- Email: [email protected]
Flow rate measurements of a fibrous material using a pressure drop technique
Flow Measurement and Instrumentation 11 (2000) 177–183 www.elsevier.com/locate/flowmeasinst
Flow rate measurements of a fibrous material using a pressure drop technique Srikanth Venkatasubramanian a, George E. Klinzing b
a,*
, Brian Ence
b
a Chemical/Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, PA 15260, USA CertainTeed Corporation, Insulation Group, 1400 Union Meeting Road, PO Box 1100, Blue Bell, PA 19422-0761, USA
Received 27 August 1999; accepted 2 September 1999
Abstract The measurement of flow rates of solids in gas–solid flows is a challenging task. Fibrous materials are known to change in character as they flow and, as such, can compound the difficulty of measurement of such flow rates over that of powders and granular materials in the transport condition. The principle of measuring the pressure loss experienced by a gas–solid flow in a straight section of piping was used here to test a metering device for fibrous solid flow which has been shown to work for a wide range of powders and granular materials in our laboratories. The flow rates obtained for the fibrous material had a high degree of reproducibility suggesting the possibility of widespread industrial application of the pressure drop flow-metering device. 2000 Published by Elsevier Science Ltd. Keywords: Flow rate; Fibrous; Pressure drop
1. Introduction Reliable metering of solids is of paramount importance in industry for effective operation and control of gas–solid transport systems. An abundance of techniques and devices such as turbine flow meters, Coriolis flow meters, Laser Doppler velocimetry, acoustic signal cross-correlators, dielectric meters, heat addition measurements, and vibration flow meters have been developed and tested in research laboratories for solids flow metering in pneumatic transport systems. However, most of these techniques are impractical in an industrial scenario because of their cost, intrusive nature, or effectiveness and as such pose severe limitations from a practical or operational viewpoint. Therefore, there is a great need for reliable, on-line, continuous, and non-intrusive measurement of solids mass flow rate in industrial operations. The specific problem examined here is the metering of a fiberglass blowing operation of CertainTeed Inc. This problem is studied with a view to developing a
* Corresponding author. Tel.: +1-412-624-9630; fax: +1-412-6249639. E-mail address: [email protected] (G.E. Klinzing). 0955-5986/00/$ - see front matter 2000 Published by Elsevier Science Ltd. PII: S 0 9 5 5 - 5 9 8 6 ( 0 0 ) 0 0 0 1 7 - 0
strategy for metering the amount of material delivered to the customer site from the insulation delivery system (Volu-matic) as opposed to the amount fed to the delivery system. Pneumatic conveying characteristics are usually represented as a plot of pressure drop per unit length versus superficial gas velocity at specific solid flow rates in a Zenz-type state diagram [1].
2. Measurement principle In 1924, Gasterstadt [2] showed that there is a linear relationship between the ratio of pressure drop per unit length of pipe with suspension flow to the pressure drop per unit length with the gas alone and the solids loading ratio when plotted at constant gas velocity. The development of this equation is as follows. The total pressure drop (⌬Pt) for fully developed flow in a straight section can be given as the sum of the pressure drop of the gas alone (⌬Pg) and an additional pressure drop due to solids (⌬Ps). Therefore the pressure drop per unit length can be determined by dividing the pressure drop by the length and expressed as:
178
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
Nomenclature D dp fg fs L ⌬Pg ⌬Ps ⌬Pt Re Rep Ug Up Wg Ws
diameter diameter of particle gas friction factor solids friction factor length pressure drop of gas alone solids pressure drop total pressure drop Reynolds number of the gas Reynolds number of the particle gas velocity particle velocity gas flow rate solids flow rate
Greek a e m rg rp
pressure drop ratio voidage solids loading ratio gas density density of the particles
⌬Pt ⌬Pg ⌬Ps ⫽ ⫹ L L L
(1)
The pressure drop due to the gas flow in a pipe of length L, diameter D, and with an average velocity Ug can be given as: ⌬Pg ergU2g ⫽fg L 2D
(2)
Similarly, the relationship for the extra pressure drop due to the solids in a fully accelerated suspension flow can given as: ⌬Ps (1−e)rsU2p ⫽fs L 2D
(3)
The voidage e can be expressed as a function of solids loading ratio (m) as: e⫽1⫺
冉 冊冉 冊
r g Ug 4Ws ⫽1⫺ m 2 pD rsUp r s Up
(4)
Substituting these relationships into the equation for pressure drop per unit length, the relationship for specific pressure drop ratio a can be given as:
冉 冊 冉 冊 冉 冊冉 冊冉 冊
