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While many boiler rooms and power plants have been swapping out old coal and oil-fired burners/boilers for natural gas and bio-fuels, there still will always be the need for oil burners/boilers in the world. With this need, come the multiple options of how to burn the oil. While there are many different variations, the two main methods of oil atomization for the burner are pressure and air atomization.

Pressure atomization depends on the oil pressure inside the nozzle tip to spray a fine mist of oil, very similar to a Windex spray bottle. The micronized oil droplets are flung into burner head, where they are thoroughly mixed with the combustion air and ignited. As mentioned above, the pressure at the oil nozzle is the key factor in the atomization process, therefore your oil pump and pressure regulator are the key components in this system. The pump needs to be able to meet the gallons per hour (gph) requirement for the burner/boiler to meet their load capacity. The pressure regulator is set in accordance to the firing rate which is normally between 100-300 (psi). The turndown ratio for a pressure atomizing burner is normally only 3:1 or 4:1.

Air atomization adds another variable to the equation. Like pressure atomization, the oil is pumped through the system and into the oil nozzle. There, the oil is sheared by a intersecting stream of air. These two elements are mixed rapidly and forced out the nozzle tip into the burner head where they mix with the combustion air and ignited. As mentioned above, not only is the oil pressure of importance, as seen in the pressure atomization, but the injection air is as well. The oil and atomizing air are both varied based on the firing rate of the burner. While the oil pressure will remain around 100 (psi), the atomizing air pressures can range from 5-75 (psi) based on the design of the burner and firing rate. The turndown ratio for an air atomized burner is normally 6:1 or 8:1.

Both main forms of oil atomization are acceptable and widely used throughout the industry. Depending on the application and resources available one may be better suited than the other. Pressure atomization requires a more powerful pump and motor assembly to create the amount of pressure at the oil tip, while air atomization does not. Air atomization requires an air compressor to be able to create the atomizing air needed to shear the oil in the tip while pressure atomization does not. The whole piping rack for the oil is not as complicated and requires fewer parts for the pressure atomization while air atomization requires piping for air, solenoid valves, air pressure regulators, etc. Air atomization allows for a better turndown ration, which will allow a burner to run at a lower rate while keeping up with the load, while the pressure atomized burner will have to run at a higher or lower rate and either vent the excess steam or cycle on and off to keep the base load. As mentioned before, the application and resources available will determine whether pressure or air atomization will be the better choice for an oil burner application.


Robert Bohn, Mechanical Engineer


Are there any fuel oil handling systems that don’t need return pumps? Are there any advantages to not having return pumps?  While it is true that I have worked on system designs and executions that have not included return pumps, my general answer is “no,” there are not any advantages.  And there are many different functional reasons why, especially within Mission Critical and First Response handling systems.  The purpose of this white paper is to list several different uses for return pumps under various circumstances, and to demonstrate how every single installation can benefit from them.


We all know that return pumps are great for keeping things under control, even if humans seem determined to find a way to spill fuel—running supply pumps in “hand,” or opening valves that should remain closed, and preventing a day tank overflow.  This is the most basic function of a return pump, and by far the most popular reason for their inclusion in both older and more modern designs.


FUEL RETURN:  Sending fuel from a day tank, to a main tank

Fuel return is a return pump’s most basic function.  During typical, normal operation, the return pump will be a lonely piece of equipment, but will also be one of the important safety features within the entire fuel oil handling system (FOHS), potentially saving very significant environmental and financial damages.


On many basic systems, the return pump will only activate when a “high” level float switch is activated, or similar signal, from a Controller.  An improvement is to have the return pump interlocked with a “high” level switch, which will start the return pump even in the event the Controller is offline or disabled.


Be sure to size the return pump appropriately to the application.  Calculate the highest possible rate of fuel entering the day tank.  On multiple day tank systems, this may mean the highest flow rate possible from all supply pumps running simultaneously, while filling that one particular day tank, and no others.  The return pump should be sized at a significantly higher rate than that potential high-flow rate.  How much higher will depend on pump type, available flow rates, etc., but should be 150-200% of the highest flow rate possible from the supply pumps.



INSPECTIONS AND REPAIRS:  Emptying a day tank for repair or replacement

What if you find fuel in your day tank’s secondary containment?  After looking for the usual leaking suspects, such as a threaded connection, weld, valve, etc., you may come to the conclusion that your primary tank is leaking.  Whether you intend on inspecting the tank, repairing or replacing it, you will need to drain it first.  But, if you have only a top/side-mounted overflow, how do you get the fuel out?


