Introduction
Air cleaning or disinfecting devices (ACDDs) are widely used in central air-conditioning systems (CACS) to separate dust, airborne microorganisms or prevent gas contamination. In general, these devices are classified according to many different national and international standards. However, China lacks a national standard for testing and classifying these ACDDs used in CACS. Due to the extensive occurrence of Severe Acute Respiratory Syndrome (SARS), since 2003 a set of health management methods for public central air-conditioning systems have been established by the Ministry of Health of The People's Republic of China to strengthen public sanitation administration. It was demonstrated that ACDDs, employed in CACS, had an important influence on contamination control. The test system for these ACDDs was ambiguously mentioned as an aerodynamics test rig in these methods, but its principle and operation instructions were not presented, making it difficult for ACDD manufacturers to construct their own experimental setups to test products.
Data on airborne microorganisms are increasingly frequently reported in scientific literature because of the known adverse health effects of such particles. ACDDs employed in CACS have become an important issue because of related health effects. The aim of the study was to design and construct a multifunctional experimental setup that can determine various ACDDs, such as general ventilation air filters, high efficiency particulate air (HEPA) filters and air line cleaner, removal efficiencies for different test aerosols, e.g. atmospheric dust, airborne aerosol and formaldehyde, etc. For the quick determination of the removal efficiency of ACDDs, the new multifunctional experimental setup operates with two instruments, such as optical particle counters, Andersen viable particle samplers and gas samplers, which simultaneously perform on-line measurements of the test aerosol concentration in the raw and clean gas flow of a test ACDD. With this test rig, performance of ACDDs, such as the removal efficiency, dust holding capacity and the pressure drop, can be easily determined following an exact testing procedure. The measurements are either done with synthetic dust of definite composition or atmospheric dust of the ambient air. The efficiency is determined gravimetrically, photometrically or indirectly by following spectral analysis or germiculture. Previous investigations for air filters were conducted with an apparatus of similar design. The obtained experimental results and valuable experience led to the construction of this new setup. Current national standards for HEPA and ULPA filters are also beneficial to design and construct the multifunctional setup (EN1822 (1998); IEST-RP-CC-001.4(2005); ASHRAE 52.2 (1992)).
Experimental setup
The experimental setup, as shown in Figure 1, is made up of three main parts: the aerosol generation unit, the flow channel and the actual measuring unit.
Multifunctional - this set-up can satisfy different size ACDDs with various nominal flow rates. Multi-performance indexes, such as pressure drop, counting efficiency (or weighing efficiency) for atmospheric dust (or artificial dust), gas (formaldehyde, ammonia, toluene, etc) removal efficiency, microorganisms ((bacteria, fungal spores and pollen) cleaning efficiency for artificial microbes (or atmospheric microbes), can be obtained. It can be employed as not only a general ventilation filter performance test rig but also as a HEPA and ULPA performance test platform. The flow diagram of performance test system of this new experimental setup is shown in Figure 2.
Intelligent - the ACDD performance test software used is based on Visual Basic programming language. The interactive GUI program includes a real-time data display, operating mode display and a trend curve display. This control system can auto-regulate air flow rate according to customer requirements. By changing the air flow, the face velocity of the test can be altered and adjusted to different nominal face velocities ranging from 0.5 to 8.0 m/s which are typical for ACDD used in CACS. This test rig can also be used as a general ventilation air filters performance test system.
Aerosol generation unit
Two types of generators were used. A suspension nebuliser served as an aerosol generator for dispersing particles which are first suspended in liquids, widely used with many aerosol substances that include but are not limited to DEHS (di-ethyl-hexyl-sebacate), DOP(di-octyl-phthalate), PSL (Poly-Styrol Latex) and microbial suspension. A second aerosol generator was used to disperse powders, working with a rotating brush and a pressurised air flow to disperse a definite amount of powder at a highly constant rate and mainly employed to produce artificial dust or fungal conidia. Behind the aerosol generator an aerosol neutralizer was installed because the particles generally carry high electrical charges after the initial aerosol generation process. The test aerosol concentrations can be varied in a wide range by adjusting the generator and the mixing air flow. The fluctuations in concentration of the aerosol production on the measurements are of no influence because the particle flux upstream and downstream of the test filter is measured simultaneously.
Flow channel
The flow channel was composed of an air inlet handling box, upstream mixing duct, downstream mixing duct, plenum chamber, air volume flow rate measurement channel, exhaust air handling chamber and exhaust fan.
