EVALUATION OF LOCAL VENTILATION SYSTEMS
Different available measurement instruments and evaluation methods are described in Chapter 12. Some specific methods to evaluate local ventilation systems are described in this section. All local ventilation systems should be evaluated regularly. The evaluation procedures can be divided into detailed and simple, as well as direct and indirect, procedures. The detailed procedures need special instruments and competence, whereas it should be possible to use the simple procedures every day. Since the simple procedures do not measure directly the performance of the exhaust, it is usually necessary to calibrate a simple procedure by using a detailed procedure.1
The evaluation methods could be direct, e. g., measuring a containment index, or indirect, e. g., measuring pressure loss or velocity distribution. The direct methods are used to measure the performance of a hood or an inlet during periodic preventive maintenance. Indirect methods are used for verifying or checking on a daily basis (routine checks). How often each method is used depends on the availability of instrumentation and qualified personnel, since direct measurement of a hood’s performance can be both expensive and difficult. On the other hand, indirect methods are usually easier to use arid can sometimes include inexpensive, continuously monitoring instruments (pressure gauges or velocity indicators).
For some hood types, measurements usually seen as indirect method, are used to measure the hood’s performance to determine regulatory compliance. For example, regulations specify minimum and maximum face velocities for laboratory fume hoods and static pressure (negative) inside enclosed hoods. Continuously monitoring instruments can be connected to alarms that sound when the measurement is outside the specified limits.
Capture efficiency is the fraction of generated contaminant that is directly captured by the hood. Measurement of capture efficiency involves measuring concentration of process-generated contaminant or a tracer material. Using process-generated contaminant requires use of instruments suited to each specific contaminant and its conditions (temperature, pressure, concentration, form, etc.). In order to facilitate these measurements, a tracer is often substituted for the process-generated contaminant. The tracer is usually a gas (sulfur hexafluoride, nitrous oxide, helium, or similar), but an aerosol (particles) can also be used (potassium iodide, polystyrene particles, microbiological particles, etc.). The chosen tracer should be as similar to the real contaminant as possible, but at the same time should be non-toxic, easily and inexpensively measured, and have a low or undetectable background concentration.
Since it is nearly impossible to find an ideal tracer, all measurements made with tracers deviate from measurements made with real contaminants. The efficiency methods, when using real contaminants, are as near to direct measurement of function as is possible. Use of tracers can also give direct measurements, but the conditions of the tracer may differ so much from the real contaminant that these measurements must be seen as indirect.
A simple evaluation can be done by checking the airflow rate into the opening, presuming that the source characteristics, the placement ol the exhaust, and other parameters (crossdraft, work routines, supply airflow rate, etc.) have not changed since the detailed evaluation was done. Naturally, it is necessary initially to perform the simple evaluation at the same time that the detailed evaluation is performed. The flow rate into the exhaust opening can be measured in many different ways. These include pressure drop across the opening, flow measurement in the duct downstream of the hood, measuring air velocities in the opening plane, etc. Handbooks and textbooks on ventilation2’’’ and ACGIH Industrial Ventilation Chapter 94 have descriptions of different flow-measuring devices.
Since the static pressure loss for a hood is dependent on form and flow rate it can be used alone to monitor the flow rate into the hood. If the flow rate and the pressure loss were measured at the same time as the efficiency, the pressure loss can be used for monitoring hood performance. These methods are also described in the literature mentioned above.
Another simple way to evaluate the exhaust is to use smoke to visualize the air streamlines. It is sometimes possible to see how far an exhaust reaches by observing smoke movement when it is generated at different distances from the hood opening. This is better achieved by using the source (if it generates particles or hot gases) and a suitably placed lamp to look directly at the capture of the contaminants. Some gases can also be visualized, either because of density differences or because of differences in refractive index or light absorption.5
In theory it should be possible to calculate the capture efficiency without measurements. There have been some attempts to do that, using CFD, but the problems with air movements and source characteristics have shown that it will be a long time before it is possible to calculate hood capture efficiency in the design stage.
Different methods to measure efficiency are described in Section 10.5.2. Methods to visualize contaminants and airflow are described in Section 10.5.3.
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