13 Apr 2016

Packing machines for gypsum

 

A number of different factors have to be taken into consideration in the selection of bag filling systems for mortar and gypsum products. The filling process and filling time are essentially determined by the product and material characteristics, as well as by the chosen filling system. By means of measured bulk density characteristics such as flowability, aeration behaviour, bulk density, grain size distribution, fineness and wear behaviour, mortar and gypsum products can be evaluated regarding their filling behaviour. This measured data can be used for the selection of a suitable filling system.

Selection procedure

In order to be able to make a reasonable pre-selection of a powder filling system, the characteristics of the bulk material have to be determined. For this purpose, a standard dust test report is used. By means of the determined features in the dust test reports a classification regarding the filling system can be made with a modified Geldart diagram.

After the pre-selection of the filling principle either by turbine or by air filling principle the bulk material is examined at a test filling station regarding the filling times, weighing accuracy and the mass flow in coarse and fine flow. The evaluation of the results either confirms the assumed pre-selection or other filling systems have to be tested. For the filling tests the characteristics such as bag volume and air permeability of the valve bags to be used must be available (bag test certificate). Figure 1 shows the selection procedure in each steps.

These individual selection steps are described in detail:

Bulk material data – Dust test report: The material characteristics are of decisive importance for the selection of a suitable filling system. A specially created dust test report has been used to judge the bulk material for many years at Claudius Peters.

The particle size analysis of the bulk material is examined as well as the different densities of different pieces of the material (e.g.: loosely poured, shaken, fluidised bulk material density, the behaviour in a fluidised condition with both load conditions ‘non-stirred’ and ‘stirred’ including the gas-retention behaviour when interrupting the gas supply and the flow behaviour on an aeroslide). An experienced laboratory assistant needs approximately four hours to determine the above data. Additional examinations regarding sorption or wear behaviour may also be necessary.

Classification according to Geldart: On the basis of his own measurements and those of others, Geldart developed a classification system that allocates bulk materials into four groups – A, B, C, D – with regard to their fluidsation behaviour in gas/solid fluidised beds. Figure 2 shows this classification scheme. The ordinate represents the difference between the particle density ρP and the gas density ρF; the x-axis represents a characteristic particle diameter, here simplified and plotted with adequate accuracy dS,R50 = dS (at residue on screen R = 50% by mass). Geldart describes the behaviour of the four bulk material categories as follows:

Group A) The particle size is 10-20μm, the particle density is less than 1400kg/m³. Before a bubble formation the bed expands to double or triple the bed thickness at the minimum fluidisation velocity. Most pulverised bulk materials come into group A.

Group B) The particle size is 40-500μm. The particle density is approximately 1400-4500kg/m³. Bubble formation starts immediately when exceeding the minimum fluidisation velocity.

Group C) These are very small and therefore very cohesive particles with a size less than 20-30μm. Due to the strong cohesive power it is very difficult to fluidise these particles by means of mechanical agitation.

Group D) The particle size is above 600μm with a very high particle density. A very high volume flow is necessary for mixing which risks abrasion.

Filling systems for valve bags: Depending on the field of application and the specific filling characteristics of the various products, Claudius Peters offers three different filling systems for valve bags. The standard turbine, a horizontal rotating turbine can be used for free flowing bulk materials such as cement, lime, gypsum or fine building materials up to a grain size of 4mm.

 

  • Claudius Peters' powder packaging machine with automatic bag magazine (right).

A vertical rotating turbine can be used for the same range of applications, but as well as for fine/coarse building materials with a higher coarse portion. The air filling system is used for all free-flowing bulk materials including coarse building materials up to a grain size of 10mm.

Horizontal rotating turbine filling system: The characteristics and advantages of the horizontal rotating turbine are mainly the conical material inlet, the direct coupling of the motor at the impeller (no V-belt is necessary), the housing sealing via labyrinth seal with scavenging air, 4.0kW motor capacity and a simple dismantling procedure in case of maintenance requirements (Figure 3).

Design of impeller for horizontal rotating turbine system: The design of the impellers has an influence on the filling velocity of the filling turbines. For this reason special designs have been developed to improve the filling behaviour. The design shown in Figure 4 is characterised by angled impeller blades and presses the material with an increased filling pressure into the filling spout/bag.

These so-called ‘pressure impellers’ can increase the filling capacity by 10-15%. They are used in particular for materials with high Blaine numbers and therefore for gypsum. Retrofitting of existing impellers is possible.

Vertical rotating turbine filling system: The vertical rotating turbine has a motor capacity of 5.5kW and a V-belt drive (Figure 5). By means of a frequency converter for a horizontal and vertical rotating turbine the rotary speed of the turbine motors can be varied. It is therefore possible to find an optimal adjustment regarding the filling times of the different products. These parameters can be adjusted in the plant control room and guarantee a reasonable conversion of the machine parameters in case of a change of product.

