PSU

Building Thermal and Mechanical Systems Laboratory

Environment

A/E

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 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

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

LCC

44F 70F

PW, 1st & Op $

PW, Op $

1st $

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 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 LoadDOAS Parallel

Load Mix with 10,000 scfm OA in Atlanta.

SAT

F

DOASCC Load

Ton

ParallelSys. Load

Ton

Peak LoadTon

70 27 23.4 50.455 39 9 4844 47 0 47

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

6 56’

1

22’

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 hoursSAT

F

DOASCCTH

DOASSen Cool

TH

ParallelSys.TH

Combined

TH70 48,150 27,000 87,600 135,75055 83,100 77,500 37,100 120,20044 110,700 114,600 0 110,700

Parallel system 1st cost reduction with SAT

SAT F

FCU CRCP Sen. Wheel Chiller

70 (ref) $0 $0 $0 $0 55 -$51,200 -$43,200 $0 $0 44 -$85,300 -$72,000 -$20,000 $0

Hypothesis 1 confirmed, low SAT best

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 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 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 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

CostsSA

DB/ DPChiller1st k$

ChillerOp. k$

FCU1st k$

FCUOp. k$

CRCP1st k$

CRCPOp. k$

55/ 55 93 13.3 354 6.3 518 .655/ 44 103 15.8 204.6 3.6 173 .644/ 44 101 15 168 3 141 .5

Hypothesis 2 confirmed, low SA-DPT best

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 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

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

Qenv

OA, 15-20 scfm

44-55F

IG/ft2

Floor area /person

Balance Point IG/ft2=QDOAS/ ft2 + Qenv/ ft2 -Qsen/ ft2

Overcooling with the DOASGraphic 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:

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 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.