A New Phase Transition Heat Exchanger for Gas Water Heaters

inventions

Article A New Phase Transition Heat Exchanger for Gas Water Heaters

Zhoutuo Tan, Yunjia Yao, Chengning Yao, Jiapei Yang, Yougang Ruan and Qiuwang Wang *

Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (Z.T.); [email protected] (Y.Y.); [email protected] (C.Y.); [email protected] (J.Y.); [email protected] (Y.R.) * Correspondence: [email protected]; Tel.: +86-29-8266-5539

Received: 4 May 2018; Accepted: 7 June 2018; Published: 13 June 2018

Abstract: Gas water heaters take a major part in the Chinese water heater market, while the existing water heater have either low efficiency because of the single utilization of energy or the high failure rate that is caused by low temperature corrosion. A new structure of heat exchanger in the gas water heater is proposed is this article, the heat transfer method of which is not only forced convection, but also phase change heat transfer, which provides a higher heat transfer coefficient in condition of the same heat exchange area, and consequently promote the efficiency of the heat exchanger. An experimental study is carried out to compare the difference between the new water heater and the existing one on efficiency, and result shows that the new water heater is 6% higher in efficiency. Besides, this kind of water heater has a gentle temperature change when a sudden decrease or increase of gas flow rate occurs. An economic analysis is produced in order to predict the economic efficiency of the new heat exchanger. As a result, the new heat exchanger of the water heater can significantly promote the heat transfer efficiency and decrease the failure rate, and it is more economic efficient than the existing one.

Keywords: efﬁcient heat exchanger; gas water heater; phase transition heat transfer

1. Introduction Water heater has taken a big amount in energy consumption in people’s daily life. According to the U.S. Department of Energy’s (DOE) 2010 Building Energy Data Book, residential water heating consumes 13.1% of the energy that is delivered to residential buildings [1]. Gas water heaters account for 36.19% of Chinese water heater market, with the electric water heaters, solar water heater, and air-source heat pump heaters taking 47.36%, 8%, and 1%, respectively [2]. Water heating accounts for 39% of total energy consumption and a gas water heater takes 91% of the gas consumption in a normal family in the Unites States of America (USA) [3]. Consequently, the efficiency of the water heater is required to be high enough to fulfill the goal of economy and environment protection. The working principle of the core part of the existing gas water heater is shown in Figure1a. It is a finned tube heat exchanger through which the high temperature gas flows through the fin side of the heat exchanger to heat the water inside of the heat exchange coiler. The major heat transfer method takes the limited space heat transfer in the oxygen-free copper heat exchanger, and the high temperature smoke flows through the shell gap and the radiation and convective heat transfer in the limited space that occurs in the pipeline. The velocity of flue gas is low and it is laminar flow. When the heat flux density is low, the heat cannot be utilized sufficiently enough, which will result in a waste of energy. The approximate efficiency of the existing heat exchanger of gas water heater is 66~74%, which does not meet the requirement of energy conservation [4].

Inventions 2018, 3, 37; doi:10.3390/inventions3020037 www.mdpi.com/journal/inventions Inventions 2018, 3, 37 2 of 10