Air filling system: The air filling system is characterised by the following features: pressure chamber without angle, separate aeration via lower and upper air, automatic chamber aeration, pneumatic operated shut-off flap with blow-up sealing collar, swivelling filling chamber bottom for simple cleaning when changing the material and automatic self-cleaning, with shock pressure aeration of the filling channel (Figure 6).

Application of the Geldart diagram to select the filling system: By combining the determined characteristics of the Geldart diagram with practical experiences and filling tests in the test and research centre, Claudius Peters has arrived at the following qualitative conclusions:

Group A bulk materials can be filled without any problem with a turbine filling system. Optimisation regarding the filling times have to be examined and discussed in detail.

Group B bulk materials can be filled without any problems with an air filling system. The maximum particle diameter must not exceed 10mm.

Group C bulk materials cannot be allocated to any filling system without further examination. However, suitable measures, such as aeration of the bulk material in the feeding hopper or air choc impulses in the filling spout may have a positive influence on the filling behaviour.

Group D bulk materials have to be examined by means of filling tests. In general filling with an air filling system with sufficiently fine particles is possible, in cases where the maximum particle diameter is not larger than 10mm.

By knowing the particle density, particle diameter and the classification in the Geldard diagram the bulk materials can quickly and easily be evaluated with regard to their quality and their suitable filling system (Figure 7). Quantitative statements concerning the filling times, weighing accuracy, capacities and other parameters have to be determined by individual filling tests.

Valve bag test report: It is possible to select the air permeability of the bags and the volume of the bags according to the material that they are made from. The measurement consists of the connection to the pressure air net with control valve, suspended solid matter throughput measuring devices for various measuring ranges, various valves and manometers as well as a filling spout with inflatable sleeve for sealing.

The valve bag is applied with a test pressure of 5000Pa on the inside via the filling spout. The air leaking from the complete bag via lining, joints and perforation is determined as volume flow. The determined values are noted in a test report and compared to reference data.

The so-called K-value, the permeability coefficient, is determined as characteristic data for the evaluation of the bags. This value is a quotient of the measured permeability of the bag in Nm³/h and the volume of the bag in litres. Reference values for K permit the evaluation of the bag for the planned material to be filled. For fine products with high air retention capacity, the bag should have an air permeability and therefore a permeability coefficient of above 1.9.

A first statement can be given in regard to the suitability of the bag for the specified product but final filling tests with the bags and actual material to be used must be performed.

Filling tests and evaluation: For the right selection of the optimum turbine filling principle (horizontal or vertical driven turbine) both filling principles will be examined on the basis of reference products. By means of suitable filling equipments the bulk materials are tested in so-called filling tests regarding their flow behaviour and their filling behaviour.

The decisive selection criteria are the filling times and the weighing accuracy as well as the mass flows of the coarse and fine flow. The following reference products have been chosen: Portland cement, wall plaster and a mixture of plaster and dry mortar. The characteristic data of the dust test certificate is summarised in Table 1.

  Portland Cement Wall plaster Plaster-dry mortar
Bulk density (kg/m3) 1.33 1.2 0.8
Apparent density (kg/m3) 3.15 2.67 2.78
Max. average grain Korn (µm) 278 255 905
Average grain d50 (µm) 20 23 25
Blaine (cm2/g) 3092 3217 7065
Deaeration time non-stirred (s) 39 19 28
Deaeration time stirred (s) 58 34 50

Above – Table 1: Summary results of bulk tests for three different powders.

Determination of the filling system

The determined filling times and weighing accuracies illustrate an advantage of the horizontal rotating turbine for cement and wall plaster (Tables 2-4). The filling times are quicker than for the vertical rotating turbine.

The capacities to be expected (bag/h) are 10–12% higher on average. As the weighing accuracies for horizontal turbines are within the values regulated by law, a turbine drive with horizontal rotating impeller is recommended for these materials.

For plaster with a very high number of Blaine (>7000cm²/g) no significant difference between the horizontal and the vertical rotating turbine is visible. Since both filling systems supply nearly identical values for the filling time and the weighing accuracy, additional requirements such as residual discharge and cleaning regimes have to be taken into consideration.

Table 2 Horizontal rotating turbine impeller Vertical rotating turbine impeller
Air pressure hopper (bar) 0.2 0.6
Air impulses in coarse flow (-) 3 3
Nominal weight (kg) 21 21
Average weight (kg) 21.015 21.003
Average filling time (s) 5.448 6.149
Mass flow (coarse / fine) (kg/s) 4.437 / 2.437 4.643 / 1.873
Max. admissible deviation of the average value from the nominal value according to MID 2004/22/EG (kg) +/- 0.05242 +/- 0.05242

Above – Table 2: Filling test summary results for Portland cement.

