2. Objectives
To Design, Fabricate & Performance Evaluation of an
Waste Heat Driven Desiccant De-Humidifier
Silica Gel ( Absorbent)
Process Cycles
Absorption ( Moisture Air ) & Regeneration (Hot Air)
4. Chamber Design
Made up of 2.5mm thick sheet of aluminum.
Houses the cake assembly
Water flow tubes from the two heat exchangers are
connected to the cake assembly through the chamber side
walls.
Air Inlet & Outlet Pipe Sections Connected at Both
Ends
5. Cake Assembly
Made in Square Shape of 250mmX150mmX15mm Size
Configured as Heat Exchanger with fins and single pass tube
Silica gel of 0.3mm diameter filled in between the fins space (375grams)
Wire mesh is soldered on both sides of the cake assembly to hold the silica gel intact and
prevent atmospheric air leakage
2 no's of cake is assembled to the chamber in such a way that the process enters and exits
through the wire mesh only
6. Humidity Station
Placed in Tank outlet, Chamber Inlet and Chamber outlet sections
Kimo Sensor ( Measures – RH% and Temp)
Dry bulb Temperature and Wet bulb Temperature
Humidity Ratio found from Psychrometric chart using the values obtained from the
readings at these stations and hence performance evaluation is obtained
7. Water Tank
Water is filled and heated to 65deg C to create moisture
Fitted with condensing cone plate – numerous holes were made to water droplets and pass
only the moisture air to the system
8. Heat Exchanger
Two heat exchangers which are connected in parallel are heated to room temperature of
25deg C and at 65deg C respectively.
The heat exchangers water flow lines are connected to the dehumidifier’s beds through
inlet and outlet tubes.
The heat exchangers supply cold water to the bed inlet during the absorption process and
hot water during the regeneration process,
and these processes are carried out by operating the directional control valves manually.
9. Suction Blower
Suction blower as shown in fig is assembled to
the systems air outlet duct section in-order to
draw the process air and pump away the
dehumidified air from the system to the outlet
where it was needed.
The flow is inside the blower is kept at laminar
flow to have a steady desired level of de
humidified air. (36Kg/hr)
10. Agilent Data Logger
The software is a Windows®-based application.
Uses spreadsheet environment to define measurement data to be collected.
Simply identify the measurements which are to be acquired, initiate the process, and see
the data displayed in real-time.
The new quick graph feature provides easy to use graphing with many options for
graphing the data.
11. Principle of Operation
The dehumidification is been carried out with
combination of two different process which is carried
out alternatively in a given time cycle and they are
named as Regeneration process and Absorption
process
12. Absorption Process
Adsorption is a process , where moisture is condensed and held on the
surface of the material without any change in the physical or chemical
structure of the material.
The adsorbent material can be reactivated by heat.
Silica gel - SiO2 - is a hard, adsorbent, crystalline substance and very
porous.
Voids are about 50 - 70% by volume and adsorb water up to 40% of its
own mass.
The bulk density of silica gel is 480 - 720 kg/m3.
The specific heat capacity is 1.13 kJ/kgK.
13. Absorption Process
Water in tank is heated to 65deg C to create moisture air
The moisture air is then passed through the condenser assembly inside the water tank
Where the condensed water droplets will drip down to the tank and only moisture air passes through the holes on it
The moisture air then pass through the humidity station located at the outlet of the tank and inlet of the chamber
where measurements ( dbt, wbt, RH% & T ) were observed through the thermocouples and the KIMO sensors
Heat exchanger supply water flow to the bed at 25deg C
Moisture process air passes through the beds impregnated with silica gel
The silica gel absorbs the moisture in air
Then the dehumidified air pass through the humidity station located at the exit of the chamber and again the
measurements are observed
The de-humidified air is then fed to the required space through the suction blower which is fitted into the system’s
flow line
Process air leaves the dehumidifier as dry air.
14. Regeneration Process
Regeneration cycle helps to bring the silica gel filled in the
beds achieve its initial state or completely dry so that it will
be used to absorb the moisture in the air for the subsequent
cycle of operation.
