Hydraulics and Pneumatics

Air only relationships Introduction Although few reliable or universal models currently exist for predicting the pressure drop for gas–solid flows in pipelines, models for the single phase flow of a gas are well established. Although discussion will generally be in terms of air, the models presented will work equally well with the appropriate value of the specific gas constant for the particular gas being considered. Gas constants for a range of gases were presented in Section 9.2.3.2 with Table 9.2. Empty conveying pipeline pressure drop values, for air only, will provide a useful datum for both the potential capability of a system for conveying material and the condition of the pipeline (considered in Chapter 20). Air only pressure drop values for the conveying pipeline also provide a basis for some first approximation design methods for the pneumatic conveying of materials (see Chapter 18). Air supply and exhaust or venting pipelines can be of a considerable length with some ...

The influence of pressure The influence that air pressure has on volumetric flow rate is shown graphically in Figures 9.3–9.5. These are plots of volumetric flow rate, at the reference atmospheric pressure of 1.013 bar absolute, against actual volumetric flow rate. To simplify the problem an isothermal situation has been assumed in order to isolate the influence of pressure, that is: T 1 T 0. Once again this is a linear relationship. A series of lines representing the relationship for different air pressures is given on each graph, and each one illustrates the relationship for a different type of system. One is a low positive pressure system, another is for the use of high pressure air, and the third relates to vacuum conveying. System influences In Figure 9.3 the pressure ranges from 0 (atmospheric) to 0.8 bar gauge and so is appropriate to low pressure, typically dilute phase, conveying systems. If an air flow rate of 25 m3/min at free air conditions is considered it can be see...

Steam sterilization Routine sterilization is a requirement for any system using a sterile filter. It is often carried out between batches or in a continuous process plant at prescribed intervals. In batch operation provided that the filter is kept pressurised and a small air bleed is allowed to flow continuously, several batches can pass before sterilization is carried out. The steam used must be saturated and free from additives or contamination, which implies its own sterile filter, see Figure 5. Steam sterilization is shown in the lower diagram of Figure 3. A well designed system should last for 12 months before cartridge replacement is necessary; this probably means of the order of 100 sterilization cycles (according to the recommendations of the manufacturer). Typical cycles are shown in Table 1.The cost of replacement cartridges is small compared with the cost of a failure of the batch due to contamination, so replacement times should not be extended beyond 12 months. ...

BREATHING AIR FILTRATION The purity requirements for compressed air used for respiration and hygienically critical applications such as hospital patient recovery and life support in hazardous environments such as mines, underwater tanks, etc, must be free of toxic or irritating ingredients, odours dust and solid particles. Numerous organisations and agencies around the world have established specifications pertaining to the purity of compressed air for human respiration. Although variations in requirements exist, mainly due to the method and type of application, it can be seen from Table 1 that there is substantial agreement between the various standards. is undecided. but is likely to be 5 mllm' (parts per million- ppm) in U.K. and 15 mllm' in other European countries. There shall be no toxic or irritating ingredients, odour. dust or solid particles. For the U.K., 10% of the values included in EH40/94 Occupational Exposure Limits, published by the Health and Safety...

Carbon dioxide This is naturally present in the atmosphere in various concentrations depending on the environment and may be tolerated at much higher levels than the CO, but will impair psychomotor functions if the levels are high. It is readily removed by a variety of desiccants, both adsorptive and absorptive, with the absorptive type predominating. These materials have a limited capacity and therefore need replacing or reactivating at regular intervals. Nitrous oxide and nitrogen oxide These are present in the air as effluents of industrial processes and by combustion within the compressor. They irritate the nasal passages and can be detected in very small concentrations. They can be removed by an appropriate grade of activated carbon or by molecular sieves. Odours These can be a problem, even when the compressor is correctly sized, sited and well maintained. The presence of odour is to some extent subjective and is therefore unreliable when used as the only means of monit...

STERILE AIR AND GAS FILTERS The demand for sterile compressed air increases with the adoption of advanced technolo­gies which were unknown a few years ago. The selection of a sterilization filter for a compressed air system can be a difficult task. The production of proteins, vaccines, antibodies, hormones, vitamins and enzymes involves high technology processes which require aseptic and sterile supplies of gases or liquids throughout the manufacturing cycle. The production and packaging of many dairy and food products such as beer, yoghurt, cream and cheese use compressed air or carbon dioxide. The nature of these products makes them susceptible to contamination by micro-organisms held in the compressed air or gas. Any product that can be contaminated by airborne bacteria must be protected. In the case of food and chemicals produced by fermentation, bacteria would cause serious defects and rejection of the product. In the fermentation and pharmaceutical industries, compressed...

