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A Novel Measuring Instrument for Pyro-Processes
0263±8762/00/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 78, Part A, July 2000
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES K. H. CHAN (ASSOCIATE MEMBER), J. H. GOODFELLOW, Y. R. GOH (ASSOCIATE MEMBER), V. NASSERZADEH (ASSOCIATE MEMBER), J. SWITHENBANK (FELLOW) and D. S. TAYLOR* Shef®eld University Waste Incineration Centre (SUWIC), University of Shef®eld, UK *King®eld Electronics Ltd., Chester®eld Trading Estate, Derbyshire, UK
A
unique self-contained data acquisition unit consisting of multiple sensing elements and recording electronic components installed in a heat resistant capsule is currently being developed at SUWIC. This unit can be introduced into a process system with the raw feed material and experiences the same conditions as the material being processed, as it simultaneously measures the temperature, gas compositions and heat ¯uxes in the system. At the end of the process, the instrument can be recovered, and the data recorded and stored in its memory unit downloaded to a computer. This instrument is applicable to pyro-processes such as incineration, power generation and steel processing, as well as to the food industry. Keywords: combustion; heat protection; in situ measurement; instrumentation; microcomputer
in various forms4. Their microporous structure gives them a thermal conductivity even lower than that of ceramic ®bre, promising to reduce the necessary Ball Instrument capsule size signi®cantly. Nickel/chrome alloys, with proven resistance to corrosion and creep at the temperatures involved, are also commercially available5. Ultimately then, the key to development of a practical Ball Instrument is full exploitation of the physical and thermal characteristics of materials, and determination of the best geometry for them to be applied. This is being achieved by the combined application of mathematical analysis, Computational Fluid Dynamic (CFD) modelling, and practical experimentation.
INTRODUCTION Most processes that use sensors for recording and control have them located in positions ®xed when the plant was built. However, within the lifetime of a plant, there are often occasions when more information and data are needed, to study the operating conditions and dynamics of the process. Measurements of process variables made in close proximity to processed material are also more favoured for detailed research than those made remotely. If there were a device available that could be introduced into an existing process to make such measurements, without modi®cation to the process equipment, it would be of signi®cant value to the engineering community. Such a device is the subject of this research program. It is generically described herein as a `Ball Instrument’, because the original concept had that kind of package in mindÐa compact, rugged device, able to be dropped into, and to survive, the harsh environments found in many pyro-processes. There are already commercial examples of in-process data loggers for temperature recording1, but they are designed for applications with well-managed transport mechanisms. Examples of this are steel billet pusher furnaces, glass lehrs and car-type tunnel kilns. Unfortunately, the design of their thermal protection makes their use inappropriate in more dynamic and unavoidably chaotic pyro-processes. Portable ¯ue gas analysers of compact dimensions are also available2, but they are designed to be used in relatively benign ambient conditions. They also need signi®cant sample conditioning to be applied. Fortunately, compact ceramic sensors for measurement of ¯ue gas species concentration have become available, which may overcome these dif®culties. They are able to detect and measure concentrations of various gases with great selectivity, and are inherently rugged3. Very advanced thermal insulation materials, capable of operation at temperatures up to 12008 C., are also available
GEOMETRICAL DESIGN OF THE INSTRUMENT When in use, the ball instrument is exposed to high temperatures, and for most pyro-processes, this is typically about 10008 C, with heat ¯ux ranging from 2 to 5 kW m ±2. Hence, one of the most important design considerations for this instrument is the heat protection technique applied to protect the electronics at the core of the unit, since it is intended that the device be introduced into a furnace/kiln/ incinerator with the raw feed, be allowed to pass through the system, and be recovered at the furnace exit. The duration of the time the instrument will spend in the process is typically between 40 and 60 minutes6. The size and geometry of the ball instrument must be optimized to fully exploit the thermal properties of the insulating materials employed. To reduce the heating rate to the core of the instrument, the ratio of the exposed surface area per unit volume must be minimized. A sphere is the ideal shape for lowering the rate of heat transfer, but the fabrication of a spherical casing at this early stage of the instrument development is dif®cult and is neither time nor cost effective. Hence, cylindrical and cube shaped capsules were made for theoretical and experimental investigations on geometrical designs of the unit. 783
CHAN et al.
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Material Selection
Figure 1. Description of the Ball Instrument.
Before a casing of either shape is constructed, the optimum design ratio, which minimizes the rate of heat transfer from the exposed external surface to the internal surface in the core, must be determined. Based on Fourier’s Law of heat conduction7, the one dimensional heat transfer rate at steady state is given in equation (1). ¶T q = kA (1) ê ¶x For a general thermal insulation system, the three dimensional heating rate of the unit q, assuming quasisteady state is given by: qcy = 2kp(T2
+ (hr2
ê ê
T1 ){2r1 r2 (H Hr1 )(r2
h)ê
ê
ê r1 ) ê
1
1
ln[(r2 h)(r1 H)ê 1 ]}
(2)
1
T1 )(x1 x2 )(x2 x1 )ê (3) ê ê where subscripts cy and cu refer to the cylindrical and cube shaped casing respectively, Figure 1. The corresponding life (i.e., the maximum duration which the instrument can be exposed to high temperature) of each unit is given by: qcu = 12k(T2
tcy =
»lr12 (r kr2 2 ê
tcu =
»lx21 (x 12kx2 2 ê
r1 )(T2
T1 )ê ê
x1 )(T2 ê
1
T1 )ê
(4) 1
(5)
Using equations (1) to (4), the optimum design ratios of the cylindrical unit were found to be 1.5, 2 and 2 for r2 /r1 , H/r2 and h/r1 , respectively. The optimum ratio of x2 /x1 for the cube casing was found to be 1.5. The values for r1 and x1 for the optimum ratios were obtained from the following equations: p r1 = 3A1 (T2 T1 ) (6) ê p x1 = 6 A1 (T2 T1 ) (7) ê where, A1 is given by kt/»l. EXPERIMENTAL PROGRAM The main objective of the preliminary tests was to evaluate the best means of protection for the electronics in the core of the instrument. Various heat protection techniques and their associated problems were addressed accordingly. Selection and testing of suitable electronic components and sensors was also carried out. Time dependent calculations on heat conduction of the plaster insulation were carried out using FLUENT code.
