Active and Passive Desiccants in DOAS: Selection and Control

1. Active and Passive Desiccants in DOAS: Selection and Control ASHRAE Seminar 11Dallas Meeting 2007 Stanley A. Mumma, Ph.D., P.E. Professor Emeritus Penn State University, @ Univ. Park, PA sam11@psu.edu Web: http://doas-radiant.psu.edu

2. Presentation Outline • DOAS defined, • Selecting the Supply air Conditions: flow rate, DBT, DPT, • Equipment available to achieve the desired SA thermodynamic state point, • Touch on a few Control Issues, • Mumma preferred equipment choices, • ASHRAE Headquarters Building Renovation.

3. DOAS Defined for this presentation 20-70% less OA,than VAV High Induction Diffuser Cool/Dry Supply DOAS Unit W/ Energy Recovery Building With Sensible and Latent cooling decoupled Parallel Sensible Cooling System

4. Key DOAS points: • 100% OA delivered to each zone via its own ductwork. • Flow rate generally as spec. by Std. 62.1-2004 or greater (LEED, Latent. Control) • Employ TER, per Std. 90.1-2004. • Generally CV. • Use to decouple space S/L loads—Dry. • Rarely supply at a neutral temperature. • Use HID, particularly where parallel sys does not use air.

5. Key DOAS points • EW control to achieve a degree of air side economizer, to be discussed later. • Terminal reheat?—allowed by Std. 90.1. • What about space heating? • DOAS and displacement ventilation (DV) are not competitors! • Other configurations? Never put other AC equipment in series with DOAS! • Terror resistance?

6. How should the OA be introduced in DOAS-FCU applications? Parallel, GoodSeries, Bad

7. “SMART” Ceiling DOAS-Fan coil arrangement. DOAS and FCU’s back in parallel for SMART cost effective performance

8. How do DOAS’s perform under the threat of terrorist activities? 1,000 ft2 Exterior Zone 1 9,000 ft2 Exterior Zone 2 10,000 ft2 Interior Zone 3 Facility floor area and zoning

9. VAV System Schematic Filter, hf 3 2 1

10. DOAS Schematic Filter, hf 3 2 1

11. How do DOAS’s perform under the threat of terrorist activities? External release, filter efficiency VAV-80%, DOAS-98%

12. How do DOAS’s perform under the threat of terrorist activities? External release, filter efficiency for equal conc or exposure

13. How do DOAS’s perform under the threat of terrorist activities?

14. Selecting the supply air DPT Qlatent=0.68*scfm*Dw (grains). If all latent load from people @ 205 Btu/person, then Dw=10 gr/lb with 30 scfm/person, requires 51F DPT if space 75F 50% RH (W=65 gr/lb) or Dw=15 gr/lb with 20 scfm/person, requires 48F DPT if space 75F 50% RH or Dw=30 gr/lb with 10 scfm/person, requires 39F DPT if space 75F 50% RH

15. Selecting the supply air DPT or Dw=60 gr/lb with 5 scfm/person, requires -2F DPT if space 75F 50% RH i.e. SA has only 5 gr/lb (65-60) , or is extremely dry! Not even a consideration for commercial applications—Mechanical or Desiccant!

16. EW 5 196 RA 4 2 Space 1 3 OA CC PH 168 Dw=15 grains, with 20 cfm/person 140 112 84 5, 75F, 50%, 65 gr/lb Humidity ratio (grains/lb) 56 4, 48F DPT, 50 grains 28

17. DOAS equipment on the market today 4850 gr

18. DOAS equipment on the market today varies 48-55+50 4850

19. 196 168 140 112 84 Humidity ratio (grains/lb) 56 28 EW SW 6 7 RA 4 Space 2 5 3 1 OA CC PH 2 Dual Wheel Arrangement 3 7 6 4 5

20. DOAS equipment on the market today Desiccant added for 3 reasons:1. 45F CHWS still works2. achieve DPT < freezing3. reduce or eliminate reheat Type 3

21. 7 6 5 1 4 7 3 1 2 2 6 3 5 4 DOAS needs Enthalpy 4 > 3

22. DOAS equipment on the market today EW effec. Control’d, w/ bypass damper Type 3 Desiccant wheel 7891 Heater 84 F DBT148 gr/lb 7498 7891 EW 7698 5051 82129 7558 544344 DPT RA when unoccupied, EA when occupied

23. 3 2 1 1 8 6 4 5 8 7 3 4 2 7 5 6

24. Type III Desiccant Wheel Heating

25. DOAS equipment on the market today 76.5 F50 gr/lb

26. 2 4 1 3 2 1 2 4 3

27. 1 4 3 0 100% OA 2 9 0 Cold Deck 7 8 Neutral Deck 5 6 2 Return Terminal Heating, Typ. Schematic of the IECDDVAV System Zone, Typ. Region of interest

28. Significant chiller size reduction with EW at design

29. A few control issues

30. h=35.7 btu/lbm 80% - 85% of OA cooling load could be saved if wheel on: in this case almost 50% of coil load Reheat adds significant cooling load, beside wasting heating energy. h=27.5 btu/lbm

32. What has ASHRAE sponsored research found? censored Office: 1 story 6,600 ft2 Retail: 1 story 79,000 ft2

33. Base Case: DX, 350 cfm/ton

34. DX (400 cfm/ton) with Desiccant

35. DOAS w/ Desiccant +DX 350 cfm/ton 400 cfm/ton

36. DOAS w/ EW +DX 350 cfm/ton CC 400 cfm/ton CC

37. Performance for office, based upon 62.1-2004 ventilation req’d Humidity Control (Occ. Hours >65% RH) Annual Op Cost vs Base DX LCC: Equipment 1st + 15 yr Gas and Electric $, 1,000’s 2004 dollars

38. Performance for retail, based upon 62.1-2004 ventilation req’d Humidity Control (Occ. Hours >65% RH) Annual Op Cost vs Base DX (%) LCC: Equipment 1st + 15 yr Gas and Electric $, 1,000’s 2004 dollars

39. NIST findings July, 2005 NIST Finding: The more complex DOAS system modeling still showed latent cooling being provided by the WSHPs in the zones.Reason: Condensation will occur if the cooling surface temperatures (WSHP’s surface temp = 45F) are below the space DPT. This is a design/modeling problem, not a DOAS problem!

40. Mumma Preferred equipment choices • Always consider dual path DOAS to the spaces, and use where it makes sense. • I have yet to find a DOAS application where EW’s should not be used, when controlled properly. • In most situations, use mechanical refrigeration to dehumidify, even if it means increasing the ventilation rate above the Std. 62.1 minimums. Choice is supported by the ASHRAE research project results discussed above. • To achieve the low temperature chilled water economically, use OPAC where cost effective. • Can these preferences be seen in practice? One potential case follows!

41. How is ASHRAE HDQ renovation “walking the talk”?

42. How is ASHRAE HDQ renovation “walking the talk”? • Single wheel Dedicated Outside Air System (DOAS) supplying cold and dry (50 gr/lb) ventilation air throughout with >75% effective EW. • Off Peak AC used with favorable utility rates. Reduces demand changes and facilitates low CHWS temperature operation. • Chilled ceiling used throughout the 2nd level. • Dual Path, low temperature VAV serving the first floor. • Learning Center, DOAS with PTAC.

43. Questions