Time, Temperature and Turbulence  

  Combustion efficiency can be further explained in terms of the three T’s; Time, Temperature and Turbulence.

Time

For example, in oil heat systems, the amount of time oil vapor has to combust (or reside in the flame front or burning zone) has been improved dramatically with the advent of flame retention burners.  The primary difference between this type burner and the older conventional style burner is that the flame retention burners violently spin the air/fuel mixture resulting in better mixing.  This reduces the amount of excess combustion air necessary to insure each fuel droplet is completely surrounded by oxygen and burns completely.   As the amount of combustion air is reduced, efficiency increases.

O₂ readings within the manufacturer’s specifications document that the burner’s flame retention properties are operating as designed and engineered.

With gas fired systems, air and fuel mixing occurs inside the burner.  Again, increasing the amount of time the fuel and air have the opportunity to thoroughly mix will help insure complete combustion.

There is a common misconception that the shutter on an atmospheric burner controls combustion air intake.  While some degree of combustion air does enter through the shutters, draft remains constant.  As the shutters are closed, more combustion air is drawn in through the ‘secondary’ air intake, thus, little combustion air control is provided.

Adjustable air intake shutters on atmospheric burners primarily control the velocity at which the fuel/air mixture moves down the burner throat or ‘mixing area’.

This can be demonstrated by taking a combustion test while shutters are adjusted.  Unless the shutter is completely open or closed, little difference in the O₂ / CO₂ readings will be observed.

 Increasing the amount of time flue gases are in the heat exchanger also increases the amount of time for heat transfer to occur.

O₂ and stack temperature readings within the manufacturer’s specifications document that the flue gases are moving through the heat exchanger at a velocity which allows time for maximum heat transfer while still permitting the introduction of additional combustion air.

Temperature

As the temperature difference (DT or Delta T) between the source of heat and the material being heated increases, so does the rate of heat transfer.  This heat transfer rate is measurable in forced air systems and boilers.  By reducing the amount of combustion air introduced into the combustion process to the absolute minimum necessary, we increase the DT between the flame/flue gases and the distribution air or boiler water.

Radiant heat energy is much more effective in transferring heat than convective/conductive heat transfer.  It is primarily put off by the flame itself and is directly related to flame temperature.  As excess air is reduced, the higher flame temperature generates more radiant heat energy.

The burn rate in combustion process is very sensitive to temperature.  If flame temperature is increased by 10%, the rate of combustion more than doubles.  Unfortunately, the same increase in flame temperature also increases production of NO_x gases by more than 10 times when sufficient O₂ is available.

To estimate the actual flame temperature, continue the horizontal line from the top of fuel to the point where it intersects the line corresponding to the percent of excess air.  From the point of intersection, draw a vertical line up to the point where it intersects the combustion air temperature line (80°F); Then draw a horizontal line to the vertical axis – temperature rise °F.

For example:

A natural gas burner operating with 25% excess air (4.5% O₂) has an estimated flame temperature of 2930°F plus 80°F (combustion air temperature) equals 3010°F.

Note:  Deduct approximately 11% for gas and 4% to 6% for oil due to water vapor in the flue gases.

Note that in this example  burning natural gas, a 25% excess air translates to a 4.5 O₂ reading and a 20% excess air translates to a 4.0% O₂ reading on the analyzer.  In other words, a ½ percent decrease in the O₂ reading represents an almost 200° increase in flame temperature.

O₂ readings within the manufacturer’s specifications document that the flame temperature has been maximized for the most efficient radiant heat production and transfer.

Turbulence

Turbulation of the fuel, air and heat source provides for more complete combustion by keeping these components in contact with each other for a longer period of time.

Agitation of flue gases in a heat exchanger serves to provide a continual circulation of hotter flue gasses in contact with the heat exchanger surfaces.  Typically, heat exchanger surfaces have a wide variety of irregular surfaces incorporating bumps, ridges etc. to provide this effect.  Boilers and domestic hot water heaters commonly have turbulators that provide for this mixing process.  These surfaces also produce eddy currents that recirculate flue gases and increase the amount of time those flue gases remain in the heat exchanger. 

These turbulators and irregular heat exchanger surfaces are designed to be most effective at the appliance’s full firing rate.  Under firing a burner results in the smaller volume of flue gases taking the path of least resistance through the boiler thus reducing the scrubbing and mixing of the flue gases against the fire side of the heat exchanger.

Again, O₂ readings within the manufacturer’s specifications verify that the burner is firing at full capacity, maximizing efficient heat transfer.