ORIGINAL

Performance of casting aluminum-silicon alloy condensing heatingexchanger for gas-fired boiler

Weixue Cao1& Fengguo Liu2

& Xue-yi You1

Received: 6 June 2017 /Accepted: 9 January 2018 /Published online: 29 January 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractCondensing gas boilers are widely used due to their high heat efficiency, which comes from their ability to use the recoverablesensible heat and latent heat in flue gas. The condensed water of the boiler exhaust has strong corrosion effect on the heatexchanger, which restricts the further application of the condensing gas boiler. In recent years, a casting aluminum-silicon alloy(CASA), which boasts good anti-corrosion properties, has been introduced to condensing hot water boilers. In this paper, the heattransfer performance, CO and NOx emission concentrations and CASA corrosion resistance of a heat exchanger are studied by anefficiency bench test of the gas-fired boiler. The experimental results are compared with heat exchangers produced by Honeywelland Beka. The results show that the excess air coefficient has a significant effect on the heat efficiency and CO and NOx emissionof the CASAwater heater. When the excess air coefficient of the CASA gas boiler is 1.3, the CO andNOx emission concentrationof the flue gas satisfies the design requirements, and the heat efficiency of water heater is 90.8%. In addition, with the increase ofheat load rate, the heat transfer coefficient of the heat exchanger and the heat efficiency of the water heater are increased.However, when the heat load rate is at 90%, the NOx emission in the exhaust gas is the highest. Furthermore, when thetemperature of flue gas is below 57 °C, the condensation of water vapor occurs, and the pH of condensed water is in the2.5~5.5 range. The study shows that CASA water heater has good corrosion resistance and a high heat efficiency of 88%.Compared with the heat exchangers produced by Honeywell and Beka, there is still much work to do in optimizing andimproving the water heater.

1 Introduction

With the world energy revolution and China’s energy structureadjustment, as well as worldwide attention to the quality of theatmospheric environment, the development of new technolo-gies in efficient energy use is getting more attention. Thereplacement of coal-fired boilers by gas boilers is importantto the energy saving and environmental protection in Chinabecause a condensing heat exchanger is good for the reductionof heat loss and harmful gas emissions in the boiler system.The condensing heat exchanger has also received attention forits effective waste heat recovery.

The effects of the tube stages, gas velocity and the Teflon-lined tubes were considered to study the convection and dif-fusion process on the gas side to better understand the heattransfer from the condensation on the horizontal tubes [1]. Thecondensation, re-evaporation and the associated transfer in themembrane were also tested to find the performance of themembrane [2]. The possible reaction steps of the acidic con-densate were discussed with regards to the advantages anddisadvantages of the absorbers in the condensing boiler [3].The forced convection heat transfer with water vapor conden-sation was studied both theoretically and experimentally, andthe Reynolds number and bulk vapor mass fraction werefound to be important factors for the condensation process[4]. A new type left–right symmetric internally finned tubewas proposed and investigated numerically and experimental-ly. The results showed that the excess air coefficient, thecooling flow rate and inlet temperature of the water, and theRe number have significant effects on the convection-condensation of heat transfer [5].

The problem of corrosion in the boiler is also a hot researchtopic. The theoretical background for the corrosion was

* Xue-yi Youxyyou@tju.edu.cn

1 Tianjin Key Laboratory of Indoor Air Environmental QualityControl, School of Environmental Science and Engineering, TianjinUniversity, Tianjin 300072, China

2 School of Energy and Safety Engineering, Tianjin ChengjianUniversity, Tianjin 300384, China

Heat and Mass Transfer (2018) 54:1951–1960https://doi.org/10.1007/s00231-018-2284-8

studied numerically and experimentally [6]. The resultsshowed that the flue gas velocity directly affects the corrosion.A new thermal conductive Nylon-12 polymer heat exchangerwas introduced to avoid cost and corrosion concerns in metal-lic designs [7]. The dew point temperature of sulfuric acid wasdiscussed numerically. It was found that the dew point tem-perature of sulfuric acid decreases with an excess air ratioreduction, the increase of wall temperature, and the sulfuricacid vapor condensation on the surface of ash particles [8].

