Abstract This experiment predicts the performance of an industrial scale heater that is to be used in an incineration plant. This was achieved by performing heat transfer experiments on the CFC, Forane R141b-1, 1 dichloro 1-fluroethane. After formation of thermal and physical properties and relationships, it is found that an industrial scale system will take 7.5 minutes to boil 175 kg of the CFC at 80% of the critical flux. 1 Introduction Immersion heaters are primarily used in industry and also commercially; commonly used in industry as fuel pre-heaters and process plant pre-heaters. Commercial uses include kettles and central heating boilers. This experiment focuses on a process stream pre-heater functioning in a high temperature incineration plant. There are different ways of disposing hazardous compounds produced in laboratories and industrial processes. However, incineration is the most feasible and therefore the most common in regards to chlorofluorocarbons (CFCs). Incineration is preferred over other methods as the process involves complete destruction and denaturing of the harmful compound. Heat that is produced in the incineration can also be used to heat other fluid streams in the same chemical plant or process; this increases the sustainability of incineration. The CFC present in the waste stream in this experiment is Forane R141b-1,1 dichloro 1-fluoroethane and exists as a liquid. Physical and thermal properties of this CFC are determined using ASPEN properties. It is expected that that as the temperature difference between the heater and process fluid is increased, the nature and rate of the heat transfer from the heater to the process fluid changes considerably. There are three principal behaviors of boiling that all feature in this experiment. Convective boiling occurs when the heater surface is slightly hotter than the liquid i.e. small temperature difference; convective currents transport the slightly warmed liquid to the surface where evaporation takes place with minimal bubbling or boiling. Nucleate boiling sees the formation of bubbles and vigorous boiling as the heater surface becomes hotter. This causes high turbulence and hence high rates of heat transfer. Film boiling occurs when the temperature difference is above the critical surface-liquid temperature difference, and the liquid is unable to wet the surface any further. This causes a significant reduction in the heat transfer coefficient. This experiment predicts the performance of an industrial scale heater that is to be used in an incineration plant. This is achieved by performing heat transfer experiments on the waste product from the laboratory scale apparatus. The relationships between physical and thermal properties of the waste stream as a function of temperature are obtained; this data then contributes to calculating the theoretical critical heat flux. The experimental critical heat flux is determined directly using the apparatus and is compared with the theoretical value. The experimental and theoretical values are represented as a function of temperature difference, where temperature difference is the difference in temperature between the heater and the process liquid. Having this information and these relationships enables the predictions of energy and time requirements to be made if the process is to be scaled up to an industrial process in an incinerator plant. 2 Theory In order to calculate theoretical and experimental values for critical heat flux, other variables are calculated. Surface tension, 𝜎, saturation pressure, 𝑃!, saturated vapour density, 𝜌! and latent heat, ℎ!", are found using the physical property graphs provided using the known experimentally determined value of the saturation temperature, 𝑇!. It is assumed that the saturation temperature is the temperature of the Forane R141b-1,1 dichloro 1-fluroethane when the voltage is 0 V.

Other variables such as power input, 𝑃!" , heat flux, 𝑈, temperature difference, Δ𝑇, heat transfer coefficient, ℎ, and surface area, 𝐴 are calculated from experimental data. The liquid density, 𝜌!, is determined from the ASPEN Properties software. To calculate power input;

𝑃!" = 𝐼𝑉 [W] To calculate surface area; where 𝑟 and 𝑙 are radius and length respectively;

                                         𝐴 = 𝜋𝑟! + 2𝜋𝑟𝑙 = 𝜋𝑟(𝑟 + 2𝑙) [m2]

From (2.1) and (2.2), heat flux can be calculated;

                                   𝑈 = !!"!

[W m-2] Temperature difference is defined as; where subscripts HE and liquid denote heating element and liquid respectively;

Δ𝑇 = 𝑇!" − 𝑇!"#$"% [℃]

From (2.3) and (2.4), heat transfer coefficient can be calculated;

                               ℎ = !!!

[W/m2 ℃]

Theoretical critical heat flux is calculated using the Zuber equation; where 𝑔 is acceleration due to gravity;

                                 𝑈!"# =!!!"!!!"

∙ !"(!!!!!)!!!

!!∙ !!

!!!!!

!!

