Active beams versus VAV with Reheat
Analysis of May 2013 ASHRAE Journal article
Ken Loudermilk
Vice President, Technology & Developement
Active Chilled Beams
Pre-treated
Primary Air
0.4 to 0.7 in. SP
1 Part
Air handling unit
Entrained
Room Air
2 to 4 Parts
Supply Air to
Room
3 to 5 Parts
Comparison of water to air as a heat transfer medium
14“ x 14“
Air Duct
1“ Dia. Water
Pipe
Cost to transport cooling with water 15 to 20% that of air
Typical active beam cooling operation
Pre-treated primary air
Primary airflow requirement is the greater of:
• Volume flow rate needed to maintain mandated ventilation to space
• Volume flow rate needed to offset space sensible heat gains
• Sensible cooling contribution
• Drive induction of room air through coil
• Volume flow rate needed to maintain space dew point temperature
Typical active beam cooling operation
ACB with 55˚F primary air
33% of space sensible heat
removed by primary air
67% of space sensible heat
removal by water
Active beams with a DOAS vs. VAV with reheat
ASHRAE Journal, May 2013
• Office/classroom building at UC Davis
• 56,500 ft2 building
• Sensible loads average 19.5 Btu/h-ft2
• Occupancy is one person per 275 ft2
• Compares VAV + reheat to ACB system
with DOAS
• Analyzes and compares
•
System first cost
•
System energy use
•
Other benefits of VAV + R
Active beams with a DOAS vs. VAV with reheat
ASHRAE Journal, May 2013
• Sensible design (outdoor air)
•
100˚F DB/70˚F WB (54˚F dew point)
•
Humidity ratio W = 62.2 grains/lbm-DA
•
Enthalpy h = 33.8 Btu/h-lbm
• Off peak operation (outside air)
•
50% indoor sensible load
•
77˚F DB/59˚F WB (46˚F dew point)
•
Humidity ratio W = 46.3 grains/lbm-DA
•
Enthalpy h = 25.8 Btu/h-lbm
Active beams with a DOAS vs. VAV with reheat
ASHRAE Journal, May 2013
ACB system
•
100% OA (DOAS) air handling unit
VAV + reheat system
•
Mixing AHU with VFD
• No energy recovery!
• Equipped with airside economizer
• Primary air 63˚F, 54˚ DP (W = 2.7 grains)
• Primary air 55˚F, 52˚ DP (ΔW = 8 grains)
• 0.15 CFM/ft2 for ventilation
• 0.15 CFM/ft2 for ventilation
• 0.53 CFM/ft2 for latent cooling!!
• 0.18 CFM/ft2 for latent cooling
• Primary airflow
•
Primary airflow
• 30,000 CFM (0.53 CFM/ft2)
•
• Constant air volume, no set back
• Normal VAV turndown ratio of 6:1
• No DCV provisions
• Interior terminals DCV (allows shut off)
50,000 CFM (0.88 CFM/ft2)
Active beams with a DOAS vs. VAV with reheat
ASHRAE Journal, May 2013
Performance
comparison of
systems as
described
Primary air conditions and flow rates as described by authors
Design airflow rates in CFMPA per square foot
VAV System
ACB System
55˚ DB/52˚ DP
ΔW = 7.9 grains
63˚ DB/54˚ DP
ΔW = 2.7 grains
Interior Space
Perimeter Space
Interior Space
Perimeter Space
Ventilation
0.15
0.15
0.15
0.15
Dehumidification
0.18
0.18
0.53
0.53
Sensible Cooling
0.60
1.75
0.22
0.53
Resultant Airflow
0.60
1.75
0.53
0.53
Avg. 0.88 CFM/ft2
Avg. 0.53 CFM/ft2
Air handling unit configurations as described
OA Heating
Coil
Filters
Cooling
Coil
Fan
Array
Filters
(0.15 to 0.88
8,300 CFM
CFM/Ft2)
63⁰F
30,000 CFM
30,000 CFM
Cooling Fan
Array
Coil
55⁰F
8,475 to
50,000 CFM
(0.53 CFM/Ft2)
Bypass
Damper
8,300 CFM
30,000 CFM
VFD
30,000 CFM
Note: 100% OA, no energy recovery!
Relief
Fan
0 to 41,700
CFM
recirculation
8,475 to
50,000 CFM
Note: OA requirement only 0.15
CFM/Ft2, 16% of design airflow rate!
