Assessing Energy Efficiency of Compression Heat Pumps in Drying Processes when Zeotropic Hydrocarbon Mixtures are Used as Working Agents

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1 MATEC Web of Conferenes 73 DOI: / ateonf/ Assessing Energy Effiieny of Copression Heat Pups in Drying Proesses when Zeotropi Hydroarbon Mixtures are Used as Working Agents Alexander Shurayts 1, Alexander Rule 2,*, and Elena Usahea 3 1 Yuri Gagarin State Tehnial Uniersity of Sarato, , st. Politehniheskaya, Sarato, Russian Federation 2 Publi Liited Copany Giproniigaz, , st.kiroa, Sarato, Russian Federation 3 Sientifi Tehnologies Ltd, , st. Mosow highway, Sarato, Russian Federation Abstrat. Presents the results of studies of innoatie aterials in the field of renewable energy.the paper proposes a design and a forula for assessing energy effiieny of the heat pup air dryer, whih uses zeotropi hydroarbon ixtures of saturated hydroarbons as a working agent and applies the priniple of a ounter-urrent heat exhanger with a ariable teperature of both the working and the drying agents. Energy effiieny of the heat pup is ahieed by eans of obtaining a greater part of heat fro renewable energy soures, in this ase by ooling the air and ondensing the water apors in the heat pup. A onduted analysis identified orrelations in establishing the arginal real oeffiient of perforane of the opression heat pup dryer running on zeotropi hydroarbon ixtures and operating a yle with ariable teperatures of both the working and the drying agent in the eaporator and the ondenser of the heat pup. Aording to the established orrelations, the arginal real oeffiient of perforane of the opression heat pup dryers running on zeotropi hydroarbon ixtures of 40 ol% of R600a and 60 ol% of R601 is 1.92 ties higher than that of the sae dryers running on only R600 (n-butane). 1 Introdution Drying agriultural and other types of produe, inluding grassy forage, egetables, fruits, grains, tiber, pottery and other aterials, aounts for a signifiant energy onsuption. A literature reiew [1-5] deonstrates that grassy forage, grains, egetables, fruits, different types of tiber, and other aterials are dried ost properly and effetiely with heat pups at low teperatures using the air as the drying agent with its oplete reirulation and dehydration. Energy effiieny of the heat pup is ahieed by eans of obtaining heat fro a renewable energy soure, in this ase by ooling the air and ondensation of water apors in the heat pup. Aording to [1,5,6], the aerage rate of * Corresponding author: ritaitrofanoa@yandex.ru The Authors, published by EDP Sienes. This is an open aess artile distributed under the ters of the Creatie Coons Attribution Liense 4.0 (

2 MATEC Web of Conferenes 73 DOI: / ateonf/ return of suh installations is 3-4 years. An optial type of engine for the opressor of the industrial heat pup dryer is a gas engine [1], whih allows for a onsiderable inrease in the obustion effiieny of the priary fuel by eans of useful heat extration fro flue gases, ondensation of water apors in the, and the use of heat fro ooling systes and engine lubriation. Existing types of heat pups use substanes with a onstant boiling point as a working agent, whih is not energy effiient when low-teperature heat soures, suh as ground and outdoor bodies of water, hae a high oluetri heat apaity [1,2,4], and therefore their teperature reains onstant or aries slightly. At the sae tie, when a soure or sink for heat has liited oluetri heat apaity, for exaple, the drying air [7], whose teperature hanges signifiantly while it is being dehydrated in the eaporator and being heated in the ondenser, using a substane with a onstant boiling point as a working agent [8,9] is haraterized by redued effiieny of non-renewable energy (eletriity fro theral power plants, gas, et.) used to drie the opressor. 2 Heat pup dryer design onept and desription The proposed design of the heat pup dryer that applies a oplete reirulation of air and runs a yle with ariable teperatures of both the working and the drying agents that flow in opposite diretions, is an energy effiient drying solution (fig. 1). The design onept is as follows. The working agent hosen is a two-oponent zeotropi ixture 4, whih, when boiled in eaporator 1, inreases its teperature fro the initial alue at the inlet of eaporator tubes unit 3 to the final teperature alue at the outlet. Contrary to that, huid air 5, while flowing throw shell side 2 in the opposite diretion, redues its teperature fro the initial alue at the outlet to the final teperature alue at the inlet of eaporator tubes unit 3. During its transition fro apor to liquid in ondenser 9, the zeotropi ixture redues its teperature fro the initial alue at the inlet to ondenser tubes unit 11 to the final alue at the outlet. Contrary to that, dehuidified air 5, flowing throw shell side 10 in the opposite diretion, inreases its teperature fro its initial alue, at the outlet, to the final alue at the inlet to ondenser tubes unit 11. The hange in the teperature of boiling and ondensation of the zeotropi ixture, as well as the hange in the alue of the teperature differene of the beginning and end of boiling or ondensation, an be ahieed by hanging the perent oposition of one of its oponents. Figure 1 shows the design and the operating priniples of the gas engine heat pup. Fig. 1. Design of the heat pup dryer running on a zeotropi hydroarbon ixture with a gas engine drie: 1 eaporator; 2 eaporator shell side; 3 eaporator tubes unit; 4 working agent ade fro a two-oponent zeotropi ixture; 5 air; 6 tubes of the apor phase of working agent 4 used to onnet to the outlet of eaporator tubes unit 3; 7 opressor; 8 tubes of the apor phase of working agent 4 used to onnet to the outlet of opressor 7; 9 ondenser; 10 2