A great amount of work has been done to promote the efficiency of the water heater. Aguilar C[5] gives a conclusion that to promote the thermal and combustion efficiencies of water heaters, time, temperature, and turbulence (3Ts) should be improved of thr water heater, Lowering the flue gases temperature by 25 ◦C reduces fuel consumption by 1% and improves the thermal efficiency by the same percentage [6]. Galitsky C [7] analyzed the combustion process and found that decreasing the excess air by 15% enhances the heating efficiency by 1%, and Cx Cm [8] found that water heater efficiency could be promoted by 2.5% if the excess O2 level is decreased by 1%. Hongbin Y I et al. [9] optimized the air inlet structure of the burner in order to improve the combustion situation, which provides a slight increase on efficiency of 1.06%. S Tajwar et al. [10] changed the surface design of the flow channel and improved the turbulence of combustion gases. S Eiamsa-Ard et al. [11] enhanced the heat transfer in tube by adding a helical tape inserts and improved turbulence of the flow in tube, which has been widely used by water heater producers to promote the thermal efficiency. SJ Craig [12] found that directing flue gas by baffle could also bring high thermal and combustion efficiency. These methods can enhance the heat transfer process and slightly promote the efficiency of a gas water heater. The main way to improve the efficiency of the gas water heater is to add an additional heat exchanger to grasp the heat of the exhausted flue gas and the latent heat of water vapor [13], which has been widely used by water heater producers like A.O. Smith and Ariston. This kind of heaters is called a condensing water heater, which consumes about 30% less energy than a traditional heater [14]. The efficiency of the condensing water heater is 21.1~28.7% higher than that of a traditional one [15]. Although the addition of condensing heat exchangers at the flue gas outlet can further utilize a small amount of water vapor that is generated by flue gas combustion, low temperature corrosion often occurs on the finned tube and causes a reduction of the thermal efficiency of the heat exchangers [16–20]. Other attempts are tried by changing the whole structure and heat transfer method of water heaters. In the gas water heater that was disclosed in U.S. Patent Publication No. US2007/0133963 A1 [21], a closed vacuum chamber full of particles is used as a core heat transfer member. The heat transfer coiler for heating the water is buried in the heat conduction particles in the cavity. The heat of the high temperature gas passes through the wall of the closed cavity in the form of heat conduction, then heat is conducted into heat conduction particles, and then into the coiler. Phase transition heat transfer is another method transmitting high heat flux, which is gradually applied in water heaters recent years. In the electric water heater that was disclosed in Chinese patent CN201510250211 [22], the heat transfer coiler is buried in a closed vacuum cavity containing water. Water is vaporized by heating tube and cooled by heat transfer coiler. In patent CN201610352526 [23], an electric water heater that is based on vacuum phase-change principles is also disclosed, consisting of several independent heating modules. Each module works as a vaporization and condensation unit. It has the advantages of high heat transfer coefficient, fast heating speed, isolation of water and electricity, safety, and compactness. The invention provides a highly efficient heat exchanger component for rapid heat production in view of the problem that the heat utilization of the gas water heater in the prior art is low and the heat production speed is slow. The main heat transfer method for the new heat exchanger is not only forced convection, but also phase transition (see Figure1b), which has not been found in public reporting. InventionsInventions2018 2018,,3 3,, 37 x FOR PEER REVIEW 33of of 1010 Inventions 2018, 3, x FOR PEER REVIEW 3 of 10

(a) (b) (a) (b) Figure 1. (a) Working principle of the existing heat exchanger in gas water heater; and, (b) Phase Figure 1. (a) Working principle of the existing heat exchanger in gas water heater; and, (b) Phase Figuretransition 1. ( ina) the Working new heat principle exchanger of the in existinggas water heat heater exchanger. in gas water heater; and, (b) Phase transition in the new heat exchanger in gas water heater. transition in the new heat exchanger in gas water heater. 2. Materials and Methods 2. Materials and Methods 2. MaterialsDue to the and high Methods temperature of the flame, the finned tube cannot contact with the flame directly. Due to the high temperature of the flame, the finned tube cannot contact with the flame directly. The Dueflue gas to the needs high to temperature be cooled before of the it flame, rises up the to finned the finned tube cannottube, which contact is withmade the of flamecopper directly. in case The flue gas needs to be cooled before it rises up to the finned tube, which is made of copper in case Thethat flue the gasheat needs exchanger to be cooledgets burned before and it rises result up in to low the finnedheat flux. tube, Phase which transition is made heat of copper transfer in caseis an that the heat exchanger gets burned and result in low heat flux. Phase transition heat transfer is an thateffective the heat heat exchanger transfer method gets burned whose and heat result transfer in low coefficient heat flux. is Phasegenerally transition high and heat it transferis suitable is anfor effective heat transfer method whose heat transfer coefficient is generally high and it is suitable for effectivetransferring heat high transfer heat method flux. By whose imitating heat heat transfer pipe, coefficient a vacuum is copper generally cavity high is and produced it is suitable based foron transferring high heat flux. By imitating heat pipe, a vacuum copper cavity is produced based on transferringphase transition high mechanism. heat flux. By imitating heat pipe, a vacuum copper cavity is produced based on phase transition mechanism. phaseThe transition new gas mechanism. water heater consists of two heating parts, of which the inner part is a vacuum The new gas water heater consists of two heating parts, of which the inner part is a vacuum cavity.The There new gasis a watercoiler in heater the upper consists part of of two cavity. heating At parts,down ofpart which of the the cavity, inner belo partw is the a vacuum coiler is cavity. There is a coiler in the upper part of cavity. At down part of the cavity, below the coiler is cavity.phase transition There is a medium, coiler in thewhich upper is generally part of cavity. water Ator downrefrigerant. part ofPorous the cavity, medium below is immersed the coiler isin phase transition medium, which is generally water or refrigerant. Porous medium is immersed in phasethe liquid transition phase medium,transition whichmedium is generallyin order to water increase or refrigerant.the number Porousof boiling medium core and is immersedaccelerate th ine the liquid phase transition medium in order to increase the number of boiling core and accelerate the thevaporization liquid phase rate. transition The bottom medium of the in order cavity to is increase the heating the number plate that of boiling is heated core directly and accelerate by high vaporization rate. The bottom of the cavity is the heating plate that is heated directly by high thetemperature vaporization flame. rate. The The external bottom part of the of cavity the heater is the heating is finned plate tube that surrounding is heated directly the cavity. by high The temperature flame. The external part of the heater is finned tube surrounding the cavity. The temperaturestructure of flame.the vacuum The external cavity is part shown of the in heater Figure is 2 finneda and the tube experimental surrounding matter the cavity. is shown The structurein Figure structure of the vacuum cavity is shown in Figure 2a and the experimental matter is shown in Figure of2b the. vacuum cavity is shown in Figure2a and the experimental matter is shown in Figure2b. 2b.