Authors‘ performance conclusions
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
VAVR system as described
55
8,475
41,525
17%
49.9
0.0
45.6
5.5
88.0
ACB system as described
63
30,000
0
100% 40.5
23.5
26.0
8.7
89.9
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
VAVR system as described
55
8,475
16,525
34%
13.0
0.0
6.6
2.0
19.4
ACB system as described
63
30,000
0
100% 15.6
5.1
26.0
1.4
41.2
Sensible design conditions
(100% sensible & latent space loads)
Shoulder season operation
(50% sensible, 80% latent space load)
Energy use
comparable
ACB system
more than
double
Authors‘ performance conclusions
2
kBtu/ft -year
30
25
0.1
2.3
20
5.1
0.0
1.0
1.5
15
10
Pumps
Fans
Heating
District cooling
17.9
12.8
5
0
ACB Design as
Described
VAV Reheat
Authors‘ performance conclusions
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
VAVR system as described
55
8,475
41,525
17%
49.9
0.0
45.6
5.5
88.0
ACB system as described
63
30,000
0
100% 40.5
23.5
26.0
8.7
89.9
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
VAVR system as described
55
8,475
16,525
34%
13.0
0.0
6.6
2.0
19.4
ACB system as described
63
30,000
0
100% 15.6
5.1
26.0
1.4
41.2
Sensible design conditions
(100% sensible & latent space loads)
Shoulder season operation
(50% sensible, 80% latent space load)
Energy use
comparable
ACB system
more than
double
Actual performance comparisons
100
88.0
89.9
80
79.2
70
60
52.0
50
41.2
40
30
19.4
20
10
Sensible Design Performance
Latent Design Performance
as designed
ACB system
VAVR system
as designed
ACB system
VAVR system
as designed
ACB system
0
VAVR system
Operational Energy use, kW
90
Shoulder Season Performance
What’s wrong with this picture?
Waterside sensible
cooling
Primary Air Sensible Cooling
System Design
TPA (˚F) Btu/h-ft2
Btu/hCFM
CFM/ft
% Space
Sensible
Cooling
Btu/hCFM
% Space
Sensible
Cooling
2
Latent Cooling Requirement
(all airside)
Btu/h- ΔW
grains
ft2
Ventilation
Btu/hCFM
CFM/ft2
CFM/ft2
VAVR system as
described
55.0
19.5
22.0
0.89
100%
NA
NA
2.2
11.0
7.5
0.30
0.15
ACB system described
63.0
19.5
13.2
0.53
36%
23.6
64%
2.2
6.0
4.1
0.53
0.15
•
•
•
ACB system primary airflow rate as designed is driven by space latent load
combined with low ΔW
Primary airflow rate is 75% greater than that typically required by properly
designed ACB systems
Beam water side cooling capacities (23.6 Btu-h-CFM) as designed are far
lower than those (40 to 50 Btu/h-CFM) in properly designed ACB systems
Active beams with a DOAS vs. VAV with reheat
ASHRAE Journal, May 2013
Performance
comparison with
properly designed
ACB system
Air handling unit modifications for ACB system
OA Heating
Coil
Filters
Cooling
Coil
Fan
Array
Filters
55⁰F
Cooling Fan
Array
Coil
8,475 CFM
(0.3 CFM/Ft2)
16.667 CFM
30,000 CFM
(0.3
55⁰F
16.667 CFM
CFM/Ft2)
Bypass
Damper
8,300 CFM
16,667 CFM
VFD
16,667 CFM
Reduce SAT to 55˚F
Lower PA dew point allows primary airflow
reduction of 45% and an associated fan
energy reduction of 70%!
Relief
Fan
8,192 CFM
recirculation
Introduce mixing at AHU
Mixing results in an additional 30%
reduction in chiller energy!
16,667 CFM
Leveling the playing field
Waterside sensible
cooling
Primary Air Sensible Cooling
System Design
2
TPA (˚F) Btu/h-ft
Btu/hCFM
CFM/ft
% Space
Sensible
Cooling
Btu/hCFM
% Space
Sensible
Cooling
2
Latent Cooling Requirement
(all airside)
Btu/h- ΔW
grains
ft2
Ventilation
Btu/hCFM
CFM/ft2
CFM/ft2
VAVR system as
described
55.0
19.5
22.0
0.89
100%
NA
NA
2.2
11.0
7.5
0.30
0.15
ACB system described
63.0
19.5
13.2
0.53
36%
23.6
64%
2.2
6.0
4.1
0.53
0.15
Properly designed
ACB system
55.0
19.5
22.0
0.30
33%
44.0
67%
2.2
11.0
7.5
0.30
0.15
•
Same primary air conditions used for both systems
• ACB system primary airflow rate reduced from 30,000 CFM to 16,950 CFM!
