A boiler is a significant contributor to the overall cost of operation in any facility, commercial or industrial. This, because of its large consumption of energy both fuel and electricity, the cost of which often exceeds the initial cost of the equipment by four times every year! The burner and controls play a very significant role in this energy outlay.
The burner itself is a combustor, forming a chemical reaction as it combines fuel and air in the presence of heat; initially with the pilot, and then spontaneously as the chemical reaction continues.
There are various types of burners found on boilers and they vary based on the manufacturer and application. This article addresses larger industrial steam boiler applications and focuses on firetube and watertube designs using the Integral and gun-type burners and employing either rotary or blade type air damper system.
The best time to evaluate a burner and whether an upgrade or replacement may be required, is during the boiler’s annual inspection when it is down, cooled and completely opened, but before this all occurs, the boiler operator or qualified technician should carefully note the following:
- Boiler operating pressure, and is it holding steady during the process?
- Is the Burner cycling more than 5-10 times per hour?
- Take combustion readings including the stack temperature based on the operating pressure as it should be holding at or below approximately 100 Deg.F over the saturated steam temperature. Other readings include Excess air, CO2, and possibly NOx readings depending on local codes and other requirements.
- How are the fuel and air mix controlled? If it’s a single point positioning system using linkages and cams, look for deflection, wear, and slippage.
- Check the flame as it modulates from low to high fire. Is it luminous and clean looking or is it mostly yellow with evidence of smoke? Then when the boiler is down and open does the furnace have soot deposits?
- What was the turndown of the burner? Burners can range from 4:1 to 10:1 dependent on size and technology.
When a Burner Upgrade or Replacement Makes Cents
- High excess air readings above 4%: If this is the case, it’s possibly because the burner spends most of its time at low fire where it is difficult to maintain optimum O2 levels. In this case, the burner/boiler is probably oversized for the load and needs to be derated if it will not result in condensing of the flue gas in the boiler or stack. The other possibility is poor combustion air control as the burner modulates between low and high fire. This can be remedied with better fuel/air ratio control such as replacing the single-point positioning system using linkages and cams with a parallel positioning system using independent electrically driven actuators with a central controller.
Another consideration may be adding O2 Trim if the boiler room air fluctuates in temperature, the calorific fuel value fluctuates, and/or the stack draft varies.
Remember, a 2% increase in excess air will result in a 1% loss in efficiency. If the annual fuel bill of the boiler is $250,000 that’s a $2,500 loss just because the excess air isn’t properly controlled.
- Frequent cycling is another indication of an oversized burner/boiler combination for the load or, it could be the burner does not have enough turndown for the load variations, and cycles accordingly. This typically results in 15% loss on average due to the frequent pre and post combustion purges for safety purposes. Back to our $250,000 fuel bill, that’s $37,500 lost!
- Combustion air fan motor: Is it high-efficiency? What about variable speed drive? These can result in attractive electrical cost savings especially with Variable Speed Drive. This is because a speed reduction results in a cubic reduction in energy consumed.
- Burner refractory condition as it relates to the dry oven, throat tile and furnace liner. What is often misunderstood, is that this refractory is only for the boiler’s protection, but often not appreciated is it also assists in forming and producing proper combustion, and its condition is therefore doubly important.
Burner Replacement Considerations
If the boiler is determined to be in good shape, and the existing burner cannot be economically upgraded, burner replacement should be seriously considered. However, certain considerations are in order before asking for a proposal:
- The burner needs to match the furnace dimensionally because it provides the Time for combustion to complete.
- Does the burner mounting plate need to be replaced?
- How will NOx control be handled if required? Any emissions standards in your area?
- Will the new burner fit in the space after mounting, allowing room for maintenance and removal if necessary?
- Will the existing burner management control (Programmer) require an upgrade?
This decision to upgrade or replace a burner is not really a simple one if it is going to be done to best benefit the facility in which it supplies its energy.
Professional assistance is often the best option; assuring the dollar invested is for the right reason(s), and thereby further assuring the best financial and operational outcome.
PBBS has professional boiler room experts that can make appropriate recommendations and even help you with leveraging local energy-saving rebates. Contact us today.