(a) (b) (a) (b) Figure 2. (a) Designed structure of the vacuum cavity; and, (b) Experimental structure of the vacuum Figure 2. (a) Designed structure of the vacuum cavity; and, (b) Experimental structure of the vacuum cavity. cavityFigure. 2. (a) Designed structure of the vacuum cavity; and, (b) Experimental structure of the vacuum cavity. ThereThere is is a a shell shell that that is is made made of of steel steel outside outside the the cavity cavity in orderin order to containto contain the the cavity cavity and and locate locate the wholethe whole cavityThere cavity above is a above shell the that burner.the burner. is made Between Between of steel the outsidethe cavity cavity and the and thecavity the shell shellin thereorder there is toa iscontain flue a flue gas gasthe passage passagecavity where and wher locate ae seconda secondthe whole heat heat exchange cavity exchange above pipe pipe the is arranged.is burner. arranged. Between After After the thethe bottom cavitybottom heat and heat exchange the exchange shell there plate plate ofis a the of flue the body gas body ispassage heated is heated wher by e theby a heatthe second heat source, heatsource, the exchange fluethe gasflue pipe risesgas is rises from arranged. from the perimeter theAfter perimeter the of bottom the of outside theheat outside exchange bottom bottom of plate the innerofof thethe cavity innerbody is alongcavity heated thealongby flue the gas flueheat passage, gassource, passage and the the, flueand heat thegas is heatrises transferred is from transferred the to theperimeter heatedto the heated fluidof the in fluidoutside the in second the bottom second heat of exchange heatthe innerexchange tube. cavity along the flue gas passage, and the heat is transferred to the heated fluid in the second heat exchange

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InventionsInventions 20182018,, 33,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 1010 The main structure of the heat exchanger is shown in Figure3a and the experimental device is shown tube.tube. TheThe mainmain structurestructure ofof thethe heatheat exchangerexchanger isis shownshown inin FigureFigure 33aa andand thethe experimentalexperimental devicedevice isis in Figure3b. The geometrical parameter of the heater is shown in Table1. shownshown inin FigureFigure 33bb.. TheThe geometricalgeometrical parameterparameter ofof thethe heaterheater isis shownshown inin TableTable 1.1.

((aa)) ((bb))

FigureFigure 3.3.3. (((aaa))) DesignedDesignedDesigned structure structurestructure of ofof the thethe external externalexternal part; part;part; and, and,and, (b) ( Experimental(bb)) ExperimentalExperimental structure structurestructure of the ofof external thethe externalexternal part. partpart Table 1. Geometrical parameter of the heater. TableTable 1.1. GeometricalGeometrical parameterparameter ofof thethe heater.heater. Geometry Geometry Geometry GeometryGeometry Value (mm) GeometryGeometry Value (mm) GeometryGeometry Value (mm) Parameter ValueValue (mm)(mm) Parameter ValueValue (mm)(mm) Parameter ValueValue (mm)(mm) ParameterParameter ParameterParameter ParameterParameter CavityCavityCavity LengthLength Length 200200 200 Total Total Length LengthLength 240240240 DiameterDiameterDiameter of pipe ofof pipepipe 10 1010 Cavity Width 120 Total Width 160 Fin thickness 0.5 CavityCavityCavity WidthWidth Height 120120 250 TotalTotal Height WidthWidth 280160160 FinFin Fin thicknessthickness 18 × 0.520.5 CavityCavity HeightHeight 250250 TotalTotal HeightHeight 280280 FinFin 1818 ×× 22