In a traditional gas boiler, the temperature of the exhaustgas is so high that it causes a great deal of energy waste.Furthermore, the large emission of harmful gases also pollutesthe atmosphere. A low emission premixed cylindrical burnerwas numerically and experimentally investigated [9]. How todevelop and set up an efficient and corrosion-resistant con-densing heat exchanger in gas utilization equipment with alarge utilization rate of gas energy, long service life and lowemission of air pollutants is an important research topic in-volving gas boilers. For this reason, a new mix flow-surfacebox-type condensing heat exchanger is proposed. This newtype of heat exchanger is made of cast aluminum-silicon alloy(CASA) for its anticorrosion properties. The performance ofthe condensing heat exchanger is tested in detail by consider-ing the characteristics and effects of heat transfer, corrosionand flue gas emission on the heat exchanger. The performanceof the heat exchanger is compared with heat exchangers pro-duced by Honeywell and Beka.

2 CASA heat exchanger

The CASA heat exchanger shown in Fig. 1 is tested experi-mentally. The main material of the CASA is Al and Si, whichmake up approximately 85% and 11% of the material, respec-tively. The other materials in the heat exchanger are Mn andMg. The CASA heat exchanger is a double-pipe heat inter-changer, in which the flue gas is surrounded by a water jacket,and the flue gas and water flow in opposite directions. Theflue gas scours the inner wall surface of the heat exchanger toform convective heat transfer. To enhance the heat transfer,many pin-shaped ribs are arranged on the inner side of thewater jacket. The minimum spacing between the needle-shaped ribs is 3 mm. The fin area of the heat exchanger is1.08 m2, and the fin efficiency is 83.6. The internal structureis shown in Fig. 2. After silicone resin treatment to resist theacidic corrosion in the condensing gas-fired heater, the inter-nal wall of the CASA heat exchanger is approximately 5 mmthick. Meanwhile, the V-shaped groove shown in Fig. 3 isprovided on the inner side of the water channel in order toincrease the heat transfer area of the water channel.

The heat exchanger is formed by a rectangular parallelepi-ped with a hollow cylinder. The inlet and outlet of the flue gasand water are shown in Fig. 1. Premixed combustion gas

flows through the inlet of flue, and is injected into the com-bustion chamber through the orifices on the surface of theburner. The gas is ignited by the ignition device to form a hightemperature combustion gas. Then, the high-temperature fluegas and pin fins fully complete their heat exchange, and theflue gas is finally discharged to the flue gas outlet.

At the same time, in Fig. 3, the cold water enters the heatexchanger through the water inlet, is heated by the flue gas,changes flow direction seven times, and leaves the heat ex-changer through the water outlet. In the left part of the waterchannel (area 1), the cold water is preheated by the convectionheat transfer between the cold water and low-temperature fluegas. The preheated water changes the flow direction two timesand flows into the middle water channel (area 2) to be furtherheated by the high temperature flue gas. The heated watercontinues flowing to the outlet of water through the waterchannel located on the outside of the combustion chamber(area 3).

The chemical energy of the combustion releases high tem-perature gas and the heat of the gas is absorbed by the coldwater to output heat energy. The improvement of the heattransfer efficiency, and corrosion resistance and the reductionof the pollutant emissions of the heat exchanger are the re-search topics in this paper.

9 4

5

32

6 7

8

1

Fig. 1 The structure of CASA heat exchanger. 1-Flue gas inlet, 2-Fluegas outlet, 3-water inlet, 4-water outlet, 5-outfall, 6-water turn port, 7-shell, 8-pin fin, 9- H fin

R35

R8557

100

420

275

45

Area 1 Area 2 Area 3

Water Gas

Fig. 2 The internal structure of the CASA heat exchanger

1952 Heat Mass Transfer (2018) 54:1951–1960

3 Experiment and method

The experimental platform is established according to the re-quirements of GB 20534-2010 Gas-fired heating and hot wa-ter combi-boilers (National standard of China). The experi-mental scheme is arranged using the orthogonal test method[10]. The impermeability, security and reliability of the airpassage, circuit and water channel are checked carefully.Figure 4 shows the experimental platform. To study the prop-erties of the CASA heat exchanger, two brands of pre-mixedheat exchangers of Honeywell and Beka are also tested. Thepre-premix test system includes a MINI Venturi mixer, FIMEfan, Honeywell PX42 proportional valve and other major ac-cessories. After the experimental platform is set up, the igni-tion test is carried out to validate the performance of the ex-perimental system.

