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DOAS Core Technologies - Munters · DOAS Core Technologies ... dew point hits a lower limit ... •...

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1 Section 4 of 6 DOAS Core Technologies Desiccant dehumidification Air-to-air energy recovery Evaporative cooling
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1

Section 4 of 6

DOAS Core Technologies

• Desiccant dehumidification

• Air-to-air energy recovery

• Evaporative cooling

Munters Technical Training - October 2016

Desiccant dehumidification

2

HybridDX + DesiccantDehumidifier

IndustrialDesiccantDehumidifier

Munters Technical Training - October 2016

Desiccant vs. cooling dehumidification

3

Munters Technical Training - October 2016

Industrial desiccant dehumidification

4

Munters Technical Training - October 2016

Basic desiccant wheel performance

5

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

6

• 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

7

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

8

Bypass Air

Munters Technical Training - October 2016

Process air temperature

9

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

10

Precool coil

Munters Technical Training - October 2016

Reduce process air velocity

11

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

12

Higher velocity provides adequate drying using smaller

wheels

Munters Technical Training - October 2016

Increase reactivation air temperature

13

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

14

Condenser heat reactivation

Munters Technical Training - October 2016

Process air temperature rise:First, understand the “total heat” of air

15

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

16

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”

17

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

18

Post-purge Pre-purge

Munters Technical Training - October 2016

Condenser heat-reactivated desiccant behavior (General pattern - specifics vary)

19

Note that the leaving air temperature is close to the same as the entering temperature.. but much drier

Munters Technical Training - October 2016

Heat recovery wheels - “Peak Load Reducers”

20

Munters Technical Training - October 2016

Heat transfer vs. moisture transfer efficiency

21

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

22

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

23

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

24

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

25

• 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


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