Standard Guide for Small-Scale Environmental Chamber Determinations of Organic Emissions from Indoor Materials/Products

Importancia y uso:

4.1 Objectives—The use of small chambers to evaluate VOC emissions from indoor materials has several objectives:

4.1.1 Develop techniques for screening of products for VOC emissions;

4.1.2 Determine the effect of environmental variables (that is, temperature, humidity, air speed, and air change rate) on emission rates;

4.1.3 Rank various products and product types with respect to their emissions profiles (for example, emission factors, specific organic compounds emitted);

4.1.4 Provide compound-specific data on various organic sources to guide field studies and assist in evaluating indoor air quality in buildings;

4.1.5 Provide emissions data for the development and verification of models used to predict indoor concentrations of organic compounds; and

4.1.6 Develop data useful to stakeholders and other interested parties for assessing product emissions and developing control options or improved products.

4.2 Mass Transfer Considerations—Small chamber evaluation of emissions from indoor materials requires consideration of the relevant mass transfer processes. Three fundamental processes control the rate of emissions of organic vapors from indoor materials; evaporative mass transfer from the surface of the material to the overlying air, desorption of adsorbed compounds, and diffusion within the material. A more detailed discussion of emission mass transfer theory can be found in Guide D8141.

4.2.1 The evaporative mass transfer of a given VOC from the surface of the material to the overlying air can be expressed as:

where:

ER   =   emission rate, mg/h, A   =   source area, m², k_m   =   mass transfer coefficient, m/h, VP_s   =   vapor pressure at the surface of the material, Pa, VP_a   =   vapor pressure in the air above the surface, Pa, MW   =   molecular weight, mg/mol, R   =   gas constant, 8.314 J/mol-K or Pa m³/mol-K, and T   =   temperature, K.

Thus, the emission rate is proportional to the difference in vapor pressure between the surface and the overlying air. Since the vapor pressure is directly related to the concentration, the emission rate is proportional to the difference in concentration between the surface and the overlying air. The mass transfer coefficient is a function of the diffusion coefficient (in air) for the specific compound of interest and the level of turbulence in the bulk flow.

4.2.2 The desorption rate of compounds adsorbed on materials can be determined by the retention time (or average residence time) of an adsorbed molecule:

where:

τ   =   retention time, s, τ_o   =   constant with a typical value from 10⁻¹² s to 10⁻¹⁵ s, and Q   =   molar enthalpy change for adsorption (or adsorption energy), J/mol.

The larger the retention time, the slower the rate of desorption.

4.2.3 The diffusion mass transfer within the material is a function of the diffusion coefficient (or diffusivity) of the specific compound. The diffusion coefficient of a given compound within a given material is a function of the compound's physical and chemical properties (for example, molecular weight, size, and polarity), temperature, and the structure of the material within which the diffusion is occurring. The diffusivity of an individual compound in a mixture is also affected by the composition of the mixture.

4.2.4 Variables Affecting Mass Transfer—While a detailed discussion of mass transfer theory is beyond the scope of this guide, it is necessary to examine the critical variables affecting mass transfer within the context of small chamber testing:

4.2.4.1 Temperature affects the vapor pressure, desorption rate, and the diffusion coefficients of the organic compounds. Thus, temperature impacts both the mass transfer from the surface (whether by evaporation or desorption) and the diffusion mass transfer within the material. Increases in temperature cause increases in the emissions due to all three mass transfer processes.

4.2.4.2 The air change rate indicates the amount of dilution and flushing that occurs in indoor environments. The higher the air change rate the greater the dilution, and assuming the outdoor air is cleaner, the lower the indoor concentration. If the concentration at the surface is unchanged, a lower concentration in the air increases the evaporative mass transfer by increasing the difference in concentration between the surface and the overlying air.

4.2.4.3 Air Speed—Surface air speed is a critical parameter for evaporative-controlled sources as the mass transfer coefficient (k_m) is affected by the air speed and turbulence at the air-side of the boundary layer. Generally, the higher the air speed and turbulence, the greater the mass transfer coefficient. In a practical sense for most VOCs, above a certain air speed and turbulence, the resistance to mass transfer through the boundary layer is minimized (that is, the mass transfer coefficient reaches its maximum value). In chamber testing, some investigators prefer to use air speeds high enough to minimize the mass transfer resistance at the surface. For example, air speeds of 0.3 m/s to 0.5 m/s have been used in evaluating formaldehyde emissions from wood products. Such air speeds are higher than those observed in normal residential environments by Matthews et al.,4 where in six houses they measured air speeds using an omni-directional heated sphere anemometer with a mean of 0.07 m/s and a median of 0.05 m/s. Thus, other investigators prefer to keep the air speeds in the range normally found indoors. In either case, an understanding of the effect of air speed on the emission rate is needed in interpreting small chamber emissions data.

4.3 Other Factors Affecting Emissions—Most organic compounds emitted from indoor materials and products are non-reactive, and chambers are designed to reduce or eliminate reactions and adsorption on the chamber surfaces (see 5.3.1). In some cases, however, surface adsorption can occur. Some relatively high molecular weight, high boiling compounds can react (that is, with ozone) after being deposited on the surface. In such cases, the simultaneous degradation and buildup on and the ultimate re-emission from the chamber walls can affect the final chamber concentration and the time history of the emission profile. Unless such factors are properly accounted for, incorrect values for the emission rates will be calculated (see 9.4). The magnitude of chamber adsorption and reaction effects can be evaluated by way of mass balance calculations (see 9.5).

4.4 Use of the Results—It is emphasized that small chamber evaluations are used to determine source emission rates. These rates are then used in IAQ models to predict indoor concentration of the compounds emitted from the tested material. Consultation with IAQ modelers may be required to ensure that the small chamber test regime is consistent with the IAQ model assumptions. The concentrations observed in the chambers themselves should not be used as a substitute for concentrations expected in full-scale indoor environments.

Subcomité:

D22.05

Referida por:

D7911-19, D8141-22, D6177-19R24, D7143-24, D6669-19, D7339-18, D8142-23, D6670-25, D7859-19, D6330-20, D7706-17R23, D6178-19R24, D6803-19, E1333-22, D8591-24, D8445-22A, D8345-21

Volúmen:

11.07

Número ICS:

13.040.01 (Air quality in general)

Palabras clave:

indoor air quality; indoor sources; indoor materials; indoor products; small chamber testing; environmental test chambers; organic emissions; emission factor; emission rate; mass transfer;