AU662456B2

AU662456B2 - A thermal oil heater

Info

Publication number: AU662456B2
Application number: AU52343/93A
Authority: AU
Inventors: Arthur Maurice Meredith
Original assignee: HEATMASTER TECHNOLOGY Pty Ltd
Current assignee: Individual
Priority date: 1993-01-07
Filing date: 1993-12-10
Publication date: 1995-08-31
Anticipated expiration: 2013-12-10
Also published as: AU5234393A

Description

I i I sar I--gruc- 662456

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION For a Standard Patent

ORIGINAL

Name of Applicant: Actual Inventor: ARTHUR MR2URICE ERD ARTHUR MAURICE MEREDITH 04 a r ooze wo o or a o o *o 0 Address for Service: WRAY ASSOCIATES, Primary Industry House, 239 Terrace, Perth, Western Australia, 6000.

Adelaide Attorney code: WR Invention Title: .m Details of Associated Provisional Application: No: Sa 9 U 6 i PL7000 PL6700 a'.

aaa~dJ The following statement is a full description invention, including the best method of performing it me:of this known to S 'I'W 0j; 1?A 1 2 The present invention relates to a heater and in particular relates to a heater that is used for heating thermal oil.

However, it is to be understood that the eiiater of this invention may be utilised in heating other liquids, provided that the heating is only conductea to temperatures below the boiling points of those liquids.

Thermal oil is often used as a heating medium where temperatures up to about 260 0 C are required in industry, and where the relatively bulky and expensive facilities of steam generation are not warranted. Indeed, thermal oil can be heated to these high temperatures by vessels that are not rated as pressure vessels due to the heating being conducted at a temperature under the boiling point of the thermal oil.

Further, thermal oil heaters are generally easier to maintain and to operate than other types of heaters, and the thermal oil itself is relatively easy to handle and to transfer.

4 4 However, most traditional forms of thermal oil heaters use coiled tubing wound into a spiral type combustion chamber where the heat passes down the combustion chamber within the coil and returns on the outside of the coil. The oil is then pumped through the coil to be heated by the flame of the combustion chamber. In this respect, in order to maintain .the appropriate temperature, the flame is usually provided by a fully modulating burner which is both expensive in capital cost and requires a relatively highly qualified person to commission, operate and maintain the burner.

Further, this traditional type of thermal oil heater usually requires a surge tank or a separate storage vessel that allows the oil to circulate and to hold the temperature.

Such surge tanks are generally adapted to be able to cope with different demands placed on the heater and thus are also relatively expensive. Further still, the capital cost of winding an appropriate sized coil of high quality seamless -3tube into a combustion chamber is normally not warranted by any users other than the largest users. Thus, there is again a reasonably large capital expense to be outlaid.

It is an aim of the invention to provide a heater that may at least be used as a thermal oil heater and that, in such use, overcomes or at least partly alleviates the difficulties referred to above.

The present invention provides a heater for heating a liquid, the heater being characterised by the provision of an internal combustion chamber having a burner located at one end thereof and having heat exchange tubes located thereabout wherein the combustion gases exit the combustion chatiber at said one end and are passed into the heat exchange tubes in order to heat the liquid as it flows past the heat exchange surfaces of both the heat exchange tubes and the outer surface of the combustion chamber.

The heater of the present invention preferably comprises an outer shell that defines a heat transfer chamber therewithin, oo:the outer shell preferably being generally elongate and S 20 substantially cylindrical. Within the heat transfer chamber there may be provided the array of longitudinally extending heat exchange tubes together with the cylindrical combustion achamber, such that the tubes are arranged generally about the combustion chamber.

The combustion chamber is preferably closed at its other end by a generally concave portion that may assist in the return of the combustion gases to said one end thereof.

In the preferred form of the invention, there is provided a combustion gas distribution header located about the one end 1 30 of the combustion chamber. The distribution header is in fluid communication with both the combustion chamber and the inlet ends of the heat exchange tubes. In this form, the at 4 outlet ends of the heat exchange tubes are in fluid communication with a flue exhaust that may be located towards the other end of the heat transfer chamber. Further, the heat transfer chamber preferably includes a liquid inlet, arranged to allow entry to the heat transfer chamber of the liquid to be heated, together with a liquid outlet.