3 MATEC Web of Conferenes 73 DOI: / ateonf/ ondenser 9 shell side; 11 ondenser tubes unit; 12 tubes of the liquid phase of working agent 4 used to onnet to the outlet of ondenser tubes unit 11; 13 teperature regulator reduing the teperature of the saturated liquid phase of working agent 4; 14 tubes of the liquid phase of working agent 4 onneting regulator 13 with the inlet of eaporator tubes unit 3; 15 gas engine to drie the opressor; 16 fan; 17 heat pup station; 18 drying station; 19 drying produts; 20 reirulation dut. 3 Seletion of the working agent Working substanes used as oponents of a zeotropi ixture in heat pup dryers should not ontribute to the depletion of the ozone layer of the atosphere [10] and liate hange [11], should be fireproof and ost-effiient, aailable for use and hae no negatie ipat on the huan body, they should hae ariable alues of boiling points and ondensation, whih are the ost suitable for the drying proess of grass, grain and siilar produts. Currently, there are no working agents that an perfetly satisfy all these requireents. For exaple, freons R11, R12, R22 widely used in the past, whih satisfy the low-teperature drying onditions of agriultural and other siilar produts and ould be used as oponents of a zeotropi ixture in heat pup dryers, hae negatie effets on the ozone layer of the atosphere and ontribute to liate hange. In this respet, the hoie of oponents for the zeotropi ixture is arried out indiidually, releant to the lowteperature heat pup air dryer onsidering all positie and negatie fators of their effet. Based on the results of the onduted analysis, zeotropi ixtures of saturated hydroarbons, onsisting of R600a (isobutane) and R601 (n-pentane) eet these requireents ore fully. R600a and R601 are onsidered to be opletely safe in ters of their effet on the ozone layer of the Earth. These gases do not ause the greenhouse effet, do not hae affet liate hange or the huan body, are extrated diretly fro natural gas, and are uh heaper than other working agents. Howeer, using zeotropi ixtures an result in a redued heat eission oeffiient in the inner surfae of the heat exhange tubes unit the working agent syste of the eaporator and the ondenser of the heat pup (fig. 1) due to redued aporization enters and dereased bubble departure diaeter, opared to eah of the oponents of the ixture [12]. The alulations, arried out in aordane with [12-14], deonstrate that the redution of the oerall heat eission oeffiient does not exeed 5.5%, sine the ain speifi weight in the total balane of its alue takes the heat eission oeffiient in the outer surfae of the finned heat exhanger tubes unit the drying air syste, whih is haraterized by relatiely low heat eission in ase of fored air flow. Suh heat eission redution rate and a subsequent inrease in apital inestent in heat exhange surfaes of the eaporator and the ondenser in this ase are as not as signifiant as the redued operating osts due to an inreased oeffiient of opression heat pup dryer perforane, whih is explained below. The drawbak of saturated hydroarbons, when used as oponents of zeotropi ixtures, is that they pose a fire hazard. In this respet, industrial heat pup dryers should be equipped with soke detetors. At the sae tie, when used as a drie for the opressor of the gas engine, whih is ost preferred in the drying proess, a soke detetor is andatory, based on the seurity requireents for gas supply ites [15,16]. Apart fro that, in the ase of odern heat pup dryers with a sall filling apaity (35-50 g), een if the entire working agent leaks out, its onentration in a spae of 15 3 will be ten or ore ties lower than %, the lowest inflaability liit of R600a and R601 ixtures [17,18]. Gien the oerall balane of adantages and disadantages, urrently ore than 35% of household refrigerators in Europe run on isobutane R600a. The low-boiling oponent of the ixture, R600a has the boiling point of t = C in the 3

