EP2918909B1

EP2918909B1 - A liquid fuel portable heater having a thermoelectric generator, and according control method

Info

Publication number: EP2918909B1
Application number: EP15155313.8A
Authority: EP
Inventors: Enzo Giaretta; Stefano Verani; Davide Galletti; Ugo Romagnani; Sergio Lorenzi
Original assignee: Dantherm SpA
Current assignee: Dantherm SpA
Priority date: 2014-03-13
Filing date: 2015-02-17
Publication date: 2018-11-21
Anticipated expiration: 2035-02-17
Also published as: US20160258618A1; EP2918909A1; US20190024886A9

Description

Scope of the invention

The present invention relates to a liquid fuel portable heater optimised to reduce the consumption of electricity needed for it to operate.

State of the art

Liquid fuel portable heaters generally comprise a fuel tank and a combustion chamber into which a fuel aerosol is introduced, taken from the fuel tank, which is nebulised through a nozzle nebulizer. Such known devices use an electric pump to withdraw the liquid fuel from the tank and bring it to the nebulizer nozzle at a preset pressure. Said heaters may further comprise further electrical devices needed to operate, such as a fan to convey the combustion air into the combustion chamber or to diffuse heated hot air from the combustion chamber into the environment. Solenoid valves, safety systems, electrical controls, requiring electricity to operate may also be present.
Said known heaters therefore require a significant supply of electricity to operate all the internal electrical devices, among which the aforesaid electric pump.
By way of example, a typical heater with a thermal power output of 20kW, or 68,200 BTU/h, may require electrical power of indicatively 50 W to operate its electrical equipment, about half of which is needed to operate the pump alone.
Conversely, in order to facilitate moving or transporting the portable heater, and in order to use the heater in places where there is no mains electricity, connection cables to the mains need to be avoided to permit standalone operation of the heater.
To meet this need, attempts have been made to develop heaters which have a source of electricity on board, such as electric batteries or converter devices of the temperature difference between the combustion chamber and the external environment into electricity, such as for example thermoelectric cells, making the operation of the heater independent of the mains electricity supply.
Since the amount of electricity required by the known heaters is very high, the known heaters require high-capacity and therefore very bulky and heavy batteries.
Furthermore, in the case of using thermoelectric cells to convert thermal energy into electricity, it would be necessary to install a very large number of cells on board, requiring extensive support surfaces around the combustion chamber to house them, entailing very large dimensions of the heater and a consequent increase in costs.
These technical limitations conflict with the need to provide a small size and lightweight portable heater with a high autonomy of operation.
Consequently, to reduce the size and weight of the heater, the need is strongly felt to reduce the consumption of electricity absorbed by the electric devices on board, among which the electric pump for liquid fuel which powers the burner of the heater.
Said pump is used to generate a pressure of liquid fuel upstream of a nebulizer nozzle high enough to nebulise the fuel through the nebulizer.
At said pressure the nozzle nebulises the fuel by generating an aerosol with the combustion air, thus feeding the combustion of the flame triggered by an ignition system.
The flow capacity of the pump must be equal to or greater than the output flow from the nebulizer nozzle, at a given pressure. A drop in capacity would in fact lead to a consequent decrease in pressure and the loss of the correct amount of fuel nebulised.
The typical pumps for burners used according to the prior art, classified by flow and pressure, are usually over-sized in order to cater to a wide range of sizes of burners to which they can be fitted.
This over-sizing is materialised in excessive work by the pump, well above the actual needs of the system; said extra work is dissipated in the form of recirculation and discharge of the surplus flow upstream of the pump.
The power consumption of the pump is obviously related to how much flow it is able to deliver at a certain pressure, obviously net of output and pressure drops.
In light of the above, and in particular of the fact that the pumps fitted on known heaters are typically oversized in relation to actual needs, it follows that these heaters have a significantly higher electrical consumption than necessary, weighing heavily on the operative power reserve of battery-powered heaters or on the size thereof or on any self-generating electrical devices, such as thermoelectric cells.
With reference to a typical flow-pressure trend of a pump generally used in the prior systems, for example shown in figure 1A, where the pressure is shown on the x-axis and the flow on the y-axis, a system using only a portion of such performance merely uses these curves at a certain pressure, diverting the excess flow and thus wasting the electricity consumed.
For example, if a portable heater is designed to be powered by nebulised fuel at 8 bar, thus consuming 2 litre/hours of fuel, and is fitted with a pump the characteristic curve of which shows a flow of 20 litre/hours at 8 bar, it follows that about 9/10 of the electricity consumed to power said pump is unnecessary, the fuel being mainly discharged and not introduced by the nebulizer for combustion.
The need is therefore felt to reduce the consumption of electricity absorbed by the pump, and in the case of an over-sized pump being used, to avoid the need to divert the excess flow of fuel back towards the tank.

