Comparison and analysis of test methods for laboratory fume hood

 

Comparative analysis of EN 14175, ANSI / ASHRAE 110 and JB / T 6412-1999

 

Li Siwei, Liu Dong, School of mechanical engineering, Tongji University

 

Abstract: in this paper, the test methods of exhaust hood in European standard (EN 14175), American Standard (ASHRAE 110) and Chinese standard (JB / T 6412-1999) are compared. The differences of basic test conditions, surface wind speed test method and pollutant concentration test method in each standard are analyzed in detail. After comprehensive comparison, the conclusion is that ASHRAE 110 is the best The test method in this paper has high comprehensive evaluation.

 

introduction

 

Exhaust hood is an important equipment to control pollutants in laboratory. At present, in order to ensure the safety and health of laboratory personnel, the requirements for Laboratory environmental control are higher and higher. People pay more and more attention to the efficiency and safety of exhaust fume hood, and formulate corresponding exhaust fume hood standards. For example, the United States formulated ANSI / ASHRAE 110-1995 [1] in 1995, revised it [2] in 2003, and published BSR / ASHRAE 110; In 2003, European standard EN 14175 was formulated in Europe; in 1999, Chinese standard JB / T 6412-1999 was also formulated to guide and standardize the production and use of domestic exhaust cabinets.

 

In this paper, the test methods of exhaust hood mentioned in en 14175, ANSI / ASHRAE 110-1995 (including some amendments in BSR / ASHRAE 110p, hereinafter referred to as ASHRAE 110) and JB / T 6412-1999 are compared.

 

Main test items

 

The air distribution in the exhaust cabinet must be reasonable, in order to control the pollutants emitted from the cabinet, and make it smoothly discharged to the outside, instead of escaping to the indoor through the operation port of the exhaust cabinet, which endangers the health and safety of laboratory personnel. In order to evaluate the performance of the exhaust hood, although there are differences in the standard rules formulated by various countries, they all have two basic elements: the surface wind speed and the pollutant control concentration.

 

In 1992, American National Standards Institute (ANSI) and American Industrial Hygiene Association (AIHA) formulated the ANSI / AIHA z9.5-1992 laboratory ventilation standard, emphasizing that the surface wind speed is an important index to evaluate the performance of exhaust hood. The so-called surface wind speed is the control wind speed on the operation surface of the exhaust cabinet. It is generally required that the surface wind speed should be controlled within the range of 0.4 ~ 0.6m/s. The air velocity is too small, which indicates that the control effect of the exhaust fan on the operation surface is not enough, and the pollutants in the cabinet are easy to get rid of the predetermined air discharge path and escape from the exhaust cabinet through the operation surface; If the surface wind speed is too high, it is easy to produce turbulence in the area near the outlet in the cabinet, which affects the air distribution form of the exhaust cabinet. Pollutants may accumulate in the corner of the exhaust cabinet or escape from the operation surface of the exhaust cabinet [5, 6]. The ideal surface wind speed test results should make the surface wind speed in a predetermined range, and the wind speed distribution on the operating surface as uniform as possible.

 

However, the research shows that even if the surface wind speed meets the requirements, the control of pollutants by the exhaust hood may not be able to achieve the expected goal. From 176 exhaust hood test reports conducted according to ashrae110, it is found that the correlation coefficient between the exhaust hood surface wind speed and the control concentration of tracer gas is only 0.24 [7]. Therefore, in 1995, ANSI and ASHRAE (American Society of heating, refrigeration and Air Conditioning Engineers) introduced the ANSI / ASHRAE 110-1995 laboratory exhaust hood performance test method, which not only insisted on the surface wind speed test, but also first proposed another index to evaluate the performance of exhaust hood: pollutant concentration test. At present, most countries have listed the pollutant test as another very important evaluation index in the exhaust hood test. Pollutant control concentration refers to the tracer gas concentration on the operation surface when the tracer gas is released at a speed of 2 lgmin in the exhaust cabinet during normal operation. In ASHRAE 110 and JB / T 6412-1999, the tracer gas concentration at the breathing belt of the dummy in front of the cabinet is used as the control concentration, and the release rate of the tracer gas in the cabinet is 4 L / min; in en 14175, the tracer gas concentration on the operating surface is used as the control concentration, and the release rate of the tracer gas in the cabinet is 2 L / min. Compared with the surface wind speed, the pollutant control concentration can more directly reflect the pollutant control effect of the exhaust hood. However, because there are many chemical substances in nature, the pollutant control concentration only reflects the control situation of the exhaust hood for the chemical and physical properties similar to the tracer gas, so the pollutant control concentration can not fully reflect the performance of the exhaust hood. Therefore, the current national standards mainly use the surface wind speed and pollutant control concentration to evaluate the performance of the exhaust hood, and add some other evaluation indexes according to their own national conditions.