• Beam water side cooling capacity (44 Btu-h-CFM) increased by 86%
Primary air conditions and flow rates for modified ACB system
Design airflow rates in CFMPA per square foot
VAV System
ACB System
55˚ DB/52˚ DP
ΔW = 7.9 grains
55˚ DB/52˚ DP
ΔW = 2.7 grains
Interior Space
Perimeter Space
Interior Space
Perimeter Space
Ventilation
0.15
0.15
0.15
0.15
Dehumidification
0.18
0.18
0.18
0.18
Sensible Cooling
0.60
1.75
0.20
0.60
Resultant Airflow
0.60
1.75
0.20
0.60
Avg. 0.88 CFM/ft2
Avg. 0.30 CFM/ft2
Actual performance calculations
Sensible design conditions
(100% sensible & latent space loads)
VAVR system as described
ACB system as described
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
55
63
8,475
30,000
0.0
23.5
45.6
26.0
5.5
8.7
88.0
89.9
100% 28.7
24.4
4.5
7.6
62.1
41,525 17% 49.9
0
100% 40.5
Modified ACB system
30% less than VAVR
54.5 38% less than VAVR
Primary air @ 55 ˚ F DB , 53 ˚F DP 55
16,667
0
Mixing at air handling unit
55
8,475
8,192
51%
21.6
24.4
4.5
6.8
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
0.0
14.3
26.0
26.0
2.9
7.0
48.2
79.2
100% 28.5
15.3
4.5
5.9
51.5 6% more than VAVR
31.8 34% less than VAVR
Latent design conditions
(75% sensible, 90% latent space load)
VAVR system as described
ACB system as described
55
63
8,475 29,025 23% 26.6
30,000
0
100% 40.2
Modified ACB system
Primary air @ 55 ˚ F DB , 53 ˚F DP 55
16,667
0
Mixing at air handling unit
55
8,475
8,192
51%
10.3
15.3
4.5
3.9
SAT
OA
RA
OA
AHU
cooling
Beam
cooling
Fan
Pumps
Total
energy
˚F
CFM
CFM
%
kW
kW
BHP
BHP
kW
55
63
8,475
30,000
0.0
5.1
6.6
26.0
2.0
1.4
19.4
41.2
Shoulder season operation
(50% sensible, 80% latent space load)
VAVR system as described
ACB system as described
16,525 34% 13.0
0
100% 15.6
Modified ACB system
Same as VAVR
Primary air @ 55 ˚ F DB , 53 ˚F DP
55
16,667
0
100%
8.4
6.1
4.5
2.0
19.3
Mixing at air handling unit
55
16,667
0
100%
8.4
6.1
4.5
2.0
19.3 Was more than double
* Air handling unit operating in economizer mode
Sensible Design Performance
Latent Design Performance
AHU mixing
22.0
ACB system,
55˚F PA
19.4
ACB system,
as designed
40
ACB system
20
VAVR system
AHU mixing
ACB system,
55˚F PA
52.0
ACB system,
as designed
80
ACB system
50
VAVR system
AHU mixing
ACB system,
55˚F PA
60
ACB system,
as designed
88.0
ACB system
90
VAVR system
Operational Energy use, kW
Actual performance comparisons using modified ACB system
100
89.9
79.2
70
62.1
54.5
51.5
41.2
30
31.8
22.0
10
0
Shoulder Season Performance
Active beams with a DOAS vs. VAV with reheat
ASHRAE Journal, May 2013
System cost
comparisons
Authors’ installed cost comparison of the systems as designed
Material cost ($)
Labor cost ($)
Equipment cost ($)
Subcontractors ($)
Lbs. of ductwork (lbs.)
Chilled water piping (LF)
Hot water piping (LF)
VAV system as designed
ACB system as designed
Cost or Qty.
Cost/CFM
Cost or Qty.
