Patent Description:
Diesel fired coolant heaters are essentially water heaters. They are typically installed in commercial, industrial and marine applications to preheat engines to facilitate starting in cold weather or to provide comfort heat to the passenger compartments. They burn liquid fuels to generate heat which is then transferred to the coolant system of the target application. Coolant is then circulated throughout the system to deliver the heat to the desired locations and thus transferred to the engine or heat exchangers.

In cold weather, engines can be difficult to start because the oil becomes more viscous, causing increased resistance of the internal moving parts, while cold diesel fuel does not atomize and ignite as readily. Cold engines work inefficiently, resulting in increased wear, decreasing useful engine life. To overcome these issues, heated coolant is circulated through the engine, heating the engine block, internal components and oil within.

In cold weather, when vehicles are stationary, the engines are typically idled to generate heat to keep the engine and passenger compartments warm. Utilization of a coolant heater eliminates the need to idle the engine, thus reducing the overall fuel consumption, corresponding emissions and provides a reduction in engine maintenance. Heat generated by the heater is transferred to the engine directly by circulating coolant through the engine block.

In some cases, newer commercial engines are very efficient but need to operate within specific operating temperatures to ensure proper operation of the emissions control equipment. In some applications, the engine loading is low and thus it never reaches the required operating temperature. Diesel fired coolant heaters are utilized to add heat to the engine to maintain or increase the operating temperatures so that the emissions control equipment operates correctly.

In cold temperatures, hydraulic equipment must be cycled gently until it warms up, otherwise it can be damaged. Heated coolant can be provided to heat hydraulic system reservoirs and equipment to enable faster operation in cold temperatures, reducing potential component life damage.

Heat can also be applied with such heaters to temperature sensitive loads such as cooking grease in rendering trucks or for the transportation of waxes or foodstuffs which may solidify in cold temperatures. An example of a combustion control apparatus for ensuring optimum combustion conditions in a boiler is described in <CIT>.

It is an object of the present invention to provide an improved vehicle heater and controls therefor.

There is accordingly provided a heater according to claim <NUM>.

Preferred embodiments are described in dependent claims <NUM>-<NUM>.

Referring to the drawings and first to <FIG> and <FIG>, there is shown a vehicle heater <NUM>. The heater <NUM> includes a housing <NUM>, a pump which in this example is a coolant pump <NUM>, and a heat exchanger <NUM>. The heat exchanger <NUM> has a plurality of legs, for example, legs <NUM> and <NUM> shown in <FIG> for mounting the heat exchanger on a support frame <NUM>. The housing <NUM> includes a controller cover <NUM> which covers a controller <NUM> shown in <FIG>. There is also a motor which in this example is an electric motor <NUM>. The electric motor <NUM> powers an air compressor <NUM> and a fuel pump <NUM>, both of which are shown in <FIG>. Referring back to <FIG> and <FIG>, the heater <NUM> further includes an air intake <NUM> which receives combustion air for the heater and an exhaust system <NUM> which discharges exhaust gases from the heater. There is also an air filter <NUM> shown in <FIG>. The heater <NUM> further includes a fuel line connector <NUM> for connecting the heater to a fuel tank <NUM> of a vehicle via a fuel line <NUM> as shown in <FIG>.

As best shown in <FIG> and <FIG>, the heat exchanger <NUM> includes a cylindrical combustion chamber <NUM> and an outer jacket extending about the combustion chamber, which in this example is a coolant jacket <NUM>. The coolant pump <NUM> circulates a liquid, which in this example is engine coolant, through the heat exchanger <NUM> in order to heat the coolant. In particular, the coolant is fed through the coolant jacket <NUM> of the heat exchanger <NUM> via a conduit <NUM>. The coolant is then heated by combustion of fuel in the combustion chamber <NUM>. The coolant may be a mixture of water and antifreeze.