4 MATEC Web of Conferenes 73 DOI: / ateonf/ atospheri pressure and the high-boiling oponent R601 has the teperature of t = C. Now we estiate the hange of teperature onditions for the heat pup, designed for air drying, operating on a zeotropi ixture of R600a (isobutane) and R601 (n-pentane), ontaining 40 ol% of R600a. 4 Estiating teperature onditions of the working and the drying agents In order to estiate teperature onditions of the heat pup, we need to know the following: 1) hange in the urrent teperature of the saturated zeotropi ixture of R600a and R601 in the eaporator and the ondenser of the heat pup, inluding its initial and final alues, in relation to the degree of dryness X in the ariation range fro 0 to 1; 2) hange in the teperature of the drying air in the eaporator and the ondenser of the heat pup, inluding its initial and final alues, in relation to the teperature and flow olue of the saturated zeotropi ixture. The nature of teperature t hanges in the eaporator and the ondenser of the heat pup, inluding its initial and final alues for the zeotropi ixture of R600a and R601, for whih Dalton s law, Raoult s law [17,18], Konoalo s first law [19], the Antoine equation [20,21] for saturated apor pressure for pure oponents are appliable, an be estiated depending on the degree of dryness X, aording to [20]: ψ 1-ψ Х=Р + β ξ ВR601 АR601 - С R601 β= Р +t -10 ξ = Р -10 ВR600a АR600a - С R600a +t (1) (2) (3) where: Р is the absolute total pressure of the ixture, Pa 105; is the R600a ontent in the initial liquid and apor phases of the ixture, with whih it enters the eaporator and the ondenser of the heat pup, ol.%; АR600a, ВR600a, СR600a, АR601, ВR601, СR601 are oeffiients typial for saturated hydroarbons R600a and R601 within a ertain teperature range; t is the boiling point of an indiidual hydroarbon (R600a and R601), C. Based on the ost aurate experiental data on the saturated hydroarbons R600a and R601, [21, 22] gies the oeffiients АR600a, ВR600a, СR600a, АR601, ВR601, СR601 in forulas (2) and (3). Differenes, resulting fro applying alulation forulae (1) (3) based on Dalton s laws, Raoult s law and the Antoine equation for zeotropi ixtures of saturated hydroarbons at the absolute pressure up to 1.0 MPa, to the atual easureent data, are 4.5% [23]. Changes in the teperature of eaporated and ondensed ixture in the range of the dryness degree X fro 0 to 1.0 result in a hange in the teperature of the heated or ooled drying air. The initial teperature alue of a liited heat apaity enironent, in this ase 4