Thus, in operation, the heat transfer chamber may be filled with, in the preferred embodiment, thermal oil, such that the oil flows around and about each of the heat exchange tubes, the outer surface of the combustion chamber, and the outer surface of the distribution header. The outer surface of the combustion chamber and the outer surface of each of the heat exchange tubes and the distribution header thus become the heat exchange surface area via which heat is transferred from the combustion gases to the thermal oil. The thermal oil may then be passed to any particular application where heating is required with the oil.

During operation, the flame from the burner preferably extends about half way along the length of the combustion 20 chamber such that the combustion gases are forced towards the end of the chamber and then move to the outside of the chamber to be forced back along the walls of the chamber towards the burner. By leaving the end of the combustion 0 4 chamber nearest the burner open, and by providing a distribution header immediately thereabout, the combustion gases may exit the combustion chamber and may transfer into S. the distribution header. The combustion gases will then be transferred along the length of the heat exchange tubes to exit via the flue exhaust.

S 30 Such a configuration provides an even distribution of heat to the combustion chamber and eliminates the fierce heat of the radiant flame temperatures. Thus, a simple high-low fire burner will suffice to provide the combustion capacity of this heater and there is no need to be operating with a fully Ft i I: modulating burner. However, it will be appreciated that any suitable type of burner may be used.

In particular, a preferred type of burner is that referred to as a "sliding high-low burner". This burner allows for operation between high and low settings without the need for prior on-off switching and thus is capable of more accurately maintaining a liquid such as thermal oil at a particular temperature.

A sliding high-low burner is preferably controlled by an air/ gas ratio valve and may alter from high to low fire by a pulse from a digital temperature controller. The air/gas ratio valve may thus be altered up or down by this thermostatic pulse with a high level of control.

Preferably, the controlling means is able to provide an initial low temperature control to the burner until the thermal oil reaches a predetermined temperature, after which the burner operates at a constant high temperature. In this respect, because the thermal oil has a reasonably high o C 2 viscosity at lower temperatures, which makes the oil difficult to pump through the system, it is unwise to initiate heating at the high temperature due to the possibility of local heating and the generation of "hot- S. spots".

Preferably, the controlling means is a digital-type 25 controller such as that marketed under the trade name •EUROTHERM. The use of such a controller allows the burner to i be operated on low until the temperature of the thermal oil reaches about 90 0 C. At this point, the thermal oil will circulate adequately and the burner may be operated on high.

The controlling means preferably also includes various safety devices such as a lock-out control to prevent over-heating of the thermal oil, a lock system to prevent an unauthorised o'4

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-6person from operating the controller, and an additional thermostat controller set at a fixed level to prevent the incorrect inputting of a lock-out temperature causing the heater to overheat.

In a further preferred form of the invention, the liquid inlet is arranged adjacent the combustion chamber, having a series of outlets therealong to evenly distribute the liquid along the length of the heat transfer chamber. Thus, and again in the case of the preferred example, the thermal oil will flow about the outside of the combustion chamber and about the outside of the heat exchange tubes to pass into the outlet and then to be used as necessary.

Further, an additional distribution means may be utilised to ensure a flow of the liquid about the combustion gas distribution header. In this respect, the external surface temperature of the header may tend to rise to an unacceptably high level if the liquid thereabout remains stagnant.

In the preferred form of the invention, a thorough SCdistribution of the liquid across all of the heat transfer Cc 20 surfaces is highly preferred in order to minimise the potential for the creation of hot-spots which, in the case of thermal oil, may lead to the carbonisation of the thermal Soil. Thus, the specific means used to distribute the liquid about and over the heat transfer surfaces will preferably distribute the liquid to all areas of the heater and will provide areas that may be hotter with larger volumes of liquid.

