Sharp JX-9400 Informações Técnicas descarregar pdf (Página 140)

Air sampling and analysis

Air at the four locations is sampled with a pump through small tubes ﬁlled with

an adsorbing medium (for example, activated charcoal, T

ENAX). The sampling

rate is about 0.1 l of air per minute. VOCs accumulate in the compound by

absorption, as long as the medium is not saturated.

The sampling tube is then hermetically sealed and taken to the laboratory

for further analysis. The VOCs are desorbed by heating the tubes and they

are stored in a cold trap. The content of the trap is then injected in the

column of a gas chromatograph. A ﬂame ioni zation detector (FID) is used to

detect and measure the amount of each compound, while a mass spectrograph

is used to help identify the compound (Mogl et al., 1995). An FID analyser can

also be used on site to sample and analyse the air but, if pulse injection is used,

this analysis should be performed at periodical and short intervals at the four

locations shown in Figure 6.10.

Which VOCs?

The ‘natural’ concentration of VOCs is often (and hopefully!) not large enough

to provide accurate measurement of transfer rate. Therefore, a mix of various

VOCs should be injected in the extract duct in order to obtain concentrations

that are signiﬁcantly larger than concentrations in outdoor air. Criteria to

determine the VOCs’ mixture are:

The compounds are selected on their occurrence in buildings. Sources are

paints, paper, solvents, carpets and human emissions.

They are representative of the diﬀerent organic famil ies with the character-

istic of their functional group and sa turation degree. The result is diﬀerent

boiling point and polarity.

They are easy to analyse and the results are signiﬁcant. The concentrations

chosen must be under the saturation limit of the T

ENAX sampler.

The compounds should be easy to manipulate

They should not give a long-lasting and bad smell in the room and should

present an acceptable toxicity at the measured concentrations.

Lists of VOCs found most often in oﬃce buildings, as well as proposals for

VOC cocktails can be found in the literature (Brow n et al., 1994; Hodgson,

1995; Maroni et al., 1995; Molhave et al., 1997; Wolkoﬀ et al., 1997; van der

Syringe

Hot air blower (200 °C)

Stainless steel

turnings

Ø 6 mm copper tube

Figure 6.11 Flash evaporation device for injecting the VOCs

Source: Roulet et al., 2000.

Contaminants in Air Handling Units 119

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