Problem-Free Air Conveying Part III

Consider the possible effects of ambient conditions on operations and the potential impact of the system on product characteristics

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Temperature. High temperature products can be conveyed quite successfully and conveying gas at any temperature can be used. Compatibility with system components is the determining factor. Conveying air velocities also have to be guaranteed if there are significant temperature changes.

The evaluation of gas and product temperatures presents the difficulty. At the feeding point, for example, cold air may be used to convey a high temperature product. Along the conveying line, there will be a move towards thermal equilibrium between the air and product, as well as heat transfer from the pipeline to the surroundings. Since conveying times are very short, it is unlikely that equilibrium will be established. It is quite possible, therefore, for the surface of the particles to be "cold" and the inner core to be "hot." Because of this, it is often possible to use filter cloths in these high temperature situations. By the same reasoning, the product in the reception hopper could be very hot once equilibrium has been established there.

The maintenance of conveying air velocities is particularly important in these situations, but their evaluation can be difficult. Particle temperature transients represent a complex three-dimensional heat-transfer conduction problem and should only be attempted by an expert. However, since air density increases as temperature decreases, the maintenance of air velocities is only likely to be a problem when a very high temperature gas is used to convey a cold product. In this case, the temperature gradient effect could override the pressure gradient influence on air density.

Wet products. Fine products that are wet will tend to coat the pipeline and gradually block the line. If the product is not too wet, heating the conveying air can relieve the problem. Difficulty may be experienced in discharging a wet product from a hopper.

The conveying characteristic
As in many plant situations, troubleshooting would be relatively straightforward if you know what information is required, and can obtain high quality data.

The three major variables that specify the operating point of a pneumatic conveying system are: solids mass-flow rate; gas mass-flow rate; and pressure gradient (pressure drop per unit length).

One way of presenting these variables is to plot solids mass-flow rate against the mass flow rate of gas, as shown in Figure 2. This graphical form is referred to as the conveying characteristic or performance map. A conveying characteristic applies to a particular bulk material and a particular pipeline.

 

A Typical Conveying Characteristic

Figure 2. This performance map for cryolite plots the three major pipeline variables: solids mass-flow rate, gas mass-flow rate and pressure drop per unit length.

 

In this representation, the third variable, conveying-line pressure drop, is presented as a set of curves. Each curve represents a line of constant conveying-line pressure drop. The shape of these curves varies and depends on the conveying capability of the particular material. A comparison of different conveying characteristics shows that the shape of the curves is governed by the mode of conveying, which itself is determined by the physical properties of the material being conveyed.

The extent of the performance envelope for a conveying characteristic is bounded by four limits:

1. The lower limit due to the air-only pressure drop for the pipeline;

2. The right-hand limit, which is governed by the volumetric capacity of the air mover; (Using a larger capacity machine would increase this limit but rarely offers an advantage, because this simply limits the rate at which material can be conveyed.)

3. The upper limit can be due to either the pressure rating of the air mover or the maximum rating of the solids feed device (which is the case here); and

4. The limit to the left-hand side of the characteristic is normally the most important as it marks the boundary between flow and no flow. For a system to operate without possibility of a blockage, the operating point must be to the right of this boundary.

Some materials possess physical characteristics that prohibit conveying in non-suspension modes of flow in conventional pipelines. In such cases, the limit of the pressure drop curves to the left-hand side of the graph corresponds to a minimum velocity. In this case, the material remains predominantly in suspension. Typically, this minimum velocity would be about 15-18 m/s (3,000-3,600 ft/min). These systems are often referred to as dilute phase systems.

The conveying air velocity is a critical parameter. The velocity at the point where material is fed into the pipeline is particularly important. If the velocity is too low, pipeline blockage may occur. If the velocity is too high, the rate at which material can be conveyed will be restricted and problems such as particle attrition and erosion may result. It, therefore, is essential to know the conveying air velocity in order to assess the performance of a system and the potential for optimization and uprating.

On-line measurement
To determine the system operating point on the conveying characteristic graph, you must have data on the air flow rate, the material flow rate, and the conveying line pressure drop. In addition, depending on the application, measurements of temperature may be required.

However, most pneumatic conveying systems include very little diagnostic instrumentation. In many cases, a simple pressure gauge mounted on the air supply line is all that is available.

Material mass-flow rate is certainly the most difficult measurement. Generally, only an average conveying rate can be obtained. An estimate of the air flow rate can be found if performance curves for the air mover are available. In some cases, these estimates can provide enough information to identify a particular problem. In other cases, estimates can be so inaccurate that at best they are unhelpful and at worst are actually misleading.

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