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Improving the hygienic design of equipment in handling dry materials
13 Improving the hygienic design of equipment in handling dry materials K. Mager, Quest International, The Netherlands
13.1
Introduction: principles of hygienic design
Several guidelines for equipment design have been published in order to produce foods in a hygienically acceptable way. The principles are to prevent the contamination of food products by substances that would adversely affect the health of the consumer. In that respect these guidelines describe design principles based on: · smooth product contact surfaces; · no dead areas; and · the avoidance of condensation in the equipment. These principles of design for equipment in the food industry are originally based on the handling of liquids (EHEDG, 2004). However, food products with other product characteristics may also need to be taken into consideration. This means that the principles of design should also count for dry particulate materials. It is important, therefore, to realise what differences there are in the characteristics of powders as compared with liquids.
13.2
Dry particulate materials and hygienic processing
In this chapter dry particulate materials, more commonly called powders, fall in the size range of less than 10 m for ultrafine powders up to a several millimetres for agglomerates and granulates. Generally, powders are defined as consisting of individual particles that have a diameter smaller than 150 m. Larger particulates are often composed of many smaller particles, and
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substructures can be achieved spontaneously by the natural phenomena of adhesion and electrostatic forces. However, stronger structures are best achieved by forcing particles to bind together using moisture or especially added binders while the particles are being fluidised or mixed. A definition of a dry product in the sense of microbial stability is not so easily given, since it can change slightly from product to product. As a rule of thumb, one can say that when the water activity is below 60%, little to no microbial growth will occur. Dry materials can be characterised by both their single particle (such as shape and size) and their bulk (such as bulk density and flowability) characteristics. It should be emphasised that the bulk characteristics of industrial dry materials are at least as important as their single particle characteristics, and for each material, the most important characteristics influencing materials handling will vary. Flowability is an important characteristic for dry material retention in equipment, and generally improves with: · · · · ·
increase in particle size and particle sphericity; decrease of moisture content; decrease in fines content; decrease in surface stickiness; decrease in neutralisation of surface energy/charge.
Hygienic processing also influences the dry material quality properties of: · · · ·
aroma; chemical, biological or physical activity; colour; flavour.
In general it can be stated that, based on their characteristics, powders have the tendency to stick to product contact surfaces and are more likely to remain in the process line as compared with liquids. Also, lump formation and hygroscopic properties are important parameters in enhancing this effect. As mentioned earlier, an increased moisture content in the powder (and remaining powder residues!!) can cause serious proliferation of microorganisms. The above-mentioned effects have to be taken into serious consideration when designing equipment processing powders.
13.3
Cleaning regimes
The criteria for hygienic design of equipment and plants for dry materials handling depend upon the moisture content of the dry material and the method of cleaning. Whether the equipment is cleaned wet or dry has a significant effect on the design criteria. If wet cleaning procedures are applied the design has to fulfil the general requirements for equipment in the liquid area as described in several
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EHEDG documents (EHEDG 2001, 2003, 2004). If only dry cleaning procedures are applied, less stringent requirements can be allowed as will be described in the following sections. However, it is important to establish that it is possible to suffice with dry procedures only. Sometimes factories do infrequent wet cleaning and it is known from trend analyses that microbiological contamination occurs after these periods. If wet cleaning procedures are applied it is extremely important that the equipment is dried immediately, because: · remaining wet spots can be the cause of lump formation in the subsequent batch; · the proliferation of microorganisms in the wet spots can contaminate the powders. Moreover, the combination of powders and water provides an ideal source for microbiological growth! It is particularly important that critical areas such as dead legs, sharp corners are behind seals and gaskets are locations that can be dried within a reasonable time in order to avoid the favourable conditions for microbiological contamination. In this sense it should be emphasised that the need of wet cleaning should be taken into serious consideration. Wet cleaning is a critical hazard in the dry material handling area and dry cleaning procedures are preferred in all respects. Dry cleaning is applicable for dry food material contact surfaces where: · dry material remaining in the equipment as loose layers or dust covering does not present any risk of degrading the quality of the dry material subsequently produced; · possible cross-contamination of dry material during a production change to another material presents no problem to the quality or safety of the dry material subsequently produced; · dry material remaining in the equipment does not present any risk of microbial growth occurring due to the prevailing moisture content, temperature and humidity conditions; · dry material is non-hygroscopic and non-sticky. Dry cleaning procedures include the use of vacuum cleaners, brushes and scrapers. However, procedures can also be applied in which the equipment is rinsed with `neutral' agents such as salt and starch.