In order to achieve the objective with minimum physical size of the Ball Instrument, protective casings made from refractories with low thermal diffusivity, such as ®ne ®nishing plaster, mastics (castable ceramic), VIX (castable refractory), ceramic ®bre blanket, ceramic ®bre paper, Himalayan 2600 (refractory brick), and Cellobond (ceramic-cement composite blocks) were tested. More advanced insulators, such as Microtherm microporous insulation will be tested in due course. Besides the use of insulation, it was believed that the protective casing could also be kept cool by the ablation effect, in which ablative resin decomposes, vaporizes and hence removes heat from the instrument capsule8. Therefore, protective casings made from Resin LY5052 as the ablative cooling medium were also investigated. A protective casing can only minimize the rate of heat transfer, it does not prevent some heat from reaching the core of the instrument. Hence, further means of heat protection for the electronics at the core of the instrument was vital. This secondary method of heat protection was provided in the form of a liquid heat sink at the core of the unit. Water, contained in a stainless steel shell, was used to serve as this heat sink, since it has moderate boiling point, high heat capacity and high latent heat of vaporization9. A selection of the specimens fabricated and the type of heat sink incorporated in each specimen is summarized in Table 1. Each specimen was tested by placing it in a furnace, temperature-controlled at 10008 C, for 40 to 90 minutes. The typical heat ¯ux encountered in this furnace was about 1 kW m ±2. The temperatures of the furnace and the core of the ball were measured using K-type mineral sheathed thermocouples, and continuously logged by an automated data acquisition system. Figure 2 shows typical core and furnace temperatures obtained during the experiments. In the ®rst few minutes, only the insulating casing of the unit was being heated, hence no increase in the core temperature was initially observed. The water in the core was then heated steadily until it reached its boiling point. For a further period of approximately 20 minutes, depending on the type of insulating material used to construct the casing, the absorption of latent heat associated with boiling of the water kept the core temperature constant at 1008 C. Once all the water in the core had been driven off, the core temperature started increasing again. In the example shown in Figure 2, the ball instrument would have been able to operate in a furnace at 10008 C for at least 60 minutes before its electronics became susceptible to thermal damage caused by excessive core temperature. The performance of each specimen was determined, based on the time taken to fully evaporate the water at its core. The average heat transfer rate was derived directly from the latent heat of water and this drying time. By replacing the rate of heat transfer, qcy and qcu in equations (2) and (3), respectively with these calculated values, the effective conductivity, keff of each ball was evaluated. It should be noted that the effective thermal conductivity calculated using equations (2) and (3) differ from the thermal conductivity of the insulation material quoted by the manufacturers. This is because the heat transfer Trans IChemE, Vol 78, Part A, July 2000
Trans IChemE, Vol 78, Part A, July 2000
12 13 14 15 16 17 18 19 20 21 22 23
5 6 7 8 9 10 11
4
1 2 3
No
Cube
Cylindrical
Cylindrical
Shape
Glass Fibre Reinforcement ±
Plaster
Himalayan 2600
±
±
± Extra moisture ± Glass Fibre Reinforcement ± ± Extra moisture Extra moisture ± ± Ceramic Fibre Reinforcement ± ±
Additional
±
VIX
Cellobond VIX_CFB
Resin LY5052
Ceramic Fibre Disc Himalayan 2600 Mastics
CF Paper Roll
Plaster
VIX
Refractory
Insulation
70 vol% of water in a S.S. shell
70 vol% wet ceramic ®bre module
Fress plaster + water
70 vol% of water in a stainless steel (S.S.) shell
70 vol% of water in a stainless steel (S.S.) shell
Central core
3
1630 1840 1470 1700 1700 250 550 790 1700 1700 1700 620
1700 1700 1700 1700 1700 1700 1700
1700
1700 1700 1700
cm
V
18.8 15.6 13.0 15.0 15.0 9.0 11.0 10.0 12.0 12.0 12.0 11.4
15.0 15.0 15.0 15.0 15.0 15.0 15.0
15.0
15.0 15.0 15.0
cm
H
13.4 11.0 10.0 10.0 11.0 ± ± ± 8.0 7.0 6.0 9.4
9.0 9.0 9.0 9.0 9.0 9.0 9.0
9.0
9.0 9.0 9.0
cm
h
Table 1. Information on specimens tested for ef®ciency evaluation.
10.5 6.05 6.0 6.0 6.0 3.0 4.0 10.0 12.0 12.0 12.0 3.7
6.0 6.0 6.0 6.0 6.0 6.0 6.0
6.0
6.0 6.0 6.0
cm
X2 or r2
5.1 3.25 5.0 3.5 4.0 ± ± ± 8.0 7.0 6.0 2.7
3.0 3.0 3.0 3.0 3.0 3.0 3.0
3.0
3.0 3.0 3.0
cm
X1 or r1
2.0 1.9 1.2 1.7 1.5 ± ± ± 1.5 1.7 2.0 1.4
2.0 2.0 2.0 2.0 2.0 2.0 2.0
2.0
2.0 2.0 2.0
1.8 2.6 2.2 2.5 2.5 3.0 2.8 1.0 1.0 1.0 1.0 3.1
2.5 2.5 2.5 2.5 2.5 2.5 2.5
2.5
2.5 2.5 2.5
X2 /X1 H/X2 or r2 /r1 or H/r2
0.22 ± 0.26 0.26 0.26 ± ± ± 0.48 0.48 0.48 0.40
0.07 0.07 0.07 0.40 0.08 ± ±
0.48
0.26 0.26 0.48
ktrue @258 C
0.33 0.25 0.46 0.40 0.43 ± ± ± 0.09 0.12 0.08 0.37
0.22 0.24 0.29 0.26 0.38 0.33 0.63 0.29
0.41 0.31 0.23
keff
1.31 1.68 0.75 1.17 1.10 0.93 0.76 1.89 1.93 1.76 1.40 1.40
1.89 1.73 1.45 1.57 1.11 1.25 0.79 1.62
1.00 1.32 1.82
g
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES 785
CHAN et al.
786
Figure 2. Typical temperature history of the furnace and the central core of the Ball Instrument, Ball No. 22.
phenomena encountered during the experiment occurred with the system in unsteady state. However, these simple equations are suf®cient for evaluating the casing design and the geometry’s in¯uence on the rate of heat transfer, at this stage of the instrument’s development. To compare the performance between the specimens tested, a variable called relative ef®ciency was formulated. The performance of each specimen was evaluated from the time taken for the water contained in the core of the specimen to evaporate during the experiment, normalized to the temperature of the furnace and the total volume of the specimen. Comparisons between specimens could therefore be made, based on their performance relative to the performance of Ball 1. Hence, the relative ef®ciency for each specimen can be given by: 1 Relative efficiency, g = (tnorm )(tnorm )ê(Ball No.1)
=
tdry T24 V
tdry T24 V
1 ê
(7) (Ball No.1)
The effective conductivity and relative ef®ciency of each specimen is also listed in Table 1. Programmable Data Acquisition Controller The device to be used for in situ acquisition and logging of temperature data is a CMOS microcontroller, selected and programmed by King®eld Electronics Ltd., our industrial partner in this aspect of the research. It incorporates analog to digital converters and a serial interface, and has very low power requirements. It is capable of operating over a remarkably wide temperature range, with an upper limit of 1258 C, making it ideally suited to use in this application. By encapsulating the circuit board and its components in a water-resistant epoxy-resin based compound after manufacture, the controller is able to be fully immersed in water, without risk of short-circuits. This data-logger’s ®rmware program monitors the pins on its sensor/data socket, and is able to detect whether it is required to begin logging, or needs to monitor the serial interface for incoming commands. The software, which effectively de®nes the data-logger’s function, can be changed during manufacture. Its activities include the scaling and conversion to engineering units, of data from the selected sensors. These include thermocouples, cells
Figure 3. Functional testing of the thermocouple connected to the Programmable Data Acquisition Controller (PDAC).
