The forgotten technology
November 1st 2009 The statistics about the amount of energy consumed by compressed air use in industry are common knowledge. Plus, it is a well known rule of thumb that at least 85% of the energy used in a typical water-cooled compressor can be recovered as hot water.Why then, with Atlas Copco Compressors' recent announcement that it has been certified by TÜV as able to recover 100% of the energy used, is energy recovery technology still under-used in industry? Peter Lattaway, product support manager, explains
Surprisingly, something that is
frequently overlooked, ignored, or
rejected is the provision of an energy
recovery system. It could be due to a toonarrow
focus on application requirements or
perhaps it is rejected because of the
perceived complexity of installation and
integration into an existing manufacturing
process. And also, it may be that because of
the age of an existing compressor
installation, it is considered that the cost of a
retrofitted energy recovery system could not
be justified.
However, the fact remains that all
compressors generate heat and many
industries can benefit from some form of
heat recovery in the form of hot water.
The physics of energy recovery
The theory: All energy used to compress air
is lost as heat. A small amount of this heat
energy is then lost from the compressor
package by radiation, as well as small energy
losses in other components. Ambient air
contains moisture and in the compression
and cooling process this moisture condenses
and releases 'condensation' heat or latent heat.
When the latent heat cancels the other losses,
the recovered energy in the form of hot water
is equivalent to the electrical input energy.
The test: The independent TÜV Institute
supervised tests on Atlas Copco's ZR 55-750
'Carbon Zero' range of oil-free compressors,
with built-in energy recovery systems,
resulting in their being the first in the world
to be certified 'net zero energy consumption'
at specific conditions. The official conclusion
was '100% of the electrical energy consumed
could be recovered and net power (energy)
consumption of the ZR compressor with
built-in energy recovery at specific design
conditions is zero'.
The practical application:Heat from the
air compression process is generated and
transferred to the cooling water through the
compressor's internal components. At 10bar
(145psig) and with inlet water at 20°C
(68°F), the transferred recoverable heat
contribution from these components is 12%
from the oil cooler, 9% from the high
pressure and low pressure compressor
elements, 37% from the intercooler and 42%
from the aftercooler. The result: 100% heat
recovery in the form of water at 90°C.
General applications for this hot water
output can be easily found in such areas as
showers or washroom duties. But it is in
those applications, where hot water or steam
is an inherent part of the production process
where the biggest energy saving
opportunities occur. Such requirements are
often found in such industries as food,
pharmaceutical and the petrochemical
industry but can occur in many differing
situations. It should also be noted that airborne
heat recovery is also easily achieved
and this can be used for applications such as
space heating.
Savings with a Carbon Zero
compressor
To illustrate what substantial financial
savings and environmental contribution a
Carbon Zero compressor can make, a typical
application scenario involves a 132kW
compressor with a heat equivalent of
132kJ/s, running 8000 hours per year.
In addition to the main duty of supplying
compressed air, its hot water output can be
utilised as pre-heated feed water to an oilfired
boiler, thus saving boiler fuel. These
calculations are based on a boiler efficiency
of 90%, the calorific value of heating oil at
41,200 kJ/l, and a fuel cost of 45p/litre:
Heating oil saved = 132 x 3600 / (41,200 x
0.9) = 13l/hour
Heating oil saved over the course of a year
= 104,000l
Cost of fuel = £0.45/l x 13 l/h = £5.85/hour
Yearly savings = £5.85 x 8000 hours per
year = £46,800/year
Equivalent CO2 = 104,000 x 2.518 kg CO2
per litre† = 261,872 kg or 262t of CO2
This carbon saving is approximately
equivalent to taking 87 average cars off the
road.*
The challenge ahead
There is clearly ample evidence that energy
savings in compressed air production benefit
plant operators' bottom line and the planet as
a whole. The challenge ahead is the need to
inform and educate compressed air users
about the substantial rewards of using Carbon
Zero technology and the need to integrate it
within their manufacturing systems.
† Energy conversion factor taken from
Carbon Trust's Food Industry Factsheet
CTL018: Energy and Carbon Conversions
* Based on the carbon emissions of 3
tonnes of CO2 per year for an average car
with average annual mileage
100% energy recovery?
IP&E spoke to Atlas Copco regarding the fact that although it is technically possible to
achieve 100% heat recovery it is not going to happen in most everyday conditions. The
TÜV conducted its test at specific design conditions of 40°C ambient temperature and 70%
RH. The question remains is the 100% claim not just a marketing stunt? "No it's not, there
is a marketing element to it and it is pitched that way in order to generate interest. There is a lot of debate about heat recovery and if we have added to the debate and made people aware of the vast savings that are out there then even if they are unable to obtain the full 100% then it's guilty as charged", admitted Lattaway. More articles from Atlas Copco Ltd: | 