 

Overview of national standards

 

2.1 ASHRAE 110 method of testing performance of laboratory furniture hood

 

ASHRAE 110 is the most widely used exhaust hood standard in the world. Its purpose is to provide quantitative and qualitative test methods to evaluate the control ability of laboratory exhaust hood to pollutants. The quantitative test method is the surface wind speed test and pollutant control concentration test described above, while the qualitative method is the flow display test.

 

Flow display test is to use visual smoke to directly display the flow mode of air flow in the exhaust cabinet. In ASHRAE 110, flow display test is divided into two types: Micro flue gas test and large flue gas test. This qualitative test method has the advantages of simple operation and intuitive results, but it also has its limitations. First of all, strong subjectivity. Although ASHRAE 110 is constantly strengthening the standardization of its test results description, for example, the latest BSR / ASHRAE 110p (the revised version of ANSI / ASHRAE 110-1995) not only increases the content of mobile display test, but also standardizes the description method of its test results. The description is still quite subjective and its reliability is reduced. Second, the results cannot be quantified. No further comparison can be made between two results with the same description. Finally, the scope of human eye observation is limited, for its small changes or escape can not be distinguished. In view of the above limitations, the flow display test is always only an auxiliary test, and can not become a necessary evaluation index.

 

2.2 Chinese standard (JB / T 6412-1999)

 

JB / T 6412-1999 specifies the type, basic parameters and dimensions, technical requirements, test methods and inspection rules, as well as marking, packaging and storage. In addition to the test content of ASHRAE 110, the resistance test is added in the test method part. The purpose of resistance test is to measure the resistance of the exhaust hood, which can indirectly reflect whether the design of the exhaust hood is reasonable and whether the air flow is smooth from another side.

 

The measuring principle is to use pitot tube to measure the total pressure value and dynamic pressure value in the exhaust pipe, then calculate the wind speed in the pipe from the dynamic pressure value, and then calculate the local resistance of each joint and the resistance along the pipe according to the wind speed. After subtracting the dynamic pressure, local resistance and resistance along the way from the total pressure value measured at the beginning, the rest is the resistance of the exhaust hood.

 

2.3 European standard (EN 14175 fame cupboards)

 

The description of EN 14175 is the most detailed among all the current exhaust fume hood standards, with a total of six parts, namely: terminology, safety specifications and operation requirements, standard test methods, field test methods, installation and maintenance recommendations and VAV exhaust fume hood. Among them, the standard test method describes in detail the test method of exhaust hood test in the test room, which is divided into five parts: air flow test, sliding door test, air flow indicator test, structure and material test and lighting test. The air flow test part is the same as ASHRAE 110 In contrast, the flow display test is omitted, and the anti-interference test, ventilation efficiency test and pressure difference test are added to reflect the performance of the exhaust cabinet more accurately and comprehensively.

 

The purpose of anti-interference test is to investigate whether the operator walking in front of the cabinet will affect the pollutant control ability of the exhaust cabinet. In the test, a flat plate is moved back and forth to simulate the possible impact on the operation of the exhaust cabinet when the operator operates in front of the cabinet, and then the change of tracer gas concentration on the operation surface is recorded during the movement.

 

The purpose of ventilation efficiency test is to investigate the time required for the exhaust cabinet to completely discharge the pollutants in the cabinet under the condition of normal ventilation in the laboratory, that is, the attenuation of the pollution concentration inside and outside the cabinet when there is no pollutant in the cabinet.

 

Differential pressure test is to measure the gauge pressure in the exhaust pipe with a micro manometer to see whether it meets the predetermined differential pressure.

 

Comparison and analysis of test methods

 

Although some test items are the same, such as surface wind speed test and pollutant control concentration test, the test requirements specified in various standards are not the same. The following test methods are compared and evaluated.

 

3.1 Basic test conditions

 

The preparation before the test determines the reliability and accuracy of the test to some extent. In the three standards mentioned above, corresponding requirements have been made for basic test conditions. See Table 1 for details.

 

Table 1 Comparison of basic test conditions
 

It can be concluded from table 1 that the differences in this part mainly focus on the size of test chamber, temperature, pressure, background concentration of tracer gas and requirements for avoiding interference. In the test of the exhaust hood, the size of the test chamber will mainly affect the test results of the pollutant concentration and the air distribution on the operation surface. The influence of too small test room size has two aspects. First, it will make the air supply outlet of the laboratory ventilation system too close to the operation surface of the exhaust hood, and its air flow velocity is too large to meet the above-mentioned basic test conditions, which will affect the control of pollutants by the exhaust hood [5]; Second, because the amount of tracer gas is certain, the size of the test room is too small, which means that the concentration of tracer gas in the room will be too large to meet the final requirements of the test. On the contrary, if the size of the test room is too large, the concentration measurement results will be too small, forming a false image, and it is considered that the exhaust cabinet is very effective for the control of pollutants.

