GB2349454A

GB2349454A - Radiant heater for food display unit with infrared source and emitter plate

Info

Publication number: GB2349454A
Application number: GB9906928A
Authority: GB
Inventors: Trevor Douce
Original assignee: COUNTERLINE Ltd
Current assignee: COUNTERLINE Ltd
Priority date: 1999-03-26
Filing date: 1999-03-26
Publication date: 2000-11-01
Also published as: GB9906928D0

Abstract

A radiant heater includes a housing containing an infrared radiation source (26) such as a quartz halogen lamp. The housing has an opening which is partially covered by an emitter plate (30) and may be made from perforated metal. The emitter plate (30) absorbs a proportion of the radiation from the infrared source, either directly or by reflection from reflector plates (28) and top plate (22). This causes the plate to heat up to at least 100{C and emit infrared radiation of a longer wavelength than the direct source radiation. The heater may be used to maintain the temperature of heated food in a display unit, where the radiant heaters are mounted on the underside of the shelves (4) and the canopy (6), as longer wavelength radiation is better absorbed by the food.

Description

HEATING APPARATUS The present invention relates to heating apparatus and particularly, although not exclusively, to a heated food display.

There is a growing market for ready prepared hot meals and snacks offered for sale for both immediate and home consumption. A particular development in this market is the offering of such meals and snacks for sale on a self-service basis, for example in supermarkets. As such, there is a need for a display unit which can maintain a product at a desired temperature whilst presenting the product attractively, so that it can easily be seen by potential customers, and allowing the product to be easily and safely removed from the display.

One existing display comprises a number of vertically spaced shelves.

The top surfaces of each shelf, on which products are placed, is heated.

The underside of each shelf is polished. The idea is that the polished underside of the shelves reflects heat radiated from the shelf immediately below back towards that shelf to keep products placed on the shelf warm.

In practice, however, the amount of heat reflected is low and the food is mainly kept warm from its underside.

Another method of keeping food warm, used mainly in catering, is to place infrared lamps over the food. These lamps can be very bright and hence uncomfortable to look at. To ensure that the light they produce is directed towards the food they must be equipped with parabolic or near parabolic reflectors. This makes the lamps bulky and unsuitable for positioning above vertically spaced shelves.

In most food service areas and, in particular, in retail shops selling space is at a premium and the ability to display food for sale on vertically spaced shelves, stacked one above the other, is essential.

Using a number of separate infrared lamps can produce an uneven distribution of heat. The light produced by infrared lamps is of a relatively short wavelength and can be reflected, rather than absorbed, by food.

It is an object of the present invention to provide an improved heated display unit for food. It is a particular object to provide a display unit which is both more efficient and effective in transmitting heat to food than known units and which allows food to be displayed on a number of vertically spaced shelves.

According to the present invention there is provided a radiant heater comprising a housing having an opening, an infrared radiation source disposed in the housing and an emitter plate partially covering the opening.

Since the emitter plate partially covers the opening it will absorb a proportion of radiation emitted by the infrared source. This will cause the temperature of the emitter plate to increase and it will, in turn, emit radiation itself.

The heater is preferably arranged so that, in use, the emitter plate reaches a temperature of at least 100 C, more preferably about 130 C.

The infrared radiation source preferably emits at least some visible light. The infrared radiation source preferably comprises an infrared lamp, more preferably a quartz halogen lamp. More than one lamp may be used.

It is preferable that the heater includes a controller, for example a phase angle controller, which allows the output of the lamp or lamps to be adjusted.

Since the emitter plate would typically be at a much lower temperature than the filament of the lamp the emitter plate will emit infrared radiation of a longer wavelength than the lamp. Provision of an emitter plate enables the heater to emit a particularly broad spread of wavelengths of radiation compared with a heater which relies only on a lamp. The lamp will emit some visible and some relatively short wavelength infrared radiation a proportion of each of which will pass through the opening of the housing, missing the emitter plate. A proportion of the remainder of the radiation emitted by the heat lamps will be absorbed by the emitter plate, as described above, leading to the emission of relatively long wavelength infrared radiation by the emitter plate. It is preferred that the emitter plate is arranged to absorb approximately 50% of the radiation emitted by the infrared radiation source.

Since the heater will emit some visible radiation there is no need to provide a separate light source to illuminate products. The relatively long wavelength infrared radiation emitted by the emitter plate is better absorbed by foodstuffs.

