IE55661B1 - Radiant heaters - Google Patents

Description

2 5566 1 The heating of industrial buildings offers a considerable challenge to designers. If fuel is used to warm the air inside the building, then all air inside the building must be heated to provide a 5 comfortable environment within the lower 2 metres - the area which is occupied by people.

By the very nature of warm air, it is less dense than the cooler air within the building and therefore the hot air leaving the heating source must rise. It is 10 this occurrence which produces the wasteful stratification effect, where roof void temperatures can exceed the comfort level by as much as 20°C.

Heat losses from the roof area are, therefore, very high since the temperature gradient is increased by 15 up to 20°c. In older buildings, where insulation values are much lower than today's modern structures with 0.7W/m2 values, up to 90% of the heat generated within the building when passed to air as the transmitting medium can be lost from the upper, warmer 20 parts of the building.

Radiant heating can provide at least equal and in most .cases improved comfort conditions with large energy ^savings over systems using warm air.

Radiation is one of the basic mechanisms by which 25 energy is transferred between regions of different temperature. It is distinguished from other methods of heat transfer, conduction and convection, by the fact that it does not depend upon the presence of an intermediate material to act as a carrier of energy. On 30 the contrary, energy transferred by radiation is impeded by the presence of a material in the space 3 between. Energy transferred by radiation is the consequence of energy carrying electro-magnetic waves.

Energy transmitted by radiation is converted into heat when it is absorbed. The energy carrying electromagnetic waves must, therefore, strike solid objects in order to be converted into heat energy.

The rate of radiant energy .emission by a surface is dependent upon its absolute temperature. The rate of emission from one body to another is governed by their different absolute temperatures and this relationship is determined using the Stefan Boltzmann Law.

Over the years, three distinct forms of radiant heating system have developed which operate in three distinct temperature bands. 1) Low temperature 60 - 175°C 2) Medium temperature 150 - 450°C 3) High temperature 800 - 950°C The low temperature range comprises all systems using water or steam as the initial heat transfer medium.

The medium temperature range uses electric sheathed elements or radiant tubes (direct fuel fired and recirculated hot air ducts with all black surfaces).

The high temperature range uses incandescent electric or gas heated surfaces.

Low Temperature Since, as a result of the Stefan Boltzmann law, radiant output is related to absolute temperature to the fourth power, the total heat output in a radiant form is low. 4 Indeed, a greater proportion of the heat is given to convection. Since this is natural convection, it can only rise and add very little to the comfort of a building at low level.

Medium Temperature Equipment designed for operation in this range has high combustion efficiencies and, because it operates below incandescent temperatures, can use steel as the heat transfer medium, which under the conditions used has an 10 emissivity near to that of a black body. In this temperature band, the proportion of radiation to convection is much more favourable and represents optimum radiant efficiency because it is possible to use a large proportion of the heat of the combustion.

High Temperature This equipment is invariably in the form of a gas fired surface combustor. Whilst the peak radiant output is high due to incandescent temperatures, much of the heat cannot be extracted due to the high exhaust gas 20 temperature (900°C surface: 900°C exhaust gas).

There will now follow a comparison of some medium temperature radiant heaters.

Central Plant Systems In these, using either a heat exchanger or direct fired 25 unit, hot air is circulated within a duct-work system designed to run throughout a building to provide overall heating. Standard practice is to contain the ducts (about 15-75cm and larger) within an insulated holding trunking having a central feed and twin return ducts positioned side by side. The duct-work, usually 30 5 galvanised steel, is exposed on the underside and is often painted to improve its radiant efficiency. It is efficient, purpose-designed and expensive, and cannot normally be used for 'zoned' applications, where heat 5 is only required in a certain zone.

The other major disadvantage is that a breakdown results in the loss of the heating system in total.

Continuous Tube Systems These comprise standard mild steel tubes with 10 reflectors fitted above where appropriate, suspended at high level with individual burner units fitted at regular intervals. Combustion takes place within the tubes, the exhaust being extracted at the ends of the tube runs, although several runs can be interconnected. 15 However, water condenses within the tubes and this must be drained. Zoned operation is possible but, under these circumstances, the relative efficiency drops. Such systems tend to be more costly to install than systems designed around 'unit' radiant tube heaters 20 (see below).

Individual breakdown of a burner results in the lowering of the average surface temperature of that tube run with subsequent lowering of efficiency.

Dnit Radiant Tube Heaters 25 There are a number of manufacturers producing heaters of this type. System design is achieved by using an appropriate number of individual units which can be interconnected electrically to provide excellent 'zoned' control, maximising fuel utilisation.

Most units comprise a U-tube radiator system, an atmospheric burner and exhaust fan, at respective ends of the limbs of the U-tube, the flame from the burner extending into one limb and the fan sucking out hot air from the other limb. Some burners are totally enclosed with fully automatic spark ignition and flame failure detection; also with a choice of air inlet including filters or duct spigot for external air entry. A reflector is fitted above the radiator tube.

