Session C5: Ventilation Choices

4-Pipe VAV vs. Active Chilled Beams

Steven T. Taylor, PEPrincipalTaylor Engineering, Alameda, CAstaylor@taylor-engineering.com

Learning Objectives

• Explore the pros and cons of using active chilled beams versus 4-pipe VAV systems;

•Become familiar with different lab types as they define which system type is more appropriate;

•Understand the difference between a load-dominated, hood-dominated, and ventilation-dominated lab space; and

•Understand the cost and energy implications of two popular mechanical systems used in laboratories.

Agenda

• System schematics• 4-pipe VAV (4PVAV) • Active Chilled Beams (ACB)

• Important fundamentals: Lab types• Ventilation-dominated• Hood-dominated• Load-dominated

• Case study: Lab in Oakland, CA• Cooling loads• Energy use

• Other factors• Conclusions

4-pipe VAV Schematic

Lab Exhaust fans

VFD

VFD

VFD

Air Handler

FSD

Lab air valves -typ

Cond. drain

Change-over valves

Control valves

Sub

-du

ct

Outdoor Air

Min. Flow Bypass

Dampers

Supply Air Duct

Hot Water Coil

No chilled water coil at air handler in “dry” climates

Chilled water coil with condensate drain at zones

“Dry”: <63F Dewpoint (75F/65%RH) except for a few hundred hours/year

DenverPortland

Includes most of west coast, southwest, mountain states (ASHRAE B and C Climate Zones)

Changeover Piping

Why Changeover instead of two coils?

•Much higher hot water temperature drop ΔT• Improved boiler efficiency with condensing boilers• Smaller piping and HW pumps• Lower HW pump energy

• Less space required• Lower air pressure drop

• Lower fan energy

• Less expensive• Eliminated HW coil• Smaller HW piping

Active Chilled Beam Schematic

Lab Exhaust fans

VFD

VFD

VFD

Air Handler

FSD

Lab air valves

Active Chilled Beam

Sub

-du

ct

Outdoor Air

Supply Air Duct

Hot Water Coil

Chilled water coil required at air handler

Active chilled beams (changeover piping same as 4PVAV)

Lab Types

•Ventilation-dominated• Where the minimum ventilation rate (air changes per hour

ACH) is greater than hood exhaust rates and rates required for cooling

•Hood-dominated• Where the hood exhaust rate exceeds those required for

cooling or minimum ventilation

• Load-dominated• Where the airflow needed for cooling exceeds the hood

exhaust and minimum ventilation rates

Only “load-dominated” labs benefit from ACBs

• ACB induction cooling increases cooling capacity at a lower outdoor air rate• ~2.5 times the sensible load that primary outdoor air can provide alone

• So labs that require 6 ACH to 15 ACH for cooling require only 6 ACH of outdoor air

• Reduces fan energy and energy needed to condition outdoor air

How prevalent are “load dominated” labs?

UC Davis Lab Plug Loads (from Labs21 Minimizing Reheat Energy Use in Laboratories)

•Varies significantly from lab to lab•Occupied lab vent

typically 6 to 10 ACH• Equates to ~5.8 to

9.7 w/ft2 total load

• Some labs may be designed for high w/ft2, but seldom operate there

Equivalence of ACH to plug loads assuming a 9 foot ceiling and 55°F supply air temperature, 75°F space temperature

ACHLoads w/ft2

2 1.94 3.96 5.88 7.7

10 9.712 11.614 13.5

Example Lab

• 55000 ft2 Lab in Oakland, CA

• 6 ACH occupied, 4 ACH unoccupied

• Air-to-water heat pump/chillers• 2-pipe changeover since there is no simultaneous heat/cool

with 4PVAV and not enough hours with ACB system to be cost effective

• Lab mix (which favors ACBs)• 22% ventilation dominated • 78% load dominated labs• 0% hood dominated labs