Here the heat exchanger which is operating at 65deg C is
used to fed the hot water to the two cake assembly
15. Regeneration Process
Air is drawn into the chamber through atmospheric air intake section
Heated to high temperature of 70deg C through heating element
Water tank is isolated from the chamber
Heat exchanger supply water flow to bed at 65deg C
The hot air then pass through the humidity station located at the inlet of the chamber
where measurements ( dbt, wbt, RH% & T ) were observed
Hot air then passes through the beds and silica gel releases the moisture from the bed
The hot air then pass through the humidity station located at the outlet of the
chamber where measurements ( dbt, wbt, RH% & T ) were observed
Regeneration air leaves the dehumidifier as warm wet air.
16. Experimental Results
0.00
20.00
40.00
60.00
80.00
0.00 200.00 400.00 600.00 800.00 1000.00 1200.00 1400.00 1600.00 1800.00 2000.00
Hot Air Temp @ Chamber Outlet
Temp
Initial Condition
The chart shows the temperature gradient of the desiccant bed when hot air flows
through the cold bed, the silica gel absorbs the heat from hot air and then releases the
moisture from it and thus gets completely dried.
After 750secs the temperature starts to increases gradually, during this heating period,
the silica gel is attaining its dried stature and the system is ready for the absorption cycle
or to dehumidify the air
17. Experimental Results
Initial Condition
It illustrate that the effective period of adsorption ranges between 10 and 12 min for
the presented hot air conditions. After this period, it is noticed that the exit humidity of
dehumidified air seems to be nearly constant. This means that successive adsorption will
consume more power for the air blowing system
18. Experimental Results
Process Cycle
28.500
29.000
29.500
30.000
30.500
31.000
31.500
32.000
32.500
0.00 1000.00 2000.00 3000.00 4000.00 5000.00 6000.00
Humidity Ratio ω ( Grams Moisture /Kg
Dry Air)@ System Outlet
Humidity Ratio ω ( Grams
Moisture /Kg Dry Air)@
System Outlet
20.00
22.00
24.00
26.00
28.00
30.00
32.00
34.00
36.00
38.00
0 1000 2000 3000 4000 5000 6000
Humididty Ratio By Interplating
Dbt & Wbt
Humidity Ratio By KIMO Sensor
19. Experimental Results
Approximately 5 to
10gms Moisture /Kg
Dry Air is been reduced
0.00000
5.00000
10.00000
15.00000
20.00000
25.00000
30.00000
35.00000
0.00 1000.00 2000.00 3000.00 4000.00 5000.00 6000.00
Humidity Ratio ω ( Grams
Moisture /Kg Dry Air)@ System Outlet
Humidity Ratio ω ( Grams
Moisture /Kg Dry Air) @ System Inlet
Humidity Ratio ω ( Grams
Moisture /Kg Dry Air) @ Tank Outlet
28.500
29.000
29.500
30.000
30.500
31.000
31.500
32.000
32.500
0.00 1000.00 2000.00 3000.00 4000.00 5000.00 6000.00
Humidity Ratio ω ( Grams Moisture /Kg
Dry Air)@ System Outlet
Humidity Ratio ω ( Grams
Moisture /Kg Dry Air)@
System Outlet
20. Conclusion
Absorption / Desorption operations of a packed bed have been investigated.
Silica gel applied as the working desiccant.
The following conclusions can be summarized:
1. The absorption rate has its maximum value at the beginning of absorption, and then
rapidly decreases to its minimum value for a period of time. This period is dependent
on the bed parameters and air inlet conditions.
2. The average absolute moisture removal is 5 to 10gms/Kg dry Air
3. The start-up period of desorption is discussed and highlighted. It is found that a
transient period 720secs (after few tests )from absorption to desorption is needed.
4. The bed design parameters can be selected in accordance with the objective function of
the process. To maximize the drop in air humidity it is recommended to use desiccant bed
with higher bed sizes.
