TMdrive-MVG Series Catalog - Tmeic.com

Energy Savings Payback Calculations
Appendix. Energy Savings Payback Calculations
Replacing a mechanical speed control device with an adjustable speed drive usually produces large energy savings plus a
reduction in maintenance costs. This appendix outlines how the energy savings can be calculated as follows:
1. Calculate the cost of energy used by the electric drive speed control system, outlined on this page.
2. Calculate the cost of energy used by the mechanical speed control system, outlined on the next page.
The difference is the energy cost savings. Typical power consumption curves for pumps and fans are shown below.
Power (pu)
1.0
0.8
o
0.6
0.4
0.2
0
Pump with
throttling valve
Energy
savings
o
o
o
o
20% 40% 60% 80% 100%
o
Electrical
ASD
RPM/Flow (pu)
Example: Pump power savings using electric drive (ASD)
Power (pu)
1.0
0.8
0.6
0.4
0.2
0
Fan with
damper
Energy
savings
Fan with inlet
guide vanes
Electrical
ASD
0% 20% 40% 60% 80% 100%
RPM / Flow (pu)
Example: Fan power savings using electric drive (ASD)
Below is an example of the energy cost calculation for a pump driven by a motor and electric drive. The calculation for the
mechanical system is similar and is described on the next page. Since energy consumption varies with speed and flow, you
need the load profile table which shows the number of hours running at the various flows. Refer to the example below.
Energy Cost for Electric Drive Speed Control
Step 9
Step 8
Step 6
Energy Cost
Input kW
$0.07/kWh
1,212
Energy
Drive
Cost
Efficiency
$371,500/yr
96.5%
Step 10
Step 7
Calculation
details
Three-Phase
Electric Supply
Electric
Adjustable
Speed
Drive
Motor
Step 4
Motor
Input kW
Shaft kW
1,169
Motor
1,119
Input
Efficiency
Shaft hp
95.7%
1,500
Step 5
Step 3
Electric Motor
Shaft
Variable speed motor
Pump
Chart & Load
Curve
Step 2
Variable speed pump
Step1
Desired
Flow
90%
Start
here
Output Flow
and Pressure
Step 1
Step 2
Step 3
Select the desired pump
output flow, for example
90%, and number of
hours/day at this flow,
for example 12.
Obtain the variable
speed pump
performance chart and
the load pressure-flow
curve, which is the flow
resistance of the
process being fed.
Find the pump input
power.
Percent Pressure (or Head)
100
Pump Performance Chart & Load Curve
Load Curve
(Process)
Pump Chart
Speed N2
1,800 rpm
80
B
1,500 hp
A
90 100
Percent Output Flow
Pump hp
2,000
Overlay the load pressure-flow curve on the pump
chart and find the pump input shaft horsepower at
the 90% flow (point B). You need the load profile.
See example below.
Pump input power at N2 rpm = 1,500 hp
at point B, 90% flow.
Daily Load Profile (example)
Operation
Hours/day
Percent
Flow
5
100%
12
90%
5
80%
1
70%
1
60%
Step 4
Convert the input shaft
horsepower to shaft kW
Conversion: Horsepower x 0.746 = kW
Input shaft kW = 1,500 hp x 0.746 = 1,119 kW
Step 5
Step 6
Obtain the electric
motor efficiency
Example: Induction motor efficiency 95.7% from
manufacturer's data sheets (find at each RPM)
Motor input power = 1,119 kW/0.957 Efficiency
= 1,169 kW
Step 7
Step 8
Obtain the adjustable
speed drive efficiency
Example: Drive efficiency 96.5% from manufacturer's
data sheets (find at each RPM)
Drive input power = 1,169 kW/0.965 Efficiency
= 1,212 kW
Step 9
Step 10
Obtain the electric
power cost
Example: Energy cost = $0.07/kWh. Calculate cost for
the hours at this flow from load profile; in this example it
is 12 hours/day.
Energy cost = 1,212 kW x 0.07$kWh x 12 hrs/day
x 365 days per year = $371,500 per year. (Repeat
calculation for other flows in load profile and total).
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