GB2205150A - Blockage detection in heat exchangers e.g. boilers - Google Patents

Abstract

A heat exchanger comprises a plurality of parallel fluid circuits (A; B), each of which contains at least two groups (21, 23, 25, 27; 22, 24, 26, 28) of heat exchange elements (10). The elements of each group are connected in parallel with each other. Successive groups of elements in each respective circuit are connected together in series through an intermediate conduit (31, 33, 35; 32, 34, 36) providing a single flow path. The exchanger has detector means (18) responsive to a pressure difference between two points, the detector means being connected between the parallel fluid circuits, the connection of the detector to each circuit being at a point (146) in that circuit which, in normal operation, is at substantially the same pressure as the point (148) to which the detector means is connected in the or each other of said circuits. The detector (18) may be a thermal switch. The heat exchanger may comprise (Figs 2 to 5, not shown) an annular array of tubes surrounding a burner space and extending between two annular headers, each divided into two pairs of spaces, one pair for each respective circuit A, B. <IMAGE>

Description

HEAT EXCRANGERS This invention relates to heat exchangers comprising a plurality of parallel fluid circuits each comprising at least two groups of heat exchange elements in which the elements of each group are in parallel with each other, successive groups of elements being connected together in their respective circuit through a conduit providing a single flow path.

Heat exchange takes place between a first fluid flowing in the parallel circuits, and a second fluid around the outside of the heat exchange elements. A non-limiting example of such a heat exchanger is one incorporated in a module of a boiler for heating water for a central heating system.

Typically the heat exchanger may comprise a series of manifolds in each circuit, the manifolds being connected by tubes, usually with an extended heat transfer surface provided externally, these tubes constituting the heat exchange elements.

The performance of the heat exchanger will be impaired if one of the elements becomes blocked, for example by foreign matter or by excessive scaling. Because there are two or more parallel circuits, and because in each circuit there are always two or more heat exchange elements in parallel with each other, blockage of a single element may easily go undetected for some time. In the typical application of the heat exchanger to a central heating boiler, the initiation and cessation of combustion is determined by the demand for hot water, controlling the operation of the boiler through suitable thermostatic controls.If flow of water in one circuit is reduced by virtue of a heat exchange element becoming blocked, thereby reducing the thermal efficiency of the heat exchanger, combustion will merely be prolonged, thereby using more fuel than necessary. It will in many circumstances not be possible to detect, even from knowledge of increased fuel consumption, that anything is amiss. Even if in the opinion of the operator something is amiss it may not be clear that the cause is a blocked heat exchange element; and even if this cause is established, the location of the blocked element may not be easy to find without careful and timeconsuming visual inspection of all the elements.

In the case of a modular central heating boiler, for example of the kind described ln our United Kingdom patent No. 1556813, which is typically used In industrial or commercial premises, an advantage of the modular arrangement (whereby the boiler consists of two or more self-contained modules, each with its own heat exchanger, burner and control system) Is that one module may be taken out of service for inspection, maintenance and repair without closing down the whole boiler. However, it is clearly desirable to reduce as far as possible the time during which a module is necessarily out of service.If the heat exchanger of that module has a comparatively large number of heat exchange elements, therefore, it is desirable to be able not only to detect promptly a blockage in any one of the elements, but also to be able to locate that element promptly so that the matter may be rectified as quickly as possible. This may largely be achieved if the fault is indicated as soon as it occurs.

According to this invention, in a heat exchanger comprising a plurality of parallel fluid circuits each comprising at least two groups of heat exchange elements in which the elements of each group are in parallel with each other, successive groups of elements being connected together in their respective circuit through a conduit providing a single flow path, there is provided detector means responsive to a pressure difference between two points, the detector means being connected between the parallel fluid circuits, the connection of the detector means to each circuit being at a point in that circuit which in normal operation is at substantially the same pressure as the point to which the detector means is connected in the or each other of said circuits.

