drop durin R-245fa vaporization in a BPHE. The effects of refrierant ass flux, heat flux, saturation teperature, and superheat deree at the outlet are investiated. 2. EXPERIMENTAL APPARATUS AND PROCEDURES 2.1 Apparatus The experiental apparatus, as shown in Fi.1, consists of three ain loops such as a refrierant loop and two water loops (one for the evaporator heatin and the other for the condenser coolin). Besides, a data acquisition syste is also included to obtain different test conditions. The refrierant loop contains a refrierant pup (the diaphra pup), a ass flow eter, an evaporator (the brazed plate heat exchaner (BPHE)), a turbine, a condenser and a receiver. The refrierant pup with chaneable stroke piston is driven by a otor which is controlled by a variable-frequency drive. The liquid flow rate of R245fa is varied by a chaneable stroke piston, and it also can be further adjusted by a variable-frequency drive throuh chanin the current frequency. The R245fa flow rate is easured by a ass flow eter with an accuracy of ±0.2 percent. In the evaporator, enery balance is used to calculate the heat transferred fro the hot water to the refrierant. The R245fa vapor can o throuh the turbine or a valve, both of which can reduce the vapor pressure. Meanwhile, a condenser is used to condense the R245fa vapor fro the turbine by a cold water loop. After condensin, the liquid refrierant flows back to the receiver. The pressure of the refrierant loop can be controlled by varyin the flow rate and teperature of the cold water in the condenser, and it also can be chaned by adjustin the openin size of the valve in the loop. Fi.1 Scheatic diara of experiental syste The BPHE studied in this paper, as scheatically shown in Fi.2, consists of 6 plates. The plate surfaces are staped to becoe rooved with corruation type of herrinbone. Each plate is 2.22 thick and the pitch between the plates is 3.2. More detailed eoetrical characteristics of the BPHE tested are shown in Fi.2 and Table 1. Fi.2 Scheatic diara of the plate Table 1 Geoetrical characteristics of the evaporator 2

3 Plate lenth, L () 158 Plate width, W () 76 Noinal area of the plate, A ( 2 ) Corruation type Herrinbone Anle of the corruation, β(derees) 60 Corruation aplitude, b () 1.5 Corruation pitch, p () 2.22 Nuber of plates 28 Channels on refrierant side 14 Channels on water side 13 The water loop in the experiental syste for evaporator heatin contains a water therostat with a 18kW heater and a 500w pup which is used to drive the hot water to the plate heat exchaner with a specified water flow rate. The water flow rate is easured by a turbine flow eter with an accuracy of ±0.5 percent. Another water loop desined for condensin the R245fa vapor contains a coolin tower with a coolin capacity of 20kW. A 500w pup is used to drive the coolin water to the condenser. The water flow rate is also easured by a turbine flow eter with an accuracy of ±0.5 percent. The data acquisition syste includes a recorder and a 24V power supply. The 24V power supply is used to drive the water flow eter and differential pressure transducer to output an electric current 4-20A. The recorder is a 80 channel YOKOGAWA GP20 recorder which is used to record the teperature and voltae data, and it also can be used to analyze the data collected Procedures The experient is conducted in ters of the heat flux exchaned between hot water and R245fa, the ass flux of the R245fa, the teperature of the hot water inlet and the saturation vapour pressure of R245fa. Before each test, the evaporator and the condenser are fed with water at a constant teperature, respectively. Then, the teperature and flow rate of the hot water for the evaporator are adjusted to keep the saturation vapour pressure at a desired value, and the heat flux also can be calculated fro the hot water. Finally, the refrierant flow rate in the evaporator can be controlled by adjustin the voluetric pup. Once the teperature, pressure and flow rate steady state conditions are achieved at the evaporator inlet and outlet both on refrierant and water sides, all the readins are recorded for a set tie, and the averae value durin this tie is calculated for each paraeter recorded. 3. DATA REDUCTION 3.1 Heat transfer The procedures to calculate the overall heat transfer coefficient in the evaporator are described in the followin. Firstly, the total heat transfer rate between the counter flows in the brazed plate heat exchaner (BPHE) is calculated fro the hot water side, e, w e, w p( e, w, i e, w, o W c T T ) (1) Then, the vapor quality at the evaporator inlet is evaluated fro the refrierant state at the pre-evaporator throuh enery balance. The heat transfer rate is calculated fro the water side, p, w p, w p( p, w, i p, w, o W c T T ) (2) The heat transfer to the refrierant in the pre-evaporator contains two parts, one part is used to heat the refrierant to the saturation teperature (sensible heat transfer), and the other part is used on the evaporation process (latent heat transfer). Thus where, p w p, r, (3) p, r r ( p, r, o p, r, i W h h ) (4) The overall heat transfer coefficient U for the evaporation of R-245fa in the BPHE can be expressed as 3