a⫽
⌬Pt/L ⌬Ps/L ⫽1⫹ ⫽1 ⌬Pg/L ⌬Pg/L
⫹
fs Up rsUp m fg Ug rsUp−rgUgm
(5)
For dilute phase gas–solid mixtures, rsUpÀrgUgm can be assumed, therefore the relationship for specific pressure drop ratios can be given as: a⫽1⫹
冉 冊冉 冊冉 冊
fs Rep D m fg Re dp
(6)
Combining the constants, the Gasterstadt relationship can be simply given as: a⫽1⫹Km
(7)
The parameter K can be empirically determined because of the number of factors that it depends on: material characteristics (particle size distribution, mean particle size, pipe characteristics, particle density), pipe characteristics (inside diameter, internal roughness), configuration (horizontal or vertical), and conveying gas properties (velocity, density, viscosity). Gasterstadt [2] performed experiments with different types of wheat in a 90 mm I.D. pipe and with air velocities from 12 to 28 m s⫺1 and found the above relationship to be valid and K dependent on gas velocities. He found that for gas velocities above 20 m s⫺1, the parameter K became independent of gas velocity. In the case of vertical flow the expression is similar but includes an additional term due to gravitational effects. Cabrejos and Klinzing [3] investigated this relationship proposed by Gasterstadt for a dilute phase test system for granular materials. They compared data from their experiments with those from other researchers re-
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
plotted in this format to reveal the same linear relationship between a and m. Some of the data which exhibited this relationship include those from Hinkle [4] and Rizk [5]. Experimental values of K for horizontal flow were found to be in the range 0.7364 to 0.7718 while those for vertical flow were typically higher. Based on this analysis, it can be concluded that for gas velocities significantly above saltation (typically 50%), K becomes independent of gas velocity, implying that very few experiments are required to determine K for a particular system. Thereby, knowing K, the mass flow rate of solids being conveyed can be calculated from:
冉 冊冉
Ws⫽mWg⫽
冊
a−1 pD2Ugrg K 4
(8)
In this work, this relationship will be used to experimentally verify the application to the flow of fibrous material as a solids flow meter.
3. Experimental 3.1. Test rig layout and equipment description A 0.0472 m (12 in) Volu-matic feeder provided by CertainTeed was used for the insulation system delivery. This system is provided with a Gardner Denver blower, a helical screw for moving the fragmented bales of insulation, and a blow-through rotary valve-type arrangement which also serves to chop the fragments into fine fiber aggregates. The conveying line consists of a flexible hose section of length 30 m (100 ft) and diameter 0.0118 m (3 in) attached to a spool piece of flexible hose of length about 0.0945 m (2 ft) placed at the outlet of the rotary valve arrangement. Control of the airflow can be achieved by placing different amounts of weights on the relief valve at the blower outlet. Solids flow rate can be controlled by changing the size of the inlet opening to the rotary valve-type arrangement and also through a gear speed lever that controls the speed of the rotary valve arrangement. The air–solid separation at the other end of the conveying line was achieved by the use of a special in-line filter assembly. Care was taken to ensure that there are no leaks in the end section of the conveying line, especially in the vicinity of the filter, to avoid dusty conditions in the laboratory. 3.2. Pressure and weight measurements For developing the pressure drop-based flow meter for the application, three kinds of sensor measurements are required: differential pressure for obtaining specific pressure drops, gas velocity, and measured values (instantaneous) of solids flow rate since the
179
chopping/conveying process can produce unsteady twophase flows. The solids throughput can be obtained from the rate of change of weight of the filter bag assembly. Fig. 1 shows a typical configuration of the test rig for pressure drop flowmeter testing in the horizontal configuration. Differential pressures are recorded at locations indicated in Fig. 1 during the test runs. While static pressure measured depends on the transducer location with respect to the collector and feeder and the radial position, differential signals depend on the spacing between the taps. Pressure measurements were made using Omega Pressure transducers (silicon diaphragm type with 8 V DC excitation; Model 162PC01D, 6.87 kPa (1 psi) differential). These were chosen because of their superior performance in previously conducted tests at our laboratories, low hysteresis, high degree of linearity, and the convenience of a high-level voltage output signal (1–6 V) that can be fed directly to a data acquisition system. Pressure taps are located