ASHRAE Atlanta paper 3

1

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 _at_ 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 _at_ 45F CHWT 0.79

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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 2TP
• 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

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

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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. (50078-55
  gt20078-44) or (11,500gt6,800)

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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
Overcooling with the DOAS, the energy
balance/person
Balance Point IG/ft2QDOAS/ 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
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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.

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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.