TheThe working working process process of of the the heat heat exchangerexchanger is is shownshown inin FigureFigure 4 4.4.. AfterAfterAfter thethethe gas gasgas water waterwater heater heaterheater is isis started,started, the the water water in in the the inlet inlet pipe pipe enters enters the the vacuum vacuum cavity cavity and and heated heated by by the the vapor vapor phase phase transiti transitiontransitionon medium,medium, thethe mediummediummedium condensed condensedcondensed on onon the thethe heat heatheat transfer transfertransfer pipe pipepipe and andand drop dropdrop back backback to toto the thethe bottom bottombottom of ofof the thethe cavity cavitycavity to tobeto bebe vaporized vaporizedvaporized again. again.again. Then, ThenThen water,, waterwater flow flowflow through throughthrough finned finnedfinned pipes pipespipes between betweenbetween the thethe cavity cavitycavity and andand the thethe outer outerouter shell shellshell to tobeto be heatedbe heatedheated again againagain by by theby thethe flue flueflue gas. gas.gas. In the InIn mainthethe mainmain time, timetime the,, flue thethe gasflueflue was gasgas cooledwaswas cooledcooled by the byby heat thethe exchange heaheatt exchangeexchange plate platefirst,plate and first,first, then andand cooled thenthen cooledcooled by the byby finned thethe finnedfinned tube, andtube,tube, finally andand finallyfinally flows flowflow out toss outout the toto environment. thethe environment.environment.

FigureFigure 4. 4. WorkingWorking process process of of the the new new gas gas heat heat exchangerexchanger withwith addition addition ofof condenser.condensercondenser..

AdditionalAdditional characteristicscharacteristics ofof thethe newnew heatheat exchangerexchanger hahaveve beenbeen foundfound duringduring analysisanalysis andand Additional characteristics of the new heat exchanger have been found during analysis and experiment.experiment. WhenWhen ccomparedompared withwith thethe existingexisting heatheat exchangerexchanger ofof gasgas waterwater heater,heater, thethe flueflue gasgas experiment. When compared with the existing heat exchanger of gas water heater, the flue gas passagepassage ofof thethe newnew heatheat exchangerexchanger isis muchmuch narrowernarrower asas aa resultresult ofof thethe vacuumvacuum cavitycavity insideinside thethe passage of the new heat exchanger is much narrower as a result of the vacuum cavity inside the body,body, whichwhich resultresultss inin aa higherhigher velovelocitycity ofof thethe flueflue gasgas,, andand consequentlyconsequently bringsbrings aa higherhigher heatheat body, which results in a higher velocity of the flue gas, and consequently brings a higher heat transfer transfertransfer coefficient, coefficient, shown shown in in Figure Figure 5. 5. Moreover, Moreover, the the new new heat heat exchanger exchanger shows shows lower lower coefficient, shown in Figure5. Moreover, the new heat exchanger shows lower sensitivity on heat sensitivitysensitivity onon heatheat fluctuation.fluctuation. WhenWhen changingchanging thethe gasgas flow,flow, thethe waterwater temperaturetemperature ofof thethe newnew heatheat exchangerexchanger wwillill changechange muchmuch slowerslower thanthan thethe existingexisting one.one.

Inventions 2018, 3, 37 5 of 10

ﬂuctuation. When changing the gas ﬂow, the water temperature of the new heat exchanger will change muchInventions slower 2018, 3 than, x FOR the PEER existing REVIEW one. 5 of 10 Inventions 2018, 3, x FOR PEER REVIEW 5 of 10