The emitter plate preferably comprises a perforated metal plate, for example an aluminium or stainless steel plate. The plate is preferably at least 1mm thick, more preferably about 3mm thick. The outer dimensions of the plate preferably substantially fill the opening in the housing.

For the avoidance of doubt it should be understood that the emitter plate could comprise one or more non-perforated plates which, alone or together, partially fill the opening of the housing.

To enhance ernission of relatively long wavelength infrared radiation by the emitter plate the side of the plate facing away from the housing is preferably coloured black. This could be achieved by painting the plate with black, preferably matt black, paint. Alternatively, the plate could be coated with coloured PTFE. The opposite side of the emitter plate, that facing the infrared source, preferably has a brushed or polished finish.

To ensure good efficiency the housing preferably includes a reflector arranged to direct radiation produced by the lamp towards the opening. In one embodiment, the housing is produced from sheet metal and the inside surfaces of the housing are polished to act as a reflector.

To further enhance efficiency insulating material is preferably disposed behind the reflectors, or around the housing, to minimise heat loss.

To prevent a person accidentally touching the emitter plate a further perforated screen, or mesh, is preferably disposed in front of the emitter plate. This screen is preferably of lighter construction than the emitter plate and preferably obscures a lower proportion of the area of the opening than the emitter plate.

As well as enabling a wide spread of wavelengths to be produced from only one type of infrared source the emitter plate confers other, important, advantages.

As the emitter plate absorbs some of the radiation emitted by the lamp it reduces the likelihood of persons being dazzled which could be harmful to their eyesight. This is important where the heater is positioned at or near eye level.

A result of the screening effects of the emitter plate is that it is possible to use a compromised shape of reflector whilst still directing the bulk of the emitted light and heat in a desired direction. For instance, the reflector and therefore housing may be made sufficiently thin so that the heater can be conveniently mounted between vertically spaced shelves.

The emitter plate also enables an even heat distribution to be produced over a wide area. Preferably, the region of the emitter plate adjacent to the lamp has a lower transmissivity than other regions. This prevents a hot spot forming under the lamp. Preferably, the emitter plate comprises a central region of low transmissivity and a peripheral region of higher transmissivity. The transmissivity of the emitter plate may be varied by varying the spacing of apertures formed through the plate.

The heater unit is preferably comprised in a heated food display unit.

The heated food display unit preferably comprises a surface, onto which food may be placed, and a heater spaced above the surface arranged to radiate heat towards the surface. The surface itself is preferably also heated, for example by an electric element. To prevent the risk of injury through contact with the surface a punched metal plate or grid is preferably spaced above the surface, by approximately 10mm.

The surface preferably comprises a shelf. The display unit preferably comprises a plurality of vertically spaced shelves with radiant heaters mounted on their undersides arranged to radiate heat towards the surface of the shelf below. The radiant heaters could be arranged to heat the top surfaces of each shelf.

In use the plate or grid forming the surface of each shelf would typically become heated to around 90 C, so, to further reduce the risk of injury foodstuffs to be displayed are preferably placed onto Nylon trays which, in turn, are placed onto the shelves. The Nylon trays will reduce the amount of conducted, but not necessarily radiated, heat and would typically have a surface temperature of about 50 C.

It is preferable that the unit is arranged to maintain packaged foodstuffs at around 70 C, particularly no less than 65 C.

In an alternative embodiment the infrared radiation source comprises one or more electrical heater coils. Since these coils emit no, or very little, visible radiation it may be necessary to provide a visible light source. A fluorescent light source would be suitable and could be installed either above or below the emitter plate.

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a perspective view of a heated food display unit according to the invention; Figure 2 shows an exploded perspective view of a radiant heater; Figure 3 shows a side view of the heater of Figure 2; Figure 4 shows a plan view of the heater of Figure 2; Figure 5 shows an underplan view of the heater of Figure 2, with the emitter plate removed; Figure 6 shows a cross-section taken along the line A-A of Figure 5 with ray paths drawn on; Figure 7 shows a plan view of the emitter plate of Figure 2; and Figure 8 shows an exploded perspective view of a shelf of the display unit of Figure 1.

Referring to Figure 1 a heated food display unit comprises a body 2 on which are mounted three vertically spaced shelves 4. The body 2 includes a canopy 6 which partially overlies and is spaced above the top shelf and a base portion 8 having a display surface 10 which underlies the lowermost shelf.

