Saturday, August 8, 2015

All about Oil Burner Nozzles

All about Oil Burner Nozzles


Why do we use Burner Nozzles?

For a better understanding of how a nozzle fits into the performance of an oil burner, let us first review the steps in the process of efficient combustion.

  • Like all combustible matter, the oil must first be vaporized converted to a vapour or gas before combustion can take place. This is usually accomplished by the application of heat.
  • The oil vapour must be mixed with air in order to have oxygen present for combustion.
  • The temperature of this mixture must be increased above the ignition point.
  • A continuous supply of air and fuel must be provided for continuous combustion.
  • The products of combustion must be removed from the combustion chamber.

The simplest way to burn fuel oil is the old vaporizing pot type burner in which heat is applied to a puddle of oil, thus vaporizing the fuel. These vapours are then burned after mixing with the proper amount of air. In most applications, this method of vaporizing is too slow for high rates of combustion and cannot be controlled in the low rates, high leads back to the original question of why nozzles are used. One of the functions of a nozzle is to atomize the fuel, or break it up into tiny droplets which can be vaporized in a much shorter period of time when exposed to high temperatures. This booklet will be concerned primarily with the high-pressure atomizing nozzle since it is the most common in the Oil Heat Industry.

What does the nozzle do?

The atomizing nozzle performs three vital functions for an oil burner:

Atomizing As just discussed, it speeds up the vaporization process by breaking up the oil into tiny droplets something like 55-billion per gallon of oil at a pressure of 100-
psi (standard in the industry). The exposed surface of a gallon of oil is thereby expanded to approximately 690,000 square inches of burning surface. Individual droplet sizes range from .0002 inch to .010 inch. The smaller droplets are necessary for fast quiet ignition and to establish a flame front close to the burner head. The larger droplets take longer to burn and help fill the combustion chamber.

Metering A nozzle is designed and dimensioned so that it will deliver a fixed amount of atomized fuel to the combustion chamber within approximately plus or minus 5% of rated capacity. This means that functional dimensions must be controlled very closely. It also means that nozzles must be available in many flow rates to satisfy a wide range of industry needs. Under 5.00 GPH, for example, over 20 different flow rates and 6 different spray angles are considered standard.
Patterning A nozzle is also expected to deliver the atomized fuel to the combustion chamber in a uniform spray pattern and spray angle best suited to the requirements of a specific burner. More details on patterns and angles later.

So how does a Burner Nozzle work?

Now that we know what a nozzle is supposed to do, let’s see how it does it. But before we do, let’s take a look at the cutaway showing the functional parts of a typical Delavan nozzle. The flow rate, spray angle and pattern are directly related to the design of the tangential slots, swirl chamber and orifice.


Cross section of a burner nozzle


First, a source of energy is needed to break up the oil into small droplets. Therefore pressure is supplied to the nozzle, usually from a motor-driven pump at 100-150 psi. But pressure energy alone doesn’t do the job. It must first be converted to velocity energy and this is accomplished by directing the pressurized fuel through a set of slots which are cut in the distributor at an angle, or tangentially, to create a high velocity rotation within the swirl chamber. At this point, about half of the pressure energy is converted to velocity energy. As the oil swirls, centrifugal force is exerted against the sides of the chamber, driving the oil against the orifice walls, leaving a void or core of air in the centre. The oil then moves forward out of the orifice in the form of a hollow tube. The “tube” becomes a cone shaped film of oil as it emerges from the orifice, ultimately stretching to a point where it ruptures and throws off droplets of liquid.


Nozzle Selection.

T match nozzles to a burner takes experience trial and error and a good understanding, but if you are simply replacing a burner nozzle you can read the small number on the side of a nozzle and re-order the correct one. There number normall look something like 0.60 40 EH. The size 0.60, the angle 40 degrees and the pattern EH.

To insure consistent quality, every burner quality branded nozzle like Danfoss or Delevan is tested for flow rate and spray angle on modern, high instrumented test stands. Spray quality is observed during testing for uniformity, balance and flutter.