3.1 Experimental arrangement

The condensing gas water exchanger system is shown inFig. 5. The test instruments, along with their measuring rangeand precision, are listed in Table 1. The gas water heater is afull premixed condensing gas water heater made of the CASAmaterial, and the power is 24 kW. The type of the gas used inthis experiment is 12 T (12 T means a natural gas whoseWobbe index is 50.73 MJ/m3). In accordance to the standardof gas-fired heating and hot water combi-boilers (GB 20534-2010), the parameters such as valve opening and electricalsignal input are adjusted to study the effects of these changeson the whole system. In the process of testing, the character-istics of the condensing water heater at different condensingtemperature, the different excess air rate, and the heat load areanalyzed.

A TESTO 350 flue gas analyzer, which has a wide detec-tion range, complete detection parameters and high precision,

is used for detecting the concentration of NOx, CO, and oxy-gen in the flue gas, as well as the flue gas’ temperature. Animage of the flue gas analyzer is shown in Fig. 6. Table 2shows the test parameters and the corresponding measurementaccuracy.

The experimental system is divided into five parts, namely,the gas supply, water system, air inlet, combustion and fluegas chamber and experimental result detection. In this

Fig. 4 The exchanger on the experimental platform

U-tube manometer

Flowmeter

Pressure gauge

Thermometer

Water pump

Plate heat exchanger

GasP

T

T

TP

T

TT

TP

Fig. 5 The sketch map of experimental system

1

23

4

65

Inlet of cold water

Outlet of hot water

7

Fig. 3 The flow direction of the water in CASA heat exchanger

Heat Mass Transfer (2018) 54:1951–1960 1953

experiment, both the flue gas passage and water channel areequipped with a cut-off valve. When the system runs abnor-mally, the path is shut down in time to ensure the safety of theexperiment. The parameters for various detection methods,the usage of instruments and the experimental settings are allimplemented under the guidelines described by GB 25034-2010.

The static pressure before the gas proportional valve is setto 2000 ± 50 Pa, the dynamic pressure is controlled to 2000 ±20 Pa, and the water pressure inside the hot water circuit isapproximately 0.1 Mpa. The content of CO and CO2 of theindoor air is controlled within 0.002% and 0.2%, respectively.The lab temperature is 20 °C ± 5 °C. The temperature of theinlet water is controlled at 20 °C ± 2 °C. The temperaturedifference between the indoor air and the inlet water is notmore than 5 °C.

Only one variable is changed at a time, with the remainingconditions unchanged when the experiment is carried out. Atthe beginning of the experiment, due to the low temperature ofthe body system, the flue gas composition is not accurate.Typically, the experimental system is run for 10 min at the fullload state before the measurement starts.

3.2 Theoretical analysis and calculation

Heat load refers to the heat energy released from the full com-bustion of gas per unit time. Heat load is one of the mostimportant parameters used to measure the heating capacityof a gas boiler. Here, the measured gas consumption has beenconverted into the heat load at the standard atmospheric pres-sure by the following eq. [11].

ϕ ¼ 1

3:6� Q1 � V � pa þ pm

pa � pg

�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

101:3þ pg101:3

� pa þ pg101:3

� 288

273þ tg� d

dr

s

ð1Þ

where ϕ represents the converted actual heat load (kW); Q1

represents the inferior calorific value at the reference condi-tions, MJ/m3; V represents the gas flow rate, m3/h; pa repre-sents the atmospheric pressure, kPa; pm represents the relative

static pressure of gas, kPa; pg represents the gas pressure be-fore the valve, kPa; tg represents the gas temperature, °C; drepresents the relative density of dry test gas, kg/ m3; and drrepresents the relative density of the reference gas, kg/ m3.

In this experiment, the gas and water heat exchanger is acounterflow type exchanger, and the method of heat transfereffectiveness-number of transfer units (ε ‐NTU) is used tostudy the performance of the heat exchanger. The heat transfereffectiveness and the number of transfer units of the mixed-flow heat exchanger is calculated by the following eqs. [12]:

NTU ¼ KsAGcð Þg

ð2Þ

ε ¼ 1−exp −NTU 1−Crð Þ½ �1−Crexp −NTU 1−Crð Þ½ � ð3Þ

Cr ¼Gcð ÞgGcð Þw

ð4Þ

where G is the mass flow rate of the water, kg/s; C is thespecific heat of the water, J/(kg°C); Ks is the convective heattransfer coefficient of heat exchanger, K/m2°C; A is the heattransfer area, m2; and Cr is the ratio of heat capacity. Thesubscript of g and w represents gas and water, respectively.

