How do you determine the size of a Filter Unit?

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Have you ever had to choose the right extractor system for one of your customers and you weren’t quite sure which one to choose? Then this post will be very handy.
If you’re already an expert in extractor systems, perhaps some useful ideas will come out by reading this post or, even better, you can send me a message and teach me something!

Let’s start from a fact

Choosing an extractor system is not an easy task, there are a number of factors to consider. This is why I’d like to give you a few suggestions, which are not exhaustive though certainly handy: you’ll be able to complete your next quote on your own by knowing how to choose the right solution.


First of all, just few and yet fundamental technical considerations

foto Hp
For the sake of simplicity, let’s say that an extractor system consists of two elements: the fan and the filter unit. Here I’ll focus on choosing the filter unit, while in the next post I’ll tell you how to choose the fan.
The Filter Unit is the part of the extractor system where the drawn dust and chips are conveyed. What it is used for? It is designed to retain the dust and chips and let out the filtered air.
If the Filter Unit is too small it will not be able to effectively filter the amount of air and dust it receives from the fan: we’ll see a large amount of dust coming out from the ejection vent, the filter sleeves will quickly become clogged and the suction force will be affected.
A very large Filter Unit involves a very expensive purchase and maintenance and the cost is not justified.
An extractor system of the right size ejects extremely clean filtered air, allows for long operating time without extraction losses, ensures the removal of all the dust and chips produced during the processing stage, leaving a clean, healthy and comfortable operating environment for everyone working in it
The aim of this information is to provide you with the means to choose the right Filter Unit.


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Compliant with regulations
The reference guidelines impose a series of minimum requirements that come in handy for our story, by defining:
– The type of processable dust
– The type of fabric filter that can be used
The crossing (filtration) rate
– The required control systems
– The cleaning systems
– An accurate filtration rate based on the type of dust to be removed.



In our case, the filtration rate is the value that helps us calculate the size of the filter.



Let’s leave out some of these points for the time being and just focus on what we need to achieve our purpose: the crossing or filtration rate.
The filtration rate is a value that indicates the speed at which the air full of dust passes through the fabric filter. Clearly, the greater this speed, the worse the result in terms of filtering efficiency. The lower the filtration rate, the better the quality of the ejected air. The filters will be able to retain a large amount of dust, the fabric filter will last longer, it will not be affected by pulling or stretching due to the impact force of the air and the degree of pollution in the atmosphere will be minimal.



The regulation establishes the maximum crossing speed with reference to the type of dust to be removed with the extractor, i.e. based on their particle size: the coarser the dust, the higher the accepted speed. Very fine dust (for example, the dust resulting from smoothing processes) requires extremely low filtration rates.


Here below is an explanatory table:

Position Particle size (μm) Crossing rate
A ≥ 60 0,03 → 0,04 m/sec
B 30 ÷ 60 0,025 → 0,033 m/sec
C 5 ÷ 30 0.02 → 0,03 m/sec
D < 5 0.013 → 0.02 m/sec

This table is very important, as it allows us to define the size of our filter based on a mathematical formula:
Let’s suppose we have to create an extractor system for position A:
– dust with an average particle size ≥ 60 µ

– overall flowrate of 8000 m3/h
(You don’t know how to calculate the overall flowrate for a system)? I’ll teach you soon how to do this!)



This is the formula you need to use:

FLOWRATE  :  3.600  :  SPEED

In our example we’ll have:
8000: 3600: 0.04 = 55.55 m2


This means that if we want to work at a speed of 0.04 m/sec (as allowed by the regulation), we need a filter with a filtering surface of at least 55 m2.
Likewise, if we were to choose a filter to remove very fine dust, for example dust resulting from smoothing processes or powder associated with food like flour or sugar, with a particle size of approximately 20 µ, we would be in position C.



In this example, our formula would give the following result:
8000: 3600: 0.02 = 75 m2
This means that if we want to work at a speed of 0.03 m/sec, we need a filter with a filtering surface of at least 75 m2.



As you can see, the process is rather simple and consists of a few steps:
1. Calculating the required overall flowrate
2. Analysing the dust to be filtered
3. Choosing the relative speed required by the regulation
4. Applying the mathematical formula


What we suggest above is a simple, effective and safe formula in line with regulations that you can use to choose the right type of filter to offer your customers.


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