at the top of the pipe as placement at the bottom will corrupt the signal below saltation. Male Swagelock adapters mounted on the top of the pipe were used to connect 0.00098 m (1/4 in) PVC tubing to the transducers. The length of the connecting tubing for each transducer was kept relatively uniform to ensure the same extent of damping in all pressure measurements. A piece of filter cloth material was placed over the Swagelock fitting serving as a filter to eliminate the build up of fiberglass in the pressure tap and associated tubing. This action yielded consistent pressure transducer signals. The weighing apparatus for obtaining the solids flow rate is detailed schematically in Fig. 1. The assembly consists of an electronic platform scale (Ohaus Champ Multi-functional bench scale 1–45.4 kg (100 lb) range) equipped with a desk-mounted LCD-indicator unit. Apart from possessing excellent resolution for the range of conditions used (0.00908 kg (0.02 lb)), the scale has overload protection features and an adjustable averaging level. The scale was also equipped with a standard RS232C bidirectional interface which can be used for computerized data collection. This modular arrangement permitted easy enhancements or modifications under various operational conditions. Gas velocities were measured using a hot wire anemometer. Even at the maximum available solids throughput with the minimum gas velocity available on this machine, the solids flow was still found to be completely suspended with no saltation or sliding layer. The gas velocity measurement was also found to be fairly independent of the feeder opening settings as long as there was sufficient head of material above the inlet opening. 3.3. Data acquisition and analysis All the voltages generated from the pressure transducers as well as the weighing assembly were fed to a
180
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
Fig. 1. Schematic of test rig for metering fiberglass flow.
Pentium 90 MHz PC fitted with a ATMIO-16 Plug and Play Data Acquisition board from National Instruments. Lab Windows v4.0 for Windows 95, a Visual C/C++ compatible rapid application development (RAD) tool from National Instruments, along with its specialized Clanguage libraries, was used to create the user interface and perform the signal analysis. The pressure signals were suitably scaled and preprocessed to zero baseline errors using software methods. Typically pressure signals are sampled at 512 Hz (1024 samples) and weigh assembly readings were taken every 5 seconds.
vs. time trace to obtain steady mass flow rate data. Therefore, it was possible to obtain multiple steady values of the solids flow rates during a single run. To establish the reproducibility of the pressure transducer measurements, the transducers were interchanged between sections. It was found that the pressure drop readings for each section remained the same indicating that the transducers were consistent in their readings.
4. Results and discussion
3.4. Test methodology Since the proposed meter will be used on larger-scale systems with higher solids throughput, a representative subset of the higher solids throughput range on the settings for the speed control lever and the feeder scale were used. Multiple runs were performed for several of the machine settings. The runs were found to have a reproducibility of ±20%. To ensure accuracy of the recorded weights, it became necessary to shake the filter bag periodically to compact and redistribute the solids collected. During this period the weight trace was unstable. This portion of the trace was not used to determine the mass flow rates. Straight lines were fitted with a 90% correlation or greater to sections of the weight
The results of tests are summarized in Figs. 2 and 3 for two different types of insulation material for all three sections of the horizontal piping arrangement tested. It is evident from these plots that the K values are not significantly different under corresponding machine settings for the two different materials. The correlation and hence the prediction of mass flow rate using this metering technique are dependent on the position of the hose. If we assume that under the field conditions the hose is emptied completely at the end of the run, it is reasonable to expect that the best determination of the flow rate will be obtained from a set of taps located reasonably close to the feed point. The observed difference in pressure drop readings for different positions could be explained
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
181
Fig. 2. Pressure drop flowmeter data for InsulSafe III insulation material (alpha1, alpha2 and alpha3 correspond to data from differential pressure transducers P1, P2, and P3 placed at increasing distances from the feed point as shown in Fig. 1.