Heat efficiency refers to the percentage of the total heat ofgas combustion used by the water heater. The heat efficiencyis calculated by

ηt ¼M � C � tw2−tw1ð Þ

V � Q1� 273þ tg

288� 101:3

Paþ Pg−S

� 100% ð5Þ

where ηt is the heat efficiency at temperature t, %, t (=tw2 ‐tw1); C is the specific heat of water, C =4.19 × 10−3 MJ/kg·K;M is the water mass flow, kg/min; tw2 is the temperature of theoutlet water, °C; tw1 is the temperature of the inlet water, °C;Q1 is the measured low calorific value of the gas, MJ/m3, m3;

Fig. 6 TESTO 350 flue gas analyzer

Table 1 The experimental parameters of the test instruments

Instrument Measuring range Precision

U-tube manometer 0~6000 Pa 10 Pa

Electronic scales 0~1000 kg 0.01 kg

Second chronograph – 0.01 s

Gas flowmeter 0.01~6 m3/h 0.1 L

Mercurial thermometer 0~100 °C 0.2 °C

Thermometer 0~100 °C 0.1 °C

1954 Heat Mass Transfer (2018) 54:1951–1960

V is the measured gas consumption, MJ/m3; pa is the atmo-spheric pressure during the test, kPa; pg is the gas pressure inthe gas flow meter during test, kPa; tg is the gas temperatureinside the gas flow meter, °C; and S is the saturated watervapor pressure at temperature tg, kPa.

NOx and CO are the most important pollutants in flue gas.During the measurement of NOx and CO concentration, it isnecessary to convert the NOx and CO concentration of theactual combustion into a reference concentration when excessair ratio α = 1. The conversion process is carried out by eqs.(6) and (7). The excess air ratio α is the ratio of the amount ofair used for actual combustion to the amount of air used fortheoretical combustion. The excess air ratio α is calculated byeq. (8). The volume proportion of oxygen in the air is approx-imately 21%.

COα¼1 ¼ COð Þm � 21

21− O2ð Þmð6Þ

NOx α¼1ð Þ ¼ NOx � 21

21− O2ð Þmð7Þ

α ¼ 1

1−O2ð Þm21

ð8Þ

where COα = 1 is the volume fraction of CO in flue gas afterconversion, %; (CO)m is the volume fraction of CO in the fluegas, %; (O2)m is the volume fraction of O2 in the flue gas, %;NOx(α = 1) is the volume fraction of NOx in flue gas after con-version, %; and NOx is the volume fraction of NOx in the fluegas, %.

4 Results and discussion

4.1 The effect of excess air factor

The excess air rate of the water heater not only affects theconcentration of CO and NOx in the flue gas but also has agreat influence on the heat transfer efficiency. In this experi-ment, the heat load of the designed system is 24 kW. Theeffect of the excess air coefficient on the heat efficiency andon the CO and NOx emission concentration were investigatedunder that heat load. According to the China standard, thelimits of NOx and CO concentration of GB 25034-2010 are39.9 ppm and 210 ppm, respectively. These are always

considered as general limits of emission in the thermal systemfor 12 T natural gas.

The nature gas enters the combustion chamber through the1.7 mm nozzle and forms a flue gas after burning. Amongthem, the content of water vapor in the flue gas is related tomany factors such as the combustion of the gas, the excess airratio and the condensation temperature. When the excess aircoefficient is 1, 1.1, 1.2, 1.3, 1.4 and 1.5, the volume fractionof water vapor in flue gas is 18.7%, 17.15%, 15.84%, 14.71%,13.74% and 12.80%, respectively. At the same time, the dewpoint temperature of the flue gas is also different when thepartial pressure of water vapor in the flue gas is different.For example, when the partial pressure of water vapor is0.05 bar, 0.1 bar, 0.2 bar, 0.4 bar and 0.5 bar, the dew pointtemperatures is 32.6 °C, 45.5 °C, 59.7 °C, 75.4 °C and80.9 °C, respectively.

