In large cities, it is necessary to clean the air to avoid health problems. Every year, air pollution has been increasing worldwide. The population increase, the growth of cities, urbanization, industrial development, transportation, and the consumption of energy resources cause pollution. The restoration of the atmosphere in such spaces to its original conditions does not occur naturally.
The air quality directly affects human health because the respiratory system is unable to process the polluting particles that float in the polluted air. Therefore, it is very important to act on this subject. The city’s population, according to the studies carried out worldwide, spends about 80% of their time in enclosed spaces. In these spaces, the concentration of pollutants in the air can be up to 20 times higher than in outdoor spaces. Therefore, it is important to clean the indoor air [1,2,3,4,5].
We propose to develop air filters manufactured with the microequipment technology (MET) . These filters can clean the air in enclosed spaces, providing a safeguard for citizens’ health.
The filters available in the current market are costly; their manufacturing cost makes it practically impossible to implement them in offices, homes, schools, common areas of hospitals, and public transport.
MET can use most of the available materials . The amount of material used to make microcomponents is less than that used by larger machines. The energy for manufacturing is also lower because micromotors consume small amounts of energy, and the necessary torque for their operation is much smaller. We have proposed and previously used MET for the development of the minifilters and microfilters.
Health services recognize that the environment plays an important role in the development of respiratory diseases. The US Environmental Protection Agency (EPA) is in charge of regulating the air quality in closed and open spaces [7,8]. According to the EPA, the levels of pollutants indoor could be 100 times higher than those outdoor. Volatile organic compounds (VOCs) are organic chemicals in the air. In the respiratory system, for example, the pollutants and VOCs to be studied specifically are cigarette smoke, radon, carbon monoxide, nitrogen dioxide, formaldehyde, cleaning agents, animal waste, and dust mites . VOCs are also biologically generated . Many health problems are caused because of poor indoor air quality; usually, these health problems are not recognized without the adequate tools until several years have passed [2,9]. Another interior microenvironment where human beings spend most of their time is car cabin; according to the studies conducted by Zhang et al. in new cars in China , the amount of VOCs is higher than that established by the Chinese National Quality Standard of Air in Interiors. According to the studies carried out by Mukund et al. , the levels of VOCs in the air inside automobiles can be up to eight times higher than that in the vicinity of the monitored space, representing an important health problem. During certain activities, indoor levels of VOCs may reach 1,000 times that that of the outside air. Respiratory, allergic, or immune effects in infants or children are associated with man-made VOCs and other indoor or outdoor air pollutants . VOCs contribute to the formation of the tropospheric ozone and smog [5,7,8]. Secondhand smoke is a public health problem. In 2006, the term “involuntary smoke” emerged because nonsmokers do not want to breathe tobacco smoke . The smoke tends to remain inside closed spaces for 1.5–2 h.
Exposure to radon is considered the second cause of lung cancer after the consumption of tobacco. Therefore, it is necessary to know the burden of disease due to radon exposure. In Galicia, Spain, a study was carried out on the effect of radon on the development of cancer in the local population .
Carbon monoxide poisoning is one of the leading causes of death worldwide. In the United States, several cases of death from carbon monoxide poisoning are reported, particularly in adverse situations, such as after a hurricane or flood, when electric energy generators based on hydrocarbons are used in interior spaces without adequate precautionary measures. This is because a ventilation system is required to evacuate the emissions of combustion to ensure good quality air inside the house .
Nitrogen dioxide (NO2) is one of the air pollutants produced during combustion where nitrogen is oxidized. In exterior areas, the main sources of NO2 are the emissions from automobiles. At home, all devices that work with fuels based on hydrocarbons such as heaters, ovens, and stoves produce NO2.
Formaldehyde is an air pollutant commonly found in buildings that use wood, especially for the resins used for their bonding. It is also found in plastics, textiles, carpets, furniture, pesticides, paints, glues, and cleaning products. To perform daily cleaning, chemical products are commonly used; however, many of them contain harmful substances that impair air quality.