The means used to distribute the liquid preferably also allows for a volumetric flow rate of liquid through the heater that is sufficient for the heat of the combustion gases to be removed by the liquid without the liquid becoming stagnant. If the residence time of the liquid is too high, the liquid will be unable to remove the surface heat quickly a- A f;- 1 7 enough and the potential for the generation of hot spots, and thus the carbonisation of thermal oil is increased.

Thus, it is desirable to control the ratio of heat to volumetric flow of liquid in order to minimise the amount of stagnant liquid within the heater.

The heater of the present invention is preferably used in conjunction with an expansion tank that does not continuously expose the high temperature liquid to oxygen but that does open to atmosphere, and is one that may include a safety system that will switch the burner of the heater off if the liquid level becomes too low.

Such an expansion tank preferably includes three chambers, the first chamber being directly connected to the heat transfer chamber of the heater, the second chamber including a smaller vent line to the heat transfer chamber. of the heater and the third chamber including the vent to C C atmosphere. In this form, the expansion of the thermal oil e e primarily occurs in the first chamber, each of 'the chambers being separated by downwardly extending baffles, and 20 resulting in the coolest oil being included in the third chamber which vents to atmosphere.

S.Furthermore, the atmospheric vent preferably includes a Vrr type overflow so that the condensate fills the V and traps air movement such that there is no continuous intake of 25 oxygen.

Further, the expansion tank may include a safety cutoff AS.. switch that is provided by a float type stainless steel contact to the thermal oil, the switch allowing the power to qil the burner to be disconnected if the level of thermal oil is too low in the expansion tank. Further still, the expansion tank is preferably mounted above the heater to ensure the provision of a continuously flooded system, and is preferably J aki rfrbymutdaoetehae oesr h l- i II -i i- l -1 8 mounted on the suction side of the pump responsible for circulating the liquid through the heater.

There may also be provided a flow switch for detecting when there s no flow of thermal oil through the heater. In this way, in the event that the flow of thermal oil is inadvertently stopped while the burner remains operational, the flow switch will detect the lack of flow of thermal oil and turn the burner off.

A preferred embodiment of the present invention will now be described in relation to the accompanying drawings. However, it is to be understood that the following description is not to limit the generality of the above description.

In the drawings:- Figure 1 is a schematic, perspective, sectional view of 15 a heater in accordance with a preferred embodiment of ~the present invention; ~Figure 2 is a side sectional view of the heater of Figure 1; Figure 3 is an end sectional view of the heater of Figure 1; Figure 4 is a schematic flow diagram of the heater of it t Figure 1 in use; and 'i Figure 5 is a sectional view of a preferred type of expansion tank for use in the flow diagram of Figure 4.

€h *Figure 1 illustrates a heater 10 having a shell II that 25 defines a heat transfer chamber 12, the heat transfer-chamber Liii 12 including therewithin an array of heat exchange tubes 14 Stogether with a combustion chamber 16.

f Located at one end of the heater 10, and of the combustion chamber 16, is a burner 18 responsible for the flame 20 that

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9 in use extends about half way along the length of combustion chamber 16.

Also located at that end of the heater 10 (and the combustion chamber 16) is a combustion gas distribution header 22 that is in fluid communication with the interior 24 of the comoustion chamber 16 and with the inlet ends 26 of each of the heat exchange tubes 14. As can be seen, the ditribution header 22 serves to support and mount those inlet ends 26 of the heat exchange tubes 14.

Figure 2 illustrates many of those same features, and as such those same features have been referred to with common reference numerals. However, Figure 2 also illustrates a schematic representation of the passage of the combustion gases within the interior 24 of the combustion chamber 16.

As can be seen by the various arrows, the gases return to the open end of the combustion chamber by passing about the lame t II and along the walls thereof to pass into the distribution header 22. As each of the inlet ends 26 of the heat exchange tubes 14 are in communication with the distribution header, the combustion gases are urged into and along thc 'teat exchange tubes 14 to exit those tubes at their respective outlet ends 28. The combustion gases then exhaust via the flue exhaust The burner 18 has a flame tube assembly 19 that protrudes beyond the front surface 21 of the distribution header 22, i preferably by an amount in the order of 10mm to 20mm but more preferably by.15mm. This protrusion assists in allowing the flame to jump the width of the distribution header 22-to pass into the interior 24 of the combustion chamber 16.