Figure 5. Velocity profile of the flue gas. Figure 5. Velocity profileprofile ofof thethe flueflue gas.gas. 3. Results and Analysis 3. Results and Analysis 3.1. Thermal Performance 3.1. Thermal Performance An experimental study has been carried out to test the behavior of the new heat exchanger and An experimentalexperimental studystudy hashas been been carried carried out out to to test test the the behavior behavior of of the the new new heat heat exchanger exchanger and and to to compare the new heat exchanger with the existing one. Figure 6 shows the water temperature rise compareto compare the the new new heat heat exchanger exchanger with with the the existing existing one. one. Figure Figure6 6shows shows the the water water temperature temperature rise rise of the new heat exchanger when the gas flow is 0.01 m3/min and the water flow is 0.002 m3/min, of thethe new new heat heat exchanger exchanger when when thethe gasgas flow flow is is 0.01 0.01 m m3/min3/min and and thethe waterwater flowflow is is 0.002 0.002 mm33/min,/min, 0.003 m3/min, 0.004 m3/min, and 0.005 m3/min, respectively. 0.003 m3/min, 0.004 0.004 m m3/min,3/min, and and 0.005 0.005 m m3/min,3/min, respectively. respectively.

75 75 5 m3/min 70 3 35 m3/min

) 70 3 C 65 3 m3/min )  2 m /min (

C 65 3  60 2 m3/min ( 4 m /min 60 3 55 4 m /min 55 50 50 45 45 40 40 35 35 30 Outlet water temperatureOutlet water 30 Outlet water temperatureOutlet water 25 25 20 20 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Time (s) Time (s) Figure 6. Outlet water temperature rise. Figure 6.6. Outlet water temperature rise. Figure 7 shows the comparison of the heat quantity absorbed by water between the new heat Figure 7 shows the comparison of the heat quantity absorbed by water between the new heat exchangerFigure and7 shows the existing the comparison one when of the heatgas low quantity is 0.01 absorbed m3/min and by water water between flow is the0.002 new m3/min, heat exchanger and the existing one when the gas low is 0.01 m3/min and water flow is 0.002 m3/min, exchanger0.003 m3/min, and 0.004 the existingm3/min, oneand when0.005 m the3/min. gas lowHeat is that 0.01 is m absorbed3/min and by waterwater flowis calculated is 0.002 by: m3 /min, 3 3 3 0.003 m3/min, 0.004 m /min3 , and 0.005 m /min.3 Heat that is absorbed by water is calculated by: 0.003 m /min, 0.004 m /min, and 0.005 m /min. Heat that is absorbed by water is calculated by: Q= m c ( T T ) , (1) · p out in (1) Q= m cp ( T out T in ) , Q = mcp(Tout − Tin), (1)   where ·m is mass flow rate, cp is water heat capacity of water, and Tout and Tin is temperature of outlet p out in where mmis is mass mass flow flow rate, rate,cp cis is water water heat heat capacity capacity of of water, water, and andT Tout andand TTin is temperature of outlet ·  water and inletinlet water,water, respectively.respectively.m mcan can be be calculated, calculated as, as follows: follows: water and inlet water, respectively. m can be calculated, as follows:   · ·  m= ρV V,, (2 (2)) m  V , (2) ·  V is is water water flow flow and andρ ρis is water water density. density. V is water flow and ρ is water density. Different from the existing heat exchanger, the water flow has a significant effect on the Different from the existing heat exchanger, the water flow has a significant effect on the efficiency of the new heat exchanger. It is found that the heat absorbed by water in the new heat efficiency of the new heat exchanger. It is found that the heat absorbed by water in the new heat exchanger is higher than the existing one and it increases from 6630 W to 7000 W with increase of the exchanger is higher than the existing one and it increases from 6630 W to 7000 W with increase of the water flow, while the heat of the existing one fluctuates between 6340 W and 6550 W. The thermal water flow, while the heat of the existing one fluctuates between 6340 W and 6550 W. The thermal efficiency of the new heat exchanger is 94%, while the existing one is 88% through converting. The efficiency of the new heat exchanger is 94%, while the existing one is 88% through converting. The

Inventions 2018, 3, 37 6 of 10

Different from the existing heat exchanger, the water flow has a significant effect on the efficiency of the new heat exchanger. It is found that the heat absorbed by water in the new heat exchanger is higher than the existing one and it increases from 6630 W to 7000 W with increase of the water flow,

