Chemical Engineering December 2018 - 40

FIGURE 2. Throughout the entire compressed-air
installation, energy use can be
optimized according to recommendations
from ISO 11011
Supply side
Efficiency in
generation
Distribution
side
Efficiency in
distribution
Demand
side
Efficiency in
use of
compressed air
started. Users can recover heat from existing
installed compressors or choose
to add the heat-recovery component to
newly purchased compressors (Figure 1).
ISO 11011 (Compressed Air - Energy
Efficiency - Assessment) is a
worldwide standard covering compressed-air
energy efficiency. It guides
users on how to perform a system audit
for compressed air units. It is part of
ISO 50001.
ISO 11011 focuses on the total cominstallation,
pressed
air
covering
the
process from the generation of compressed
air, through its distribution and
eventual end use. This is illustrated in
Figure 2.
Energy efficiency is always a top priority
and a strong incentive when seeking
solutions to further reduce a facility's
carbon footprint. A full understanding of
a process' needs is required to select the
best technology that will realize optimal
efficiency in energy recovery throughout
all phases of the air-compression process.
In the compressor room, users
have many solutions available to optimize
the air supply by controlling different
compressor parameters and looking
at opportunities to recover heat from the
compression process.
In the lifetime of a compressor, the
initial investment and installation of the
equipment will account for around 10-
15% of the total cost. On average, end
users may spend another 10-15% of
the cost on maintenance over the compressor's
lifetime. The rest, a staggering
70-80%, goes to energy costs. With
that in mind, optimizing a machine's
energy usage will have a major impact
on the installation's total cost of ownership.
Consider that a 35% reduction in
energy consumption can typically be
achieved by exchanging load/unload
compressors with variable-speed-drive
(VSD) compressors:
* For a 250-kW compressor running
for 8,000 hours, the energy cost
will amount to roughly $200,000
per year, or $2,000,000 over 10
years, assuming an energy cost of
$0.10/kWh
* In the case of a 35% reduction in
energy, this will result in a savings of
$700,000 over a 10-year period
If an energy-recovery system is also
installed, users will save even more.
The additional energy savings will come
from reduced or excluded fuel consumption,
or other energy sources for
heating water.
However, energy recovery does not
always result in heat, even when heat
is required, and often, there may not
be sufficient quantities of heat available.
The quantity of recovered energy will
vary over time if the compressor has a
variable load. In order for recovery to be
feasible, a relatively stable heat-energy
demand is needed. Recovered wasteheat
energy is best utilized to supplement
energy supplied to the system, so
the available energy is always utilized
when the compressor is operating.
Heat recovery potential
When compressors are equipped with
heat recovery or energy recovery capabilities
(Figure 3), users may have
several questions that typically include
the following:
* How much heat (kW) can be
recovered?
* Will the energy recovery system affect
the compressor's performance?
* What is the hot water temperature
level that can be reached?
FIGURE 3. A compressor with energy
recovery capabilities can contribute to
overall lower lifecycle costs for the entire
installation
40
Engineers can answer the first of
these questions by calculating the energy-recovery
potential of their process
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Chemical Engineering December 2018

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