,1 Thus, by filling the heat transfer chamber 12 with thermal oil via the liquid inlet 32, the heat of th- combustion gases is transferred from those combustion gases via the heat 2 V 1 i 41 10 transfer surfaces of each of the combustion chamber, the heat transfer tubes and the distribution header to the thermal oil. Thus, the thermal oil exiting via the liquid outlet 39 is at an increased temperature as required.

In this respect, there are preferably a number of outlets 34 provided along the length of the liquid inlet 32 to allow an even distribution of the thermal oil along the length of the heater in order to distribute the thermal oil over all of the heat transfer surfaces. These outlets 34 may be provided as slots that extend about a half circumference of the liquid inlet 3' as is evident from Figure 3.

The slots 34 closest to the distribution header 22 are of a greater size than the slots further away from the distribution header 22, allowing a 7reater volume of thermal oil to pass through the combustion chamber at regions where radiant heat is highest. Indeed, the open area of the larger slots may be as much as twice the open area of the smaller slots.

The slots 34 in the liquid inlet 32 have a combined open area which is equal to the cross-sectional area of the inlet 32 itself and preferably the inlet is of a diameter that allows I f;"or distribution of thermal oil with a velocity in the order t -1 Sof 1.5 ms The slots 41 illustrated in the liquid outlet 39 are 25 similarly of greater size adjacent the distribution header 22 than are the slots away from the distribution header 22.

"However, for the liquid outlet 39 it is preferred to -have a combined open area which is at least to 1.5 times the crosssectional area of the outlet 39 itself, while the outlet preferably remains of a diameter that allows for collection and transfer of thermal oil with a velocity in the order of -i ms 'Iil 12. Also illustrated in Figures 2 and 3 is an additional distribution means in the form of distribution pipes 33 and a saddle collector 35. The distribution pipes 33 are in the form of Venturi pipes and are located such that a portion of the thermal oil delivered by the liquid inlet 32 is directly and evenly distributed over the external surface of the distribution header 22. This additional flow ensures that there is adequate flow of the thermal oil about the external surface of the header 22 so that the likelihood of the creation of stagnant hot-spots is reduced.

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r ~~49 r 9 99 9 *9 9r 11t 4r 99

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9C*9 .440r *4 4( The saddle collector 35 is added to assist in generating flow from bottomi to top about the external surface of the header, the flow col]:.cted thereby being transferred along the outlet 39. In another form, the saddle collector 35 may simply be provided as a straight pipe extending outwardly from the liquid outlet 39, again having holes therein to receive the flow of oil about the distribution header.

In a preferred form of the invention, the heat exchange tubes 14 may be 40mm diameter seamless tubes welded to the headers at each end thereof. The tubes are preferably rated at about 28 kW/hr per square metre surface area and the tubes may be fitted with swirl baffles to create a more turbulent flow thereabout to increase the heat transfer efficiency, to provide a greater surface contact, and to reduce the 25 formation of hot spots.

When using thermal oil in particular, the oil must be pumped through the heater and thus the velocity of the thermal oil over the heat transfer surface areas must be appropriate in order to provide an adequate degree of heat transfer, without however heating the liquid to an undesirable point or to above its boiling point. In this respect, it is preferred that thermal oil is not heated over 3161C as any heating of thermal oil above that temperature tends to carbonise the thermal oil and the oil will thus degenerate rapidly, forming i i 12 carbon on the thermal oil side of the combustion chamber which would insulate that surface and cause higher temperatures to be generated. In this respect, it has been determined that an appropriate temperature rise across the heater, namely from the liquid inlet to the liquid outlet, will be in the order of 14 0

C.

