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Dispelling the positive pressure pneumatic conveying myths

When more can be less

When the productivity of a positive pressure pneumatic conveying system declines, or when the production of a site has increased and the system is asked to convey more material, then the first inclination of its operators is often to increase the flow or the pressure of the compressed air. This is understandable, in part because it seems like an easy and cost-efficient fix. Unfortunately, increasing the flow or pressure of the blower or compressor (or simply adding another) is rarely the best solution and, in many cases, can even lead to problems such as blockages. The reason is that, when it comes to positive pressure pneumatic conveying systems, more can be less. 

Blockages, high energy bills and unloading times: identifying the problem

Imagine you are pouring material into a funnel. An even flow will make sure that the material smoothly goes through the narrow end. However, if too much material is added at the same time, then that narrow end will clog up. This means that, in order to increase the volume of material that is transported, it would require a larger funnel opening in our example. And, only after enlarging the funnel opening, a greater flow will help you convey more material.

 

This is similar to a pneumatic conveying system. In most cases, the first thing you need to do is not to increase the size, pressure, or flow of the blower or compressor but rather the diameter of the pipeline.

That is why, when you notice that the performance of your pneumatic conveying system is declining, you first have to identify the problem.

 

In some cases, you may find that a leak is causing the problem or maybe your compressed air installation is simply getting old. Increasing the flow or pressure in these cases is also not a good solution. If there is a leak, this just wastes energy, and, if the blower is getting too old, then it has to be replaced. The good news is that, in either case, new pipelines do not have to be installed, which would be much more time-consuming and expensive.

To better help you troubleshoot these problems and optimize your system’s performance, let’s look at the common myths and misperceptions related to pneumatic conveying.

Myth #1: Increasing the flow will allow you to transport more material

This is almost always false. Since the pipeline size matters much more than the flow rate in determining how much material can be moved, increasing the flow often leads to unnecessary blockages that disrupt your operation and can result in costly delays. Only after adjusting the pipeline diameter to the right size, a greater flow will help you move more material in a smooth and efficient way.

Myth #2: Increasing the pressure will improve the performance of your positive pressure pneumatic conveying system

This is equally incorrect for much of the same reason. You cannot force more material through a pipeline and a pneumatic conveying system than it is designed for. If it experiences blockages or the performance is lacking, then the cause is most likely either a mistake in the system design or a blower or compressor that no longer delivers an optimal pressure and flow.

In this case, replacing the blower (instead of getting a bigger one) or adjusting your system, is the better solution. 

Myth #3: To boost the output of your pneumatic conveying system, you only need to increase the pipeline diameter

While it is true that increasing the pipeline diameter is often the only (or certainly the best) way to boost the volume of material that your system can convey, once you take this step, it is very possible that you then also need a larger blower or compressor.

The reason is obvious: Larger pipes mean that more pressure and flow are required to move the material – especially because it has to be moved at a certain speed in order to avoid blockages. If the full potential of the previous compressed air system has not been fully utilized, then it may be able to handle larger pipes. However, in many cases, an upgrade of your compressed air installation will be required. 

Myth #4: Using a medium-pressure compressor with ancillary equipment is better than using a blower or low-pressure compressor

This is absolutely false. Pneumatic conveying does not require a lot of pressure, so it makes no sense to use a medium-pressure compressor (which supplies air as of 4 bar(g) and up) together with a pressure vessel and a pressure reducer instead of a low-pressure compressor or a blower (which supply air from atmospheric pressure up to 4 bar). Even if this medium-pressure setup is already present at your site to provide compressed air to, for instance, control the valves, it is usually still more cost-effective to invest in a dedicated low-pressure system for your pneumatic conveying applications.

The reason is obvious: The larger compressor consumes a lot more energy, which increases your operating costs. Investing in a correctly sized compressor will, in most cases, give you a quick return on investment thanks to the energy savings. Per 1 bar you are compressing above the actual demand, you are wasting 7% of energy. 

Myth #5: Truck-mounted compressors can get the job done as well as external compressors, and, if not, an external compressor can easily be added

Truck-mounted compressors, for example, those for cement transporters, are often not an ideal solution because they are an “off-the-shelf" product instead of one selected for the specific application. They are very small and have low pressure and flow, meaning they are not up for the job.

It is much better to simply use an external blower or compressor for the job, which can greatly reduce loading and unloading times and ensure that your operation runs more smoothly.

Some users opt to add a second compressor or blower to support the truck-mounted compressor. But this does not have any advantages. On the contrary: it can lead to some of the aforementioned blockage problems. 

Making the right decision

If you don’t fall prey to these popular myths and misperceptions – and keep this information in mind when evaluating your pneumatic conveying system and making investment decisions – you will often be able to save money and do more with less.