13.4
Design principles
Compared with liquids, dry materials handling must take into account the possibility of material lump formation, creation of dust explosion conditions, high moisture deposit formation in the presence of hot air, and material remaining in the equipment after plant shutdown (even if a degree of self-emptying is achieved). Powders tend to stick in the process equipment more than liquids.
Improving the hygienic design of equipment in handling dry materials
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13.4.1 Materials of construction Construction materials coming in contact with food (including associated adhesives) must be food grade (Food and Drug Administration (FDA) or European Food Safety Authority (EFSA) approved or national equivalent). Selection of construction materials depends greatly upon the dry materials, method of cleaning and cleaning agents to be used. The abrasive characteristics of powders can particularly affect the product contact surfaces. Metals Hygienic dry materials handling is best conducted with product contact surfaces of stainless steel. Suitable grades are SS 304, 304L (EN 1.4301/1.4306) and SS 316, 316L (EN 1.4401/1.4404). The 316 grades are more resistant to chloridecontaining solutions, especially under wet and hot conditions. Aluminium and aluminium alloys (coated and non-coated) might also be used as dry material contact surfaces where only dry cleaning is applied. However, the abrasive characteristics of the processed powder shall be considered in this choice. Moreover, if aluminium is specified from an operational or weight aspect, there is a potential corrosion problem when a wet cleaning procedure is applied. Carbon steel can also be considered as a contact surface in components involving dry processing and dry cleaning operations. Non-metals Plastics (e.g. polycarbonate, polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polyacrylamide (PA) and polytetrafluoroethene (PTFE)) and elastomers (e.g. nitrile butyl rubber (NBR), viton, ethylene propylene diene monomer (EPDM) and silicon rubber) may be used. When in contact with dry materials they must retain their original surface condition and conformational properties when exposed to the processing conditions of temperature and humidity, and also during cleaning operations. It is important to realise that plastics and elastomers in particular are sensitive to abrasive powders and therefore the contact surfaces should be minimised as much as possible. Glass is a hygienic material, but should not be used because of the risk of breakage and subsequent difficulty in detecting broken glass in dry materials. It is recommended that the glass is replaced with another material, e.g. polycarbonate. Non-metallic surfaces used in dry materials handling can create electrostatic charges on the material. This can cause surface adhesion by small particles. Electrostatic effects during dry materials handling in pneumatic conveying systems and non-metallic equipment parts, for example, can be problematic, and therefore special attention should be paid to accessibility and cleaning in such systems. 13.4.2 Product contact surfaces Product contact surfaces should be smooth and resistant against both dry material contact and against liquid chemicals used in wet cleaning. Product
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contact surfaces therefore should be free of crevices, pitting, pinholes and any hairline cracking that can cause material penetration and cleaning difficulties. A roughness standard of Ra 0:8 m is recommended where there is a risk of microbial growth associated with high moisture content in the dry material or wet cleaning. As the surface roughness of cast materials and carbon steels does not meet the recommended figure above, the cleanability of the components made with these materials require further investigation in relation to the actual dry material being handled. In order to carry out a dry cleaning operation, contact surfaces should be fully accessible for safe manual cleaning and inspection. In order to carry out a hygienic wet cleaning operation, contact surfaces should not be horizontal, but have a slight slope to facilitate drainability of cleaning solutions. The possibility for product contact on sharp internal corners (r 6 mm) and recesses, etc., where dry material can accumulate, should be avoided. Windows and inspection ports mounted in product contact surfaces should be flush with the surrounding surfaces to minimise dry material build-up. When using nonmetallic materials as contact surfaces, the porosity of the materials should be investigated with regard to their ease of cleanability. 13.4.3 Static seals (gaskets) for duct and flange connections Static seals should be of an elastic material, have a non-porous surface and be cleanable. They should be mounted to create a flush surface without any crevice with the surrounding metallic body (Fig. 13.1a). The seal material should be abrasion-resistant to the dry material being handled. In the case of dry processing and dry cleaning only, closed cell-foamed non-absorbing materials for gaskets or seals can be applied. Open foam material is not allowed. Static seals should be clean before assembly and the possibility for penetration of dry material into the gasket or seal during equipment operation should be avoided. Misalignment of ducts should be avoided as dry materials can be trapped on the misaligned ridges (Fig. 13.1b). Assembly of seals and gaskets for vessels of large diameter require special attention to prevent operational problems, especially air and liquid (washing) leakage and material dust emissions to atmosphere. PTFE can be used as a static seal in combination with an elastomer (food grade, FDA or EFSA approved or national equivalent). The PTFE should be of high-density resilient quality. Metal-to-metal contact duct assemblies (Fig. 13.1c) and paper-type gaskets between flanges can be applied where a plant operates at atmospheric pressure and requires no wet cleaning. 13.4.4 Flexible connections One of the biggest hygienic design concerns in the dry materials handling area is that of the flexible connections in process lines. Flexible connections between duct ends are always liable to cause dry material build-up between the flexible
Improving the hygienic design of equipment in handling dry materials
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Fig. 13.1 Examples of static flange seals for dry products: (a) hygienically designed seal usable for wet cleaning, (b) seal creating a gap and misalignment, (c) metal-to-metal flange joint (only for dry cleaning).