Inventionswhile the 2018 heat, 3, x of FOR the PEER existing REVIEW one fluctuates between 6340 W and 6550 W. The thermal efficiency6 of 10 of Inventionsthe new 2018 heat, 3, exchanger x FOR PEER REVIEW is 94%, while the existing one is 88% through converting. The heat that6 of 10 is heatabsorbed that is by absorbed water in by the water new heatin the exchanger new heat keeps exchanger increasing, keeps but increasing increases, but more increases and more more slowly and heat that is absorbed by water in the new heat exchanger keeps increasing, but increases more and morewhile slowly water flowwhile increases. water flow Seemingly, increases. there Seemingly is a highest, there value is a highest of heat value absorption of heat foe absorption the new heat foe more slowly while water flow increases. Seemingly, there is a highest value of heat absorption foe theexchanger new heat and exchanger it depends and on it the depends water flow.on the water flow. the new heat exchanger and it depends on the water flow. 7100 7100 7000 7000 6900 6900 New heater 6800 ExistingNew heater heater 6800 Existing heater 6700 6700 6600 6600 Absorbed heat (W) heat Absorbed 6500 Absorbed heat (W) heat Absorbed 6500 6400 6400 6300 6300 2.0 2.5 3.0 3.5 4.0 4.5 5.0 2.0 2.5 Water3.0 flow3.5 (m3/min4.0 ) 4.5 5.0 Water flow (m3/min) Figure 7. Absorbed heat of two heat exchangers. Figure 7. Absorbed heat of two heat exchangers. The flue gas temperature of the two heat exchanger is investigated and is shown in Figure 8. The flue flue gas temperature of the two heat exchanger is is investigated investigated and and is is shown shown in in Figure Figure 8.. The exhaust gas temperature of the new heat exchanger varies form 81 °C to 76 °C, while that of the The exhaust gas temperature of the new heat exchanger varies form 81 ◦°CC to 76 °C◦C,, while that of the existing one is from 97 °C to 92 °C. It is found that exhaust gas temperature of the new heat existing one one is is from from 97 97◦C °C to 92to ◦ 92C. It°C. is foundIt is found that exhaust that exhaust gas temperature gas temperature of the new of heat the exchangernew heat exchanger is approximate 16 °C lower than existing one, and both of the exchangers’ exhaust gas exchangeris approximate is approximate 16 ◦C lower 16 than °C lower existing than one, existing and both one, of and the exchangers’both of the exhaustexchangers’ gas temperatureexhaust gas temperature decrease with the increase of water flow. temperaturedecrease with decrease the increase with ofthe water increase flow. of water flow.

95 New heater ) New heater C 95 Existing heater  ) (

C Existing heater  ( 90 90

85 85

80 80 Exhaustedtemperaturegas Exhaustedtemperaturegas 75 75 2.0 2.5 3.0 3.5 4.0 4.5 5.0 2.0 2.5 Water3.0 flow 3.510-3 (m4.03/min) 4.5 5.0 Water flow 10-3 (m3/min) Figure 8. Exhausted flue gas temperature. Figure 8 8.. ExhaustedExhausted flue flue gas temperature. Additionally, an experiment testing the sensitivity about temperature change of the two heat Additionally, an experiment testing the sensitivity about temperature change of the two heat exchangers.Additionally, The flow an experimentof the gas will testing have thea sudden sensitivity change about from temperature 0.01 m3/min change to 0.005 of them3/min two, heatand exchangers. The flow of the gas will have a sudden change from 0.01 m3/min to 0.005 m3/min, and thereexchangers. is a significant The flow difference of the gas of water will havetemperature a sudden change change betwe fromen 0.01the two m3/min heat exchangers to 0.005 m 3(see/min, in there is a significant difference of water temperature change between the two heat exchangers (see in Figureand there 9). When is a significant gas flow decreased, difference ofthe water outlet temperature water temperature change of between the existing thetwo heat heat exchanger exchangers goes Figure 9). When gas flow decreased, the outlet water temperature of the existing heat exchanger goes down(see in immediately Figure9). When and gas reach flowes decreased,steady condition the outlet in 20 water s, while temperature the water of temperature the existing heatof the exchanger new one down immediately and reaches steady condition in 20 s, while the water temperature of the new one decreasesgoes down slowly immediately and stabilized and reaches in 70 steadys. condition in 20 s, while the water temperature of the new decreases slowly and stabilized in 70 s. one decreases slowly and stabilized in 70 s.