A return pump makes this process much faster and less messy, sending the good, clean fuel exactly to where you want it—back into the main tank.  You are also eliminating potential problems with big, messy barrels, which will contaminate the fuel and cause you to dispose of hazardous materials, increasing the hassle—not to mention the expense—even more.



FUEL FILTRATION:  Cleaning ALL of the fuel, not just the fuel in the main tank

One of the most common reasons for emergency generators to fail to run is bad fuel.  The best way to prevent bad fuel is to filter it.  ALL of it.  This means polishing the fuel that is trapped in supply lines, return lines, and day tanks.  Generators get their fuel directly from their day tanks, but most fuel is only polished in the main tank!


Return pumps are great for circulating fuel.  Modern control systems can be programmed to run polishing sequences, including activating return pumps (and supply pumps) that will circulate the fuel through the day tanks and allow it to be cleaned at the main tank(s).  Running a day tank “turnover” sequence, in combination with a filtration/polishing sequence, ensures much cleaner fuel throughout your entire fuel system.  The best generators on Earth won’t run on bad fuel!*


*ALL Mission Critical facilities should be on a strict fuel filtration and polishing regimen (Please look out for other white papers about Filtration from Preferred Utilities).  In fact, any facility that requires a backup generator of any type has a responsibility to ensure, to the best of their ability, that the generator runs in an emergency.  That’s what they’re there for!



THERMAL MANAGEMENT:  Decreasing fuel supply temperatures for generators

Generators use diesel fuel oil not only for internal combustion, but also for cooling the engine’s injectors.  Less than half of the fuel that the generator draws from the day tank is actually used for combustion; the remainder of the fuel is returned to the day tank by the engine, and at a higher temperature.  As continuous running creates a continuous fuel temperature increase, this can adversely affect the performance of the generator, up to, and including, generator shutdown.


There are two main contributors to overheating a day tank’s fuel:

  1. Ambient temperature.  Perhaps the day tank is outside, or on a rooftop, in a warm climate, or all of the above.  If the fuel in the day tank is already at 95 degrees F, for example, it’s going to rise fairly rapidly when the generator engine starts.  Unfortunately, in places like California, many power outages occur during the hottest days of the year, due to excessive demand on the grid.
  2. Day tank size versus generator engine size.  A large generator engine paired with a small day tank will increase the day tank fuel temperature quickly, regardless of any other environmental conditions.  Local, state, or national code may inhibit the installation of larger day tanks.


Return pumps can assist in decreasing fuel temperatures for generators under both scenarios.  By simply circulating the hot fuel out of the day tanks, and replacing it with cooler fuel from the main tank(s), the generators will continue to run, and run more efficiently.


This fuel circulation can be automated.  The day tanks can be equipped with Resistance Temperature Detector (RTD) probes, which will monitor the day tank temperature.  When the day tank temperature reaches a pre-determined threshold, the RTD will signal the master control system (“Controller”), which will start a day tank cooling sequence.  We sometimes refer to a day tank cooling sequence as, “level bouncing.”  A level bouncing sequence would look something like this hypothetical example:

  1. RTD reports temperature threshold met on Day Tank 1 to Controller.
  2. Controller activates Day Tank 1 Return Pump.  Day Tank 1 begins to pump out.
  3. Day Tank 1 reaches “Supply-Pump-On” lower level, which creates a Call For Fuel.
  4. Controller turns Return Pump off.
  5. Controller activates Supply Pump, and opens Day Tank 1’s inlet valve.
  6. Supply Pump fills Day Tank 1.
  7. Day Tank 1 reaches “Supply-Pump-Off” high level.
  8. Controller deactivates Supply Pump.
  9. RTD monitors temperature, and…
    1. RTD reports temperature threshold met on Day Tank 1 to Controller, and sequence repeats… or…
    2. RTD reports temperature acceptable.  No action occurs.

Return pumps are useful for far more than just pushing fuel back to a main tank.  They are an integral part of any system and do not only save us from a messy cleanup and a lot of explaining, but also enable us to truly and completely clean a system, cool a day tank, or just do a more thorough inspection.


For more information, or if you have any questions, please contact:


Lee Carnahan

District Sales Manager, West


209.890.9993 cell



Last summer a facility in Texas spilled 3,500 gallons of diesel fuel intended for one of their emergency generators. The fuel was pushed up through a day tank vent, ran across their parking lot, and into a pond adjacent to their property. The clean-up team recovered about 2,100 gallons of fuel out of the pond, but at a cost of about $300,000.