material and metal duct surface. In smaller diameter devices, duct ends are connected with rubber or plastic sleeves. Ring clamps for mounting flexible connections should be placed close to or right at the duct end to minimise dead areas for dry material build-up as demonstrated in Fig. 13.2. The plastic sleeve must allow small axial and radial movements without generating axial forces. The flexible material should have a smooth surface that minimises surface build-up of dry material. Larger diameter duct ends are often connected with rubber type profiles mounted with flanges. These devices are most probably never removed and cleanability is difficult. As such these are critical areas in the process line. This is one of the areas where improvements are needed and present a challenge for the current engineers.
Fig. 13.2 Examples of flexible connection duct ends (right). One ring clamp close to the pipe end used for smaller diameters; crevice not totally avoidable (left). Application of two clamps, one of which is mounted directly at the pipe end to avoid any crevice (middle).
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13.5
Handbook of hygiene control in the food industry
Types of equipment in dry material handling areas
Typical equipment in the dry material handling area includes, e.g., powder blenders, grinders, rotary valves, powder discharge systems, fluid bed dryers and spray dryers. In principle all the design criteria as mentioned earlier shall be implemented in these equipment. However, typical hazards in all parts of the total process line are considered as well. For example: · In the spray driers and fluid bed system special attention should be paid to the air inlet system. The drying air in fluid bed systems and spray dryers should be filtered to avoid a direct product contamination. EHEDG recommendations are that the filters should be at least of class EU-7 for hot drying air. In the downstream part of the spray dryer the powder has to be transported with cold air and this shall be filtered with an EU-10 filter. The air outlet system can also be a critical area. Dust is collected in these filters and frequently pulsed back into the product. Bag filters in the fluid bed system can be especially contaminated when the cleaning procedures are not carried out according to strict procedures. · The dust extraction system in the powder charge cabinets should be designed in such a way that lumps in the exhaust line cannot fall back into the powder (see Fig. 13.3). · Rotary valves cannot be cleaned in place and therefore special attention has been paid in order to design retractable rotor devices in order to enhance the cleaning procedures.
Fig. 13.3 Powder charge cabinet (CIP = cleaning-in-place).
Improving the hygienic design of equipment in handling dry materials
13.6
227
Conclusions: Improving hygiene in processing powders
It is clear that in the powder-handling area, special design criteria and process procedures are required. A dedicated (pre-) HACCP (Hazard Analysis of Critical Control Point) study should be part of the development process during the design phase of the process lines. Decisions on the required cleaning procedures (wet or dry cleaning) are crucial. In the case that only dry cleaning procedures are to be applied, less stringent design criteria might be possible. On the other hand, the equipment itself in the powder-handling area is often not of a hygienic design when wet cleaning procedures have to be applied. Rotary valves, sieves, bag filters, flexible connections are all examples of equipment and process line components that are still a challenge for equipment manufacturers to improve.
