PSU

1. A/E Environment Selecting the Supply Air Conditions for a Dedicated OA System Working in Parallel with Distributed Sen. Cooling Equip. PSU Kurt M. Shank, M.S. &Stanley A. Mumma, Ph.D., P.E. College of Engineering Department of Arch. Engineering Penn State University @ University Park, PA Building Thermal and Mechanical Systems Laboratory

2. Presentation Outline • Objective • Present 3 hypotheses, regarding SAT, SA-DPT, and Terminal Reheat • Load, Energy, and Cost impact of SAT • Load, Energy, and Cost impact of SA-DPT • Terminal Reheat and SAT • Conclusions and Recommendations

3. Objective • Challenge the current practice of supplying air from dedicated OA systems at a neutral temperature (~70F). • Develop a methodology for selecting the proper supply air conditions.

4. Hypothesis 1: Load, Energy, & Cost will decrease with DBT PW, 1st & Op $ PW, Op $ LCC 1st $ 44F 70F

5. System Wide Impact of DBT on Load, Energy, and Cost • Assumptions • Atlanta, GA data; 12 hr/day, 6 day/wk. • 10,000 scfm of OA • Supply air DPT, 44F • 20 scfm of OA/person • Resulting space DPT, 52F • Space condition, 78F, 40% RH • No terminal reheat required, i.e. space not overcooled with ventilation air (relaxed later)

6. System Wide Impact of DBT on Load, Energy, and Cost • Assumptions, Continued • Constant design sensible load, split between the DOAS and the parallel system; i.e. reduce SAT (greater sensible cooling done) and reduce the load on the parallel system (there-fore size). • Fan Coil first cost, $6/cfm • Ceiling Radiant Panel cost, $8/sq. ft. • Sensible Wheel in DOAS, $2/scfm OA DOAS Parallel Building Load

7. Load Mix with 10,000 scfm OA in Atlanta.

8. Reason Peak Load Increased with Increasing SAT • Because of less than 100% effectiveness at the enthalpy wheel, only about 80% of the sensible cooling done on the return air (state 5-6) by the supply air (state 3-4) is able to be recovered by the enthalpy wheel (state 6-2). Consequently, the more reheat, the greater the cooling required when the parallel system is considered. Illustrated on the next slide.

9. Reason Peak Load Increased with Increasing SAT, illustrated Path from 6-6’ is the increase in reheat, and the path 2-2’ is the reduction in coil load. Since it is shorter than 6-6’, the cooling coil load is not reduced as much as the cooling capability of the supply air when reheated. 1 2 2’ 6 6’ 5

10. Energy Mix with 10,000 scfm OA in Atlanta, 3744 hours

11. Parallel system 1st cost reduction with SAT Hypothesis 1 confirmed, low SAT best

12. Hypothesis 2, Load, Energy, & Cost will decrease with DPT • Assumptions: • Atlanta, GA data, 12 hr/day, 6 day/wk. • 10,000 scfm OA • Building Sensible Load, 75 Tons (representative of a 60,000 sq ft building, served by an all air system with a design supply air flow rate of 0.6 scfm/sq.ft. at 55F)

13. System Wide Impact of DPT on Load, Energy, and Cost • Assumptions, continued: • Allowable space RH range, 40-60% for acceptable IAQ. (Sterling and Sterling) • Chiller capacity drops 10% when the chilled water temp. drops from 45 to 40F. • Chiller kW/ton increases by 10% when the chilled water temp. drops from 45 to 40F. • Chiller kW/ton @ 45F CHWT: 0.79

14. System Wide Impact of DPT on Load, Energy, and Cost • Assumptions, continued: • Fan Coil and CRCP performance as below 30 Key: CRCP, Btu/hr per ft2 FCU, Btu/hr per cfm HT 10 65 55F CHWT

15. System Wide Impact of DPT on Load, Energy, and Cost • Assumptions, continued: • FCU fan efficiency, 74% and 2”TP • FCU & CRCP pumps, 80% eff., water temp rise 5F, and pressure drop 30 ft water. • Chiller installed 1st cost, $1000/ton • Energy costs, $0.09/kWh

16. System Wide Impact of DPT on 1st and energy Costs Hypothesis 2 confirmed, low SA-DPT best

17. Hypothesis 3, Terminal Reheatwill be needed sparingly if at all • Issues: • Terminal Reheat is permitted where it is required to meet Std. 62--Which is why so many all air VAV systems use terminal reheat • VAV box minimums are set to meet the ventilation requirements. The minimum setting will always be at or above that required by the DOAS system since “zc “ in Eq. 6.1 will always be less than or equal to 1.

18. Hypothesis 3: Terminal Reheat • Issues continued: • If “zc “ = 0.4 and a space needs 200 scfm of OA, then the box minimum must be 500 scfm. “zc “ for a dedicated OA system is always 1, so it will deliver the 200 scfm. • A room served by a VAV box with a minimum setting of 500 scfm at 55F is prone to overcool the space sooner than the dedicated OA system supplying 200 scfm of air at either 55 or 44F. (500*[78-55] >200*[78-44]) or (11,500>6,800)

19. Overcooling potentialwith the DOAS • Assumptions: • Envelope UA, 0.09 Btu/hr-F/ft2 of floor area • Summer OA, 90F • Winter OA, 20F • Ventilation, 15 or 20 scfm/person • Occupancy Density, 0-90/1000 ft2 • Internal generation, Lights, equipment; 0-15 W/ft2

20. 44-55F OA, 15-20 scfm Qenv IG/ft2 Floor area /person Overcooling with the DOAS, the energy balance/person Balance Point IG/ft2=QDOAS/ ft2+ Qenv/ ft2 -Qsen/ ft2

21. Overcooling with the DOASGraphic from the energy bal. Example: 20 people per 1000 ft2 ,4 W/ft2, If the IG less than 4 W/ft2 with an occupancy density of 20, the DOAS will overcool; if more, need parallel cooling. 15 20 Summer 4 IG, W/ft2 Winter 0 90 0 Occupancy/1000 ft2

22. Conclusion: • The 3 hypothesis verified • For many building applications, terminal reheat is seldom if ever needed with 55F or even 44F SAT from the DOAS. • Old Paradigm of supplying the air at a neutral temperature, in dedicated OA applications, should be abandoned.

23. Recommendation • The supply air DPT should be low enough to maintain the space RH no higher than 40%, about 44F in many cases. • The supply air dry bulb temperature should be at 55F or below. • Where Occupancy densities are very high, and terminal reheat is frequently required, use recovered heat.