Thus if a heat exchange element In one of the circuits becomes blocked, the resulting change in pressure of the fluid flowing in that circuit, at the point in the circuit to which the detector means is connected, causes a differential pressure to arise between that circuit and the other circuit or circuits. The detector means is preferably connected to suitable means for giving a warning, which can by known means be arranged to indicate whlch cIrcuIt contaIns the blockage. At the same time, the detector means can initiate shutdown of the system, in readiness for the blockage to be located and corrected.

It will be noted that the blockage need not be complete, but need only be sufficient to cause a large enough pressure differential to be set up between the circuits to enable the detector means to operate.

The detector means is preferably in the form of a thermal switch, arranged to respond to changes in temperature, caused by a flow of fluid as a result of the pressure difference between the two circuits which, in turn, results from the occurrence of a blockage in one of them. The thermal switch Is preferably so located that fluid reaches it from the two circuits at a temperature only a little above ambient temperature. A relatively small difference between the temperatures in the parallel circuits of the heat exchanger can then represent a substantial change as compared with the excess over ambient of the normal temperature sensed by the thermal switch.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings of this application, in which: Figure 1 is a diagram illustrating prIncipal parts of a module for a central heating boiler, including a heat exchanger in one embodiment according to the invention; Figure 2 is a diagrammatic representation showing a rear header of one form of heat exchanger according to the invention; Figure 3 is a diagram similar to Figure 2 but showing a front header of the same heat exchanger; Figure 4 is a simplified sectIonal view showing one possible construction of the heat exchanger of Figures 2 and 3, taken on the line IV-IV in Figure 5, with a small portion (marked "X" in Figure 4) shown as a cut-away section on the line IVA-IVA in Figure 5; and Figure 5 is a simplified sectional view taken on the line V-V in Figure 4.

Referring to Figure 1, a heat exchanger comprises two parallel fluid circuits A and 5. Each circuit A, B comprises four groups of heat exchanger elements 10. In circuit A, these groups are indicated at 21, 23, 25 and 27 respectively. In circuit 3, the corresponding groups of heat exchange elements are indicated at 22, 24, 26 and 28 respectlvely. The heat exchange elements in this example contain water to be heated. This water flows through the groups of heat exchange elements in each circuit in the order quoted above.

Each group of heat exchange elements comprises four of the elements 10, arranged in parallel with each other. The first groups 21, 22 of the two circuits A, B are connected together at their inlet side by a common feed conduit 30 receiving unheated water from a water inlet 38. Similarly, the fourth groups 27, 28 are connected together at their outlet side by a common delivery conduit 37 supplying heated water to a water outlet 39.

Each successive group of elements in each circuit is connected with the next group through an intermediate conduit providing a single flow path.

For circuits A and 3, these intermedIate conduits comprise, respectively, a first intermediate conduit 31, 32; a middle conduit 37, 34; and a second intermediate conduit 35, 36.

Figure 1 shows the heat exchanger in association with a burner 12, supplied with a mixture of fuel and air through an inlet duct 100, with the combustion gases heating the water in the heat exchanger by indirect heat exchange. Fuel is supplied to the duct 100 through a fuel valve indicated at 14, and air through a forced-draught fan indicated at 16. The components shown in Figure 1, together with a conventional control system not shown, constitute a boiler or a module for a boiler, for central heating purposes or for heating domestic or process hot water, or for any of these purposes.

A detector 18 is connected through a tube 19 to a tapping indicated at 146 into the middle conduit 33 of circuit A, and through a tube 20 to a tapping indicated at 148 into the middle conduit 34 of circuit B. The tubes 19 and 20 are of quite small cross-section, but are interconnected. The detector 18 consists of a thermal switch, arranged to deliver an electrical signal to a switching circuit indicated at 40.