Fig. 3. Pressure drop flowmeter data for Optima fiberglass insulation material (alpha1, alpha2, and alpha3 correspond to data from differential pressure transducers P1, P2, and P3 placed at increasing distances from the feed point as shown in Fig. 1.
182
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
as follows. The effect of the bend prior to the straight section might have contributed to the overall pressure loss for the section. In order to have the pressure recovery from a bend a sufficient distance from the bend is required. The tests performed could have had the effect of the bend present in their readings. It should also be noted that the ratios of the pressure drops, the specific pressure drop, given by the alpha value, would have the bend effect present for the gas alone flow as well as the fiberglass and gas together which would tend to mask the bend effect. Another reason for the difference between the pressure drop readings in the straight section could be that the fiberglass itself was changing character as it is being conveyed through the corrugated hose and traversing different bend arrangements. The fiberglass leaves the feeder in a state that consists of clumps or aggregates of the material. CertainTeed had noted that the corrugated tubing does affect the condition of the material with a certain length of tubing being required before the material reaches its maximum insulating quality. The flow process is probably making the aggregates less compact having a higher voidage. The last section of piping yielded results that had the lowest correlation between the loading and specific pressure drop. This section contained material that had traveled the farthest and gone through two bend arrange-
Fig. 4.
ments. This material could be the least compacted in the flow process. Figs. 4 and 5 presents the horizontal section P1 which is closest to the feed point, giving the values of the specific pressure drop ratio at given loading conditions for the two types of insulating materials tested. Measuring the flow rate at the beginning of the piping gives the user ease in obtaining a reliable flow rate in a section of the piping that can be specified as being both horizontal and straight and not under the influence of a bend. The material in this section most probably is in the state of clumps of fiberglass. Eq. (8) is the working expression for the calculations of the solids flow rate. The value of alpha is determined, then the specific pressure drop, and then from the figures presented the value of the loading can be found. Eq. (8) can then be used to calculate the solids flow rate. In order to improve the stability in the measured flow rates of solids it is suggested that the tests be conducted over longer periods of time (around 30 min). Short times for the test seem to introduce significant variations in the weight of the material collected and thus the flow rates measured. Testing with the solids flow meter on plastic pellets had been carried out at an industrial site for long time-periods (30 min or more) with improved stability in the flow rates measured.
Recommended pressure drop flowmeter correlation data (transducer P1 data) for InsulSafe III fiberglass insulation material.
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
Fig. 5.
183
Recommended pressure drop flowmeter correlation data (transducer P1 data) for Optima fiberglass insulation material.
5. Conclusions
References
The conveying of fibrous materials has some unique challenges especially in their ability to change character as they are conveyed through various geometries. This work has shown how this changing character is expressed by the pressure drop measurements made at various sections of the conveying line. By using the initial section of the line after the feeder, the specific pressure drop measurements can be used to obtain a value of the solids flow rate of the fibrous material. It has been our experience that testing at an industrial site for over 30 min improves the stability of the results obtained.
[1] F. Zenz, D.F. Othmer, Fluidization and Fluid–Particle Systems, Chapman and Hall, London, 1960, 314pp. [2] J. Gasterstadt, Die experimentelle Untersuchung des pneumatischen Fordervorganges, V.D.I. Zeitschrift 68 (24) (1924) 617– 624. [3] F.J. Cabrejos, G.E. Klinzing, Novel solids pressure drop flow meter. Paper presented at the ASME Winter Annual Meeting, 1992. [4] B.L. Hinkle, Acceleration of particles and pressure drop encountered in horizontal pneumatic conveying. Unpublished PhD dissertation, Georgia Institute of Technology, 1953. [5] F. Rizk, Pneumatic transport of plastic granules in horizontal ducts—the simultaneous effects of gravity, pipe material, and solid properties, particularly in the optimal operating range. DoktorIngeneiurs Dissertation, University of Karlsruhe, 1973.
