Combustion Blog

There are many different kinds of burner control systems, each unique to its design, performance and intention. One such system uses what some people refer to as a Bias Proportioning Valve system or Ratio Regulator control. In either referred name, it’s the same. The ration regulator is used for burner control where the fuel gas and combustion air need to be proportional throughout the burner’s fired range. The control of gas flow is accomplished by an air pressure signal applied to the top of the regulators diaphragm. As the combustion pressure is increases, the pressure force is transmitted to the regulator diaphragm by a loading line and causes an opening movement of the regulator valve. As the gas flow through the valve increases, so does the pressure on the outlet side of the valve. Things come into balance between the inlet and outlet pressures and the result is the ratio of the outlet gas pressure and air signal pressure reach about 1:1.

 
Since we all need some means of adjustment, so do ratio regulators. There is “a bias adjustment” that is used to vary the gas flow when setting the burner at low fire. This adjustment is used to increase or decrease the gas flow which will result in a more rich or lean combustion mixture. Since all regulators usually have a flow capability that is greater than the burners needs at high fire, a gas flow limiting orifice is applied. This orifice is placed between the ratio regulator outlet and gas inlet to the burner in order to trim the high fire or maximum flow setting. The key ingredient to success with ratio regulators is to keep the two adjustments separate and adjusted only at the appropriate point of control of each. The Bias Adjustment on the regulator should only be made at the low fire setting. Once this is accomplished, it should not be changed unless the burner is at the low fire setting. The adjustment of the ALO or limiting orifice is made only at the high fire point of the burners set up. To adjust either one while the system is at any intermediate point of control ends up changing everything. If that is done, all bets are off on where that low fire setting ends up or high fire!

 
To learn more about what regulator to select, things you need to know like “what is the right inlet fuel pressure setting?” or how to calculate pressure drops and selection of the right regulator. These can be found in the Eclipse Product Catalog. Go to the selection for Shutoff and Control Valves, click on 742- Eclipse Ratio Regulators. The easy steps there apply to any proportioning style ratio regulator and taking the time to check it out can save a lot of time trying to set things up. The burner manufacturer selects the right valve for the burner, but it’s always good to know how it works, why it works and how to set it all up. Figure 1 below shows in color how a typical ratio regulator looks, how the gas flows, the air signal is applied, diaphragm and springs. Figure 2, is a line drawing of how a typical proportioning control system is connected. Check out the Tech Notes section of the Eclipse Engineering Guide and it explains many of the different types of control set ups! Now all the mystery is gone, it’s just another kind of regulator!

Thanks to all of our workshop attendees that all gained a greater knowledge or combustion.

We had over 40 attendees from 15 different companies including:

• Buhler Aeroglide
• Heatec Inc.
• Owens Corning

During the course of your endeavors to keep burner systems, furnaces and any gas fired heating equipment going, you hear someone ask or comment “what is the turn down”?

To understand what is meant is sometimes a challenge. If it is to refer to the burner system, the definition is:
"The range of input rates within which a burner will operate."
The maximum a burner can fire depends on how the burner is designed. The minimum the burner will "turn down" is based on that maximum rate. Some burners can have as much as a 50:1 or more turn down while other types of burners are designed for only 2 or 3:1 turndown.

For an oven or furnace, it is the maximum to minimum range it is expected to operate. This may take into consideration things such as load, idle running or both.

The big deal in all of it comes when the burner system is attached to a furnace or oven. The expectations of the performance of a particular oven or furnace may be much different from the performance of the burner. It could be that at one time, they were matched quite well, but as expectations changed, such as production level increases, decreases or the type of product being processed the effects to furnace turn down changes. These can all contribute to pushing the system, both burner and oven to new higher limits or new lower limits.  

So how do you know when things just are not going right, if it’s a burner problem or something more?  The key is to understand the relationship between oven turndown and burner turndown.  It is important to know the difference of one to the other.   With the help of the simple chart shown, you can ask yourself those questions and visualize just where the problem may be. First think of the furnace or oven’s expected turn down range. This would be the operational temperatures expected to be achieved. On the chart, this is the range shown as Furnace Turndown in the top blue bar. This range spans the two side blue bars of the graph.  If you find your burner seems to be slow to respond to load increases or cannot reach temperature settings, it just may be the burner system turn down is not matched to the expected turn down of your oven.  This may require re-thinking your burner system.

If you find your burner system just cannot reach the lower set points or reach the higher set points, this may be a combination of lacking capacity and turn down capability.  Many other factors can be considered, but now you can see that turn down of the oven or furnace may not be the same as the turn down of the burner system. Sometimes the mission of the oven or furnace has changed. This could be a result of higher expected production levels, faster through puts or even product or process changes.  “Just because you want to, doesn’t mean you can” But at least you know what turn down means, and another tool in your tool box of knowledge!