Table 3 Horizontal rotating turbine impeller Vertical rotating turbine impeller
Air pressure hopper (bar) 0.1 0.3
Air impulses in coarse flow (-) 0 3
Nominal weight (kg) 20 20
Average weight (kg) 21.019 19.998
Average filling time (s) 5.361 6.106
Mass flow (coarse / fine) (kg/s) 3.577 / 3.269 4.546 / 1.804
Max. admissible deviation of the average value from the nominal value according to MID 2004/22/EG (kg) +/- 0.04992 +/- 0.04992

Above – Table 3: Filling test summary results for wall plaster.

Table 4 Horizontal rotating turbine impeller Vertical rotating turbine impeller
Air pressure hopper (bar) 0 0
Air impulses in coarse flow (-) 3 3
Nominal weight (kg) 14 14
Average weight (kg) 14.003 14.004
Average filling time (s) 7.553 7.582
Mass flow (coarse / fine) (kg/s) 3.00 / 0.23 2.73 / 0.502
Max. admissible deviation of the average value from the nominal value according to MID 2004/22/EG (kg) +/- 0.03744 +/- 0.03744

Above – Table 4: Filling test summary results for plaster-dry mortar.

Modular design and weighing system

Due to their special modular design, exchange of the filling modules and installation of additional modules to increase the capacity is easy and possible at any time for all of Claudius Peters’ packing plants. Furthermore, the dedusting air is directly fed upwards from the socket via channels to the central dedusting system.

Consequently, the amount of dust arising is reduced after the filling procedure to nearly dust-free levels, ensuring an environmentally-friendly operation.

Each filling module in the packer is equipped with a separate independent electronic weighing system. These weighing systems are connected externally with the Claudius Peters Pactron Master cement terminal via a data bus. The individual control parameters can be adjusted from this terminal. The electronic weighing system controls the dosing procedure and optimises the adjusting parameters with respect to disconnecting points for each filling procedure. The dosing procedure can be divided into the following phases:

Phase I: Bag identification and zero taring;

Phase II: Start of the coarse flow metering under permanent monitoring of the filling flow quantity;

Phase III: Fine flow metering for maintaining high weighing accuracy (constant mass flow);

Phase IV: Check weighing with simultaneous ascertainment of the fine flow and post-run quantity. Resulting from this, optimisation of the switching-off points;

Phase V: Bag discharge.

Bag-sealing with ultrasound

Ultrasonic valve sealing technology: The expected high levels of cleanness during storage and transport of the bagged material led to the development of the ultrasonic sealing technology. For this purpose special bag valves are used which are covered on the inside with a fusible plastic.

The valve is fixed between an ultrasonic probe and a plate for the welding procedure. A generator produces ultrasonic vibrations that are transferred through the ultrasonic probe to the fixed valve. The heat generated by the ultrasonic melts the plastic and the valve is sealed.

For the filling of the material, so-called inflation sleeves are used which seal the valve to the outside during the filling process and prevent material from remaining in the bag valve. Furthermore, an exact positioning of the bags after pulling off from the spouts is of great importance. Especially developed bag chairs fulfil this function and guarantee a closing rate of nearly 100%. The filling process can be seen in Figure 8.

Due to this dust free filling and the subsequent bag sealing with ultrasound, an expensive dust recovery and recycling facility becomes redundant.

Range of application and capacities of packing machines with ultrasonic welding: Packing machines with ultrasonic welding can be used for filling of all flowing materials. The dust-free filling procedure as well as the tight closing of the bags can be used for nearly all building materials and cement products.

The filling times of the packing machines depend mainly on the bagging weight as well as on the corresponding product characteristics. Spout capacities of 200–240 bag/h can be realised. Table 5 shows the capacities of different applications, machine types and products.

Product / weight 6-Sp rotary packer – Horizontal turbine without ultrasonic welding (bags/hr) 6-Sp rotary packer – Vertical turbine with ultrasonic welding (bags/hr) 6-Sp air packer air filling system with ultrasonic welding (bags/hr)
Ordinary Portland cement / 25kg 2340    
White cement / 25kg 2250    
Cement plaster / 30kg   1250  
IP 14 cement plaster / 30kg   1260  
KC5050 / 25kg     1320
80% sand 20% cement / 25kg     1330

Below – Table 5: Capacities of different applications, machine types and products. 

Automatic bag application

The Claudius Peters’ automatic bag applicator (below) offers a uniform application capacity with reduced personnel. With a capacity of up to 4500 bag/hr, empty bags can be fed to the applicator from the cassette magazine or from the single/double roller magazine. The automatic bag applicators are suitable for different bag sizes and bag specifications. A change to the corresponding size can be carried out automatically. By means of a rotary table, a variable arrangement of the bag magazine is possible. This opens up the possibility of retrofit installations into existing plants.

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