Molds that are developed easily on trees and leaves on the outside of homes can be taken to closed environments inside homes, schools, or business centers with relative ease, where it represents a danger to the health of the lungs. Approximately, 100 molds have been identified as potentially harmful to human health, and only a few are commonly found in indoor environments . Mold can affect the respiratory tract. A study reveals that about 21% of asthma cases are due to excessive exposure to mold.
New tools, printers, and computer equipment started filling offices, and the number of staff inside offices also increased dramatically along with the hours of work. It is believed that these changes resulted in a higher concentration of air pollutants inside buildings . The concentration of pollutants in buildings depends mainly on the circulation of air. To achieve displacement of the air, ventilation systems are used. The cost and the noise produced by ventilation systems are the two negative aspects of the implementation of this solution. To solve the problem of the energy used, the implementation of renewable energy, passive filters, and maintenance is proposed . Passive filters such as the proposed microfilters can control the levels of contaminants in closed spaces, proving to be a benefit for the health of large city inhabitants. Therefore, it is important to emphasize the relevance of this topic for the society because our health and the health of future generations are being considered. It is important to analyze the environments where people spend more time, such as offices, schools, and homes. Furthermore, it is important to consider the different factors such as ventilation, air conditioning systems, the renewal of the air, cleaning products used, pet excrements, the design of the building, and external contaminants .
This article is structured as follows: Section 2 describes the methods of air purification for interiors. Section 3 presents the design of the microchannel filter and its manufacturing process. Section 4 presents the theoretical analysis of pressure drop in the filter. Section 5 summarizes results and discussion. Section 6 describes the MET. Section 7 presents new proposals. Section 8 concludes this article.
2 Air purification of interior
Air pollutants in enclosed spaces are generated from both exterior and interior sources. Inside, they are generated by smoking, burning wood in fireplaces, stoves, water heaters, enamels, aerosols, and pet animal wastes. Outside, they are generated by the emissions caused by the traffic of internal combustion engines of vehicles, industrial waste in communities near factories, natural disasters such as floods and fires, etc. . Most air filtration processes are based on sorption, which is the retention of one substance by another when in contact. The different sorption filtration mechanisms are described in this article.
Sorption filtration removes contaminating gases from indoor air using solid absorbers. It is the most commonly used technology. Most commercial products are based on this technology. The effectiveness of cleaners is based on adsorption technology, which depends on the properties and the amount of sorbents, the packing density of the sorbent layer, the speed and air flow through the sorbent media, the properties of the VOCs, and the conditions of the environment such as humidity and temperature. Depending on the requirements of application, adsorbents such as activated carbon, zeolite, and alumina activated with various packing densities can be used as filtration media. Sorption filters are found in most commercial automobiles to purify air in the passenger cabin. The service life of these filters is determined by the adsorption capacity of the filter element. New sorption technologies involve a desorption stage to clean the filter element and to increase the life of the filtration system .
Ultraviolet photocatalytic oxidation sorption removes gaseous pollutants by chemical reactions on the catalytic surface of a semiconductor under UV irradiation. The resulting hydroxyl radicals are a highly reactive species that can oxidize VOCs adsorbed to the surface of catalysts .
Currently, the most widely used photocatalyst for purification is TiO2. Depending on the type and the level of concentration of the treated pollutants, the generation of harmful intermediate products and derivatives should be considered [20,21].
Air ionizers create charged air molecules under the application of a power source. The recent developments of an air ionization process control with devices that use dielectric barrier discharge to generate nonthermal plasmas have resulted in applications to clean indoor environments . VOCs are eliminated by a complex series of oxidation reactions with CO2 and water products. The efficiency in the destruction of VOCs depends on the ion density, the treatment time, and the chemical structure of the VOCs . The most widely used method for the generation of ions in the air is by thermal plasma reactors . Ozone is a powerful oxidant. Theoretically, it can react with several VOCs found indoors. VOCs that react with ozone fast enough can produce other pollutants, aldehydes, and organic aerosols .