Flow rate measurements of a fibrous material using a pressure drop technique Srikanth Venkatasubramanian a, George E. Klinzing b
a,*
, Brian Ence
b
a Chemical/Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, PA 15260, USA CertainTeed Corporation, Insulation Group, 1400 Union Meeting Road, PO Box 1100, Blue Bell, PA 19422-0761, USA
Received 27 August 1999; accepted 2 September 1999
Abstract The measurement of flow rates of solids in gas–solid flows is a challenging task. Fibrous materials are known to change in character as they flow and, as such, can compound the difficulty of measurement of such flow rates over that of powders and granular materials in the transport condition. The principle of measuring the pressure loss experienced by a gas–solid flow in a straight section of piping was used here to test a metering device for fibrous solid flow which has been shown to work for a wide range of powders and granular materials in our laboratories. The flow rates obtained for the fibrous material had a high degree of reproducibility suggesting the possibility of widespread industrial application of the pressure drop flow-metering device. 2000 Published by Elsevier Science Ltd. Keywords: Flow rate; Fibrous; Pressure drop
1. Introduction Reliable metering of solids is of paramount importance in industry for effective operation and control of gas–solid transport systems. An abundance of techniques and devices such as turbine flow meters, Coriolis flow meters, Laser Doppler velocimetry, acoustic signal cross-correlators, dielectric meters, heat addition measurements, and vibration flow meters have been developed and tested in research laboratories for solids flow metering in pneumatic transport systems. However, most of these techniques are impractical in an industrial scenario because of their cost, intrusive nature, or effectiveness and as such pose severe limitations from a practical or operational viewpoint. Therefore, there is a great need for reliable, on-line, continuous, and non-intrusive measurement of solids mass flow rate in industrial operations. The specific problem examined here is the metering of a fiberglass blowing operation of CertainTeed Inc. This problem is studied with a view to developing a
* Corresponding author. Tel.: +1-412-624-9630; fax: +1-412-6249639. E-mail address: [email protected] (G.E. Klinzing). 0955-5986/00/$ - see front matter 2000 Published by Elsevier Science Ltd. PII: S 0 9 5 5 - 5 9 8 6 ( 0 0 ) 0 0 0 1 7 - 0
strategy for metering the amount of material delivered to the customer site from the insulation delivery system (Volu-matic) as opposed to the amount fed to the delivery system. Pneumatic conveying characteristics are usually represented as a plot of pressure drop per unit length versus superficial gas velocity at specific solid flow rates in a Zenz-type state diagram [1].
2. Measurement principle In 1924, Gasterstadt [2] showed that there is a linear relationship between the ratio of pressure drop per unit length of pipe with suspension flow to the pressure drop per unit length with the gas alone and the solids loading ratio when plotted at constant gas velocity. The development of this equation is as follows. The total pressure drop (⌬Pt) for fully developed flow in a straight section can be given as the sum of the pressure drop of the gas alone (⌬Pg) and an additional pressure drop due to solids (⌬Ps). Therefore the pressure drop per unit length can be determined by dividing the pressure drop by the length and expressed as:
178
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
Nomenclature D dp fg fs L ⌬Pg ⌬Ps ⌬Pt Re Rep Ug Up Wg Ws
diameter diameter of particle gas friction factor solids friction factor length pressure drop of gas alone solids pressure drop total pressure drop Reynolds number of the gas Reynolds number of the particle gas velocity particle velocity gas flow rate solids flow rate
Greek a e m rg rp
pressure drop ratio voidage solids loading ratio gas density density of the particles
⌬Pt ⌬Pg ⌬Ps ⫽ ⫹ L L L
(1)
The pressure drop due to the gas flow in a pipe of length L, diameter D, and with an average velocity Ug can be given as: ⌬Pg ergU2g ⫽fg L 2D
(2)
Similarly, the relationship for the extra pressure drop due to the solids in a fully accelerated suspension flow can given as: ⌬Ps (1−e)rsU2p ⫽fs L 2D
(3)
The voidage e can be expressed as a function of solids loading ratio (m) as: e⫽1⫺
冉 冊冉 冊
r g Ug 4Ws ⫽1⫺ m 2 pD rsUp r s Up
(4)
Substituting these relationships into the equation for pressure drop per unit length, the relationship for specific pressure drop ratio a can be given as:
冉 冊 冉 冊 冉 冊冉 冊冉 冊
a⫽
⌬Pt/L ⌬Ps/L ⫽1⫹ ⫽1 ⌬Pg/L ⌬Pg/L
⫹
fs Up rsUp m fg Ug rsUp−rgUgm
(5)
