Differential Effects of Various Cleaning Solutions on the Cleaning and Regeneration Performance of Commonly Used Polyester Fiber Material Air Filters

Abstract

The regeneration and performance of air filter materials are important means of conserving energy and reducing emissions in civil public buildings. The repeated regeneration and use of air filters can not only effectively increase the lifespan of the filters but also reduce the phenomenon of filter disposal and incineration after reaching or not reaching the replacement cycle, further reducing resource waste and air pollution and thereby directly or indirectly reducing carbon emissions. In this study, polyester fiber air filters commonly used in civil public buildings were selected as the research object, and the regeneration performance and structural parameters of water and a cleaning solution were investigated under various cleaning conditions. The results show that the filtration efficiency when cleaning with water was higher than that when cleaning with the cleaning solution. The filtration efficiency for PM₁₀, PM_2.5, and PM_1.0 increased from 0.3% to 3.5%, 0.7% to 6.3%, and 0.1% to 4.6%, respectively. Water could be used twice for cleaning for PM₁₀ and once for PM_2.5, and the cleaning solution could only be used once for cleaning for PM₁₀. The counting filtration efficiency of 0.3 to 2.5 μm particulates showed a relatively significant change. The resistance after cleaning with water was higher than that after cleaning with the cleaning solution. For the quality factor (QF) value of PM₁₀, the cleaning solution had a slightly higher cleaning effect, while for the QF values of PM_2.5 and PM_1.0, water had a slightly higher cleaning effect. In practical use, it is recommended to first use a cleaning solution and then water for subsequent cleaning. This study provides data support for the use of filters to achieve the dual carbon goal.

1. Introduction

The relevant literature shows that people spend 90% of their time indoors [1], and, as a place with relatively high human activity, the health of the indoor environment in civil public buildings is of utmost concern [2]. In the pandemic era, air filters were widely used and popularized as an effective tool to remove air pollutants [3,4,5]. However, currently, the air filters on the market are often in the form of combination filters, with ordinary filter materials used to purify particulate matter and activated carbon materials used to adsorb and purify harmful gases in the air. After the microorganisms in the air are intercepted by the filter material, they attach to the filter material and grow and reproduce. If the air filter is not replaced and cleaned in a timely manner, the growing microorganisms will not only release a large amount of harmful gases but will also re-enter the building with the supply airflow of ventilation and air conditioning, thereby polluting the indoor air environment [6]. Therefore, the performance parameters and regeneration performance of air filters have become the focus of current research.

Relevant scholars have also conducted extensive research [7,8,9,10,11,12,13]. Some studies have mainly focused on improving the performance parameters of filters, such as developing both new [7] and composite materials [8], as well as optimizing filter structures [9] and combinations [10]. Although some research results have been obtained, the issue of performance differences in filters in practical use has not been effectively resolved, and the goals of high efficiency and low resistance have not been achieved. Other studies have mainly focused on the demand for the frequent replacement of air filters, the regeneration methods of air filters [11], the performance changes after regeneration [12], and the economic costs and operational difficulties [13]. This is because the commonly used solution is to replace the air filters, and the replaced filter materials are discarded or incinerated [14], which may pollute the environment and waste resources. The most commonly used regeneration method that has been extensively studied is the water cleaning method [15] because, in practical engineering, this method is convenient, accessible, and has relatively low economic costs. Therefore, research on water cleaning is currently a hot topic.