Botanical cleaning of air removes pollutants from the air in indoor environments through plants and their soils through biological processes. It is the newest technology among those presented in this work. It is novel and is currently not available in the North American market. The results obtained show that botanical cleaning can significantly reduce formaldehydes and VOCs under controlled conditions [26,27,28]. The botanical air purifier works through on–off cycles because it requires a lapse of biodegradation to continue working . Therefore, the characterization of the system determines the characteristics of the operation cycle.
3 Microchannel filter manufacturing
For the manufacture of the microchannel filter support, X-660 laser cutting machine from Universal Laser Systems was used. The specifications indicate that for a depth of 0.25, it is possible to work at 1,000 ppi at a speed of 0.1 s per step .
To make the cuts, it is necessary to work with files with DWX or DWF extension of AutoCAD. Most cutting equipment work with these types of files.
There is an interesting approach for manufacturing parts with the 3D-printing process . Recycling metallic powders used in the additive manufacturing process is essential for reducing the process cost, manufacturing time, energy consumption, and metallic waste.
3.1 Design of microchannel support base
The microchannel filter works through the principle of sorption. The air (fluid in gas phase) or water (fluid in liquid phase) passes through the filter. Therefore, we developed a two-phase filter. Sorption is carried out by water. Later, the solid particles are separated by decantation. VOCs suspended in the air can be accumulated and used as a raw material to make fuels.
The general characteristics of the microchannel filter are described in this article (Figure 2).
The manufacturing material is the acrylic glass that allows the observation of air and water flow with the naked eye.
We used standard screws of an eighth inch and a sixteenth to form the microchannel between the screw and the walls of the device (Figure 3).
The control volume through which fluids and particles pass through is obtained from the empty space between the screw and the support (Figure 4). The microchannel is obtained as the counterpart of the elements that comprise it, as shown in Figure 5 (cross section of the microchannel).
The cross section of the microchannel is 0.16 mm2, with a perimeter of 1.84 mm. The length of the sides is 0.61, 0.62, and 0.62 mm. These two sides are formed by the screw cord and by the acrylic support. Two thirds of the surface of the microchannel is made of steel and one third is made of acrylic. The commercial stainless steel presents good permeability . However, acrylic has greater water absorption (approximately 0.3%) . The instruments for flow measurement made with steel and acrylic have a water absorption percentage of 0.35%.
An isometric cross-sectional view allows us to better understand the design of the microchannel, where it can be seen that the contact surface is maximized because of the pitch of the M4 roll cord (0.8) .
To calculate the effective area of the filter, we used 0.16 mm2 multiplied by five, and in this case, 0.8 mm2 (Figure 6).
3.2 Microchannel filter-based cell
To construct microchannel filter arrangements with adaptive geometry, the basic design of the fundamental unit of the filter is isolated. This is obtained by optimizing the materials, and a support material and a standard screw are required, as shown in Figure 7.
3.3 Microchannel filter matrix
The microchannel cells are arranged to construct a filter in geometries necessary to cover the air intake area in closed-air intake systems (Figure 8).
The microfilter design depends on the type of particles to be separated from the air, the maximum tolerable pressure drop in the microchannel flow, the filter life, the cost of filter manufacturing, the amount of energy and the material used for its fabrication, operating conditions, and resistance to corrosion .
3.4 Microchannel filter prototype
The following proposal was developed after evaluating each one of the aforementioned characteristics. In Figure 9, we present the microfilter prototype and describe it in the following sections.
The prototype was fabricated with material that can be easily acquired commercially. Therefore, its manufacturing cost in upscaling can be reduced.
The support or the filter base was fabricated from a crystal acrylic cell (colorless) on which the structure of the microchannel was subsequently placed. In this case, during filter functional tests, the operation can be observed with the naked eye. The fluid crossing the canal can be observed because of the acrylic material, which has duly verified specification sheets and is available in the local market. Its use is growing because of its cost-to-benefit ratio, low manufacturing cost, and excellent physical properties.
