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Rotating heat exchangers

Rotating heat exchangers are not themselves sources of contaminants, but they

may transfer contaminants from exhaust to supply air with entrained air, and

through possible leakage around the wheel at the separation wall. Leakage from

exhaust to supply was measured by the author in several units, and found to be

negligible in most cases (see Chapter 5, ‘Leakage through heat exchangers’).

A part of the extract air is indeed entrained to the supply duct by the

rotation of the wheel, as shown in Figure 6.5. All the exhaust air contained

in a sector of the wheel is entrained back into the supply air.

This is avoided by installing a purging sector, which returns this vitiated air

back to the exhaust duct (see Figure 6.6). This chamber covers a sector of about

58, in which the outdoor air passes through the wheel, makes a 1808 turn in the

purging chamber, passes back in the wheel and ﬁnally leaves the air handling

unit by the exhaust air duct. This cleans the wheel from contaminants accumu-

lated when passing in the extract air, before entering the outdoor air. Note that

this device functions properly only when the sense of rotation of the wheel is

3.5

4.5

5.5

200 300 400 500 600 700

Odour intensity

Surface concentration of bacteria

[thousands of CFU/cm

]

Figure 6.4 Bacteria concentration at inner surface of a humidiﬁer

correlated with the odour intensity

Note: CFU ¼ colony forming unit.

Source:Mu

ller et al., 2000.

Supply air

Extract ai

Rotation air

Figure 6.5 Some extract air is entrained in the supply airﬂow by the

rotation of the wheel

Contaminants in Air Handling Units 111

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Sharp JX-9400 Technical Information Seite 132