Balancing Variable Flow Systems Taylor

8/11/2019 Balancing Variable Flow Systems Taylor

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Balancing Variable FlowHydronic Systems

Steven T. Taylor, PEJeff SteinTaylor EngineeringAlameda, CA


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Agenda

Balancing Issues Why balance? Balancing Options

Piping System Analysis

Results

Controllability Flow during Transients Energy costs First costs

Ranking

Recommendations Resources: Balancing Variable Flow Hydronic Systems,

Steve Taylor and Jeff Stein, October 2002, ASHRAE Journal


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Balancing Issues

Ensure adequate flow available at all coils to meetloads

Less than design flow may be adequate most of the time Ensure differential pressure across control valves is not

so high as to cause erratic control Two-positioning Unstable control at low loads

Cost considerations

First costs (installed costs and start-up costs) Pump energy costs (peak demand and annual) Rebalancing costs (if any) as coils are added to system


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Balancing Options

1. No balancing

Relying on 2-way control valves to automatically providebalancing

2. Manual balance Using ball or butterfly valves and coil pressure drop

Using calibrated balancing valves (CBVs)3. Automatic flow limiting valves (AFLVs)

4. Reverse-return

5. Oversized main piping

6. Undersized branch piping7. Undersized control valves

8. Pressure independent control valves Not studied in our ASHRAE paper


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Piping Systems Analysis

Heating system 540 gpm 400 VAV reheat coils Constant speed pumps

Based on actual building inOakland Cooling system

1,200 gpm

20 Floor-by-floor AHUs Variable speed pumps All valves: 2-way modulating

Analyzed using Pipe-Flo


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HW Piping Floor Plan


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Typical Coil Piping

Options 1, 4, 5, 6, & 7

Option 2


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Typical Coil Piping

Option 3

Option 8


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Option 1: No Balancing

Advantages No balancing labor Coils may be

added/subtracted

without rebalance

Disadvantages Imbalance during

transients or ifsetpoints are

improper Control valves near

pumps can be over-pressurized,

reducingcontrollability


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Option 2: Manual w/CBVs

Advantages Valves can be used for

future diagnosis (flow

can be measured)

Reduced over-pressurization of controlvalves at low flow

Disadvantages Added cost of calibratedbalancing valve

Higher balancing cost Complete rebalance may

be required if coils

added/subtracted Slightly higher pump head

due to balancing valve

Coils may be starved ifvariable speed drives areused without DP reset

Slightly higher pumpenergy depending on flowvariations and pumpcontrols


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Starved Loads with CBVs and Fixed DP

Setpoint: Design Condition

VFDLoad

Load

DP

100 GPM

5 PSI D

100 GPM

5 PSI D

5 PSI D

28 PSI D, Cv=19

5 PSI D

2 PSI D

12 PSI D38 PSI D45 PSI D

PUMP CLOSE LOAD REMOTE LOAD

20

60

50

40

30

10

0

70

PRESSURE

PSIG


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Starved Loads with CBVs and Fixed DP

Setpoint: No Remote Flow Condition

56 GPM

1. 6 PSI D

0 GPM

0 PSI D

1. 6 PSI D

8. 8 PSI D

12 PSI D

0 PSI D

VFDLoad

Load

DP

12 PSI D12 PSI D19 PSI D

PUMP CLOSE LOAD REMOTE LOAD

20

60

50

40

30

10

0

70

PRESSURE

PSIG


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Option 3: Automatic Flow Limiting

Valves


added/subtracted withoutrebalance

Disadvantages Added cost of strainer andflow limiting valve

Cost of labor to clean strainerat start-up

Higher pump head andenergy due to strainer andflow limiting valve

Valves have custom flowrates and must be installed incorrect location

Valves can clog or springscan fail over time

Control valves near pumpscan be over-pressurized,reducing controllability


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Option 4: Reverse-return


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Reverse Return Configurations

C/C

C/C

C/C

C/C

Reverse return riser

(elevation)

Reverse return on f loor

(plan)

H/C

H/C

H

/C

H

/C

H/C

H/C


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Option 4: Reverse-return



No significant over-pressurization of controlvalves close to pumps.