For dilute phase gas–solid mixtures, rsUpÀrgUgm can be assumed, therefore the relationship for specific pressure drop ratios can be given as: a⫽1⫹
冉 冊冉 冊冉 冊
fs Rep D m fg Re dp
(6)
Combining the constants, the Gasterstadt relationship can be simply given as: a⫽1⫹Km
(7)
The parameter K can be empirically determined because of the number of factors that it depends on: material characteristics (particle size distribution, mean particle size, pipe characteristics, particle density), pipe characteristics (inside diameter, internal roughness), configuration (horizontal or vertical), and conveying gas properties (velocity, density, viscosity). Gasterstadt [2] performed experiments with different types of wheat in a 90 mm I.D. pipe and with air velocities from 12 to 28 m s⫺1 and found the above relationship to be valid and K dependent on gas velocities. He found that for gas velocities above 20 m s⫺1, the parameter K became independent of gas velocity. In the case of vertical flow the expression is similar but includes an additional term due to gravitational effects. Cabrejos and Klinzing [3] investigated this relationship proposed by Gasterstadt for a dilute phase test system for granular materials. They compared data from their experiments with those from other researchers re-
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
plotted in this format to reveal the same linear relationship between a and m. Some of the data which exhibited this relationship include those from Hinkle [4] and Rizk [5]. Experimental values of K for horizontal flow were found to be in the range 0.7364 to 0.7718 while those for vertical flow were typically higher. Based on this analysis, it can be concluded that for gas velocities significantly above saltation (typically 50%), K becomes independent of gas velocity, implying that very few experiments are required to determine K for a particular system. Thereby, knowing K, the mass flow rate of solids being conveyed can be calculated from:
冉 冊冉
Ws⫽mWg⫽
冊
a−1 pD2Ugrg K 4
(8)
In this work, this relationship will be used to experimentally verify the application to the flow of fibrous material as a solids flow meter.
3. Experimental 3.1. Test rig layout and equipment description A 0.0472 m (12 in) Volu-matic feeder provided by CertainTeed was used for the insulation system delivery. This system is provided with a Gardner Denver blower, a helical screw for moving the fragmented bales of insulation, and a blow-through rotary valve-type arrangement which also serves to chop the fragments into fine fiber aggregates. The conveying line consists of a flexible hose section of length 30 m (100 ft) and diameter 0.0118 m (3 in) attached to a spool piece of flexible hose of length about 0.0945 m (2 ft) placed at the outlet of the rotary valve arrangement. Control of the airflow can be achieved by placing different amounts of weights on the relief valve at the blower outlet. Solids flow rate can be controlled by changing the size of the inlet opening to the rotary valve-type arrangement and also through a gear speed lever that controls the speed of the rotary valve arrangement. The air–solid separation at the other end of the conveying line was achieved by the use of a special in-line filter assembly. Care was taken to ensure that there are no leaks in the end section of the conveying line, especially in the vicinity of the filter, to avoid dusty conditions in the laboratory. 3.2. Pressure and weight measurements For developing the pressure drop-based flow meter for the application, three kinds of sensor measurements are required: differential pressure for obtaining specific pressure drops, gas velocity, and measured values (instantaneous) of solids flow rate since the
179
chopping/conveying process can produce unsteady twophase flows. The solids throughput can be obtained from the rate of change of weight of the filter bag assembly. Fig. 1 shows a typical configuration of the test rig for pressure drop flowmeter testing in the horizontal configuration. Differential pressures are recorded at locations indicated in Fig. 1 during the test runs. While static pressure measured depends on the transducer location with respect to the collector and feeder and the radial position, differential signals depend on the spacing between the taps. Pressure measurements were made using Omega Pressure transducers (silicon diaphragm type with 8 V DC excitation; Model 162PC01D, 6.87 kPa (1 psi) differential). These were chosen because of their superior performance in previously conducted tests at our laboratories, low hysteresis, high degree of linearity, and the convenience of a high-level voltage output signal (1–6 V) that can be fed directly to a data acquisition system. Pressure taps are located at the top of the pipe as placement at the bottom will corrupt