Nozzle Types or Spray pattern.

Hollow Cone Nozzle Types.

Hollow cone nozzles can be used in burners with a hollow air pattern and also for use in small burners (those firing 1.00 GPH and under), regardless of air pattern. Hollow cone nozzles generally have more stable spray angles and patterns under adverse conditions than solid cone nozzles of the same flow rate. This is an important advantage in fractional gallonage nozzles where high viscosity fuel may cause a reduction in spray angle and an increase in droplet size. Hollow nozzles produce a spray which delivers fine droplets outside the periphery of the main spray cone. These fine droplets greatly enhance ignition and create a stable flame for use with flame retention burners.

Solid Cone Nozzle Types

Solid nozzles produce a spray which distributes droplets fairly uniformly throughout the complete pattern. This spray pattern becomes progressively more hollow at high flow rates, particularly above 8.00 GPH. These nozzles may be used in larger burners (those firing above 2.00 or 3.00 GPH) to provide smoother ignition. They can also be used where the air pattern of the burner is heavy in the centre or where long fires are required.

Semi solid Nozzle Types

Semi Solid nozzles produce a spray which is neither truly hollow nor solid. These nozzles frequently can be used in place of either solid or hollow cone nozzles between .40 GPH and 8.00 GPH, regardless of the burner’s air pattern. The lower flow rates tend to be more hollow. Higher flow rates tend to be more solid.


Here is a table showing the codes for solid, semi solid and hollow nozzle codes for Danfoss, Delavan, Hago and Fluidics.


Flow Rate

Atomizing nozzles are available in a wide range of flow rates, all but eliminating the need for specially calibrated nozzles. Between 1.00 GPH and 2.00 GPH, for example, seven different flow rates are available. Generally, with hot water and warm air heat, the smallest firing rate that will adequately heat the house on the coldest day is the proper size to use and the most economical. Short on-cycles result in low efficiency. Another guideline is to select the flow rate that provides a reasonable stack temperature regardless of the connected load. (According to the New England Fuel Institute, aim for a stack temperature of 400°F or lower on matched packaged units or 500°F or lower on conversion burners.) If the boiler or furnace is undersized for the load, it may be necessary to fire for the load and ignore the efficiency

Spray Angles

Spray angles are available from 30° through 90° in most nozzle sizes to meet the requirements of a wide variety of burner air patterns and combustion chambers. Usually it is desirable to fit the spray angle to the air pattern of the burner. In today’s flame retention burner, it is possible to fire more than one spray angle with good results. Generally, round or square combustion chambers should be fired with 70° to 90° nozzles. Long, narrow chambers usually require 30° to 60° spray angles.


Effects if Pressure on Burner Nozzle Performance.

Normally, 100 psi is considered satisfactory for the fixed pressure supplied to the nozzle, and all manufacturers calibrate their nozzles at that pressure.

It is interesting to observe the sprays of a nozzle at various pressures. At the low pressure, the cone-shaped film is long and the droplets breaking off from it are large and irregular. Then, as the pressure increases, the spray angle becomes better defined. Once a stable pattern is formed, any increase in pressure does not affect the basic spray angle, measured directly in front of the orifice.

At higher pressure, however, you will note that beyond the area of the basic spray angle, the movement of droplets does make a slight change in direction inward. That’s because at this point the air pressure outside the spray cone is higher than that on the inside, which deforms the spray inward.

Pressure has another predictable effect on nozzle performance. As you would expect, an increase in pressure causes a corresponding increase in the flow rate of a nozzle, assuming all other factors remain equal. This relationship between pressure and flow rate is best shown in the table on page 13.

Increasing pressure also reduces droplet size in the spray. For example, an increase from 100 to 300 psi reduces the average droplet diameter about 28%. One last word on the subject: if pressure is too low, you may be under-firing the burner. Efficiencies may also drop sharply because droplet size is larger and the spray pattern changed. If pressure isn’t carefully checked, the marking on the nozzle becomes meaningless. Pressures of more than 100 psi are sometime desirable, but rarely do burners operate at anything less.

