Dedicated Outdoor Air Systems (DOAS)

Dedicated Outdoor Air Systems (DOAS) Fresno ASHRAE Chapter Sept. 14, 2011 Stanley A A. Mumma Mumma, Ph Ph.D., D P P.E. E Prof. Emeritus, Architectural Engineering Penn State University, Univ. Park, PA sam11@psu.edu Web: http://doas-radiant.psu.edu 1 Key Presentation Points Problems with common VAV systems. DOAS defined. Parallel sensible terminal equipment choices. DOE Report: DOAS ranks first. System Selection Matrix. Issues Conclusions. 2

Current HVAC system of choice: VAV OA Std. VAV AHU VAV Space 1, VAV w/ single air delivery path 3 Inherent Problems with VAV Systems Poor air distribution y control Poor humidity Poor acoustical properties Poor use of plenum and mechanical shaft space Serious control problems, particularly with tracking return fan systems Poor energy transport medium: air Poor resistance to the threat of biological and chemical terrorism Poor and unpredictable ventilation performance 4

. vent’n performance. Poor & unpredictable OAB 3,600 cfm OA ? AHU 6,000 cfm % OAB ?60 1,500 cfm OA 2,250? (900 1,350) No! Eq. for OA? No! Why not? OAreq’d 900 cfm OA 3,600? OA (6,000-OA)*0.225 3,600 based on table 6-1 OA 2,903, 30% more, but no Z 900/1,500 1 LEED point i t Z1 0.6 2,903-(900 1,350) 653 more than table 6-1 value Where does the 653 cfm go? 4,500 cfm OAreq’d 1,350 cfm Over vent ? , cfm,, Unvit 1,350 Z2 0.3 Unvit ratio 0.225 1,350/6,000 5 Can VAV limitations be overcome? OA 2,250 AHU 2,250 cfm How is the space load handled, when 6,000 cfm required for a VAV? % OAB 100 Condition of supply air, DBT & DPT? 900 cfm 1,350 cfm OAreq’d 900 cfm OAreq’d 1,350 cfm Z1 1 Z2 1 6

DOAS Defined for This Presentation 20%-70% less OA, than VAV DOAS Unit w/ Energy Recovery Cool/Dry Supply Parallel P ll l Sensible Cooling System High Induction Diffuser Building with Sensible and Latent Cooling Decoupled Pressurization 7 Key DOAS Points 1. 100% OA delivered to each zone via its own ductwork 2. Flow l rate generally ll as spec. b by Std. d 62.1 or greater (LEED, Latent. Ctl) 3. Employ TER, per Std. 90.1 4. Generally CV 5 Use 5. U to d decouple l space S/L loads—Dry l d D 6. Rarely supply at a neutral temperature 7. Use HID, particularly where parallel system does not use air 8

Total Energy Recovery (TER) Wheel 9 High Induction Diffuser Provides complete p air mixing g Evens temperature gradients in the space Eliminates short-circuiting between supply & return Increases ventilation effectiveness 10

Parallel Terminal Systems DOAS air Induction Nozzle S C Sen Cooling li C Coilil Radiant Cooling Panels Room air Chilled Beams Fan Coil Units Air Handling Units CV or VAV VRV Multi-Splits Unitary ACs i.e., WSHPs 11 12

DOAS with Parallel VAV Std. VAV AHU OA Economizer OA Outdoor air unit with TER VAV Space 2, DOAS in parallel w/ VAV 13 VAV Problems Solved with DOAS/Parallel VAV Poor air distribution Poor humidity control Poor acoustical properties Poor use of plenum and mechanical shaft space Serious control problems, particularly with tracking return fan systems Poor P energy ttransportt medium: di air i Poor resistance to the threat of biological and chemical terrorism Poor and unpredictable ventilation performance 14

DOAS with Parallel FCU Other ways to introduce OA at FCU? Implications? OA Outdoor air unit with TER FCU Space 3, DOAS in parallel w/ FCU 15 Parallel vs. Series OA introduced for DOAS-FCU applications? Parallel, Good Series, Bad 16

Advantages of the correct paradigm parallel FCU-DOAS arrangement At low sensible cooling load conditions, the terminal equipment may be shut off—saving fan energy The terminal device fans may be down sized since they are not handling any of the ventilation air, reducing first cost The smaller terminal fans result in fan energy savings The cooling coils in the terminal FCU’s are not derated since they y are handling g only y warm return air,, resulting g in smaller coils and further reducing first cost. Opportunity for plenum condensation is reduced since the ventilation air is not introduced into the plenum near the terminal equipment, for better IAQ 17 VAV Problems Solved with DOAS/Parallel FCU Poor air distribution Poor humidity control Poor acoustical properties Poor use of plenum and mechanical shaft space Serious control problems, particularly with tracking return fan systems Poor P energy transport medium: d air Poor resistance to the threat of biological and chemical terrorism Poor and unpredictable ventilation performance 18

DOAS with Parallel Radiant, or Chilled Beam OA Outdoor air unit with TER Radiant Panel Space 3, DOAS in parallel w/ CRCP 19 VAV Problems Solved with DOAS/Radiant-Chilled Beam Poor air distribution Poor humidity control Poor acoustical properties Poor use of plenum and mechanical shaft space Serious control problems, particularly with tracking return fan systems Poor energy transport medium: air Poor resistance to the threat of biological and chemical terrorism Poor and unpredictable ventilation performance 20