Summary of the invention

A purpose of the present invention is to make available a liquid fuel portable heater which makes it possible to satisfy the above needs and at least partially overcome the drawbacks mentioned with reference to the prior art.
In particular, a task of the present invention is to make available a liquid fuel portable heater able to operate independently of the mains electricity grid, and which at the same time has extremely limited dimensions and weight, making it easy to move.
The purpose of the present invention is to provide a liquid fuel portable heater comprising an electric pump for supplying the fuel, controlled so as to greatly reduce the electricity absorbed by the pump, at the same nebulisation pressure of the fuel.
A further purpose of the present invention is to provide a liquid fuel portable heater which despite comprising an oversized fuel pump with respect to the required flow rate, makes it possible to reduce or avoid the work in excess of the pump, thus the excess energy consumption corresponding to the difference between the nominal flow of the pump and flow required by the nebulizer.
Another aim of the invention is to provide a liquid fuel portable heater comprising an electric pump for fuel supply, able to avoid the need to divert part of the flow of fuel in excess of that required for combustion, even when the pump is oversized.
According to another aspect, the purpose of the present invention is to provide a method for controlling the power supply of an electric pump for a liquid-fuelled, portable heater, so as to reduce the energy consumption of the pump, adapting such consumption to the actual flow of liquid fuel required by the nebulizer, even in the case in which said pump is oversized.
These and further purposes and advantages are achieved by means of a liquid fuel portable heater according to claim 1.
Said portable heater comprises an electric control unit configured so that when the heater is on, said control unit actuates the electric pump by supplying it with a sequence of pulses with a non-zero voltage, and pause intervals with a substantially zero voltage alternating with said pulses, wherein the average duration of the pulses is less than the average duration of the pause intervals.
This provision makes it possible to reduce the amount of electricity absorbed for the same amount of fuel actually conveyed to the combustion chamber.
In this regard, one can define duty cycle as the percentage of area under the curve of the power supply voltage of the pump in the operative interval of the heater, or over a period in the case of periodic power supply, compared to the total area which would be under the curve if the pump were supplied with a constant voltage equal to the maximum value of the power supply voltage over the entire operative interval or period of the heater.
In other words, the work cycle, or duty cycle, can also be seen as the ratio of the duration of ON with respect to the time interval of operation of the heater.
In the case of a standard frequency power supply such as 50 Hz, the average duration of the pulses, or ON duration is roughly equal to the average duration of the pause intervals or OFF duration, the pump thus having a duty cycle of 50%.
The energy absorbed by the pump is proportional to the value of the duty cycle.
Consequently by reducing the ON duration compared to the OFF duration over the same period, or same operative interval of the heater, the value of the duty cycle and thus the electricity absorbed by the pump in the unit of time is reduced.
According to one embodiment, the control unit is configured to adjust the frequency of the sequence of actuation intervals or pulses of the pump, especially at frequency values below 50Hz.
This provision makes it possible to reduce the number of ON portions as well as the duration of the single supply portions for the number of pauses, further reducing the area under the power supply curve of the pump and thus further reducing the electricity absorbed by the pump. In other words, the reduction of electricity absorbed by the pump during an operative interval of the heater is the result of a combined action of a reduction of the average duration of the pulses or supply portions compared to the average duration of pauses, and a reduction in the frequency of the pulses.
Tests conducted on a standard pump suitable to be powered at 50 Hz and with a duty cycle of 50%, showed optimal operating and energy-saving results adjusting the frequency to 10 Hz and adjusting the ON duration to achieve a 12% duty cycle. In such conditions the electricity consumption of the pump was reduced from 24W, in normal operation at 50 Hz and 50% duty cycle, to 5.7W, thus leading to substantial energy savings.
This high energy saving, also taking into account the energy requirements of the other electrical devices on board needed to operate, has the material effect of at least doubling the operation autonomy of the heater, if running on batteries, or halving of the number of thermoelectric generation cells needed.
According to one embodiment, the control unit is configured to modulate the ON duration compared to the OFF duration over each period, and thus to modulate the duty cycle, and additionally or alternatively, to also modulate the frequency of the power supply sequence of the pump over time, in order to provide the correct flow of liquid fuel to the nebulizer at a pressure not below a predefined minimum threshold of nebulisation pressure.
For example, said minimum predefined nebulisation pressure threshold may be about 8 bars. Such pressure value guarantees a correct nebulisation by the nebulizer.
This way the problem of having to divert a portion of excess fuel generated by an oversized pump to bring it back to the tank is resolved. In fact by adjusting the duty cycle and frequency the pump is powered only when and as required to ensure a flow and pressure to the nebulizer sufficient for said nebulisation. No overworking is produced, no excess electricity is wasted unnecessarily.
According to one embodiment, the heater according to the invention comprises an expansion chamber for pressure waves, positioned between the pump outlet and the nebulizer in order to store excess fuel in output from the pump and release it gradually to the nebulizer at a pressure not below a predefined minimum threshold of nebulisation pressure.
Advantageously, the expansion chamber acts as a real volumetric temporal reserve of pressurised fuel simultaneously performing two technical tasks.
In fact, according to a first advantageous aspect said expansion chamber acts as a reserve and accumulator of the excess flow of fuel dispensed by the pump in the active ON phase, enabling the continuous dispensing of fuel toward the nebulizer during the off phase of the pump.
According to another advantageous aspect, the expansion chamber acts as a pressure equalizer, thus as a regulator of pressure peaks in the circuit downstream of the pump.
According to another aspect of the invention, the aforesaid purposes and advantages are satisfied by a method according to claim 9.