I was called to the site two weeks after the spill and took these pictures of the pond. It’s amazing how resilient nature can be in Texas. The only damage I could see to the pond was browned grass below the waterline. Now, ten months later, the pond appears to have fully recovered.


The generator fueling system for this facility was installed in 2013. From an inspection of the day tanks, all the instrumentation and safety devices met the required NFPA and local fire codes. However, I did not recognize the systems integrator who did the PLC controls. I suspected there was an error in the PLC program exacerbated by a system design that didn’t anticipate something going wrong.


The facility owner brought in a couple of sharp corporate engineers to autopsy the existing controls. They found errors in the PLC programming logic. A level sensor failed, showing a low fuel level in the day tank, so the PLC controls energized supply pumps to re-fill the day tank from the main storage tank. With the level sensor stuck, the PLC controls ignored all the other instrumentation indicating the tank was full, continued pumping fuel, and quickly overfilled the tank. The facility engineers thought the system started pumping fuel at about midnight. Facility staff coming on duty at 7 a.m. smelled diesel fuel, noticed the fuel on the ground, and shut off the pumps.


At first glance, the control sequences for diesel generator fueling systems are not terribly complicated, so local systems integrators are often hired to provide controls for fueling systems. However, to ensure fuel is always available to mission critical emergency generators, and fuel spills are prevented, the Preferred engineers—who specialize in the design of generator fueling systems—try to anticipate every likely failure mode:


–What happens if a level sensor gets stuck?

–What happens if an analog transmitter fails and produces 0 milliamps?

–What should the controls do if a pump fails to prove flow?

–What happens if there is a break in a fuel line, or a tank starts to leak?

–What happens if an operator manually energizes a fuel transfer pump and then goes home?


After supplying so many fueling systems over the years, all of these failures will happen. Regardless of a component failure or operator error, fuel spills are still unacceptable, and the generators still need fuel.


I did boiler controls for twenty years before learning how to design and commission fuel handling systems. NFPA boiler code dictates all the safety devices and sequences required to operate boilers. As a result, at least three separate devices must fail to run the water out of a boiler, or overpressure a boiler. NFPA code for fueling systems is much less specific. In fact, the fuel system that caused the spill at this facility didn’t violate any NFPA fuel handling codes.


In the end, this facility’s Preferred installer and consulting engineer commissioned the new Preferred fuel handling system controls. Commissioning is the process of simulating all the “What happens if…” scenarios described above and verifying the fuel system responds correctly to all imaginable upset conditions.


It’s the last thing we do on every fuel handling project.

David Eoff, BSME, MBA

Preferred Utilities, National Sales Manager


What happened the last time your house lost power? That email you were writing might have had to wait an extra half an hour, and your refrigerator might have warmed a few degrees. At most, ordinary power outages represent a minor annoyance to the home or office.

The situation is different at the massive data centers of the world. Amazon now sells over 600 items per second, and their systems are designed to accommodate up to 1,000,0000 transactions per second. At this scale, a 20 minute power outage at one of the data centers powering its store could cost Amazon millions of dollars in lost revenue.

To avoid this sort of catastrophe, the world’s big data centers strive to meet the Uptime Institute’s “Tier-Standards,” specifying various levels of guaranteed data processing availability, reliability, and redundancy. Meeting these standards requires avoiding single-points of failure — all components must have redundant backups.

One of the most critical components, of course, is the power supply system: without power, the flow of data grinds to a halt. Although massive data centers pull their power from the public electric grid, they must have redundant systems of backup power ready to go. Stored power in batteries is important, but the real backup system is the diesel generator.

Managing the reliability and redundancy of their generator systems is a significant challenge for data centers. It’s an unfortunate reality that components break and systems fail tests. At many data centers, the fuel system supplying the generator will have components from a legion of vendors, not one of whom will understand (or take responsibility for) the whole system. This can make troubleshooting routine systems failures a nightmare.

Working with a company that provides a fully integrated system is essential – from the fuel tanks and pump systems to the monitoring devices and control systems. Therefore if a problem arises, data centers have a single support call to make. A single source contact will understand how the pieces work together and can quickly solve problems. It’s the difference between working with a parts manufacturer with a few engineers on staff, and an engineering design firm that manufacturer’s the parts.