The front of the body 2 of the display unit is open to allow access to the shelves 4. The front edges of each shelf 4 include an upstanding barrier 12 along their front edges to prevent items slipping off. The front edge of each shelf also includes provision for the display of point of sale (POS) material, generally 13, for example prices and product descriptions.

The rear of the body 2 of the display unit is closed by way of doors (not shown) which allow access to be gained to the rear of the shelves 4, for example for restocking.

A radiant heater is mounted on the underside of each shelf 4 and the canopy 6. The heaters are arranged to radiate heat towards the top of each shelf 4 below, and in the case of the lowermost shelf towards the display surface 10, to heat or maintain the temperature of products placed on the shelves 4 and display surface 10.

Although not critical to the invention the length of the shelves would typically vary between around 500 and 1800mm. The vertical spacing of the shelves would typically be such that there is a 50mm clearance between the top of a product placed on one shelf and the underside of the shelf immediately above. The shelves 4 (including heaters) are approximately 70mm thick.

The type of radiant heater employed is illustrated in Figures 2 to 7.

These Figures show a heater which is generally square in plan view. It will be appreciated, however, that the heater could be produced as an elongate rectangle of similar shape to the shelves. Alternatively, more than one heater may be secured to the underside of each shelf.

Referring to Figures 2 to 7 the heater comprises a housing, generally 20. The housing comprises a generally rectangular top plate 22 formed from rigidised stainless steel sheet. Mounted on the sheet is a quartz lamp support 24 which, in turn, supports a quartz halogen lamp fitting 26 which receives a 300 watt quartz halogen lamp. The support 24 comprises a channel section. The number of lamps employed will, of course, depend upon the size of the unit. Typically, it is preferred that 300 watt quartz halogen lamps are mounted at 250mm intervals along the display.

Secured around the periphery of the top plate 22, at an angle of approximately 45 to the plate 22, are four reflector plates 28. The reflector plates are also formed from stainless steel sheet. The top plate and reflector plates are polished on one side to give good reflective properties. The polished side of the plates form the inside of the housing 20. The outer edges of the reflector plates 28 define an opening in the housing.

An insulating material (not shown) is disposed behind the top plate 22 and reflector plates. Disposed in the opening is an emitter plate 30.

The emitter plate 30 comprises a 3mm thick stainless steel plate which substantially fills the opening and is secured to the housing in such a manner that it can be easily removed, without the need of tools, to allow access to the lamps 26. The upper face of the plate 30, directed towards the housing 20 has a brushed finish. The lower face of the plate 30 has a matt black finish. The plate 30 is perforated with an array of obround slots.

The density of the slots varies across the plate 30. Particularly, there is a central, generally rectangular, region 32 of lower density, where there a fewer slots per unit area, surrounded by a peripheral margin 34 of higher density where there are a greater number of slots per unit area.

Approximately 24% of the area plate in the central region and 55% of the area in the peripheral margin region is removed. Thus, the emitter plate 30 has a greater transmissivity towards its edges than its centre. The width of the margin is about 1/6 the side of the emitter plate.

In operation, light emitted by the lamps 26 is reflected by way of the top plate 22 and reflector plates 28 towards the emitter plate. This can be seen in Figure 6. A proportion of the light passes through the slots in the emitter plates 30 which will serve to both heat and illuminate products placed below. Some of the remaining proportion of emitted light will be absorbed by the emitter plate 30 causing the plate to heat up, typically to around 130o C. The emitter plate 30 will then, also, emit infrared radiation.

But, since it is at a significantly lower temperature than the filaments of the lamps, this radiation will be predominantly of a longer wavelength. Since the lower face of the emitter plate 30 is coloured matt black it will be more efficient at emitting this relatively long wavelength radiation than its top surface so that a greater proportion of the radiation will be directed away from the heater.

Emission of long wavelength infrared radiation is particularly advantageous as it is more efficient absorbed by products placed below the heater, to a greater depth, than shorter wavelength infrared radiation.

The variable density of slots in the emitter plate 30 ensures that a more even spread of radiation is emitted by the heater than would otherwise be the case. As there are fewer slots in the region directly below the lamp 24, where the emitted light is most intense and a greater number of slots around the periphery, where the light falling on the plate is less intense, the result is that light transmitted through the emitter plate is evenly distributed.