Plexiglas acrylic sheets are widely marketed in Mexico and are properly characterized. Its physical and mechanical specifications have been documented. This brand of acrylic was selected for manufacturing the first generation of the prototype. The properties of the material may vary due to thickness.
Plexiglas properties (%)
|Property||Method||Plexiglas G||Plexiglas MC|
|Weight lost in drying||0.1||0.1|
|Weight gained in immersion||0.2||0.3|
|Lost water soluble||0.0||0.0|
|Change of dimensions in immersion||0.0||0.0|
|Water absorption (weight gained) after immersion||D229 D570|
It is necessary to consider the water absorption of acrylic because it can be observed from Table 1 that the prolonged exposure to an aqueous medium changes the percentage of water absorbed by the material. It is necessary to consider this because the absorption properties affect the performance of the filter during its life.
Taking the aforementioned factors into consideration, it was decided to use 0.236 inch plexiglass. The main advantages of this material were its mechanical properties, the wide available documentation, the standardization of its manufacturing processes, and the quality control.
The laser engraving technology presents an important area in microchannel manufacturing because it can significantly reduce the manufacturing time of the filter. Current equipment can fabricate channels up to 50 µm [33,36] and can use various materials from glass to polycarbonate. It is possible to make channels with complex geometries in three dimensions with clean cuts. The roughness resulting from the cut is much lower than that obtained by other manufacturing processes. Therefore, for the prototype working scale proposed at this project stage, the precision of the equipment is enough.
Integrating laser technology into MET equipment could lead to a whole new area of opportunities for the manufacture of sorption filters by computer numerical control (CNC) methods.
At the mesoscale, a precision of 0.1 mm is relatively necessary. At the microscale, a tolerance close to 0.1 µm is required to achieve good results.
To determine the tolerance of this component, it is necessary to establish the assembly points with the different components of the device. The support has an acrylic tube on the outside, in the internal part the support has the five holes of ¼ inch (0.635 cm).
3.5 Tolerance analysis between the support and the acrylic tube
In the assembly, the base dimension of the acrylic tube must be considered as the base dimension overall. The placement of the support inside the tube will depend on the precision with which the acrylic tube is manufactured. For fine adjustment, an epoxy adhesive is used. The thinness of the adhesive film compensates for the dimensional difference between the tube and the support. The acrylic tube of 2 inch outside diameter has a tolerance of ±0.015 inch. The support is manufactured considering the positive tolerance of the inside diameter of the tube, and in this case, 1¾ + 0.015 inch .
3.6 Tolerance analysis of the structural support base
The base tolerance is provided by the specifications of the ¼ inch screw, which indicates the following characteristics .
The structure of the filter channel is formed by taking advantage of the screw cord of a ¼ inch screw, with the limitation of working only with screws under manufacturing standards.
A series of configurations of the screws are fabricated for a wide variety of applications, such as the screw head and the type of thread. For the application to a microchannel structure, the captive screw is of particular interest because this screw type is without a head, which allows the passage of air without obstruction (Figure 10) .
The type of the rope and the material of the screw are considered for the selection of the screw characteristics to be used and the thickness of the support.
Because the support has a thickness of 6 mm, the commercial screw length closest to this value was selected.
Regular thread was selected to work with the lowest possible pressure drop. Because a fine thread has a greater number of turns per inch, as presented in Table 2, where the first column indicates the product key; the second describes the diameter, the number of turns of string per inch, and the length of the screw; the third describes the price, and finally, the last column indicates the quantity of pieces per package (Table 2).