Additional Benefits of DOAS/Radiant-Chilled Beam Beside solving problems that have gone unsolved for nearly 35 years with conventional VAV systems, note the following benefits: Greater than 50% reduction in mechanical system operating cost compared to VAV Equal or lower first cost Simpler controls Generates up to 80% of points needed for basic LEED certification 21 DOAS Equipment on the Market Today I: Equipment that adds sensible energy recovery or hot gas for central reheat II Equipment II: E i t that th t uses total t t l energy recovery III: Equipment that uses total energy recovery and passive dehumidification wheels w ee s IV: Equipment that uses active dehumidification wheels, generally without energy recovery 22

DOAS Equipment on the Market Today 48 50 gr 23 90 EW Space .024 168 CC 80 2 140 .020 020 % 40 W lb Bu et ) (F 70 112 .016 3 84 .012 60 2 5 4 0% 56 .008 008 50 40 28 .004 40 50 60 HUMIDITY RATIO (Lbv/Lba) Hot & humid H h id OA condition 70 80 90 DRY BULB TEMPERATURE (F) 100 Humidity ratio (grains/lb) PH % 4 3 60 2 1 80 % OA 196 .028 5 RA 120 24

DOAS & Energy Recovery ASHRAE Standard 90.1 and ASHRAE’s new Standard for the Design Of High Performance Green Buildings (189.1) both require DOAS systems to utilize TER for almost the entire USA regardless of system size as illustrated in the next slide. They also require that the total effectiveness be at least 50 and 60% respectively. The Stds permit the use of class 1-3 air w/ TER. 25 Climate Zone 60% TER Req’d Std. 189.1-2009 1A, 2A, 3A, 4A, 5A, 6A, 7, 8 (Moist E. US Alaska) 6B 1B, 2B, 5C 3B, 3C, 4B, 4C, 5B Design Air flow when 80% OA 0 cfm (all sizes require TER) 1,500 cfm 4,000 cfm 26 5,000 cfm

48.7 M 18.7 M 85.4 M 141.6 M 27 1,603 hours in hot/humid region. 70% ε EW reduces peak load from 69T to 44T, (25T reduction) and saves 11,680 TH or 1,168/yr g 10,000 , cfm OA,, assuming 1 kW/T, 0.10/kWh 2234 hours in the triangle 4923 hours in dry region 28

Selection l matrix 29 Max points, 272: VAV 53%, DOAS-Rad 90% Sys. Alts IAQ (5) (wtg) 1st Op. DBT Ctl. Plenum (5) (4) (3) depth (5) FCU w/ DOAS 5/25 7/35 1/4 1/3 6/30 8/8 1/4 1/3 AHU Future Maint Ductwork (1) Flex (4) (3) (2) Noise (2) Total Score 6/12 1/2 126 VAV, HW RH 4/20 5/25 3/12 5/15 2/12 4/4 5/20 7/21 2/4 7/14 145 LT VAV, HW RH 4/20 6/30 4/16 6/18 3/30 4/4 6/24 7/21 3/6 7/14 183 FPVAV, HW RH 2/10 4/20 5/20 4/12 4/20 8/8 3/12 3/9 4/8 2/4 123 FPVAV, Chw recool 1/5 3/15 6/24 3/9 5/25 8/8 4/16 2/6 7/14 3/6 128 LT DDVAV 3/15 2/10 2/8 2/6 1/5 4/4 2/8 4/12 1/2 5/10 80 UFAD 6/30 1/5 7/28 8/24 8/40 4/4 8/32 5/15 8/16 4/8 202 CRCP-DOAS 8/40 8/40 8/32 7/21 7/35 8/8 7/28 8/24 5/10 8/16 254 Category Feature rating/score System performance in a category (i.e. 1st cost) rating 1-8 (8 Best): i.e. FCUw/ DOAS meeting 1st cost earns a 7 Importance weighting of a category 1-5 (5 most important) Score: in a cell: product of importance weighting and system performance. i.e. for CRCP-DOAS in the category of Op , the score is 4*8 32 Conventional VAV 145 pts: DOAS-Rad 254 pts 30

DOAS Issues Reserve capacity EW issues, including control SA Conditions C diti 30% surplus OA for a LEED point Lost air side economizer Filtration/Terror resistance P Pressurization/floor i i /fl component 62.1/unbalanced flow @ EW Toilet Exh/recirc. Air Direct/indirect evap. cool 31 32

Conclusion DOAS offers the following benefits: – Assured ventilation performance. – Excellent IEQ IEQ. – Low energy use compared to all air systems. – Much simpler controls compared to VAV. – Competitive first cost. Congratulations to those of you already designing/building/using DOAS !!!!!!!! 33 34

Poor air distribution Poor humidity control VAV Problems Solved with DOAS/Parallel FCU Poor acoustical properties Poor use of plenum and mechanical shaft space Serious control problems, particularly with tracking return fan systems Pd 18 Poor energy transport medium: air Poor resistance to the threat of biological and chemical terrorism