However, in practical use, polyester fiber air filters are used to protect the end air filters [16], often placed in front as primary filters. In addition, due to their relatively low production cost, more mature preparation technology, and more stable performance in long-term applications, their usage is also more extensive, and they are currently one of the most commonly used air filtration materials in the market. After a period of use, polyester fiber air filters may adsorb and filter a large amount of solid particles with different sizes, gas pollutants, and microorganisms. They stay in some places, such as on the surface and in the deep layers of the fibers, resulting in increased resistance, decreased filtration efficiency, and even an inability to function properly. In addition, due to the diversity of the various pollutants in the air [17], filter materials are also prone to bacterial growth; this makes it difficult to remove impurities, such as particles and microorganisms, which adhere to the inside and outside of the fibers. During the usage cycle, filter materials may exhibit changes in surface particles, harmful gases, and microorganisms because changes in meteorological parameters, such as temperature and humidity, cause them to adhere firmly to the surface or interior of the fibers. This often results in partial cleaning, with some pollutants remaining attached to the fibers [18]. Therefore, although the current technology used is the water cleaning method, there are still some shortcomings in terms of the specific cleaning effect, the changes in filtration performance after cleaning, and the number of cleaning cycles. Previous research has not provided targeted results or suggestions. For some difficult-to-clean filters, a cleaning solution is used for treatment; however, chemicals are present in cleaning solutions [19], and there are currently no studies on their impact on the regeneration of filter fiber materials. Additionally, there is currently no relevant result or conclusion on whether the regeneration effect of cleaning solutions is better than that of other solutions. Furthermore, there is a serious lack of research on the water cleaning method and the regeneration performance of the polyester fiber air filters commonly used in civil public buildings after using different cleaning solutions.

Therefore, in response to the above practical problems, the authors of this paper took the most commonly used polyester fiber material air filter as the research object, examined water and cleaning solutions under various cleaning conditions, and tested and analyzed the filtration regeneration performance and structural parameters of the filter materials. This study provides a reference for the regeneration and utilization of air filter fibers under the dual carbon goal.

2. Materials and Methods

2.1. Evaluation of Performance Parameters

The cleanliness of air filter materials was calculated using Equation (1) [11]:

$${\eta }_1=(1-{\displaystyle \frac{m}{M}})\times 100\%$$

(1)

where ${\eta }_1$ is the cleanliness (%); m is the mass of residual dust after cleaning (g); and M is the initial mass of the fresh filter (g).

Filtration efficiency was calculated using Equation (2) [20]:

$$\eta ={\displaystyle \frac{C_1-C_2}{C_1}}\times 100\%$$

(2)

where $\eta$ is the filtration efficiency (%), and C₁ and C₂ are the concentrations of particulate matter before and after filtration (μg/m³).

Counting efficiency was calculated using Equation (3) [20]:

$${\eta }_i=(1-{\displaystyle \frac{N_{2i}}{N_{1i}}})\times 100\%$$

(3)

where ${\eta }_i$ is the counting efficiency (%), and $N_{1i}$ and $N_{2i}$ are the average counting concentrations of the particle size before and after filtration (particle/L).

Filtration resistance was calculated using Equation (4) [20]:

$$\Delta P=P_2-P_1$$

(4)

where P₁ and P₂ are the static pressure before and after filtration (Pa).

The filtration efficiency decay rate was calculated using the following Equation (5) [16,21]:

$$K={\displaystyle \frac{{\eta }_n-{\eta }_0}{{\eta }_0}}\times 100\%$$

(5)

where ${\eta }_n$ is the filtration efficiency, %, and ${\eta }_0$ is the filtration efficiency of the fresh filter material, %. When K > 0, the filtration efficiency increases, and when K < 0, the filtration efficiency decreases.

The quality factor value (QF) was calculated using the following Equation (6) [22]:

$$QF={\displaystyle \frac{-\ln \left(1-\eta \right)}{\Delta p}}$$

(6)

where η is the filtration efficiency, %, and $\Delta P$ is the filtration resistance, Pa.

2.2. Experimental Instruments

A GRIMM1.109 Portable Aerosol Spectrometer, supplied by Beijing Saak-Mar Environmental Instrument Ltd., Beijing, China, was used to measure the concentrations of particles before and after air filtration. The concentration measurement was 2,000,000 P/L, the measurement range was 0.1~100,000 μg/m³, and the repeatability was 5%. An HD2114P.0 Portable Micromanometer, supplied by DeltaOHM Co., Ltd., Selvazzano (PD), Italy, was used to measure filtration resistance. Its accuracy was ±2%, reading + 0.1 m/s, and its pressure range was ±0.4% F.S. An HD37AB1347 Indoor Air Quality Monitor, supplied by DeltaOHM Co., Ltd., Selvazzano (PD), Italy, was used to measure velocity. Its accuracy range was ±3%. A JSM-6510LV scanning electron microscope, supplied by Japan Electronics Co., Ltd., Tokyo, Japan, was used to analyze the fiber structure morphology inside the material. Its magnification was 5~30 million times, and its resolution was up to 3.0 nm. The concentrations obtained five minutes before and after testing were used as the average concentrations to reduce experimental errors. The experimental platform is shown in Figure 1.