the signal below saltation. Male Swagelock adapters mounted on the top of the pipe were used to connect 0.00098 m (1/4 in) PVC tubing to the transducers. The length of the connecting tubing for each transducer was kept relatively uniform to ensure the same extent of damping in all pressure measurements. A piece of filter cloth material was placed over the Swagelock fitting serving as a filter to eliminate the build up of fiberglass in the pressure tap and associated tubing. This action yielded consistent pressure transducer signals. The weighing apparatus for obtaining the solids flow rate is detailed schematically in Fig. 1. The assembly consists of an electronic platform scale (Ohaus Champ Multi-functional bench scale 1–45.4 kg (100 lb) range) equipped with a desk-mounted LCD-indicator unit. Apart from possessing excellent resolution for the range of conditions used (0.00908 kg (0.02 lb)), the scale has overload protection features and an adjustable averaging level. The scale was also equipped with a standard RS232C bidirectional interface which can be used for computerized data collection. This modular arrangement permitted easy enhancements or modifications under various operational conditions. Gas velocities were measured using a hot wire anemometer. Even at the maximum available solids throughput with the minimum gas velocity available on this machine, the solids flow was still found to be completely suspended with no saltation or sliding layer. The gas velocity measurement was also found to be fairly independent of the feeder opening settings as long as there was sufficient head of material above the inlet opening. 3.3. Data acquisition and analysis All the voltages generated from the pressure transducers as well as the weighing assembly were fed to a
180
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
Fig. 1. Schematic of test rig for metering fiberglass flow.
Pentium 90 MHz PC fitted with a ATMIO-16 Plug and Play Data Acquisition board from National Instruments. Lab Windows v4.0 for Windows 95, a Visual C/C++ compatible rapid application development (RAD) tool from National Instruments, along with its specialized Clanguage libraries, was used to create the user interface and perform the signal analysis. The pressure signals were suitably scaled and preprocessed to zero baseline errors using software methods. Typically pressure signals are sampled at 512 Hz (1024 samples) and weigh assembly readings were taken every 5 seconds.
vs. time trace to obtain steady mass flow rate data. Therefore, it was possible to obtain multiple steady values of the solids flow rates during a single run. To establish the reproducibility of the pressure transducer measurements, the transducers were interchanged between sections. It was found that the pressure drop readings for each section remained the same indicating that the transducers were consistent in their readings.
4. Results and discussion
3.4. Test methodology Since the proposed meter will be used on larger-scale systems with higher solids throughput, a representative subset of the higher solids throughput range on the settings for the speed control lever and the feeder scale were used. Multiple runs were performed for several of the machine settings. The runs were found to have a reproducibility of ±20%. To ensure accuracy of the recorded weights, it became necessary to shake the filter bag periodically to compact and redistribute the solids collected. During this period the weight trace was unstable. This portion of the trace was not used to determine the mass flow rates. Straight lines were fitted with a 90% correlation or greater to sections of the weight
The results of tests are summarized in Figs. 2 and 3 for two different types of insulation material for all three sections of the horizontal piping arrangement tested. It is evident from these plots that the K values are not significantly different under corresponding machine settings for the two different materials. The correlation and hence the prediction of mass flow rate using this metering technique are dependent on the position of the hose. If we assume that under the field conditions the hose is emptied completely at the end of the run, it is reasonable to expect that the best determination of the flow rate will be obtained from a set of taps located reasonably close to the feed point. The observed difference in pressure drop readings for different positions could be explained
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
181
Fig. 2. Pressure drop flowmeter data for InsulSafe III insulation material (alpha1, alpha2 and alpha3 correspond to data from differential pressure transducers P1, P2, and P3 placed at increasing distances from the feed point as shown in Fig. 1.