Brief description of the drawings

Further characteristics and advantages of the present invention will, in any case, be evident from the description given below of its preferred embodiments, made by way of a non-limiting example with reference to the appended drawings, wherein:

figures 1 and 2 show two voltage waveforms at fixed frequency for powering an electric pump for a heater, according to the prior art;
figure 1A shows a typical pressure-flow trend of an electric pump;
figures 3, 4 and 5 show details of voltage supply trends for an electric pump for a heater, according to the present invention;
figure 6 shows a schematic view of a heater according to the invention;
figure 7 is a block diagram of the heater in figure 6.

Description of some preferred embodiments

With reference to the figures, reference numeral 100 globally denotes a liquid fuel portable heater according to the invention.
In this description, the pulse sequence of the pump power supply is also called sequence of power intervals, or sequence of ON.
Consequently the duration of the pulse is the duration of the power interval and also the ON duration.
Similarly, the pause intervals are also called non-powered intervals or OFF intervals.
In addition, the interval in which the heater is switched on and thus the interval during which liquid fuel is dispensed from the nebulizer into the combustion chamber and in which combustion takes place will also be referred to as the heater operative interval,
In this description, the liquid fuel is sometimes referred to as fuel.
The heater 100 comprises a combustion chamber 101, for example of a substantially tubular shape, into which a liquid fuel, or fuel, such as diesel fuel, in the form of aerosol 15, in particular as a nebulised mixture of fuel and air, is conducted.
The heater 100 thus comprises a nebulizer 13 to form the fuel aerosol and to continuously nebulise it into the combustion chamber 101, in which a flame 16 is generated. Nebulisation is performed upon reaching a predefined fuel pressure upstream of a nebulizer nozzle.
Outside and around the combustion chamber a casing 102, or skirt, is present forming an interspace 103 around the combustion chamber 101, suitable to be crossed by a flow of air 104 which thermally insulates the combustion chamber from the outer casing, and simultaneously heats and forcibly enters an environment to be heated.
To form such forced flow of air 104 a fan 105 placed upstream of the nebulizer 13 and interspace 103 may be used. Part of said flow may be diverted into the combustion chamber 101 to provide combustion air for combustion.
The fan 105 is driven by an electric motor 105 '.
According to the invention, the heater comprises a tank 6 placed on board the heater, in particular under the combustion chamber housing 101 or casing 102.
A frame, not shown in the drawings connects and joins all the components of the heater 100, so as to move it easily in one body.
The heater may comprise wheels 7 positioned to facilitate its movement.
The heater 100 further comprises an electric pump 10 having an inlet 11 for receiving the fuel from the tank 6, and an outlet 12 to send said liquid fuel to the nebulizer 13 in input to the combustion chamber 101.
As shown in Figure 7, the heater comprises a supply duct 43 of the fuel which connects the tank 6 to the pump 10, to transfer to the pump 10 the fuel 44 contained in the tank 6.
A supply duct 45 fluidically connects the pump 10 to the nebulizer 13.
A compensation duct 46 fluidically connects the supply duct 45 to the expansion chamber 14.
According to one embodiment, the heater 100 further comprises an electricity source 24, in particular mounted on board and thus integrated in the heater. This electricity source is dimensioned to supply electricity to all the electrical components on board, such as the pump 12, a control unit, a motor 105' of the fan 105, circuit boards, solenoid valves.
The electricity source may be an electric battery, or storage battery for example a rechargeable battery.
The heater 100 may comprise at least one direct conversion cell of a temperature differential into electric energy (not shown) mounted on the heater 100 to receive a temperature differential between the inside of the combustion chamber and the outside environment, in which said electricity produced by said at least one conversion cell is suitable to electrically power said electric pump and said control unit 20.
For example, the at least one direct conversion cell is a Seebeck cell.
The heater 100 may comprise a Stirling motor positioned to take the temperature differential between the combustion chamber and the environment and convert it into mechanical energy, for example to drive an electricity generator.
