Energy Efficient Fluid Flow
Pumping System Fundamentals
Welec = V DPtotal / [Effpumpx Effdrivex Effmotor ]
• 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
Pumping System Savings Opportunities
Welec = V DPtotal / [Effpumpx Effdrivex Effmotor ]
• Reduce volume flow rate
• Reduce required pump head
DPstatic
DPvelocity
DPelevation
DPfriction
• Increase pump, drive, motor efficiency
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 Gain
Increase Initial Reservoir Level
• Welev = V DPelevation difference between inlet and outlet
• Reducing elevation difference reduces work to
overcome elevation by 20%
Minimize Friction
Use 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 Friction
Use 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 Friction
Use 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
Fan w/ Inlet Vanes
By-pass loop By-pass damper Outlet valve/damper
Inlet vanes
(No savings)
(No savings)
(Small savings) (Moderate savings)
Efficient Flow Control
Close Bypass Valve
dP
VFD
Trim impellor for
constant-volume
pumps
Slow fan for
constant-volume
fans
VFD for
variable-volume
pumps or fans
Energy Efficiency of Flow Control
100%
Power (%)
80%
60%
40%
20%
0%
0%
20%
40%
60%
80%
Volume Flow Rate (%)
By-pass
Outlet Damper
Variable Inlet Vane
Variable Frequency Drive
100%
Pump/Fan and System Curves
DP
Pump/Fan Curve
System Curve
V
W = V DP = area of rectangle
Bypass Flow: Zero Energy Savings
DP
Pump/Fan Curve
System Curve
V2 = V1
• When bypassing, V through pump is constant
• Thus, pump work is constant and no savings
V
Throttle Flow: Small Energy Savings
Throttled System Curve
DP
Design System Curve
V2 = V1 / 2
V1
V
• With throttling and inlet vanes, V decreases but P increases
• Thus, net decrease in W (area under curves) is small
Reduce Pump/Fan Flow: Big Energy Savings
DP
Pump/Fan Curve
System Curve
V2 = V1 / 2
•
•
•
V1
V
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%
Three Ways to Reduce Pump/Fan Flow
Close Bypass Valve
dP
VFD
Trim impellor for
constant-flow
pumping
applications
Slow fan for
constant-flow
fan applications
Install VFD for
variable-flow
pumps or fans
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 valve
B: Max flow with valve open
C: 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 damper
B: Max flow with damper open
C: Damper open and fan speed (RPM) reduced
Variable Flow Pumping:
Process Cooling Loop
bypass /
pressure
relief
valve
cooling
tower
dP
cooling
water to
process
loads
7.5 hp
pump
city water
make-up
25 hp
pump
reservoir
warm
water
cool
water
process water return
VSD
• W2 = W1 (V2/V1)3
• Reducing flow by
50% reduces
pumping costs by
87%
Variable Flow Pumping:
HVAC Chilled Water Loops
Chilled Water Supply
AHU 1
Secondary
Chilled Water
Pumps
Chilled Water Return
AHU 2
AHU 3
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
B
C
A: Flow throttled by partially closed valve
B: Max flow with valve open
C: Valve open and pump slowed by VFD
Wsav for throttle to VFD = A – C
Wsav for bypass to VFD = B – C
Wsav 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
Reason: Wfluid = V DP = k V3
• Example:
– Current: Two pumps in parallel for four hours
– Recommended: One pump for six hours
– Estimated Savings: $500 /yr
Optimize Efficiency of Pumps/Fans
Correct Fan Inlet/Exit Conditions
No
Yes
Resize Over-sized Pumps
• Pump operating at offdesign point M
• Eff = 47%
• Replace with properly
sized pump
• Eff = 80%
• Savings: $14,000 /yr
Fluid Flow Summary
•
Reduce Required Pump/Fan Head
–
–
–
–
•
Employ Energy Efficient Flow Control
–
–
–
–
•
Reduce excess elevation head
Use larger diameter pipes
Use smoother pipes/ducts
Use long-radius elbows and low-friction fittings
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