As shown in Fig. 7, when α is in the 1.2 ~ 1.5 range, theconcentration of CO and NOx in the corresponding flue gas isobviously reduced with the increase of α. With the increase ofα, most of the gas in the combustion chamber are fullycombusted and oxidized into CO2 and H2O. Only a smallportion of gas has little oxygen to react with and releases asCO to cause excessive pollution emissions. The fraction ofCO in the flue gas decreases slowly when α > 1.3. Whenα = 1.3, the CO and NOx emissions in the flue gas satisfiesthe requirement of GB 6932-2010.With the further increase ofα, the increase of the air flow rate shortens the combustiontime and the gas is not fully combusted, which results in lowheat efficiency and gas waste. If α > 2, the velocity of airflowis larger than the speed of flame propagation may cause aflame-out and unsafe combustion. Figure 7 shows that whenα increases from 1.2 to 1.3, the heat efficiency of the waterheater gradually increases. When α is increased to 1.3, theheat transfer efficiency reaches its largest value, 90.8%, andthe concentration of CO and NOx in the flue gas emission alsomeet the corresponding national standard. Afterwards, an in-crease in α causes the heat efficiency to decrease.

The dew point of the flue gas is not only related to theamount of water vapor in the flue gas but also to the compo-nents of the gas and the excess air coefficient of the combus-tion of the gas. Here, taking the combustion of CH4 as anexample, when the excess air coefficient is 1.0, 1.5 and 2.0,the partial pressure of water vapor in the flue gas is 0.19, 0.13and 0.10 bar, and the dew point temperature of the flue gas is58.8 °C, 49.9 °C and 45.5 °C, respectively. The latent heat

Table 2 The test parameter andmeasurement accuracy of Testo350 flue gas analyzer

Test items Measuring range Sensing element Precision

Flue-gas temperature 0~650 °C thermocouple 1 °C

Oxygen 0~25% electrochemical transducer 0.10%

NOx 0~4000 ppm Infrared equipment 1 ppm

CO 0~10,000 ppm electrochemical transducer 1 ppm

Heat Mass Transfer (2018) 54:1951–1960 1955

recovery is realized mainly through the condensation of watervapor in the flue gas. The condensation rate of the water vaporin the flue gas is related to the partial pressure of water vaporin the flue gas and the water vapor pressure on the surface ofthe water film in the heat exchanger. When the flue gas tem-perature is below the dew point, the water vapor in the flue gasbegins to condense. Taking CH4 as an example, when theexcess air coefficient is 1.0, the dew point temperature of theflue gas is 58.3 °C, and the partial pressure of water vapor inthe flue gas is 0.19 bar. At this point, if the flue gas is con-densed to 51 °C, the water vapor in the flue gas begins tolocally condense until the partial pressure of the water vapordrops from 0.19 bar to 0.14 bar, at which point the condensa-tion rate of water vapor is (0.19-0.15)/0.19 × 100% = 26%. Ifthe flue gas temperature continues to decrease to 20 °C, thecondensation rate at this time is 87%. It should be noted thatafter condensation of water vapor, acidic gas pollutants in theflue gas also have an impact on the condensed water, and thepollutants generate acidic ions in the condensed water.

For the case of α =1.3, the concentrations of CO and NOxemissions from the CASA heat exchanger are compared withthose of the other two international brands heat exchangers,Honeywell and Beka, at a heat load of 24 kW. The results areshown in Fig. 8.

The heat exchangers produced by Honeywell and Beka arewell known for their high heat transfer efficiency and lowemissions. From Fig. 8, it can be seen that the concentrationof CO and NOx emissions are higher than those of the heatexchangers produced by Honeywell and Beka. In spite of that,the concentrations of CO and NOx emission from the CASAheat exchanger meet the national standards, but there are stillsome improvements required for the CASA heat exchanger.

4.2 The effect of load rate

For different load rates, the working condition of water heateris different. When it is at a partial load rate, the volume of gasand air is put into the water heater. At this time, the concen-tration of CO and NOx emission show different regularity.

