Patent Publication Number: US-2002005686-A1

Title: Heated food storage and display cabinet

Description:
[0001] The invention relates to a heated food storage and display cabinet and is primarily concerned with a heated food storage and display cabinet for use in supermarkets. In particular the invention relates to an improvement in a heated food storage and display cabinet described in our International patent application no. PCT/GB99/04379.  
       [0002] Changes in working patterns have led to changes in eating habits particularly with respect to what might be called the traditional evening meal where a family sits around a table and eats home-cooked food. It is now becoming increasingly commonplace for convenience meals to be eaten in the evening while watching television from an easy chair. That has, in turn, led to an increase in take-away food outlets where a person can collect a hot ready-cooked meal to eat at home. For some time, supermarkets have offered a range of prepared food which can be taken home, heated or cooked, and then eaten. However, there is now increasing competition from take-away food outlets which supermarkets would like to address by supplying hot ready-cooked meals themselves. One of the problems faced by supermarkets is how to keep such food hot whilst making it readily accessible to customers and an object of the present invention is to provide a cabinet which will be suitable for such a purpose.  
       [0003] In U.S. Pat. No. 3,942,426 there is described a heated sandwich bin with air curtains. The bin is not intended for display of packaged heated food which customers select and take away but is intended for use by fast food outlets where sandwiches need to be kept hot without spoiling so that those who are serving food to a customer can do so quickly. The bin has heated air curtains at front and rear. Whilst some of the air diffuses inwardly of the bin, the air does not actually flow over the sandwiches themselves to avoid drying them out. One object of the present invention is to provided a heated food storage cabinet which enables air to flow over packs of food.  
       [0004] Attempts have been made to provide open fronted refrigerated food display cabinets which use an air curtains to help prevent cooled air escaping through the open front. Whilst the present invention is not concerned with food refrigeration, examples of refrigerated cabinets can be seen in U.S. Pat. No. 2,993,349 and U.S. Pat. No. 5,755,108. In U.S. Pat. No. 2,993,349, an open fronted refrigerated display cabinet has an upwardly open-fronted flue at its rear and a blower for moving air over a refrigerating unit and upwardly through the flue. A second blower is provided at the upper end of the cabinet which draws air upwards through the flue and directs it downwards as a curtain across the open front of the cabinet and towards an air inlet at the bottom of the cabinet for recirculation. The open front of the flue does not, however, guarantee that a sufficient amount of cooled air will reach the food stored at the top of the cabinet or that the flow of air will be reasonably even throughout. Moreover, any food overhanging the back of the shelves so as to project into the cabinet will interfere with upset the flow of air along the flue. Moreover, a flow path to the second blower does not encourage air to flow over food at the upper end of the cabinet.  
       [0005] U.S. Pat. No. 5,755,108 similarly provides an upward flow of cooled air at the rear of the cabinet and a curtain of air across an open front. Whilst in this case the problem of an open fronted flue does not exist, the construction is rather complex. For example, the upward flow of cooled air for circulation within the cabinet relies either upon the way in which air from a single outlet in the base of the cabinet can flow through a gap behind the lowest shelf or it is necessary to provide two completely separate air flow channels at the rear of the cabinet, one exclusively for air to be used as the air curtain and the other exclusively for supplying air for cooling food in the cabinet. In the former case, any food projecting across the gap behind the shelf will upset air flow to the remainder of the cabinet and, in the latter case, the need to provide two air flow channels takes up space thereby decreasing the amount of space available for food storage and makes manufacturing more complex. Moreover, as in U.S. Pat. No. 2,993,349, the flow path to a blower at the upper end of the cabinet used to produce the air curtain does not encourage air to flow over food at the upper end of the cabinet.  
       [0006] According to an aspect of the invention there is provided a heated food storage and display cabinet comprising an open fronted chamber in which packs of food can be stored, an upwardly extending enclosed air duct having a plurality of outlets over a substantial length of the duct leading from the duct to the chamber, a first flow inducing means for directing air upwardly through the duct and a second flow inducing means for producing a curtain of air which is directed across the open front of the chamber and heater means, the first flow inducing means causing the air to pass over the heater means, through the duct, through the outlets and into the chamber, the cabinet having a glazed, openable door in which at least part of the said duct is defined, the openable door enabling packs of food to be placed in the cabinet other than through the open front.  