able to detect and measure gas species, and sensors to measure incident radiation. The program also allows the data-logger’s sampling intervals to be input, and for data to be collected from its memory, by issuing commands from a computer equipped with a serial interface. Preliminary testing of the data logger (temperature logging only) showed that the electronic components could operate satisfactorily between the freezing point and boiling point of water, Figure 3. The mini data logger was boiled in water and subsequently placed in melting ice while it was still taking external (thermocouple) and internal (surface of the circuit board) temperature measurements. Despite slight discrepancies in the external temperature measurement compared with an independent thermometer (which was recti®ed in later versions), the data-logger proved itself capable of operation while situated in an ambient temperature ranging from 0 to 1008 C. This showed the feasibility of using both the latent heat of fusion and of evaporation of ice/water at the central core, should that prove necessary. DISCUSSION Comparisons between the relative ef®ciencies of balls 1, 14, 15 and 16, all of which were fabricated using VIX insulation with 70% of the core volume ®lled with water, showed that the best performance was given by Ball 15 which had design ratios of 1.7, 2.5 and 2.9 for r2 /r1 , H/r2 and h/r1 , respectively. Although it may appear that the design ratios giving the second best performance, Ball 16, were closer to the aforementioned optimum, the actual difference between Balls 15 and 16 was small (i.e., 1 cm for h and 0.5 cm for r1 ). Comparisons between cube-shaped specimens made from the same material; Balls 20, 21 and 22, showed that the ball with the optimum design ratios, Ball 20, gave the best performance. As can be seen from Table 1, Balls 10 and 11 were the specimens fabricated from ablative resin LY5052. When compared to most other specimens of the same geometry, these showed a low performance. This was believed to indicate that the received heat ¯ux in this application was not high enough to fully exploit the bene®ts of ablation, which is more suited to high heat ¯ux conditions with relatively short contact time10. There was also judged to be a possible future problem from using ablative material, of Trans IChemE, Vol 78, Part A, July 2000
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES interference produced by mixing of pyrolysis products from the resin with gas samples being presented to gas sensors. There would also be a need for replacement of the entire barrier capsule after each use. Hence, the resin was not considered suitable for this ball instrument application. The insulating abilities of the different refractory materials were evaluated using the effective thermal conductivity, keff. Since this effective conductivity was geometry- speci®c in the nature of its de®nition, only the results obtained from the same geometry specimens were comparable, i.e., Balls 1 to 11. These specimens all had water contained in a stainless steel shell to serve as a heat sink at their core. The specimens having the lowest effective thermal conductivity of 0.22 W m ±1 K ±1, Balls 3 and 4, used insulation comprizing plaster reinforced with glass ®bre. This excellent performance was mainly due to having a high moisture content within the structure, originating from the casting process. Unfortunately, an earlier use of excess water had also reduced the strength of the plaster, causing cracks to appear in the original specimens thus fabricated. By reinforcing the plaster insulation with glass ®bre, this cracking was avoided, even after the balls had been tested in the furnace. Balls 2, 7, and 8 were fabricated speci®cally to study the effect of having a very high moisture content in the insulation material. The results showed a slight improvement in the overall performance. Separate tests had shown that insulating casings made from plaster could be reused by re-introducing water into the dried material. Ceramic ®bre insulation has excellent insulating properties, is resistant to chemical attack and has a usable service temperature beyond 13008 C, even in its standard grade. In this application it proved to have good characteristics, as illustrated by Ball 5, which was fabricated by using layers of ceramic ®bre paper. Other refractories such as the Himalayan 2600, VIX and mastic were generally disappointing in this application. Most of the specimens tested, (i.e., Balls 1 to 18 and 23) were charged with a volume occupying 70% of the water capacity provided at the core. A series of separate experiments had been conducted to investigate the effect of boiling the water contained in stainless steel shells of various sizes, without insulation. These experiments showed that the time to evaporate all the water was reduced if the initial water content was too high. This was due to the vigorous boiling action purging out a large quantity of the liquid water from the core. This surging characteristic was investigated, with the result that is shown in Figure 3. From experimental observation and calculation, the surging problem only became signi®cant when operating in conditions towards the right hand side of the dotted line. From Figure 4, it is evident that at heat ¯uxes ranging from 2 to 5 kW m ±2, which are expected in typical ball instrument applications, 70% of the capacity of the core should be the maximum volume of water charged to avoid the surging problem. Localized hot spots were also detected during the tests when the core was almost dried. These were found using irreversible temperature-indicating stickers attached to a dummy circuit board. The corners of this printed circuit board were found to reach a temperature of more than 1308 C, in some of the experiments, 5 minutes before the core was totally dried. To avoid the occurrence of such hot spots on the circuit board, and any spillage of water from the core during Trans IChemE, Vol 78, Part A, July 2000
787
Figure 4. Amount of water loss by surging, against quantity of water.
tumbling of the capsule, it would not be possible to employ a simple outlet hole as the route for steam to escape from the core. An alternative technique to the use of a water-®lled cavity was to make the core from a porous material able to hold water within its structure. This technique was investigated by testing several specimens with ceramic ®bres placed within the core to help to retain the water, these being Balls 20 to 22. The diffusion mechanism and high heat exchange rate between the steam and water within the ®brous structure makes localized hot spots unlikely to occur. It should be noted that ceramic ®bre occupied 30% of the core volume, leaving 70% of the capacity for waterÐ the same as the limit imposed by the surging problems mentioned earlier. An additional bene®t of using this technique is a reduction in the rate of heat transfer to the core thanks to the insulating properties of the added ceramic ®bre. Of all the specimens tested, Ball 20 proved to have the highest relative ef®ciency. It had the optimum design ratios and was made from glass ®bre reinforced plaster, with a ceramic ®bre core holding 70 vol.% of water. CFD MODELLING OF TRANSIENT HEAT FLOW The transient heat ¯ow in the capsule structure was modelled analytically using Laplace Transform Solution of the Fourier’s Law11 and numerically using CFD, Fluent version 4.5212. A plaster insulated Ball Instrument was simulated as a cube, comprizing 26 ´ 26 ´ 26 conducting cells. The central water core was also modelled, as a block of conducting cells with the appropriate heat capacity and a very high value of heat conductivity. The predicted temperature contours of the plaster insulation at the time when the central core reached the water boiling point (1008 C), indicated to be after 42 minutes, are displayed in Figure 5. The external surface was assumed to be at a constant 10008 C and the central core having just reached 1008 C. FURTHER DEVELOPMENT OF THE BALL INSTRUMENT Moving from the relatively well-controlled environment of a test-furnace to the less predictable and more turbulent conditions encountered in most pyro-processes places new demands on the equipment to be employed. The ®rst of these is mechanical, needing a physical capsule to be
CHAN et al.