The air inlet handling box consisted of low efficiency filter, medium efficiency filter, HEPA filter and air conditioning unit. The three class filters are used to reduce basic contamination level and provide clean air, especially when test high efficiency ACDDs, such as HEPA or ULPA filters.
The upstream mixing flow channel, a tubular flow channel, should be long enough, generally ten times flow channel equivalent diameter as mentioned below, to insure that the aerosol produced by the suspension nebuliser is adequately mixed with the flow channel air and no droplets remain in the aerosol. The aerosol flows in a perpendicular direction towards the ACDD holder. The holder for the sample can be removed without changing the position of the two concentration measuring instruments. This allows a large number of tests in a short time. Through the holder and downstream mixing duct, test aerosol enters into a plenum chamber. Behind the plenum chamber there are air volume flow rate measurement channels. Further downstream an exhaust air handling chamber, containing backup HEPA filters, collects all the particles that pass through. Behind the exhaust air handling chamber the air flow is controlled by a fan with variable speed control.
Main characteristics of this flow channel are given as following:
- Negative pressure - the total air flow channel operates under negative pressure which is produced by the installed exhaust fan. Otherwise, some harmful test aerosols may be able to leak through the air duct and protection will be compromised.
- Modular structure - as with the experimental setup, it is able to perform experiments not only with dust particles but also with airborne biological particles (bacteria, fungal spores and pollen) and gas contamination (formaldehyde, ammonia, etc.). For this purpose the apparatus has to be easy to dismantle for cleaning or disinfecting all crucial parts of the apparatus. Therefore, the flow channel is designed as several modular parts. For example, the air inlet handling box can be disassembled easily and moved away from the upstream mixing duct because of four versatile wheels installed at the bottom of the box brackets. This is beneficial to test aerosol conversion operations between atmospheric dust and other manual aerosols. In addition, the plenum chamber and exhaust air handling box are also fitted with wheels.
Actual measuring and control unit
The schematic diagram of the data acquisition and control system of the experimental setup is shown in Figure 3. Across the ACDD holder the pressure drop ΔP of the test ACDD is measured by a pressure difference sensor. The aerosol concentration upstream and downstream of the test ACDD is measured using two identical instruments. One is located before and the other behind the test ACDD. Test aerosol concentration is measured upstream and downstream from the ACDD using two measuring instruments in parallel. If the level of the upstream concentration exceeds the measuring range of the instrument then a dilution system shall be included between the sampling point and the instrument. Various apparatuses can be employed to measure upstream and downstream aerosol concentration. For example, optical particle counters (OPC), dust gauge, gas sampler and microbial sampler are used to measure number, weight, gas and microbial concentration, respectively. The removal efficiency E(d) can be calculated with equation 1.Where
d is particle size,
Cup(d) is concentration in raw airflow,
Cdown(d) is concentration in clean airflow.
One can see in this equation that the removal efficiency E(d) indicates the fraction of particles of size d that are collected in a ACDD if challenged with dust of concentration Cup(d).
The test air temperature and relative humidity can be monitored by temperature and humidity sensor installed in the middle of the plenum chamber. The relative humidity of the air can be varied between 20% and 80% by controlling air conditioning unit. Behind the plenum chamber the air flow is monitored by two different orifice meters for different volume range.
Previous investigations shown that test aerosol sampling volumes are very small compared with the cross section of the flow channel, the sampling can be regarded as representative because the aerosol should be uniformly dispersed across the flow channel. Inhomogeneities in the aerosol flow caused by the test ACDD vanish rapidly, thus their influence on the measurements can be disregarded because the measuring volumes have a sufficient distance to the test ACDD.
The operating instructions provided by the test equipment manufacturer should be followed. All test instruments should be calibrated per manufacturers' instructions. The accuracy of all measurements is ±10% of reading unless otherwise specified. It is important that the proper flow rate through the ACDD be established prior to test, the uniformity of airflow approaching the ACDD must be established as part of qualifying the test system.
Results and discussion
To perform an accurate ACDD performance test, it is necessary that the challenge aerosol concentration be uniform over the entire upstream and downstream cross-section of the sampling port. In addition, verification of consistency of concentration is indispensable for the precise determination of the fractional efficiencies and was done by simultaneous measurements with the two instruments when no test ACDD was placed in the ACDD holder.
