A line filter is a device that is used for the treatment of compressed air. Its main mission is to clean the compressed air of impurities of all kinds, even bacteria.
Filters are the great allies in compressed air or gas installations. With them, we adopt the air/gas quality to the requirements of each plant. Usually, the way to estimate the level of air quality is done following the parameters of ISO 8573-1.
This regulation regulates the maximum level of pollutants in the compressed air, in terms of the amount of moisture, particles and residual oil.
In this article, we will only treat the level of contaminating particles, oils, water in liquid phase and bacteria.
The environment from which the air sucks the compressors is very variable. However clean the atmosphere surrounding a compressor may seem, it may contain some element in suspension that contaminates our compressed air system. From the simplest installation to the most complex, such as medicinal air, we must analyze the maximum permissible pollution limits and select the appropriate filtering system for each case.
By their application, they could be classified according to the following table:
– Pre-filters . They are the first filters that are installed after the compressors. Its main mission is to eliminate the coarse particles that could be sucked by the compressor. They are, in themselves, the protective filters of the filtering network that is installed next. Most manufacturers offer these filters with coalescing capacity, that is, they can remove a small amount of water and oil while removing solid particles. Its filtering capacity is usually greater than 1 micron and 1 mg / m3 of oil.
– Intermediate filters . They are installed after the pre-filters and their mission is to protect the different accessories and equipment that are installed in the air network. Its characteristics are determined according to the air quality required. So we will have:
- Filters of particles. They are like pre-filters but with a greater degree of filtering. For particles up to 0.1 microns and for oil or water up to 0.1 m3 / m3. At the outlet of the adsorption dryers, these filters are usually placed with a degree of 1 micron.
- Active carbon filters. Dedicated to the elimination of vapors and odors of oil. They do not eliminate them completely, but the residual they leave is very small. On average 0.003 mg / m3.
- Active carbon towers. They are used to reduce to the maximum the residuals of oil vapor and odors, with the idea of being used in human breathing systems.
- Catalyst filters. Used in human breathing systems for the elimination of Carbon Monoxide (CO).
– Final filters . In this group we include all the specific protection filters of the system before its final use. As an example, we can consider:
- Regulating filters. They are small filters that are installed on the pneumatic drive machine. They are formed by a particle filter and a pressure regulator. For machines that need lubrication, the filtering equipment itself assembles a lubricator. These equipment are commonly known as FRL.
- Bactericidal filters. Widely used in medicinal applications or in laboratories. They are used for the elimination of certain bacteria in the compressed air. Its maintenance requires periodic sterilization. Depending on each manufacturer, this can be done with steam or by autoclave. They have a very short lifespan, determined by the maximum number of sterilizations allowed.
The structure of the filters is very similar between the different manufacturers. It mainly changes the internal design, to improve the losses of load, the quality of the materials of the filtering elements, the degree of efficiency and the design of the element itself.
On the section of a set of filters from the manufacturer Kaeser, we will explain the main parts:
1. The body . Manufactured in different materials, it is usually cast aluminum. It is divided into two parts, the upper one includes the connections to the pipe and the lower one is removable to be able to change the filter element. They can be supplied manufactured in steel and in special stainless steel designs.
2. Filter element . It is the key part of the filter. According to its design, it can be used for different jobs. It is very important to know its efficiency, particle size and its coalescence capacity.
3. Condensate purge system . In the section, you can observe an internal purge of a buoy, which acts by floating on the condensate accumulated in the lower part. They can be supplied with other types of different purges, such as electronic, capacitive or manual.
4. Differential pressure gauge . This accessory will indicate the level of dirt that the filter element has. There are many different models, with voltage-free contacts for alarms or transmission of the differential pressure value through a 4 to 20 mAmp signal.
5. Modular connection system . It is very useful when in the same installation the pre-filters and post-filters can be combined because it facilitates the assembly and reduces the load losses.
6. Liquid level indicator . In the last generations of filters are not usually included, due to the fragility of the indicator and the risk of leaks, but they are very useful to see if the purge system is working properly.
One of the first things that we have to calculate when designing the compressed air installation is the pressure required for the compressor. This involves making a study of the pressure required in the machinery to be used and adding to it the pressure equivalent to the load losses, which both the interconnection pipe and the different equipment installed in it will create.
One of the equipment that is going to create that loss of load is the filters. We have to consider that these accessories have a charge loss value when they are new and a different one when they are used. Our installation will be more efficient the better we determine the number of necessary filters and their characteristics. We must also look at the design of the same because in the market we can find solutions that improve the performance of the filtering equipment.
In the photograph, we see an example with a filter of Boge Compressors. Just in the air intake has installed an ingenious system, which reduces the loss of pressure that occurs when the compressed air enters the filter and makes a sudden change of direction to go to meet the filter element.
The pressure drop value produced by the filters may seem small, but remember that it is constant, that is to say, that a pressure loss of 200 gr., Will have to be compensated with a higher consumption in the compressor for the rest of the life of the installation.
To select the filter appropriate to the installation we are designing, we must take into account several aspects:
- The quality of the compressed air we want to achieve. This is a vital aspect of our selection because it will determine the choice of type of filters, a degree of filtration and number of filters to be installed.
- The size of the filter. It will be determined by the flow and pressure of the compressed air system.
- Type of filter. We will analyze if we can use filters with aluminum or steel housings, if necessary a sterilization process or the control system of saturation and quality of the compressed air.
We recommend that the selection of the filters be made by the technicians of the manufacturers, but if it is a simple installation and sufficient documentation is available, it can be done taking into account the data of that documentation.
Manufacturers usually include in their catalogs correction tables on the indicated flow. We can see as an example, below.
These tables show us the efficiency of the filter, depending on the working pressure. Most data on the flow indicated in the catalogs refer to a pressure of 7 bar g. When it is necessary to work at a different pressure, the flow will have to be corrected according to the aforementioned pressure. In the previous table we see that for pressures below 7 bar g, the filtering capacity is reduced and on the contrary to higher pressures the filtering capacity increases.
Example. Assume a filter with a filtering capacity at 7 bar g of 1.2 m3 / min. This is a very standard flow rate in 3/8 “or ½” filters. Applying the correction factors of the previous table we will obtain the actual filtering capacity. Let us suppose the calculation of two pressures, 5 bar g, and 10 bar g:
- For 5 bar g. 1.2 x 0.85 = 1.02 m3 / min
- For 10 bar g. 1.2 x 1.19 = 1.42 m3 / min
This effect occurs due to the comprehensible capacity of the gases. As the design of the filter is thought to 7 bar g, when the pressure decreases, the volume increases and therefore the loss of pressure in the filter. Which would force us to reduce the filtering rate or choose a larger filter?
For the same reason, by increasing the pressure the volume decreases making our filter able to filter a greater flow. In this case, we could select a lower filter size and save on the purchase amount.
One last important fact is that the information referred to in the specifications of the filters is always considered at a temperature of 21ºC, this means that with an air cooling system in the compressor, the ambient temperature should be 10ºC for most of The manufacturers.
Any higher temperature will harm the oil removal by filtering because this factor influences the separation capacity. Under normal operating conditions, a filter chain composed of pre-filter, post-filter, and activated carbon filter can leave an oil residual of approximately 0.003 mg / m3.
If the application for which we need compressed air is very sensitive to oil contamination (laboratory applications, food, medicine, instruments, etc.), we recommend not opting for a filtering system alone but using compressors free of suitable oil to the process.