“Cap and Trade” – whether it eventually comes to pass or not – is another good reason to look at your process heating applications to make sure you are getting the best return on your energy dollars.
There are many techniques and devices around that will reduce your energy usage, reducing or reclaiming heat that is currently lost, at varying capital costs payback periods. But the first thing that should be done is to look at the operating parameters of the systems – take a look at the basics. These are low cost first steps that will save energy immediately and allow you to more accurately assess the benefits of the higher cost options that are available.
Quality
Identify any quality issues and correct the faults causing them. In scrapping or reworking a faulty item you are throwing away 100% of the energy used to produce that item. These faults may be caused by poor temperature uniformity in the oven or furnace, or insufficient/excessive residence times. An oven uniformity check will highlight these problems.
Correcting the process faults that affect quality will save energy and reduce scrap as well as improving your customer relations.
Measure and Monitor
Know your energy enemies and tackle the big abusers first. But you must make sure you are tackling the real gas hogs by metering the gas flows to each of your individual systems and monitoring those meters over time. Select good meters with temperature and pressure compensation and you may be surprised to find out which part of the process is the real villain in your system. The meters used to identify the big users will provide an ongoing tool that will provide data allowing continuous performance monitoring.
Air/Fuel Ratio
Regularly check the air and gas pressures at each burner and compare to the manufacturer’s recommendations. These pressures are indicative of the flow of combustion air and fuel to the burner. All fuels need a specific amount of air for complete combustion. This is the stoichiometric air/gas ratio. Any additional air is referred to as excess air. Burners are designed to operate at a certain level of excess air. All the air that passes through the burner will be heated up to the temperature of the oven or furnace and will then be thrown away through the exhaust system. So the lower the excess air level at which the burner operates, the less hot air will be thrown away and the more efficient the burner system will be. But it is not good practice to reduce the excess air levels below the manufacturer’s recommendations as this could result in incomplete combustion – a waste of fuel – and the formation of Carbon Monoxide. So regular checking of the air and fuel pressures at each burner is required to ensure that the burner is operating at its design ratio.
Excess air is very expensive. Every 10% extra air above stoichiometric uses 0.2% extra fuel for each 1000F of exhaust temperature. 0.2% may seem to be a paltry number, too little to worry about but, for example, an aluminum melting furnace operates with an exhaust temperature around 22000F and 20% extra air will be throwing away an additional 8.8% of its fuel input just heating up that extra air. This is a major energy waste that can be saved by simply tuning the controls so that the burners operate at their correct ratio.
Any extra air getting into the oven or furnace is an energy waster irrespective of its source. The additional air has to be heated up to the oven operating temperature and is then thrown away. Running with too high a chamber suction, leaking seals on doors or access ports, will all add air to the system that will waste energy and the leaks should be addressed? Maintain the seals and the insulation of the furnace.
These leaks may also produce cold spots in the chamber resulting in quality issues that in turn waste more energy.
Reduce your fuel input, reduce your CO2 emissions and reduce your carbon footprint.
Good Maintenance
All burners are designed to create air and fuel flows at velocities selected to promote good mixing and flame retention. Burners that are not cleaned regularly will suffer from a build up of deposits from carbon or contaminants in the air that will restrict openings and so change the design velocities. This can result in poor mixing, poor combustion, and the formation of carbon deposits which will accelerate the downward spiral. The burner will become difficult to start and unreliable in operation and the poor combustion will waste energy. Pull and clean the burner regularly to return it to its original design conditions. Check the air and fuel filters. The burner control system will assume that a certain pressure at the burner head equates to a specific flow and controls the ratio on this premise. Dirty filters produce a higher pressure drop for the same flow so a linked-valve burner control system will be unable to maintain the optimum ratio. This will cost energy.
The problem may be overcome by switching to a ratio regulator control system or a mass flow control system that meters the air and fuel flows and controls them independently to maintain the ratio.
Safety Checks
Annual checks of the burner safety systems are one of the many duties we are committed to by the NFPA codes that FM measure safety systems against. As part of its major role in improving safety standards the codes direct us to check the integrity of the safety shut-off valves. Double block and vent arrangements in burner gas lines have been common for many years. The two blocking safety valves are arranged in series so that a gas leakage due to the failure of either valve is prevented by its partner valve. The vent valve is a normally open valve, open when the two main valves are closed and closed when the burner is in operation and the two main valves are open. The vent line is run to a safe area (usually at roof level). There can be a tremendous energy cost when the vent valve fails. Leaking gas is piped away to the safe area away from personnel for safety reasons. That is also away from anyone who might notice the leak and correct it. This often means that that leak will not be detected until the next scheduled burner safety check, and that could be up to a year away.
The vent line size may vary from ½-inch to 1.5-inch and, because they are normally open, will leak large volumes of gas to atmosphere continuously whenever the burner is operating. They should be checked regularly.
These are the areas that should be addressed in order to optimize the efficiency of your existing operation. Once this is done then the more exotic techniques for energy reduction can be considered, costed and compared to their cost benefit. There are systems for recovering energy from the exhaust gasses and recycling it back into the oven by preheating the combustion air, either through recuperation or regeneration. Or we can recover the energy by preheating the load before it enters the oven. There are mass flow control systems that ensure that excess air is kept to a minimum by monitoring oxygen levels in the exhaust gasses. We can reduce the amount of air required for combustion by raising the oxygen levels in the combustion air supply. We can take that concept a stage further and replace all the air and use an oxy-fuel burner system.
All these are well proven options and have particular benefits that make them better suited to some applications rather than others. It is possible to save very significant amounts of energy; saving 50% of fuel usage is not unusual when regenerative burners are applied to aluminum melters for example. There will be a fuel saving technique suitable for most processes and the benefits of each can be addressed in detail in future papers. But in all cases the first step is to look at your existing set up and optimize its operation by taking a fresh look at the basics.
Industrial Thermal Systems, Inc.
Robb Jackson, President
Tony Kelly, Combustion Specialist, tkelly@industrialthermal.com
Tony Kelly worked for a major European combustion company for 30 years before joining Industrial Thermal Systems in 1999.
Phone: 1-513-561-2100
Industrial Thermal Systems supply and service combustion systems for all process heating applications with an emphasis on improving their efficiency. Operating out of Cincinnati, Ohio, for more than 25 years they upgrade existing and supply new burner systems, fuel trains, and control panels. Their training facility houses a classroom area, flame safeguard fault simulation and demonstration units and burner set ups for hands on exercises.
 |
All comment postings require your name and email address for user verification only. Your email address will not be used or distributed for any other purpose.
No advertising is permitted and will result in the post being deleted and/or banning. Please click "REPORT" to report any inappropriate posts. blog comments powered by Disqus
Permission to Reproduce:
Unless otherwise stated, the copyright and similar rights to all material published on this website are owned by The Manufacturers Group Inc. DBA Manufacturing & Technology eJournal( mfrtech.com ). Reproduction of any article in print, electronic or any other form must acknowledge mfrtech.com as the Source and include a link to http://www.mfrtech.com
 |
