It gives information about Desiccant-Based Air Conditioning Systems : Introduction, Sorbents & Desiccants, Dehumidification, Liquid Spray Tower, Rotary Desiccant Dehumidifiers, Evaporative-Cooling Air Conditioning Systems, Thermal Storage Air Conditioning Systems, (Theoretical treatment), Heat Pump Systems: Heat Pump Cycle, different heats pump Circuits.

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Mechanical Engineering
Subject:- Heating, Ventilation and Air
Conditioning (ME 411)
B.Tech.Mechanical
Purushottam W. Ingle
Assistant Professor

2. 1
• Advanced Air-conditioning Systems
• Desiccant-Based Air Conditioning Systems : Introduction, Sorbents &
Desiccants, Dehumidification, Liquid Spray Tower, Rotary Desiccant
Dehumidifiers, Evaporative-Cooling Air Conditioning Systems, Thermal
Storage Air Conditioning Systems, (Theoretical treatment), Heat Pump
Systems: Heat Pump Cycle, different heats pump Circuits.
P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
9/4/2023

3. Desiccant-Based Air Conditioning Systems
• Desiccants are commonly used to keep products dry and stable
• Desiccant cooling systems are heat-driven cooling units and
they can be used as an alternative to the conventional vapor
compression and absorption cooling systems.
• The operation of a desiccant cooling system is based on the use
of a rotary dehumidifier (desiccant wheel) in which air is
dehumidified. The resulting dry air is somewhat cooled in a
sensible heat exchanger (rotary regenerator), and then further
cooled by an evaporative cooler.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

4. • The resulting cool air is directed into a room. The
system may be operated in a closed cycle or more
commonly in an open cycle in ventilation or
recirculation modes.
• A heat supply is needed to regenerate the desiccant.
Low-grade heat at a temperature of about 60–95°C is
sufficient for regeneration, so renewable energies such
as solar and geothermal heat as well as waste heat
from conventional fossil-fuel systems may be used.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

5. Sorbents and Desiccants
• Sorbents are materials which attract and hold certain vapor/liquid
substances. The process is referred to absorption if a chemical
change take place and as absorption if no chemical change occurs.
• Desiccants, in both liquid and solid forms are a subset of sorbents
that have a high affinity to water molecules liquid desiccants absorb
water molecules, while solid desiccants adsorb water molecules and
hold them on their vast surfaces.
• While desiccants can sorb water in both liquid and vapor forms, The
sorption driving force for both liquid and solid desiccant is a vapor
pressure gradient.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

6. • Adsorption (in solid desiccants) and absorption (in liquid
desiccants) occur when the water vapor partial pressure
of surrounding air is larger than that at the desiccant
surface. when an air stream is brought in contact with a
desiccant, water vapor from the air is attracted by the
desiccant, the air is dehumidified and the water content
of desiccant rises.
• As the water sorbed by the desiccant increases, the
sorption rate decreases and finally stops when sorption
equilibrium is reached.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

7. • For dehumidification to be resumed, water must be removed from
the desiccant by heating.
• This process is referred as desorption, reactivation/regeneration.
• The heat of sorption (or desorption) is generally higher than the
latent heat of vaporization of water it approaches the latter as
sorption equilibrium is reached.
• Some typical liquid desiccants are water solutions of Calcium
Chloride [CaCl], Lithium Chloride [LiCl], Lithium Bromide [LiBr] and
triethylene glycol.
• Common solid desiccants are silica gel, molecular sieves (zeolites),
activated alumina and activated carbon.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

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11. Cyclic Dehumidification
• Dehumidification by vapour compression system is accomplished by
cooling the air below the dew point and the reheating it. The
performance is greatly hindered when desired outlet dew point is
below 40 °F due to frost formation on the cooling coils.
• Desiccant dehumidification is accomplished by direct exchange of
water vapor between the air stream and a desiccant material due to
water vapor pressure difference Fig. shows the cyclic operation of a
desiccant dehumidification system.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

12. Continue…..
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
In sorption (1-2), dry and cold desiccant (point-1) sorbs moisture since the
vapour pressure at the surface is lower than that of the air stream. During this
process the moisture content (loading/uptake) increases. The surface vapor
pressure increases and liberated heat of sorption raises.
The desorption (2-3), the desiccant is subjected to a hot air stream and
moisture is removed and transferred to the surrounding air. The surface
vapour pressure is increased and the desiccant temperature rises due to the
added heat. The cycle is closed by cooling (3-1).
The desiccant is cooled while its moisture content is constant and the surface
vapour pressure is lowered. The above cycle of sorption, desorption and
cooling can be modified by combining the sorption process with cooling to
approach isothermal rather than adiabatic sorption.