The heater 100 comprises an electrical control unit 20 configured to power the electric pump, or control an electricity supply of the pump, with the heater on, with a sequence of pulses 115, 115' with a non-zero voltage, and pause intervals 116 with a substantially zero voltage alternating with said pulses, wherein the average duration of the pulses 115, 115' is less than the average duration of the pause intervals 116. In particular, such average durations are evaluated for the same operative interval of the heater.
The average duration of the pulses 115, 115' is preferably less than 2/3 of the average duration of the pause intervals 116, or, even more preferably less than half the average duration of the pause intervals 116.
In this context, the average duration of the pulses is understood as the quotient of the sum of the durations of all the pulses and the total number of pulses in the operative interval of the heater.
Similarly, the average duration of the pause intervals is understood as the quotient of the sum of the durations of all the pause intervals and the total number of pauses in the operative interval of the heater.
In other words, the electric control unit 20 is configured to control an electricity supply of the electric pump 10, with the heater on, in a sequence of power intervals 115 having an ON duration of T1, and non-powered intervals 116 having an OFF duration of T2, wherein the sum of the durations of the ON intervals T1 is less than the sum of the durations of the OFF intervals T2 in the operative interval of the heater.
Some possible examples of voltage trends V as a function of time t to power the pump 10, according to the invention, are shown in figures 3, 4 and 5, while figures 1 and 2 show possible electric voltage trends as a function of time using a traditional power supply. Along the x-axis 121 is the time, and along the y-axis 122 the electrical voltage is shown.
In particular, the prior trend shown in figures 1 and 2, with a sinusoidal waveform in figure 1 and with a square waveform in figure 2, is at fixed frequency and maximum constant voltage, for example at 50Hz and has a duty cycle of approximately 50%.
The provision of reducing the on duration compared to the OFF time reduces the duty cycle and thus the electricity absorbed by the pump compared to that traditionally used.
In other words, according to an embodiment, the control unit 20 is configured to adjust, or modulate, the ON duration compared to the OFF duration to obtain a duty cycle of less than 50%.
In other words again the duty cycle value, or percentage value of the integral of a voltage/time curve of said sequence of pulses in an operative interval of the heater with respect to the integral of a hypothetical voltage/time curve with a direct, constant voltage, with an amplitude equal to the maximum amplitude of said voltage/time curve of said sequence of pulses in the same operative interval of the heater, is less than 50%.
According to an embodiment, the control unit 20 is configured to adjust, or modulate, the ON duration compared to the OFF duration, or, in other words, the duration T1 of the pulses compared to the duration T2 of the pause intervals, to obtain a duty cycle, with a value of less than 40%, in particular less than 30%, for example less than 20%. Experimental tests have shown particularly favourable behaviour of the pump at such values of the duty cycle.
According to an embodiment, the control unit 20 is configured to adjust, or modulate, the ON duration compared to the OFF duration to obtain a duty cycle, with a value between 10% and 40%, for example between 20% and 30%.
According to a preferred embodiment, the control unit 20 is configured to perform a duty cycle of about 12%. At this value of duty cycle, it has been found that the power absorbed by the pump, although advantageously greatly reduced in value, allows the supply of an adequate fuel flow and pressure for nebulisation through the nebulizer 13.
According to an embodiment, the control unit 20 is configured to adjust the frequency of the sequence of actuation intervals of the pump 210, 211, 212 in the operative interval of the heater, especially at frequency values below 50Hz.
This makes it possible to further reduce the power absorbed by the pump 10. In fact, reducing the frequency of the supply portions, or sequence of pulses, means further reducing the area under the curve in the operative interval of the heater and thus reducing the energy absorbed by the pump. Said reduction of the frequency, in conjunction with the reduction in the average duration of the pulses compared to the average duration of pause intervals, allows very high energy savings to be achieved.
According to an aspect of the invention, the control unit is configured to adjust, or reduce the frequency of the pulse sequence, to values between 10Hz and 40Hz, for example between 10Hz and 30Hz, preferably to about 10 Hz. Experimental tests have shown particularly advantageous results in terms of energy saving by reducing the frequency of the pulse sequence to the above values. In fact in said pulse frequency ranges, in conjunction with the reduction in the average duration of the pulses compared to the average duration of the pause intervals, there is an evident reduction of electricity consumption while continuing to provide a fuel flow and pressure suitable for proper operation of the heater.