Thus, by determining the kilowatt rating of the heater and knowing that an average specific heat of thermal oil is 2.6, an appropriate volumetric flow rate of thermal oil through the heater can be determined to give rise to a 14 0

C

temperatute increase. For example, for a heater rated at 440 kilowatts, a volumetric flow of about 12.0 litres per second will provide the desired temperature increase. Thus, the ratio of burner net input to volumetric flow rate of oil being pumped over the heat transfer surfaces is preferably a ratio of about 36.4 Kw/hr per litres/second.

r Further, such a volumetric flow of oil through the heater assists in preventing oxidation of the oil and in maintaining the oil in relatively good quality.

The internal combustion chamber itself is preferably of a size that is large enough for the combustion to be complete to meet the various gas codes or to ensure burning of oil or gas fuels. The chamber should also be of a size to ensure that there is no impingement of the flame which might cause S 25 "chilling".

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The volume of the combustion chamber may also be determined to be dependant upon the net burner input. The ratio-of net; burner input to the volume of the combustion chamber is

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preferably in the order of 1000 kW/hr per cubic metre.

Further, the volume of the plenum chamber 29 for the gases exhausting from the exhaust flue 30 may also be determined to be dependant upon the net burner input. Preferably, the

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4~ li 13 ratio of net burner input to the volume of the plenum chamber is in the order of 2000 kW/hr per cubic metre.

Thus, it will be appreciated that the ratio of the volume of the plenum chamber to the volume of the combustion chamber is about 1 to 2, the combustion chamber preferably being about twice the volume of the plenum chamber.

It has also been determined that the storage volume is preferably limited such that the entire volume of the vessel empties within a maximum of about 1.6 minutes, but preferably in about 1.2 minutes. For example, using the preferred ratio of burner net input to volumetric flow rate of 36.4 Kw/hr per litres/second, and for a storage volume of 2770 litres of thermal oil, a rate of 1400 kW/hr requires 38.46 litres/ second which for 2770 litres requires 1.2 minutes. Thus, the volume of thermal oil per kW/hr net burner input is about 2 to 1, and from this the preferred storage volunme can be determined for a given net burner input.

*9 o The combustion chamber is preferably constructed of mild steel and has a wall thickness suitable to assist in preventing the creation of hot spots. It has been found that for the heating of thermal oil from 80 kW/hr to 440 kW/hr net input, the walls are required to be in the order of 12mm (minimum), whereas for heating from 440 kW/hr to 1500 kW/hr the minimum thickness should be about 16mm and for 1500 kW/hr 25 to 3500kW/hr the minimum thickness should be about It should also be noted that where high temperatures occur .such as those experienced in this heater, it may be necessary i to provide allowance for expansion of the outer shell. Thus, any legs supporting the heater should include the appropriate means connecting them to the heater which allow for such expansion.

r 14 Illustrated in Figure 4 is an example of the manner in which the heater of the present invention may be utilised. This particular application is the sterilisation of a section of soil or for thewarming of an area of soil in order to assist with plant propagation. The area of soil to be warmed or sterilised would be about the heat tubing Thus, Figure 4 illustrates the heater 10 being connected such that the heated oil passes through tubing 42 for its use in tubes 40 and return to the heater for subsequent reheating.

The oil is pumped via pump 44 about that system.

A flue gas exhaust 46 is illustrated that would be connected to the flue exhaust 30 as illustrated in Figure 2. Further, a gas supply 48 is shown connected to the burner 18. This of course is where a gas fired burner is utilised.

.O 15 Figure 4 also illustrates an expansion tank 50 havinig first, second and third chambers (52, 54 and 56 respectively). The :.first chamber 52 is in direct communication with the heat transfer chamber 12 (as shown in Figure 2) via line 58 and is S.of a relatively large diameter to ensure that the expanded hot oil transfers through this line and not through vent line that communicates with the second chamber. This vent line 60 is included in the second chamber primarily to prevent an airlock.

The first chamber 52 is thus the hot chamber while the 25 temperature of the oil in the second and third chambers progressively decreases to be relatively cool in the third chamber 56. As it is this third chamber 56 that vents to S. atmosphere via the V-type vent 62, there is a need to ensure that the oil in the third chamber is at a relatively low temperature.

