Efficiency Measurements

Performance tests for local ventilation systems are classified in the following three ways: type, commissioning, and in-use tests.

The procedures and requirements for the type and commissioning tests are covered in national and international standards—for example, for laboratory fume cupboards, welding fumes, and kitchen hoods.

Type testing is carried out in special laboratories under controlled condi­tions using tracer methods. The methods used provide a means of comparing the efficiencies of various local exhausts. Type testing is a method for comparing the efficiencies of various products. It also provides a method for checking the design of airflow rates for general ventilation, and the development of individ­ual products.

The commissioning tests are typically performed before a local ventilation system is put into use; the purpose is to confirm that the design conditions have been achieved. Hence, commissioning tests apply to methods used in type test­ing, but in a simple and modified way. In-use tests concentrate on the exposure effects of a local ventilation system.

For example, the type test of a laboratory fume hood includes determina­tion of the concentration at various points across the opening of the hood by us­ing various tracer source locations inside the hood. The commissioning test could concentrate on the measurements taken at one point in the opening with one source location.

In-use tests deal with the concentration measurements in the worker’s breathing zone for various hood applications. The performance tests presented here are typically in-use tests, where normal work procedures are ongoing.

The initial performance test for all local ventilation systems is a smoke test, which provides easy airflow visualization between the source and the hood. It helps to identify, with little effort, the main features of airflow patterns. Such a test, recorded by a video camera, allows performance comparisons to be made before and after improvements. Real contaminant or tracer gas measurements are necessary in the case of more detailed testing.

For exterior hoods, the measurement of capture velocity provides a quick check of the ideal design conditions. However, it must be remembered that cap­ture velocity is not a direct measure of the ability of an exterior hood to provide personnel protection. Other efficiency measures are required in order to evalu­ate its performance in practice. The following two efficiency measurements could be useful: capture efficiency and occupational hygiene efficiency. These measures complement each other.

Contaminants captured by an exterior hood can cause exposure if allowed to enter the hood after passing through the operator’s breathing zone. The cap­ture efficiency describes the percentage of the generated contaminant that is cap­tured directly by the hood. Occupational hygiene efficiency describes the effect of the use of the hood on the operator’s exposure to the contaminant. The occu­pational hygiene efficiency is performed with the exterior hood working and not working. For example, many wood-processing machines produce large volumes of wood chips and dust and cannot operate without the exhaust hoods in use. With very toxic contaminants, it is essential to work with the hood exhaust op­erating at all times.

For enclosures, velocity measurements, in the plane of the opening, of­fer a quick check on the design conditions. Flowever, the opening velocity is not a direct measure of the ability of an enclosure to provide personnel pro­tection. Other measures of efficiency are required and depend on use of the enclosure. In the case of safety cabinets and laboratory hoods, allowance factors for protection and leakage are applied to ensure complete safety when in use.

For exterior hoods and enclosures, the measurement of the breathing zone concentration provides a method of comparing the effects of changes in the supply and exhaust airflow rates,