Referring back to <FIG>, there is a burner head <NUM> mounted on an end of the combustion chamber <NUM>. The burner head <NUM> has a nozzle <NUM> which in this example is a two fluid siphon-type air atomizing nozzle. Fuel from the tank <NUM> is drawn into the fuel pump <NUM> via the fuel line <NUM>. The fuel is then discharged from the fuel pump <NUM> towards a fuel control valve, which in this example is a proportional control valve <NUM>, via a conduit <NUM>. The fuel is then provided to the nozzle <NUM> via a conduit <NUM>. The nozzle <NUM> utilizes compressed air received from the air compressor <NUM> via a conduit <NUM> to break up the fuel and deliver a highly atomized spray of fuel into the combustion chamber <NUM>. An igniter <NUM> ignites the atomized fuel to produce a flame <NUM>. Combustion air for the combustion reaction is supplied to the combustion chamber <NUM> by a blower assembly <NUM> which includes a blower <NUM> and a blower motor <NUM> for powering the blower. The heat generated by the combustion reaction is transferred to the coolant flowing through the heat exchanger <NUM> and then circulated throughout the vehicle coolant system.

As best shown in <FIG>, the combustion chamber <NUM> in this example has a double wall formed by a cylindrical inner wall portion <NUM> and a cylindrical outer wall portion <NUM>. The cylindrical inner wall portion <NUM> and the cylindrical outer wall portion <NUM> are spaced apart from each other by an annular space <NUM> which provides a passageway between the wall portions. A plurality of apertures <NUM> extends through the inner wall portion <NUM> and communicates with the space <NUM>. In this example, the apertures <NUM> are arranged in spaced-apart, annular rows <NUM> and <NUM> which extend circumferentially about the inner wall portion <NUM>. The apertures <NUM> permit air to enter the combustion chamber <NUM> from the space <NUM>.

Referring back to <FIG>, there is a first set of fins <NUM> extending radially inwardly from the coolant jacket <NUM> to the combustion chamber <NUM>. The fins <NUM> facilitate the transfer of heat from the combustion chamber <NUM> to the coolant jacket <NUM> and thus the coolant flowing through the coolant jacket. In this example, the fins <NUM> comprise a single, cylindrical member which is annular in profile. The cylindrical member is an aluminum casting in this example but may be of other metals formed in ways other than casting. The fins <NUM> extend from near a first end <NUM> of the combustion chamber <NUM> to a position near a second end <NUM> of the combustion chamber <NUM>. In this example, each of the fins <NUM> tapers in profile from the second end <NUM> of the combustion chamber <NUM> to the first end <NUM> thereof. Accordingly, the fins <NUM> are thinner near the first end <NUM> of the combustion chamber <NUM> than near the second end <NUM> of the combustion chamber <NUM>. The fins <NUM> are also spaced further apart from adjacent fins near the first end <NUM> of the combustion chamber <NUM> than near the second end <NUM> thereof. This is caused by using a single annular casting for the fins <NUM> in order to facilitate removal of the casting from a mould. However, the result is that the spacing between the fins <NUM> is less optimal near the first end <NUM> of the combustion chamber <NUM>.

Referring now to <FIG>, there is a second set of fins <NUM> which extends from a position near the first end <NUM> of the combustion chamber <NUM> part way towards the second end <NUM> thereof. In this example, the fins <NUM> also comprise a single, cylindrical member which is annular in profile and of aluminum casting as best shown in <FIG>. However, the fins <NUM> may also be of other materials and be in other configurations in other examples. Each of the fins <NUM> is positioned between two adjacent fins of the first set of fins <NUM> to reduce spacing between the fins of the set of fins <NUM> and accordingly optimize heat transfer between the combustion chamber <NUM> and the coolant jacket <NUM>.

The nozzle <NUM> is shown in greater detail in <FIG> and includes a hex body <NUM>, a stem <NUM>, a cap <NUM> and a distributor <NUM>. The stem <NUM> has an axial bore <NUM> through which fuel from the fuel tank <NUM>, shown in <FIG>, flows in the direction indicated by arrow <NUM>. Referring back to <FIG>, there is also a disparager assembly <NUM> and a seal in the form of an O-ring <NUM> which is disposed between the disparager assembly <NUM> and the distributor <NUM>. The disparager assembly <NUM> includes an outer barrel <NUM> and an inner rod <NUM> which are concentric with each other. The outer barrel <NUM> has a threaded inner wall portion <NUM> and the inner rod <NUM> has a threaded outer wall <NUM>. The threaded inner wall portion <NUM> of the outer barrel <NUM> and the threaded outer wall <NUM> of the inner rod <NUM> have different thread pitches which creates a tortuous flow path for the fuel as it flows through the disparager assembly <NUM>. This disrupts the flow of gas bubbles within the fuel stream, thereby breaking up larger gas bubbles into smaller gas bubbles prior to passing into the distributor <NUM>. The sizes of the gas bubbles are sufficiently reduced after passing through the disparager assembly <NUM> to avoid disrupting the fuel flow to the combustion chamber <NUM>. Otherwise, the combustion process may be interrupted which may cause the heater <NUM> to stumble or flame out. Compressed air supplied from the air compressor <NUM>, shown in <FIG>, flows through the nozzle <NUM> as indicated by arrow <NUM> in <FIG> and interacts with the fuel, causing the fuel to break up into an atomized spray <NUM> consisting of small droplets of fuel. The small droplets of fuel are evaporated by the heat of combustion and form a combustible gas which, when mixed well with air, is burned in the combustion chamber <NUM> shown in <FIG>. The degree of atomization of the fuel is dependent upon the supplied air pressure from the air compressor <NUM>.