Installation is relatively easy and costs are low. However, such systems hitherto cannot effectively be used above about 10 metres from floor level since they do not provide any significant heating at floor level when above about this height and their heat distribution at floor level is not always uniform.

According to the present invention there is provided a radiant heater comprising:- a) a tube; b) a burner communicating with one end of the tube; c) suction means communicating with the other end of the tube for withdrawing hot air from the tube; and d) a heat reflective housing which receives the tube, the housing comprising:- i) a top reflector surface inside which is the tube; ii) first and second side reflector surfaces joined to the top reflector surface; and iii) first and second end reflector surfaces joined to the top reflector surface, the first and second side reflector surfaces and the .first and second end reflector surfaces extending below the lowest point of the tube by a distance which is at least 6 cm.

The said distance could be in the range from 6cm to about 2.5 metres.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic, perspective view of an example of a radiant heater according to the invention; Figures 2 and 3 are details of what is shown in Figure 1; Figures 4 to 10 show alternatives to what is shown in Figure 3; Figure 11 is a perspective view of the reflective housing of a further example of a radiant heater according to the invention; and Figure 12 shows a detail of what is shown in Figure 11.

Referring first to Figure 1, a radiant heater comprises first and second reflector portions 1 and 2; first and δ second side reflector plates 3 and 4; first and second end reflector plates 5 and 6; first and second steel 0-tubes 7 and 8 received by the first and second portions 1 and 2 respectively; end reflector plates 9 and 11 for 5 the reflector portion 1 and end reflector plates 10 and 12 for the reflector portion 2, the ends of the tube 7 protruding through the plate 9 and the ends of the tube 8 protruding through the plate 10; first and second gas (or oil) burners and control units 13 and 14 10 communicating with the ends of tubes 7 and 8 respectively nearer a V-shaped channel 15 between portions 1 and 2; and first and second suction fan units 16 and 17 communicating with the ends of tubes 7 and 8 respectively which are further from the channel 15 15. The portions 1 and 2 are made by bending plates of a suitable heat-reflective metal such as aluminium and these portions and the side reflector plates 3 and 4, end reflector plates 5 and 6 and end reflector plates 9 and 11 and 10 and 12 (all of which are also made of 20 such a heat-reflective metal) are assembled together by bolting, or riveting, or welding, and using appropriate metal brackets. The length of the heater is typically at least 2 metres.

In use of the heater, gas (or oil) is burnt at the 25 burners and control units 13 and 14, and hot air is sucked through "the tubes 7 and 8 by the suction fan units 16 and 17. The provision of two U-tubes 7 and 8 provides for a greater heat output than with the use of a single tube for a given length of heater and the θ provision of side reflector plates 3 and 4, end reflector plates 5 and 6 and end reflector plates 9 and 11 and 10 and 12 and the V-shaped channel 15 results in a reduced (and more uniform) spread of heat radiated by 5 the tubes 7 and 8, meaning that the heater may be disposed a greater height above ground to give a useful heating effect at ground level compared with a conventional radiant heater. Typically, the radiant heater may be held at least 10 metres above the ground, 10 (typically in the range from 15 metres to about 30 metres above the ground) to give a useful heating effect at the ground. Also, the overall shape of the radiant heater is such as to restrict the loss of heat due to convection.

Figure 2 shows one of the end reflector plates 9 and 10. The distances A-B and F-G are about 15cms; the distances B-C and E-F are about 6.4 cms; the distances C-D and D-E are about 12.3 cms; and the distance A-G is about 61 cms. Each of the angles a is 154°. The 20 distance between the centres of the limbs of the U-tube 7 or 8 is about 30.5 cms.

Reference numeral 18 denotes a box-section fastening bracket.

Figure 3 is an end-view of the reflective housing 25 comprising reflector portions 1 and 2, the side reflector plates and end reflector plates. The angle 9 (that is the angle between the line joining the centres of the limbs of the U-tube 7 and the line joining the centres of the limbs of the U-tube 8) is 160° and the 30 distance H-I is about 1.1 metres.

There will now be described with reference to Figures 4, 5, 6, 7, 8, 9 and 10 alternative configurations for the reflective housing, each of these figures being an 10 end view corresponding to Figure 3. In each of the alternative configurations, the reflector portions 1 and 2 are structurally identical with the reflector portions 1 and 2 respectively of the foregoing 5 embodiment.

In Figure 4, the angle -9 is 140° and the distance H-I is about 1.04 metres. Reference numerals 19 and 20 denote extension plates for the end reflector plate 5, one or both of which may be fitted to the end reflector 10 plate 5. There would, of course, be a corresponding extension plate or plates for each of the end reflector plate 6 and the side reflector plates 3 and 4. Each of extension plates 19 and 20 is about 61 'cms by about 30.5 cms.

In Figure 5, the angle Θ is 120° and the distance H-I is about 91.4 cms.

