Section 4 of 6 DOAS Core Technologies Desiccant dehumidification Air-to-air energy recovery Evaporative cooling 1
Desiccant dehumidification Industrial Desiccant Dehumidifier Hybrid DX + Desiccant Dehumidifier 2
Desiccant vs. cooling dehumidification 3
Industrial desiccant dehumidification 4
Basic desiccant wheel performance Source: The Dehumidification Handbook - 2nd Edition (Munters, 1991-2002)..and leaves VERY dry, and hotter Process air enters at moderate temperature and humidity 5
Predicting desiccant DH performance Key variables Inlet air moisture Inlet air temperature Reactivation air temp. Process air velocity Source: Chapter 6 - Desiccant Dehumidifier Performance The Dehumidification Handbook - 2nd Edition (Munters, 1991-2002) 6
Changing process air moisture 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 7
That s one good reason for bypass air......save energy by not drying any more air than necessary Bypass Air 8
Process air temperature The lower the temperature in......the lower the moisture of the leaving process air 9
That s one good reason for precooling the air... Smaller (less expensive) desiccant wheel does more more work Precool coil 10
Reduce process air velocity 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 11
That s one good reason commercial desiccants operate at higher velocity: No need for deep drying, so use smaller, less expensive wheels Higher velocity provides adequate drying using smaller wheels 12
Increase reactivation air temperature 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 13
That s one good reason for using condenser heat: When less drying required don t overheat reactivation for the required moisture removal Condenser heat reactivation 14
Process air temperature rise: First, understand the total heat of air Increasing total heat (Enthalpy) Increasing Increasing latent heat Latent heat: The heat that was needed to evaporate the water vapor in the air. More water vapor.. more latent heat sensible heat 15
Changing air s moisture content changes the air temperature along a line of constant total heat 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 16
Desiccant process air temperature rise: Constant total heat + reactivation heat carryover Moisture removal temp rise Heat carryover temp rise 17
Reducing heat carryover is one reason for purge sectors for the desiccant wheel Post-purge Pre-purge 18
Condenser heat-reactivated desiccant behavior (General pattern - specifics vary) Note that the leaving air temperature is close to the same as the entering temperature.. but much drier 19
Heat recovery wheels - Peak Load Reducers 20
Heat transfer vs. moisture transfer efficiency 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 21
Wheel stops during moderate temperature hours, to avoid overheating the building Greatest benefit is peak load reduction... Major reduction in size and cost of heating and cooling DOAS components 22
Ventilation air moisture load continues without load reduction whenever over-recovery of heat is problematic In other words... passive desiccant wheels cannot dry the air at all times DOAS systems still need to include a dedicated DH component 3317 hours without moisture load reduction (Palm Beach) 23
Evaporative cooling as DOAS components Direct evaporative cooling Low-energy cooling, by adding moisture to supply air Indirect evaporative cooling Low-energy cooling, by adding moisture to exhaust or outdoor air......while removing supply air heat through a plate or tube heat exchanger 24
Summary - DOAS component technologies 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 25