GB2146157A - Automatic blocked drain safeguard for combustion systems in which flue gasses are condensed - Google Patents

Abstract

In a boiler (or like burner) wherein the heat exchange is sufficiently good to require condensate from the flue gasses to be drained from the flue, a safety arrangement comprises an alternating current flame detector of the kind which employs the rectifying action of the flame in combination with a moisture detector located in the condensate drain and connected electrically to the flame detector electrodes, the fuel valve control system being sensitive to either the loss of the flame or the detection of a build up of moisture in the condensate drain to shut off the fuel supply.

Description

SPECIFICATION Automatic blocked drain safeguard for condensing combustion systems The present invention relates to an automatic blocked drain safeguard for condensing com bustion systems.

As the real cost of fossil fuels increases there is a corresponding demand for improve ments in the thermal efficiency of water heat ing appliances used in domestic, commercial and industrial situations.

This demand has led to the development of more efficient heat exchangers which actively cause condensation of water vapour in exhaust products, the resulting condensate being drained safely away, often via a dedicated drain pipe.

As part of a comprehensive automatic operating system for this type of water heater it is important to be able to ensure,that a blockage in the drain pipe does not lead to a build up of condensate within the appliance which could lead to mechanical damage or faulty operation.

It is an object of the present invention to provide a reliable, low cost means of detecting and reacting to a blocked drain situation in such a system.

The invention relies for its effectiveness and low cost on the prior need and existence of an electronic rectification type flame detector used during the boiler ignition and operating stages to provide safe automatic control. This well established means of detecting hydrocarbon flames relies on an electrically conductive electrode being placed in contact with the flame, this electrode having the ability to withstand flame temperatures and being electrically insulated from the metal of the earthed burner.

Provided that the flame is also in contact with earthed metal, then the existence of a flame results in a detectable electrical conduction path between the electrode and the earthed metal via the flame.

If then an alternating voltage is applied between the electrode and earth, the flame is found to partially rectify this voltage and cause a direct current to be produced of typically 0.2-20 ,uA, depending on the size of the flame and other associated factors. This small current is then amplified to signal the presence of a flame.

It has now been found that, if the alternating voltage is supplied from a high impedance source, typically of 0.2-20 MPa, the previously described D.C. flame current is largely unaffected but an additional leakage resistance of the same order or lower than the voltage source impedance existing between the flame detection electrode and earth can cause a significant reduction in the D.C. current produced by a given flame.

In accordance with one aspect of the present invention, there is provided an automatic blocked drain detection system for condensing combustion systems of the type having a boiler flame detection arrangement in which a voltage is applied between a flame detection electrode and earth, the presence of a flame causing a current to flow between the flame detection electrode and earth and the presence or absence of this current being used to determine the switching state of a switching circuit controlling the supply of fuel to the boiler burner(s), and wherein, for the detection of a blockage in the condensate drain pipework of the boiler, a second electrode is fitted into the drain pipework in a position which will ensure contact with the condensate if the latter pipework becomes blocked thereby, the latter electrode being connected electrically to the flame detection electrode but insulated from the drain pipework, which is earthed, the arrangement being such that if the drain becomes blocked a low resistance path is established from the second electrode to earth via the condensate effective to cause the switching circuit to adopt its switching state corresponding to a flame failure condition and therefore resulting in the automatic shutdown of the boiler.

In accordance with a second aspect of the present invention, there is provided a combined flame control and blocked drain detector for a condensing combustion system, comprising a first electrode which is insulated from earth and which is disposed so as to lie within the flame from an earthed burner during normal operation, a second electrode which is electrically connected to the first electrode and positioned in the drain pipework of the combustion system such that, if that pipework should become blocked by condensate, the condensate provides a low resistance path between the second electrode and earth, a high impedance A.C. source supplying an alternating voltage to the first and second electrodes, and a detector circuit coupled to said electrodes, the detector circuit being adapted to respond to the presence of a predetermined D.C. current between the first electrode and earth resulting from partial rectification of the alternating voltage by the flame to signal normal operation of the combustion system but the presence of said low resistance path, should the pipework become blocked by condensate, being arranged to modify said D.C. current sufficiently to cause the detector circuit to signal a flame failure condition.

Thus, under normal running conditions the second electrode fitted in or near the drain is not in contact with condensate and so the flame detection and supervision functions of the automatic control system can respond directly to the state of the flame. This means that the boiler is allowed to operate as long as heat is required and a flame is present, shut down only occuring if heat is no longer required or the flame goes out. However, with the drain blocked, the level of the condensate rises until it makes electrical contact with the drain electrode.This causes a low resistance path to be formed between the electrode and earth via the condensate, of the order typically of 2-200K. This low resistance leakage path is arranged to effectively suppress the operation of the flame detector, causing it to signal a flame failure condition, resulting in the automatic shutdown of the appliance.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows diagrammatically one embodiment of a blocked drain safeguard in accordance with the invention, including its relationship with the other parts of the automatic combustion control system; and Figure 2 shows in more detail one embodiment of the electrical circuitry for performing automatic shutdown in the event of a blocked drain condition.

