GB2332508A

GB2332508A - Heat exchanger for combi boilers

Info

Publication number: GB2332508A
Application number: GB9827707A
Authority: GB
Inventors: Roy Bratley
Original assignee: CARADON IDEAL Ltd
Current assignee: CARADON IDEAL Ltd
Priority date: 1997-12-18
Filing date: 1998-12-16
Publication date: 1999-06-23
Also published as: GB9726703D0; GB9827707D0

Abstract

A block-type heat exchanger 20 has a main body 21 and a detachable end plate 22. The main body and the inner face of the end plate define conduits 11 for the flow of water for a central heating circuit. Gas flueways 10 extend through the main body, and water in the conduits is heated by heat exchange with hot combustion gas flowing through the flueways. The end plate supports tubes 23 projecting into the conduits forming part of a domestic hot water supply line. Water in the supply line is heated by heat transfer from the water in the conduits. The relatively high thermal inertia of the heat exchanger simplifies control of the heat exchanger water temperature and can help reduce time-lag in the provision of domestic hot water.

Description

1 2332508 1 HEATEXCHANGER This invention relates to heat exchangers and is

parficularly concerned with heat exchangers for domestic gas boilers of the type capable of supplying both heated fluid for a central beating circuit and heated potable water for a domestic supply circuit without the need for additional storage, i.e. so called combi' boilers.

In a combi boiler, the fluid, e.g water, for the central heating circuit and potable water in the domestic supply circuit are heated on demand. Consequently, the supply of heated water is not instantaneous, and to reduce the response time between the demand for and the supply of heated potable water efforts have been made to reduce the thermal inertia of heat exchangers. As a result many combi boilers have low water content copper heat exchangers. The low water content, and a resultant sensitivity of the heat exchanger to water flow rate necessitate the use of a complex control system to synchronise water flow and burner ignition. The control system is complicated further by the need for a modulated heat input which aims to match the gas combustion rate to the heat output requirement. The complexity of the control system means that it is prone to malfunction, leaving the boiler inoperative.

It is the aim of the present invention to address this problem and thereby improve the reliability of combi boilers. Accordingly, the invention provides a block-type heat exchanger for a combi boiler, the heat exchanger comprising a body having a first passageway for the flow of fluid for a central heating circuit, a second passageway for the flow of potable water for domestic supply, and a flueway in heat transfer relation with the passageways for the flow of a -aseous heat exchanae medium.

A block-type heat exchanger compnses one or more cast modules, c) but herein the term encompasses other forms of heat exchanger with equivalent In characteristics.

Contrary to previous developments of comb, boilers the then-nal inertia of the heat exchanger is increased by the incorporation of the first and second passageways into the body of the block type heat exchan er. However, as a result of the increased thermal inertia the synchronisation of the fluid now through the passageways and burner ignition is not of critical importance, and a relatively simple control system can be employed. The maintenance of the potable domestic water above a predetermined level when there is no demand keeps the response time for the supply heated potable water at an acceptable level.

Preferably, the heat exchanger comprises temperature sensing means for providing a signal indicative of the temperature of water in the first passageway and activating the burner to heat the water in the heat exchanger in Z7 c response to said water temperawe falling below the predetermined level. The use of a single signal indicative of the water temperature allows further simplification of the control system and therefore a single signal is preferred. However, the invention encompasses temperature sensitive means providing more than one signal indicative of the water temperature at different locations in the heat exchanaer or on the outlet for potable water.

In a preferred embodiment, the second passageway is disposed within the first passageway so that the fluid m the first passageway is in direct heat exchange relation with the gaseous medium in the flueway, and the water m the second passageway is in direct heat exchange relation with the fluid in the first t7 passageway, i.e. the gaseous medium and the potable water are in indirect heat exchange relation. The second passageway may define a sinuous path through the first passageway, and the water in the second passageway may make one or more passes along the length of the first passageway.

Three or more first and second passageways may be provided to Ifol C 1 1 ensure un -m heating and cooling in both passageways and thus further s'mpl'f, 1 J the required control system, The invention also provides a combi boiler having a gas burner, and a block-type heat exchanger comprising a body having a first passageway for the c C7 flow of fluid for a central heating circuit and a second passageway for the flow of potable water for domestic supply, and a flueway in heat transfer relation with the passageways foi- the flow of a gaseous heat exchange medium.