[W m-2]

By comparing the theoretical and experimental value of critical heat flux, an indication of the system’s efficiency is gained. The theoretical value will be higher because there is no account of losses that are indefinitely present in a real working system. The system in this experiment is not completely insulated which means there is heat loss to the surroundings; this lowers its efficiency. This is just one example of several physical parameters that effect efficiency. It is expected that there will be an increase in power input as temperature difference increases. This is due to more energy per unit time needed to heat the system as the heating element and the Forane R141b-1,1 dichloro 1-fluroethane drift further apart in temperature. 3 Method The apparatus used for this experiment features a strong glass cylinder containing the Forane R141b-1,1 dichloro 1-fluroethane. Embedded in the cylinder is a water-cooled coil, this is where the vapour condenses back to a liquid and droplets can be observed here. Electrical current is supplied infinitely from a variable transformer. The bottom of the cylinder features a copper thimble containing a high watt density element. This is responsible for heating the liquid. Copper is used as it is a superb conductor of heat and in this case inert to the Forane R141b-1,1 dichloro 1-fluroethane. The thermal action of the system is determined by the cooling water flowrate and temperature.

(2.1)

(2.2)

(2.3)

(2.4)

(2.5)

(2.6)

The experimental procedure is as follows; • Switch on the system, observe the saturation temperature as the voltage is set to 0 V • Set the cooling water flowrate to 2 g s-1 and increase the voltage by 5 V intervals until

50 V is reached. Manually record all experimental parameters whilst observing the behavior of the Forane R141b-1,1 dichloro 1-fluroethane to determine which type of boiling is occurring.

• Once 50 V is reached, increase the cooling water flowrate to 4 g s-1 and increase the voltage by 10 V intervals until the voltmeter and thermometer “cut out”. This usually happens near 170℃ for this apparatus. Keep recording all parameters.

• Between each voltage interval, allow the system to reach stability i.e. only record parameters when fluctuation is minimal if not zero.

• After using the apparatus, switch off electrical supply, circulate cooling water until pressure is atmospheric and switch of the electronic thermometer.

4 Results

Figure 1. Plot of density of Forane R141b-1, 1 dichloro 1-fluroethane as a function of temperature at 1 atm using values found with ASPEN Properties software. Line of best fit included.

Figure 2. Plot of logarithmic heat flux as a function of logarithmic temperature difference with line of best fit.

Figure 3. Plot of logarithmic heat transfer coefficient as a function of logarithmic temperature difference for nucleate boiling with line of best fit.

Figure 4. Plot of logarithmic heat transfer coefficient as a function of logarithmic temperature difference for film boiling with line of best fit.

1210

1220

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0 10 20 30 40

Den

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-3]

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0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

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3.45

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0 0.5 1 1.5 2 2.5

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[W

/m2 ℃

]

Log Temperature Difference [℃]

The saturation temperature, 𝑇!, is found to be 24℃ when the voltage is 0 V. From this, the following parameters are taken graphically from the physical property graphs; surface tension, 𝜎 = 0.0182 N m-1, saturation pressure, 𝑃! = 75 kN m-2, saturated vapour density, 𝜌! = 3.7 kg m-3 and Latent heat, ℎ!" = 226.5  kJ kg-1

5 Calculations The flowing example calculations correspond to the first row of data shown in Table 1. Known dimensions (𝑟 = 0.00635 m and 𝑙 = 0.034 m) of the apparatus allow surface area to be calculated first; recalling equation (2.2);

𝐴 = 𝜋𝑟 𝑟 + 2𝑙 = 𝜋 0.00635 0.00635 + 2 0.034 = 0.00148 m2

Power input is calculated from equation (2.1);

𝑃!" = 𝐼𝑉 = 0.08 ∙ 5 = 0.4 W Heat flux is now determined with equation (2.3);

𝑈 = !!"!= !.!

!.!!"#$= 270 W m-2

Temperature difference is calculated from equation (2.4);

Δ𝑇 = 𝑇!" − 𝑇!"#$"% = 35 − 30.0 = 5.00  ℃ Heat transfer coefficient is now calculated from equation (2.5);

ℎ = !!!= !"#

!.!!= 54 W/m2 ℃

Finally the theoretical critical heat flux is calculated using the Zuber equation.

𝑈!"# =!!!".!∙!.!

!"∙ !.!"∙!.!"#$ !"#$!!.!

!.!!

!! ∙ !"#$

!"#$!!.!

!! = 219300 W m-2

The experimental heat flux for this experiment is 210000 W m-2, which is taken from Figure 2. Comparing the two values, the expected higher theoretical value is considered as 100% efficiency in an ideal system. However when operating this real system, efficiency is found to be very good; !"!!!!