At Preferred Utilities we specialize in fuel systems—it’s what we do all day, every day—we pride ourselves on designing reliable systems that reduce the need for support calls in the first place. Data center engineering teams are generalists and great at looking at the big picture, so when it comes to fuel systems, they often aren’t able to immerse themselves in the details the way our engineers do. We know the code compliance specs, how to make sure the tank size is correct, and how to optimize virtually any scenario to help data centers at all Tier levels to keep the their fuel, power, and data flowing.

If your company or industry requires this kind of technical expertise, you can reach Preferred Utilities Manufacturing Corporation at (203)-743-6741. We are dedicated to your success. People. Products. Results.


RFO-headerDiscussions of sustainable energy don’t often include food flavorings. However, the same process that creates liquid smoke—the stuff you can buy at the grocery store to add a smoky flavor to just about anything—can produce liquid wood, a very environmentally friendly fuel.

You may not have heard of liquid wood because, until very recently, it was quite difficult to burn effectively. Preferred Utilities Manufacturing changed this.

Liquid smoke is part of a family of products whereby wood is converted from a solid into a liquid. Wood pulp is heated in the absence of oxygen during a process called pyrolysis. This produces bio-oil—or liquid wood.

Unlike petroleum or natural gas, liquid wood fuel is a 100% renewable resource: the wood used to create the fuel can be balanced by replanting new trees. Liquid wood is also carbon efficient because the replanted trees offset carbon emissions, which eliminates the need to purchase separate carbon offsets. As a result, liquid wood is 81 percent more carbon efficient than natural gas, and 88 percent more carbon efficient than petroleum.

Once it’s being properly fed to the burner, liquid wood behaves pretty much just like traditional fuel oils. This means that existing boiler equipment can be retrofitted for use with liquid wood, dramatically decreasing conversion costs compared to other biofuels.Green Oil

So why haven’t we seen the widespread adoption of liquid wood as a fuel oil? After all, the basic chemistry isn’t new—liquid smoke has been around for more than 100 years. Ensyn, an Ontario biofuel firm, has become adept at producing competitively priced liquid wood fuels, but very few companies have been able to offer reliable systems to burn these fuels, and none have been successful in the marketplace—until now.

Ranger-Brochure-ClipOne of the keys to burning liquid wood is the pump system that delivers the fuel to the boiler. Liquid wood has to arrive in the boiler at much higher and more specific pressures than natural gas or petroleum, and because it is highly acidic, the pipes must be high-grade stainless steel. This all requires advanced pumping and monitoring equipment, combined with the engineering chops to put the whole system into place. That’s where Preferred Utilities shines.

As a hybrid engineering/manufacturing firm, Preferred is uniquely equipped to devise and implement customized solutions to help commercial and residential properties including universities, colleges, hospitals, and more convert their boilers to liquid wood. Compared to other biofuels that can’t be retrofitted to existing systems, such as wood chips or pellets, the logistics and upfront investment of converting to liquid wood for heating fuel is quite reasonable.

But handling the fuel is one thing. Burning it? Another thing entirely. We’re talking about a substance that is 25% water with the consistency of lemon juice. Burning it effectively presents a significant challenge. That’s why Preferred Utilities developed the Ranger Combustion System. As of May 2017, Preferred Utilities burners are the only known burners capable of effectively and reliably firing liquid wood. There are several installations in Ohio, Vermont, and Maine currently burning this fuel with Preferred Ranger Burners.


Liquid wood also presents an opportunity to go green quickly. It can take years to transition to carbon neutral, but a liquid wood conversion can be completed in a matter of months. We have found that in many cases this extraordinary fuel source can reduce carbon emissions by about 80 percent. For more information about the potential of using liquid wood at your establishment, contact Preferred Utilities at (203) 743-6741.





Preferred Featured in Today's Boiler


The Spring 2015 edition of Today’s Boiler magazine published Dr. Jianhui Hong’s article “NOx Emissions Reduction Strategies.” In the article Dr. Hong explains the importance of low NOx and provides solutions for complying with low NOx emissions regulations.

He explains that “NOx is a term used to include two important air pollutants: NO (nitric oxide) and NO₂ (nitrogen dioxide). These pollutants are sometimes called mono-nitrogen oxides…NOx is a term used to include two important air pollutants: NO (nitric oxide) and NO₂ (nitrogen dioxide). These pollutants are sometimes called mono-nitrogen oxides.”

NOx gases are harmful in a number of ways. Exposure to NOx gases is harmful to human health by irritating the mucous membranes and penetrating the lungs, “causing oxidizing damage to the tissues.”

