Aspects Related to the Quality of Extract or Exhaust Air

In many of these aspects, the Extract Air Classification, summarized in Table 9.22, is a useful tool to assist in system design. Applications of these classifica­tions are described in Annex A of prEN 13779.

Categories for exhaust air (EHA) are also defined in prEN 13779. Nor­mally the EHA category for air to be discharged outdoors is the same as ETA, but if the system includes reliable and monitored cleaning equipment, the cate­gory can be one lower. With these provisions, air from industrial processes with very high pollution level, in category ETA 4 can be cleaned to EHA 3 levels. Examples of ETA and EHA Classification Applications

Reuse of Extract Air

The quality of extract air is crucial for its use as recirculated air. The clas­sification described in prEN 13779 is also valid for industrial applications. Air even possibly containing any carcinogenic compound is automatically of category ETA 4 (“very high pollution levels”) and must not be used, not even after cleaning, as recirculated air (should be applicable normally only in category ETA 1, and in some cases ETA 2, after cleaning) or transfer air (applicable in categories ETA 1 and ETA 2). This means that reuse of ex­tract air in any industrial application is limited to relatively clean indus­trial environments) (extract air in category ETA 1 or 2), and even then the quality of air must be continuously controlled and monitored in a reliable way.


FlH TABLE 9.22 Extract Air Categories (ETA)

Pollution level code Description Example

Extract air with low pollution levels

ETA 1 Air of the same quality as Offices, storage rooms,

Outdoors with respect to public service places. No

Humidity. From rooms major pollution sources,

With pollutant sources including smoking,

From humans and building material.

Extract air with moderate pollution levels

ETA 2 Air from occupied spaces Smoking lounges,

That have impurities in eating areas,

Excess of F. TA 1.

Extract air with high pollution levels

F. TA 3 Spaces in which moisture, Toilets, kitchens, garages,

Chemical processes, etc. tunnels, car parks, sol —

Substantially lower air vent areas, laboratories,


Extract air with very high pollution levels

ETA 4 Air containing impurities and Industrial processes,

Odors detrimental to laboratories, smoking

Health, in concentrations lounges,

Higher than regulations allow.

Distances and Locations of Openings

Rough principles are presented in prEN 1377.9. The general princi­ple is to discharge air with high pollutant levels (category EHA 3 or 4) from openings above the roof level, and upward. Only relatively clean air can be discharged through outer walls. Openings for supply air should be located at the highest practicably possible distance from con­tamination sources outdoors or from exhaust air.

Pressure Conditions

As a general rule, the system must be designed so that contaminated ex­tract or exhaust air cannot mix with supply air through leakages. This entails requirements for pressure conditions, especially for heat recovery units. For category ETA 1, no limitations are needed: for ETA 2 and 3 using air-to-air heat exchangers for heat recovery, a positive pressure in the supply air side is required (ETA 2, in average; ETA 3, throughout). When extract air is of cate­gory ETA 4, only systems using an intermediate heat transfer medium should be applied. Positive pressure should be avoided in air ducts at least tor cate­gory ETA 3 and must be avoided altogether for category ETA 4.