Business: Industrial

Getting Ideas for Energy Management Opportunities
2.2.3 Managing compressed air

Compressed air is the most expensive utility in a foundry. The foundry industry
uses a great deal of compressed air for production purposes. It is a safe and
convenient form of energy, frequently taken for granted and overlooked as a
possible savings option.

Compressed air is, mistakenly, often considered “free” by
those using it, because free air is being used from the atmosphere.The electrical
cost of compressed air may run to 70% or more of the total system’s annual operating
costs, while maintenance and depreciation may take 15–20% each. Therefore,
it is clear that compressed air is one technology where energy efficiency improvements
are directly related to financial savings. On average, the savings may
be found
in fixing:

• Leaks – 25%;
• Poor applications – 20%;
• Air lost in drainage systems – 5%; and
• Artificial demand – 15%.

What remains is the net useful compressed air
usage – only 35%! The above
breakdown of losses varies with the company involved. In some systems,
the leaks alone may account for 60%.

The compressed air leak losses can be
calculated during a no-consumption period, using the formula, V_L =
[V_C × t] / T, where V_L = the
volume of leak loss, V_C = the capacity of the compressor at
full load in m3/min, t = time in seconds of full-load compressor operation
(i.e., total full-load
measuring
time), and T = total measured, elapsed time.

A rule of thumb is that leaks should
not be higher than 5%.

Your investigations should concentrate on the above areas.

It should start with a quick, simple scan of the system. Its purpose is to
optimize the existing system, leading to savings in energy and money. Each part of the system should be investigated for possible improvements and savings options. However, for best results, do not just consider it as a sum of individual components such as compressors, dryers, filters, coolers and auxiliary equipment.

Take an overall view
and think dynamically in terms of pressures versus volumes, rates of change
in pressure, etc., to optimize the system effectively. This approach will result in a considered, thoughtful audit of the compressed air system, to
include:

• Analysing demand and matching capacity to demand;
• Controlling peak demand events;
• Correcting poor applications and waste in using
  compressed air;
• Identifying and correcting leaks;
• Controlling and managing the entire system;
• Optimizing the maintenance program;
• Sensitizing users to correct practices
  and savings opportunities; and
• Monitoring results, performance and costs of
  the compressed air system’s operations.

Together with a brief description
of the various issues, we list some remedial EMOs, and indicate whether they
would likely be in the category
of housekeeping items of zero or little cost ($), low
cost ($$) or retrofit high-cost
capital items ($$$).

Analyse the demand

• Identify critical users and analyse their needs regarding
  compressed air pressure, volumetric flow, frequency of use and duration of
  the usage events.
  That will
  help in designing eventual custom-fitted solutions and minimize
  affecting other users in the system ($).

Control peak demand

• Provide adequate compressed air storage capacity to reduce
  cycling.
• Consider installing additional compressed air tanks ($).
• Consider replacing
  part of the air distribution network with large-diameter piping to stabilize
  air supply and enable reduction of air pressure ($$–$$$).

Correct poor
applications

• Replace vacuum generators using compressed air, pneumatic motors,
  cooling by blowing compressed air, and open blowing, with other equipment
  giving the
  same
  results but at lower costs ($–$$).
• Do not cool hot
  castings with compressed air. If need be, install
  a low-pressure blower for the job.

Eliminate waste

• Generate compressed air at the lowest possible pressure suitable
  for the
  task ($).
• Never generate at too high a pressure only to reduce it to a lower operating
  pressure later ($).
• Do not compensate with higher pressure
  for poorly maintained air tools
  or undersized air distribution lines ($).
• Consider using high-efficiency blow
  nozzles (reducing air consumption
  by at least 50%) ($).
• Consider using a different nozzle type and configuration
  when blowing off water after annealing ($–$$).
• Minimize losses of compressed
  air in various pieces of measuring and controlling equipment using it:
  install section valves ($$).
• Consider dual pressure control for off-shift operation
  ($$).
• Switch off compressors when not needed ($).

Eliminate leaks

• Think of compressed air as you would water: stop the leaks
  at once ($).
• Use the listening method after normal working hours ($).
• Invest in an ultrasound
  listening device to identify leaks (e.g., Ultraprobe 2000™ ($).
• Consider
  purchasing a compressed air leak tester to detect pressure drops because
  of leaks and to measure the compressor capacity ($$).
• Consider implementing an
  automatic leak-measuring process, to be done on weekends, through a computerized
  control, regulation and monitoring system and installation of enough section
  valves ($$-$$$).

