An Introduction To Oil Burners For Heating Systems

An Introduction To Oil Burners For Heating Systems

introduction
An oil burner is a mechanical device used to prepare the oil for burning in heating appliances such as boilers, furnaces, and water heaters. The term oil burner is somewhat of a misnomer because
this device does not actually burn the oil. It combines the fuel oil with the proper amount of air for combustion and delivers it to the point of ignition, usually in the form of a spray.
The fuel oil is prepared for combustion either by vaporization or by atomization. These two methods of fuel oil preparation are used in the three basic types of oil burners employed in commercial,
industrial, and residential heating. The following are the three basic types of oil burners: 

1. Gun-type (atomizing) oil burners.
2. Vaporizing (pot-type) oil burners.
3. Rotary oil burners.
Gun-type atomizing oil burners are available as either low-pressure or high-pressure types (see Figure 1). Both are used in residential heating applications with the latter being by far the more popular of the two. 

FIG 1

The advantage of the vaporizing (pot-type) oil burner is its low operating cost. It is the least expensive to use(figure 2 ), but it has limited heating applications. It is currently used only in small structures
located in milder climates. Vaporizing burners can be divided into the three following types:
1. Natural-draft pot burners. figure 7
2. Forced-draft pot burners. figure 6
3. Sleeve burners. figure 8

FIG 2 

Rotary oil burners are commonly used in the heating systems of commercial or industrial buildings, although they can and have been used for residential heating applications (see Figures 3).
 The following types of rotary oil burners are available for heating purposes:
• Vertical rotary burners . figure 9
• Horizontal rotary burners  . figure 10
• Wall-flame rotary burners

FIG 3

1- Gun-Type Oil Burners
Gun-type, high-pressure atomizing oil burners are sometimes called sprayers or atomizing burners because they spray the fuel oil instead of vaporizing it. They are also referred to as gun or pressure oil burners because the oil is forced under pressure through a special gun-like atomizing nozzle. The liquid fuel is broken up into minute liquid particles or globules to form the spray.

Construction Details
The principal components and parts of a gun-type, high-pressure atomizing oil burner used in residential and light commercial oil heating systems are illustrated in Figures 4,5

FIG 4

FIG 5

The components and parts of a typical gun-type oil burner can be divided into the following
categories:
1. Burner control.
2. Primary safety control.
3. Gun assembly.
4. Ignition transformer.
5. Burner motor and coupling.
6. Fuel pump.
7. Combustion air blower.
2- Vaporizing (Pot-Type) Oil Burners
Figures 2,6 and7 show a typical vaporizing (pot-type) oil burner. The fuel oil is vaporized for combustion by heating it from below. The vaporized fuel oil rises vertically where it is burned at
the top. The following are the two basic types of vaporizing, or pot, oil burners:
1. The natural-draft pot burner.
2. The forced-draft pot burner.
In the former, the air necessary for combustion is provided by the chimney. The forced-draft pot burner relies on both the chimney and a mechanical device (e.g., a fan) for the air supply.
Sleeve burners (also referred to as perforated sleeve burners) represent a third type of vaporizing, or pot, burner. Although these burners are used mostly in conjunction with small oil-fired equipment
(e.g., kitchen ranges and space heaters), they can also be employed to heat a small house, if outside temperatures do not become too low.

FIG 6 The forced-draft  

FIG 7 The natural-draft 

FIG 8 Sleeve burners

3- Rotary Oil Burners
Rotary burners operate with low-pressure gravity and are available in a number of designs depending on the different conditions of use. In each case, the operating principle involves throwing the oil by centrifugal force. Rotary oil burners can be classified either as rotary nozzle or rotary cup burners. The essential components of the rotary nozzle burner are shown in Figure 11. Air pressure acting on the propeller causes the nozzle assembly to rotate at a very high speed. Oil is supplied through the hollow shaft to the nozzles, and the rotary motion causes the oil to be thrown off in a fine spray by centrifugal force. The flame from this spray heats up the metal vaporizing rim
hot enough to vaporize the oil spray as it comes in contact with it. Being thoroughly mixed with air, a blue flame is produced. On some designs, the spray vaporized by the vaporizing rim is superheated
by passing through grilles.