A clear understanding of the present invention will be gained with reference to the following detailed description of a preferred embodiment, given by way for example only with reference to the accompanying drawings in which:

C -- Fi,c;ure 1 shows a partially sectioned plan view of a known heat exchangerl Figure 2 shows a section taken along line A-A in Figure 2; C) Cl I Figure 3 shows an exploded perspective schematic illustration of an embodiment of a heat exchanger according to the invention, and Figure 4 shows side elevations of alternative embodiments of potable water passageways.

The known heat exchanger shown in Figures 1,2 is formed as a generally rectangular casting defining three upwardly directed elongate flueways 10 for flow of a gaseous heat transfer medium, in particular hot flue gases from a gas burner of a domestic gas boiler, and four conduits 11, the conduits being adjacent to and in direct heat exchange relationship with the gas flueways, for flow of a fluid to be heated through the heat exchanger between water inlet and outlet ports. The heat exchanger may be cast in one piece, as shown, or it could be formed from a number of separate castings.

Extending along each gas flueway 10, continuously from adjacent a lower entrance end 12 to an upper exit end 13), are heat exchange fins 14 formed integrally with the walls of the flueway. The fins on opposed sides of each flueway may be aligned, as shown in Figure 1, or they may be staggered so that C) I Z-1 4 the tip of each fin confi-onts or extends into the space between the an opposed pair of adjacent fins.

A baffle (not shown) may project into the flueways to increase the turbulence of the passing gases, and thus Improve the efficiency of the heat exchanae between the gas and the fluid.

An embodiment of a heat exchanger according to the invention, generally designated 20, is illustrated schematically in Figure 33. Three parallel finned flueways 10 extend laterally of a main body 2 1. Adjacent to and in direct heat transfer relation with the flueways are four conduits I I for the flow of water for a central heating circuit. An end plate 22 of the main heat exchanger body 21 is laterally detachable from the body 21 and supports four vertically spaced apart serpentine like tubes 2') arranged to be received in the conduits 11.

Each tube 2' 3 defines a separate fluid flow path through the associated conduit I I and has an inlet 26 and an outlet 27 communicating with respective inlet and outlet pipes 24,25 of a domestic supply circuit. The tubes 23 are housed within the conduits I I so that the water in the tubes 23 is heated primarily by heat exchange with the surrounding water in the conduits 11. The water in. the conduits I I is heated by heat exchange with the gaseous heat transfer medium in the flueways 10, and thus the water in the tubes 23 3 and the gaseous heat exchange medium are in indirect heat transfer relation. Suitable apertures or openings are provided in the end plate, which can serve as a manifold, for the communication of the tubes 2-33 with the domestic supply circuit via supply pipes 24,25.

The tubes 23 are externally finned to increase the efficiency of heat exchange between the water in the conduits I I and the tubes 2-3 3. The tubes in the outer conduits, i.e. the conduits which are only bordered on one side by a flueway, may make fewer passes through the conduit than the tubes in the inner conduits.

For instance, the tubes for the inner conduits may make four passes (Figure 4b) and the tubes in the outer conduits may make two passes (Figure 4a). As the outer conduits are only heated from one side, the output of the outer conduits is reduced approximately by half In place of the tubes, heat exchange plates could be employed. The plates could be formed by a pair of confronting metal sheets with a circuinjacent seal and a serpentine-like path pressed in one or both of the sheets. Plate inlet and outlet ports may be located at a common side of the plate to communicate with supply pipes 24,25.

The main body 21 and the end plate 22 may be initially cast in a single piece, and subsequently separated, or they may be distinct castings. Similarly the main body 21 could be of modular construction, but a single casting is preferred because it avoids the need to provide seals the between modules. The end plate can be releasably fastened by any suitable means of the main body 2 1, or it can be perinanently secured to the main body 2 1.

The conduits I I are wider than the conduits of known block-type heat exchangers for combi boiler to accommodate the tubes or plates 23. For instance the width of the conduits may be increased by between 40% and 80%, and the width is preferably increased by around 60%. The width of the conduits is preferably between 22 min and 30 mm. Also the stays 15 of the known heat exchangers shown in Figures I and 2 are not provided, or may be repositioned, particularly in the outer conduits which may require some reinforcement to resist the water pressure, to allow insertion of the tubes or plates 23. The resultant increased water capacity of the heat exchanger increases the thermal inertia of the heat exchanger. To keep the response time, i.e. the time lag between a demand for heated potable water arising and being satisfied to an acceptable level, the water in the tubes or plates 23) is maintained above a predetermined level during penods when there is no demand for water either for the central heating circuit or for the domestic supply circuit. Also, the increased thermal inertia provides a buffer 1 6 against undesirably rapid heating of the water in the conduits and therefore a complex control system synchronising burner ignition and water flow is not required.