Polyester fiber filter materials (G4, certification EN779, ISO9001) were obtained from GuangDong Fresh Filter Co. Ltd., Foshan, China. The fiber diameter was 20.56 ± 0.05 µm, the filling rate was 6.27 ± 0.03%, and the porosity was 93.73 ± 0.03%. Under repeated exposure to dust, the changes in regeneration performance after cleaning were examined using both water and a cleaning solution. The water used was urban tap water, and the cleaning solution selected was a Dili brand 827 m air conditioner cleaning agent, which was diluted four times according to the instructions for use; the pH values of the two are shown in Figure 2. A specific cleaning image is shown in Figure 3.

3. Results and Discussion

3.1. Influence of Filtration Velocity on Filtration Efficiency

A dust-loading experiment was conducted using pipeline dust in a ventilation system [11]. When the final resistance of the filter material reached twice the initial resistance, dust emission was stopped [23]. The experimental conditions were set to room temperature (temperature: 20.2–26.7 °C; humidity: 40.5–62.7%), making them more representative. The above steps were repeated, and the changes in filtration efficiency with filtration velocity under the different cleaning solutions are shown in Figure 4.

In Figure 4, it can be seen that, within the testing range, the filtration efficiency shows a trend of first increasing and then decreasing with the increase in the filtration velocity. In the cleaning test using water, the filtration efficiency range for PM₁₀ is 42.3% to 59.4%, the filtration efficiency range for PM_2.5 is 17.4% to 30.4%, and the filtration efficiency range for PM_1.0 is 7.7% to 18.4%. The filtration efficiency range for PM₁₀, PM_2.5, and PM_1.0 using the cleaning solution is 39.2% to 59.2%, 14.9% to 30.7%, and 6.2% to 18.2%, respectively. The filtration efficiency when using water for cleaning is higher than that when using the cleaning solution. The filtration efficiency for PM₁₀, PM_2.5, and PM_1.0 increases from 0.3% to 3.5%, 0.7% to 6.3%, and 0.1% to 4.6%, respectively, and the effect on fine particulate matter is significant. This is because the same water cleaning method is used for cleaning; however, it can be observed in Figure 2 that the cleaning solution has weak alkalinity [24], which can cause certain damage to the fiber structure, resulting in an increase in the pore size between fibers and a reduction in the collision between particles and fibers. Compared with water, the filtration effect decreases. This conclusion is consistent with the literature [24], verifying the findings of this study. In addition, it can be observed in the figure that the filtration efficiency of the polyester filter material reaches its maximum at 1.1 m/s. At this time, the filtration efficiencies when using the water cleaning method 0 to 3 times are as follows: for PM₁₀, 59.4%, 57.8%, 52.4%, and 50.4%, respectively; for PM_2.5, 30.4%, 27.7%, 24.6%, and 25.2%, respectively; and for PM_1.0, 18.4%, 15.4%, 13.5%, and 13.9%, respectively. The filtration efficiencies when using the air conditioning cleaning solution 0 to 3 times are as follows: for PM₁₀, 59.2%, 57.8%, 49.2%, and 46.8%, respectively; for PM_2.5, 30.7%, 25.8%, 22.5%, and 21.2%, respectively; and for PM_1.0, 18.2%, 13.5%, 10.3%, and 9.3%, respectively. Therefore, these data, combined with the trend in the graph, confirm that 1.1 m/s is the optimal filtration velocity. Under this condition, the changes in the filtration performance of different cleaning solutions at different cleaning times are shown in Figure 5.