The heat transfer coefficient of the heat exchanger and theoverall heat efficiency of the water heater also show differ-ences for different load rates. For the condensing water heater,due to the low temperature of the flue gas, the condensedwater appears on the surface of the heat exchanger. The con-densate water absorbs some of the SO2 and NOx in the fluegas to form an acid condensate. This acidic solution corrodesthe wall of the heat exchanger. The pH value of the condensedwater shows different characteristics at different heat loadrates.

The relationship between the concentration of CO andNOx emissions and the heat load rate is shown in Fig. 9.The concentration of CO is increased from 92.5 ppm to189.3 ppm when the heat load is changed from 30% to100%. The concentration of NOx shows first an increasingtrend followed by a decreasing trend. When the heat load rateis increased from 30%~90%, the concentration of NOx emis-sion is gradually increased. When the heat load rate is 90%,the concentration of NOx is at its maximum value of19.25 ppm. If the heat load rate is further increased to 100%,the concentration of NOx drops to 16.8 ppm. When the heatload rate is in changed from 90%~100%, the decrease of NOxconcentration is mainly related to the formation mechanism ofNOx. When the combustor temperature is high, NOx isdecomposed into N2 and O2. For the studied load rate, theconcentration of CO and NOx emissions meets China’s na-tional standard.

For different heat load rates, the amount of gas entering theburner and the interior temperature of the burner are different.The heat transfer coefficient of the heat exchanger and the heatefficiency of the water heater are also different. Figure 10shows the variation of the heat transfer coefficient and heatefficiency at different load rates. The heat transfer efficiencyand the heat efficiency show the same trend, and both of themincrease with the increase of the heat load rate. In the case of apartial heat load rate of 30%, the heat efficiency is still morethan 86%, which is a high energy efficiency.

With the increase of the heat load rate, the temperature of theflue gas and the pH value of the condensate are also increased,as shown in Fig. 11. For a 12 T natural gas, the condensingtemperature is 57 °C when the excess air rate is 1 [13]. Whenthe flue gas temperature is below 57 °C, the condensation ofwater vapor occurs, and the pH of the condensed water in-creases with the increase of the heat load rate. Since the con-densate water contains and acidic solution, the pH of the con-densate water varies with the condensation temperature and heatload rate. The lower the condensation temperature is, the largeramount of condensate water appears. If the corrosion resistanceof the heat exchanger is stronger, there are more dissolved Hions in the condensed water. With the increase of the heat loadrate and the temperature of the flue gas, the pH value of thecondensed water is increased. The experimental results showthat the pH value of the condensed water is in 2.5~5.5 range.

Fig. 7 The effect of α on the heat efficiency and CO and NOx emission

1956 Heat Mass Transfer (2018) 54:1951–1960

4.3 Effect of flue gas temperature on heat transferefficiency

To achieve high heat efficiency with the water heater, onemethod is to make full use of the waste heat in the flue gas.The waste heat includes the sensible enthalpy of the flue gasand the total enthalpy of the vapor. The ratio of the heat of thecondensation to the sensible heat of the flue gas is approxi-mately 11%. The energy balance of the gas combustion isshown in Fig. 12, which assumes that the enthalpy of the fluegas is 100% and that the heat of the condensation is 11%. Theheat efficiency of the water heater can be improved by using acondensing heat exchanger.

For a water heater, the heat efficiency is directly determinedby the flue gas temperature. When the temperature of flue gasis lower than that of the dew point, the water vapor in the fluegas becomes condensation. The condensed water not onlyeffectively recovers the latent heat of the water vapor but alsoabsorbs the particles and NOx. This process achieves bothenergy saving and emission reductions.

When the water vapor in the flue gas begins to condenseinto water, the condensation absorbs some of the NOx andparticulate matter in the flue gas. At this point, the concentra-tion of the particles in the flue gas is reduced, which causes theconcentration at the exhaust port to decrease. In Fig. 13, theNOx concentration in the flue gas increases with temperature,and the CO concentration in the flue gas decreases with tem-perature. When the flue gas temperature is below 70 °C theconcentration of NOx increases at a slow rate as the flue gastemperature increases. When the temperature is over 70 °C,the formation rate of NOx is affected by the high temperatureof the flue gas.

In general, the NOx concentration in flue gas increases withtemperature, and the CO concentration in flue gas decreaseswith temperature. In this paper, the concentration of NOx andCO in flue gas is related to the combustion temperature of thegas, the excess air coefficient, the heat load rate and the heattransfer efficiency of condensing heat exchanger.