Usually lower pump headdue to reverse-return piping

having lower pressure dropthan mains (due to largerpipe)

Disadvantages Added cost of reverse-returnpiping

Not always practicaldepending on physical layout

of system


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Option 5: Oversized Main Piping

Standard main design

C/C

C/C

C/C

C/C

Oversized main riser

6

6

6

6

C/C

C/C

C/C

C/C

2

6

2

6

3 3

4 4


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Option 5: Oversized Main Piping



Reduced over-pressurization of controlvalves close to pumps

Lowest pumphead/energy due to

oversized piping, nobalance valves

Increased flexibility to addloads due to oversizedpiping

Disadvantages Added cost of larger piping


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Option 6: Undersized Branch Piping

Advantages No balancing labor Reduced cost of smaller

piping

Coils may beadded/subtracted withoutrebalance

Reduced over-pressurizationof control valves close topumps where piping hasbeen undersized

Disadvantages Limited effectiveness andapplicability due to limitedavailable pipe sizes

High design and analysis costto determine correct pipe

sizing Reduced flexibility to add

coils where piping has beenundersized

Coils may be starved ifvariable speed drives areused without DP reset

Slightly higher pump energydepending on flow variationsand pump controls


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Option 7: Undersized Control Valves

Advantages No balancing labor Reduced cost of smaller

control valves

Coils may beadded/subtracted without

rebalance Reduced over-pressurization

of control valves close topumps where control valveshave been undersized

Improved valve authoritywhich could improvecontrollability where controlvalves have beenundersized

Disadvantages Limited effectiveness andapplicability due to limitedavailable control valve sizes(Cv)

High design and analysis costto determine correct controlvalve sizing

Coils may be starved ifvariable speed drives arewithout DP reset

Slightly higher pump energydepending on flow variationsand pump controls


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Option 8: Pressure Independent Control

Valves

Advantages

No balancing labor Coils may beadded/subtracted withoutrebalance

No over-pressurization ofcontrol valves close topumps

Easy valve selection flowonly not Cv

Perfect valve authority willimprove controllability

Less actuator travel andstart/stop may improve

actuator longevity

Disadvantages

Added cost of strainer andpressure independent controlvalve

Cost of labor to clean strainerat start-up

Higher pump head andenergy due to strainer andpressure independent controlvalve

Valves have custom flowrates and must be installed incorrect location

Valves can clog or springscan fail over time


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PICVs May Improve T?

NBCIP Test Lab (as reported

by manufacturer)


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Controllability & Transients

Percent of design flow

(percent of design coil sensible capacity)

with all control valves 100% open

Maximum

pressure drop

of control valverequired for

design flow,

feet

Maximum flow

through closest coil

Minimum flow

through most

remote coil

Balancing Method

CHW HW CHW HW CHW HW

1 No balancing 20.5 44.4 143%

(106%)212%

(119%)

73%

(89%)75%

(96%)

2 Manual balance

using calibratedbalancing valves

0 0 100%

(100%)100%

(100%)

100%

(100%)

100%

(100%)

3 Automatic flowlimiting valves

20.5* 44.4* 100%(100%)

100%(100%)

100%(100%)

100%(100%)

4 Reverse-return 1.2 10.4 103%

(100%)

150%

(109%)

99%

(100%)

85%

(97%)5 Oversized main

piping7.0 20.9 122%

(103%)173%

(112%)94%

(99%)82%

(97%)

6 Undersized branchpiping

19.5 NA 142%(106%)

NA 73%(100%)

NA

7 Undersized controlvalves

8.0 NA 120%(103%)

NA 86%(89%)

NA


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Ranks

Balancing Method

Controllability

(all conditions)

Pump Energy

CostsFirst Costs

1 No balancing 7 3 3

2 Manual balance using calibrated

balancing valves4 6 6

3 Automatic flow limiting valves 7 7 7

4 Reverse-return 2 2 5

5 Oversized main piping 3 1 4

6 Undersized branch piping 6 4 2

7 Undersized control valves 5 4 18 Pressure independent control

valve1 8 87


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Conclusions & Recommendations

for Variable Flow Hydronic Systems

Automatic flow-limiting valves and calibrated balancing valves are

not recommended on any variable flow system Few advantages and high first costs and energy costs Reverse-return and oversized mains may have reasonable pump

energy savings payback on 24/7 chilled water systems

Undersizing piping and valves near pumps improves balance and

costs are reduced, but significant added engineering time required

Pressure independent valves should be considered on very large

systems for coils near pumps

Cost is high but going down now with competition

When costs are competitive, this may be best choice for all jobs For other than very large distribution systems, option 1 (no

balancing) appears to be the best option

Low first costs with minimal or insignificant operational problems


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Questions

Balancing Variable Flow Systems Taylor

Documents

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