The optimum operating conditions occur at a supply frequency of approximately 10Hz. In such conditions the power absorbed by the pump is minimal but still permits the supply of a fuel pressure to the nebulizer, above the minimum nebulisation pressure, thus permitting an optimal nebulisation.
Such duty cycle and frequency values can be modulated jointly or separately, this way it is possible to achieve the best performance balance in relation to the type of pump used, with the least consumption of electricity.
However, the combination thereof permits the maximum energy saving.
In fact, a pump suitable to be powered at a frequency of 50Hz with a duty cycle of 50% resulting in a power consumption of 24W, is still able to provide sufficient flow and pressure of the fuel to the nebulizer if powered at a frequency of 10Hz with a duty cycle of 12%, leading to the optimal result of a consumption of just 5.7W. This is an energy saving so high that it doubles the power reserve of the heater if running on battery, or halves the size if powered by a Seebeck cell.
According to an embodiment, the control unit 20 is configured to modulate the ON duration T1 compared to the OFF duration T2 of each period T so as to provide sufficient liquid fuel to the nebulizer 13 at a pressure not lower than a predefined minimum threshold of nebulisation pressure.
In particular, said minimum threshold is about 8 bar.
According to an embodiment, the control unit 20 is configured to modulate the frequency of said sequence 210, 211, 213 in the time unit so as to supply the liquid fuel to the nebulizer 13 at a pressure not lower than said predefined minimum threshold of nebulisation pressure.
According to an embodiment, the control unit 20 is configured to vary over time the duration of the individual power intervals having an ON duration T1, and the ratio between the duration of each power interval and an OFF duration T2 of the interval that precedes and/or follows the power interval, and/or to vary the frequency of said sequence 210, 211, 213 over time, for example in a differentiated manner from one period to another, for example to compensate for any variations in the flow demand by the nebulizer and any variations in the flow, in order to ensure a uniform and constant flame over time.
The electric control unit 20 may comprise a closed loop control with feed-back on the fuel pressure measured upstream of the nebulizer. In this case the control unit is configured to automatically modulate the ON duration T1 and OFF duration T2 of each period T, and to modulate the frequency of the actuation intervals in the unit of time, so that the pressure measured upstream of the nebulizer is substantially equal to a predefined set-point value, for example not less than the predefined minimum threshold of nebulisation pressure.
Furthermore, according to an embodiment, the heater 100 may comprise pressure sensors, or pressure gauges 39 arranged to detect the pressure value of the fuel upstream of the nebulizer, and, for example, to send the corresponding information to the closed loop control.
Alternatively, the electric control unit 20 may comprise an open control in which the minimum nebulisation threshold value is mechanically set by the characteristics of the nebulizer. In this case the nebuliser permits nebulisation only above said minimum threshold and does not permit nebulisation below said minimum threshold.
The nebulizer 13 may comprise a calibrated pressure non-return valve 17, for example at the minimum threshold value for nebulisation, and a calibrated nebulizer nozzle 18.
This way the nebulizer valve 17 opens only when it reaches the minimum nebulisation threshold. Such a nebulizer prevents the involuntary leakage of fuel when the pressure at the nebulizer is less than the minimum threshold for nebulisation. This makes for considerable safety in use.
The power interval 115 may be formed of a single pulse to perform a pumping cycle, or the power interval 115 may be formed of plurality of successive electrical pulses close together to perform a corresponding plurality of pumping cycles. In particular, figure 3 shows an example of a power interval 115 consisting of a single electrical pulse 115 ', while figures 4 and 5 show examples of a power interval 115 consisting of 3 pulses 115'.
In other words, the sequence of pulses to power the pump may comprise a plurality of successive electrical pulses 115' close together, for example, to perform a corresponding plurality of pumping cycles.
In general a number of pulses may be used such as to generate a flow rate and fuel pressure such as to permit a continuous and uniform nebulisation to the nebulizer.