In operation, the thermal switch 18 is responsive indirectly to a pressure difference between the two middle conduits 33 and 34. In this connection it will be appreciated that, in normal operation of the heat exchanger, the static pressure in the conduit 33 is substantially the same as that in the conduit 34. Suppose, however, that any one of the heat exchanger elements 10 becomes blocked by a piece of foreign matter. If the blocked element is in group 21 or 23, the pressure at the conduit 33 will decrease, so that a lower pressure will now obtain In conduit 33 than in conduit 34. This causes a resultant flow of water in the tubes 19 and 20, from circuit B to circuit A. This in turn causes the temperature at the thermal switch 18 to change.In direct response to this change of temperature, the switch 18 actuates the switching circuit 40 to close the fuel valve 14 and stop the forced-draught fan 16, so closIng down the system. At the same time, it causes a visual or audible alarm to operate, so alerting the operator to the presence of a blockage, which can then be sought and rectified.

The thermal switch will operate whether the flow in the tubes 19 and 20 is from circuit B to circuit A or vice versa. For example, if the blocked tube is in group 25 or 26, or in group 22 or 24, the resulting pressure change in the circuit concerned will cause water to flow from circuit A to circuit B.

A bypass pipe 140 is provided to connect the tubes 19 and 20, so as to provide a permanent bypass in parallel with the portion of tubing carryIng the thermal switch 18. The effect of this bypass is to desensitise the blockage detection system to the extent required to eliminate the effect of slight pressure imbalances between circuits A and 3 due to manufacturing tolerances.

Other refinements of this system may optionally be introduced so as to locate the particular group of heat exchange elements containing the blockage. One example of this is illustrated diagrammatically in Figure 1, in which the first intermediate conduits 31 and 32 are shown connected together by a small-bore tube 42, and the second intermediate conduits 35 and 36 similarly by a smallbore tube 44. A pair of pressure transduces 46, 48 is located at or near opposite ends of the tube 42, another pair 50, 52 similarly with respect to the tube 44, and a further pair of pressure transducers 54, 56 is similarly placed in respect of the tubes 19 and 20 respectively.All of these transducers are connected to a suitable electronic comparator circuit 58, which compares the signals from the pressure transducers of each pair to detect the direction of flow in each of the tubes 42 and 44 and that in the tubes 19 and 20.

Thus, if, for example, the flow in all three cases is from circuit A to circuit 3, the blockage will be In group 27 of circuit A. If flow in tubes 19, 20 and 42 is from circuit B to circuit A but that in tube 44 is from A to 3, the blockage will be in group 26 of circuit 3. Aga-in, if the blockage is in group 21 of circuit A, flow will be from B to A in all of the tubes. The comparator circuit 58 can be made to actuate a suitable indicator (not shown) to identify the location of the group of elements affected.

The heat exchanger shown by way of example in Figures 2 to 5 is a tubular, parallel-flow heat exchanger suitable for use in a module of a modular industrial central heating boiler. The module has a central gas burner 112 of cylindrical construction, supplied with a mixture of gas and air (in the manner mentIoned in connection with Figure 1) by a forceddraught fan not shown, through the inlet duct 100.

The heat exchanger itself incorporates a water system comprising two parallel circuits as in Figure 1. In each circuit there are again four successive groups of heat exchange elements, the groups in each circuit being in series with each other and each group comprising four heat exchange elements, 110, in parallel with each other.

Each heat exchange element 110 consists of an externally-finned tube, fixed at one end to a rear header 102, and at the other end to a front header 104, of the heat exchanger. The headers are circular, and the elements 110 are arranged in two concentric rings, so that the combustion gases fom the burner 112, which is coaxial with the rings of elements 110, pass radially outwards through these rings.

The rear header 102 is subdivided by four equally-spaced, generally radial bulkheads 106 into four manifold chambers, namely an inlet chamber 130, an outlet chamber 137, and two middle chambers 133 and 134, both of which physically separate the inlet chamber frm the outlet chamber. The inlet and outlet chambers are common to the two parallel circuits A and B (Figure 1), while the chambers 133 and 134 are part of circuit A and of circuit 3 respectively. A water inlet 138 and a water outlet 139 are connected into the chambers 130 and 137 respectively.