When NOx reacts with water or water vapor, “it forms nitrous acid (HNO₂) and nitric acid (HNO₃). These acids in the rain can make ‘acid rain.’ Acid rain can damage plants and man-made structures such as buildings, bridges, and outdoor sculptures.”

In highly populated areas, “NOx emissions from combustion processes are primarily in the form of NO. In the air, NO reacts with oxygen to produce NO₂. In the presence of sunlight, NOx can react with hydrocarbons, especially VOC (volatile organic compounds) in the air to form ground-level ozone, which is an important ingredient of smog. The reddish brown color of the hazes hanging over the skies of some major cities comes from NO₂ gas… It can cause irritation to eyes, noses, throats, and lungs. It can even cause asthma and other chronic lung diseases such as emphysema and chronic bronchitis.” NOx can also combine with moisture in the air which “induces changes in phytoplankton and produces toxic brown or red algal blooms (i.e. “red tides”). The algal blooms can cause the death of other plants and marine animals in the water.

Thermal NOx, Fuel NOx, and Prompt NOx create the conditions for NOx emission.

Dr. Hong presented five solutions for the combustion community to reduce NOx emissions.

  • Flue Gas Recirculation “targets the thermal NOx by reducing the peak flame temperature and also oxygen concentration…The use of external FGR increases the requirements for the combustion fan” which “become a significant factor in the overall costs of the burners (including fixed costs and operating costs).”
  • Steam/water injection “works similarly to external FGR. It targets thermal NOx by reducing peak flame temperature and oxygen concentration.”
  • Ultra lean premixing “aims to reduce the flame temperature by staying away from stoichiometric condition. Ultra Lean Premixing, if used alone, has the downside of high oxygen level (up to 9%) in the flue gas, and the loss of fuel efficiency due to the very high excess air.”
  • Air Staging supplies combustion air in two or more stages. “The general goal is to reduce flame temperature, and create fuel rich conditions in the early stages, before the final stage of air is supplied.”
  • Fuel Staging supplies fuel in two or more stages. “The general goal again is to reduce peak flame temperature. This technique is often combined with Ultra Lean Premixing to overcome the efficiency issue of the latter.”

In conclusion, Dr. Hong cites that although NOx emissions can be controlled by the mentioned techniques, “the most cost-effective methods tend to be combustion modifications, especially using low NOx and ultra-low NOx burners.”

Dr. Hong’s greatest contribution to the combustion community is his ability to present solutions for common problems in the industry and the stringent regulations placed on emissions.

Read the full article here.



Danbury, CT – A boiler system functions as a critical component to the continuous operation of a facility.  The loss of a boiler can cause disruption of operation and significant loss. Thus, it is important to maintain safe, reliable, and efficient operation while minimizing any downtime of the boiler system.

A boiler system consists of many sub-systems working in harmony, such as the boiler, the burner and its control, boiler control including feed water and draft control, fuel oil handling system (if burning oil is required), water treatment, fuel gas booster system (for areas with low supply gas pressure) etc.These sub-systems are sometimes procured from multiple sources.  In order to deliver the safe, reliable and efficient service that the end user expects, it is advantageous to adopt a “full system integration” approach.

Possible Problems

A boiler system in general could have many modes of failures.  Failures in water level control have serious implications on the longevity of the boiler and in safety (the sudden inrush of feedwater to a baked-dry boiler could lead to a steam explosion). Water treatment failures can decrease the longevity and efficiency of the boiler. Boiler operators need to understand these dangers. Among all sub-systems, the burner system is by far the most sophisticated subsystem in a boiler system. The burner system has many modes of failure that require extensive training and/or experience for the boiler operators to fully understand.

When a boiler system is not delivering satisfactory performance to the end user, it is sometimes difficult to pinpoint the exact cause of the problem. The following example is used to illustrate this difficulty. Sometimes a burner makes a low frequency noise, often called a combustion rumble. The rumble could be a nuisance or discomfort to the operators and residents nearby, or could even cause damage to property. Potential causes of the rumble include, but are not limited to:

  1. The burner has poor stability at certain firing rates; or the burner’s window of operation is too narrow. This could be related to the design or manufacturing of the burner.
  2. The air/ fuel ratio is improper due to poor commissioning or lack of maintenance.
  3. The servos used by the burner control may have poor accuracy or repeatability.
  4. The linkage between servos and dampers may be loose.
  5. The system does not have oxygen trim to ensure consistent excess air levels. Any variation in draft, ambient temperature, fuel gas composition, building ventilation (affecting building inside pressure vs. outside ambient pressure), or wind speed blowing on outlet of chimney, can affect the amount of combustion air supplied by the fan.
  6. Lack of draft control.  Severe draft variation may cause the air/fuel ratio to go out of range.  This is definitely a challenge if the system does not have an oxygen trim system; it can be a problem even with an oxygen trim if the draft variation is too severe for the oxygen trim to compensate.
  7. The “acoustic coupling” between the burner and the boiler’s fire chamber and the subsequent space the flue gas flows through.
  8. The fuel gas booster could surge and cause the gas pressure to oscillate, beyond the pressure regulator’s ability to regulate.
  9. The boiler room’s ventilation system could be improperly designed. When windows and doors are shut, a significant negative pressure can develop in the boiler room, causing a drop in combustion air supply and air/fuel ratio.
  10. Fuel gas supply pressure and composition can fluctuate, especially if the fuel gas is from an alternative fuel source, such as land fill gas or, to a lesser degree, digester gas.
  11. Burner components may not work well together. For example, the gas regulator may be over-sized for the flow rates of the burner.

Problems with the Multiple-Vendor Approach

Fully integrated custom controlsWhen the subsystems are procured from many different vendors piece-meal (by the general contractor or the end user) and no engineering firm takes responsibility for integrating these subsystems, it may be difficult to identify the party responsible for correcting the problem. This often results in blame shifting among different parties, ultimately frustration for the end user.

For example: in a piece-meal approach, the burner may be supplied by a burner company, the controls may be supplied by a company that is solely dedicated to burner controls and knows little about the combustion behaviors of the particular burner. The specifications do not call for a draft control or oxygen trim, when in reality one or both of those may be required for the site conditions and requirements. The booster, if there is one for the job, may be supplied by yet another vendor, the commissioning may be done by a contractor, the ventilation system of the boiler room may not have been designed properly to avoid high negative building pressure.  The troubleshooting process itself is further complicated by the diverging interests of the different parties involved.

Sole Source Responsibility

The most important advantage of the full system integration approach is that the integrator must accept sole source responsibility. If the burner system does not perform, the integrator is responsible for correcting the problem. There is no blame shifting among different suppliers.XPlus

A burner system supplier that adopts the full system integration approach is inclined to build a long term relationship whenever it sells a job. The supplier would look at the specific conditions and requirements of the customer, and look for the best solution tailored for the job instead of chasing the latest trendy requirement in specifications. For example, it may be tempting to ask for a 12:1 or higher turndown from the burner system, but can the non-condensing boiler operate at 12:1 or higher turndown without condensation and corrosion problems?  Is 10:1 or 8:1 turndown enough for the job? In another example, does the system require a draft control device to work? Can the burner deliver satisfactory performance without the draft system?

A supplier adopting the full system integration approach would look at total costs of ownership (the fixed costs and the operating costs) for the boiler system, instead of focusing on the fixed costs. In today’s corporate procurement practices, too often the one responsible for buying the boiler system is not the one paying the energy bill, hence there is less incentive to consider the total costs of ownership.

For example, a burner capable of operating at 1.5-2.5% oxygen during the majority of its operation time can lead to significant savings in fuel costs.  If a vendor offers a burner system without use of  oxygen trim, is the burner operating at consistent excess air levels all year round? Does the lack of oxygen trim mean conservatively high excess air levels? In another example, a fiber mesh burner may be used to meet 9 ppm NOx requirements without FGR, but the additional costs of fuel due to the very high excess air levels (typically 8-9% oxygen dry in flue gas) and the costs of replacing filters and fiber mesh combustion heads need to be factored in when purchasing a burner system. In another example, a burner constructed with flimsy, low grade sheet metals may need frequent service and replacement parts, while a burner constructed out of durable steel can provide years of service beyond the normal warranty periods.