The formation mechanisms of NOx and CO are stronglyrelated to the concentration of the NOx and CO in the flue gas.

Fig. 9 Effect of different heatloads on the CO and NOxconcentration in flue gas

Fig. 8 Concentration of CO andNOx emission of the heatexchangers (CASA, Honeywelland Beka) when α = 1.3 and heatload = 24 kW

Heat Mass Transfer (2018) 54:1951–1960 1957

NO gas accounts for more than 90% of the NOx gas generatedby the gas combustion. When the combustion condition issatisfied, only a small amount of CO can be formed in thecombustion process. When the amount of air needed for com-bustion is insufficient, CO will be produced. With the increaseof combustion temperature, some of CO is oxidized to CO2

and the concentration of CO will decrease.The water vapor of the exhaust gas also has some impact on

the NOx and CO concentration. The water vapor of the ex-haust gas with the dew point is 58.8 °C begins to condensewhen its temperature is lower than 57 °C [14]. Due to theinfluence of the partial pressure difference of the gas concen-tration [14], the NOx could be partly dissolved in the con-densed water. When the flue gas temperature is higher thanits dew point of 58.8 °C, the NOx dissolved in the condensedwater is decreased, and the concentration of NOx in the fluegas is increased. The NOx concentration in the flue gas in-creases with temperature, and the CO concentration in flue gasdecreases with temperature, as shown in Fig. 13.

4.4 Comparison of three water heaters

Three heat exchangers, i.e., Honeywell, Beka and CASA,are analyzed on the same platform. The experimental

natural gas is 12 T, and its calorific value is 36,000 kJ/Nm3. The nominal gas flow is 2.40 Nm3/h, and the ratedload input is 24 kW.

The heat efficiency (η), the heat transfer coefficient (K), thetemperature difference between the water supply and return(△Th), the concentration of CO and NOx of flue gas, flue gastemperature (Tgas), and the time from the ignition to the nor-mal working condition are all measured, and all of the objec-tive variables are taken into account. The results are shown inFig. 14. The time from the ignition to the normal workingcondition reflects the heat efficiency and the efficiency ofthe water heater. The time from ignition for the CASA heatexchanger is 75 s, and the time from ignition for Honeywelland Beka are 62 s and 65 s, respectively.

The heat efficiency of the Honeywell and Beka heat ex-changers is 89%, which is slightly higher than that of theCASA, which is 88%. The CASA water heater satisfies therequirements of national standards. The heat transfer coeffi-cient the Honeywell, Beka and CASA heat exchanger are 59,57, and 54 W/(m2·K), respectively. Although the △Th is asmall difference in the three heat exchangers, the heat transfercoefficient of CASA is 8.5% and 5.3% smaller than those ofthe Honeywell and Beka heat exchangers, respectively.Compared with the other two heat exchangers, there is stillspace for optimization in the structural design of the CASAwater heater.

Regarding the concentrationofCO in fluegas, the concen-tration in the CASA heat exchanger is 168.5 ppm, while theHoneywell andBeka heat exchangers have concentrations of

Fig. 13 Effect of exhaust gas temperature on the concentration of CO andNOxFig. 11 Effect of heat load on flue gas temperature and condensate pH

Fig. 10 Effect of heat load on heat transfer coefficient and heat efficiency

CH4 + 2O2 = Energy + 2H2O + CO2

Total energy

111%

Chemical energy

100%

Latent heat

11%

Total

energyCondensing heat

+=

Fig. 12 Energy conservation diagram of gas combustion

1958 Heat Mass Transfer (2018) 54:1951–1960

156.7ppmand155.4ppm, respectively.TheconcentrationofNOx in the CASA heat exchanger is 15.21 ppm, while theconcentrations in the Honeywell and Beka heat exchangerare 4.75 ppm and 12.57 ppm, respectively. It is found that theconcentrationsofCOandNOxinthefluegasoftheCASAheatexchanger are higher than those of Honeywell and Beka heatexchangers even if they all satisfy the national standard.