13.7
References
(2001) Document No. 22. General Hygienic Design Criteria for the safe processing of dry particulate materials, March. Campden & Chorleywood Food Research Association, Chipping Campden. EHEDG (2003) Document No. 26. General Hygienic Engineering of plants for the processing of dry particulate materials, November. Campden & Chorleywood Food Research Association, Chipping Campden. EHEDG (2004) Document No. 8. Hygienic equipment design criteria, April. Campden & Chorleywood Food Research Association, Chipping Campden. EHEDG
13.1
Introduction: principles of hygienic design
Several guidelines for equipment design have been published in order to produce foods in a hygienically acceptable way. The principles are to prevent the contamination of food products by substances that would adversely affect the health of the consumer. In that respect these guidelines describe design principles based on: · smooth product contact surfaces; · no dead areas; and · the avoidance of condensation in the equipment. These principles of design for equipment in the food industry are originally based on the handling of liquids (EHEDG, 2004). However, food products with other product characteristics may also need to be taken into consideration. This means that the principles of design should also count for dry particulate materials. It is important, therefore, to realise what differences there are in the characteristics of powders as compared with liquids.
13.2
Dry particulate materials and hygienic processing
In this chapter dry particulate materials, more commonly called powders, fall in the size range of less than 10 m for ultrafine powders up to a several millimetres for agglomerates and granulates. Generally, powders are defined as consisting of individual particles that have a diameter smaller than 150 m. Larger particulates are often composed of many smaller particles, and
Improving the hygienic design of equipment in handling dry materials
221
substructures can be achieved spontaneously by the natural phenomena of adhesion and electrostatic forces. However, stronger structures are best achieved by forcing particles to bind together using moisture or especially added binders while the particles are being fluidised or mixed. A definition of a dry product in the sense of microbial stability is not so easily given, since it can change slightly from product to product. As a rule of thumb, one can say that when the water activity is below 60%, little to no microbial growth will occur. Dry materials can be characterised by both their single particle (such as shape and size) and their bulk (such as bulk density and flowability) characteristics. It should be emphasised that the bulk characteristics of industrial dry materials are at least as important as their single particle characteristics, and for each material, the most important characteristics influencing materials handling will vary. Flowability is an important characteristic for dry material retention in equipment, and generally improves with: · · · · ·
increase in particle size and particle sphericity; decrease of moisture content; decrease in fines content; decrease in surface stickiness; decrease in neutralisation of surface energy/charge.
Hygienic processing also influences the dry material quality properties of: · · · ·
aroma; chemical, biological or physical activity; colour; flavour.
In general it can be stated that, based on their characteristics, powders have the tendency to stick to product contact surfaces and are more likely to remain in the process line as compared with liquids. Also, lump formation and hygroscopic properties are important parameters in enhancing this effect. As mentioned earlier, an increased moisture content in the powder (and remaining powder residues!!) can cause serious proliferation of microorganisms. The above-mentioned effects have to be taken into serious consideration when designing equipment processing powders.
13.3
Cleaning regimes
The criteria for hygienic design of equipment and plants for dry materials handling depend upon the moisture content of the dry material and the method of cleaning. Whether the equipment is cleaned wet or dry has a significant effect on the design criteria. If wet cleaning procedures are applied the design has to fulfil the general requirements for equipment in the liquid area as described in several
222
Handbook of hygiene control in the food industry
EHEDG documents (EHEDG 2001, 2003, 2004). If only dry cleaning procedures are applied, less stringent requirements can be allowed as will be described in the following sections. However, it is important to establish that it is possible to suffice with dry procedures only. Sometimes factories do infrequent wet cleaning and it is known from trend analyses that microbiological contamination occurs after these periods. If wet cleaning procedures are applied it is extremely important that the equipment is dried immediately, because: · remaining wet spots can be the cause of lump formation in the subsequent batch; · the proliferation of microorganisms in the wet spots can contaminate the powders. Moreover, the combination of powders and water provides an ideal source for microbiological growth! It is particularly important that critical areas such as dead legs, sharp corners are behind seals and gaskets are locations that can be dried within a reasonable time in order to avoid the favourable conditions for microbiological contamination. In this sense it should be emphasised that the need of wet cleaning should be taken into serious consideration. Wet cleaning is a critical hazard in the dry material handling area and dry cleaning procedures are preferred in all respects. Dry cleaning is applicable for dry food material contact surfaces where: · dry material remaining in the equipment as loose layers or dust covering does not present any risk of degrading the quality of the dry material subsequently produced; · possible cross-contamination of dry material during a production change to another material presents no problem to the quality or safety of the dry material subsequently produced; · dry material remaining in the equipment does not present any risk of microbial growth occurring due to the prevailing moisture content, temperature and humidity conditions; · dry material is non-hygroscopic and non-sticky. Dry cleaning procedures include the use of vacuum cleaners, brushes and scrapers. However, procedures can also be applied in which the equipment is rinsed with `neutral' agents such as salt and starch.