The “full system integration” approach requires an integrator to have in-depth understanding and strong product offerings in all of the following areas:

  1. Boiler controls. The boiler controls ensure safe and smooth operation (water level control, burner firing rate based on temperature or pressure, draft control if necessary). It should have the capability to manage the lead-lag control of multiple boilers to ensure the boilers are operating at maximum efficiency.
  2. Fuel oil handling systems (main tank, day tank, pump sets, filtration, leak detection, etc.)
  3. Burners–especially those designed for both high efficiency and low emissions at the same time. The end user should not be forced to choose between high efficiency and low NOx.  High turndown (such as 10:1) helps avoid cycling of the boiler, and low excess air minimizes loss of heat to flue gas. Use of FGR is acceptable, but the incremental costs of running a larger motor due to FGR should be factored in. Advanced designs of burners can achieve mandated NOx emissions with less, little, or no FGR (depending on the NOx levels required).
  4. Burner controls.  The burner must be equipped with the latest Burner Management/ Combustion Control Systems (BMS/CCS) to assure that safety aspects are in accordance with the latest requirements of NFPA 85 and CSD-1. When high efficiency or tight emissions are required,  an oxygen trim system should be included, and parallel positioning or fully metered control should be used in lieu of jackshaft. The combustion control and the servos should be designed to modulate the controlled fluids (air, fuel, FGR etc.) in a coordinated manner.  For example, if the air servo cannot move fast enough to be in sync with the fuel servo, then the fuel servo needs to be slowed down in modulation, and vice versa.
  5. Commissioning and maintenance.  The burner system is commissioned and maintained by qualified service technicians that are knowledgeable about all the subsystems.
  6. Technical support and spare parts. These should be available from nearby locations.

Preferred Utilities Manufacturing Corporation has earned a reputation for accepting single-source responsibility. We firmly believe in the advantages of full system integration. Compared to the piece-meal approach, the benefits of full system integration make the choice clear. If you believe the same way, please contact us about your next project.


Be sure to check out Preferred Utilities’ contributing article to Today’s Boiler written by Robert Frohock, PE.

From 5 Things You Might Have Missed In NFPA-85″

“Reality isn’t always what you’d assume when it comes to boilers, their controls, and their plants. Several lesser-known aspects could prove useful, so take some time and crack the rest of the code.”

You can view the online version of the magazine by clicking this link (page 10).

Robert Frohock, P.E. is the engineering manager for Preferred Utilities Manufacturing Corp. (Danbury, CT). Find out more at www.preferred-mfg.com.


By David Eoff

Danbury, CT – In 1964, Preferred Instruments published an article in the Fuel Oil & Oil Heat magazine. During that time, draft controls were used primarily to control excess draft from tall chimneys and lower excess air to conserve fuel. (Heating oil was 25¢ a gallon in 1964!) Additional benefits included more reliable burner performance, reduced burner emissions, and increased safety by tripping a boiler off line if the draft turned positive.

Check out the first page of that article below.

Draft Control, 1964

Draft Control, 1964

Today, draft controls are still common on all types of boilers,  but for very different reasons. Namely, boiler construction. Since then, many more boilers were of brick-set construction, required to be run at negative draft or balanced draft pressure. Because the furnaces were not air tight, the furnace walls were kept cool by a constant stream of cool air drawn in by the slightly negative pressure of the furnace. Allowing these furnaces to “go positive” for even a short amount of time could result in damage to the boiler casing or injury to boiler operators. Boilers made in this era typically had tall stacks to induce a negative pressure (or draft) in the boiler, or induced draft fans. To control the negative pressure generated by a tall stack or an induced draft fan, stack outlet dampers were installed and controlled to maintain a setpoint typically about 0.1” negative pressure measured at the back of the furnace. Then as now, proper draft control was also important for flame stability and maintaining the correct fuel air ratio in the boiler.

There are still many balanced draft boilers in operation that require draft controls for the same reasons they did in 1964. However, even airtight forced draft boilers built today often need draft controls to help stabilize burners using flue gas recirculation for NOx control. Flue gas recirculation is often induced by the combustion air fan. If the stack draft is too negative, the forced draft fan will not be able to induce enough flue gas to meet the required NOx emissions. If the stack draft is not repeatable, the fan will induce varying amounts of flue gas recirculation that will make the fuel air ratio control unstable. Burners utilizing high flue gas recirculation rates, and ultra low NOx burners have narrow limits of flammability and require precise fuel air ratio control. The combustion controller can’t precisely control the air flow through the burner if the boiler draft is constantly changing.

Boilers that operate with excessively negative pressure will draft too much air through the furnace, resulting in poor burner turndown and poor efficiency because excess air cannot be controlled–especially at lower firing rates. When these older boilers are retrofitted with low NOx burners using flue gas recirculation, the high draft condition needs to be controlled because the recirculated flue gas is diluted by fresh air (called tramp air) that leaks through the boiler casing. The diluted flue gas is less effective at reducing NOx. To meet typical NOx guarantees, effective draft controls need to be installed, and the boiler casings often need to be repaired to reduce air leakage.