Uniformity of concentration
According to EN 1822[5], the sampling should be representative, which is the case when the aerosol concentration measured from the sample does not deviate by more than 10% from the mean value determined in accordance with the following demand. The aerosol input and the mixing duct shall be so constructed that the aerosol concentration measured at individual points of the duct cross section directly in front of the test ACDD shall not deviate by more than 10% from the mean value obtained from at least 9 measuring points spread evenly over the duct cross section. The location of the nine uniformly spaced sampling locations is determined by dividing the face area that the air flows through into squares or rectangles with the minimum possible aspect ratio (i.e., as close to square as possible). The sampling locations are the centre of each square or rectangle. For this experimental setup, locations of nine measuring points are shown in Figure 4; the fifth measuring point is the representative sampling point (RSP).
An example of verification of aerosol uniformity is given in Table 1. It can be observed that, for whether upstream or downstream sampling, the variations in nine measurements of the particle number concentration are relatively small and in the range of ±10% of the mean value. Sampling concentration at the representative sampling point, the fifth point in Figure 4, can represent the whole cross section aerosol concentration.Consistency of concentration
According to EN 1822, the mean aerosol concentrations determined at the upstream and downstream sampling points without the test ACDD in position shall not differ from each other by more than 5%. Otherwise, larger measuring errors would occur due to unreasonable flow channel size, poor air duct tightness or incorrect location of sampling point. A case study of this verification for the test rig is given in Table 2. It can be founded that the experimental setup meets the above requirement. It indicates that the design of air flow channel is reasonable. From Table 2, it can be also seen that upstream concentration is a little larger than simultaneous downstream concentration. This phenomenon may be explained by particle diffusion, sedimentation, collision and conglutination.
Conclusions
The aerodynamic experimental setup is designed and constructed to determine the performance of air cleaning or disinfecting devices used in central air-conditioning system. As a multifunctional test rig, it can be widely employed to measure various performances. These experimental results will be of great interest for air cleaning processes in many fields, such as clean room technologies and infection control technologies. In addition, the simultaneous in situ measurement of test aerosol concentration upstream and downstream of the test ACDD with two identical instruments allows a statistically reliable fractional efficiency determination in a short time. The first experiments with the new experimental setup for determining the fractional efficiencies of fibrous filters proved the apparatus is a valuable tool for characterising air filters.
Future work includes the removal efficiency measurements for bioaerosols (airborne bacteria, fungal spores and pollen) and gas contamination. Biological particles and gas contamination show different aerodynamic behaviour and adherence properties than dust particles and this will consequently result in different ACDD collection efficiencies. In general, removal efficiency of bioaerosol and gas contamination can not be obtained directly compared with dust particles, which may lead to some loss error, is this error significant? Moreover, natural decay of these two aerosol is usually faster than dust particles, especial fine particles. Exploring how to eliminate this effect on the removal efficiency is another issue.
Contact:
Guoqing Cao and Yizhao Zhang
Institute of Building Environment and Energy Efficiency
China Academy of Building Research
Email: cgq78@hotmail.com
Please contact the authors for references.
This work was supported by the grants of China National Key Technology R&D Programs (2006BAJ02A10).
Notes for Figure 1:
- Low efficiency filter; 2. Medium efficiency filter; 3. HEPA filter; 4. Air conditioning unit; 5. Air inlet handling box; 6. Electric self-closing valve;7. Aerosol inlet in the duct; 8. Upstream mixing flow channel; 9. Upstream sampling point; 10. Static pressure probe; 11. Upstream part of the test Air cleaning or disinfecting device holder; 12. Test Air cleaning or disinfecting device;13. Downstream part of the test Air cleaning or disinfecting device holder; 14. Static pressure probe; 15. Downstream mixing flow channel; 16. Downstream sampling point; 17. Flexible connection; 18. Plenum chamber; 19-20. Air volume flow rate measurement flow channel; 21-22. Orifice meter; 23-24. Electric self-closing valve; 25. HEPA filter; 26. Exhaust air handling chamber; 27. Exhaust fan with variable speed control; 28. Fan room
Notes for Figure 3:
1~28. the same as fig.1; 29. Aerosol generator; 30. Aerosol neutraliser; 31. Upstream sampler; 32. Pressure difference sensor; 33. Downstream sampler; 34. Temperature and humidity sensor; 35-36. Pressure difference sensor; 37. Frequency changer; 38. Computer.