13.4
Design principles
Compared with liquids, dry materials handling must take into account the possibility of material lump formation, creation of dust explosion conditions, high moisture deposit formation in the presence of hot air, and material remaining in the equipment after plant shutdown (even if a degree of self-emptying is achieved). Powders tend to stick in the process equipment more than liquids.
Improving the hygienic design of equipment in handling dry materials
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13.4.1 Materials of construction Construction materials coming in contact with food (including associated adhesives) must be food grade (Food and Drug Administration (FDA) or European Food Safety Authority (EFSA) approved or national equivalent). Selection of construction materials depends greatly upon the dry materials, method of cleaning and cleaning agents to be used. The abrasive characteristics of powders can particularly affect the product contact surfaces. Metals Hygienic dry materials handling is best conducted with product contact surfaces of stainless steel. Suitable grades are SS 304, 304L (EN 1.4301/1.4306) and SS 316, 316L (EN 1.4401/1.4404). The 316 grades are more resistant to chloridecontaining solutions, especially under wet and hot conditions. Aluminium and aluminium alloys (coated and non-coated) might also be used as dry material contact surfaces where only dry cleaning is applied. However, the abrasive characteristics of the processed powder shall be considered in this choice. Moreover, if aluminium is specified from an operational or weight aspect, there is a potential corrosion problem when a wet cleaning procedure is applied. Carbon steel can also be considered as a contact surface in components involving dry processing and dry cleaning operations. Non-metals Plastics (e.g. polycarbonate, polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polyacrylamide (PA) and polytetrafluoroethene (PTFE)) and elastomers (e.g. nitrile butyl rubber (NBR), viton, ethylene propylene diene monomer (EPDM) and silicon rubber) may be used. When in contact with dry materials they must retain their original surface condition and conformational properties when exposed to the processing conditions of temperature and humidity, and also during cleaning operations. It is important to realise that plastics and elastomers in particular are sensitive to abrasive powders and therefore the contact surfaces should be minimised as much as possible. Glass is a hygienic material, but should not be used because of the risk of breakage and subsequent difficulty in detecting broken glass in dry materials. It is recommended that the glass is replaced with another material, e.g. polycarbonate. Non-metallic surfaces used in dry materials handling can create electrostatic charges on the material. This can cause surface adhesion by small particles. Electrostatic effects during dry materials handling in pneumatic conveying systems and non-metallic equipment parts, for example, can be problematic, and therefore special attention should be paid to accessibility and cleaning in such systems. 13.4.2 Product contact surfaces Product contact surfaces should be smooth and resistant against both dry material contact and against liquid chemicals used in wet cleaning. Product
224
Handbook of hygiene control in the food industry
contact surfaces therefore should be free of crevices, pitting, pinholes and any hairline cracking that can cause material penetration and cleaning difficulties. A roughness standard of Ra 0:8 m is recommended where there is a risk of microbial growth associated with high moisture content in the dry material or wet cleaning. As the surface roughness of cast materials and carbon steels does not meet the recommended figure above, the cleanability of the components made with these materials require further investigation in relation to the actual dry material being handled. In order to carry out a dry cleaning operation, contact surfaces should be fully accessible for safe manual cleaning and inspection. In order to carry out a hygienic wet cleaning operation, contact surfaces should not be horizontal, but have a slight slope to facilitate drainability of cleaning solutions. The possibility for product contact on sharp internal corners (r 6 mm) and recesses, etc., where dry material can accumulate, should be avoided. Windows and inspection ports mounted in product contact surfaces should be flush with the surrounding surfaces to minimise dry material build-up. When using nonmetallic materials as contact surfaces, the porosity of the materials should be investigated with regard to their ease of cleanability. 13.4.3 Static seals (gaskets) for duct and flange connections Static seals should be of an elastic material, have a non-porous surface and be cleanable. They should be mounted to create a flush surface without any crevice with the surrounding metallic body (Fig. 13.1a). The seal material should be abrasion-resistant to the dry material being handled. In the case of dry processing and dry cleaning only, closed cell-foamed non-absorbing materials for gaskets or seals can be applied. Open foam material is not allowed. Static seals should be clean before assembly and the possibility for penetration of dry material into the gasket or seal during equipment operation should be avoided. Misalignment of ducts should be avoided as dry materials can be trapped on the misaligned ridges (Fig. 13.1b). Assembly of seals and gaskets for vessels of large diameter require special attention to prevent operational problems, especially air and liquid (washing) leakage and material dust emissions to atmosphere. PTFE can be used as a static seal in combination with an elastomer (food grade, FDA or EFSA approved or national equivalent). The PTFE should be of high-density resilient quality. Metal-to-metal contact duct assemblies (Fig. 13.1c) and paper-type gaskets between flanges can be applied where a plant operates at atmospheric pressure and requires no wet cleaning. 13.4.4 Flexible connections One of the biggest hygienic design concerns in the dry materials handling area is that of the flexible connections in process lines. Flexible connections between duct ends are always liable to cause dry material build-up between the flexible
Improving the hygienic design of equipment in handling dry materials
225
Fig. 13.1 Examples of static flange seals for dry products: (a) hygienically designed seal usable for wet cleaning, (b) seal creating a gap and misalignment, (c) metal-to-metal flange joint (only for dry cleaning).