13. Cyclic dehumidification
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17. Liquid Desiccant Spray tower
• A desiccant solution from the sump or reservoir is continuously
sprayed downward in the absorber, while air, the process stream
moves upward.
• The air is dehumidified and the desiccant solution absorbs moisture
and is weakened. In order to maintain the desired solution
concentration, a fraction of the solution from the reservoir sump is
passed through the regenerator, where it is heated by the heating coil
and gives up moisture to the desorbing air stream.
• The strong, concentration solution is then returned to the reservoir
sump. The heat liberated in the absorbed during dehumidification is
removed by the cooling coil to facilitate continuous absorption.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

18. Continue….
• The process air stream exists at a relatively low temperature. If sufficiently low
water temperature is available (an underground well, for example), the process
stream could provide both sensible and latent cooling.
Advantages
1)The system is controlled to deliver the desired level of dry air by adjusting the
solution concentration.
2)concentrated solution can be economically stored for subsequent drying use.
3)The system can serve as a humidifier when required by simply weakening the
solution.
4)When used in conjunction with conventional A/C systems, humidity control is
improved and energy is conserved.
9/4/2023 P.W. Ingle Department Of M chanical Engineering, Sanjivani COE, Kopargaon

19. • Disadvantages
• Some desiccants are corrosive
• Response time is relatively large
• Maintenance can be extensive
• Crystallization may be problem.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

20. Rotary Desiccant Dehumidifiers
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

21. Continue …
• typical rotary solid desiccant dehumidifiers is shown in Fig.Unlike the
intermittent operation of packed towers, rotary desiccant
dehumidifiers use a wheel (or drum) that rotates continuously and
delivers air at constant humidity level.
• Desiccant wheel typically consist of very fine desiccant particles
dispersed and impregnated with fibrous or ceramic medium shaped
like a honeycomb/fluted corrugated paper. The wheel is divided into
two segments.
• The process stream flows through the channel in one segment, while
the regenerating (or reactivating) stream flows through the other
segment.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

22. Continue…
• Desiccant Material
• The desired desiccant properties of for optimum dehumidification
performance are a suitable isotherm shape and a large moisture
sorption capacity.
• The isotherms of silica gel are almost linear. The moisture sorption
capacity is high. The desiccant is reactivated at relatively low
temperatures and is suitable for moderate dehumidification.
• Molecular sieves have very steep isotherms at low relative humidity.
The desiccant is reactivated at relatively high temperatures and its
used for deep dehumidification.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

23. Continue …..
• The Desiccant Wheel
Some considerations for selection of desiccant wheel are
• Appropriate desiccant materials
• Large desiccant content
• Wheel depth and flute size
• Size and cost
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

24. Continue…..
• The following is a range of typical parameters for rotary desiccant
wheels :
• Rotation speed : 4 to 10 RPM
• Desiccant fraction : 70 to 80 %
• Flute size : 1 to 2 mm
• Reactivation segment : 25 to 30 % of wheel
• Face velocity : 300 to 700 fpm
• Reacting temperature : 100 to 300 °F
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

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26. Continue …..
• With the help of chiller the liquid desiccant is chilled.
• Liquid chilled desiccant A desiccant solution from the sump is continuously
sprayed downward in the absorber, while air, the process stream moves
upward.
• The air is dehumidified and the desiccant solution absorbs moisture and is
weakened and diluted.
• Diluted solution again passed through heat exchanger and passed through the
regenerator where it is heated by the heating coil and gives up moisture to the
desorbing air stream.
• The strong solution further cooled in heat exchanger and further it is passed
through the chiller and get cooled and then sprayed to get processed air.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

27. Solid packed tower system
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

28. Continue …..
• In first cycle, the ambient air which is precooled, comes in contact with solid
desiccant chamber (solid packed tower) in which sorption takes place, cooled
and dehumidified air is used for application.
• Simultaneously the reactivated air is passed through the heater and then
through the second tower in which desorption takes place and exhaust air is
out.
• Then in next cycle the ambient air is made to flow through second tower where
the sorption takes place and then that air is used for the application. With the
help of valve provided on upper side the chamber are used alternately.
• So the same chamber is used for sorption and desorption process. Separate
cooler is used for the cooling purpose.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

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30. Continue ….
• Dotted line is reactivated air. Process air outlet: Where 75 percent of
the air that entered the process inlet is returned as warm, dry,
processed air for drying. Reactivation air outlet: Where 25 percent of
the process inlet air or separate intake is exhausted as extremely
warm, humid air. This air is normally exhausted outside of the
building.
• The regeneration of the desiccant material is carried out by using a
second air stream, called reactivation or regeneration air.
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