Commercial screw characteristics and prices
|XAOS06006||1/4–20 × 1/4||$2.08||100|
|XAOS06009||1/4–20 × 3/8||$2.55||100|
|XAOS06013||1/4–20 × 1/2||$3.32||100|
|XAOS06016||1/4–20 × 5/8||$4.63||50|
|XAOS06019||1/4–20 × 3/4||$5.10||50|
|XAOS06025||1/4–20 × 1″||$8.04||50|
|XAOS06038||1/4–20 × 1.1/2||$11.16||50|
|XAOS06051||1/4–20 × 2″||$11.73||50|
The screws have an error of ±0.01%. Starting from this margin of error, we determine the tolerance of the holes that will contain each of the screws. If we add 0.01% to the dimension of the screw, we can obtain the maximum value of the diameter of the hole, i.e., 0.254 + 0.00254 inch. The sum of this is 0.25654 inch. For the minimum value, we will have to subtract 0.00254 inch from 0.254 inch, and the result is 0.25146 inch. The dimension that ensures the assembly is the minimum. Therefore, the hole must be of 0.25146 inch with a tolerance of 1%. The adjustment of the screw is carried out by means of sanding of the material.
For the assembly when using the established tolerances, three cases are presented, which are as follows:
First case: The screw does not easily enter the hole and is adjusted by sanding.
Second case: The screw just enters, and it is not necessary to perform an additional procedure.
Third case: The screw does not enter, and the force assembly is performed, where a certain amount of material is removed from the screw as it moves through the hole. Figure 11 shows the assembly of the microchannel filter elements.
The microchannel filter assembly serves to verify the manufacturing tolerance of each of the parts that make up the filter (Figure 12).
Assembly by adhesion is carried out after the adjustment between the screws and the support. A homogeneous epoxy glue film is applied to form a solid connection between the acrylic support and the carbon steel from which the screws are made (Figure 13).
Then, the pipe is inserted into the filter inlet and outlet for the two working fluids, air and water, so that the interaction takes place. It was decided to place the fluid inlets perpendicularly with respect to the horizontal microchannels.
The front view shows the alignment between feeders and microchannels, as shown in Figure 14.
Two types of assembly tests were performed:
Air test: For the air test, a compressor of 250 ps pressure and 3 hp of power was used. The system is fed with a constant air flow. Measurements are performed at the inlet and outlet. The seal between the elements, acrylic tube, and microchannel filter are verified.
Water test: For water testing, a water pumping system with a minimum flow rate is used to evaluate the seal between the microchannel filter and the acrylic tube.
In this test, the leakage of the working fluid is analyzed. Water in the joints between the screws and the support is analyzed.
The sealing process between the filter and the acrylic pipe requires special attention because commercial sealants work at regular pressure well below those required for filters of these characteristics.
The pressure drop of the microchannel filter is very large due to the low porosity of the filter. Therefore, it is necessary to perform micromachining at smaller scales.
It is necessary to make filters with fully controlled specifications such as the diameter and the length of the channel and to test them with specialized instruments in the range of operation of the microscale to understand the behavior of the air flow at the microscale.
Increasing the porosity of the microchannel filter is essential so that this device does not represent a significant pressure drop within an air conditioning system.
In the design of microchannel filters, it is necessary to pay special attention to the mechanism of particle separation. This depends on the operating range of the filter. The filter works within the pressure range of the air supply equipment. The viability of the filter depends on this value because it will take more or less energy to move air through the filter.
3.7 Microchannel filter characterization system design
It is necessary to characterize the microchannel filter to determine the behavior of the filter absorption control variables. Therefore, a bank of characterization of microchannel filters is proposed with the following arrangement (Figure 15).
With the test bench, the microchannel filter can be dynamically analyzed, thus determining the operating range of the filter [38,39]. The manufacturing materials, the pressure drops, the amount of energy required moving the air inside the filter, the air purification efficiency, and other important features in this type of equipment will be evaluated.
It is possible to scale the microfilter up to 1/16 inch without complications because M4 screws are commercially manufactured. However, for smaller scales, it is necessary to manufacture screws of smaller dimensions. Hence, it is proposed to work with MET technology developed by E. Kussul and coauthors [6,30,31,32,40]. To reach 10 µm of the cross-sectional area in the microchannel, it is necessary to separate 10 µm particles and air.