For the anti-corrosion mechanism of casting aluminum-silicon alloy, the pH value of the condensate water is selectedas the standard measurement of corrosion degree. In the con-densed liquid on the ally surface, the corrosion of the CASA iscaused by a micro battery, which is formed by the large cath-ode and the small anode. On the surface of the alloy, thecontinuous condensation liquid film inhibits the diffusion ofoxygen, so the diffusion reaction of the cathode becomes themain control step. The anode has a small dissolution at thesurface active point of the alloy. At the same time, the cathodetakes an oxygen absorption reaction, and the chemical reac-tion at this time is as follows:

Al−3e−→Al3þ

O2 gð Þ þ 2H2Oþ 4e−→4OH− aqð Þ

In the condensed liquid, more and more Al3+ is generated,white Al (OH)3 begins to deposit on the alloy surface, andfurther dehydration reaction takes place to form the passiv-ation layer of Al2O3.

Al3þ þ 3OH−→Al OHð Þ32Al OHð Þ3→Al2O3 þ 3H2O

The small radius Cl- has strong penetrating ability. Evenin the tiny pores of the oxidizing protective film, the

chlorine ions can pass through, and react with the metalsof the body to form corrosive products containing chlorine.At the same time, the chloride ion has strong adsorptioncapacity, which adsorbs on the surface-active position ofmetal and reacts with the metal oxide film, so that the metaloxide film is dissolved gradually, and eventually lead to thecorrosion of aluminum metal. In addition, the presence ofnon-erosive anions also has a certain corrosion inhibitioneffect. When the sulfate ion exists, the sulfate ion canweaken the adsorption of chloride ions on the surface ofaluminum and silicon by competitive adsorption. At thesame time, the sulfate ion is an oxide anion, which canpromote the repair of the oxide film and enhance the cor-rosion resistance of the alloy.

There are many ways to characterize the corrosion inten-sity, such as the change in the rates of the corrosion area, thecorrosion weight and the corrosion depth. Due to the incon-venience of dismantling the experimental gas-water heater,the measurement of the abovementioned corrosion rates isnot suitable herein. For a comprehensive consideration, thepH value of the condensate water is selected as the standardmeasurement of corrosion degree. When the surface of theheat exchanger is corroded by the condensed acid solution,the pH value of the etching solution shows to what level thecondensed acid solution can chemically corrode the surfacemetal of heat exchanger. As the surface of the heat exchang-er corrodes, the acidic condensate water will become a saltsolution and the pH value of the condensed water increasesas a result. Figure 14 shows that the pH of the condensedwater of the CASA is 3.4, while the condensed water ofHoneywell and Beka is 4.6 and 4.7, respectively. Thesefindings indicate that the reaction rate on the surface of theCASA heat exchanger and the condensed acid solution islower than those of Honeywell and Beka. Therefore, the

Fig. 14 Comprehensiveperformance of the heatexchangers of Honeywell, Bekaand CASA

Heat Mass Transfer (2018) 54:1951–1960 1959

anti-corrosion performance of the CASA heat exchanger isbetter than those of Honeywell and Beka.

5 Conclusions

A large problem plaguing condensing water heaters is corro-sion on the inner surface of the heat exchanger. For this rea-son, a heat exchanger made of CASA is proposed, and thecorresponding experiments are carried out to investigate theperformance of the heat exchanger. The effects of the excessair rate, the heat load rate, and the temperature of the flue gasare discussed. The comparison of the comprehensive perfor-mances of CASA, Honeywell and Beka heat exchanger wereconducted. The main results were summarized as follows:

(1) The excess air rate has a significant impact on the heatefficiency of the water heater and the concentration ofCO and NOx in the flue gas. When α = 1.3, the CO andNOx emissions in the flue gas satisfy the national stan-dard, and the heat efficiency of water heater is 90.8%.

(2) With the increase of heat load rate, the heat transfer co-efficient of the heat exchanger and heat efficiency of thewater heater are also increase accordingly. When the heatload rate is approximately 90%, the NOx in the exhaustgas is the highest.

(3) When the temperature of flue gas is below 57 °C, thecondensation of water vapor occurs, and the pH of thecondensed water is in the range of 2.5~5.5. The anti-corrosion performance of the CASA heat exchanger isbetter than those of Honeywell and Beka.

(4) The CASA water heater achieves better corrosion resis-tance, and the heat efficiency is only 88%. Consideringthe performance of the heat exchanger, there is still anumber of opportunities for optimization and improve-ment when compared with the heat exchangers producedby Honeywell and Beka.

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