FIG 9 Vertical rotary burners

FIG 10 Horizontal rotary burners

FIG 11 

The rotary cup oil burner (see Figure 12) contains a coneshaped cup that rotates on ball bearings carried by a central tube. The fuel is supplied to the cup through this tube. In operation,
drops of oil, issuing from the oil feed tip, come into contact with the cup as shown; by centrifugal force the drops are both flattened into a film and projected toward and off the rim of the cup, as
shown in Figure 13. Because the rim is surrounded by a concentric opening of the casing, the oil is met by the surrounding blast of primary air with which it mixes, giving the proper mixture for
combustion.

FIG 12

FIG 13

4- Combination Oil and Gas Burners
Some oil burners are available with combination oil and gas firing accessories that make it possible to use either of these fuels in thesame burner. This is particularly advantageous in areas where lowcost gas is sometimes available. The combination gas and oil burner illustrated in Figure14 contains independent ignition and control systems for gas or oil.
One convenience built into these combination burners is that the oil burner components and parts are standard and require only conventional service procedures. The safety features include a standard cadmium sulfide detection cell and primary relay control.

FIG 14

Flame-Retention Head Burners

Most oil furnaces and boilers prior to 1980 were installed with cast-iron head burners that had an efficiency rating of only about 60 percent. The efficiency of these cast-iron head burners can be

increased by reducing the firing rate. This can be accomplished by reducing the burner nozzle size, but the size reduction is controlled by the minimum firing rate for the appliance.

FIG 15 

Many conventional oil furnaces and boilers are being retrofitted with flame-retention head oil burners. A flame-retention head oil burner is designed to mix the air and fuel more efficiently than the traditional iron-head units. As a result, the amount of excess air required for good combustion is significantly reduced, resulting in a hotter and cleaner flame. In these units, the nozzle size can be reduced more than one size to achieve the maximum firing rate for the burner. The lower limit of the firing rate of a flame-retention head burner is governed by the flue gas temperature leaving the furnace or boiler.

Note

Never reduce the nozzle size below the minimum firing rate listed on the manufacturer’s rating plate. As a rule, it is a good idea not to reduce the nozzle more than one size if the conventional ironhead burner is retained.

High-Static Oil Burners
High-static oil burners are improved versions of flame-retention burners. They have an increased efficiency of 20 percent over flame-retention burners, and the high-static pressure developed in
these burners allows them to run at even lower excess air levels.

Fuel Nozzle  And Oil Burner Calcs
 there are several different types of oil burners, such as vaporizing pot type, low pressure gun type, high pressure gun type, and several types of rotary burners. The fundamental processes upon which all of these different burners are based are the same, however. The process of combustion may be thought of in the following steps:
1. The oil must be vaporized, since all combustible matter must be converted to a vapor or gas before       combustion can take place. This is usually accomplished by the application of heat.
2. The oil vapor must be mixed with air in order to have oxygen present for combustion.
3. The temperature of the mixture must be increased above the ignition temperature.
4. A continuous supply of air and fuel must be provided for continuous combustion.
5. The products of combustion must be removed from the combustion chamber.
The simplest type of burner is the vaporizing pot type. In this type of burner, heat is applied to a puddle of oil, causing vapors to be given off from the surface of the fuel. These vapors are then burned after mixing with the proper amount of air.

FIG 16

The simplest method of doing this job with light oils is by the use of nozzles
How A Nozzle Works
Separation of oil into small droplets requires the application of energy. In the case of nozzles, this energy is supplied in the form of pressure, usually from an appropriately designed motor driven pump. Pressure energy as such will not break up oil it must first be converted into velocity energy. This is done by supplying the fuel under pressure
1- pressure 
As might be expected, an increase in pressure increases the discharge rate of the nozzle, all other factors remaining equal. The relationship between the pressure and discharge from a nozzle is a fundamental one. The theoretical discharge from any orifice or nozzle is given by the equation

Flow Rate = CA (2gh)⁰′⁵

where :
C  is a dimensionless coefficient for the particular nozzle in question.
A  is the area of the nozzle orifice.
H  is the pressure head applied to the nozzle.
g  is acceleration of gravity.
This fundamental equation is modified by various factors encountered in nozzle design, but from it we arrive at a simple formula, which is of value to anyone using nozzles.