!"#$%%∙ 100 = 95.8  %

Considering the scale up calculation. Determine the time taken to boil 175 kg of the CFC assuming operation is at 80% of critical flux and the area available for heat transfer is 0.5 m2.

80% of critical flux;219300 ∙ 0.8 = 175440 W m-2

𝑃!" = 175440 W m-2 multiplied by area; 175440 ∙ 0.5 = 87720 W = 87720 J s-1

Latent heat multiplied by mass; 226500 ∙ 175 = 39637500 J ∴ !"#!$%&&

!""#$= 452 s = 7.5 minutes

6 Discussion Figure 1. shows how the density of Forane R141b-1, 1 dichloro 1-fluroethane decreases linearly as temperature is increased at 1 atm. This trend is expected and is common for fluids as density is a function of temperature and pressure. The same decreasing relationship can be found for different pressures. The nature of this decrease depends on the physical properties of the fluid, the CFC in this case shows a perfectly linear trend over the temperature range, however other fluids express a decreasing polynomial curve. Figure 1. could be extrapolated to predict densities for other temperatures as the trend line is perfectly linear and uniform. Figure 2. displays a strong positive relationship between heat flux and temperature difference i.e. as temperature difference increases the heat flux increases. This is expected and normal as more work input is needed by the system as the temperature of the heating element and CFC become further apart in isochoric conditions. There are however a few minor outliers displayed which are the result of errors in data reading and heat loss to the surroundings.

Figure 3. expresses a steady increasing trend, as temperature difference increases the heat transfer coefficient abides the same pattern. Figure 3. focuses on the nucleate boiling stage corresponding to a voltage range of 25 – 140 V. Nucleate boiling is the predominant type of boiling present throughout this experiment which is reflected in the graph as each data point is at a minimal distance from the linear trend-line i.e. a very strong correlation and low regression. The system is not subjected to major change if at all during most of the nucleate boiling period and therefore fluctuation or radical values are unlikely; instead, gradual and linear increases in temperature voltage and current. Figure 4. shows a sharp decline in heat transfer coefficient as temperature difference increases. This is also expected, graphically this means that less and less heat can be transferred to the CFC from the heating element as the temperature approaches its pinnacle. Film boiling is established in Figure 4. meaning that the liquid is unable to wet the surface any further. This causes a significant reduction in the heat transfer coefficient as explained in the introduction. With any real working system, there are inevitable factors of error present that are not accounted for in an ideal system. This experiment features errors within heat loss to the surroundings. This is difficult to quantify but could be reduced by insulating the apparatus, particularly the glass cylinder containing the CFC and host to the heat transfer. The most significant errors present in this experiment are found with the devices used to record data. Not only are certain instruments hard to read; they feature graduated increments that could show greater accuracy and hence greater sensitivity. Purely from a visual aspect, parallax induced errors in taking readings especially for the electronic thermometer. The electronic thermometer features 2 ℃ quantifying potential error of ± 1 ℃ . Similarly, all other thermometers features 1℃ increments and ±0.5℃ error. Small error of ±0.025 A are present in the current readings as the ammeter features 0.05 A increments. Perhaps the largest and most significant apparatus errors are within the voltmeter, 5 V increments and ±2.5 V error and also the pressure gauge which features 10 kN m-2 increments and ±5 kN m-2 error. 7 Conclusion The experiment aimed to predict the performance of an industrial scale heater that is to be used in an incineration plant. This was achieved by performing heat transfer experiments on the waste product from the laboratory scale apparatus; in this case the CFC waste product being Forane R141b-1, 1 dichloro 1-fluroethane. The relationships between physical and thermal properties of the CFC have been formulated and values for experimental and theoretical heat flux have been concluded. With the account of errors, the approximate elapsed time for an industrial scale system to boil 175 kg of the CFC at 80% of the critical flux will be 7.5 minutes. Nomenclature 𝐴 Surface Area [m2] 𝑇!"# Temperature Outlet [℃] ℎ Heat Transfer Coefficient [W/m2 ℃] 𝑇!"#$"% Temperature of Liquid [℃] ℎ!" Latent Heat [kJ kg-1] 𝑇!" Temperature of Heating Element [℃] 𝐼 Current [A] 𝑇! Saturation Temperature [℃] 𝑙 Length [m] ∆𝑇 Temperature Difference [℃] 𝑚 Cooling Water Flowrate [g s-1] 𝑈 Experimental Heat Flux [W m-2] 𝑃 Pressure (gauge) [kN m-2] 𝑈!"# Theoretical Heat Flux [W m-2] 𝑃!" Power Input [W] 𝑉 Voltage [V] 𝑃! Saturation Pressure [kN m-2] 𝜌! Vapour Density [kg m-3] 𝑟 Radius [m] 𝜌! Liquid Density [kg m-3] 𝑇!" Temperature Inlet [℃] 𝜎 Surface Tension [N m-1] References COULSON JM, RICHARDSON JF, (1996) Chemical Engineering Volume 1 – Fluid Flow, Heat Transfer and Mass Transfer, 5thEd, Butterworth-Heinemann: Great Britain, pp 412 – 417.