788
Figure 5. Temperature contours within the plaster insulation after exposure to 10008 C environment for 42 minutes.
provided having an exterior structure capable of resisting mechanical damage from crushing or impact, even in ambient temperatures beyond 10008 C. Inconel 601 was chosen to construct the capsule exterior basket, since it has a background of use in pyro-processes like incineration, where exposure to reducing and oxidizing atmospheres and to acidic products of combustion is routinely experienced13. A sectional arrangement drawing of the ®rst prototype is shown in Figure 6. The use of water as a heat sink had been shown to have signi®cant advantages in the earlier research, but needed to be retained by a porous material to avoid coolant loss during tumbling, whilst allowing freedom for water vapour to escape. Balsa wood was selected for this duty in the initial prototype, with a volume of water comprizing almost 70% of its volume being able to be injected into and retained by its structure, by using a high pressure pump to apply a pressure loading of around 69 bar for several minutes. The main insulation layer in the prototype comprized `Saf®l’ alumina ®bre blanket, in a 25 mm thickness when packed, positioned between an inner cage of expanded metal into which the instrument container could be placed, and the outer Inconel basket. Physical isolation and mechanical resilience was thus provided for the circuit canister.
The Ball Instrument illustrated here was designed primarily for the collection of temperature data using its specially developed data-logger. For application to gas analysis, the sensors to be employed need to be sited where there will be a lower temperature, requiring changes to the capsule’s construction. The intention is to position a chosen part of the mass being utilized as a heat sink at an appropriate radial distance from the core, with layers of insulation employed outside it, and between it and the circuit board itself. The balance of heat sink mass will still be sited at the core, but will not be deliberately allowed to reach such high temperatures as presently experienced. Active pre-cooling of the mass employed at the core might also eventually prove to be necessary to comply with the lower required ambient temperature limits when using gas sensors. The design, optimization, construction and testing of such arrangements form a major part of the ongoing project. CONCLUSION A unique, self-contained data acquisition unit is currently being developed at SUWIC. This will incorporate multiple sensing elements and an associated data-logger, housed in a compact heat-resisting capsule and equipped with an onboard power supply, being thus able to make in situ measurements and store the data for subsequent computer up-loading and analysis. Gypsum plaster, reinforced with glass ®bres, has been demonstrated to be an excellent transient heat barrier for the electronics, with ceramic ®bre materials as an alternative. Water was selected for use as a heat sink material because of its ability to absorb heat at a temperature below the service temperature of the mini data logger. The water is to be held within ceramic ®bres or balsa wood to limit its mobility and the shape or physical orientation of the capsules for application have been optimized. Using the above con®guration and aforementioned materials at optimized geometry, the core had been successfully kept below 1258 C for a time exceeding 90 minutes. This proves the practicality of the concept. Following on from this feasibility study, a fully engineered prototype capsule has been constructed, to address the problems imposed by the physical demands of a pyro-process such as the incineration of solid waste. NOMENCLATURE q k A T x qcy qcu T2 T1 r1 r2 h H tcy
Figure 6. The prototype Ball Instrument.
tcu
heat transfer rate, W thermal conductivity, W heat transfer area, W m ±1 K ±1 temperature, K denotes some local position with respect to x coordinates, W heat transfer rate into a cylinder, W heat transfer rate into a cube, W external surface temperature (furnace temperature), K internal surface temperature of the insulation (core temperature), K radius of the internal can (water core), m radius of the whole cylindrical unit, m height of the internal can (water core), m height of the whole cylindrical unit, m measuring duration/drying time of the central heat sink for a cylinder, s measuring duration/drying time of the central heat sink for a cube, s
Trans IChemE, Vol 78, Part A, July 2000
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES » l x1 x2 A1 tnorm V tnorm(Ball No.1) g
density of the heat sink material (central core), kg m ±3 latent heat of evaporation of the heat sink material (central core), J kg ±1 length of the internal cube (central core), m length of the whole cubic unit, m heat transfer parameter, de®ned as kt/»l, m2K ±1 normalized surviving time, s m ±3 K ±4 total volume of the unit,m3 normalized surviving time for specimen Ball No.1, s m ±3 K ±4 relative ef®ciency,
789
8. Tran, H. K., Rasky, D., 1994, Thermal response and ablation characteristics of light-weight ceramic ablators, J Spacecraft and Rockets, 31: 993±998. 9. Lide, D., 1996, Handbook of Chemistry and Physics, 76th edition, (CRC Press Inc). 10. Kirk-Othmer, 1992, Encyclopaedia of Chemical Technology, Vol. 7, 4th Edition, (John Wiley and Sons). 11. Carslaw, H. and Jaeger, J., 1971, Conduction of Heat in Solids, 2nd edition, (Oxford University Press). 12. Fluent Europe, June 1993, Fluent User’s Guide V4.2. 13. Company Product Handbook of Inco Alloys International, Holmer Road, Hereford HR4 9SL, UK. Tel: +44(0) 1432 382200.
REFERENCES 1. Literature from Datapaq Ltd., Deanland Ho Cowley Road, Cambridge, Cambridgeshire, CB4 4GX. Phone No.: +44 (0) 122-342-3141, 1999, Datapaq Furnace TrackerTM. 2. Literature from Ati Ltd., 1st Floor, 237±239 Oldham Road, Springhead, Oldham OL4 4QR. Phone No.: +44(0)161-624-0200, 1999, Porta Sens II Gas Detector. 3. Nenov, T. G. and Yordanov, S. P., 1996, Ceramic Sensors, (Technomic Publishing Co). 4. WebsiteÐhttp://www.microtherm.uk.com/Microporous insulation 5. Literature from Inco Alloys Ltd., Holmer Road, Hereford HR4 9SL. Phone No.: +44(0)143-238-2200, 1999, Inco Alloys International Product Handbook. 6. Williams, P. T., 1997, Incineration of municipal waste with energy recovery, Annual Short Course on Incineration of Municipal Waste with Energy Recovery, University of Leeds. 7. Holman, J. P., 1992, Heat Transfer, 7th edition, Chapter 1, (McGraw Hill).
Trans IChemE, Vol 78, Part A, July 2000
ACKNOWLEDGEMENT The authors would like to thank the Engineering and Physical Science Research Council (EPSRC) for its ®nancial support for this project and the technical and ®nancial contributions from the industrial collaborators; King®eld Electronics Ltd, UK and ABB Corporate Research, Switzerland.
ADDRESS Correspondence concerning this paper should be addressed to Dr K.-H. Chan, Department of Chemical and Process Engineering, University of Shef®eld, Mappin Street, Shef®eld S1 3JD, UK. This paper was presented at the IChemE’s Research 2000 conference, held at the University of Bath, UK, 6±7 January 2000. The manuscript was received 27 September 1999 and accepted for publication after revision April 2000.
