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Dedicated Outdoor Air Systems (DOAS) and Building Pressurization

Dedicated Outdoor Air Systems (DOAS) and Building Pressurization. ASHRAE Annual Conference Albuquerque Session 6 Monday June 28, 2010. Stanley A. Mumma, Ph.D., P.E. Prof. Emeritus, Architectural Engineering Penn State University, Univ. Park, PA sam11@psu.edu.

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Dedicated Outdoor Air Systems (DOAS) and Building Pressurization

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  1. Dedicated Outdoor Air Systems (DOAS) and Building Pressurization ASHRAE Annual Conference AlbuquerqueSession 6Monday June 28, 2010 Stanley A. Mumma, Ph.D., P.E. Prof. Emeritus, Architectural Engineering Penn State University, Univ. Park, PA sam11@psu.edu Web: http://doas-radiant.psu.edu

  2. Presentation Outline • Importance of building pressurization. • Impact of TER on building pressurization. • Estimating building pressurization needs. • Ratio of pressurization air flow to total OA for various occupancy categories. • Impact of unbalanced flow on TER performance in DOAS applications. • New DOAS configuration? • Research needs. • Conclusions.

  3. Is building pressurization important? • Yes or no? • Do you employ it? • Why pressurize? • Limit moisture migration through the envelope, summer and winter? • Limit hot or cold locations around the perimeter—thermal comfort? • How do you determine the required flow?

  4. Source of pressurization air? • Is the OA specified by Std. 62.1 sufficient? • Is toilet exhaust part of the pressurization air requirement? • Does the use to total energy recovery have any bearing on the issue of pressurization?

  5. An example of a pressurized building without TER

  6. Example of the same building with DOAS and TER • Notice how the toilet exhaust is handled? • Why, and how is it different than an all air system—even with heat recovery. • The TER flow is unbalanced!! Does it matter?

  7. 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 Building Pressurization

  8. How much air flow is required for building pressurization? • Well, it depends--right? On What? • Building tightness. • Building use. • Construction quality. • Wind velocity and direction. • Stack effect. • Method of automatic control—if any. • So how do you know what to design for?

  9. How much infiltration would you expect if no pressurization—excluding toilet exhaust?Consider normal practice construction! • It depends  • None • ½ ACH • 1 ACH • > 3 ACH • What is the basis for your opinion? • Is ACH a floor area or wall area concept?

  10. Published air tightness recommendations. ref: given in paper Note: Leakage is per unit area of exterior perimeter wall

  11. Do we attempt to control a building pressure at 50 Pa? • 249.1 Pa = 1” H2O. • So 50 Pa = 0.2” H2O—clearly much higher than we attempt to pressurize buildings. • Most buildings using pressurization differential control have 0.03” H2O, or 7.5 Pa. as the set point • So the answer is no! • Can we modify the published air leakage rates to reflect 7.5 Pa rather than 50?

  12. Converting published 50 Paleakage rates to 7.5 Pa. Applying the parabolic relationship: Flow, cfm/ft2 wall area

  13. Converting leakage rates at 7.5 Pa. from wall area to floor area. • Assume 15,000 sq ft building with 10 ft high walls. • Assume the length to width ratio is 2:1. • Resulting wall area is 5,195 ft2 • Leakage for office = 0.1057*5,195=549 cfm • Converting to a floor area bases:549/15,000=0.037 cfm/ft2 floor area (office) • Similarly for school:Leakage is: 0.066 cfm/ft2 floor area.

  14. Is there another place that OA is specified on a cfm/ft2 floor area basis? • Is 0.037 and 0.066 cfm/ft2 floor area about the same order of magnitude as other floor area based sources? • If expressed as an ACH, what would we get? • Is it coincidence that the pressurization flow is of the same order of magnitude as the floor component of ASHRAE Std. 62.1?

