GB2091868A

GB2091868A - Air heater

Info

Publication number: GB2091868A
Application number: GB8202485A
Authority: GB
Original assignee: Deutsche ITT Industries GmbH; ITT Industries Inc
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1981-01-28
Filing date: 1982-01-28
Publication date: 1982-08-04
Also published as: NL8100389A

Abstract

In an air heater wherein the combustion products from a burner 11 pass firstly through the tubes of a main heat exchanger 12 and thereafter through the tubes 1 of a secondary heat exchanger 13, air to be heated is blown horizontally over the tubes of both heat exchangers by a fan 15. The secondary heat exchanger tubes 1, which have a flattened cross section are provided with depressions 5 and 6. Specific dimensions for the height, width etc. of the tubes 1 are disclosed. <IMAGE>

Description

SPECIFICATION Heat exchanger The invention relates to an air heaterfor heating air from the products of combustion of e.g. gas or oil.

Such heaters have a heat exchanger and the combustion gases are made to flow by a fan, said heat exchanger being provided with a distribution box for the combustion gases, one or a number ofjuxtaposed channels being connected to said distribution box, which channels lead to a receiving box from which the gases leave the heat exchanger.

It is known to have an afterconnected heat exchanger or heat recovery unit, to transfer further heat which is still present in the combustion gases coming from the main heat exchanger to the air to be heated.

A drawback of this known arrangement is that the heat recovery from combustion gases to the air to be heated is only moderate, so that the overall efficiency of the arrangement is not optimal.

It is an object of the invention to provide a heat recovery unit giving a higher efficiency than the prior arrangements and which moreover can be manufactured in a simple way with relative low expenses.

Many attempts are already made to increase the amount of additional heat which is recovered from combustion gases cooled in a main heat exchanger by means of an afterconnected heat exchanger. To that effect the course was always persued to decrease as much as possible the velocity of the combustion gases in the afterconnected heat exchanger, to lengthen the time in which the heat transfer can take place. Accordingly the channels of such a heat exchanger were relative widely dimensioned. However, it has appeared - contrary to all expectation - that a considerably higher heat transfer takes place if the combustion gases in the afterconnected heat recovery unit are conducted quickly through a tube assembly with a total passage cross-section which is smaller, by an order of magnitude, than that of the main heat exchanger.

The efficiency attained in this way is at least 5% higher than that of any of the prior art heat recovery units with a comparable volume, simplicity and price.

According to the present invention there is provided a heat recovery unit for recovering heat from the products of combustion coming from main heat exchanger in an air heater the heat recovery unit being provided with a distribution box for the entering products of combustion, several juxtaposed tubes connected to the distribution box, said tubes leading to a receiving box from which the products of combustion leave the heat recovery unit, characterized, in that the tubes have an inner width of 2 to 5 mm, an inner depth of 100 to 500 mm, that the distance between the distribution box and the receiving box is 200 to 800 mm, that the distance between the tubes is 20 to 250 mm and that the air to be heated is conducted through the spaces between the tubes.

According to a further aspect of the present invention there is provided an air heater having a generally box-shaped construction with a throughpassage for air to be heated, a main heat exchanger comprising several primary tubes extending across the passage and a heat recovery unit comprising several secondary tubes extending across the passage behind the primary tubes, a first fan for forcing the air to be heated through the passage from the back to the front, and a second fan for forcing the products of combustion first through the primary tubes and then through the secondary tubes, the internal cross-sectional area of the primary tubes being large in relation to that of the secondary tubes so the products of combustion pass through the secondary tubes significantly more quickly than they do through the primary tubes.

The invention will be further explained with reference to the drawings.

Figure 1 shows a heat recovery unit according to an embodiment of the invention.

Figure 2 shows an air heater to which the unit of Figure 1 is applied.

In Figure 1 is a heat recovery unit comprising a box 2a open at the front and rear, in which at one side four tubes 1 (secondary tubes) are connected to a distribution box 2 (of which the front- and upperside are not drawn for the sake of clearness) and at the other side to a receiving box 3. The supply of combustion gases which are already cooled by the (not shown) main heat exchanger takes place at the rearside of the distribution box. When passing through the tubes 1 they are further cooled and discharged at the front side of the receving box. The air to be heated is blown through the space enclosed by the tubes, the receiving and distribution boxes 3 and 2 and the walls 4 interconnecting said boxes.

The tubes 1 are provided with long or local impressions 5 or 6 with a depth so that the major faces of a tube are kept at the proper distance. Tube 1 is preferably made from a single channel, but the channel also can consist of several sections placed one after the other.