Boiler construction today is almost entirely different, but draft controls are still required in many applications for mostly different reasons. Boiler combustion chambers now are entirely steel encased and air tight. The burners always include forced draft fans sized to pressurize the burner windbox, the furnace, and sometimes even part of the stack. The boiler and breeching are designed to withstand this positive pressure without the need for cooling air.

Today draft controls are required to accurately maintain draft conditions in the furnace and compensate for changes in outside conditions including:

  • Changes in ambient air temperature
  • Changes in stack temperature as the boiler warms up or changes firing rates
  • Changes in wind velocity blowing across the stack
  • Changes in draft conditions caused by multiple boilers connected to a common breeching.

Precise draft control is required now because we expect the burner’s fuel-air ratio controller to hold excess air levels typically below 15% at high fire to conserve fuel. The fuel-air controller can’t effectively maintain low excess air levels when draft conditions are changing.

Just as importantly, modern low NOx burners are more sensitive than their 1964 counterparts. Ultra low NOx burners are extremely sensitive to draft conditions (and ambient temperature, stack oxygen, phase of the moon, operator’s attitude, etc.) Too much draft can cause the burner to run lean, become unstable, and flame out. Too little draft can cause the burner to burn back into the burner internals and damage equipment. Most burner manufacturers require draft controls be installed with their burners if any of the following conditions are present:

  • Boiler stack is taller than 100 ft. (sometimes 50 ft. is the limit)
  • An induced draft fan is running in the stack
  • Two or more boilers share a common stack

If your applications meets one of the above conditions and you don’t install draft controls, the burner manufacturer will offer little help if you experience burner stability problems during start-up. A typical response will be, “Install draft controls and call us if the problem persists.”

To meet the increasingly demanding requirements of modern low NOx burners, draft controls have become much more sophisticated.

–Draft range transmitters have replaced the diaphragms in direct-sensing draft controllers. This is an important advance because the sensing line and diaphragms in direct-sensing draft controllers were prone to fill with condensation and quit working. Transmitters have this same weakness, but are easier to install higher than the breeching tap to ensure they stay dry. Draft transmitters usually include filters to help reject boiler pulsation and transmit just the boiler draft. Once the draft signal is digitized, it is easier to manipulate in a digital controller.

–PID controllers have replaced proportional only controllers. Although the derivative component of the PID control is rarely used, the integral component helps the controller respond faster to quick load changes. Floating point, and gap PID controls utilize a lower proportional gain when the draft is close to setpoint to help eliminate controller oscillation during normal operating conditions.

–Firing rate is often used as a feed-forward input to the draft controller. During commissioning, the technician determines the best draft damper position at 20%, 40%, 60%, 80%, and 100% firing rates. During quick load changes the draft controller monitors the burner firing rate and quickly moves the stack damper to these predetermined positions. As the firing rate begins to level out, the PID controller takes over again to trim the damper position to hold the draft setpoint for that load.

–Modern draft controllers have an adjustable start position—a separate damper position or draft setpoint used only for burner lightoff. If the technician is fighting an unstable pilot, he can position the draft damper or draft setpoint where he needs it to ensure a stable pilot flame and reliable main burner lightoff.

–A digital draft controller can generate a high or low draft pressure alarm, a low draft pressure shutdown contact to the burner management system, and can communicate digitally to a plant-wide control system.

Although positive pressure boiler designs have reduced the need for draft controls, the sensitivity of low NOx burners has actually increased the use of draft controls in recent years. As burner performance standards for low excess air, low NOx operation have increased, the performance requirements for draft controllers have increased proportionately. As the inset article at the beginning of this post shows, Preferred Instruments was one of the earliest providers of boiler draft controls. Today, Preferred Instruments continues to manufacture the most advanced draft control products available to handle any draft control application.

The JC-22D stand-alone draft controller interfaces with most burner management system and combustion controller to safely monitor and control furnace draft in virtually any application.

The Preferred JC-22D


The JC-22D stand-alone draft controller interfaces with virtually any burner management system and combustion controller to safely monitor and control furnace draft in virtually any application.”


BurnerMate Universal

The BurnerMate Universal boiler controller incorporates burner management, combustion control, feedwater control, and draft control for complete boiler control in one easy-to-use package. The BMU incorporates all the draft control functions of the JC-22D. Because it is integrated with the other boiler control functions, the only field device required is a draft transmitter and draft damper actuator.

The BurnerMate Universal

The BurnerMate Universal