The temperature of the water in the heat exchanger is maintained above a predetermined level by any suitable temperature sensitive mechanism, e.g a thermostat incorporating a thennistor or a fluid-filled capillary strip. The thermostat triggers the firing of the gas burner when the temperature of the water in heat exchanger drops below a level which would result in an unacceptably long response time. Water may be passed through the boiler without flow around the central heat circuit or through the domestic supply circuit, a by-pass flow way interconnecting an outlet of the heat exchanger with the corresponding inlet thereof being provided for this purpose. Alternatively, the temperature of water in the heat exchanger may be cycled between an upper and a lower temperature without flow through the heat exchanger. The position of the thermostat is important because the increased thermal inertia of the heat exchanger affects the controllability of the cycling. The thermostat should be located in or on the body of the heat exchanger such that there is adequate thermal influence in terms of water temperature when heat is removed from the heat exchanger by cool water from the central heating circuit or the domestic supply circuit, and also when heat is supplied to avoid overheating of water in either flow path and the unwanted noise associated with overheating. Ideally the thermostat is positioned in a conduit 11 in close proximity to the cold water inlet of a tube or plate 23. Typically a demand for potable water for domestic consumption takes priority over a demand for water for the central heating system. A pump is provided to circulate the water around the central heating circuit. Mains pressure is utilised to pressurise the domestic supply circuit.

The temperature of water pumped through the heat exchanger from the central heating circuit is maintained above a threshold temperature by a waxfilled thermostat, or another suitable temperature sensitive valve. The wax stat 7 prevents cold water from the central heating circuit reducing the temperature of the water in the heat exchanger when there is no demand for hot water. The temperature of water in the heat exchanger may be cycled between the maximum r - equired for the central heating system, e. g. 82'C, and. say, 65'C in order to provide potable hot water at around 60'C, The minimum thermal capacity of the described heat exchanger embodiment 0. 33 Kg water equivalent per I KW maximum water output. Each tube or plate 23 has an 8KW output giving the three conduit heat exchanger an equivalent output to known combi boiler heat exchangers.

To avoid overheating of the water in either flow path, water can be circulated through the conduits I I and the tubes or plates 2_3) when the gas burner is ignited, two by-pass flow ways then being provided. However, the water temperature may be cycled without flow through the heat exchanger.

Of course, modifications to the described embodiments are possible without departing from the scope of the claims.

8

Claims

A block-type heat exchanger for a combi boiler having a gas burner, the heat exchanger comprising a body having a first passageway (11) for the flow of a fluid for a central heating circuit, a second passageway (23)) for the flow of potable water for domestic supply, and a flueway (10) in heat transfer relation with the passageways (11,23) for the flow of a gaseous heat exchange medium.

? A block-type heat exchanger according to claim 1 wherein the water in the second passageway (233) is in direct heat transfer relation with the fluid in c 1 the first passageway (11) and is in indirect heat transfer relation with the gaseous heat exchange medium in the flueway (10).

A block-type heat exchanger according to claim 2, wherein the C1 second passageway is disposed within the first passageway.

c 1 c 4. A block-type heat exchanger according to any one of the preceding claims, wherein the heat exchanger comprises a main body (2 1) and an end plate (22) secured to the main body.

5, A block-type heat exchanger according to claim 4 wherein the main body (2 1) is of unitarv construction.

6. A block-type heat exchanger according to claim 4 or 5, wherein the second passageway is defined by means (23) mounted on the end plate (22).

7. A block-type heat exchanger according to claim 6, wherein the means mounted on the end plate comprises a tube (23).

9 A block-type heat exchanger according to any one of the preceding claims having a plurality of flueways disposed between adjacent passageways.

8.

9. A block-type heat exchanger according to any one of the preceding c claims having temperature sensing means for providing a signal indicative of the CP fluid temperature in the or each first passageway.

10. A combi boiler having a gas burner and a block type heat exchanger according to any one of the preceding claims.

11. A combi boiler according to claim 10 insofar as it is dependent on claim 9, wherein the burner is actuated in response to the fluid temperature falling below a predetermined level.