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES K. H. CHAN (ASSOCIATE MEMBER), J. H. GOODFELLOW, Y. R. GOH (ASSOCIATE MEMBER), V. NASSERZADEH (ASSOCIATE MEMBER), J. SWITHENBANK (FELLOW) and D. S. TAYLOR* Shef®eld University Waste Incineration Centre (SUWIC), University of Shef®eld, UK *King®eld Electronics Ltd., Chester®eld Trading Estate, Derbyshire, UK
A
unique self-contained data acquisition unit consisting of multiple sensing elements and recording electronic components installed in a heat resistant capsule is currently being developed at SUWIC. This unit can be introduced into a process system with the raw feed material and experiences the same conditions as the material being processed, as it simultaneously measures the temperature, gas compositions and heat ¯uxes in the system. At the end of the process, the instrument can be recovered, and the data recorded and stored in its memory unit downloaded to a computer. This instrument is applicable to pyro-processes such as incineration, power generation and steel processing, as well as to the food industry. Keywords: combustion; heat protection; in situ measurement; instrumentation; microcomputer
in various forms4. Their microporous structure gives them a thermal conductivity even lower than that of ceramic ®bre, promising to reduce the necessary Ball Instrument capsule size signi®cantly. Nickel/chrome alloys, with proven resistance to corrosion and creep at the temperatures involved, are also commercially available5. Ultimately then, the key to development of a practical Ball Instrument is full exploitation of the physical and thermal characteristics of materials, and determination of the best geometry for them to be applied. This is being achieved by the combined application of mathematical analysis, Computational Fluid Dynamic (CFD) modelling, and practical experimentation.
INTRODUCTION Most processes that use sensors for recording and control have them located in positions ®xed when the plant was built. However, within the lifetime of a plant, there are often occasions when more information and data are needed, to study the operating conditions and dynamics of the process. Measurements of process variables made in close proximity to processed material are also more favoured for detailed research than those made remotely. If there were a device available that could be introduced into an existing process to make such measurements, without modi®cation to the process equipment, it would be of signi®cant value to the engineering community. Such a device is the subject of this research program. It is generically described herein as a `Ball Instrument’, because the original concept had that kind of package in mindÐa compact, rugged device, able to be dropped into, and to survive, the harsh environments found in many pyro-processes. There are already commercial examples of in-process data loggers for temperature recording1, but they are designed for applications with well-managed transport mechanisms. Examples of this are steel billet pusher furnaces, glass lehrs and car-type tunnel kilns. Unfortunately, the design of their thermal protection makes their use inappropriate in more dynamic and unavoidably chaotic pyro-processes. Portable ¯ue gas analysers of compact dimensions are also available2, but they are designed to be used in relatively benign ambient conditions. They also need signi®cant sample conditioning to be applied. Fortunately, compact ceramic sensors for measurement of ¯ue gas species concentration have become available, which may overcome these dif®culties. They are able to detect and measure concentrations of various gases with great selectivity, and are inherently rugged3. Very advanced thermal insulation materials, capable of operation at temperatures up to 12008 C., are also available
GEOMETRICAL DESIGN OF THE INSTRUMENT When in use, the ball instrument is exposed to high temperatures, and for most pyro-processes, this is typically about 10008 C, with heat ¯ux ranging from 2 to 5 kW m ±2. Hence, one of the most important design considerations for this instrument is the heat protection technique applied to protect the electronics at the core of the unit, since it is intended that the device be introduced into a furnace/kiln/ incinerator with the raw feed, be allowed to pass through the system, and be recovered at the furnace exit. The duration of the time the instrument will spend in the process is typically between 40 and 60 minutes6. The size and geometry of the ball instrument must be optimized to fully exploit the thermal properties of the insulating materials employed. To reduce the heating rate to the core of the instrument, the ratio of the exposed surface area per unit volume must be minimized. A sphere is the ideal shape for lowering the rate of heat transfer, but the fabrication of a spherical casing at this early stage of the instrument development is dif®cult and is neither time nor cost effective. Hence, cylindrical and cube shaped capsules were made for theoretical and experimental investigations on geometrical designs of the unit. 783
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Material Selection
Figure 1. Description of the Ball Instrument.
Before a casing of either shape is constructed, the optimum design ratio, which minimizes the rate of heat transfer from the exposed external surface to the internal surface in the core, must be determined. Based on Fourier’s Law of heat conduction7, the one dimensional heat transfer rate at steady state is given in equation (1). ¶T q = kA (1) ê ¶x For a general thermal insulation system, the three dimensional heating rate of the unit q, assuming quasisteady state is given by: qcy = 2kp(T2
+ (hr2
ê ê
T1 ){2r1 r2 (H Hr1 )(r2
h)ê
ê
ê r1 ) ê
1
1
ln[(r2 h)(r1 H)ê 1 ]}
(2)
1
T1 )(x1 x2 )(x2 x1 )ê (3) ê ê where subscripts cy and cu refer to the cylindrical and cube shaped casing respectively, Figure 1. The corresponding life (i.e., the maximum duration which the instrument can be exposed to high temperature) of each unit is given by: qcu = 12k(T2
tcy =
»lr12 (r kr2 2 ê
tcu =
»lx21 (x 12kx2 2 ê
r1 )(T2
T1 )ê ê
x1 )(T2 ê
1
T1 )ê
(4) 1
(5)
Using equations (1) to (4), the optimum design ratios of the cylindrical unit were found to be 1.5, 2 and 2 for r2 /r1 , H/r2 and h/r1 , respectively. The optimum ratio of x2 /x1 for the cube casing was found to be 1.5. The values for r1 and x1 for the optimum ratios were obtained from the following equations: p r1 = 3A1 (T2 T1 ) (6) ê p x1 = 6 A1 (T2 T1 ) (7) ê where, A1 is given by kt/»l. EXPERIMENTAL PROGRAM The main objective of the preliminary tests was to evaluate the best means of protection for the electronics in the core of the instrument. Various heat protection techniques and their associated problems were addressed accordingly. Selection and testing of suitable electronic components and sensors was also carried out. Time dependent calculations on heat conduction of the plaster insulation were carried out using FLUENT code.