Hollow fibre filter Hollow fibres were originally developed for dialysis and have now been adapted for compressed air. The filter is arranged with a smaller spread of hole sizes, but with a larger number of membranes. This gives a greater retention ability. Due to a closer pore distribution than conventional membrane filters, the number of pores per unit area of filter is greater, which extends the service life. A closer pore distribution means that the largest pore size is much reduced. A conventional membrane filter has a pore size at least 0.3 micron larger than a hollow fibre filter. The development of a membrane filter element with a rated pore size ofO.I micron and a reduced pore distribution means the difference between the retention of viruses and bacteria. The construction of the hollow fibre filter means that it is economical to manufacture smaller elements for laboratory use. Housings Sterile filter cartridges must be fitted into a pressure holding housing. The method ...

Breathing apparatus standards BS 4275, which has already been referred to, includes recommendations for the selection and maintenance of respiratory protective apparatus. It is currently being revised to bring it into line with current legislation. BS 4667 can be studied, particularly Part 3 which deals with compressed air line breathing apparatus, which is used in industry for paint spraying and for work in hazardous atmospheres. It is however being superseded by a range of European Standards, in particular: BSEN 139, BSEN 270, BSEN 271, prEN 1835, prEN 12419 Compressed air apparatus is supplied with clean air through a small diameter hose which is connected to a pressure reducer attached to the wearer. A larger hose takes air from the reducer to the face piece. The air supply may be continuous and controlled at a higher rate than the maximum needed during inhalation or it may be controlled by a demand valve. Of these there are two types, one which opens only when there is a negati...

Carbon monoxide This is the most insidious danger faced by compressed air users. As a contaminant of compressed air, carbon monoxide is usually introduced via the intake port of the compressor. The compressor power drive itself (exhaust fumes from the internal combus­ tion engine) can be a major source of CO. In addition, thermal decomposition of the compressor lubricant due to high local temperatures can result in the formation of CO within the compressor. The toxicity of carbon monoxide has, by the nature of the CO gas molecule, a 300 times greater affinity for the haemoglobin molecule than does oxygen. At levels of only 2% CO/haemoglobin involvement in the blood stream, disturbances to the psychomotor functions can occur, but this does not normally become significant until 5% levels are reached. At elevated pressures, however, absorption may be more rapid and complete, and any impairment to the higher functions may be disastrous. A 2% involvement corresponds to about 10 ppm ...

COMPRESSED AIR FILTRATION The properties of compressed air make it a versatile, secure and economic medium that is economic to produce and handle. Compressed air is used either as a carrier medium for the transport of energy to the point of use where its potential and kinetic energies can be converted into a driving force for pneumatic equipment, or as a process medium itself (eg breathing air) or for processing purposes (agitating, mixing, packaging, conveying and pressurising). These applications can be found discussed in other chapters in the book. For most applications of compressed air, a requirement of air quality exists, expressible as a permissible level of contaminants. The purity demands of high precision complex and fully automated pneumatic systems and of chemical, biochemical, biomedical, electronic, pharmaceutical and food process­ ing applications increase year by year, requiring a continuously improving level of air treatment. The degree of treatment of air for ...

Inducted contaminants Industrial stationary compressors are normally installed in separate rooms away from factory contaminants, drawing in air from the outside atmosphere as in Figure 3. The level of dust concentration is likely to be of the order of I0-50 mg/m 3• It is standard practice to fit the compressor with an intake filter, having an efficiency of the order of 99.9%, based on the dust concentration present in the ambient air (see Tables 3 and 4). The intake filter can be expected to pass all particles smaller than 5 micron as well as a proportion of the larger sizes. It will also pass atmospheric water vapour and all gases, bacteria and viruses. Types of intake filters Intake filters (Figure 4) commonly used are: • Paper filters- with renewable elements used on all types of compressors. Filtering efficiency is high (over 99%) with a typical pressure drop of 2.5 to 3.5 mbar if correctly sized. They are not recommended for use with reciprocating compressors unless...