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Section 4 of 6
DOAS Core Technologies
• Desiccant dehumidification
• Air-to-air energy recovery
• Evaporative cooling
Munters Technical Training - October 2016
Desiccant dehumidification
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HybridDX + DesiccantDehumidifier
IndustrialDesiccantDehumidifier
Munters Technical Training - October 2016
Basic desiccant wheel performance
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Process air enters at moderate temperature
and humidity
..and leaves VERY dry, and
hotter
Source: The Dehumidification Handbook - 2nd Edition (Munters, 1991- 2002)
Munters Technical Training - October 2016
Predicting desiccant DH performanceKey variables
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• Inlet air moisture
• Inlet air temperature
• Reactivation air temp.
• Process air velocity
Source: Chapter 6 - Desiccant Dehumidifier PerformanceThe Dehumidification Handbook - 2nd Edition (Munters, 1991- 2002)
Munters Technical Training - October 2016
Changing process air moisture
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The lower the moisture in...
...the lower the moisture of the leaving process air
Note: This is different behavior than cooling-based DH, in which outlet dew point hits a lower limit regardless of inlet moisture.
Moisture leaving cooling-based DH is limited by coil temperature
Munters Technical Training - October 2016
That’s one good reason for bypass air......Save energy by not drying any more air than necessary
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Bypass Air
Munters Technical Training - October 2016
Process air temperature
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The lower the temperature in...
...the lower the moisture of the leaving process air
Munters Technical Training - October 2016
That’s one good reason for precooling the air...Smaller (less expensive) desiccant wheel does more more work
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Precool coil
Munters Technical Training - October 2016
Reduce process air velocity
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The slower the process velocity...
...the lower the moisture of the leaving process air
BUT: when there’s no VALUE to deep drying (as in commercial buildings) Higher velocity allows smaller, less expensive wheel and a smaller unit
Munters Technical Training - October 2016
That’s one good reason commercial desiccants operate at higher velocity: No need for deep drying, so use smaller, less expensive wheels
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Higher velocity provides adequate drying using smaller
wheels
Munters Technical Training - October 2016
Increase reactivation air temperature
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The hotter the reactivation temperature...
...the lower the moisture of the leaving process air
BUT: when there’s no VALUE to deep drying (as in commercial buildings) There’s NO NEED to heat to high temperatures
Munters Technical Training - October 2016
That’s one good reason for using condenser heat:When less drying required—don’t overheat reactivation for the required moisture removal
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Condenser heat reactivation
Munters Technical Training - October 2016
Process air temperature rise:First, understand the “total heat” of air
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Increasingsensible heat
Incre
asing
tota
l hea
t
(Ent
halpy
)Increasinglatent heatLatent heat: The heat that was needed to evaporate the water vapor in the air. More water vapor.. more “latent” heat
Munters Technical Training - October 2016
Changing air’s moisture content changes the air temperature ≈ along a line of constant total heat
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Spray water into the air..
.. the air cools down..
...because the heat in the air is “consumed” to evaporate the water
Remove water from the air..
.. the air heats up..
...because the heat that originally evaporated the water is returned to the air
Munters Technical Training - October 2016
Desiccant process air temperature rise:Constant total heat + “reactivation heat carryover”
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Moisture removal
temp rise
Heat carryovertemp rise
Munters Technical Training - October 2016
Reducing heat carryover is one reason for “purge” sectors for the desiccant wheel
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Post-purge Pre-purge
Munters Technical Training - October 2016
Condenser heat-reactivated desiccant behavior (General pattern - specifics vary)
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Note that the leaving air temperature is close to the same as the entering temperature.. but much drier
Munters Technical Training - October 2016
Heat transfer vs. moisture transfer efficiency
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6-10 RPM adequate for heat transfer
18-22 RPM needed for equal moisture transfer
To avoid over-recovery of heat, wheel slows down, greatly reducing moisture transfer
Munters Technical Training - October 2016
Wheel stops during moderate temperature hours, to avoid overheating the building
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Greatest benefit is peak load reduction...Major reduction in size and cost of heating and cooling DOAS components
Munters Technical Training - October 2016
Ventilation air moisture load continues without load reduction whenever over-recovery of heat is problematic
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3317 hours without moisture load reduction (Palm Beach)
In other words... passive desiccant wheels cannot dry the air at all times—DOAS systems still need to include a dedicated DH component
Munters Technical Training - October 2016
Evaporative cooling as DOAS components
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Direct evaporative coolingLow-energy cooling,
by adding moisture to supply air
Indirect evaporative coolingLow-energy cooling,
by adding moisture to exhaust or outdoor air...
...while removing supply air heat through a plate or tube heat
exchanger
Munters Technical Training - October 2016
Summary - DOAS component technologies
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• Desiccant dehumidifiers• Dries air very deeply - Sometimes not necessary to dry all supply air
• Adds heat to process air - Some cooling is needed
• Hybrid DX-Desiccant configuration is especially efficient (each kW dries the air twice, and dries with very little temperature rise)
• Heat recovery wheels• Excellent way to reduce peak loads and save equipment cost
• Cannot always remove moisture—a dedicated DH component is absolutely necessary in a DOAS unit
• Evaporative coolers• Direct evaporative coolers provide low-cost, low-energy cooling—
but add moisture to supply air. Best for dry climates.
• Indirect evaporative coolers cost more, but provide very low-cost cooling without adding moisture to the supply air