Figure 5 better illustrates the expansion tank 50 and shows the first, second and third chambers 52, 54 and 56 in 15 communication with the expansion line 58, the vent line and the V-type atmospheric vent 62 respectively. As can be seen, the level of oil (as indicated by the broken line A) is maintained constant across each of the three chambers by virtue of the downwardly extending baffles 64, each baffle 64 having intermediate vents 66 to allow the gases at the top of the chambers to be able to equalise. In this respect, it is these gases that vent through the V-tube 62 and cause the condensate liquid 64 to form in the elbow thereof forming a seal against the continuous ingress of air but nonetheless allowing the appropriate venting.

A gauge glass 68 is al~o shown in Figure 5, the gauge glass being provided essentially to give an indication as to the level of the oil in the chambers 52, 54 and 56. However, it will be appreciated that a float type arrangement may be included in chamber 56 that will provide a warning to an o. operator if the level of oil falls to an unacceptably low level.

Thus, it can be seen that the present invention provides a S 20 heater that provides a relatively inexpensive method of continuously heating thermal oil for many different applications that is both efficient and inexpensive. In this respect, the lack of coiled heat exchange tubes and the lack of the need for a fully modulating burner significantly decrease the expense of the heater of this invention.

I, "The heater is simple to operate and does not require highly v skilled or highly qualified technicians to either commission, maintain or install. Furthermore, the heater does not operate under high pressures that would be categorised as operation of a pressure vessel. This simplifies many of the operating procedures, significantly increases safety and reduces the overall cost of the heater.

16 Finally, it will be understood that there may be other modifications and alterations that may be made to the configurations described herein that are also within the scope of the present invention.

