Air flow rate evaluation
Introduction
The selection of a fan, blower or compressor is probably one of the most important decisions to be made in the design of a pneumatic conveying system. It is often the largest single item of capital expenditure and the potential conveying capacity of the plant is dependent upon the correct choice being made. The rating of the fan, blower or compressor is expressed in terms of the supply pressure required and the volumetric flow to be delivered. Any error in this specification will result in a system that is either over-rated, is not capable of achieving the desired material flow rate, or will cause a pipeline blockage and convey nothing.
For an existing system it is often necessary to check the performance, particularly if operating problems are encountered, or changes in material or conveying distance need to be considered. Here it is the conveying line inlet air velocity that is important. Since the determination of conveying line inlet air velocity and the specification of air requirements is so important for the successful operation of pneumatic conveying systems, all the appropriate models are derived and presented for reference purposes.
Although it is air that is generally referred to, materials can be conveyed with any suitable gas. Constants are included in the equations that will correctly account for the type of gas used when evaluating the volumetric flow rate required. Air mass flow rate is also considered, as it is a useful working parameter with regard to solids loading ratio, and as its value remains constant it is particularly useful in equations of continuity.
Supply pressure
The delivery pressure, or vacuum, required depends essentially upon the working pressure drop needed over the length of the conveying pipeline. The pressure drop across the gas–solids separation device can usually be neglected, but if a blow tank is used for feeding the material into the pipeline then an allowance for the pressure drop across the feeding device will have to be made. Consideration will also have to be given to the pressure drop in any air supply and extraction lines, and to the need for a margin on the value of conveying line pressure drop required to convey the material through the pipeline at the specified rate.
The magnitude of the conveying line pressure drop, whether for a positive or a negative pressure system, depends to a large extent on the conveying distance and on the solids loading ratio at which the material is to be conveyed. For short distance dilute phase conveying a fan or blower would be satisfactory, but for dense phase conveying or long distance dilute phase conveying, a reciprocating or screw compressor would be required. The pressure drop is also dependent upon the conveying gas velocity and a multitude of properties associated with the conveyed material.
Volumetric flow rate
The volumetric flow rate required from the fan, blower or compressor depends upon a combination of the velocity required to convey the material and the diameter of the pipeline to be used. Pipes and fittings are generally available in a range of standard sizes, but velocity is not so clearly defined.
For convenience the velocity at the end of the pipeline could be specified, for in the majority of cases compressors are rated in terms of ‘free air delivered’, and the pressure at the end of a pipeline in positive pressure systems, in most applications, will be sufficiently close to atmospheric for this purpose. It is, however, the velocity at the start of the line that needs to be ascertained for design purposes.
The problem is that air, and any other gas that is used for the conveying of materials, is compressible and so its density, and hence volumetric flow rate, is influenced by both pressure and temperature. If the plant is not located at sea level, the influence of elevation may also have to be taken into account. As a result of the compressibility with respect to pressure, stepped bore pipelines are often employed and so these are given due consideration.
In negative pressure systems the air at the start of the conveying line is approximately at atmospheric pressure, and it decreases along the length of the conveying line to the exhauster. For this type of conveying system, therefore, the minimum velocity that needs to be specified occurs at the free air conditions. Exhausters, however, are generally specified in terms of the volumetric flow rate of the air that is drawn into the air mover, and not free air conditions, and so it is essentially the same problem in evaluating air flow rates as with positive pressure conveying systems.
The influence of velocity
A conveying plant is usually designed to achieve a specified flow rate. Material flow rate can be equated to the solids loading ratio and the air mass flow rate. The air flow rate, in turn, is proportional to air velocity and pipe bore. Since these three parameters also have an effect on the compressor rating, it is extremely important that the correct air mover specification is made. The relationship between the various parameters that link the compressor rating and material flow rate is demonstrated with the path analysis shown in Figure 9.1.
Figure 9.1 also illustrates the importance of conveying air velocity in this relation- ship, as it influences both the supply pressure and the volumetric flow rate of the compressor. This helps to explain why conveying air is one of the most important variables in pneumatic conveying, and why it needs to be controlled fairly precisely.
If, in a dilute phase conveying system, the velocity is too low, it is possible that the material being conveyed will drop out of suspension and block the pipeline. If, on the
other hand, the velocity is too high, bends in the pipeline will erode and fail if the material is abrasive, and the material will degrade if the particles are friable. Velocity also has a major influence on the conveying line pressure drop, and hence on the mass flow rate of the material conveyed through a pipeline. The range of velocity, therefore, is relatively narrow, particularly in dilute phase systems, varying from a minimum of about 15 m/s to a maximum of around 30 m/s.
For dense phase conveying the conveying line inlet air velocity can be as low as 3 m/s, but this depends upon the solids loading ratio at which the material is conveyed and the nature of the conveyed material. If the velocity drops below the minimum value the pipeline is likely to block. It is important, therefore, that the volumetric flow rate of air, specified for any conveying system, is sufficient to maintain the required minimum value of velocity throughout the conveying system.
Material influences
It should be noted that in evaluating conveying air velocities and volumetric air flow rates in pneumatic conveying applications, the presence of the material is disregarded in all cases. The conveying air velocity is essentially the superficial value, derived simply by dividing the volumetric flow rate by the pipe section area, without taking account of any particles that may be conveyed.
In dilute phase conveying, and at low values of solids loading ratio, the influence of the conveyed material will have negligible effect in this respect. At a solids loading ratio of 100, however, the material will occupy approximately 10 per cent of the volume at atmospheric pressure and so the actual air velocity will be about 10 per cent higher. At increased air pressures and solids loading ratios the percentage difference will be correspondingly higher.
It would be a very complex and time consuming process to evaluate actual air velocities and so for convenience the superficial air velocity is universally employed.
Critical values such as the minimum conveying air velocity and conveying line inlet air velocity are mostly derived from experience and experimental work. In such cases it is the superficial value of air velocity that is used.
Compressibility of air
As with the flow of air only in a pipeline, or single phase flow, the flow of a gas–solid mixture will also result if there is a pressure difference, provided that a minimum value of conveying air velocity is maintained. Material flow with the conveying air will be in the direction of decreasing pressure, whether it is a positive pressure or a vacuum conveying system.
Since air is compressible, the volumetric flow rate of the air will gradually increase from the material feed point at the start of the pipeline, to the material discharge point at the end of the pipeline. In a single bore pipeline the conveying air velocity will also gradually increase over the length of the pipeline.
This means that it is the value of the conveying air velocity at the material feed point, or the start of the pipeline, that is critical, since the value of the conveying air velocity will be the lowest at this point, in a single bore pipeline. In determining the necessary volumetric flow rate of air, therefore, it is the condition prevailing at the start of the pipeline, in terms of pressure and temperature that must be taken into account.