1 Stanley A. Mumma, Ph.D., P.E. Prof. Emeritus, Architectural Engineering Penn State University, Univ. Park, PA Web: Contaminant Transport and Filtration Issues with DOAS ASHRAE Louisville Meeting June 23, 2009

2 Objective  Dispel the common perception that DOAS is inferior to all air system when contaminant flushing rates from occupied spaces are considered.  Shift the DOAS filter selection paradigm.

3 DOAS Defined OA DOAS Unit w/ TER, CC, & Filtration Cool/Dry Supply Parallel Sensible Cooling System High Induction Diffuser Building with Sensible and Latent Cooling Decoupled RA

4 Facility floor area and zoning for case study 10,000 ft 2 Interior Zone 3 1,000 ft 2 Exterior Zone 1 9,000 ft 2 Exterior Zone 2 10 ft high ceilings--all zones

5  VAV system: –Supply air (SA) flow rate, 16,000 cfm, of which 4,000 cfm is OA. – Perimeter zones 1 and 2, each receives 1 cfm/ft 2 of supply air via a shut off box VAV system. Since the SA is 25% OA, then each perimeter zone is receiving 0.25 cfm/ft 2. – Interior zone 3 receives 0.6 cfm/ft 2 of supply air via a VAV system, i.e cfm/ft 2 OA  DOAS: –OA flow for the facility, 4,000 cfm uniformly distributed in each zone, or 0.20 cfm/ft 2. Case Study Conditions

6 Transient analysis is based upon the following simplifying assumptions:  Well mixed zones.  No interzonal transfer.  Contaminants stay suspended.  VAV system is analyzed while operating in the minimum OA mode (4,000 cfm OA), and a supply airflow rate of 16,000 cfm. Releases during full economizer mode (resulting in very high peak space concentrations when releases occur near the OA inlet) will not be presented.  The capacitance of the duct system, and its associated influence on the transient response, is neglected.

Filter,  f VAV System Schematic

Filter,  f DOAS Schematic

9 Transient dimensionless concent’n from release near OA inlet w/  f = 0% VAV Zone 3 DOAS VAV Zone 1 & 2

10 Transient concent’n from release near OA inlet w/ VAV  f = 80% & DOAS  f = 98% NOTE: Exposure w/ DOAS is 1/425 that of VAV. VAV Zone 3 DOAS VAV Zone 1 & 2

11 DOAS filter  necessary to match the space concent’n/exposure after 60 min. for a given VAV filter . Ref: OA intake occurances. Paradigm shift!! =  = exp = conc.

12 Filter Optimization Assumptions  Same SA flows as in transient analysis: VAV 16,000 cfm; and DOAS 4,000 cfm.  Assume that the filter loading profile is exponential, causing the average pressure drop over the life of the filter to be: [(Dploaded opt - Dpclean opt)/2]*Fill-ratio Fill-ratio used.  Reference replacement frequency is 3 months for VAV and 6 months for DOAS  fan/motor combined efficiency, 60%.  Electrical cost, $0.10/kWh.  Term of analysis, 5 years.  Fan operating hours per year, 4160 hours.  Neglect the time value of money and inflation.

13 Merv 11 Filter 75%  Merv 13 & 14 Filter 94 & 98%  Merv 16 Filter 99.7%  Filters used in the Optimization

14 Optimal Sol. For VAV & DOAS Filters w/ Equal 1hr Conc’n System Filter eff. Optimal Face area, ft 2 Months between filter change Filter 5 yr. 1 st cost, $ Fan 5 yr. Op cost based on filter avg DP, $ Total 5 year Cost, $ VAV $753$3,246$3,999 DOAS $519$870$1,389 VAV $3,959$3,611$7,570 DOAS $717$1,350$2,067 VAV $5,368$5,650$11,018 DOAS $1,052$1,392$2,444

15 DOAS filter Optimization based on Monthly Replacement System Filter eff. Optimal Face area, ft 2 Months between filter change Filter 5 yr. 1 st cost, $ Fan 5 yr. Op cost based on filter avg DP, $ Total 5 year Cost, $ VAV $753$3,246$3,999 DOAS944.5/9.31/6.5$1,635/ $519 $1,401/ $870 $3,036/ 1,389 VAV $3,959$3,611$7,570 DOAS983.5/8.81/6.4$1,823$2,130$3,953 VAV $5,368$5,650$11,018 DOAS /8.91/6.5$2,981$2,137$5,118 $

16 Conclusions 1.A common attitude that contaminant flushing is a problem with DOASs is not warranted when the proper DOAS filter is selected, i.e. one better than the filter used in a comparably performing VAV system. 2.The old “equivalence” paradigm needs to change. 3.Guidance is offered, at least for one set of conditions, on DOAS filter efficiencies. And a method is provided for use on other sets of conditions. 4.Finally, contrary to conventional wisdom, the selection of better and more expensive DOAS filters resulted in optimized performance costing less— far less —than VAV systems.

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18 VAV Governing Equations

19 DOAS Governing Equations