Inventions 2018, 3, 37 7 of 10 Inventions 2018, 3, x FOR PEER REVIEW 7 of 10

60 20 Inventions 2018, 3, x FOR PEER REVIEW 7 of 10 New heater 55 ) Existing heater C  ( 6050 2015 ) New heater /min 5545 3 )

Existing heater m ( C   ( ) 5040 1510    /min 45 3

35 m (  40 10   30 5 Gas flow  Outlet water temperature temperature water Outlet 3525 Gas flow

3020 50 Gas flow

Outlet water temperature temperature water Outlet 0 50 100 150 200 250 300 350 25 Gas flow Time (s) 20 0 Figure 9.0 Water50 temperature100 150 change200 250 with300 fluctuation.350 Figure 9. Water temperature change with fluctuation. Time (s) 3.2. Economic Analysis Figure 9. Water temperature change with fluctuation. 3.2. Economic Analysis According to thermal performance of the new heater, a 6% promotion on thermal efficiency has According3.2.been Economic produced to Analysis thermal when compared performance to the ofexisting the new heater. heater, An aeconomy 6% promotion analysis onis proposed thermal efficiencyto obtain has beenthe produced economicAccording when efficiency to thermal compared of performancethe tonew the heater. existing of theFor heater.newsingle heater, user, An economyathe 6% heat promotion of analysis water on that thermal is proposedis required efficiency toper obtain dayhas the economicbeencan be produced efficiency calculated when of as: the compared new heater. to the For existing single user,heater. the An heat economy of water analysis that isis requiredproposed per to obtain day can be calculatedthe economic as: efficiency of the new heater. For single user, the heat of water that is required per day Qd c p V d  t , (3) can be calculated as: Qd = cpρVd∆t, (3) where cp is heat capacity of water, 4.2 kJ/(kg∙K). ρ is water density, kg/m3. Vd is volume of water, and Q c V  t , 3 (3) wheremc3p. Δist is heat temperature capacity ofrise water, of water, 4.2 °C. kJ/(kg Daily·K).d naturalρ is p water gas d consumption density, kg/m can be. V calculatedd is volume as: of water, and m3. ∆t is temperature rise of water, ◦C. Daily natural gas consumption can be calculated as: where cp is heat capacity of water, 4.2 kJ/(kg∙K). ρ is water density, kg/m3. Vd is volume of water, and Qd m3. Δt is temperature rise of water, °C. Daily Vnaturalgd  gas, consumption can be calculated as: (4) qQgd Vgd = , (4) Qqgdη Vgd  , (4) where qg is heat value of natural gas and kJ. η is thermal efficiency of gas water heater. The total qg whereoperatingqg is heat cost value of the ofheater natural can be gas calculated and kJ. as:η is thermal efficiency of gas water heater. The total operatingwhere cost qg is of heat the value heater of cannatural be calculated gas and kJ. as: η is thermal efficiency of gas water heater. The total F V p nd , (5) operating cost of the heater can be calculatedd as: gd g Fd = Vgd pgnd, (5) where pg is price of natural gas, yuan/m3. n is number of heaters. d is working days. For a single F V p nd , −3 3 d gd g (5) family water flow is 5.676 × 10 m /min,3 temperature rise is 25 °C. Heat value of natural gas is 36,533 where pg is price of natural gas, yuan/m . n is number3 of heaters. d is working days. For a single family wkJ/kghere and pg is the price price of of natural natural gas, gas yuan/mis 2.8 Yuan/m3. n is . number Assuming of heaters.that the gasd is water working heater days. works For one a single hour water flow is 5.676 × 10−3 m3/min, temperature rise is 25 ◦C. Heat value of natural gas is 36,533 kJ/kg familya day. Thewater fixed flow cost is 5.676 of the × heat10−3 mexchanger3/min, temperature of new heater rise is 25about °C. Heat900 yuan value, while of natural the existing gas is 36 one,533 is and the price of natural gas is 2.8 Yuan/m3. Assuming that the gas water heater works one hour a kJ/kg720 yuan and thefrom price gas ofheater natural producer gas is 2.8 Ariston. Yuan/m The3. Assuming total cost thatof the the two gas heaters water heater is shown works in Figureone hour 10 day.a Thean day.d it fixed Thecan befixed cost found ofcost thethat of heatthe the heat cost exchanger exchanger would be of equalof new new when heaterheater it is is about about 2.5900 900 years. yuan yuan, , while while the theexisting existing one is one is 720 yuan720 yuan from from gas heatergas heater producer producer Ariston. Ariston. The The total total cost cost of of the the two two heatersheaters isis shown in Figure Figure 10 10 and 7000 it canan bed it found can be that found the that cost the would cost would be equal be new equal heater when when it is it about is about 2.5 2.5 years. years. 6000 existing heater 7000 182.8 yuan 5000 new heater 6000 existing heater 4000 5000 182.8 yuan 3000 4000 2000

Total cost forTotal user(yuan) single 3000 1000 2000 180 yuan 0 Total cost forTotal user(yuan) single 0 1 2 3 4 5 1000 Time (year) 180 yuan 0 Figure0 10. 1Total cost2 for 3single 4user. 5 Time (year) Figure 10. Total cost for single user. Figure 10. Total cost for single user.