Fig. 1 shows an automatic combustion control system 10 connected to a flame detection electrode 12, which, in turn, is in electrical contact with the flame(s) 14. A simplified, earthed metal burner 16, is also shown.

The alternating flame electrode voltage, with its high impedance source, is supplied by the flame detection circuit 18. Condensate produced during combustion is collected and removed via an earthed metal drain pipe 20, into which is mounted an insulated, condensate, detection electrode 22 coupled by an electrical link 24 to the flame probe 1 2.

Detection of a blocked drain by the simulation of a flame failure condition is achieved via the electrical link 24, with the subsequent shutdown of the appliance being achieved by a logic/timing circuit 26, within the automatic combustion control system 10.

Fig. 2 shows in more detail an example of a circuit for achieving detection of a flame failure condition, modified in accordance with the present invention to detect also a blocked drain condition.

An alternating supply voltage to the two probes 1 2 and 22 is connected via a high impedance source, represented by capacitance Cl.

When no flame or blocked drain exists, a filter circuit comprising resistors, R1 and R2, together with a capacitor, C2, prevents any alternating signal from entering a D.C. amplifier 28 so that no output signal is given thereby.

However, when a flame is established and brought into contact with both earthed burner 1 6 and the flame detection probe 12, the ionised gases of the flame 14 cause an alternating current to flow through the flame together with a flame-related D.C. component.

This D.C. component causes a corresponding DC bias to be produced in the probe voltage, which is separated from the AC waveform via the filter circuit R1, R2, C2 before entering the input of the D.C. amplifier. This leads to the signalling of a 'flame-on' condition via some form of semiconductor or electromechanical device 32.

In practice, this flame signal is often employed to maintain the fuel shut-off valve(s) energised, resulting in automatic shutdown of the burner if the flame signal is cut off.

If the condensate drain 20 becomes blocked while a flame is being signalled, the condensate level will rise until it makes contact with the drain probe 22. This contact results in a low resistance path being formed between the drain probe and the earthed drain, which causes both the A.C. and D.C.

components of the probe voltage to be reduced below the level necessary for detection of the flame. The resulting flame failure condition can be used to initiate complete shutdown of the burner system.

There is therefore provided by the invention a simple safeguard to ensure that the boiler is shutdown in the event that the drain pipe should become blocked.

Whereas it is assumed in the aforegoing description that the burner(s) is earthed, this is not essential. Some burners may be of materials other than metal, e.g. ceramics. In this event, an earthed electrode is positioned adjacent the burner mouth to provide a path for the D.C. current to earth.

Furthermore, it is not essential for the operating voltage to be A.C. In some embodiments the applied voltage to the electrodes can itself be D.C. without affecting the operating principle of the system.

Claims (6)

1. An automatic blocked drain detection system for condensing combustion systems of the type having a boiler flame detection arrangement in which a voltage is applied between a flame detection electrode and earth, the presence of a flame causing a current to flow between the flame detection electrode and earth and the presence or absence of this current being used to determine the switching state of a switching circuit controlling the supply of fuel to the boiler burner(s), and wherein, for the detection of a blockage in the condensate drain pipework of the boiler, a second electrode is fitted into the drain pipework in a position which will ensure contact with the condensate if the latter pipework becomes blocked thereby, the latter electrode being connected electrically to the flame detection electrode but insulated from the drain pipework, which is earthed, the arrangement being such that if the drain becomes blocked a low resistance path is established from the second electrode to earth via the condensate effective to cause the switching circuit to adopt its switching state corresponding to a flame failure condition and therefore resulting in the automatic shutdown of the boiler.

2. A system as claimed in claim 1 wherein said voltage applied to the flame detection electrode is an A.C. voltage, the presence of a flame causing a rectified D.C. current to flow between the flame detection electrode and earth.

3. A system as claimed in claim 2 wherein the switching circuit comprises a D.C. amplifier, whose input is commonly coupled to both the flame detection and blocked drain electrodes via an R.C. filter circuit, the presence and absence of said rectified D.C. current resulting in output signals of the amplifier corresponding to normal operation and a flame failure condition, respectively.

4. A combined flame control and blocked drain detector for a condensing combustion system, comprising a first electrode which is insulated from earth and which is disposed so as to lie within the flame from an earthed burner during normal operation, a second electrode which is electrically connected to the first electrode and positioned in the drain pipework of the combustion system such that, if that pipework should become blocked by condensate, the condensate provides a low resistance path between the second electrode and earth, a high impedance A.C. source supplying an alternating voltage to the first and second electrodes, and a detector circuit coupled to said electrodes, the detector circuit being adapted to respond to the presence of a predetermined D.C. current between the first electrode and earth resulting from partial rectification of the alternating voltage by the flame to signal normal operation of the combustion system but the presence of said low resistance path, should the pipework become blocked by condensate, being arranged to modify said D.C. current sufficiently to cause the detector circuit to signal a flame failure condition.

5. An automatic block drain detection system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A combined flame control and blocked drain detector for a condensing combustion system, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.