Active beams versus VAV with Reheat Analysis of May 2013 ASHRAE Journal article Ken Loudermilk Vice President, Technology & Developement

Active beams versus VAV with ReheatAnalysis of May 2013 ASHRAE Journal article

Ken Loudermilk

Vice President, Technology & Developement

Pre-treated Primary Air

Entrained Room Air

Supply Air to Room

1 Part

3 to 5 Parts2 to 4 Parts

Air handling unit

0.4 to 0.7 in. SP

Active Chilled Beams

Cost to transport cooling with water 15 to 20% that of air

1“ Dia. Water Pipe

14“ x 14“

Air Duct

Comparison of water to air as a heat transfer medium

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

Pre-treated primary air

Typical active beam cooling operation

Typical active beam cooling operation

67% of space sensible heat removal by water

33% of space sensible heat removed by primary air

ACB with 55˚F primary air

• 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


• 100% OA (DOAS) air handling unit•No energy recovery!

•Primary air 63˚F, 54˚ DP (W = 2.7 grains)

•0.15 CFM/ft2 for ventilation

•0.53 CFM/ft2 for latent cooling!!

•Primary airflow • 30,000 CFM (0.53 CFM/ft2)

• Constant air volume, no set back

• No DCV provisions

• Mixing AHU with VFD•Equipped with airside economizer

•Primary air 55˚F, 52˚ DP (ΔW = 8 grains)

•0.15 CFM/ft2 for ventilation

•0.18 CFM/ft2 for latent cooling

• Primary airflow• 50,000 CFM (0.88 CFM/ft2)

•Normal VAV turndown ratio of 6:1

• Interior terminals DCV (allows shut off)

VAV + reheat systemACB system


Performance comparison of systems as described

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

Design airflow rates in CFMPA per square foot

Primary air conditions and flow rates as described by authors

Resultant Airflow 0.60 1.75

Avg. 0.88 CFM/ft2

0.53 0.53

Avg. 0.53 CFM/ft2

VAV System

Interior Space Perimeter Space

ΔW = 7.9 grains55˚ DB/52˚ DP

ACB System



Air handling unit configurations as described

VFD 30,000 CFM30,000 CFM

8,300 CFMRelief

Fan

8,475 to 50,000 CFM0 to 41,700

CFM recirculation

Bypass Damper

Fan Array

Cooling Coil

Heating Coil

OA Filters

30,000 CFM 30,000 CFM

(0.53 CFM/Ft2)

63⁰F

Fan Array

Cooling CoilFilters

8,300 CFM

8,475 to 50,000 CFM

(0.15 to 0.88 CFM/Ft2)

55⁰F

Note: 100% OA, no energy recovery! Note: OA requirement only 0.15 CFM/Ft2, 16% of design airflow rate!

Authors‘ performance conclusions

Energy use comparable

ACB system more than

double

˚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

Sensible design conditions (100% sensible & latent space loads)

SAT OARAOAAHU

coolingBeam

coolingFan Pumps

Total energy




Shoulder season operation (50% sensible, 80% latent space load)

SAT OA RA OAAHU

coolingBeam

coolingFan Pumps

Total energy


30

25

20

15

5

0

kBtu

/ft2 -y

ear

10

2.30.1

17.9

5.1

12.8

1.51.0

District coolingHeatingFansPumps

ACB Design as Described

VAV Reheat

0.0


Energy use comparable

ACB system more than

double





SAT OARAOAAHU

coolingBeam

coolingFan Pumps

Total energy





SAT OA RA OAAHU

coolingBeam

coolingFan Pumps

Total energy

Actual performance comparisons

Operation

al Energy use,

kW

100

90

80

70

60

50

40

30

20

10

0

Sensible Design Performance Latent Design Performance Shoulder Season Performance

52.0

VAVR

sys

tem

79.2

ACB

syst

em

as d

esig

ned

41.2

19.4

VAVR

sys

tem

ACB

syst

em

as d

esig

ned

88.0

VAVR

sys

tem

89.9

ACB

syst

em

as d

esig

ned

What’s wrong with this picture?

• 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

Ventilation

System Design TPA (˚F) Btu/h-ft2 Btu/h-CFM CFM/ft2

% Space Sensible Cooling

Btu/h-CFM


Btu/h-ft2

ΔW grains

Btu/h-CFM CFM/ft2 CFM/ft2

0.15

0.150.307.5NA19.5

63.0 19.5 13.2 0.53 36% 23.6

VAVR system as described

ACB system described

55.0 22.0

Latent Cooling Requirement (all airside)

Waterside sensible cooling

Primary Air Sensible Cooling

11.0

6.0

0.89

64% 2.2

100% 2.2

4.1 0.53

NA


Performance comparison with properly designed ACB system

Air handling unit modifications for ACB system

8,300 CFM

VFD16,667 CFM16,667 CFM

Fan Array

Cooling Coil

Heating Coil

OA Filters

Bypass Damper

30,000 CFM16.667 CFM

(0.3 CFM/Ft2)

55⁰F

Fan Array

Cooling CoilFilters

Relief Fan

8,475 CFM

16.667 CFM

16,667 CFM8,192 CFM

recirculation

(0.3 CFM/Ft2)

55⁰F

Reduce SAT to 55˚FLower PA dew point allows primary airflow

reduction of 45% and an associated fan energy reduction of 70%!

