Energy efficient boiler controls and systems

By David Smrdel, manager, RoviSys Power and Energy Group

The typical boiler consumes two to three times its installed cost in fuel annually. Many upgrades can pay for themselves within 12 months. In most situations, new or updated controls will produce the most significant reduction in fuel consumption for the same output and significantly extend the life your boiler.

Beyond tuning and good maintenance practices, below are the most important ways to improve the efficiency and life of nearly all industrial boilers.

Replace mechanical, jackshaft and unreliable boiler controls

Linked fuel and air controls reduce your ability to adjust combust efficiency. Mechanical boiler controls with complex linkages wear over time, forcing operators to increase excess oxygen in order to ensure safety and thereby reduce efficiency. And controls that suffer frequent partial or complete failures, or frequent cycling, clearly cost your plant money. Modern systems with solid state sensors and independent servo motors for fuel and air give you precise, automated and consistently reliable control of your boilers. New controls most often provide the most significant increases in the efficiency of a boiler system.

Measure combustion efficiency

A natural gas burner requires only two to three percent excess oxygen to burn fuel efficiently without exhausting excessive carbon monoxide. You can measure boiler flue gas inexpensively with one-time test kits. Or, for less than a thousand dollars, you can purchase a capable hand-held analyzer that also provides stack gas temperature, combustion efficiency and other data for display and download to computer. Given that inefficient combustion can quickly waste thousands of dollars, this is an excellent option for boilers without a permanently installed analyzer.

Flue gas trim systems

Stack temperature and flue gas composition are major indicators of combustion efficiency. In any sizeable boiler where fuel or load varies, a flue gas analyzer should be installed in-line. These systems monitor one or many factors contributing to combustion efficiency, then automatically communicate changes to the combustion controls. On well-designed natural gas-fired systems, for example, O2 levels as low as about 3 percent are in reach. You can roughly expect to increase boiler efficiency one percent for each 15 percent reduction in excess air or 40 degree Fahrenheit reduction in flue gas temperature.

Flue heat recovery

Heat that goes out your flue is wasted. An economizer enables you to return this otherwise wasted heat to warm boiler feed water.

Condensate return systems

Returning steam and hot condensate to the feedwater or boiler system is essential for energy efficiency. Steam carries three to five times the heat as the same weight of water. The more steam and condensate you get back the boiler, the less cold makeup water you buy, treat and heat. Return of high purity condensate reduces energy losses from dissolved solids and blowdown operations and lessens water disposal costs.

Variable speed drives

According to the US government, motors account for nearly two thirds of the electricity used in manufacturing. Unlike boilers, electric motors are efficient over wide output ranges. Having only on/off capability is inefficient. Upgrading to variable speed drives in burner fans and water pumps can reduce energy use and improve control of any process. Make sure that your current boiler control system can be configured to control the drive speed as well as any dampers that will remain.

Automated soot blowing

Clean tubes simply transfer heat more efficiently. Soot blowers, which can be manual or automatic, clean by spraying steam into the heat exchange tubes. Automated soot blowing ensures that the cycle is maintained and does not burden operators with another task.

Automatic blowdown systems

inadequate blowdown can push boiler water into steam distribution, which increases formation of deposits. Excessive blowdown wastes energy, water and chemicals. Automatic surface blowdown systems reduce energy costs by initiating operations only when needed. This minimizes heat losses as well as reducing water treatment costs.

Boiler feedwater treatment

Dissolved solids in your boiler systems reduce heat transfer efficiency and leave deposits in your system. Dissolved gases in your water and steam increase pitting and rust, which increases maintenance costs and hastens replacement. Or these reasons, mechanical and chemical deaeration, along with regular maintenance to keep air from entering the system downline, is critical to modern boiler water protection and control. Additional feedwater and condensate return treatments might also be required to protect equipment and maintain efficiency.

Sequencing

Most boilers are more efficient at high loads. Running multiple boilers in your facility at partial load is inefficient. Automatic sequencing can maximize primary boilers before firing standby units. Sequencing can be integrated into a load balancing system that reads sensors that indicate increases and decreases in demand within the plant.

Load leveling

What if you could anticipate the need for steam and increase or decrease in advance? It can be simpler than it sounds. For instance, temperature sensors in condensate return systems are sufficient to keep some plants in balance. Systems can incorporate any measurable factor that impacts steam needs: outside temperature, upstream production rates and more.

Dave can be reached at 330-995-8209 or by e-mail.