Energy Efficient Fluid Flow
Dec 17, 2015
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