• EnergyPlus model

ACB supply air temperature control

• Ideally just low enough to prevent condensation on chilled beams

• Ideal: condensation sensor based reset

Equipment Sizing vs. Supply air Temperature

ACB 55°F SAT

ACB 58°F SAT

ACB 60°F SAT

ACB 65°F SAT

4-Pipe VAV

AHU (cfm) 39,402 39,402 39,402 39,402 61,492

HP/chillers (tons) 196 185 178 162 105

4PVAV has larger AHU but smaller chiller plant

4PVAV has only HW coil so SAT does not affect loads or energy

~63F SAT warmest to ensure spaces <65% RH

Site Energy Use Index, Btu/h/ft2

Ideal condensate reset between these two

Energy Cost Index $/ft2

$0.00

$0.10

$0.20

$0.30

$0.40

$0.50

$0.60

$0.70

$0.80

Heating Cooling Fans Pumps Heat Rejection

Co

st

[$/f

t²]

ACB 55°F SAT + Chilled Beam

ACB 58°F SAT + Chilled Beam

ACB 60°F SAT + Chilled Beam

ACB 65°F SAT + Chilled Beam

4-Pipe VAV

$3.24 /ft²$3.01 /ft² $2.90 /ft² $2.82 /ft²

$2.56 /ft²

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

$3.00

$3.50

Total

Co

st

[$/f

t²]

ACB 55°F SAT + Chilled Beam

ACB 58°F SAT + Chilled Beam

ACB 60°F SAT + Chilled Beam

ACB 65°F SAT + Chilled Beam

4-Pipe VAV

Ideal condensate reset between these two

EnergyPlus Results - Heating

•Heating energy is lower for 4PVAV • Literally no reheat• Higher for labs in load-dominated mode

in cold weather due to greater OA conditioning

• But lower for labs in ventilation-dominated mode since ACB dehumidification increases reheat En

ergy

Co

st $

/ft2

AC

B 5

5F

AC

B 5

8F

AC

B 6

0F

AC

B 6

5F

4P

VA

V

EnergyPlus Results - Cooling

• Cooling energy is lower for 4PVAV • Economizer effect in load-dominated labs

• Many hours/year with Oakland’s mild weather

• ACB could mitigate with waterside economizer but at added cost

• Single temperature chiller plant • No cooling efficiency benefit for lukewarm

CHWST to ACBs

• ACB could mitigate with dual temperature plant or DX for AHU but at added cost/complexity

Ener

gy C

ost

$/f

t2

AC

B 5

5F

AC

B 5

8F

AC

B 6

0F

AC

B 6

5F

4P

VA

V

EnergyPlus Results - Fans

• Fan energy is lower for 4PVAV • Because ACBs provide higher sensible

loads, all labs become ventilation-dominated so the system is ~CAV

• 4PVAV has cube-law performance of fans at part load• Pressure drop through entire duct

system goes down with square of flow

• Exhaust system also VAV due to high exhaust stacks – savings lower if high exhaust velocity must be maintained

Ener

gy C

ost

$/f

t2

AC

B 5

5F

AC

B 5

8F

AC

B 6

0F

AC

B 6

5F

4P

VA

V

Why does 4PVAV have lower fan energy?

Other 4PVAV Advantages:

• Net first costs are usually lower • Very high cost of the ACBs and their installation• Smaller cooling and heating plants• Even smaller piping and pumps due to higher ΔTs• Partially offset by larger AHUs & ductwork, terminal unit condensate drains

• Works for all zones• ACB system only effective in load-dominated labs

• Ventilation-dominated zones are essentially constant volume reheat• ACB cannot be used on some hood-dominated labs due to insufficient

ceiling space to house the ACBs

• More flexible for future remodels• Hoods can be added to load-dominated labs without any modifications • 4PVAV diffusers more readily relocated vs. ACBs

• Lower risk of condensation damage

Other ACB Advantages:

• Smaller supply air system if many load-dominated labs• Less roof space needed for AHUs• Less ceiling space needed for ductwork

• Possible floor/floor height reduction

•No condensate drains at terminal units

Conclusions

• 4PVAV Advantages• Usually more energy efficient

• For sure in mild, dry climates• For sure if most zones hood- or ventilation-dominated

• Usually costs less• Unless most labs are highly load-dominated

• More flexible• Works for all lab types• Less expensive to remodel

• ACB Advantages• Less space required for AHUs, duct mains• No condensate drains at terminal units

Questions?

Steve Taylor

staylor@taylor-engineering.com