Cost/CFM
$215,179
$584,058
$319,695
$252,067
38,000
310
2,085
$4.30
$11.68
$6.39
$5.04
$576,496
$1,509,349
$608,349
$647,037
28,612
10,244
9,630
$19.22
$50.31
$20.28
$21.57
Total HVAC cost ($) $1,370,999
HVAC cost ($/ft2)
$25
$3,341,231
$27
$62
$111
System ductwork configuration
2,000 FPM @
0.9 CFM/Ft2
900 FPM
0.53 CFM/Ft2
ACB System Mains
VAV System Mains
AEFF = 50,000/2,000 = 25.0
One supply riser
AEFF = 30,000/900 = 33.3
Two supply risers
Authors’ conclusion: Average duct cross sectional area 33% greater for the
ACB system
Ductwork configuration
2,000 FPM
2,000 FPM
ACB System Mains
VAV System Mains
AEFF = 16,667/2,000 = 8.3
AEFF = 50,000/2,000 = 25
VAVR system average duct cross sectional area is triple that of the ACB
system when sized for the same maximum velocity
Beam requirements
VAV system as described
Interior
zones
2
Perimeter
All zones
zones
ACB system as described
Interior
zones
Perimeter
All zones
zones
Modified ACB system
Interior
zones
Perimeter
All zones
zones
38,922
17,578
56,500
38,922
17,578
56,500
38,922
17,578
56,500
BTU/H-ft
13.4
33.0
19.5
13.4
33.0
19.5
13.4
33.0
19.5
CFMPA
23,634
26,367
50,000
13,123
16,877
30,000
7,878
8,789
16,667
CFMPA/ft2
0.61
1.50
0.88
0.34
0.96
0.53
0.20
0.50
0.29
BTU/H-CFMPA
22.0
22.0
22.0
39.6
34.4
38.0
66.0
66.0
66.0
ACB Btu/h-LF
ACB CFM PA /LF
305
550
398
1,056
1,056
1,056
7.7
16.0
10.9
16.0
16.0
16.0
Linear feet of ACB required
1,706
1,055
2,761
492
549
1,042
Area served, ft
2
• Number of beams reduced by 63% by modifying ACB system
• 63% reduction in beam piping and insulation costs
Remedies for other cost inequities
• Perimeter VAV terminals serve
multiple offices
• No indication of thermal zoning
or condensation prevention in
ACB system
• Interior VAV terminals have no
reheat provisions
• Four pipe configuration of interior
space beams
• Remedy: Eliminate heating provision on
interior beams
• Eliminates HW piping/insulation and
connection to interior beams
• Perimeter VAV terminals require
only one HW connection per
zone
• Perimeter beams require
individual HW connections
• Accomplish perimeter heating by heating
primary air
• Reduces HW piping/insulation and
connections to one per perimeter zone
Cost comparison using modified ACB system
Material cost ($)
Labor cost ($)
Equipment cost ($)
Subcontractors ($)
Lbs. of ductwork (lbs.)
Chilled water piping (LF)
Hot water piping (LF)
VAV system as designed
ACB system as designed
Modified
ACB system
Cost or Qty.
Cost/CFM
Cost or Qty.
Cost/CFM
Cost or Qty.
$215,179
$584,058
$319,695
$252,067
38,000
310
2,085
$4.30
$11.68
$6.39
$5.04
$576,496
$1,509,349
$608,349
$647,037
28,612
10,244
9,630
$19.22
$50.31
$20.28
$21.57
$320,282
$838,544
$337,978
$359,472
15,896
5,691
5,350
Total HVAC cost ($) $1,370,999
HVAC cost ($/ft2)
$25
$3,341,231
$27
$62
$1,856,277
$111
$33
Modified ACB system cost assumes the cost per CFM for the ACB system remains constant and thus system costs are
proportional to the reduced primary airflow requirements. These costs also do not include any possible reheat piping
reduction opportunities discussed before.
Installed cost comparison (ACB system with mixing AHU, no ER)
Summary Table
SYSTEM TYPE
Central Equipment Costs
Specify AHU configuration
All air VAV
Active Beams
All-air system
Total $
$/ft2
Total $
$/ft2
Mixing type without energy recovery
$554,608
$9.83
$467,782
$8.29
Beam system (pulldown menu)
Mixing type without energy recovery
Air handling units and exhaust fans
$196,642
$137,201
Chillers
$223,253
$169,152
Boilers
$37,384
$66,204
Specify perimeter heat method
Pumps
$97,329
$95,225
All air system (pulldown menu)
Air and water distribution costs
$851,323
$15.09
$1,060,439
$18.80
Single duct VAV with reheat
Beam system (pulldown menu)
Ductwork and insulation
$389,350
$237,025
Piping and insulation
$91,944
$460,106
Air terminal units
$150,109
$0
Supply/return grilles and diffusers
$92,610
$19,824
Condensation prevention (pulldown menu)
Active beams
$0
$263,000
Reset each floor chilled water supply temperature
Zone controls
$127,310
$80,485
Other costs
$197,400
$3.50
$206,400
Fire and life safety
$84,600
$84,600
Commissioning
$112,800
$112,800
$1,603,332
$28.43
2 pipe beams, primary air heating coil
$3.66
$9,000
Condensation prevention
TOTAL INSTALLED COST OF HVAC SYSTEM
$8.16
$1,734,621 $30.76
Gray cells require user input
Article in winter 2013 High Performance Buildings
•
UC Davis Health and Wellness
Center
• 73,112 ft2 of conditioned space
• LEED-NC Gold
• Occupied since March 2010
• DOAS air handling strategy
• Energy performance
• Projected 35% better than LEED baseline
• Actual 31% better than projected
• 70% less primary air than VAV system
And the hands down winner is………
Chilled beams!
Questions?
Active beams versus VAV with Reheat
Analysis of May 2013 ASHRAE Journal article