In Figure 6, the angle Θ is 100° and the distance H-I is about 74.3 cms. Reference numeral 191 denotes an extension plate for the end reflector plate 5, which 20 may be fitted if desired like the plate 19 of Figure 4, and is about 74.3 by about 30.5 cms. If the plate 191 is provided, then of course, a further extension plate will be provided for each of the end plate 6 and side reflector plates 3 and 4.

In Figure 7, the angle $ is 165° and the distance H-I is about 68.6 cms. Reference numeral 192 designates an end reflector extension plate which may be added on if desired, together with an extension plate for the end plate 6 and extension plates for the side reflector 30 plates 3 and 4. The distance I-J is about 7.62 cms.

Figure 8 shows a configuration identical with that of Figure 7 except for the shape of the end reflector 11 plate 5 (and hence the end reflector plate 6 and side reflector plates 3 and 4), the angle B being 160°.

In Figure 9, the angles Θ and 6 are 160° and the distance H-I is about 1.03 metres.

The configuration of Figure 10 is identical with that of Figure 9, the angles Θ and B again being 160°, but the end reflector plate 5 is somewhat longer, the distance H-I being about 99.1 cms.

In the above embodiments, there are different values for the angle θ'; As a general rule, it may be stated that preferably, the angle S is in the range from 90° to 180°.

There will now be described an embodiment using just a single U-tube which, again, provides a greater and more uniform radiant heat output than a conventional radiant heater using a single ϋ-tube. Referring to Figure 11, the reflective housing for the U-tube (not shown) comprises a housing made from heat-reflective metal such as aluminium in the form of: a first plate section bent to provide side reflector walls 3 and 4 with a top reflector portion 21 between them, the portion 21 being formed to have a V-shaped channel 15? and end reflector plates 5 and 6, the plate 5 having openings 22 and 23 therein for the limbs of the U-tube. The housing further includes, made of the same material, a plate 24 covering the channel 15; and fitted on top of plate 21, a plate 25 bent to form a further-channel 26, above the channel 15. The housing is held together by bolting or riveting, or welding and using suitable brackets. Again, the housing has a minimum length of about 2 metres and, referring to Figure 12, the distance K-L and 0-P are about 7.6 cms; the distances L-M and 0-N are about 21.6 cms; the distance J-Q is about 15.9 cms; 12 the distances P-J and J-K are about 15.2 cm; the angle μ is 116°; the angle φ is 115°; the angle δ is 148°; the angle r is 120°. The channel 15 communicates with the burner for supplying pre-heated air thereto 5 and the channel 26 communicates with the suction fan unit for receiving exhaust air therefrom.

In all the foregoing embodiments, the side and end reflector surfaces extend at least 6cm below the lowest point(s) of the U-tube(s).

Claims (9)

13

1. A radiant heater comprising:- a) a tube; b) a burner communicating with one end of 5 the tube; c) suction means communicating with the other end of the tube for withdrawing hot air from the tube; and d) a heat reflective housing which receives 10 the tube, the housing comprising:- i) a top reflector surface inside which is the tube; ii) first and second side reflector surfaces joined to the top reflector surface; and 15 iii) first and second end reflector surfaces joined to the top reflector surface, the first and second side reflector surfaces and the .first and second end reflector surfaces extending below the lowest point of the tube by a distance which is at 20 least 6 cm.

2. A radiant heater according to Claim 1, wherein the top reflector surface comprises first and second reflector portions forming a channel between them 25 which is substantially V-shaped in cross-section.

3. A radiant heater according to Claim 2, including a further tube, a further burner communicating with one end of the further tube and further suction means 30 communicating with the other end of the further tube for withdrawing hot air from the further tube, the first-mentioned tube being inside the first reflector portion of the top reflector surface and the further tube being inside the second reflector 35 portion of the top reflector surface, and the first and second side reflector surfaces and the first and second end reflector surfaces extending below the lowest point of the further tube by a distance which is at least 6 cm.

4. A radiant heater according to Claim 2 or 3, wherein the first and second reflector portions of the top reflector surface are disposed with respect to each other at an angle in the range from 90° to 180°.

5. A radiant heater according to any preceding claim, wherein the said distance is .in the range from 6 cm to about 2.5 metres.

6. A radiant heater according to any preceding claim, wherein the first and second side reflector surfaces converge towards each other and then diverge away from each other.

7. A radiant heater according to Claim 1, wherein the top reflector surface is provided with: a first, lower passageway above the tube, which passageway communicates with the burner for supplying hot air thereto; and a second, upper passageway, above the first passageway, the second passageway communicating with the suction means for receiving exhaust air therefrom.

8. A radiant heater according to any preceding claim, wherein the or each tube is U-shaped.

9. A radiant heater, substantially as herein described with reference to, and as shown in, Figures 1 to 10 or 11 and 12 of the accompanying drawings. F.R. KELLY & CO., AGENTS FOR THE APPLICANTS.