Said or each pulse of said power interval 115 may be in the form of a sinusoidal waveform or a square waveform.
According to an embodiment, the pump 20 is a reciprocating pump such as a piston.
According to an embodiment, such piston pump comprises a cylinder and a piston sliding inside the cylinder so as to push out the fuel in a pulsed manner towards the nebulizer. Outside the cylinder a solenoid is wound which, when crossed by electric current, generates an electromagnetic field which moves the piston between a first and a second end stroke position. In the movement from the first end stroke position to the second end stroke position the piston pump sucks the fuel from the tank, while in the opposite stroke, from the second end stroke position to the first end stroke position it pushes the fuel to the nebulizer 13.
The piston pump may comprise a spring configured to return the piston from the first end stroke position to the second end stroke position at the end of the dispensing phase.
In other words, such a piston pump provides for an active phase in which the solenoid is electrically powered, in which the piston moves from the second end stroke position to the first end stroke position, pushing the fuel to the nebulizer under pressure, and a passive return phase in which the piston moves backwards from the first end stroke position to the second end stroke position, in which the solenoid is not powered and the spring returns the piston to the second end stroke position performing the suction phase of the fuel.
With reference to figures 3 and 4 showing the voltage trend applied to the pump, in this case to the solenoid, the active phase is represented by the portion of the pulse 115 ' ranging from the minimum value of the potential, or voltage, to the maximum value of the potential, or voltage, and by the portion, while the passive phase corresponds to the pulse portion 115 ' which goes from the maximum value to the minimum value of the potential.
According to an alternative embodiment, the pump 10 may be a reciprocating pump diaphragm, so as to compensate abrupt pressure variations.
Again in an alternative embodiment, the pump 10 may for example be a gear or vane rotary pump. In this case, an example of a power supply voltage trend of the pump is shown in figure 5.
In general, the pump 10 may comprise an electrical actuator 10" and a mechanical pumping apparatus 10' mechanically connected to the actuator 10" so that said actuator 10" operates the mechanical pumping apparatus 10' to pump the fuel.
The electric actuator 10" transforms the incoming electricity into motion which is provided to the pumping apparatus.
The pumping apparatus 10' is selected from the normal existing types of devices such as piston, diaphragm, gear, vane.
The electric control unit 20 may comprise a current controller 31 and a timer 32, wherein said current controller 31 is configured to generate in output said power interval 115 having an ON duration T1, for example having a square or sinusoidal waveform, for example said power interval comprising a single pulse 115' or a plurality of pulses 115' side by side and close to each other, and wherein said timer 32 adjusts said pause interval 116, or OFF interval, having an OFF duration T2.
The heater 100 may comprise a switch 33 electrically interposed between the electricity source 24 and the electric pump 10, and in addition or alternatively, between the electricity source 24 and the control unit 20. For example the switch 33 is arranged to allow/prevent the power supply of the pump 10 and the control unit 20 simultaneously.
The electric control unit 20 and the switch 33 may be integrated on the same circuit board 37.
The heater 100 may further comprise electronic control devices 34, 35 to control further electrical devices 36', 36", 36'" placed on board the heater, such as fans 105 to generate a forced flow of air, or solenoid valves for the fuel.
Said electronic devices 34, 35 may be integrated on said circuit board 37.
According to an embodiment, the heater comprises an expansion chamber 14 in fluidic communication with the fuel between the outlet 12 of the pump 10 and the nebulizer 13. Said expansion chamber 14 is suitable to store excess fuel gradually with respect to a fuel flow actually passing through the nebulizer during the powering of the pump at the electrical pulses 115. Likewise, the expansion chamber is suitable to gradually release the excess fuel to the nebulizer 13 during the pause intervals, providing a continuous delivery of fuel through the nebulizer during an entire operative interval of the heater.
The predefined minimum threshold of nebulisation is for example about 8 bar.
The expansion chamber 14 may be a closed chamber of variable internal volume in fluidic connection with the pressurised fuel upstream of the nebulizer 13, suitable to expand containing a larger amount of fuel when the fuel pressure increases, and suitable to be compressed to release said fuel when the pressure decreases.
