Determination of Capture Efficiency
Capture efficiency of the hood (ac) is defined as the ratio of the directly captured contaminant to the amount of generated contaminant.6-9 The directly — captured contaminant is that part of the emission captured directly into the hood, and does not include that part of the emission captured after dilution into the ambient room air. The room mass balance is determined using the conditions and expressions given in Fig. 10.107 and assuming the room air is well mixed:10-11
*7nig (^vs *7vex)^o ’ (10.129)
Where
QVH — ventilation flow rate into and out of the space (m3 s_I)
Qvex = rate of extract from the hood and the associated infiltration (m3 s-i!
Q = contaminant generated (mg s_l)
Cr = pollutant concentration in room (mg nr3)
C„ = pollutant concentration outdoors (mg m-3)
Ct, = pollutant concentration in the extract, (mg m~3)
The mass balance of the hood is given by
*?mg (*7vex *vl)Cr + Qv[C(J , (10.130)
Where Qvi is the escaped flow rate of the exhaust (m3 s~x).
By assuming the outdoor concentration is zero, Eq. (10.129) can be expressed as
*?mg ^vtxi^e ~ Cr) + (<7vs *7vex)^r * (10.131 )
The right-hand side of Eq. (10.131) can be looked at as two parts: Qyex{Ce — Cr) is the directly captured part of the emission and (qm + qvex)CT is the part of the emissions escaping into the room.
|
FIGURE 10.107 Room mass balance boundaries for A Contaminant source, local exhaust hood, and general ventilation.11 |
Q<s = ventilation flow rate into and out of the space, m3 s~’
<jyl = spillage into the space from the hood, m3 s~’ qyex = rate of extract from the hood and the associated infiltration, m3 s ‘
= contaminant generated, mg sr1 (equal to qmg in the equations)
C. — pollutant concentration in the room, mg rrr3
C, == pollutant concentration outdoors, mg rrr3 Ce = pollutant concentration in the extract, mg irr3
Capture efficiency is then defined as
«C = <7vcx(Q — Cr)/qm, (10.132)
And the emission escaping into the room is given by
(1 — (Xc)qMg — (?vs • i 10.1>3 )
According to Eq. (10.130), Eq. (10.133) can also be expressed as
(1 — Otc)qmf, = qv|(C,, — Cr) . (10.1.34)
Equation (10.133) is more useful than Eq. (10.134), where the escaped airflow QV| is difficult to measure.
At least three factors must be considered when using Eq. (10.132). The first is that the room air is seldom well mixed; the second is that the emission rate is usually unknown; and the third is that measurement of Qys is often very difficult, especially in spaces where a mixture of mechanical and natural ventilation is used. If the room air is poorly mixed, a sampling strategy is needed in order to determine the room air concentration (Cr). There exist different strategies for this, and two are as follows:
The contaminant concentration measurement in the general ventilation exhaust will provide the best average value for room air concentration
The room air concentration (Cr) can be determined by measuring the concentration for a grid of points around the room.
The best way to determine the capture efficiency is to use the same contaminants that are encountered in practical applications. The usually unknown emission rate in Eq. (10.132) can be evaluated by the use of different models especially in the case of evaporation, or by weighing the waste material or by measuring the contaminant concentrations and airflow rates iri all exhaust ducts in the space and using Eq. (10.131) to calculate Qma.
Sometimes the real contaminant cannot be used for the determination of the capture efficiency, for example, if there is no suitable analysis method available for the real contaminant, or if the analysis method is too expensive. In these cases, tracer gases can be used (see Section 10.2.3.3).
In the case of gaseous contaminants, the tracer gas is selected to simulate as well as possible the properties (density, temperature) and momentum of the real contaminant. It is essential to ensure that the tracers are nontoxic, chemically nonreactive, nonadsorptive on indoor surfaces, and inexpensive. The mixing of the tracer with the actual gaseous contaminant before its release or the release of the tracer with a density near that of the air will improve the validity of the simulation. With tracers, the most difficult task in practice is the relationship of the discharge between the tracer and the real contaminant. Case-by-case techniques to release the tracer are necessary in practice. With tracer gases, the procedure for capture efficiency is described in detail in the European Standard.12 The tracer gas concentrations are measured in the exhaust duct for two release locations as illustrated in Fig. 10.108.
First, the tracer is released directly into the exhaust duct (Fig. 10.108(3). Second, the tracer is released with the same flow rate at the contaminant — generation point(s) (Fig. 10.1086). The capture efficiency (a;:) is calculated using Eq. (10.132) by taking into account that
(( ,’■ — Cr)/C. r(,f,
Where Ce is the tracer concentration (mg m~3) in the duct, when released at the source release point(s); Cref is the tracer concentration (mg m-i) in the duct, when released directly into the exhaust duct; and Cr is the tracer concentration (mg itt — ) in the room, when released at the source release point(s).