       [0007] The flow of air from the outlets provided over a substantial length of the duct provides an even distribution of heated air over food packs which will normally be placed on shelving in the chamber. As the duct is enclosed, there is no likelihood of food packs projecting into the duct and blocking air flow in the duct. Also, as the second flow inducing means draws air at least partly from the upper end of the cabinet, the air moving towards the second flow inducing means will tend to flow over food packs at the upper end which leads to efficient use of the heated air.  
       [0008] Such a cabinet is particularly good for storing hot take-away food as a customer can simply reach through the air curtain directed across the open front, pick up the food, and then remove the food from the cabinet. The effective use of an air curtain avoids the need to provide a door on the cabinet which needs to be opened and closed and is advantageous in that respect. Also, the use of air to keep the food heated is much better than supporting the food on shelves in the form of hot plates as the heat is more evenly distributed over the food. Also, hot plates require the use of heating elements to keep them hot making them difficult to relocate in a cabinet.  
       [0009] The openable door enables food to be placed in the cabinet initially and to enable staff to refill the cabinet. The openable door may be arranged in a rear wall of the cabinet. The glazing enables staff to see when the cabinet needs to be re-filled.  
       [0010] Preferably the openable door is double glazed so as to prevent heat loss and condensation. In a preferred embodiment the openable door has an inner surface which faces the chamber and a plurality of louvres is provided on the inner surface thereof, the door and louvres defining said at least part of the duct. The louvres may be formed in a panel attached to or incorporated in the openable door. Alternatively, individual louvre members may be supported on the openable door by appropriate support means. In such an embodiment the support means may comprise opposed channel members having slots adapted to receive said individual louvre members. The channel members may be formed from a similar material as the openable door, for example stainless steel. The louvre members may be formed from a plastics material and preferably a substantially transparent plastics material such as, for example Lexan.  
       [0011] According to a second aspect of the present invention there is provided a heated food storage and display cabinet comprising an open fronted chamber in which packs of food can be stored, an upwardly extending enclosed air duct having a plurality of outlets over a substantial length of the duct leading from the duct to the chamber, a first flow inducing means for directing air upwardly through the duct and a second flow inducing means for producing a curtain of air which is directed across the open front of the chamber, and heater means, the first flow inducing means causing the air to pass over the heater means, through the duct, through the outlets and into the chamber, and the second flow inducing means being arranged to draw air from an upper end of the chamber and use it in the formation of the air curtain, wherein the cabinet is further provided with an additional heater means operable to supplement the heater means to maintain a desired temperature within the chamber.  
       [0012] The additional heater means is preferably operable in response to a sensed operating condition of the cabinet, for example chamber temperature. The provision of such an additional heater means is desirable in circumstances where the voltage of electricity supplied to the cabinet is variable. Taking the example of the United Kingdom, mains electricity is supplied at a nominal voltage of 240 volts. However depending upon such factors as location and time of day, the actual voltage of electricity supplied may drop as low as 230 volts. In such circumstances the heater means may be unable to maintain a desired temperature. A drop in the chamber temperature is sensed and, via appropriate control means, the additional heater means activated. 
     
    
    
     [0013] A heated food storage and display cabinet in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which:  
     [0014]FIG. 1 is a perspective front view of one form of heated food storage and display cabinet as described in PCT/GB99/04379;  
     [0015]FIG. 2 is a vertical cross-section through the cabinet shown in FIG. 1 on line II-II in FIG. 1;  
     [0016]FIG. 3 is a vertical cross section similar to FIG. 2 showing another form of heated food storage and display cabinet as described in PCT/GB99/04379 and having an openable door;  
     [0017]FIG. 4 is a diagrammatic cross-section showing part of the door in a modified version of FIG. 3 and showing a duct cover in an open position when the door is closed;  
     [0018]FIG. 5 is a diagrammatic perspective view of the duct cover of FIG. 4 showing the way in which it covers the duct when the door is an open position;  
     [0019]Fig. 6 is a perspective view of a glazed and louvered door;  
     [0020]FIG. 7 is a side view of the door of FIG. 6;  
     [0021]FIG. 8 is a plan view of the door of FIG. 6; and  
     [0022]FIG. 9 is a cross-sectional side view of a heated food storage and display cabinet in accordance with the present invention having a glazed and louvered door. 
