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7.7 Mixing of the air streams
Generally a mixing of the air streams must be expected
with wheels. Without special precautions VDI 6022 must be
observed: 'Regenerators with wheels are to be used only
if for hygienic reasons recirculation could also be used.'
Causes for mixing of the air streams include:
■ Carryover
A specic volume of air (depending on the speed of
rotation, air velocity and wheel geometry) is rotated in the
other direction by an air stream.
■ Leakage
Leakage through the radial and transverse seals
according to the pressure gradients and the seal quality.
■ Extract air transmission
Because the storage mass is alternately in both air
streams, they each inuence the other. For example,
odours can be transmitted with the smallest particles (e.g.
cigarette smoke).
■ Substance transmission
Wheels also transmit gaseous substances. The amount
transmitted depends on the wheel type and the substance
itself. Unfortunately, few measurements are available in
this eld, and on the other hand it is known in practice that
this is not a problem for standard VAC systems.
In rare cases odourants from the extract air may be
'collected' in the wheel and under extreme fresh-air condi-
tions (very high relative humidity) may be emitted again. This
can cause odour problems. In general, this problem can be
prevented by special adjustments of the cleaning mode or
with a minimum speed of rotation.
Notice
The high tightness seal in Hoval rotary exchangers
minimises leakage. They are even certied for opera-
tion in hospitals.
7.8 Supply air humidification
The humidication downstream from the wheel must be
dimensioned to ensure that the desired setpoint value is
reached even with minimum fresh-air humidity. Because the
wheel speed is generally controlled by the supply air temper-
ature, the corresponding humidity content must be consid-
ered when dimensioning the humidier.
7.9 Corrosion
Hoval rotary heat exchangers have proven to be very
durable in VAC systems. The Hoval application consulting
service can provide information on what equipment to use for
applications where corrosion is potential danger, such as in
kitchens or specic industrial applications etc.
7.10 Application limits
Before selecting the rotary heat exchanger check that appli-
cation limits are not exceeded during operation:
Temperature -40…70°C
Pressure difference max. 2000 Pa
Pressure difference to
outside
max. 2000 Pa
Pressure drop Recommended 80 Pa to 130 Pa
Table 10: Application limits
7.11 Danger or contamination
In 'normal' ventilation systems the air streams are generally
cleaned with coarse lters. This ensures that there is no
danger of dirt build-up on the rotary heat exchanger. If this is
a potential problem with specialised applications, this must
be considered in the design:
■ Install the exchanger so it can be cleaned in its installed
position.
■ Provide inspection openings before and after the rotary
heat exchanger.
■ If possible, clean the air stream by ltering to prevent dirt
built-up or the cleaning intervals are extended.
In practice it has been demonstrated that the danger of dirt
build-up is much less than expected. Clear statements can
only be made on the basis of experience. The Hoval applica-
tion consulting service can also provide information.
7.12 Condensation in the warm air stream
If more water condenses from the warm air than the (heated)
cold air can absorb, condensate is formed. Because this
phenomenon primarily occurs in the rst third of the warm
wheel side primarily because of the thermodynamic function,
some of it is removed by the warm-air stream. This must
be considered for downstream components. In general,
condensate drip trays should be installed on the warm-air
and cold-air side. The following must also be checked or
implemented:
■ How is the condensate drained off?
■ Is there an icing hazard?
System design
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