As appears from Figure 1 the heat recovery unit is distinguished by a very simple construction lending itself well to construction in materials such as stainless steel which are resistant to corrosive materials produced if the temperature of the combustion gases possibly drops below the dewpoint so that condensation occurs. If this occurs regularly the receiving box is provided with a condensate discharge.

In certain cases it may be favourable to apply fins to the tube walls.

In Figure 2 an air heater is schematically shown having a through-passage for air to be heated indicated by double headed arrows A and consisting of a gas or oil burner 11, a main heat exchanger 12 having several primary tubes extending vertically up through the through-passage, an afterconnected heat recovery unit 13, corresponding to the one shown in Figure land a blower 15 driven by an electric motor 14. At first the combustion gases move upwardly through the main heat exchanger 12 and thereafter downwardly through the afterconnected heat recovery unit 13 to be sucked off at the discharge channels by a fan (not shown). The air to be heated is horizontally blown through the throughpassage in the air heater by means of the blower 15.

Such an arrangement lends itself well to the application of an afterconnected heat recovery unit.

The exhaust gases are driven by a fan (not shown) through an outlet 16 to be discharged through a flue (not shown). The inlet to the burner is depicted by the reference numeral 17 in Figure 2.

In Figure 1 the tubes 1 have an internal width in the range 2 to 8 mm, an internal depth (from front to back) in the range 100 to 500 mm, and a height, corresponding to the distance between the receiving box 3 and the distribution box 2, in the range 200 to 800 mm. The distance between the tubes lies in the range 20 to 250 mm. In this particular example the depth is about 160 mm and the width is about 4 mm.

The height is about 460 mm. The width of the box 2a is about 1150 mm. The unit in Figure 1 and the air heater of Figure 2 are shown only diagramatically, and the through-passage would have a width and height in the same range as that of the heat recovery unit. In this particular example the width and height of the through-passage is 1150 mm and 460 mm, respectively.

The number of primary tubes in the main heat exchanger is sixteen and the number of secondary tubes is twenty-eight. This would have a heat output of over 300,000 BTU/hr. For smaller heaters, e.g.

240,000 BTU/hr and 180,000 BTU/hr, the number of primary and secondary tubes would be 12 and 21 (respectively) and 9 and 17 (respectively). The secondary tubes would have the specific dimensions noted above.

The heat recovery unit shown in Figure 1 can have a larger number of tubes such as 1, each of smaller size, with transverse fins, and a commerciallyavailable motor vehicle radiator could be adapted for this purpose. Provided it has the same general size as the heater main heat exchanger so that it fits on the back of it, then a vehicle radiator could meet the requirements of cross-sectional size of the tubes for the primary gases, which would of course pass through the inside of the radiator, where water would normally pass, for a vehicle cooling system.

It should however be borne in mind that periodic servicing may be necessary and the inside of the tubes may need cleaning so access to the insides of the tubes should be made available, by means of for example a removable panel (not shown) over the distribution box 2.

Claims

1. A heat recovery unit for recovering heat from the products of combustion coming from main heat exchanger in an air heater the heat recovery unit being provided with a distribution boxforthe entering products of combustion, several juxtaposed tubes connected to the distribution box, said tubes leading to a receiving box from which the products of combustion leave the heat recovery unit, characterized, in that the tubes have an innerwidth of 2 to 5 mm an inner depth of 100 to 500 mm, that the distance between the distribution box and the receiving box is 200 to 800 mm, that the distance between the tubes is 20to 250 mm and that the air to be heated is conducted through the spaces betweenthe tubes.

2. A unit according to claim 1, characterized in that at least one tube within its depth dimension is subdivided into several parallel-connected sections.

3. A unit according to claim 1 or 2, characterized in that the wall of the tubes are locally provided with indentations contacting the other wall of the tube.

4. A heat recovery unit substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

5. An air heater having a generally box-shaped construction with a through-passage for air to be heated, a main heat exchanger comprising several primary tubes extending across the passage and a heat recovery unit comprising several secondary tubes extending across the passage behind the primarytubes, afirstfanforforcing the airto be heated through the passage from the back to the front, and a second fan for forcing the products of combustion first through the primary tubes and then through the secondary tubes, the internal crosssectional area of the primary tubes being large in relation to that of the secondary tubes so the products of combustion pass through the secondary tubes significantly more quickly than they do through the primary tubes.

6. An air heater having a main heat exchanger and a heat recovery unit according to any of claims 1 to 4 for recovering heat from the products of combustion leaving the main heat exchanger.