In Figure 5, it can be seen that using water and cleaning solutions to clean the polyester fiber materials has a significant cleaning effect. Overall, the filtration performance of the cleaned materials significantly decreases, and there are certain differences. The filtration efficiency decreases even more after cleaning with the cleaning solution. After cleaning with water one to three times, the filtration efficiency range is 50.4% to 57.8% for PM₁₀, 24.6% to 27.7% for PM_2.5, and 13.5% to 15.4% for PM_1.0. After cleaning with the cleaning solution, the filtration efficiency range is 46.8% to 57.8% for PM₁₀, 21.2% to 25.8% for PM_2.5, and 9.3% to 13.5% for PM_1.0. The main reason for this is that, as the number of cleaning cycles increases, the fibers become softer, which is due to the internal stress of the cleaned fibers being eliminated; this is consistent with the conclusions given in the relevant literature, thus verifying the findings of this study [25]. Of course, it is also related to the inherent properties of the material [26]. Large particles are directly washed away, whereas small particles enter the interior of the fibers, which causes the gaps between the fibers to become smaller; this results in less significant changes for large particles and more obvious filtration effects for small particles. However, the fibers of the filter material cleaned with the cleaning solution have some damage [27], thus decreasing the filtration efficiency. Figure 6 shows the effect of the different cleaning solutions on the filtration efficiency decay rates.

In Figure 6, it can be seen that, compared with the original group, the filtration efficiency decay rates after cleaning one to three times with water are as follows: for PM₁₀, 2.7%, 11.9%, and 15.3%, respectively; for PM_2.5, 8.9%, 18.9%, and 17.1%, respectively; and for PM_1.0, 16.3%, 26.3%, and 24.5%, respectively. The filtration efficiency decay rates after cleaning one to three times with the cleaning solution are as follows: for PM₁₀, 2.3%, 16.9%, and 20.9%, respectively; for PM_2.5, 15.7%, 26.7%, and 30.8%, respectively; and for PM_1.0, 25.8%, 43.4%, and 49.1%, respectively. According to the relevant standards, the filtration efficiency of air filters after cleaning should not be lower than 85% of the filtration efficiency before cleaning [23]. It can be seen that water can be used twice for cleaning for PM₁₀ and once for PM_2.5, and the cleaning solution can only be used once for cleaning for PM₁₀. The other results do not meet the standard value of reaching 85% before cleaning.

3.2. Differences in Counting Filtration Efficiency

At the optimal velocity mentioned above, the differences in the counting filtration efficiency of the polyester fiber air filtration materials after cleaning with the different cleaning solutions are shown in Figure 7.

Figure 7 shows that there is a certain difference in the counting filtration efficiency of the filter materials after cleaning with water and the cleaning solution. The counting filtration efficiency when cleaning with water is higher than that when cleaning with the cleaning solution. For particles with a diameter between 0.3 and 2.5 μm, the counting filtration efficiency changes relatively significantly, with a maximum difference of 3.8%. However, for large particles, the difference is almost negligible. The main reason for this is that, during the cleaning process, large particles fall directly under the impact of the water flow, while small particles enter the interior of the fibers, making the fiber structure more compact. The combined effect of diffusion and interception makes it easier to capture [28], but cleaning solutions can cause some damage to fibers. With an increase in the number of cleaning times, the difference in the counting filtration efficiency of particles between 0.3 and 2.5 μm becomes more significant.

3.3. Influence of Filtration Velocity on Resistance

The differences in the resistance of the filter materials under the different cleaning solutions are shown in Figure 8.

Figure 8 shows that the resistance increased significantly after cleaning with water. The resistance range after cleaning with water was higher than that after cleaning with the cleaning solution one to three times, ranging from 2.5 to 9.5 Pa. The main reason for this is that the cleaning solution is an alkaline solution, which may cause certain damage to the surface, resulting in changes in the fiber structure and a decrease in resistance. However, at the same time, the filtration efficiency did not significantly decrease, indicating that, although the cleaning solution may have had a certain impact on the surface of the fibers, the effect was relatively small. In practical use, an increase in resistance may directly increase energy consumption, leading to inadequate ventilation or an accelerated system lifespan, which needs to be addressed in a timely manner. However, the results of this study show that the effect after cleaning did not suddenly increase or increase significantly, so it did not have a significant impact on actual use. In addition, air filters are often affected by many factors during operation, and their operating status may also be dynamic. In summary, the cleaned effect meets the actual operation requirements. The results in Figure 8 were fitted, and the fitting results are shown in

Table 1. Fitting of resistance characteristics of filter materials under different cleaning times.