Fig. 3. Pressure drop flowmeter data for Optima fiberglass insulation material (alpha1, alpha2, and alpha3 correspond to data from differential pressure transducers P1, P2, and P3 placed at increasing distances from the feed point as shown in Fig. 1.
182
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
as follows. The effect of the bend prior to the straight section might have contributed to the overall pressure loss for the section. In order to have the pressure recovery from a bend a sufficient distance from the bend is required. The tests performed could have had the effect of the bend present in their readings. It should also be noted that the ratios of the pressure drops, the specific pressure drop, given by the alpha value, would have the bend effect present for the gas alone flow as well as the fiberglass and gas together which would tend to mask the bend effect. Another reason for the difference between the pressure drop readings in the straight section could be that the fiberglass itself was changing character as it is being conveyed through the corrugated hose and traversing different bend arrangements. The fiberglass leaves the feeder in a state that consists of clumps or aggregates of the material. CertainTeed had noted that the corrugated tubing does affect the condition of the material with a certain length of tubing being required before the material reaches its maximum insulating quality. The flow process is probably making the aggregates less compact having a higher voidage. The last section of piping yielded results that had the lowest correlation between the loading and specific pressure drop. This section contained material that had traveled the farthest and gone through two bend arrange-
Fig. 4.
ments. This material could be the least compacted in the flow process. Figs. 4 and 5 presents the horizontal section P1 which is closest to the feed point, giving the values of the specific pressure drop ratio at given loading conditions for the two types of insulating materials tested. Measuring the flow rate at the beginning of the piping gives the user ease in obtaining a reliable flow rate in a section of the piping that can be specified as being both horizontal and straight and not under the influence of a bend. The material in this section most probably is in the state of clumps of fiberglass. Eq. (8) is the working expression for the calculations of the solids flow rate. The value of alpha is determined, then the specific pressure drop, and then from the figures presented the value of the loading can be found. Eq. (8) can then be used to calculate the solids flow rate. In order to improve the stability in the measured flow rates of solids it is suggested that the tests be conducted over longer periods of time (around 30 min). Short times for the test seem to introduce significant variations in the weight of the material collected and thus the flow rates measured. Testing with the solids flow meter on plastic pellets had been carried out at an industrial site for long time-periods (30 min or more) with improved stability in the flow rates measured.
Recommended pressure drop flowmeter correlation data (transducer P1 data) for InsulSafe III fiberglass insulation material.
S. Venkatasubramanian et al. / Flow Measurement and Instrumentation 11 (2000) 177–183
Fig. 5.
183
Recommended pressure drop flowmeter correlation data (transducer P1 data) for Optima fiberglass insulation material.
5. Conclusions
References
The conveying of fibrous materials has some unique challenges especially in their ability to change character as they are conveyed through various geometries. This work has shown how this changing character is expressed by the pressure drop measurements made at various sections of the conveying line. By using the initial section of the line after the feeder, the specific pressure drop measurements can be used to obtain a value of the solids flow rate of the fibrous material. It has been our experience that testing at an industrial site for over 30 min improves the stability of the results obtained.
[1] F. Zenz, D.F. Othmer, Fluidization and Fluid–Particle Systems, Chapman and Hall, London, 1960, 314pp. [2] J. Gasterstadt, Die experimentelle Untersuchung des pneumatischen Fordervorganges, V.D.I. Zeitschrift 68 (24) (1924) 617– 624. [3] F.J. Cabrejos, G.E. Klinzing, Novel solids pressure drop flow meter. Paper presented at the ASME Winter Annual Meeting, 1992. [4] B.L. Hinkle, Acceleration of particles and pressure drop encountered in horizontal pneumatic conveying. Unpublished PhD dissertation, Georgia Institute of Technology, 1953. [5] F. Rizk, Pneumatic transport of plastic granules in horizontal ducts—the simultaneous effects of gravity, pipe material, and solid properties, particularly in the optimal operating range. DoktorIngeneiurs Dissertation, University of Karlsruhe, 1973.