 

As far as temperature is concerned, ASHRAE 110 is converted to 72 ± 5 ° F, which is almost consistent with the requirements of EN 14175. Theoretically, the test can be carried out at room temperature, but the test results will be different under different indoor temperatures. In order to make the test results of different exhaust cabinets comparable and repeatable, it is generally believed that the temperature of the test room should be controlled.

 

The space pressure of chemical laboratory should ensure a certain negative pressure difference relative to the adjacent area to prevent indoor pollutants from spreading out, so the pressure control requirements proposed by ASHRAE 110 are very necessary. ASHRAE 110 and en 14175 have the same requirements for the background concentration of tracer gas, but JB / T 6412-1999 has lower requirements (the upper limit of concentration specified in JB / T 6412-1999 is 0.5ppm), and the background concentration of the test can be determined by comparing with the final test results after the test.

 

Among the requirements to avoid interference, the requirements of these three standards are basically the same. It is generally recommended that the air supply speed of the room in the area 1.5m in front of the exhaust cabinet should not exceed 20% of the air speed of the exhaust cabinet. Therefore, the air supply speed in the area should be reduced as much as possible, so as to avoid the strong fluctuation of the air speed and the formation of local vortex, which will lead to the escape of pollutants. It is worth mentioning that in en 14175, it is specially pointed out that there should be no higher temperature objects in the test room, so as to avoid the influence of hot pressure on the air distribution.

 

Therefore, the requirements mentioned in en 14175 are more detailed and comprehensive.

 

3.2 wind speed test

 

As the first evaluation index, surface wind speed has been recognized by many countries, and the test process is almost the same. The differences between the standards are mainly reflected in the selection of measuring instruments, the arrangement of measuring points and data processing, as shown in Table 2.

 

It can be concluded from table 2 that the layout of measuring points and data processing requirements in each standard are basically similar, but in JB / T 6412-1999, it is specially stipulated that the number of measuring points is not less than 16, which is also to ensure that the test results are reliable enough to reflect the wind speed distribution on the whole operation surface to a certain extent. In this part, the main difference between the standards lies in the selection of anemometer and the number of data records for each measuring point.

 

According to ASHRAE 110, the minimum error of the selected anemometer is as follows:

 

According to en 14175, the minimum error of the selected anemometer is 0.03m/s and the maximum error is 0.07m/s. The range of wind speed of exhaust cabinet is 0.4 ~ 0.6m/s, and the error range of anemometer selected according to en 14175 is 0.04 ~ 0.05m/s, which is slightly larger than that of anemometer selected according to ASHRAE 110, which is 0.025 ~ 0.03m/s. In addition, if the design is not reasonable, the face wind speed of the exhaust hood may exceed 1m / s, so the provisions on the range of anemometer in ASHRAE 110 should be more reasonable.

 

In ASHRAE 110 of 1995 edition, it is stipulated that the number of readings at each measuring point is four times. In 110p, it not only increases the number of readings and specifies the reading time, but also emphasizes that the anemometer must use a bracket to fix the readings during the test, and cannot be held by hand. The purpose of this is to increase the reliability and accuracy of the test. As far as a single measuring point is concerned, it can be read 20 times or 60 times, which can ensure the accuracy of the test results. Therefore, on this premise, reading 60 times not only prolongs the test time, but also increases the workload of data processing, which has little impact on the final test results.

 

It can be seen that the selection of anemometer by ASHRAE 110 is reasonable, and the whole test method is enough

reference

　　Method of Testing Performance of Laboratory Fume Hood(ANSI/ASHRAE 110-1995) [S]. Atlanta: ASHRAE, 1995

　　Method of Testing Performance of Laboratory Fume Hood (BSR/ASHRAE 110P)[S]. Atlanta: ASHRAE,2005

　　European Committee for Standardization. Fume Cupboards (EN 14175) [S]. Brussels: European Committee for Standardization, 2003

State Bureau of machinery industry. Exhaust hood (JB / t6412-1999) [S]. Beijing: State Bureau of machinery industry, 1999

 

Que Yanzhen. Research on ventilation control of modern laboratory [D]. Shanghai: Tongji University, 2003

　　Lan B D Mclntosh. ASHRAE Laboratory Design Guide [M].Atlanta: ASHRAE, 2001

　　Dale T, Karen Maupins. Using the ASHRAE 110 Test as a TQM toolto improve laboratory fume hood performance [J]. ASHRAE Transactions, 1997, 14: 851-862

　　WUNDER J S. Personnel communication from operating experiencewith laboratory equipment[R]. Madison: University of Wisconsin, 2000