The air compressor <NUM> is shown in greater detail in <FIG> and includes an air compressor housing <NUM>, a diaphragm <NUM>, a cylinder head <NUM> and an air filter <NUM>. Referring now to <FIG>, the fuel pump <NUM> is shown in greater detail. The fuel pump <NUM> is a gerotor pump in this example but may be a different type of pump such as a gear pump in other examples. The fuel pump <NUM> is mounted on a fuel pump housing <NUM> together with the proportional control valve <NUM>. The fuel pump <NUM> has a connecting rod assembly <NUM>, shown in <FIG>, which is connected to the electric motor <NUM>.

As shown in <FIG>, the electric motor <NUM> has an output shaft <NUM> which drives both the air compressor <NUM> and the fuel pump <NUM>. In this example, the electric motor <NUM> drives the air compressor <NUM> and the fuel pump <NUM> simultaneously at the same speed. The output shaft <NUM> is provided with a moulded drive cup <NUM> which forms part of a magnetic coupling <NUM> with a cylindrical, moulded shaft follower <NUM> received within the drive cup <NUM>. The drive cup <NUM> has internal magnets <NUM> in an annular wall thereof and the shaft follower <NUM> has magnets <NUM> in an annular wall thereof. A shaft <NUM> of the shaft follower <NUM> is connected to the fuel pump <NUM>. When the output shaft <NUM> of the electric motor <NUM> rotates, the drive cup <NUM> rotates the shaft follower <NUM> which cause its shaft <NUM> to rotate the fuel pump <NUM>. There is also a moulded separator cup <NUM> located between the electric motor <NUM> and the fuel pump <NUM>. The separator cup <NUM> contains the fuel within the fuel pump <NUM> while magnetically transferring the rotational torque to drive the fuel pump. This eliminates the need for a dynamic shaft seal on the fuel pump which reduces the potential for fuel leaks. The output pressure of the fuel pump <NUM> remains constant throughout the RPM range of the pump.

<FIG> and <FIG> shows a fan assembly <NUM> which provides combustion air for the heater <NUM>. The fan assembly <NUM> includes a fan housing <NUM> which receives the blower assembly <NUM> including the blower <NUM> and the blower motor <NUM>. The fan assembly <NUM> further includes a cylindrical sleeve <NUM> and an air swirler <NUM> which is mounted on the cylindrical sleeve as best shown in <FIG>. The sleeve <NUM> is adapted to receive the nozzle <NUM>. The air swirler <NUM> has fins which extend radially outwardly from the sleeve <NUM>. The air swirler <NUM> is located in the path of the combustion air supply indicated by arrow <NUM> and forces the combustion air to swirl prior to entry into the combustion chamber <NUM> as shown in <FIG>. The swirling air <NUM> interacts with the atomized fuel spray <NUM>, shown in <FIG>, causing the air and the fuel to mix. The swirling air also creates a vortex which creates a recirculation in the combustion chamber <NUM>, causing the hot gases of combustion to interact with the new air/fuel mixture delivery. The internal recirculation zone created by the swirling air results in low velocity regions which anchor the flame. This improves mixing and flame stabilization which results in a shorter, more compact flame and lower nitric oxides.