In order to achieve the objective with minimum physical size of the Ball Instrument, protective casings made from refractories with low thermal diffusivity, such as ®ne ®nishing plaster, mastics (castable ceramic), VIX (castable refractory), ceramic ®bre blanket, ceramic ®bre paper, Himalayan 2600 (refractory brick), and Cellobond (ceramic-cement composite blocks) were tested. More advanced insulators, such as Microtherm microporous insulation will be tested in due course. Besides the use of insulation, it was believed that the protective casing could also be kept cool by the ablation effect, in which ablative resin decomposes, vaporizes and hence removes heat from the instrument capsule8. Therefore, protective casings made from Resin LY5052 as the ablative cooling medium were also investigated. A protective casing can only minimize the rate of heat transfer, it does not prevent some heat from reaching the core of the instrument. Hence, further means of heat protection for the electronics at the core of the instrument was vital. This secondary method of heat protection was provided in the form of a liquid heat sink at the core of the unit. Water, contained in a stainless steel shell, was used to serve as this heat sink, since it has moderate boiling point, high heat capacity and high latent heat of vaporization9. A selection of the specimens fabricated and the type of heat sink incorporated in each specimen is summarized in Table 1. Each specimen was tested by placing it in a furnace, temperature-controlled at 10008 C, for 40 to 90 minutes. The typical heat ¯ux encountered in this furnace was about 1 kW m ±2. The temperatures of the furnace and the core of the ball were measured using K-type mineral sheathed thermocouples, and continuously logged by an automated data acquisition system. Figure 2 shows typical core and furnace temperatures obtained during the experiments. In the ®rst few minutes, only the insulating casing of the unit was being heated, hence no increase in the core temperature was initially observed. The water in the core was then heated steadily until it reached its boiling point. For a further period of approximately 20 minutes, depending on the type of insulating material used to construct the casing, the absorption of latent heat associated with boiling of the water kept the core temperature constant at 1008 C. Once all the water in the core had been driven off, the core temperature started increasing again. In the example shown in Figure 2, the ball instrument would have been able to operate in a furnace at 10008 C for at least 60 minutes before its electronics became susceptible to thermal damage caused by excessive core temperature. The performance of each specimen was determined, based on the time taken to fully evaporate the water at its core. The average heat transfer rate was derived directly from the latent heat of water and this drying time. By replacing the rate of heat transfer, qcy and qcu in equations (2) and (3), respectively with these calculated values, the effective conductivity, keff of each ball was evaluated. It should be noted that the effective thermal conductivity calculated using equations (2) and (3) differ from the thermal conductivity of the insulation material quoted by the manufacturers. This is because the heat transfer Trans IChemE, Vol 78, Part A, July 2000
Trans IChemE, Vol 78, Part A, July 2000
12 13 14 15 16 17 18 19 20 21 22 23
5 6 7 8 9 10 11
4
1 2 3
No
Cube
Cylindrical
Cylindrical
Shape
Glass Fibre Reinforcement ±
Plaster
Himalayan 2600
±
±
± Extra moisture ± Glass Fibre Reinforcement ± ± Extra moisture Extra moisture ± ± Ceramic Fibre Reinforcement ± ±
Additional
±
VIX
Cellobond VIX_CFB
Resin LY5052
Ceramic Fibre Disc Himalayan 2600 Mastics
CF Paper Roll
Plaster
VIX
Refractory
Insulation
70 vol% of water in a S.S. shell
70 vol% wet ceramic ®bre module
Fress plaster + water
70 vol% of water in a stainless steel (S.S.) shell
70 vol% of water in a stainless steel (S.S.) shell
Central core
3
1630 1840 1470 1700 1700 250 550 790 1700 1700 1700 620
1700 1700 1700 1700 1700 1700 1700
1700
1700 1700 1700
cm
V
18.8 15.6 13.0 15.0 15.0 9.0 11.0 10.0 12.0 12.0 12.0 11.4
15.0 15.0 15.0 15.0 15.0 15.0 15.0
15.0
15.0 15.0 15.0
cm
H
13.4 11.0 10.0 10.0 11.0 ± ± ± 8.0 7.0 6.0 9.4
9.0 9.0 9.0 9.0 9.0 9.0 9.0
9.0
9.0 9.0 9.0
cm
h
Table 1. Information on specimens tested for ef®ciency evaluation.
10.5 6.05 6.0 6.0 6.0 3.0 4.0 10.0 12.0 12.0 12.0 3.7
6.0 6.0 6.0 6.0 6.0 6.0 6.0
6.0
6.0 6.0 6.0
cm
X2 or r2
5.1 3.25 5.0 3.5 4.0 ± ± ± 8.0 7.0 6.0 2.7
3.0 3.0 3.0 3.0 3.0 3.0 3.0
3.0
3.0 3.0 3.0
cm
X1 or r1
2.0 1.9 1.2 1.7 1.5 ± ± ± 1.5 1.7 2.0 1.4
2.0 2.0 2.0 2.0 2.0 2.0 2.0
2.0
2.0 2.0 2.0
1.8 2.6 2.2 2.5 2.5 3.0 2.8 1.0 1.0 1.0 1.0 3.1
2.5 2.5 2.5 2.5 2.5 2.5 2.5
2.5
2.5 2.5 2.5
X2 /X1 H/X2 or r2 /r1 or H/r2
0.22 ± 0.26 0.26 0.26 ± ± ± 0.48 0.48 0.48 0.40
0.07 0.07 0.07 0.40 0.08 ± ±
0.48
0.26 0.26 0.48
ktrue @258 C
0.33 0.25 0.46 0.40 0.43 ± ± ± 0.09 0.12 0.08 0.37
0.22 0.24 0.29 0.26 0.38 0.33 0.63 0.29
0.41 0.31 0.23
keff
1.31 1.68 0.75 1.17 1.10 0.93 0.76 1.89 1.93 1.76 1.40 1.40
1.89 1.73 1.45 1.57 1.11 1.25 0.79 1.62
1.00 1.32 1.82
g
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES 785
CHAN et al.
786
Figure 2. Typical temperature history of the furnace and the central core of the Ball Instrument, Ball No. 22.
phenomena encountered during the experiment occurred with the system in unsteady state. However, these simple equations are suf®cient for evaluating the casing design and the geometry’s in¯uence on the rate of heat transfer, at this stage of the instrument’s development. To compare the performance between the specimens tested, a variable called relative ef®ciency was formulated. The performance of each specimen was evaluated from the time taken for the water contained in the core of the specimen to evaporate during the experiment, normalized to the temperature of the furnace and the total volume of the specimen. Comparisons between specimens could therefore be made, based on their performance relative to the performance of Ball 1. Hence, the relative ef®ciency for each specimen can be given by: 1 Relative efficiency, g = (tnorm )(tnorm )ê(Ball No.1)
=
tdry T24 V
tdry T24 V
1 ê
(7) (Ball No.1)
The effective conductivity and relative ef®ciency of each specimen is also listed in Table 1. Programmable Data Acquisition Controller The device to be used for in situ acquisition and logging of temperature data is a CMOS microcontroller, selected and programmed by King®eld Electronics Ltd., our industrial partner in this aspect of the research. It incorporates analog to digital converters and a serial interface, and has very low power requirements. It is capable of operating over a remarkably wide temperature range, with an upper limit of 1258 C, making it ideally suited to use in this application. By encapsulating the circuit board and its components in a water-resistant epoxy-resin based compound after manufacture, the controller is able to be fully immersed in water, without risk of short-circuits. This data-logger’s ®rmware program monitors the pins on its sensor/data socket, and is able to detect whether it is required to begin logging, or needs to monitor the serial interface for incoming commands. The software, which effectively de®nes the data-logger’s function, can be changed during manufacture. Its activities include the scaling and conversion to engineering units, of data from the selected sensors. These include thermocouples, cells
Figure 3. Functional testing of the thermocouple connected to the Programmable Data Acquisition Controller (PDAC).