  15. TER unbalanced flow for different occupancy categories when the pressurization flow is ~1/2 ACH • Does this surprise you? • Have you observed, as I have, TER jobs returning only 30-40 % the flow expected? • How bad is 30% to 70% of OA for pressurization? Note: floor frac = Pressurization fraction

  16. hOA hSA mOA hRA hEA mRA Impact of unbalanced flow on TER performance For unbalanced flow, mOA=mRA + mPressurization Outdoor air 0 scfm Purgeor seal leakage Exhaust air Supply air Wheel Rotation Return air, including toilet exhaust

  17. Brief heat exchanger tutorial • h, efficiency ? • Circuit with min. m*Cp • Tco if HTX infinitely long

  18. Temperature profile for infinitely long HTX

  19. Heat Exchanger effectiveness, e

  20. hOA hSA mOA hRA hEA mRA For unbalanced flow, mOA=mRA + mPressurization Outdoor air 0 scfm Purgeor seal leakage Exhaust air Supply air Wheel Rotation Return air, including toilet exhaust e = mOA(hOA-hSA)/mRA(hOA-hRA) = (hEA-hRA)/(hOA-hRA) eapparent = e*mRA/mOA = (hOA-hSA)/ (hOA-hRA)

  21. Recovered MBH based upon an 85F 140 gr OA condition, an 75F 50% RA condition, and a 130” Dia EW (519 sfpm FV OA stream) Balanced Lecture Conf. rm Educ. Office

  22. What does the need for pressurization and the negative impact of unbalanced flow on the TER performance Suggest? • A new product. • With fully integrated pressurization unit and a balanced flow DOAS. • It might look like:

  23. 130” Dia At summer design conditions 124” Dia 130” Dia 117” Dia 130” Dia 112” Dia 72” Dia

  24. School Design • 20000, 14000 85F 140 gr 130" wheel • 14000, 14000 85F 140 gr 130" wheel • 14000, 14000 85F 140 gr 112" wheel • 20000, 14000 75.45F, 90.25 gr 130" wheel • 14000, 14000 75.45F, 90.25 gr 130" wheel • 14000, 14000 75.45F, 90.25 gr 112" wheel Off-Design

  25. First and Energy cost implications • Cooling energy use increases a little. • Fan energy use decreases due to EW flow resistance removal from the pressurization path. • Net impact for 40o N. Latitude locations is annual energy use is about the same for an unbalanced flow unit and the integrated unit.

  26. First and Energy cost implications • About a $ 6/scfm first cost savings for air removed from the EW and processed through the pressurization unit. In the example of an office with an OA flow of 20,000 scfm, and a pressurization requirement of 14,000, the first cost savings amounts to about $84,000. • Significantly, the low cost pressurization component is a good place to provide reserve capacity. Since about 20% reserve capacity is often provided, a 20,000 scfm OA requirement would provide another $24,000 first cost savings!

  27. Important pressurization unit control point: Maintain a constant flow, rather than a DP! • Assures balanced flow DOAS. • Provides predictable basis for equipment sizing.

  28. Advantages of integrated unit • Enables the first cost of the balanced flow DOAS to be reduced by nearly the fraction of pressurization flow. • Does not degrade the TER performance resulting from unbalanced flow • Allows reduced fan energy use since less combined supply air and purge air flow occurs on both sides of the wheel. Important since fan energy use is significant. • Allows lower operating cost, even though the cooling/dehumidification energy use may increase a little.

  29. Advantages of integrated unit • Eliminates the added installation first cost for two systems (DOAS and Pressurization). • Allows energy use for pressurization to be limited with flow measurement control. • Allows reserve capacity to be added to the pressurization unit, at much lower first cost than in the DOAS where expensive heat recovery is used. • Simplifies controls by dividing the duties.

  30. Disadvantages of integrated unit • May use more cooling energy. • Energy use results are sensitive to equipment selection, i.e. coil DP, fan h, TER selections (DP & effectiveness), air required for pressurization, cooling COP's. • May be falsely perceived as more complex. • Not very beneficial for unbalanced flows of less than 10%.

  31. Research Needs • Confirm the recommended limiting/ bounding flow rate for pressurization, i.e. 0.5 ACH (0.06 scfm/ft2 [0.3 L/s*m2]). • Confirm that the recommendation to employ fixed measured pressurization flow control, with the occasional few hours of moisture migration through the envelope when infiltration may occur, does not lead to IAQ or comfort problems. • Establish a systematic method for determining the appropriate reserve capacity.

  32. Conclusions • Building pressurization important • Adequate pressurization is achieved with ~1/2 ACH, or about 0.06 cfm/ft2 floor area. • Recommend an integrated balanced flow DOAS with a pressurization unit. • Can provide huge first cost savings without an energy penalty. • Now is the time for a new product!!

  33. Questions

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