4 U e, w A LMTD (5) Where A is the heat transfer area accountin for the actual corruated surface of the plates. The lo ean teperature difference (LMTD) is deterined by the inlet and outlet teperatures of two counterflow channels of the BPHE where LMTD T1 T2 ln( T T ) 2 (6) T 1 T e, w, i Te, wf, o 1 (7) (8) T 2 T e, w, o Te, wf, i T e wf, o T, In equations (10) and (11),, and e, wf i are the saturation teperatures of R-245fa correspondin respectively to the inlet and outlet pressures for the refrierant in the BPHE. Finally, the averae heat transfer coefficient on the refrierant side of the evaporator is derived fro the equation assuin no foulin resistances: where p (9) h U h wf w p h w is deduced fro the epirical correlation for the sinle phase water to water heat transfer in the BPHE. 3.2 Pressure drop The calculation of the frictional pressure drop p associated with the R-245fa evaporation in the BPHE is carried out on the f condition that the evaporation process is looked as a hooeneous odel. It is coputed by subtractin the acceleration pressure drop pa, the ravity pressure drop p and the anifolds and ports pressure drops pp fro the total pressure drop easured pexp p pexp p p p (13) The acceleration and ravity pressure drops are derived fro the equations f a p p G 2 ( v v ) x a l (14) p L (15) where v and v are the specific volue of liquid and vapor phase, respectively, l inlet and outlet and the averae density of the two-phase between inlet and outlet is iven as x 1 x 1 x is the vapor quality chane between the (16) The pressure drops in the inlet and outlet anifolds and ports is epirically suested by Shah and Focke, as follows f l 2 p 1.5G 2 (17) All the therodynaic properties of the refrierant R-245fa in this paper are evaluated by REFPROP RESULTS AND DISCUSSION A series of vaporization tests are conducted at four different saturation teperatures (78,85,90 and 92 ) with different evaporator outlet conditions (different derees of superheat), whereas the deree of supercoilin of refrierant at the inlet ranes between 1 and 3 deree. Table 2 ives the operatin conditions in the evaporator under test: refrierant saturation teperature T sat and pressure 4

5 p sat, inlet and outlet refrierant conditions T in,supc and T out,suph, ass flux of refrierant G, and heat flux q. All the conditions shown in Table 3 are typical for the evaporator in ORC syste. Table 2 Operatin conditions durin experiental tests T sat ( ) p sat ( bar) T in,supc ( ) T out,suph ( ) G (k/h) q(kw/ 2 ) Heat transfer The variations of evaporation heat transfer coefficient in the BPHE with different ass fluxes, heat fluxes, and superheat derees are shown in Fiures.3-5 for different saturation teperatures. Fi.3 presents the relationship between the heat transfer coefficient and the ass flux. Durin the experient the ass flux ranes fro to k/h. The heat transfer coefficients show reat sensitivity to heat flux with different saturation teperatures. Heat transfer coefficient decreases quickly with the increase of the refrierant ass flux. With saturation teperature 78, the heat transfer coefficient at ass flux around 145 k/h is two ties of the heat transfer coefficient at ass flux around 175 k/h. Fi.4 ives the effect of heat flux on the heat transfer coefficient with different saturation teperature. The sae relationship between heat transfer coefficient and heat flux is found copared with ass flux. In Fi.5 the variations of heat transfer coefficient with the superheat deree are shown. The arked decrease of the heat transfer coefficient with ass flux and heat flux is due to the increase of teperature different between the refrierant and the hot water. Fi.3-5 also indicate that saturation teperature has a positive effect on the heat transfer coefficient. Therefor, properly increasin the saturation teperature can enhance the heat transfer. Fi.3. Variations of evaporation heat transfer coefficient with refrierant ass flux Fi.4. Variations of evaporation heat transfer coefficient with heat flux 5

6 Fi.5. Variations of evaporation heat transfer coefficient with superheat deree 4.2 Pressure drop Fis.6-8 present the variations of the pressure drop with the ass flux, heat flux, and superheat deree. Different with the evaporation heat transfer coefficient, the pressure drop shows positive relation with the ass flux and heat flux, and neative relation with the superheat deree. It is noted that the hiher ass flux results in a hiher pressure drop for different saturation teperature, as shown in Fi.6. Moreover, at a hiher ass flux, the effects are stroner. Fis.6-7 also indicate that the pressure drop in the evaporator is reduced with an increase in the syste pressure ( that is for a hiher saturation teperature) with the reason that at a hiher syste pressure, the specific volue of the vapor and the viscosity of the liquid R-245fa are lower. The pressure drop decreases with the increase of the superheat deree for a sall portion of the liquid R245-fa, as shown in Fi.8. Fi.6. Variations of pressure drop in evaporator with refrierant ass flux 6