As shown in <FIG>, there are three air passages for the delivery of combustion air to the combustion chamber <NUM>. The majority of the combustion air (approximately <NUM>%) is delivered through the air swirler <NUM> as indicated by arrows <NUM>. Approximately <NUM>% of the combustion air is atomized air supplied from the air compressor <NUM> which flows through the atomizing nozzle <NUM> as indicated by arrow <NUM> to break up the fuel into droplets. The balance of the combustion air (approximately <NUM>%) is routed through the annular space <NUM> between the double wall of the combustion chamber <NUM> and delivered downstream in the combustion chamber as indicated by arrows <NUM>. This secondary air supply supplements the primary swirled air supply in conjunction with the baffle at the end of the combustion chamber <NUM> to further enhance the recirculation within the combustion chamber. The baffle and the plurality of apertures <NUM> in the inner wall portion <NUM> promote recirculation of combustion gases with the new air/fuel mixture, resulting in improved combustion.

<FIG> shows another air swirler <NUM> which may be used in the fan assembly <NUM>. The air swirler <NUM> is not mounted on the cylindrical sleeve <NUM>. Instead, the air swirler <NUM> is located near a base <NUM> of the sleeve <NUM>. The air swirler <NUM> has fins which extend upwardly from the base <NUM> of the sleeve <NUM>. The air swirler <NUM> is similarly located in the path of the combustion air supply indicated by arrow <NUM> and forces the combustion air to swirl as indicated by arrow <NUM> prior to entry into the combustion chamber <NUM> as shown in <FIG>.

Referring back to <FIG>, the exhaust system <NUM> includes an exhaust conduit <NUM> which is connected to the heater exchanger <NUM> by a flange <NUM> which is shown in <FIG>. Typically, the exhaust conduit <NUM> is connected to the exhaust of the vehicle via an exhaust pipe. There is an oxygen sensor <NUM> connected to the exhaust conduit <NUM> as best shown in <FIG>. The oxygen sensor <NUM> is also operatively connected to the controller <NUM> which is shown in <FIG>. The oxygen sensor <NUM> measures the oxygen content of exhaust gases from the heater <NUM>, thereby providing an indication of the air/fuel ratio and the status of the combustion process. <FIG> shows the oxygen sensor <NUM> and the exhaust conduit <NUM> in greater detail.

<FIG> shows the fuel control system for the heater <NUM>. The fuel control system is a closed loop fuel control system based on feedback from the oxygen sensor <NUM>. As shown in <FIG>, feedback <NUM> from the oxygen sensor <NUM> to the controller <NUM> is used to control the fuel control valve, which in this example is the proportional control valve <NUM>. In this way, the fuel delivery rate to the heater is modulated in response to the control loop. The proportional control valve <NUM>, together with the fuel pump <NUM>, provides continuously variable heat output. This is in contrast to conventional stepped control for heat output. Variable heat output control allows power consumption to be optimized.

The closed loop fuel control system allows the heat output from the heater <NUM> to be reduced or turned down while maintaining a preset stoichiometry throughout the turndown range. To reduce the heat output, the controller <NUM> reduces the speed of the blower motor <NUM> which results in a corresponding reduction in the oxygen level in the exhaust stream. To maintain the preset stoichiometry, the controller <NUM> then adjusts the proportional control valve <NUM> to reduce the fuel rate. Reducing the fuel rate in turn causes the oxygen level in the exhaust stream to increase until the target oxygen level set point is reached. The closed loop fuel control system also automatically maintains stoichiometry in situations where the air intake <NUM> or the exhaust conduit <NUM> are restricted.

A speed sensor is integrated into the electric motor <NUM> common to the air compressor <NUM> and the fuel pump <NUM>. The blower motor <NUM> is also provided with a speed sensor. The electric motor <NUM> and the blower motor <NUM> are designed to operate specific speeds associated with specific heater output levels. As the heater output is reduced in accordance with the closed loop fuel control strategy or a lower desired output is required, the motor speeds are adjusted accordingly based on the defined lookup table set out below.

The heater <NUM> is designed to operate on voltages of <NUM> to <NUM> volts where the motors are nominally rated at <NUM> volts. As the heater <NUM> supply voltage fluctuates throughout the supply nominal operating range, a closed loop speed control adjusts the motor speed to follow the required speeds defined in the above lookup table and the desired heater output setting.

The closed loop fuel control system further maintains combustion stoichiometry and resulting exhaust emissions as the operating altitude of the heater increases. As altitude increases, the air density decreases and the performance of the blower <NUM> and the air compressor <NUM> are reduced proportionally. If the fuel rate is not adjusted as the altitude increases, and resultant air flow decreases, the oxygen level in the exhaust gases will decrease and the carbon monoxide content in the exhaust gases will increase. To compensate for the reduced air density, the controller <NUM> reduces the fuel rate proportionally to maintain the specified stoichiometry or preset oxygen level target.