    
    
     [0023] A cabinet  10  comprises a bottom wall  12 , side walls  14 , a rear wall  16  and a top wall  18 . The bottom wall  12  supports an upwardly extending front wall  19  and lower front and side panels  20 ,  22  respectively which carry a rubber bumper  24  to protect against damage by supermarket trolleys. The front wall  19  has a double glazed see-through panel  26  mounted at its upper end. The panel  26  and extends upwardly towards a downwardly extending front canopy panel  28  carried by the top wall  18 . The cabinet  10  is mounted on a base  30  having castors  32  for ease of manoeuvring the cabinet into position on the floor, say, of a supermarket.  
     [0024] The cabinet  10  defines a food storage chamber  34  between the walls  12 ,  14 ,  16 ,  18 ,  19 ,  20 ,  22  having an open front  35 . A metal skin  36  formed with a multiplicity of perforations  38  is spaced inwardly of the rear wall  16 , the rear wall  16  being imperforate. The skin  36  closes the rear of the chamber  34  and extends width wise for the full width of the chamber and downwards to the bottom wall  12 . The space between the skin  36  and the rear wall  16  forms an air duct  41 , the sides of which are closed by the side walls  14 . The skin  36  carries vertically spaced baffles  42 , the baffles being inclined downwardly, with respect to the skin  36  at a suitable angle, for example 45 degrees. The baffles  42  extend for substantially the full width of the skin  36 .  
     [0025] The cabinet has slotted uprights  44  (one only of which is shown in FIG. 1) at rear corners of the chamber  34  which support shelving, three shelves  46  being illustrated in the present example. Each shelf  46  has two wire end supports  48 . Each wire support  48  is triangular and has spaced apart upper and lower left hand ends  50 ,  52 . The upper end  50  hooks into a selected slot in one of the uprights  44  and the lower end  52  locates in a slot beneath. In that way, each wire support  48  is rigidly located on the uprights  44 . A mesh shelf surface  54  is then mounted on the end supports  48 . Each mesh shelf surface  54  is provided with an upstanding projection  56  at its front end as shown clearly in FIG. 2 to help retain packs  57  of food on the shelves  46  as shown in broken lines. The mesh shelf surfaces  54  are inclined downwardly for ease of access.  
     [0026] The chamber  34  has a inclined floor  58  spaced from the bottom wall  12  of the cabinet. The floor  58  extends between the skin  36  and a position spaced from the top of the front wall  19 . A perforated metal sheet  62  extends across the space between the front wall  19  and the floor  58 . A shelf  63  having feet  63   a  stands on the floor  58 . The front of the shelf  63  is bent to form an upstanding shield  64  which projects across the perforated metal sheet  62  as is apparent from FIG. 2. Suitably mounted in the cabinet  10  beneath the floor  58  is a blower in the form a suitable air pump  66  having an inlet  68  and an outlet  70 . The inlet  68  is positioned in front of a divider  72  extending between the side walls  14  and between the bottom wall  12  and floor  58 . The outlet  70  directs air into a heating zone  74  to the rear of the divider  72 . Electrical heating elements  76  are carried on a mounting  77  suitably supported in the heating zone  74 .  