The front header 104 has solid end plates and is subdivided internally, by four axisymmetrical and generally radial bulkheads 114, into four equal manifold chambers 131, 132, 195 and 136. The chambers 131 and 132 constitute the first intermediate conduits, and the chambers 135 and 136 the second intermediate conduits, of the respective water circuits. The bulkheads 106 and 114 are so positioned and shaped that the elements 110 comprise groups of four elements.Thus, two first groups 121, 122 join the inlet chamber 130 with, respectively, the first intermediate chambers 131 and 132; two second groups 123, 124 join the chambers 131 and 132 respectively with the second intermediate or middle chambers 133, 134; two third groups 125, 126 joining the chambers 133, 174 with the chambers 135 and 136 respectively; and finally two fourth groups joining the chambers 135 and 136 with the outlet chamber 137.

In each chamber indicated in Figures 2 and 3, the elements 110 in which the direction of water flow s into the chamber are indicated with a central spot; while those in which the flow is out of the chamber are marked with a cross.

A tube 142 is tapped into the middle chamber 133 in the tapping 146, and a similar tube 144 into the other middle chamber 134 in the tapping 148. The tubes 142 and 144 are connected together by the bypass tube 40, and lead respectively to smallbore tubes 119 and 120 respectively. The tubes 119, 120 are for example of about 6 mm diameter. The tubes 119 and 120 communicate with each other at a thermal switch 118 located well away from any radiant heat from the burner. The function of the thermal switch 118 is the same as that of the switch 18 In Figure 1, and the length of the tubes 142, 144, 119 and 120 is sufficient to allow the temperature of any water flowing in them, in the event of a heat exchange element becoming blocked as described above, to fall to a value close to the ambient temperature.

A heat shield, not shown, may optionally be provided to protect the thermal switch.

The mechanical construction of a heat exchanger according to the invention may take any desired form, and the heat exchange elements need not be arranged in rings as described.

There may be more than two parallel fluid circuits, and the differential pressure detecting device such as thermal switch 18 or 118 may be connected to all these circuits. In any one circuit there ma.y be any required number of groups of heat exchange elements, and there can be any convenient number of parallel elements in any one group. The successive groups of elements in a circuit need not each have the same number of elements. The elements can take any convenient form.

The detecting device may comprise a pressure transducer for directly detecting fluid flow, instead of a thermal switch. 'It may be connected between other intermediate manifold chambers or conduits instead of the middle chambers or conduits.

Claims (9)

1. A heat exchanger comprising a plurality of parallel fluid circuits, each comprising at least two groups of heat exchange elements, the elements of each group being connected in parallel with each other, successive groups of elements in each respective circuit being connected together through a conduit providing a single flow path, the exchanger including detector means responsive to a pressure difference between two points, the detector means being connected between the circuits, the connection of the detector means to each circuit being at a point in one said circuit which, in normal operation, is at substantially the same pressure as the point to which the detector means is connected in the or each other of said circuits.

2. A heat exchanger according to Claim 1 wherein the detector means is connected to alarm means for giving visual and/or audible warning.

3. A heat exchanger according to Claim 2 wherein the alarm means is arranged to indicate which of said two points is at a lower pressure than the other point, the lower pressure being indicative of blockage at that point or upstream thereof.

4. A heat exchanger according to any one of the preceding claims wherein the detector means is arranged to initiate shut-down of the heat exchanger system if a pressure difference has been detected.

5. A heat exchanger according to any one of the preceding claims wherein the detector means is in the form of a thermal switch arranged to respond to changes in temperature caused by a flow of fluid as a result of the pressure difference between the two circuits which, in turn, results from the occurrence of a blockage in one of them.

6. A heat exchanger according to Claim 5 wherein the thermal switch is so located that fluid reaches it from the two circuits at a temperature only a little above ambient temperature.

7. A heat exchanger according to any one of the preceding claims including a bypass for desensitising the detector means.

8. A heat exchanger according to any one of the preceding claims wherein the detector means comprise a plurality of detector pairs for detecting pressure difference at various paired points and a comparator.

9. A heat exchanger according to Claim 1 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying diagrammatic drawings.