Solution	Cleaning Times	Resistance Range (Pa)	Fitting Formula	Fitting Effect
Water	0	34.5~52	$\Delta P$ = 35.7v2 − 30.4v + 36.4	R2 = 0.97523
	1	33.5~50.5	$\Delta P$ = 42.9v2 − 44.7v + 42.0	R2 = 0.98654
	2	31.5~48.0	$\Delta P$ = 39.3v2 − 38.1v + 36.7	R2 = 0.97014
	3	29.0~46.5	$\Delta P$ = 67.9v2 − 93.2v + 60.3	R2 = 0.99348
Cleaning solution	0	34.0~51.0	$\Delta P$ = 3.6v2 + 34.4v + 4.2	R2 = 0.97699
	1	31.0~46.0	$\Delta P$ = 50.0v2 − 65.0v + 51.4	R2 = 0.97523
	2	28.0~41.0	$\Delta P$ = 7.1v2 + 18.7v + 8.3	R2 = 0.99699
	3	22.0~37.0	$\Delta P$ = −17.9v2 + 71.2v − 23.2	R2 = 0.98575

.

3.4. Quality Factor of Different Cleaning Solutions

The changes in the quality factor of the filter material when using the different cleaning solutions are shown in Figure 9.

In Figure 9, it can be seen that the overall quality factor shows a trend of first decreasing and then increasing with the increase in the number of cleaning times. For the QF value of PM₁₀, the quality factor after cleaning with the cleaning solution is slightly higher than that after cleaning with water, with a maximum value of 0.0023 Pa⁻¹. However, the QF values of PM_2.5 and PM_1.0 show the opposite trend, with the quality factor after cleaning with water being slightly higher than that after cleaning with the cleaning solution, with maximum values of 0.0002 Pa⁻¹ and 0.0008 Pa⁻¹, respectively. This is because using a cleaning solution during the cleaning process can effectively remove some insoluble impurities attached to the fibers while also damaging the internal structure of the fibers. To further observe the effect of the different cleaning solutions on the fiber structure, please see Figure 10.

In Figure 10, it can be seen that, after cleaning with water, the fibers still have some protrusions or particle accumulation at the connections [29]. However, after cleaning with the cleaning solution, the fibers are relatively rough or broken, no longer presenting a naturally twisted state. This is because the cleaning solution causes some, but not serious, damage to the fibers. Therefore, in practical use, a cleaning solution should be used first to soften and decompose the impurities, dirt, etc., on the material fibers, and then water should be used for subsequent cleaning. This study provides data support for the use of filters to achieve the dual carbon goal. It also provides a foundation for ensuring air quality in special environments [30,31,32], especially in underground corrosive environments [30].

4. Conclusions

In this study, the polyester fiber materials commonly used in civil public buildings were examined after being cleaned with water and a cleaning solution. The regeneration performance and structural parameters were also tested and analyzed, and the following conclusions were preliminarily obtained:

• The filtration efficiency showed a trend of first increasing and then decreasing with an increase in the filtration velocity. The filtration efficiency when cleaning with water was higher than that when cleaning with the cleaning solution. The filtration efficiency for PM₁₀, PM_2.5, and PM_1.0 increased from 0.3% to 3.5%, 0.7% to 6.3%, and 0.1% to 4.6%, respectively. At a velocity of 1.1 m/s, the filtration efficiency of the polyester filter material reached its maximum;
• Water could be used for cleaning twice for PM₁₀ and once for PM_2.5, and the cleaning solution could only be used for cleaning once for PM₁₀;
• The counting filtration efficiency when using water was higher than that when using the cleaning solution. For particles with a diameter between 0.3 and 2.5 μm, the change in the counting filtration efficiency was relatively more significant, with a maximum difference of 3.8%;
• The quality factor showed a trend of first decreasing and then increasing with the increase in the number of cleaning times. For the QF value of PM₁₀, the quality factor after cleaning with the cleaning solution was slightly higher than that after cleaning with water, with a maximum value of 0.0023 Pa⁻¹. The QF values of PM_2.5 and PM_1.0 showed the opposite trend, with the quality factor after cleaning with water being slightly higher than that after cleaning with the cleaning solution, with maximum values of 0.0002 Pa⁻¹ and 0.0008 Pa⁻¹, respectively.

In practical use, it is recommended to first use a cleaning solution to soften and decompose the impurities, dirt, etc., on the material fibers and then use water for subsequent cleaning. This study provides data support for the use of filters to achieve the dual carbon goal.