to e 0 *0 0 o

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Claims

2. A heater according to claim 1 wherein the heater comprises an outer shell that defines a heat transfer chamber therewithin, the outer shell being generally elongate and substantially cylindrical, within the heat transfer chamber there being provided the array of longitudinally extending heat exchange tubes together with the cylindrical combustion ,chamber, such that the tubes are arranged generally about the combustion chamber. eq CI C 3. A heater according to claim 2 wherein the combustion chamber is closed at its other end by a generally concave ft portion that assists in the return of the combustion gases to said one end thereof. CI S4. A heater according to any one of claims 1 to 3 wherein the flame from the burner extends about half way along the length of the combustion chamber such that the combustion gases are forced towards the end of the chamber and then move to the outside of the chamber to be forced back along the ,walls of the chamber towards the burner, whereupon the iL combustion gases exit the combustion chamber and transfer into the distribution header, to then be transferred along the length of the heat exchange tubes to exit via a flue exhaust. 18 A heater according to any one of claims 1 to 4 wherein the burner is a sliding high-low burner.
6. A heater according to claim 5 including a controlling means capable of providing an initial low temperature control the burner until the liquid reaches a predetermined temperature, after which the burner operates at a constant high temperature.
7. A heater according to any one of claims 1 to 6 including a combustion gas distribution header located about the one end of the combustion chamber, the distribution header being in fluid communication with both the combustion chamber and the inlet ends of the heat exchange tubes, and a liquid inlet, arranged to allow entry to the heat transfer chamber of the liquid to be heated, together with a liquid outlet.
8. A heater according to claim 7 wherein the heat-transfer chamber is capable of being filled with liquid such that the .et liquid flows around and about each of the heat exchange tubes, the outer surface of the combustion chamber and the *outer surface of the distribution header, the outer surface of the combustion chamber, the outer surface of each of the heat exchange tubes and the distribution header thus becoming the heat transfer surface area via which heat is transferred from the-combustion gases to the liquid.
9. A heater according to claim 7 or claim 8 wherein the liquid inlet is arranged adjacent the combustion chamber, a having a series of outlets therealong to evenly distribute the liquid along the length of the heat transfer chamber. A heater according to claim 9 wherein the outlets are I slots, the slots closest the distribution header being of a greater size than the slots further away from the distribution header, allowing for a greater volume of liquid 1 1 19 to pass through the heat transfer chamber at regions where radiant heat is highest.
11. A heater according to claim 10 wherein the open area of the larger slots is twice the open area of the smaller slots.
12. A heater according to claim 10 or claim 11 wherein the slots have a combined open area which is equal to the cross- sectional area of the liquid inlet, the liquid inlet being a tube of a diameter that allows for passage of liquid therealong with a velocity of about 1.5 m/second.
13. A heater according to any one of claims 7 to 12 wherein the liquid outlet is arranged adjacent the combustion chamber having a series of inlets therealong to collect the liquid along the length of the heat transfer chamber.
14. A heater according to claim 13 wherein the inlets are slots, those slots near the distribution header having a greater size than the slots away from the distribution header, the combined open area of the slots being equal to at least 1.5 times the cross-sectional area of the liquid outlet, the liquid outlet being a tube having a diameter that allows for collection and transfer of liquid with a velocity of about 1.5 m/second. A heater according to any one of claims 7 to 14 wherein additional distribution means are included to ensure a flow of liquid about the combustion gas distribution header, the additional distribution means including distribution pipes in fluid communication with the liquid inlet and a collector pipe in fluid communication with the liquid outlet.
16. A heater according to any one of claims 7 to 15 wherein the ratio of burner net input to volumetric flow rate of liquid being pumped over the heat transfer surfaces is a ratio of about 36.4 kW/hr per litres/second. I F-- 20
17. A heater according to any one of claims 7 to 16 wherein the volume of the heat transfer chamber is of a size such that any liquid contained therein is capable of emptying therefrom within about 1.6 minutes.
18. A heater according to any one of claims 7 to 17 wherein the volume of liquid per kW/hr net burner input is about 2 to 1.
19. A heater according to any one of claims 1 to 18, including an expansion tank which comprises three chambers, the first chamber being directly conniected to the heat transfer chamber of the heater, the second chamber including a vent line to the heat transfer chamber of the heater and the third chamber including a vent to atmosphere, wherein expansion of the liquid primarily occurs in the first chamber, each of the chambers being separated by downwardly expanding baffles, resulting in the coolest liquid being sit. included in the third chamber.
20. A heater according to claim 19 wherein the atmospheric vent of the expansion chamber includes a V-type overflow such that condensate fills the V and traps air movement in order to prevent continuous intake of oxygen.
21. A heater according to claim 19 or claim 20 wherein the expansion tank includes a safety cut-off switch which f comprises a float type stainless steel contact in the liquid, the switch allowing the power to the burner to be disconnected if the level of liquid is too low in the expansion tank. 4 22. A heater according to any one of claims 19 to 21 wherein the expansion tank is mounted above the heater to ensure the provision of a continuously flooded system, and is also 21 provided on the suction side of a pump responsible for circulating the liquid through the heater.
23. A heater according to any one of claims 1 to 22 wherein the combustion gases exit the heater via a plenum chamber and then through an exhaust flue, the ratio of the net burner input to the volume of the plenum chamber being in the order of 2000 kW/hr per cubic metre.
24. A heater according to claim 23 wherein the ratio of the net burner input to the volume of the combustion chamber is in the order of 1000 kW/hr per cubic metre. A heater according to claim 23 or claim 24 wherein the ratio of the volume of the combustion chamber to the volume of the plenum chamber is about 2 to 1.
26. A heater according to claim 1 substantially as herein described in relation to the accompanying drawings. DATED this TENTH day of DECEMBER 1993 Applicant. l WRAY ASSOCIATES, Perth, Western Australia, o Patent Attorneys for the Applicant. t c t. i l ABSTRACT A heater 10 for heating a liquid, the heater 10 including an internal combustion chamber 16 having a burner 18 located at one end thereof and having heat exchange tubes 26 located thereabout. The combustion gases exit the combustion chamber 16 at said one end and are passed into the heat exchange tubes 26 in order to heat the liquid as it flows past the heat exchange surfaces of both the heat exchange tubes 26 and the outer surface of the combustion chamber 16. St t t rtrt It I I to I t tt E t 8 9 a i S tt 1( r1 I I r t Iirtr isl