Recommendations for Increasing Heater Efficiency

A great question came in on the Operating Fired Heaters More Efficiently and Reliably blog post:

Jim I work with natural draft heaters on a daily basis and have initiated several efficiency tests with improved burner internals. I am looking for an opportunity to optimize dual firetube treater by first off improving the combustion efficiency to 80% in each tube and then staggering the temperature controls so that one tube runs 90 to 100% of the time and the other tube only fire during high load requirements.

I sent the comment around our advanced automation consultants for any comments that they might have and I received a great reply from Lou Heavner whom you may recall from earlier posts. Lou describes how to approach optimizing these heaters:

Heater efficiency is calculated using heat loss or input/output method. Input/Output method is difficult because you have to account for lags and delays between fuel firing rate changes and the measurement of process heat absorption changes and in the specific case where there is incomplete phase change on the process side (e.g. partial vaporization) you cannot easily solve with reasonable instrumentation. The heat loss method measures heat loss in the flue gas and assumes any other losses are negligible and constant. If not, they need to be measured and added as well.

Heat loss requires knowledge of the supply air (and fuel) temperatures and the flue gas exhaust temperature as well as the composition of the fuel and flue gas, just like with a boiler. In perfect combustion, there would be no unburned fuel in the flue gas and no sensible heat losses. But due to practical considerations, there are sensible heat losses and to calculate them, you need to know the delta T between the exhaust and ambient and how much excess oxygen remains in the exhaust. Efficiency calculations made using this technique can be pretty accurate in a natural draft heater, but if there is air leakage after the combustion zone, tramp air will show up as lower efficiency due to increased O2. And there is usually an optimum cost operation where the trade-off between sensible heat losses and unburned fuel losses require some level of unburned or incompletely burned fuel leaving in the flue.

When you are ready to control, the goal is to minimize excess O2 while not allowing excessive fuel to go unconsumed. CO analyzers are often used to detect incompletely burned fuel and the goal is usually to keep it below 150 ppm or some lower target. O2 is controlled to stay as low as possible without exceeding the CO limit, which is usually 2% O2 or less for the fluegas.

You can do this with simple feedback control, but feed forward control can help do better. Information on fuel quality, if it varies, and process side temperatures and flows (the heater load demand) can be used to adjust the fuel and air for combustion to meet the heating demand at maximum efficiency. Fuel and air cross limits are often used to maintain fuel and air ratio without getting into a fuel rich condition in the firebox during load changes. But airflow is usually difficult to measure. Therefore, it is often inferred from damper position.

When evaluating an application, we would want to know what instrumentation already exists and what the process variability looks like. What efficiency are they currently obtaining? Then we would look at the control valves and any other contributors to variability to see if they warrant repair or replacement. We would similarly evaluate the instrumentation and analyzers to see if they need anything there.

Then we could evaluate the control strategy and performance and recommend reconfiguration or tuning as appropriate, which may include advanced process control (APC). The person evaluating the controls would have to weigh the cost against the improvement from better loop tuning, valve repair/replacement, CO analyzer, etc. to come up with the best solution. Dampers are often the weak link in fine control of a natural draft heater.

As my colleague Doug Simmers in Emerson’s Rosemount Analytical business noted, “The commenter is probably correct with the strategy to fire one heater full out, and bring the second unit on only when needed. Running at full fire develops the best turbulence for fuel/air mixing, and the excess O2 can be kept lower.” This is a load allocation problem when two heaters are firing simultaneously. If we can model heater efficiency for each heater as a function of load, then we could optimize the load allocation across both heaters when both must be fired. Actual testing would identify the models, uncover the best strategy, and verify or disprove this assumption.”

He may also be interested in the efficiency calculator, developed by Doug’s team.

Join the conversation and add a comment if you have experience to share.

Posted Monday, July 23rd, 2007 under Analyzers, Energy, Process Optimization.

6 comments

  1. Jerry Christenson says:

    Lou Doug Jim
    Thank you for confirming that this should work in theory. I am currently working with a customer in Peace River AB and another in Midale SK. Both have dual fire-tube treaters and we are going to install high efficiency burner internals to give them an 80% combustion efficiency. Also because we are an Electric and controls company I wanted to take things one step further with the separate temperature controls for each fire-tube. I also believe we will have less swing in the process temperature from fluid inflow.

  2. I am very glad to find such a site.
    I will add my experience by reading the information in this web site.
    THANK YOU

  3. Hi all, i just joined a brewing as a junior engineer, and have been assigned to find a way to replace a coal fired boiler with a gas fired one and i must say i really have no idea on how to go about it since the coal price has risen so much, i would like to know how viable the gas boiler is and also the supply of gas in terms of how sustainable it will be. The cost of running the gas system and for how long will it be effective

  4. Bob Sabin says:

    We can probably help or get you in contact with folks that can assist with your needs, but your message raises some questions that would best be answered to steer in the proper direction:
    – What steaming rate (in pounds per hour) would you need in this boiler?
    – What steam pressure?
    – Are there other boilers on site? If so, what steaming rate and type of fuel?
    – Are you looking to replace the unit because of steam cost, emissions issues, performance problems, expansion, or other issues?
    – Where is your plant located?
    The answers to the above will help to determine what kind of equipment you would be requiring and who would be best to address your inquiry.
    Bob Sabin
    Emerson Performance Solutions, Inc.

  5. Jerry Christenson says:

    Jim Lou and Doug
    Thanks for the efficiency calculator it works well with the other tools I use. The splitting of the fire tubes has worked very well for us with dozens of application mostly in older vessels in mature oilfields where the designed heat input is no longer required.
    I have another question regarding the heating of Heavy Oil in “1000bbl Production Tanks” (16 foot diameter 25 feet tall) Using a 10″ fire tube 18 feet long we heat a 12 to 14 foot section of the Tank cylinder. The fire-tube extends into the oil horizontally 9 feet above ground level. The heavy Oil at the fire tube is 80 degrees C, it can be as cold as 40 degrees C at the top of the tank and the water at the bottom is usually around 50 degrees C. Can anyone model the fluid flow into the tank at 20 degrees C. We have no current way to mix or blend the product in these tanks and the only aware that our competition has come up with is to force more heat into the fire tube. The 10″ tube at 500,000BTU should provide adequate heat but I believe the Oil separates and flows to the top of the tank as soon as it enters and only picks up a minimal amount of heat. There are thousands of these tanks in Western Canada with hundreds more being put into service every year. Do you know anyone who can model the fluid and temperature in this type of vessel.

  6. Ranga Seshadri says:

    Please refer to Perry’s chemical engineering handbook , 7th Edition page 11-19.
    Usually using the tank mass and heating capacity of the coil you can calculate the time to reach uniform temperature ( Use table 11-1 given in the same handbook for heat transfer calculations )

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