5 MATEC Web of Conferenes 73 DOI: / ateonf/ the air, depending on the teperature hanges, dryness fator and the zeotropi ixture flow rate, for the ase of the ounter-urrent heat exhanger, is deterined fro the balane equation between the working and drying agents in the eaporator and the ondenser of the heat pup. For the heat pup eaporator: T ai.i For the heat pup ondenser: T ai.i GaiсT ai ai.f G rx (T.f T.i). (4) G ai ai ai GaiсT ai ai.f G rх (T.i T.f ). (5) G where: T ai.i,tai.f are the initial and final teperatures of the drying air in the eaporator, K; T.f,T.i are the initial and final teperatures of the zeotropi ixture in the eaporator, K; T ai.f,tai.i are final and initial teperatures of the drying air in the ondenser, K; T,T are initial and final teperatures of the zeotropi ixture in the ondenser, K;.i.f G,G ai are the ole disharge of the zeotropi ixture and the drying air, ol/h; ai r,r are the aerage alues of latent heat of eaporation (aporization) of the zeotropi liquidapor ixture in the ondenser and the eaporator, kj/ol; ai,ai are ean speifi heat of the drying air in the eaporator and the ondenser, kj/(olk);,are ean speifi heat of zeotropi liquid-apor ixture in its boiling and ondensation teperature interals in the eaporator and the ondenser, kj/(olk); X,X are the dryness fator of the zeotropi ixture in the eaporator and the ondenser, unit frations. Estiating the teperatures of the zeotropi ixture and the drying air at gien X is arried out using the forulae (1) (3) and applying the ethod of suessie approxiations in the following anner. 1. We shall assue that the onentration alue of R600a and R601 in the zeotropi ixture equals 40 ol.%. 2. The teperature of the drying air in the ondenser outlet onsidering low teperature drying onditions equals t = 65 С. The initial and final teperature alues are shown in ai.f the figure teperature (T) entropy (S), (fig. 2). Sine the teperatures in the T-S diagra are indiated in Kelin degrees, the following notation and alue will be applied: T = 338 K. ai.f Assue the initial teperature drop between heat exhange streas at the inlet of the ounter-urrent ondenser is at a rate of t = 7 C. Then the initial teperature for zeotropi ixture of R600a and R601 in the ondenser will be t t.i = ai.f +t=65+7=72 0 С ( T.i =345 К). Then, aording to the forulae (1) (3), taking the alues of X = 1 =72 0 С and =40 ol% of (beginning of ondensation of the saturated apor phase), t.i 5

6 MATEC Web of Conferenes 73 DOI: / ateonf/ R600a, by the trial ethod we estiate the absolute pressure, whose alue, based on the results of alulations, equals Р = 0.4 MPa. 3. The final teperature alue of the ixture in the ondenser is estiated using the forulae (1) (3) and applying the trial ethod with the alue of X = 0 (end of ondensation of the saturated apor phase) and Р = 0.4 MPa, whose alue equals t.f = 55 0C ( T.f 328К ) (fig. 2). 4. Further, aording to the forula (5), for the ase of the ounter-urrent heat exhanger (fig. 2), we estiate the drying air teperature in the ondenser inlet, in relation to the teperature hange and the zeotropi ixture flow, whih is Tai.i 317К, based on the results of alulations. 5. Assue the teperature of the drying air at the eaporator outlet equals its teperature at the ondenser inlet, i.e., Tai.f Tai.i 317К. 6. Assue the absolute pressure in the eaporator is Р =0.25 MPa. Then, aording to the results of alulations based on the appliation of the forulae (1) (3) using the trial ethod gien the alue of Х=0 and absolute pressure Р =0.25 MPa, the initial teperature alue for the zeotropi ixture of R600a and R601 in the eaporator of the heat pup that ontains =40 ol.% of R600a, will be t.i =36 0С (Т.i =309 К). The alue Х=0 orresponds to a state of an equilibriu syste, when the ixture ontains only the saturated liquid phase and, ie ersa, if x=0 then the ixture ontains only the saturated apour phase. Siilarly, deterine the final teperature alue of the ixture in the eaporator t.f =54 0С (Т.f =327 К) gien the alue of Х=1 and Р =0.25 МPа. The teperature ure fro point 4 to point 1 for the ixture ontaining =40 ol.% of R600a, gien the absolute pressure Р =0.25 МPа, is shown on the teperature-entropy diagra (fig. 2.) 7. After that, aording to the forula (4), deterine the teperature of the drying air at the outlet of the eaporator, whih is Т ai.i =332 К based on the results of alulations for the ase of the ounter-urrent heat exhanger. Apply the obtained alues of the drying air teperatures Т ai.i =332 К; Т ai.f =317 К and the diretion of the air flow to the T-S diagra (fig. 2.) 8. For oparison, the dashed line in the diagra shows teperatures in the ondenser and the eaporator of the heat pup when it runs on the working agent that onsists of one substane T s.t ; they equal, respetiely, Т s.t =Т.i =345 К and Т s.t = Т.i =309 К. 6

