When VFDs are simply installed in
an existing pumping system, the energy
savings is 20 percent, on average. This
is based solely on the author’s observation in the field.
This component approach yields a
total energy efficiency gain of 28. 7 percent. However, the opportunity exists to
achieve a lot more—82 percent energy
reduction—by taking a whole-system
approach.
The opportunity exists to achieve 82
percent energy reduction by taking a
whole-system approach.
Four Steps to a
Whole-System Approach
There are four areas to look for energy efficiencies in a typical secondary
pumping system in a heating or cooling
loop in a facility. If the pumping system
was installed before 1995 (perhaps 50
percent of the installed base today), it
probably is of the three-way, constant-flow type.
In one of these typical fluid pumping systems, 8 percent of the energy is
wasted in the motor; about 29 percent
is wasted in the piping that transfers
the fluid to where it is needed; roughly
25 percent is wasted in throttling; and
20 percent is wasted in the pump. So
the actual delivered energy is only 18
percent (see Figure 1). That means
Throttle
25%
Pump
20%
Motor
8%
Figure 1
In a typical fluid pumping system, the actual
delivered energy is only 18 percent because
power is lost in the pump, piping, throttle,
and motor.
that there is an 82 percent opportunity
to reduce energy in the average pumping system.
Energy efficiencies can be achieved
in a typical closed-loop pumping system
in the following areas:
1. Variable-speed Drive. Most constant-speed, constant-flow systems
have a throttling device that is throttling
15 percent. They have a throttle valve
somewhere that just throttles the pump
back. This throttling valve dissipates an
estimated 15 percent of the pumping
power. So the first area of opportunity
is to install a VSD and open the throttling valve. Doing so improves efficiency
by 15 percent (see Figure 2).
Many VSD systems operate today
with the variable speed set at a constant reduced speed.
2. Variable Flow. Thirty years ago the
standard pumping system for cooling
was the constant-flow, constant-speed
type. It was turned on and spent its entire life pumping away at the same flow
and same pressure. Three-way valves
basically controlled the flow. Because
the heat exchanger didn’t need flow,
fluid bypassed the heat exchanger, but
the pumping system itself pumped merrily along.
For systems with three-way valves,
the second opportunity to gain efficiency is to make them variable-flow systems. The easiest way to do that is simply go into these three-way valves and
close the bypass, rendering it a two-way
valve. In this mode, the pump “rides”
its performance curve to provide the
flow needed by the system.
This step should net another 29 percent savings.
3. Flow Control. Once a pump has
been equipped with a VSD and an
open-throttle valve, the next way to
achieve energy savings is to control
flow. Switching from constant speed to
variable speed controls and maintains
a constant pressure across the pump.