able to detect and measure gas species, and sensors to measure incident radiation. The program also allows the data-logger’s sampling intervals to be input, and for data to be collected from its memory, by issuing commands from a computer equipped with a serial interface. Preliminary testing of the data logger (temperature logging only) showed that the electronic components could operate satisfactorily between the freezing point and boiling point of water, Figure 3. The mini data logger was boiled in water and subsequently placed in melting ice while it was still taking external (thermocouple) and internal (surface of the circuit board) temperature measurements. Despite slight discrepancies in the external temperature measurement compared with an independent thermometer (which was recti®ed in later versions), the data-logger proved itself capable of operation while situated in an ambient temperature ranging from 0 to 1008 C. This showed the feasibility of using both the latent heat of fusion and of evaporation of ice/water at the central core, should that prove necessary. DISCUSSION Comparisons between the relative ef®ciencies of balls 1, 14, 15 and 16, all of which were fabricated using VIX insulation with 70% of the core volume ®lled with water, showed that the best performance was given by Ball 15 which had design ratios of 1.7, 2.5 and 2.9 for r2 /r1 , H/r2 and h/r1 , respectively. Although it may appear that the design ratios giving the second best performance, Ball 16, were closer to the aforementioned optimum, the actual difference between Balls 15 and 16 was small (i.e., 1 cm for h and 0.5 cm for r1 ). Comparisons between cube-shaped specimens made from the same material; Balls 20, 21 and 22, showed that the ball with the optimum design ratios, Ball 20, gave the best performance. As can be seen from Table 1, Balls 10 and 11 were the specimens fabricated from ablative resin LY5052. When compared to most other specimens of the same geometry, these showed a low performance. This was believed to indicate that the received heat ¯ux in this application was not high enough to fully exploit the bene®ts of ablation, which is more suited to high heat ¯ux conditions with relatively short contact time10. There was also judged to be a possible future problem from using ablative material, of Trans IChemE, Vol 78, Part A, July 2000
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES interference produced by mixing of pyrolysis products from the resin with gas samples being presented to gas sensors. There would also be a need for replacement of the entire barrier capsule after each use. Hence, the resin was not considered suitable for this ball instrument application. The insulating abilities of the different refractory materials were evaluated using the effective thermal conductivity, keff. Since this effective conductivity was geometry- speci®c in the nature of its de®nition, only the results obtained from the same geometry specimens were comparable, i.e., Balls 1 to 11. These specimens all had water contained in a stainless steel shell to serve as a heat sink at their core. The specimens having the lowest effective thermal conductivity of 0.22 W m ±1 K ±1, Balls 3 and 4, used insulation comprizing plaster reinforced with glass ®bre. This excellent performance was mainly due to having a high moisture content within the structure, originating from the casting process. Unfortunately, an earlier use of excess water had also reduced the strength of the plaster, causing cracks to appear in the original specimens thus fabricated. By reinforcing the plaster insulation with glass ®bre, this cracking was avoided, even after the balls had been tested in the furnace. Balls 2, 7, and 8 were fabricated speci®cally to study the effect of having a very high moisture content in the insulation material. The results showed a slight improvement in the overall performance. Separate tests had shown that insulating casings made from plaster could be reused by re-introducing water into the dried material. Ceramic ®bre insulation has excellent insulating properties, is resistant to chemical attack and has a usable service temperature beyond 13008 C, even in its standard grade. In this application it proved to have good characteristics, as illustrated by Ball 5, which was fabricated by using layers of ceramic ®bre paper. Other refractories such as the Himalayan 2600, VIX and mastic were generally disappointing in this application. Most of the specimens tested, (i.e., Balls 1 to 18 and 23) were charged with a volume occupying 70% of the water capacity provided at the core. A series of separate experiments had been conducted to investigate the effect of boiling the water contained in stainless steel shells of various sizes, without insulation. These experiments showed that the time to evaporate all the water was reduced if the initial water content was too high. This was due to the vigorous boiling action purging out a large quantity of the liquid water from the core. This surging characteristic was investigated, with the result that is shown in Figure 3. From experimental observation and calculation, the surging problem only became signi®cant when operating in conditions towards the right hand side of the dotted line. From Figure 4, it is evident that at heat ¯uxes ranging from 2 to 5 kW m ±2, which are expected in typical ball instrument applications, 70% of the capacity of the core should be the maximum volume of water charged to avoid the surging problem. Localized hot spots were also detected during the tests when the core was almost dried. These were found using irreversible temperature-indicating stickers attached to a dummy circuit board. The corners of this printed circuit board were found to reach a temperature of more than 1308 C, in some of the experiments, 5 minutes before the core was totally dried. To avoid the occurrence of such hot spots on the circuit board, and any spillage of water from the core during Trans IChemE, Vol 78, Part A, July 2000
787
Figure 4. Amount of water loss by surging, against quantity of water.
tumbling of the capsule, it would not be possible to employ a simple outlet hole as the route for steam to escape from the core. An alternative technique to the use of a water-®lled cavity was to make the core from a porous material able to hold water within its structure. This technique was investigated by testing several specimens with ceramic ®bres placed within the core to help to retain the water, these being Balls 20 to 22. The diffusion mechanism and high heat exchange rate between the steam and water within the ®brous structure makes localized hot spots unlikely to occur. It should be noted that ceramic ®bre occupied 30% of the core volume, leaving 70% of the capacity for waterÐ the same as the limit imposed by the surging problems mentioned earlier. An additional bene®t of using this technique is a reduction in the rate of heat transfer to the core thanks to the insulating properties of the added ceramic ®bre. Of all the specimens tested, Ball 20 proved to have the highest relative ef®ciency. It had the optimum design ratios and was made from glass ®bre reinforced plaster, with a ceramic ®bre core holding 70 vol.% of water. CFD MODELLING OF TRANSIENT HEAT FLOW The transient heat ¯ow in the capsule structure was modelled analytically using Laplace Transform Solution of the Fourier’s Law11 and numerically using CFD, Fluent version 4.5212. A plaster insulated Ball Instrument was simulated as a cube, comprizing 26 ´ 26 ´ 26 conducting cells. The central water core was also modelled, as a block of conducting cells with the appropriate heat capacity and a very high value of heat conductivity. The predicted temperature contours of the plaster insulation at the time when the central core reached the water boiling point (1008 C), indicated to be after 42 minutes, are displayed in Figure 5. The external surface was assumed to be at a constant 10008 C and the central core having just reached 1008 C. FURTHER DEVELOPMENT OF THE BALL INSTRUMENT Moving from the relatively well-controlled environment of a test-furnace to the less predictable and more turbulent conditions encountered in most pyro-processes places new demands on the equipment to be employed. The ®rst of these is mechanical, needing a physical capsule to be
CHAN et al.