7 Fi.7. Variations of pressure in evaporator drop with heat flux Fi.8.Variations of pressure drop in evaporator with superheat deree 5. CONCLUSION An experiental investiation has been conducted in the present study to easure the heat transfer coefficient and pressure drop of R-245fa durin the evaporation in a brazed plate heat exchaner (BPHE) in ters of the refrierant ass flux, heat flux and the the superheat deree at the outlet of the BPHE evaporator. The results show that the heat transfer coefficient decreases with the increase of ass flux and heat flux, whereas, the pressure drop shows a opposite trend. It was also noted that the heat transfer coefficient has a sharp decline with sall increase in ass flux at the beinnin. Besides, the saturation teperature had a positive effect on the heat transfer coefficient. Finally, it was noted that at a hiher saturation teperature, the heat transfer coefficient was hiher, and the pressure drop was lower. REFERENCES Djordjevic, E., and S. Kabelac. "Flow boilin of R134a and aonia in a plate heat exchaner." International Journal of Heat and Mass Transfer (2008): Hsieh, Y. Y., and T. F. Lin. "Evaporation heat transfer and pressure drop of refrierant R-410A flow in a vertical plate heat exchaner." Journal of heat transfer (2003): Han, Don-Hyouck, Kyu-Jun Lee, and Yoon-Ho Ki. "Experients on the characteristics of evaporation of R410A in brazed plate heat exchaners with different eoetric confiurations." Applied Theral Enineerin (2003): Khan, Tariq S., et al. "Experiental investiation of evaporation heat transfer and pressure drop of aonia in a 60 chevron plate heat exchaner."international Journal of Refrieration 35.2 (2012a):

8 Khan, Mohaad S., et al. "Experiental investiation of evaporation heat transfer and pressure drop of aonia in a 30 chevron plate heat exchaner." International Journal of Refrieration 35.6 (2012b): Shah, R. K., and W. W. Focke. "Plate heat exchaners and their desin theory." Heat Transfer Equipent Desin 227 (1988). Leon, Eric W., Marcia L. Huber, and Mark O. McLinden. "NIST reference fluid therodynaic and transport properties REFPROP." (2002). Lono, G. A., and A. Gasparella. "Heat transfer and pressure drop durin HFC refrierant vaporisation inside a brazed plate heat exchaner."international Journal of Heat and Mass Transfer (2007a): Lono, Giovanni A., and Andrea Gasparella. "HFC-410A vaporisation inside a coercial brazed plate heat exchaner." Experiental Theral and Fluid Science 32.1 (2007b): Lono, G. A., and A. Gasparella. "Refrierant R134a vaporisation heat transfer and pressure drop inside a sall brazed plate heat exchaner."international journal of refrieration 30.5 (2007c): Táboas, Francisco, et al. "Flow boilin heat transfer of aonia/water ixture in a plate heat exchaner." International Journal of Refrieration 33.4 (2010): Wan, Lieke, Bent Sundén, and Raj M. Manlik, eds. Plate heat exchaners: desin, applications and perforance. Vol. 11. Wit Press, Yan, Y-Y., and T-F. Lin. "Evaporation heat transfer and pressure drop of refrierant R-134a in a plate heat exchaner." Journal of Heat Transfer (1999): Noenclature A heat transfer area of the plate, 2 Greek sybols b heiht of the corruation, β Inclination anle of the corruation Bo boilin nuber Δ difference c p specific heat capacity, J/k λ theral conductivity, w/ K d h hydraulic diaeter, ρ Density, k/ 3 G ass flux, k/ 2 s subscripts h heat transfer coefficient, w/ 2 a oentu i specific enthalpy, J/k p anifold and port i f enthalpy of vaporization, J/k f frictional L channel lenth, ravity LMTD lo ean teperature difference, i,o inlet and outlet ass flow rate, k/s l liquid phase n ch nuber of channels lat Latent heat p pressure, Pa sens sensible heat P pitch, e evaporator Pr Prandtl nuber p pre-evaporator heat flux, W/ 2 r refrierant Re Reynolds nuber w water s plate wall thickness, wall Wall/fluid near the wall T teperature, K sat saturation U W X overall heat transfer coefficient, W/ 2 K ass flow rate, k/s vapour quality 8