The heat output of the heater <NUM> is also automatically adjusted to match the ability of the vehicle coolant system to accept the generated heat. The amount of generated heat that can be transferred to the coolant is proportional to the flow rate of the coolant. If the coolant flow rate is too low, then the coolant cannot absorb all of the heat generated and the temperature rises quickly to the heater cycle off temperature and the heater cycles off. The coolant continues to circulate and because the heating cycle is very short, the coolant is only heated locally within the heat exchanger. The balance of the unheated coolant continues to circulate through the system, resulting in the unheated coolant flowing into the heater. The system temperature sensor measures the low coolant temperature and signals the heater to restart and another heating cycle begins. This frequent start/stop cycle is called short cycling. In this situation, the load never gets warm.

To prevent short cycling, the closed loop fuel control system utilizes its turndown capability to vary the heater output. As shown in <FIG> and <FIG>, the heater <NUM> is provided with temperature sensors <NUM> and <NUM>. When the temperature sensors <NUM> and <NUM> signal a call for heat, the heater <NUM> initiates a heating cycle. If the heater output is less than the heating load, the heater will run continuously or until it is shut off as it will never reach the cycle off temperature. If the heating load is less than the heater output, the heater will operate at <NUM>% output until it reaches the cycle off temperature. The control strategy dictates that the heater must run for a minimum of ten minutes after the cycle is initiated. If the elapsed cycle time is less than ten minutes, the heater will start to reduce the heat output. A PID control loop will modulate the heater output using the closed loop fuel control to maintain the coolant temperature at the cycle off temperature for the balance of the ten-minute cycle interval. At the end of the ten minutes, the heater will cycle off.

The objective of this strategy is to prevent short cycling to ensure that the maximum amount of heat can be transferred to the load. This also ensures that the heater is operated for a period of time that is sufficient to heat up the burner components and burn off fuel and combustion residue, minimizing carbon deposits inside the combustion chamber.

The heater output can be coupled to a feedback system based on an external heat exchanger to maintain a specific temperature within the heated space. Based on information supplied from the load, the heater can automatically adjust itself to maintain a desired temperature change in the system. Large temperature variations in heating systems can be considered uncomfortable. The more consistent and steady the heat, the more comfortable it can be.

The oxygen sensor <NUM> has a secondary function as a flame detection device. In particular, the oxygen sensor <NUM> measures the oxygen level in the exhaust stream to determine if a flame is present in the combustion chamber <NUM>. As shown in <FIG> during start-up and operation of the heater <NUM>, the level of oxygen in the exhaust stream as measured by the oxygen sensor <NUM> must reach prescribed limits and be maintained within the prescribed limits to indicate that a suitable flame is present in the combustion chamber <NUM>. If a suitable or "good" flame is detected, the heater <NUM> will continue to operate. If a good flame is not detected, then the controller <NUM> will shut down the heater <NUM>.

However, there are situations in which the oxygen sensor <NUM> may indicate that a flame is present in the combustion chamber <NUM> when there is no flame. For example, if the flame does not immediately ignite during ignition, fuel will continue to spray into the combustion chamber and saturate the oxygen sensor <NUM> with unburned fuel. This may cause the oxygen sensor <NUM> to potentially indicate a flame where none is present.

To overcome this problem, secondary heater performance parameters, for example, exhaust gas temperature and coolant outlet temperature, are resolved into a parameter called the EGDT which is monitored concurrently with the oxygen sensor <NUM> data. The exhaust gas temperature may be measured by a temperature sensor <NUM> shown in <FIG>. Referring now to <FIG>, if a flame is present in the combustion chamber <NUM> during ignition or operation of the heater <NUM>, the EGDT parameter is expected to rise or remain above prescribed levels. If a good flame is established at the start of combustion, the oxygen level will decrease while the EGDT value will increase. In cases where the oxygen sensor <NUM> is being deceived as to the presence of a flame, the oxygen level may decrease as normal but the EGDT will not increase, indicating a failure in flame detection and causing the controller <NUM> to indicate a fault. The concurrent monitoring of the EGDT parameter provides a secondary validation of the oxygen level reading in the exhaust stream confirming that a flame is present in the combustion chamber <NUM>.