Introduce mixing at AHUMixing results in an additional 30%

reduction in chiller energy!

Leveling the playing field

• 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%

Ventilation

System Design TPA (˚F) Btu/h-ft2 Btu/h-CFM CFM/ft2


Btu/h-CFM


Btu/h-ft2

ΔW grains

Btu/h-CFM CFM/ft2 CFM/ft2

0.15

0.150.307.5NA19.5

63.0 19.5 13.2 0.53 36% 23.6

VAVR system as described

ACB system described

55.0 22.0

Latent Cooling Requirement (all airside)

Waterside sensible cooling

Primary Air Sensible Cooling

11.0

6.0

0.89

64% 2.2

100% 2.2

4.1 0.53

NA

0.30 0.1533% 44.0 67% 2.2 7.5 0.3011.0Properly designed ACB system

55.0 19.5 22.0

0.15 0.15

0.18 0.18

0.20 0.60

Design airflow rates in CFMPA per square foot

Primary air conditions and flow rates for modified ACB system

0.20 0.60

Avg. 0.30 CFM/ft2

Ventilation 0.15 0.15

Dehumidification 0.18 0.18

Sensible Cooling 0.60 1.75

Resultant Airflow 0.60 1.75

Avg. 0.88 CFM/ft2

VAV System



ACB System



Actual performance calculations


VAVR system as described 55 8,475 41,525 17% 49.9 0.0 45.6 5.5 88.0ACB system as described 63 30,000 0 100% 40.5 23.5 26.0 8.7 89.9


SAT OARAOAAHU

coolingBeam

coolingFan Pumps

Total energy

30% less than VAVRPrimary air @ 55 ˚ F DB , 53 ˚F DP 55 16,667 0 100% 28.7 24.4 4.5 7.6 62.1

Modified ACB system

38% less than VAVRMixing at air handling unit 55 8,475 8,192 51% 21.6 24.4 4.5 6.8 54.5



Beam cooling

Fan PumpsTotal

energyLatent design conditions (75% sensible, 90% latent space load)

SAT OA RA OAAHU

cooling




SAT OA RA OAAHU

coolingBeam

coolingFan Pumps

Total energy

Same as VAVRPrimary air @ 55 ˚ F DB , 53 ˚F DP 55 16,667 0 100% 8.4 6.1 4.5 2.0 19.3

Modified ACB system

Was more than doubleMixing at air handling unit 55 16,667 0 100% 8.4 6.1 4.5 2.0 19.3* Air handling unit operating in economizer mode

Primary air @ 55 ˚ F DB , 53 ˚F DP 55 16,667 0 100% 28.5 15.3 4.5 5.9 51.5

Modified ACB system

6% more than VAVRMixing at air handling unit 55 8,475 8,192 51% 10.3 15.3 4.5 3.9 31.8 34% less than VAVR

Actual performance comparisons using modified ACB system

Operation

al Energy use,

kW

100

90

80

70

60

50

40

30

20

10

0

Sensible Design Performance Latent Design Performance Shoulder Season Performance

62.1

ACB

syst

em,

55˚F

PA

54.5 AC

B sy

stem

,

AHU

mix

ing

51.5

ACB

syst

em,

55˚F

PA

31.8

ACB

syst

em,

AHU

mix

ing

22.0

ACB

syst

em,

55˚F

PA

22.0

ACB

syst

em,

AHU

mix

ing

52.0

VAVR

sys

tem

79.2

ACB

syst

em

as d

esig

ned

41.2

19.4

VAVR

sys

tem

ACB

syst

em

as d

esig

ned

88.0

VAVR

sys

tem

89.9

ACB

syst

em

as d

esig

ned


System cost comparisons

Authors’ installed cost comparison of the systems as designed

Cost or Qty. Cost/CFM Cost or Qty. Cost/CFM$215,179 $4.30 $576,496 $19.22$584,058 $11.68 $1,509,349 $50.31$319,695 $6.39 $608,349 $20.28$252,067 $5.04 $647,037 $21.57

38,000 28,612310 10,244

2,085 9,630

$1,370,999 $3,341,231

$25 $27 $62 $111

Equipment cost ($)Labor cost ($)