Manage system

• Require users to justify using the compressed air ($).
• Institute metering of
  the usage by end-point users ($).
• Make users fiscally accountable for the compressed
  air usage ($).
• Consider installing “load shaping” – a dedicated
  demand management system to handle peaks without affecting the pressure levels,
  starting
  additional
  compressors needlessly, or leaving excess
  compressors running “just in
  case” ($$$).
• Use the central control,
  regulation and monitoring system to
  start/stop the compressors at pre-determined times during the week. One such
  program is
  XCEED™ Compressed
  Air Management System by Honeywell ($$$).

Maintain
system

• Uncontrolled compressed air quality can lead to production down time.
  Implement a regular maintenance, inspection and preventive maintenance
  program for the system’s components. Also include
  the control and monitoring equipment
  in the program ($).

Train operators

• In order to achieve operational savings and quality improvements,
  users and operators must understand the system and be aware of its operating
  costs ($).
• Delegate responsibility for ensuring that the compressed air system
  has no leaks ($).
• Request that operators mark leaks manually, as soon as discovered,
  for maintenance to fix. ($).

Monitor performance

• Install both electricity and air flow meters (vortex),
  to allow energy monitoring ($$).
• On a monthly basis, monitor, e.g.:
  • kWh/total number of labour hours in production;
  • kWh/m³ (i.e., compressor efficiency);
    and
  • m³/total number of labour hours in production.
• In addition, do the same monitoring
  in terms of dollar costs.

Other EMOs
Housekeeping

• Maintain air filters.
• Eliminate redundant couplings and hoses as potential
  sources of leaks.
• Remove obsolete compressed air distribution piping to reduce
  pressure loss, leaks and maintenance costs.
• When reciprocating and screw compressors
  are used in parallel, always maintain screw compressors at full load; when
  partial loads are required, shut down the screw compressor
  and use the reciprocating compressor instead.
• Avoid using compressed air
  when low-pressure blower air will do the job as well.
• Ensure that the system
  is dry – ensure that drainage slopes, drainage
  points and take-off points (always on top) prevent
  internal corrosion of the piping.
• Review all operations where compressed air
  power is being used and develop a list of alternative methods.
• Review the compressed
  air system and air uses annually – develop
  a checklist to simplify the task.
• Keep all air tools, connectors and hoses in
  good repair.

Low cost

• Draw intake air for both compressing and compressor cooling (if air-cooled)
  from the coolest location outside.
• Replace older, high-maintenance
  air-engine driven equipment with a new, high-efficiency type.
• When many users
  demand relatively low-pressure air, consider the economics of installing
  a separate distribution network.
• Install a pre-cooler to cool the inlet air and
  remove most of the moisture.
• Consider installing electronic condensate drain
  traps (ECDTs) to get rid of the water in the receiver and piping. No air
  is wasted when the water is ejected,
  as opposed to the standard
  practice of cracking open a receiver drain valve for continuous bleed-off.
  ECDTs are extremely
  reliable.
  The payback on the investment
  ranges from 8 to 24 months.
• Install
  a large compressed air accumulator tank to reduce compressor cycling.

Retrofit;
high cost

• Install a system pressure regulator to eliminate artificial demand
  by stabilizing pressure at the minimum required level for production. Note:
  typically,
  energy
  savings of 10% are achieved
  (e.g., XCEED™ Demand
  Expander).
• Consider installing
  rotary drum air dryers, where the heat generated by the air compressor
  is used to continually regenerate
  the air dryer desiccant, and no
  compressed air is consumed.
• Consider
  installation of an airtight plastic pipe distribution network to replace
  old steel pipes and corroded and leaking
  circuits.
• For smaller or occasional compressed air usage, consider using a combustion
  engine-driven compressor unit, which provides
  a less expensive energy input, has better part-load
  efficiency than electrical
  motors, and affords heat recovery from exhaust and the engine jacket.
• Check the
  size of the air distribution network for a “tight” fit,
  which causes excessive
  pressure losses.
• Consider replacing your compressed air dryers with a more efficient
  type, e.g., freeze dryer or rotating drum dryer.
• Consider fitting a variable
  speed drive to your fixed-speed compressor (typically, payback of less
  than two years may be obtained).
• Reduce idling losses and ensure the lowest possible
  generation pressure by constantly monitoring the end-use pressure and
  tying it to the compressor operation.
• Review compressor loading and consider whether
  installing differently sized compressors would even out the loading by
  fitting the suitably sized compressors to the momentary
  demand.

Additional information

An older technical manual, Water
and Compressed Air Systems (Cat. No. M91-6/12E), is still available at
NRCan – tel.:
(613) 996-6220 – and
remains a good reference.
For a complete listing
of other energy
management
manuals,
see Appendix
5.7. The
Web site
at www.knowpressure.org may also
furnish additional information.

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