4 Theoretical analysis of pressure drop
The pressure drop depends on the cross section and the geometrical form of the microchannel (Figure 16). The higher the cross section, the lower the pressure drop. When the cross section is high, the pressure drop is low, and the load due to the aerodynamic flow of the air that the microchannel can support is also small.
Filter simulation was realized with SolidWorks. So far, the results of the simulations performed using SolidWorks have yielded the following results (Figure 17).
As shown, there is only flow in the last holes of the filter. Therefore, it will be very important to obtain an appropriate distribution to occupy the whole filter geometry.
It is possible to observe the pressure drop in the filter (Figure 18). It will be important to have an appropriate compression source to obtain the pressure losses at the most favorable points to satisfy our objective of filtering the air.
To determine the maximum pressure drop, the flow is increased as much as possible, and the pressure difference between the inlet and outlet of the microfilter is measured. It is possible to reduce the pressure drop by reducing the dimensions of the microchannels through MET [6,30,31,32,40].
5 Results and discussion
Two types of assembly tests were carried out. The first test with air required a compressor for 110 psi of pressure and 3 hp of power. The results are presented in Table 3.
Air test results
|Flow (CFM)||Fall of pressure (psi)|
The graph of the pressure drop is shown in Figure 19. It can be seen that the higher flow pressure drop tends to 8 psi. The tests were performed at 20°C ambient temperature and humidity of 35%. To measure the flow, we use an anemometer of Amprobe model TMA-21HW and a pair of cover gauges. To obtain the flow, Mikel’s model CA-3HP compressor was used.
For the water test, a commercial water pumping system was used. With this test, joint leakage of the working fluid was analyzed. It was concluded that the filter has no joint leakage between the screws and the support.
The MET was developed by scientists from Ukraine and Mexico . Every next generation we do with previous generation, and its size is approximately two times less in comparison with the previous one. In Figure 21, we present different projections of the microCNC.
To control the position of the actuators, we used micrometers with a gear box. The prototype is presented in Figure 22.
For the automatization of microequipment, we developed different algorithms using computer vision and neural networks [40,41,42,43,44,45,46,47,48,49]. We give many references to these investigations. Readers who are interested can refer them, but those studies are not the theme of this article.
One of the first applications of the MET technology was development of microchannel recuperators for Ericsson heat engines [50,51]. Now we want to describe the new prototype of the microchannel filter.
The pressure drop of the microchannel filter is very large because of the low filter opening factor. Therefore, it is necessary to perform micromachining at smaller scales.
It is necessary to make filters with fully controlled specifications such as the diameter and the length of the channel. To test them, we need specialized instruments in the range of the microequipment operation to understand the behavior of the micrometric air flow.
We analyze the current situation as well as the commercial trends of the equipment available in the current market from those available in the domestic sphere to the hospital. The main dangers to which human beings are exposed in enclosed spaces are described to justify the development of microfilters.
The specific contributions of this study are presented here.
The interior environments as sources of diseases for the respiratory system of human beings were described. The main pollutants in closed indoor environments with which we interact daily were identified and described, and the effects of each type of pollutant on humans were described.
A reference framework was proposed for the development of microfilters for air purification in indoor spaces by considering the type of environment and the pollutants, as well as the vulnerability of the users.
Recommendations for the design of the microchannel filters for indoor air purification are proposed, implementing low-cost MET for the manufacture of the elements that constitute a microfiltration system.
The basic design of a microfiltration cell for air purification is proposed based on the technology called microchannel binary air–water flow.
The methodology developed in our group for the design and manufacture of microdevices was applied.
A prototype microchannel filter was constructed, which underwent manufacturing tests, liquid sealing tests, gas sealing tests, and water flow tests.
An evaluation of advantages and disadvantages of the microfiltration system was carried out with respect to the solutions proposed at the commercial level, where the main advantages of this device compared to current technologies were the manufacturing cost, the operating capacity, and the principle of operation by binary flow. This is because it is possible to clean the microchannel by means of water flow, which allows us to maintain the filtration quality within an optimum range of contaminant removal.
This work was partly supported by the project UNAM-DGAPA-IT102320.
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