More of this information is covered in the Eclipse Combustion Workshop.
Article by Bill Wheeler, Development Engineer
bwheeler@eclipsenet.com 

When: Tuesday, March 05, 2013 8:00 AM - Wednesday, March 06, 2013 5:00 PM

How much does YOUR air weigh?

Winter is here, and we are in the “THICK” of it. (pun intended)  Those combustion systems in service with the cold weather are working even harder to produce the required heat. When your system operator walks up to you with the complaint, “The oven is just not getting up to temperature with the same process load today!”  Or, your belt speed on that conveyor has to be slowed down to keep from running the process out of spec. Loss of production and loss of money!

With the outside air being in the low teens or even the single digit level, its density is greater or could be said, “Thicker” then it was during the summer. Airplane pilots are keenly aware of this, and calculations to pilot their aircraft safely depend a lot on what is termed “Density Altitude” conditions. In the warmth of Summer, the warmer thinner, lighter air requires more room (cubic feet per lb) as compared to the air we breathe now in the cold months of winter. Your combustion systems are the same. In the summer, those combustion air dampers and butterfly valves are adjusted to give the required cubic feet or pounds of air per cubic feet-pounds of fuel. Since the air of summer is warmer, and many combustion systems draw their combustion air from an outside duct, that change in density can be a real factor in the fuel air mixture. If those same burners are adjusted during the cold winter, and operate into summer conditions, the reverse effect occurs, lighter and less density, the combustion system is getting less pounds or cubic feet of air per pound-cubic feet of fuel.    

Air at 60F is compared to the same air at 600F.              

Looking at this from a burner adjustment level, you may need to cut back on the combustion air to regain that balance of air to fuel ratio.

A typical burner of 3.5 million btu/hr capacity running on a 8000hour/year schedule with just the temperature change from around an 85F combustion air temperature that drops to 55F (That is just a 30 degree drop in temperature) could cost almost $1,000.00 added running cost in the winter if not checked. To add to this problem, the colder denser air can increase the % excess air from let’s say 3% O2 to 5% O2 (just a two point increase). That changes your % excess air from about 15% to about 30%. That is double! And the added cost of that winter operation to $2,500.00 over normal. That’s only one burner, and if there are more, that cost can be even higher.

Things that you may want to do? Check those systems for operational efficiency when there are big changes in weather conditions like winter/summer. You may even want to think about where that combustion air is drawn from or consider pre-heating your combustion air. Pre-heating can help even in the summer on some systems and make real pay back value.  So, the question is, How much are you willing to lose? That is, how much are you “over weight” on combustion air?

For more information on the effects of temperature and altitude on combustion, check out the Eclipse Engineering Guide and check out Exothermics for tips and explanations on heat exchangers and pre-heating combustion air.   

It’s cold outside, the heat is on but is your combustion system “breathing” ok?

In the industrial, and commercial world of burners, there are a lot of systems running right now serving both process and comfort heating needs. Those burners may have been adjusted for optimum running conditions, but as the cold and inclement weather conditions prevail, some things that seem simple and ordinary can go astray.

Most natural gas combustion fuel systems incorporate a regulator to allow delivery of the fuel gas at a controlled set pressure to any given appliance.  Those regulators in many cases are located at or near the appliance and indoors.  Most codes require under certain conditions to have those pressure regulators diaphragm case be “vented” to the outdoors and in many cases that vent is either through an exterior wall or up and out through the roof. With rain, snow and even fog conditions, those vents can accumulate ice at the vent opening. Since the opening is usually out on a roof or out back of the building in some inconspicuous place, it is often overlooked and can cause effects to the regulator that can range from complete failure of the appliance to work, or cause changes to the fuel/air supply balance of a combustion system and result in loss of efficiency.  A chart is enclosed that shows in figure 1 of a typical set of values for a burner that would compare to a process oven or boiler.  The “Current” column shows a flue gas temperature of 400 degrees F (a common approximate exhaust temperature of some ovens or boilers) along with a 3% O2 reading of the flue gases, combustion air at 70F and a burner rated at 3.5million btu/hr.     The Column “New”  would represent some changes that could occur if the fuel regulator vent was clogged possibly from ice conditions, and by limited diaphragm movement, could act to “starve” the burner of fuel. This would increase the excess air to the burner and also increase the %O2 of the burner as much as 2% or more. (This chart shows it to be 5% O2, an increase of just 2%).   

The chart shows in the Annual savings column based on the fuel cost of $ 7.50 per million btu (average), that this change of %O2 can cost as much as  $2,326.00 per year.  Based on some climates, this frozen vent can cost just this one case, almost  $200.00 a month while that vent remains frozen.  In some cases, systems can eventually fail to operate at all. So, just take a moment to check those regulator vents, and save operating costs, and possibly an unwanted burner shut down. Those usually come at 2am and can cause even more grief with frozen buildings, product and water pipes or in the middle of the highest required production level when you need the system to run at its best.