The heater 100 may comprise a rigid container 51 divided into two variable volume chambers, of which a first chamber 52 is formed of said expansion chamber 14, and a second chamber 53 contains a compressible material, such as a gas, so that when the expansion chamber 14 expands, said second chamber 53 is compressed accordingly, in particular exerting an additional pressure against the expansion chamber 14.
In other words the expansion chamber 14 serves as a temporal volumetric reserve of pressurised fuel performing two tasks together.
In fact, said expansion chamber 14 performs a first task acting as a flow equalizer, i.e. as a deposit and reserve of the excess flow of fuel dispensed by the pump in the active ON phase, enabling the continuous dispensing of fuel towards the nebulizer during the OFF phase of the pump.
In addition, said expansion chamber 14 performs a second task by acting as a pressure equalizer, i.e. damping the pressure peaks in the fuel circuit downstream of the pump.
The presence of said expansion chamber 14 compensates a possible fluctuating trend of peaks and dips of the fuel pressure downstream of the pump, due to the alternate operation of the pump, for example of the piston pump. This way, moreover, the risk of non-nebulisation and consequent involuntary extinguishing of the heater is avoided. The presence of the expansion chamber also makes it possible to avoid the phenomenon known as "pipe hammer" inside the fuel ducts in the portion between the pump and the nebulizer, caused by the rapid pressure drop occurring at the end of the active phase of powering the pump.
When the pump 10 is started, the fuel fills the expandable container up to a minimum nebulisation threshold value at which the nebulizer works, after which the fuel begins to flow from the nebuliser. At each pulse of the pump, a first part of the volume of fuel contained in the expandable container flows through the nebulizer, and a second part of said volume is stored in the expandable container in order to ensure the flow in the OFF phase.
According to another aspect of the present invention, the aforesaid purposes and advantages are achieved by a method for controlling the power supply of an electric pump for a liquid fuel portable heater.
The method for controlling an electric power supply of an electric fuel pump 10 for a liquid fuel portable heater by means of an electric control unit 20 configured to power the pump,
comprises a step of controlling said power supply, with the heater on, with a sequence of pulses 115, 115' with a non-zero voltage, and pause intervals 116 with a substantially zero voltage alternating with said pulses, wherein the average duration of the pulses 115, 115' is less than the average duration of the pause intervals 116.
According to an embodiment, the method comprises a step of adjusting the duty cycle value, or percentage value of the integral of a voltage/time curve of said sequence of pulses in an operative interval of the heater with respect to the integral of an hypothetical voltage/time curve with a direct, constant voltage with a amplitude equal to the maximum amplitude of said voltage/time curve of said sequence of pulses in the same operative interval of the heater, so that said duty cycle value is less than 50%.
In particular, the on duration T1, or pulse duration, and the off duration T2, or duration of the pause interval, are chosen so as to ensure a continuous supply of fuel to the nebulizer.
A step of adjusting the frequency of said sequence of pulses 115, 115' so that said frequency of the sequence is less than 50Hz.
According to an embodiment, the method comprises a step of modulating the ON duration T1 compared to the OFF duration T2 of each period T so as to provide the liquid fuel to the nebulizer 13 at a pressure not lower than a predefined minimum threshold of nebulisation pressure.
According to an embodiment, the method comprises a step of modulating the frequency of said sequence of pulses in the time unit so as to supply the liquid fuel to the nebulizer 13 at a pressure not lower than said predefined minimum threshold of nebulisation pressure.
According to an embodiment, the control method comprises a step of varying over time the duration of the pulses with respect to the duration of the pause intervals, so as to supply the liquid fuel to the nebulizer 13 at a pressure not less than a predefined minimum threshold of nebulisation pressure.
According to an embodiment the control method comprises a step of varying over time the frequency of the sequence of pulses so as to supply the liquid fuel to the nebulizer 13 at a pressure not less than a predefined minimum threshold of nebulisation pressure.
A person skilled in the art may make modifications and adaptations to the embodiments of the device described above, replacing elements with others functionally equivalent so as to satisfy contingent requirements within the scope of the following claims.