The contaminant concentration is measured by a direct reading instrument or by some collecting method (for instance charcoal tubes). Due to the negative pressure in the exhaust duct, powerful sampling pumps are required in order to extract samples from the duct. If the concentration is uniformly distributed in the duct, one measurement point in the duct is sufficient. If the distribution of the concentration in the duct is not uniform, a survey of the concentration distribution and the corresponding duct velocities and cross-sectional areas are required. Various studies1’ have shown that distances ranging from 4 to 50 duct diameters are required, depending on the number of fittings, to ensure complete mixing in the duct.
5
(h) /v~0~0~
5
2D———— O3
FIGURE 10.108 The procedure to measure the capture efficiency by the tracer gas method, (a) The measurement of the reference concentration in the duct, when the tracer is released direcdy into the duct, (b) The measurement of the concentration in the duct, when the tracer is released from the source. / = sampling point, 2 = pump, 3 = analyzer, 4 = injection of tracer, 5 = tracer gas flow meter, 6 = tracer gas cylinder.
With particles, the contaminant concentration in the duct is determined by isokinetic sampling with subsequent laboratory analysis use of a calibrated direct reading instrument. If the concentration distribution in the duct is uneven, a complete survey of the concentration distribution with the corresponding duct velocities and cross-sectional area is required. National and ISO standards14,15 provide information on isokinetic sampling and velocity measurements. In the case of particles, the airborne emission differs from the total emission, for example in the case of granular particulate. The contaminant settling on surfaces depends on particle distribution, airflow rates, direction in the space, electrical properties of the surfaces and the material, and the amount of moisture or grease in the environment.
Test bench methods for machines not too large for test cabins have been developed in order to obtain comparative results. In the case of particles, the tracer gas describes well the behavior of aerodynamic diameter particles less than 5 to 10 |xm.9 For larger particles, correction factors should be used to modify the efficiency results obtained using the tracer gas technique.
10.5.2.1 Determination of Occupational Hygiene Efficiency
The occupational hygiene efficiency is defined as the ratio of the contaminant concentrations in the operator’s breathing zone (Cbr) with the exhaust hood operating and not operating:
Vh = (Cbr(on) — C0)/(Cbr(off) — C0), (10.136)
Where C0 is the background concentration of the room air contaminant.
This gives a direct measure of the benefits obtained by the hood. In practice, the best way to determine the occupational hygiene efficiency is to measure the actual concentrations in the operator’s breathing zone for those two cases. A tracer can also be used with the limitations described above.
10.5.2.2 Determination of the Opening Velocity
The airflow direction is evaluated by a smoke test before commencing velocity measurement at the openings. The velocity is then measured at several points. The airflow direction in each opening should be determined by a smoke test. Rotation of belts and machine parts inside the enclosure generates strong air currents that may escape from the enclosure openings near these moving parts while the other openings may exhibit inward airflow.
10.5.2.3 Determination of Protection Factor (PF)
Different protection factors have been defined.16 One method is to define it as the ratio of the concentration of a contaminant in the exhaust duct (Cj To the concentration in the breathing zone (Cbr) of a person standing in front of the enclosure, for example, a laboratory fume hood:
Where Ce and Cbr are expressed in mg nr3. This approach is useful and easy to measure. By using this method, the determined protection factors may be quite high. For carcinogenic and highly toxic materials, protection factors of 10 000 are common. Protection factors (F) based on the ratio of the breathing zone concentration (Cbr) to the release rate of the contaminant (qmg) have been used. To obtain convenient numbers for the protection factor, negative logarithms are used:
F = — log(Cbr/Qmg), (10.138)
Where Cbr is expressed in mg m~3 and Qms is in mg s-1
Protection factors based on the ratio of the breathing zone concentration
To the concentration inside the enclosure have been defined. Without complete mixing of air inside the enclosure, considerable variations in the concentration are expected. The best evaluation for enclosure concentration without complete mixing is the measurement of the concentration in the exhaust duct.
10.5.2.4 Determination of the Efficiency of Local Supply Air
Sometimes local supply air is combined with exterior hoods to protect the worker from escaping contaminants. Measuring the breathing zone concentra —
Tion as a percentage of the concentration in the exhaust airflow, with a con stant release rate of contaminant or tracer, provides a good method of comparing the effects due to changes in the local supply and exhaust airflow rates.
The dilution effect of the local air supply can be studied by measuring the concentrations in the breathing zone with and without the use of the local supply air.
To determine if the local supply air reaches the breathing zone, a tracer gas is used. The tracer concentrations at various heights below the supply unit are recorded as a percentage of the supply duct tracer concentration. This provides information on the degree of mixing taking place between the supply and ambient air.
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