material and metal duct surface. In smaller diameter devices, duct ends are connected with rubber or plastic sleeves. Ring clamps for mounting flexible connections should be placed close to or right at the duct end to minimise dead areas for dry material build-up as demonstrated in Fig. 13.2. The plastic sleeve must allow small axial and radial movements without generating axial forces. The flexible material should have a smooth surface that minimises surface build-up of dry material. Larger diameter duct ends are often connected with rubber type profiles mounted with flanges. These devices are most probably never removed and cleanability is difficult. As such these are critical areas in the process line. This is one of the areas where improvements are needed and present a challenge for the current engineers.
Fig. 13.2 Examples of flexible connection duct ends (right). One ring clamp close to the pipe end used for smaller diameters; crevice not totally avoidable (left). Application of two clamps, one of which is mounted directly at the pipe end to avoid any crevice (middle).
226
13.5
Handbook of hygiene control in the food industry
Types of equipment in dry material handling areas
Typical equipment in the dry material handling area includes, e.g., powder blenders, grinders, rotary valves, powder discharge systems, fluid bed dryers and spray dryers. In principle all the design criteria as mentioned earlier shall be implemented in these equipment. However, typical hazards in all parts of the total process line are considered as well. For example: · In the spray driers and fluid bed system special attention should be paid to the air inlet system. The drying air in fluid bed systems and spray dryers should be filtered to avoid a direct product contamination. EHEDG recommendations are that the filters should be at least of class EU-7 for hot drying air. In the downstream part of the spray dryer the powder has to be transported with cold air and this shall be filtered with an EU-10 filter. The air outlet system can also be a critical area. Dust is collected in these filters and frequently pulsed back into the product. Bag filters in the fluid bed system can be especially contaminated when the cleaning procedures are not carried out according to strict procedures. · The dust extraction system in the powder charge cabinets should be designed in such a way that lumps in the exhaust line cannot fall back into the powder (see Fig. 13.3). · Rotary valves cannot be cleaned in place and therefore special attention has been paid in order to design retractable rotor devices in order to enhance the cleaning procedures.
Fig. 13.3 Powder charge cabinet (CIP = cleaning-in-place).
Improving the hygienic design of equipment in handling dry materials
13.6
227
Conclusions: Improving hygiene in processing powders
It is clear that in the powder-handling area, special design criteria and process procedures are required. A dedicated (pre-) HACCP (Hazard Analysis of Critical Control Point) study should be part of the development process during the design phase of the process lines. Decisions on the required cleaning procedures (wet or dry cleaning) are crucial. In the case that only dry cleaning procedures are to be applied, less stringent design criteria might be possible. On the other hand, the equipment itself in the powder-handling area is often not of a hygienic design when wet cleaning procedures have to be applied. Rotary valves, sieves, bag filters, flexible connections are all examples of equipment and process line components that are still a challenge for equipment manufacturers to improve.
13.7
References
(2001) Document No. 22. General Hygienic Design Criteria for the safe processing of dry particulate materials, March. Campden & Chorleywood Food Research Association, Chipping Campden. EHEDG (2003) Document No. 26. General Hygienic Engineering of plants for the processing of dry particulate materials, November. Campden & Chorleywood Food Research Association, Chipping Campden. EHEDG (2004) Document No. 8. Hygienic equipment design criteria, April. Campden & Chorleywood Food Research Association, Chipping Campden. EHEDG