The heater <NUM> may also be provided with a backup flame detection system in the form of coolant temperature sensors <NUM> and <NUM> which are mounted on the coolant jacket <NUM> in spaced-apart locations as shown in <FIG> and <FIG>. The temperature sensors <NUM> and <NUM> measure the temperature of the coolant at two separate locations and compares the difference in temperature to a model of the theoretical temperature difference. If the measured temperature difference is outside of the range, then this may signal the lack of a flame. For example, the temperature sensor <NUM> may measure the temperature of inlet coolant while the temperature sensor <NUM> may measure the temperature of outlet coolant. The controller <NUM> senses a rise in the temperature difference between the inlet temperature sensor <NUM> and the outlet temperature sensor <NUM> and compares it to a running average of the temperature differences. The system compares the difference between the inlet and outlet coolant temperatures and the running average of the temperature differences. Depending upon the sign (+/-) of the comparison, the system can detect if a flame of the heater just came on or if it went out.

Referring now to <FIG>, the fuel delivery system of the heater <NUM> is shown. A pressure relief valve <NUM> is used to establish the fuel system operating pressure. At maximum heater output, approximately <NUM>% to <NUM>% of the total fuel flow returns to the fuel tank <NUM> over the relief valve <NUM>. The balance of the total fuel flow (approximately <NUM>% to <NUM>%) is ported through the proportional control valve <NUM> and consumed in the combustion chamber <NUM> to generate heat. As the system operates, fuel delivered from the fuel pump <NUM> passes into a separation chamber <NUM>. This allows large gas bubbles <NUM> entrained or suspended in the fuel to float up to the top of the chamber <NUM>. There is a narrow fuel passage <NUM> near the top of the chamber <NUM>. The narrow size of the fuel passage <NUM> increases the velocity of the fuel through the passage <NUM>. The gas bubbles <NUM> are carried away in the passage <NUM> through the relief valve <NUM> to the fuel tank <NUM> in the return line <NUM>.

There is also a narrow passage <NUM> located at the base of the chamber <NUM> which leads to a secondary chamber <NUM>. Larger gas bubbles such as the gas bubbles <NUM> are restricted from entering the secondary chamber <NUM> due to the narrow size of the passage <NUM>. Fuel flowing into the secondary chamber <NUM> is at the fuel burn rate which is significantly lower than the total fuel rate through the system. The velocity of the fuel is further reduced as it enters the secondary chamber <NUM>. This lowered velocity increases the residence time of the fuel in the secondary chamber <NUM>, allowing any remaining gas bubbles <NUM> to float up into the passage <NUM> and be returned to the fuel tank <NUM> in the return line <NUM>. Fuel leaving the secondary chamber <NUM> is metered through the proportional control valve <NUM> to the atomizing nozzle <NUM>.

<FIG> and <FIG> show another vehicle heater <NUM>. Like parts have like numbers and functions as the vehicle heater <NUM> described above and shown in <FIG> with the addition of "<NUM>".

Claim 1:
A heater (<NUM>) for a liquid, the heater (<NUM>) comprising:
a combustion chamber (<NUM>);
a jacket (<NUM>) for the liquid, the jacket (<NUM>) extending about the combustion chamber (<NUM>);
a fan (<NUM>) having an output communicating with the combustion chamber (<NUM>) to provide combustion air;
a fuel delivery system including a fuel pump (<NUM>);
a burner assembly connected to the combustion chamber, the burner assembly having a burner (<NUM>) mounted thereon adjacent the combustion chamber (<NUM>), the burner (<NUM>) receiving fuel from the fuel delivery system; and
an air compressor (<NUM>), the air compressor (<NUM>) having an output communicating with the burner (<NUM>) to supply compressed air thereto;
characterised in that the burner (<NUM>) includes a nozzle (<NUM>) having a disparager assembly (<NUM>), the disparager assembly (<NUM>) including an outer barrel (<NUM>) having a threaded inner wall portion (<NUM>) and an inner rod (<NUM>) having a threaded outer wall portion (<NUM>), the threaded inner wall portion (<NUM>) and the threaded outer wall portion (<NUM>) having different thread pitches to create a tortuous flow path for the fuel as it flows through the disparager assembly.