P1  is pressure at which the nozzle is calibrated.
P2  is any pressure at which it is desired to operate a nozzle other than the calibration pressure.
F1  is the calibrated flow rate at pressure Pl.
F2  is the flow rate at the desired pressure.
2- Fuel Properties and the Effects on Sprays
Specific gravity is normally used in flow calculations but in the petroleum industry the more common term is API gravity. The relationship between specific gravity and API gravity is given by the equation:

The effect of specific gravity on discharge rate (volumetric) is as follows:

Where
dl     Specific Gravity for flow F1
d2    Specific Gravity for flow F2

3-Viscosity
In small nozzles and within the limits of No. 2 fuel oil, the effects of changes in gravity are less important than the effects of changes in viscosity.
For purposes of calculations it is necessary to determine the absolute viscosity. Absolute viscosity is determined by multiplying kinematic viscosity in terms of centistokes by the specific gravity of the liquid at the same temperature. It is expressed in centipoises, which in turn may be converted into units of length, mass and time (pounds per foot per hour) for calculating Reynolds numbers
 Reynolds' number is defined as:

where :
D = orifice dimension
V = velocity of liquid
d = density of liquid
u = absolute viscosity of liquid
NR probably would explain the phenomenon at least partially if it could be correctly determined. The difficulty in determining NR lies in selection of the proper D and V. The air core in the orifice complicates both quantities.
4-Temperature 
It is importsnt to know what the relationship is between fuel temperature and viscosity.
at  temperatures, which would normally be encountered with outside tanks in the winter, the viscosity is very high. Fuels

FIG 17

DETERMINING THE VOLUMETRIC GAS FLOW RATE
The thermal furnace output of a boiler (QF) is the amount of heat supplied with the gas
in a unit of time. When taking the burner into operation the volumetric fuel flow rate should be
selected according to the nominal thermal capacity of the boiles

where:

QN  : Nom. thermal output (Kw)

nK  :   Boiler efficiency 

Volumetric gas flow rate (VBn) at STP:

where :

Hu  : Calorific value of gas (Kwh/m³)

Volumetric gas flow rate in operating condition(VBB):

Where :
pU       : Gas pressure   (mbar)
pamb   :   Barometer reading  (mbar)
pn       : Standard atmosferic pressure (mbar)
T         : Gas temperature absolute   =    (tgas+273) where  :tgas : Gas temperature relative (C⁰)

Types of Oil Burner Nozzles

The oil burner nozzle plays a vital role in atomizing the oil in the combustion chamber. Nozzles vary by GPH flow rate, spray angle, and cone type (hollow or solid). This information on common nozzle types and terms can help you make the most informed selection.

1- Hollow cone nozzles (Type A) 

can be used in burners with a hollow air pattern as well as in small burners (those firing @ 1.0 gph and under), regardless of air pattern. Type A 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. Under adverse conditions, hollow cone nozzles generally have more stable spray angles and patterns than solid cone nozzles with the same flow rate. This is an important advantage in fractional gallon nozzles as high-viscosity fuel may cause a reduction in spray angle and an increase in droplet size.

FIG 18

2- Solid cone nozzles (Type B) 

produce a spray which distributes droplets fairly uniformly throughout the complete pattern. However, this spray pattern becomes progressively more hollow at high flow rates, particularly above 8.0 gph. Type B nozzles can be used in larger burners (those firing above 2.0 or 3.0 gph) to provide smoother ignition, as well as where the air pattern of the burner is heavy in the center or where long fires are required.

FIG 19

Semi-solid nozzles (Type W) 

can frequently be used in place of either a hollow or solid cone nozzle firing at rates between 0.40 and 8.0 gph, regardless of the burner’s air pattern. Nozzles with lower flow rates tend to be more hollow, while higher flow rate nozzles tend to be more solid.

FIG 20

Atomizing nozzles 

are available in a wide range of flow rates, all but eliminating the need for specially calibrated nozzles. For example, between 1.0 gph and 2.0 gph, there are seven different flow rates 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.

FIG 21

Oil Burner Parts