In fact, it is desirable that a greater intensity of radiation is transmitted through the plate near the edges of the unit where products displayed on the shelf below are more exposed to cool ambient air.

The emitter plate also serves to reduce the intensity of stray light not directed towards the shelf below.

The top surface of each shelf 4 comprises a heated surface 40 with a perforated metal sheet or mesh 42 spaced approximately 10mm above.

The grid or mesh 42 prevents persons coming into contact with the heated surface 40. The heated surface could be formed from metal, glass or ceramic and is heated by way of electric elements secured to its underside.

The output of the elements is adjustable by way of an electronic controller.

The elements may be thermostatically controlled. The provision of a heated shelf ensures that food placed on the shelf is heated from both above and below.

The invention provides an effective and efficient food display unit which is particularly suited for use in a self-service environment. The radiant heaters with emitter plates provide for broad band emission of infrared radiation which is more likely to be effectively absorbed by food than narrow band emission. The emitter plate also allows the housing and reflector of the heater unit to be so dimensioned that it will fit within a 70mm thick shelf without the compromise reflector shape resulting in excess light and heat wastage. This allows vertically exposed shelves to be used allowing efficient use of display area.

The radiant heater uses only quartz halogen lamps to provide both light and heat. There is no need for an additional lighting system. Varying the spacing of the perforations across the emitter plate allows an even distribution of infrared and visible light across the area beneath the heater.

The thickness of the emitter plate is sufficient to prevent any possibility of a customer having a direct view of the lamps which could cause dazzling or discomfort. This feature also reduces wastage of both light and heat.

The above embodiment is described by way of example only. Many variations are possible without departing from the invention. For example, the described embodiment includes an insulating material disposed around the radiant heater housing to prevent heat loss and additional elements for heating the top surfaces of the shelves. Excess heat from the housing could be used to heat the top surfaces of the shelves so that the only heat source for the whole display is the quartz halogen lamps.

Claims

CLAIMS 1. A radiant heater comprising a housing having an opening, an infrared radiation source disposed in the housing and an emitter plate partially covering the opening.
2. A radiant heater according to claim 1 arranged so that, in use, the emitter plate reaches a temperature of 100 C.
3. A radiant heater according to either claim 1 or 2, wherein the infrared radiation source emits at least some visible light.
4. A radiant heater according to any preceding claim, wherein the emitter plate is arranged to absorb approximately 50% of the radiation emitted by the infrared radiation source.
5. A radiant heater according to any preceding claim, wherein the emitter plate comprises a perforated metal plate.
6. A radiant heater according to claim 5, wherein the region of the emitter plate adjacent to the radiation source has a lower transmissivity than other regions.
7. A radiant heater according to claim 6, wherein the emitter plate has a central region of low transmissivity and a peripheral region of higher transmissivity.
8. A radiant heater according to any preceding claim, wherein the side of the emitter plate facing away from the housing is coloured black and the opposite side of the emitter plate has a brushed or polished finish.
3. A radiant heater according to any preceding claim, wherein the housing includes a reflector arranged to direct radiation emitted by the radiation source towards the opening.
10. A radiant heater according to claim 9, wherein an insulating material is disposed behind the reflector.
11. A radiant heater according to any of claims 1 to 9, wherein an insulating material is disposed around the housing.
12. A radiant heater according to any preceding claim, wherein a perforated screen or mesh is disposed in front of the emitter plate.
13. A heated food Gisplay unit comprising a radiant heater according to any preceding claim.
14. A heated food display unit according to claim 13, comprising a heated surface onto which food may be placed and wherein the radiant heater is spaced above the heated surface and arranged to radiate heat towards the heated surface.
15. A heated food display unit according to claim 14, wherein a perforated metal plate or grid onto which food may be placed instead of the heated surface is spaced above the surface.
16. A heated food display unit according to any of claims 13 to 15 comprising a plurality of vertically spaced shelves and wherein radiant heaters are mounted on their undersides to radiate heat towards the surface of the shelf below.
17. A heated food display unit according to claim 16, wherein the radiant heaters are arranged also to heat the top surface of each shelf on which they are mounted.
18. A radiant heater substantially as herein described with reference to Figures 2 to 7 of the accompanying drawings.
19. A heated food display unit substantially as herein described with reference to Figures 1 and 8 of the accompanying drawings.