Energy Efficient Fluid Flow

Pumping System Fundamentals

• V = volume flow rate• DPtotal = pressure gain to overcome inlet/outlet affects

and friction DPstatic (pressure difference between inlet and outlet)

DPvelocity (velocity difference between inlet and outlet)

DPelevation (elevation difference between inlet and outlet)

DPfriction

• Eff = efficiencies of pump, drive, motor

Welec = V DPtotal / [Effpumpx Effdrivex Effmotor ]

Pumping System Savings Opportunities

• Reduce volume flow rate• Reduce required pump head

DPstatic

DPvelocity DPelevation DPfriction

• Increase pump, drive, motor efficiency

Welec = V DPtotal / [Effpumpx Effdrivex Effmotor ]

Fluid Flow System Saving Opportunities

• Reduce Required Pump/Fan DP• Employ Energy Efficient Flow Control • Improve Efficiency of Pumps/Fans

Reduce Pump/Fan DP

Minimize Elevation GainIncrease Initial Reservoir Level

• Welev = V DPelevation difference between inlet and outlet

• Reducing elevation difference reduces work to overcome elevation by 20%

Minimize FrictionUse Large Diameter Pipes/Ducts

Wfriction = V DPfriction

DPfriction = k / D5

Wfriction = V k / D5

Work to overcome friction varies inversely with 5th power of pipe diameter

Doubling pipe diameter reduces work to overcome friction by 97%

Minimize FrictionUse Smooth Pipes/Ducts

Wfriction = V DPfriction

DPfriction ~ friction factor f

fsteel = 0.021 fplastic = 0.018

Smoother pipes reduce work to overcome friction by: (0.021 – 0.018) / 0.018 = 17%

Minimize FrictionUse Gradual Elbows

Long radius elbows reduce work to overcome friction by 90%

Employ Energy Efficient Flow Control

Flow Control

• Systems designed for peak flow

• Systems operate at less than peak flow

• Use energy efficient method to control (reduce) flow

Inefficient Flow Control

By-pass loop(No savings)

By-pass damper (No savings)

Outlet valve/damper(Small savings)

Inlet vanes(Moderate savings)

Fan w/ Inlet Vanes

Efficient Flow Control

Trim impellor for constant-volume

pumps

Slow fan for constant-volume

fans

VFD for variable-volume pumps or fans

Close Bypass Valve

dP

VFD

Energy Efficiency of Flow Control

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%0%

20%

40%

60%

80%

100%

By-pass Outlet Damper

Variable Inlet Vane Variable Frequency Drive

Volume Flow Rate (%)

Po

wer

(%

)

Pump/Fan and System Curves

DP

V

Pump/Fan Curve

System Curve

W = V DP = area of rectangle

Bypass Flow: Zero Energy Savings

DP

V

Pump/Fan Curve

System Curve

V2 = V1

• When bypassing, V through pump is constant• Thus, pump work is constant and no savings

Throttle Flow: Small Energy Savings

• With throttling and inlet vanes, V decreases but P increases• Thus, net decrease in W (area under curves) is small

DP

V

Throttled System Curve

Design System Curve

V1V2 = V1 / 2

Reduce Pump/Fan Flow: Big Energy Savings

• W = V DP = V (k V2) = k V3

• When flow reduced by pump/fan rather than system, W varies with cube of flow• Reducing flow by 50% reduces work to overcome friction by 88%

DP

V

Pump/Fan Curve

System Curve

V2 = V1 / 2 V1

Three Ways to Reduce Pump/Fan Flow

Trim impellor for constant-flow

pumping applications

Slow fan for constant-flow

fan applications

Install VFD for variable-flow

pumps or fans

Close Bypass Valve

dP

VFD

Constant Flow Pumping:Cooling Towers With Throttling Valves

Constant Flow Pumping:Process Pumps with Throttling Valves

Constant Flow Pumping: Open Throttling Valve and Trim Pump Impellor

A: Flow throttled by partially closed valveB: Max flow with valve openC: Valve open and impellor trimmed

Constant Flow Fans:Slow Fan by Changing Pulley Diameter

Constant Flow Fans:Slow Fan by Changing Pulley Diameter

A: Flow throttled by partially closed damperB: Max flow with damper openC: Damper open and fan speed (RPM) reduced

Variable Flow Pumping:Process Cooling Loop

• W2 = W1 (V2/V1)3

• Reducing flow by 50% reduces pumping costs by 87%

warm water

cool water

cooling tower

city water make-up

7.5 hp pump

25 hp pump

reservoir

process water return

bypass / pressure

relief valve

cooling water to process loads

dP

VSD

Variable Flow Pumping: HVAC Chilled Water Loops

AHU 1 AHU 2 AHU 3

SecondaryChilled Water

Pumps

Chilled Water Return

Chilled Water Supply

Variable Flow Pumping:Open Throttling Valve and Install VFD

Full-Open Pumping:Install 2-Way Valves and VFDs

Big Cooling Towers

Big Cooling Loop Pumps

Worlds Largest Bypass Pipe

Savings From Installing VFDs

A: Flow throttled by partially closed valveB: Max flow with valve openC: Valve open and pump slowed by VFD

A

B

C

Wsav for throttle to VFD = A – CWsav for bypass to VFD = B – CWsav for bypass to VFD

W2 = W1(V2/V1)2.5

Wsav = W1 – W2

Pump Long, Pump Slow

• Identify intermittent pumping applications• More energy to pump at high flow rate for short period

than low flow rate longer• Example:

– Current: Two pumps in parallel for four hours– Recommended: One pump for six hours– Estimated Savings: $500 /yr

Reason: Wfluid = V DP = k V3

Optimize Efficiency of Pumps/Fans

Correct Fan Inlet/Exit Conditions No Yes

Resize Over-sized Pumps

• Pump operating at off-design point M

• Eff = 47%• Replace with properly

sized pump• Eff = 80%• Savings: $14,000 /yr

Fluid Flow Summary• Reduce Required Pump/Fan Head

– Reduce excess elevation head– Use larger diameter pipes– Use smoother pipes/ducts– Use long-radius elbows and low-friction fittings

• Employ Energy Efficient Flow Control – Constant flow pumping: trim impellor blade– Constant flow fans: Slow fan– Variable flow pumps and fans: Install VFDs– Pump slow, pump long

• Improve Efficiency Pumps/Fans– Correct fan inlet/exit conditions– Resize miss-sized pumps/fans