doas P3

Slide1: 

PSU Building Thermal and Mechanical Systems Laboratory 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 Selecting the Supply Air Conditions for a Dedicated OA System Working in Parallel with Distributed Sen. Cooling Equip.

Presentation Outline: 

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

Objective: 

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.

Hypothesis 1: Load, Energy, & Cost will decrease with DBT: 

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

System Wide Impact of DBT on Load, Energy, and Cost: 

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)

System Wide Impact of DBT on Load, Energy, and Cost: 

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 Building Load DOAS Parallel

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

Load Mix with 10,000 scfm OA in Atlanta.

Reason Peak Load Increased with Increasing SAT: 

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.

Reason Peak Load Increased with Increasing SAT, illustrated: 

Reason Peak Load Increased with Increasing SAT, illustrated 6 5 6’ 1 2 2’ 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.

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

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

Parallel system 1st cost reduction with SAT: 

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

Hypothesis 2, Load, Energy, & Cost will decrease with DPT: 

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)

System Wide Impact of DPT on Load, Energy, and Cost: 

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

System Wide Impact of DPT on Load, Energy, and Cost: 

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

System Wide Impact of DPT on Load, Energy, and Cost: 

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

System Wide Impact of DPT on 1st and energy Costs: 

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

Hypothesis 3, Terminal Reheatwill be needed sparingly if at all: 

Hypothesis 3, Terminal Reheat will 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.

Hypothesis 3: Terminal Reheat: 

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)

Overcooling potentialwith the DOAS: 

Overcooling potential with 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

Overcooling with the DOAS, the energy balance/person: 

Overcooling with the DOAS, the energy balance/person Balance Point IG/ft2=QDOAS/ ft2 + Qenv/ ft2 -Qsen/ ft2

Overcooling with the DOASGraphic from the energy bal.: 

Overcooling with the DOAS Graphic from the energy bal. IG, W/ft2 0 15 Occupancy/1000 ft2 0 90 Winter Summer 20 4 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.

Conclusion:: 

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.

Recommendation: 

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.