7 MATEC Web of Conferenes 73 DOI: / ateonf/ Fig.2. Coparison of a yle of the heat pup running on a ixture of R600a and R601 with a yle of the heat pup running on pure R (1-2) is the apor phase opression in the opressor; 2-3 (2-3) is the apor phase ondensation in the ondenser; 3-4 (3-4) is throttling of the liquid phase; 4-1 (4-1) is boiling of the liquid phase in the eaporator. 5 Assessing energy effiieny of the heat pup dryer Ealuate the effetieness of the heat pup dryer yle that runs on the working agent with a flutuating teperature of opared to the yle for the dryer that operates on a pure working agent of R600. The real oeffiient of perforane of the heat pup, reoended by [24] for a preliinary assessent of these yles, is estiated using the forula: r 0.74 T.a /(T.a -T.a )-(0.0032T.a T.a / T.a ) (6) where T.a, T.a are the aerage teperatures of the working agent that onsists of a zeotropi ixture in the eaporator and the ondenser, К. Aording to [24], in ase of ariable teperatures, the perforane ealuation is onduted with the help of the equialent Carnot yle. In this ase, the aerage teperature in the ondenser T.a (line 2-3) and the eaporator Т.a (line 4-1) are estiated, respetiely: 7

8 MATEC Web of Conferenes 73 DOI: / ateonf/ T.a = (T.i - T.f )/ ln (T.i /T.f ). (7) T.a = (T.f - T.i )/ ln (T.f /T.i ). (8) Putting the orresponding teperature alues T =321.4 К and Т.a.i =340.1 К, alulated using the forulae (7) and (8) for the yle , in the forula (6), we dedue the arginal real oeffiient of perforane φ r =13.3. For the yle gien the onstant teperature in the eaporator T s.t equal to the initial teperature of the ixture T.i for the yle , i.e. T = T s.t.i =309 К, and gien the onstant teperature in the ondenser T s.t equal to the initial teperature of the ixture T.i for the yle , i.e. T =T s.t.i = 345 К, the arginal real oeffiient of perforane aording to the forula (6) is φ' r =6.94. Thus, the arginal real oeffiient of perforane of opression heat pup dryers running on zeotropi ixtures that ontain 40 ol.% of R600a, is 1.92 ties higher ( φ r / φ' r =13.3/6.94) than that of the sae installation operating on a pure working agent of R Conlusions 1. The paper present a design of a heat pup air dryer that uses a zeotropi ixture of saturated hydroarbons onsisting of 40 ol.% of R600a and 60 ol.% of R601 as a working agent, and applies the priniple of a ounter-urrent heat exhanger with a ariable teperature of both the working and the drying agents. The proposed design guarantees a redued ean teperature differene between the air and the zeotropi hydroarbon ixture in the eaporator and the ondenser and, onsequently, an inreases oeffiient of perforane of the heat pup. 2. The orrelations (1) (3) were suggested for alulating the urrent teperature alues of the satu and ondenser of the heat pup, inluding the initial and final teperature alues, depending on the relatie aount of eaporated or ondensed ixture, i.e. the apor dryness degree X. 3. The equations (4) (5) were deised to estiate the initial and final alues of the drying air teperature in the eaporator and the ondenser of the heat pup, depending on the flow olue of the zeotropi ixture and the drying air, and teperature onditions of boiling or ondensation of the ixture. 4. The orrelations (6) (8) were worked out to roughly estiate the arginal real oeffiient of perforane of opression heat pup dryers running on a zeotropi ixture and operating a yle with ariable teperatures in the eaporator and the ondenser of the heat pup. Aording to the orrelations (6) (8), the arginal real oeffiient of perforane of the opression heat pup dryer running on the zeotropi ixture of 40 ol.% of R600a, φ is r =13.3, whih is 1.92 ties higher than that of the sae installation running on a pure φ' working agent of R600 with the alue r φ =6.94, i.e. r φ' / r =13.3/6.94 =

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