Appendix

Table 1. Experimental data, observations and calculated values. Calculated values are highlighted. 𝑉

[V] 𝐼

[A] 𝑇!" [℃]

𝑇!" [℃]

𝑇!"# [℃]

𝑃 [kN m-2]

𝑇!"#$"% [℃]

∆𝑇 [℃]

𝑚 [g s-1]

𝑃!" [W]

𝑈 [W m-2]

ℎ [-] Observations

5 0.08 35 17.8 19.0 3 30.0 5.00 2 0.40 270 54 Still, no sign of movement

10 0.16 38 17.8 19.1 3 30.6 7.40 2 1.60 1081 146 Few bubbles visible, convective boiling

15 0.24 39 18.0 19.5 3 33.0 6.00 2 3.60 2432 405 Few bubbles, some dripping from the heating element, convective boiling

20 0.30 40 18.0 19.7 6 33.8 6.20 2 6.00 4054 654 Few bubbles, some dripping from the heating element, convective boiling

25 0.36 42 18.0 20.0 9 34.3 7.70 2 9.00 6081 790 More bubbles, some dripping from the heating element, nucleate boiling

30 0.43 43 18.2 20.8 10 34.9 8.10 2 12.9 8716 1076 More bubbles, some dripping from the heating element, nucleate boiling

35 0.50 45 18.2 21.4 11 35.0 10.0 2 17.5 11824 1182 Some agitation in liquid, increasing number of bubbles, nucleate boiling

40 0.57 46 18.3 22.0 12 35.6 10.4 2 22.8 15405 1481 Regular dripping from heating element, more bubbles, nucleate boiling

45 0.64 47 18.3 22.8 12 35.7 11.3 2 28.8 19459 1722 Regular dripping from heating element, more bubbles, nucleate boiling

50 0.71 49 18.2 23.9 13 35.8 13.2 2 35.5 23986 1817 Regular dripping from heating element, more bubbles, nucleate boiling

60 0.85 51 18.2 22.9 13 35.5 15.5 4 51.0 34459 2223 Faster dripping from heating element, more bubbles, nucleate boiling

70 0.98 53 18.2 23.0 15 36.0 17.0 4 68.6 46351 2727 Drips are almost “stream” like, large amount of bubbling, nucleate boiling

80 1.13 55 18.0 24.2 15 36.2 18.8 4 90.4 61081 3249 Increased bubbling, nucleate boiling

90 1.25 58 17.9 25.6 17 36.8 21.2 4 113 76014 3586 Bubbling is rapidly increasing, nucleate boiling

100 1.39 60 17.9 27.0 20 37.5 22.5 4 139 93919 4174 Bubbling is rapidly increasing, nucleate boiling

110 1.53 63 17.9 28.8 22 38.5 24.5 4 168 113716 4641 Vigorous boiling has been established, nucleate boiling

120 1.66 67 17.8 30.5 27 39.2 27.8 4 199 134595 4842 Multiple droplets running down coil, “stream” dripping continues, nucleate boiling

130 1.80 71 17.8 32.6 32 40.7 30.3 4 234 158108 5218 Multiple droplets running down coil, “stream” dripping continues, nucleate boiling

140 1.94 77 17.8 34.6 41 42.1 34.9 4 272 183514 5258 Multiple droplets running down coil, “stream” dripping continues, nucleate boiling

150 2.07 86 17.6 36.8 49 44.0 42.0 4 311 209797 4995 Multiple droplets running down coil, “stream” dripping continues, some vapours, film boiling

160 2.21 100 17.6 39.1 60 46.1 53.9 4 354 238919 4433 Vapours are more visible, film boiling

170 2.33 137 17.8 39.5 62 46.6 90.4 4 396 267635 2961 System begins to cut out, film boiling