Inventions 2018, 3, 37 8 of 10

Taking China as example, there are 4.3 hundred million families and 36.19% of them use gas water heaters. It can be found that the new heater will conserve about 1.1 × 107 m3 of natural gas and save a cost about 3.1 × 107 yuan, as compared with the existing heater. Another character of the new heater that the avoidance of low temperature corrosion also contributes to the decrease cost of new heater, for a promising longer service life is obtained, which needs a further study.

4. Discussion An experimental study is proposed to compare the behavior of the new heat exchanger and the existing one. It takes nearly 150 s for the new heater to reach steady condition, which is longer than that in the existing heater, because it takes time for phase transition medium to boil in the vacuum cavity every time that it starts. The finned tube around the cavity has the ability to heat water quickly and to make up for the long starting time. The new heater has a better utilization of energy and it is more thermal-efficient than the existing one. Besides, the new heater has a best working condition and it will appear by keeping increasing the water flow and the most probable flow for best working condition is 0.01 m3/min, because it is the design working condition for the experimental heat exchanger. The exhausted gas temperature of the new heater is lower than the existing one and higher than the dew point temperature of the combusted natural gas, which is approximately 50 ◦C[8]. Low temperature corrosion occurs when the temperature exhausted gas lower than its dew point temperature. The new heat exchanger efficiently reduces the exhausted gas temperature and avoids low temperature corrosion in the meantime. With the boiling and condensing process of the phase transition medium, water temperature of the new heater decreases more slowly than that of the existing one, which is called low sensitivity of the new heat exchanger. Low sensitivity on heat source change is a good feature when it is used in a residential gas water heater because the gas flow is sometimes fluctuated in people’s house and a low sensitivity of the fluctuation will bring a satisfying user experience. With a promotion on thermal efficientcy (6%), the new heater consumes less narual gas than the existing one. Although the fixed cost of the new heater is higher than the exisiting one (180 yuan), the fixed cost difference would be recovered in about 2.5 years. With a promising longer service time of the new heater, it would be further more economically efficient.

5. Conclusions Gas water heaters are big natural gas consumer in residential buildings. The existing heater is either poor thermal efﬁciency or is likely to meet low temperature corrosion, which does not meet the requirement of energy conservation. Based on the problem, a phase transition heat exchanger is produced and an experimental study has been conducted comparing thermal behavior and using experience between the new heater and the existing one, and an economic analysis is produced to predict the economic efﬁciency. The work is shown, as follows:

(1) highly efﬁcient heat exchanger component for rapid heat production is proposed in view of the problem that the heat utilization of the gas water heater in the existing heater is low and the heat production speed is slow, with the use of the phase transition medium. (2) The heat transfer efﬁciency of the new heat exchanger in gas water heater is 6% higher than that of existing one through experiment. (3) The exhausted gas temperature of the new heat exchanger (81 ◦C~76 ◦C) is higher than the dew point temperature of combusted natural gas (60 ◦C), which avoids the low temperature corrosion of heat exchanger. (4) The new heater shows low sensitivity of heat source fluctuate and provides better using experience when there is a fluctuate of gas flow production, with the help of phase transition medium. Inventions 2018, 3, 37 9 of 10

(5) The new heater is more economic efﬁcient and for individual user the cost can be recovered in about 2.5 years. For China, about 1.1 × 107 m3 of natural gas can be conserved with the use of the new heater. (6) Avoidance of low temperature corrosion produces a longer service time of the new heater, and consequently contributes to a better economic efﬁciency, which remains to be further studied.

6. Patents There are two patents resulting from the work reported in this manuscript, the number of which is WO/2017/166557 in PCT and CN201610204147 in Chinese patent, respectively.

Author Contributions: Z.T. and Y.Y. designed and performed the experiments, C.Y. and Y.R. did the calculation and designed the heat exchanger, J.Y. analyzed the data; Q.W. contributed materials and analysis tools, and provide the idea and structure of the paper. Acknowledgments: The authors would like to thank the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No.51721004). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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