Nozzle Adjustments

Annual replacement of the nozzle is recommended. The nozzle size should match the design load. DO NOT OVER SIZE. (For determination of over sizing refer to publications listed on page 8.) Short cycles and low percent “on” time result in higher overall pollutant emissions and lower thermal efficiency.

An in-line oil filter will reduce problems due to nozzle clogging. It should be located as close as possible to the oil burner. Care should be taken to prevent air leakage into the oil suction line. Use continuous runs of copper tubing and use minimum number of joints and fittings. Always use flare fittings. Select the nozzle and spray pattern, whenever possible, using burner manufacturer’s instructions. On burner-boiler or burner-furnace matched assemblies, use the appliance manufacturer’s instructions. Bleed air from the pump and nozzle piping to avoid trapped air.

Nozzle Care and Service Tips

An oil burner nozzle is an intricate piece of hardware, designed to do an accurate job of atomizing and metering fuel oil in the spray pattern best suited to a given burner. You can help assure top performance of this vital component by following the important guidelines in this section

Until installation, keep nozzles in their original containers and preferably in a suitable box or rack. They should not be permitted to roll around in a drawer or toolbox, or carried loose in pockets. On service calls, they should be kept in a clean nozzle box.

Handle the nozzle carefully after removing from its individual container. Pick it up by the hex flats and by all means avoid touching the filter or strainer with greasy hands.
This can force foreign material into the nozzle where it can finally work its way into the slots. The possibility for problems is even greater with nozzles of lower flow rates since they have smaller slots…easier to clog

Nozzles should always be handled with clean tools… again to reduce the possibility of contamination. To properly service a nozzle and check its performance, it’s recommended that you use a pressure gauge, vacuum gauge, nozzle changer and flame inspection mirror.

Be sure the strainer or filter is in place on the nozzle before installation. Do not disassemble the nozzle before installing it because great care has been taken to make sure the nozzle is absolutely clean on delivery.

Frequently asked questions

I have a job on which it is difficult to clean up the fire. What should I do?

  • Check the fire to see whether it is off centre (see discussion of off centre fires next column).

  • Check the fan blades and if they are covered with lint and dirt, clean them.

  • If this is a conventional oil burner, check for a burned-off end cone.

  • Check the oil pressure to be sure that it is at least 100 psi.

  • Check for a plugged line filter or pump strainer..

  • If the smoke is at the outside edges of the fire, try a narrow angle nozzle.

  • If the smoke occurs at the end of the fire, it might be well to try a wider spray            angle.

  • The burner may have insufficient air capacity for this firing rate.

  • The burner may have too high air capacity for this firing rate. This would mean that the burner head couldn't handle a low firing rate.

  • It might just be a very inefficient burner, which cannot be made to burn clean.

  • The nozzle may be partially plugged, resulting in poor atomization.

  • The oil may have a high viscosity or it might be cold, resulting in a collapsing spray.

  • Get sufficient air into the boiler room. If necessary, put in an air intake pipe with a screen and storm cover.


If the burner sometimes fails to ignite smoothly and starts with a puff, what can be done to improve it?

  • Check the electrode points for proper spacing. The points should be 1/8” to 3/16” apart, 9/16” above the centreline of the nozzle and then spaced correctly forward from the face of the nozzle for each different spray angle.

  • Clean carbon and dirt from the points and from the insulators.

  • The transformer may be weak and not delivering full voltage or current. If you don’t have another transformer immediately available, file the electrodes to a sharp point. This will give a better spark.

  • Check for cracked insulators. Sometimes an insulator may be cracked under the electrode bracket and it is difficult to find. This could cause a high voltage leak, thus reducing the voltage at the points.

  • A partially plugged nozzle causing off-centre spray can cause delayed ignition.

  • Above about 2.50 gph a hollow cone spray sometimes may cause delayed ignition. Changing to a solid cone sometimes helps that situation.


  • The air setting on the burner may be wide open, thus tending to blow the fire out before it is established. Adjust the air correctly.
Information taken from Delavan re-worked by Spares Giant
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