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Figure 2 - Psychrometry of Regenerative Thermal Wheel Figure 3 - Psychrometry of Desiccant Dehumidification
the incoming air, A, is below the dew point
temperature, of the extracted air, θ
X
, there
may be some condensation in the airstream
that is coming from the occupied space so
providing increased heat exchange but, in
the case of a simple plate heat exchanger, no
transfer of water vapour.
An example of a combined sensible
and latent heat exchange (ie including an
exchange of water vapour) is shown in Figure
2. This process could be associated with a
hydroscopic thermal wheel.
Referring to Figure 2, the practical
effectiveness, ε, of the heat-recovery devices
may be described in a number of ways. In
.
terms of sensible heat exchange ε
S
= m
.
A

B

θ
A
)/ m
R

R
– θ
A
), and for latent heat exchange,
ε
. .
L
= m
A
(g
B
– g
A
)/ m
R
(g
R
– g
A
), where the
values of ε
S
and ε
L
are not necessarily equal
Many are uncertain as to how fans in an air handling system are represented on a psychrometric chart
for a device that has both sensible and latent
heat exchange. so that it can then be reused to absorb water water is made warmer than that shown in the
In terms of total heat exchange the vapour from the incoming air. figure, humidification may occur isothermally
enthalpies of the air streams may be used to (similar to the process for a steam humidifier,
. .
give ε
T
= m
A
(h
B
– h
A
)/ m
R
(h
R
– h
A
). Tempered spray a vertical process line), and if the spray is
The use of sprays and sprayed coils in comfort made warmer still, both humidification and
Desiccant dehumidification air conditioning has lost favour for many heating can be achieved at the same time.
Theoretically the use of absorbent or due to concerns for the need for onerous For the cooler water the traditional method
chemical dehumidification to remove maintenance requirements to maintain of providing low-temperature water was to
water vapour from an airstream is simply effective and healthy operation. However, spray the water onto an active cooling coil.
the reverse of the adiabatic humidification when there is a guaranteed appropriate
process and is shown in Figure 3. In HVAC operation and maintenance routine, these Face and bypass
this would typically be undertaken by a types of humidification devices are still A ‘process’ that appears to be somewhat
slowly rotating (1 to 10 revolutions per employed – particularly in industrial and under-utilised in comfort HVAC is face and
minute) framework with a matrix packing legacy systems. Adiabatic humidification (as bypass. This is not really a single process
coated with absorbent on a wheel. The would be typical of an ultrasonic humidifier) but a combination (most frequently) of a
actual performance will be determined was discussed in the article in October. If, cooling and dehumidifying process with a
by the effectiveness of the regeneration of however, the water is heated or cooled as it is mixing process where some of the air passes
the absorbent – the heating process that sprayed into the air stream it will produce the around the cooling coil and then mixes back
removes the moisture from the absorbent processes as shown in Figure 4. If the spray in with the air that has been cooled and
www.cibsejournal.com December 2009 CIBSE Journal 51
CIBSEdec09 pp50-53 cpd.indd 51 11/19/09 3:58:52 PM
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