Transcript ASHRAE Atlanta paper 3 - DOAS

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
A/E
Environment
Building Thermal and Mechanical Systems Laboratory
Presentation Outline
Objective
 Present 3 hypotheses, regarding SAT, SADPT, 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
PW, 1st
& Op $
PW, Op $
LCC
1st $
44F
70F
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 (thereDOAS
fore size).
Parallel
Building Load
– Fan Coil first cost, $6/cfm
– Ceiling Radiant Panel cost, $8/sq. ft.
– Sensible Wheel in DOAS, $2/scfm OA
Load Mix with 10,000 scfm OA
in Atlanta.
SAT
F
70
55
44
DOAS
Parallel Peak Load
CC Load Sys. Load
Ton
Ton
Ton
27
23.4
50.4
39
9
48
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
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
Energy Mix with 10,000 scfm
OA in Atlanta, 3744 hours
SAT
F
70
55
44
DOAS
CC
TH
48,150
83,100
110,700
DOAS
Sen Cool
TH
27,000
77,500
114,600
Parallel Combined
Sys.
TH
TH
87,600
135,750
37,100
120,200
0
110,700
Parallel system 1st cost reduction
with SAT
SAT
FCU
CRCP Sen. Wheel Chiller
F
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

30
Assumptions, continued:
– Fan Coil and CRCP performance as below
Key:
CRCP, Btu/hr per ft2
FCU, Btu/hr per cfm
HT
10
55F
CHWT
65
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, $0.09/kWh
System Wide Impact of DPT on
1st and energy Costs
SA
Chiller Chiller
DB/ DP 1st k$ Op. k$
55/ 55
93
13.3
55/ 44
103
15.8
44/ 44
101
15
FCU
1st k$
354
204.6
168
FCU
Op. k$
6.3
3.6
3
CRCP CRCP
1st k$ Op. k$
518
.6
173
.6
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 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
44-55F
OA, 15-20 scfm
Qenv
IG/ft2
Floor area
/person
Balance Point IG/ft2=QDOAS/ ft2 + Qenv/ ft2 -Qsen/ ft2
Overcooling with the DOAS
Graphic 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
0
Winter
0
Occupancy/1000 ft2
90
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.
