ASHRAE Journal - August 2019 - 51

TECHNICAL FEATURE

Humidity Ratio (lb Vapor/lb Air)

humidity ratio of up to
FIGURE 7 Observed dehumidification performance and target for different outdoor weather conditions at 0.8 rpm belt speed, and 110°F
0.0012. For reference,
regeneration temperatures.
a desiccant wheel
under the same conditions and at regenera93°F
tion temperatures of
86°F
150°F (66°C) would see
)
Air
75°F
an adsorption capacry
D
b
u/l
ity of 0.0049 (based
(Bt
y
p
hal
on a manufacturer's
Ent
program). While such
a desiccant wheel
outperforms the proposed belt system,
the minimum recommended regeneration
temperatures on the
Dry-Bulb Temperature (°F)
desiccant wheel are
150°F (66°C), whereas
the desiccant belt system operates at much lower
For both the range of regeneration temperatures
temperatures.
and the range of belt rotational speeds considered,
The effect of belt speed on performance is illusthe maximum sustainable change in humidity ratio
trated in Figure 6b. In general, adsorption trends
is approximately 0.0012. This is almost a full order of
upward with increased belt speed. The results suggest magnitude less than the dehumidiﬁcation requirethat quickly regenerating desiccant before it has a
ments for a design day, as described previously in Figure
chance to saturate can improve performance. Also of
4. This observed drop in humidity translates to electric
note is the difference between water vapor removed
power savings of approximately 13.7 kBtu/h (14.6 MJ/h)
from the fresh airstream and water vapor released
at design conditions as compared to the theoretical
to the exhaust airstream. This difference suggests
energy savings of 46.2 kBtu/h (48.7 MJ/h) for the same
that water is being stored in the desiccant material
conditions.
and that the system is not at steady state. The excepThe experimentally observed change in humidity
tion is at belt speeds of 0.8 rpm where the two are
is compared against the target on the psychrometric
nearly balanced as previously suggested in discussion chart in Figure 7. This is shown for both the design day
related to Figure 6a.
conditions at 93°F (34°C), as well as other outdoor
When the system is not at steady state and adsorption
temperatures. As temperatures decrease, the amount
is greater than regeneration, the system is limited by
of water adsorbed also decreases. In addition, there is
the water storage capacity of the desiccant material. The a more pronounced change in temperature at cooler
17 lb (7.7 kg) of dry desiccant contained in the proposed
weather conditions, due to the more signiﬁcant senbelt system can hold capacity of 40% of its weight in
sible heat transfer that occurs across the aluminum
moisture, or 6.8 lb (3 kg) of water. Given the mass ﬂow
sheet between the dehumidiﬁcation chambers under
rate of air and assuming dry air density of 0.0807 lb/ft3
these conditions.
3
(1.3 kg/m ) suggests that this unit might run for up
Pressure drop across the dehumidiﬁcation device was
to 140 minutes before saturation, although previous
measured for all the previously described tests and was
studies have shown an exponential decay in rates of
found to be negligible. This is likely due to the cross-secadsorption that occurs as the system approaches satutional area of the chambers and large gaps between the
ration, and which, therefore, might extend the period
desiccant belt and ducting. This suggests that the mass
signiﬁcantly.
of desiccant in the system can be signiﬁcantly increased,
A U G U S T 2 0 19

ashrae.org

ASHRAE JOURNAL

https://www.ashrae.org/

ASHRAE Journal - August 2019

Table of Contents for the Digital Edition of ASHRAE Journal - August 2019