Measurements and Analyses of the Performancs of Laboratory Fume Hoods
Doctoral thesis, 1997
When working with health-hazardous substances or unpleasant airborne pollutants, it is important to have ventilation systems that are designed and operated to protect persons working in the laboratory. In laboratory applications, the fume hood is the most commonly used item of protective equipment. This thesis is concerned with the air-handling aspects of the protective function of laboratory fume hoods.
In order to be certain that a laboratory fume hood is providing adequate protection, so that users are not exposed to health-hazardous substances, it is necessary to check its performance. There are a number of ways of doing this, but the most commonly employed method for quantitative evaluation of fume hood performance is to meter a tracer gas inside the fume hood and measure the outward leakage with an IR analyser.
Measurements presented in the thesis show that outward leakage from a fume hood has a very rapid, pulsed characteristic. Since the IR analysers that are normally used when checking the performance of a fume hood do not have sufficiently rapid response, this means that the recorded outward leakage does not fully reflect the actual leakage. In the thesis, theoretical relationships are presented that describe the inertia of IR analysers, and calculations for short-duration pulses and normal instrument time constants show that the gas concentration recorded by the instruments differ considerably from the true gas concentration.
The thesis describes two methods of determining the actual characteristic of the outward leakage from a fume hood. One method is based on a derived relationship that estimates the true gas concentration on the basis of the recorded concentration. The other way of determining the actual outward leakage process is to use a newly developed instrument with negligible inertia. The measurements that were made with this instrument clearly show that the outward leakage from a fume hood is pulsed, and that the pulses are of considerably shorter duration and have higher peak values than had previously been possible to record with traditional IR instruments.
In the thesis, results are also presented from measurements with the aim of determining how fast the flow needs to be adjusted after the position of the hood sash is changed in order to prevent outward leakage. These measurements indicate that the air flow rate should be adjusted and stabilized within 3-4 seconds in order to avoid outward leakage. Further, in order to investigate whether flow variations in the exhaust air system could affect the performance of a fume hood, measurements were made in a test room. The results show that if there are oscillations in the exhaust air system while someone is in front of the fume hood, unstable air flow through the hood occurs, sufficient to create outward leakage.
rapid concentration changes
VAV hood controllers
infrared gas analysers
laboratory fume hoods