788
Figure 5. Temperature contours within the plaster insulation after exposure to 10008 C environment for 42 minutes.
provided having an exterior structure capable of resisting mechanical damage from crushing or impact, even in ambient temperatures beyond 10008 C. Inconel 601 was chosen to construct the capsule exterior basket, since it has a background of use in pyro-processes like incineration, where exposure to reducing and oxidizing atmospheres and to acidic products of combustion is routinely experienced13. A sectional arrangement drawing of the ®rst prototype is shown in Figure 6. The use of water as a heat sink had been shown to have signi®cant advantages in the earlier research, but needed to be retained by a porous material to avoid coolant loss during tumbling, whilst allowing freedom for water vapour to escape. Balsa wood was selected for this duty in the initial prototype, with a volume of water comprizing almost 70% of its volume being able to be injected into and retained by its structure, by using a high pressure pump to apply a pressure loading of around 69 bar for several minutes. The main insulation layer in the prototype comprized `Saf®l’ alumina ®bre blanket, in a 25 mm thickness when packed, positioned between an inner cage of expanded metal into which the instrument container could be placed, and the outer Inconel basket. Physical isolation and mechanical resilience was thus provided for the circuit canister.
The Ball Instrument illustrated here was designed primarily for the collection of temperature data using its specially developed data-logger. For application to gas analysis, the sensors to be employed need to be sited where there will be a lower temperature, requiring changes to the capsule’s construction. The intention is to position a chosen part of the mass being utilized as a heat sink at an appropriate radial distance from the core, with layers of insulation employed outside it, and between it and the circuit board itself. The balance of heat sink mass will still be sited at the core, but will not be deliberately allowed to reach such high temperatures as presently experienced. Active pre-cooling of the mass employed at the core might also eventually prove to be necessary to comply with the lower required ambient temperature limits when using gas sensors. The design, optimization, construction and testing of such arrangements form a major part of the ongoing project. CONCLUSION A unique, self-contained data acquisition unit is currently being developed at SUWIC. This will incorporate multiple sensing elements and an associated data-logger, housed in a compact heat-resisting capsule and equipped with an onboard power supply, being thus able to make in situ measurements and store the data for subsequent computer up-loading and analysis. Gypsum plaster, reinforced with glass ®bres, has been demonstrated to be an excellent transient heat barrier for the electronics, with ceramic ®bre materials as an alternative. Water was selected for use as a heat sink material because of its ability to absorb heat at a temperature below the service temperature of the mini data logger. The water is to be held within ceramic ®bres or balsa wood to limit its mobility and the shape or physical orientation of the capsules for application have been optimized. Using the above con®guration and aforementioned materials at optimized geometry, the core had been successfully kept below 1258 C for a time exceeding 90 minutes. This proves the practicality of the concept. Following on from this feasibility study, a fully engineered prototype capsule has been constructed, to address the problems imposed by the physical demands of a pyro-process such as the incineration of solid waste. NOMENCLATURE q k A T x qcy qcu T2 T1 r1 r2 h H tcy
Figure 6. The prototype Ball Instrument.
tcu
heat transfer rate, W thermal conductivity, W heat transfer area, W m ±1 K ±1 temperature, K denotes some local position with respect to x coordinates, W heat transfer rate into a cylinder, W heat transfer rate into a cube, W external surface temperature (furnace temperature), K internal surface temperature of the insulation (core temperature), K radius of the internal can (water core), m radius of the whole cylindrical unit, m height of the internal can (water core), m height of the whole cylindrical unit, m measuring duration/drying time of the central heat sink for a cylinder, s measuring duration/drying time of the central heat sink for a cube, s
Trans IChemE, Vol 78, Part A, July 2000
A NOVEL MEASURING INSTRUMENT FOR PYRO-PROCESSES » l x1 x2 A1 tnorm V tnorm(Ball No.1) g
density of the heat sink material (central core), kg m ±3 latent heat of evaporation of the heat sink material (central core), J kg ±1 length of the internal cube (central core), m length of the whole cubic unit, m heat transfer parameter, de®ned as kt/»l, m2K ±1 normalized surviving time, s m ±3 K ±4 total volume of the unit,m3 normalized surviving time for specimen Ball No.1, s m ±3 K ±4 relative ef®ciency,
789
8. Tran, H. K., Rasky, D., 1994, Thermal response and ablation characteristics of light-weight ceramic ablators, J Spacecraft and Rockets, 31: 993±998. 9. Lide, D., 1996, Handbook of Chemistry and Physics, 76th edition, (CRC Press Inc). 10. Kirk-Othmer, 1992, Encyclopaedia of Chemical Technology, Vol. 7, 4th Edition, (John Wiley and Sons). 11. Carslaw, H. and Jaeger, J., 1971, Conduction of Heat in Solids, 2nd edition, (Oxford University Press). 12. Fluent Europe, June 1993, Fluent User’s Guide V4.2. 13. Company Product Handbook of Inco Alloys International, Holmer Road, Hereford HR4 9SL, UK. Tel: +44(0) 1432 382200.
REFERENCES 1. Literature from Datapaq Ltd., Deanland Ho Cowley Road, Cambridge, Cambridgeshire, CB4 4GX. Phone No.: +44 (0) 122-342-3141, 1999, Datapaq Furnace TrackerTM. 2. Literature from Ati Ltd., 1st Floor, 237±239 Oldham Road, Springhead, Oldham OL4 4QR. Phone No.: +44(0)161-624-0200, 1999, Porta Sens II Gas Detector. 3. Nenov, T. G. and Yordanov, S. P., 1996, Ceramic Sensors, (Technomic Publishing Co). 4. WebsiteÐhttp://www.microtherm.uk.com/Microporous insulation 5. Literature from Inco Alloys Ltd., Holmer Road, Hereford HR4 9SL. Phone No.: +44(0)143-238-2200, 1999, Inco Alloys International Product Handbook. 6. Williams, P. T., 1997, Incineration of municipal waste with energy recovery, Annual Short Course on Incineration of Municipal Waste with Energy Recovery, University of Leeds. 7. Holman, J. P., 1992, Heat Transfer, 7th edition, Chapter 1, (McGraw Hill).
Trans IChemE, Vol 78, Part A, July 2000
ACKNOWLEDGEMENT The authors would like to thank the Engineering and Physical Science Research Council (EPSRC) for its ®nancial support for this project and the technical and ®nancial contributions from the industrial collaborators; King®eld Electronics Ltd, UK and ABB Corporate Research, Switzerland.
ADDRESS Correspondence concerning this paper should be addressed to Dr K.-H. Chan, Department of Chemical and Process Engineering, University of Shef®eld, Mappin Street, Shef®eld S1 3JD, UK. This paper was presented at the IChemE’s Research 2000 conference, held at the University of Bath, UK, 6±7 January 2000. The manuscript was received 27 September 1999 and accepted for publication after revision April 2000.