Material cost ($)

VAV system as designed

Lbs. of ductwork (lbs.)Chilled water piping (LF)

Hot water piping (LF)

Total HVAC cost ($)

HVAC cost ($/ft2)

Subcontractors ($)

ACB system as designed

System ductwork configuration

900 FPM

0.53 CFM/Ft2

2,000 FPM @ 0.9 CFM/Ft2

ACB System MainsAEFF = 30,000/900 = 33.3

VAV System MainsAEFF = 50,000/2,000 = 25.0

Authors’ conclusion: Average duct cross sectional area 33% greater for the ACB system

Two supply risers

One supply riser

Ductwork configuration

2,000 FPM2,000 FPM

ACB System MainsAEFF = 16,667/2,000 = 8.3

VAV System MainsAEFF = 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

• Number of beams reduced by 63% by modifying ACB system

• 63% reduction in beam piping and insulation costs

Area served, ft2 38,922 17,578 56,500 38,922 17,578 56,500

BTU/H-ft2 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

CFMPA/ft2 0.61 1.50 0.88 0.34 0.96 0.53

BTU/H-CFMPA 22.0 22.0 22.0 39.6 34.4 38.0

305 550 398

7.7 16.0 10.9

1,706 1,055 2,761

ACB Btu/h-LFACB CFM PA /LF

Linear feet of ACB required

All zones

VAV system as described ACB system as described

Interior zones

Perimeter zones

Interior zones

Perimeter zones

All zones

38,922 17,578 56,500

13.4 33.0 19.5

7,878 8,789 16,667

0.20 0.50 0.29

66.0 66.0 66.0

1,056 1,056 1,056

16.0 16.0 16.0

492 549 1,042

Modified ACB system

Interior zones

Perimeter zones

All zones

Remedies for other cost inequities

• Perimeter VAV terminals serve multiple offices

• Interior VAV terminals have no reheat provisions

• Perimeter VAV terminals require only one HW connection per zone

• No indication of thermal zoning or condensation prevention in ACB system

• Four pipe configuration of interior space beams

• Remedy: Eliminate heating provision on interior beams

• Eliminates HW piping/insulation and connection to interior beams

• 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 or Qty.$320,282$838,544$337,978$359,472

15,8965,6915,350

$1,856,277

$33

Modified ACB system

Cost comparison using modified ACB system

Cost or Qty. Cost/CFM Cost or Qty. Cost/CFM$215,179 $4.30 $576,496 $19.22$584,058 $11.68 $1,509,349 $50.31$319,695 $6.39 $608,349 $20.28$252,067 $5.04 $647,037 $21.57

38,000 28,612310 10,244

2,085 9,630

$1,370,999 $3,341,231

$25 $27 $62 $111

Equipment cost ($)Labor cost ($)

Material cost ($)

VAV system as designed

Lbs. of ductwork (lbs.)Chilled water piping (LF)

Hot water piping (LF)

Total HVAC cost ($)

HVAC cost ($/ft2)

Subcontractors ($)

ACB system as designed

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.

Total $ $/ft2 Total $ $/ft2

Central Equipment Costs $554,608 $9.83 $467,782 $8.29

Air handling units and exhaust fans $196,642 $137,201

Chillers $223,253 $169,152

Boilers $37,384 $66,204

Pumps $97,329 $95,225

Air and water distribution costs $851,323 $15.09 $1,060,439 $18.80

Ductwork and insulation $389,350 $237,025

Piping and insulation $91,944 $460,106 $8.16

Air terminal units $150,109 $0

Supply/return grilles and diffusers $92,610 $19,824

Active beams $0 $263,000

Zone controls $127,310 $80,485

Other costs $197,400 $3.50 $206,400 $3.66

Fire and life safety $84,600 $84,600

Commissioning $112,800 $112,800

Condensation prevention $9,000

TOTAL INSTALLED COST OF HVAC SYSTEM $1,603,332 $28.43 $1,734,621 $30.76

All-air system

Mixing type without energy recovery

Beam system (pulldown menu)

Mixing type without energy recovery

Beam system (pulldown menu)

2 pipe beams, primary air heating coil

All air system (pulldown menu)

Single duct VAV with reheat

Specify perimeter heat method

Condensation prevention (pulldown menu)

SYSTEM TYPE

Gray cells require user input

All air VAV Active Beams

Summary Table Specify AHU configuration

Reset each floor chilled water supply temperature

Installed cost comparison (ACB system with mixing AHU, no ER)

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

Chilled beams!

And the hands down winner is………

Questions?

Active beams versus VAV with ReheatAnalysis of May 2013 ASHRAE Journal article

Documents

Active beams versus VAV with Reheat Analysis of May 2013 ASHRAE Journal article Ken Loudermilk Vice President, Technology & Developement