Claims

A liquid fuel portable heater (100) for heating air to enter in an environment to be heated, comprising:
- a frame connecting and joining all the components of the heater (100), so as to easily move the heater (100) in one body;

- a tank (6) for liquid fuel placed on board the heater (100),

- a combustion chamber (101) having a fuel inlet with a nebulizer (13) having a nebulizer nozzle (18);

- an outer casing (102) forming an interspace (103) around the combustion chamber (101), suitable to be crossed by a flow of air (104) thereby thermally insulating the combustion chamber (101) from the outer casing (102), and simultaneously heating and forcibly entering an environment to be heated;

- an electric pump (10) having an inlet (11) for suctioning said liquid fuel from the tank (6), and an outlet (12) connected to said nebulizer (13) to bring the liquid fuel to the nebulizer (13);

- an electric control unit (20) configured so that, when the heater (100) is turned on, said control unit (20) actuates the electric pump by supplying it with a sequence of pulses (115, 115') with a non-zero voltage, and pause intervals (116) with a substantially zero voltage alternating to said pulses, wherein the average duration of the pulses (115, 115') is less than the average duration of the pause intervals (116).
The portable heater according to claim 1, wherein said electric control unit (20) is configured to define a duty cycle value less than 50%, said duty cycle value being the percentage value of the integral of a voltage/time curve of said sequence of pulses (115, 115') in an operative interval of the heater (100), with respect to the integral of a hypothetical voltage/time curve with a direct, constant voltage, with a amplitude equal to the maximum amplitude of said voltage/time curve of said sequence of pulses (115', 115) in the same operative interval of the heater.
The portable heater according to at least one preceding claim, wherein said control unit (20) is configured to adjust the frequency of said sequence of pulses (115, 115'), so that said frequency is less than 50Hz.
The heater according to at least one preceding claim, wherein said control unit (20) is configured so as to vary over time the duration (T1) of the pulses with respect to the duration (T2) of the pause intervals, so as to provide the liquid fuel to the nebulizer (13) at a pressure that is not less than a predetermined minimum threshold of nebulisation pressure.
The heater according to at least one preceding claim, wherein said control unit (20) is configured to vary over time the frequency of the sequence of pulses (115, 115') so as to provide the liquid fuel to the nebulizer (13) at a pressure that is not less than a predetermined minimum threshold of nebulisation pressure.
The heater according to at least one preceding claim, wherein said electric control unit (20) is configured so that the sequence of pulses (115, 115') comprises a plurality of electric pulses (115') that are successive and close together, in particular to carry out a corresponding plurality of pumping cycles.
The heater according to at least one preceding claim, comprising an expansion chamber (14) in flow communication with the liquid fuel, interposed between the outlet (12) of said pump (10) and said nebulizer (13), said expansion chamber (14) being configured to store an excess amount of liquid fuel with respect to a flow rate of liquid fuel passing through the nebulizer (13), said excess amount of liquid fuel being generated during an electric power supply of the pump at the electric pulses (115), and being configured to gradually release said excess amount of liquid fuel to the nebulizer (13) during the pause intervals (116), providing a continuous dispensing of liquid fuel through the nebulizer (13) during an entire operative interval of the heater.
The heater according to any preceding claim, comprising at least one conversion cell (95) for the direct conversion of a temperature differential into electric power, said at least one conversion cell (95) being mounted at the heater (100) to receive a temperature differential between the combustion chamber (101) and the outer environment, said at least one conversion cell (95) being connected to electrically supply said electric pump and said control unit (20).
A method for controlling an electric power supply of a fuel electric pump (10) of a liquid fuel portable heater according to at least one of the preceding claims, by an electric control unit (20) configured to control said electric power supply,
said method comprising a step of electrically supplying said pump, when the heater is turned on, with a sequence of pulses (115, 115') with a non-zero voltage, and pause intervals (116) with a substantially zero voltage alternating to said pulses, wherein the average duration of the pulses (115, 115') is less than the average duration of the pause intervals (116).
The method according to claim 9, comprising a step of adjusting the duty cycle value, defined as the percentage value of the integral of a voltage/time curve of said sequence of pulses (115, 115') in an operative interval of the heater (100), with respect to the integral of an hypothetical voltage/time curve with a direct, constant voltage with an amplitude equal to the maximum amplitude of said voltage/time curve of said sequence of pulses (115, 115') in the same operative interval of the heater, so that said duty cycle value is less than 50%.
The method according to claim 9 or 10, comprising a step of adjusting the frequency of said sequence of pulses (115, 115'), so that said frequency of the sequence is less than 50Hz.
The method according to at least one claim 9 to 11, comprising a step of varying over time the duration of the pulses (115, 115') with respect to the duration of the pause intervals (116), and/or varying over time the frequency of the sequence of pulses (115, 115') so as to provide the liquid fuel to the nebulizer (13) at a pressure that is not less than a predetermined minimum threshold of nebulisation pressure.
The method according to at least one claim 9 to 12, comprising a step of adjusting, or modulating, the duty cycle value between 10% and 40%, preferably between 20% and 30%, more preferably about 12%.
The method according to at least one claim 9 to 13, comprising a step of adjusting, or reducing the frequency of the pulse sequence, to values between 10Hz and 40Hz, preferably between 10Hz and 30Hz, more preferably about 10 Hz.
The heater according to at least one claim 1 to 8, wherein the pump (10) is a reciprocating piston pump, or a reciprocating diaphragm pump, or a gear or vane rotary pump.