     [0027] Looking at the upper end of the cabinet  10 , the duct  41  has a closure  78  at its upper end and a perforated metal sheet  80  extends from one side wall  14  to the other and from the cross member skin  36  adjacent the closure  78  to a front cross member  82 . The front cross member  82  extends between the side walls  14  and may be constructed from an insulating material. An area between the perforated sheet  80  and the wall  18  is divided into two compartments  86 ,  88  by a sheet metal mounting  90  which carries a blower in the form of a suitable fan  92 . A baffle  94  extends between the cross member  82  and wall  18  at an angle of approximately 45 degrees and is intended to direct air towards an outlet  96  extending for the full width of the chamber  34  and formed by one or more apertures such as slots in the metal sheet  80 . A guide fin  97  projects downwardly from the sheet  80  adjacent the outlet  96 . A further guide fin  98  of translucent material extends downwardly from panel  82  adjacent the outlet  96 . The guide fins  97  extend for the full width of the chamber  34  and it will be noted that they are inclined to the vertical and are substantially parallel. A strip light fitting  100  is mounted between the canopy panel  28  and the front cross-member  82  adjacent the guide fin  98 . The translucence of the guide fin  98  ensures that the guide fin  98  will not cast a shadow from the light  100  onto food in the chamber.  
     [0028] The operation of the cabinet  10  will now be described.  
     [0029] With electric power switched on to the pump  66 , the heating elements  76  and the fan  92 , the pump  68  draws air through the front perforated sheet  62  and into the inlet  68 . The air is then blown by the pump  66  through the outlet  70 , over the heating elements  76 , through the perforations  38  in the adjacent part of the perforated skin  36  and into the air duct  41 . The heated air is driven upwards through the duct  41  and as the upper end of the duct has the closure  78 , the heated air is forced through the perforations  38  in the front skin  36  of the door  26  and into the chamber  34  as indicated by the arrows A in FIG. 2. The baffles  42  help to deflect the heated air downwardly and encourage flow of air around the food packs  57  on the shelves  46 ,  63 . The operation of the fan  92  draws some of the air near the top of the chamber  34  upwards (thereby encouraging air flow at the top of the chamber beneficial for maintaining the temperature of food packs  57  on the top shelf) and through perforations in the sheet  80  which are immediately beneath the fan. The fan  92  forces that air into the compartment  88  and through the outlet  96 . The air driven through the outlet  96  and past the fins  97 ,  98  is forced downwards and slightly inwards by virtue of the inclination of the fins  97 ,  98 . The air entering the compartment  34  through the perforations  38  in the skin  36  has a component of outward movement towards the open front of the chamber  34 . The inward direction of flow of the air from outlet  96  and the outward movement of the air from the perforations  38  results in a resultant substantially vertical downwardly moving air curtain  104  across the open front  35  of the chamber  34 . Persons requiring a pack  57  of hot food can simply reach through the air curtain  104 , pick up a pack of food and then remove the pack from the cabinet  10 .  
     [0030] The air forming the air curtain  104  flows downwards towards a collection zone  106  adjacent the perforated front metal sheet  62  where it is joined by air which has flowed over the food packs  57  in the chamber  34 . The continued operation of the pump  66  draws the air from the collection zone  106  through the front perforated sheet  62  and into the inlet  68  for recirculation. The shield  64  on the shelf  63  ensures that food packs  57  on the shelf  63  cannot slip forward and cover the perforated sheet  62 .  
     [0031] The use of the air curtain  104  helps to retain in the chamber  34  the heated air which passes over the food packs  57  from the duct  41 . In that way, heated air is retained in the chamber  34 . Also, when a customer reaches through the air curtain, the flowing air  104  forms a seal around the wrist of the customer minimising the escape of air from the chamber  34  as a food pack is being removed.  
     [0032] It will be noted that the perforations  38  in the skin  36  extend for substantially the full length and width of the duct  41 . In that way, a good supply of heated air over all the shelves  46 ,  63  of the cabinet  10  can be achieved keeping the food packs  57  evenly heated throughout the cabinet.  
     [0033] The compartment  86  above the perforated sheet  80  at the top of the compartment  34  serves as a collection zone for heated air which enters the compartment by convection through the perforated sheet  80 . Air in the collection zone tends to trickle out back through the perforations and is drawn through the fan  92  with other air from the upper end of the chamber  34 . The air which accumulates in the chamber  88  serves to provide a source of heated air for the curtain for a brief period if, for some reason, there is a short interruption of heated air from the duct  41 . For example, in order to effect an efficient heat exchange between the air and the heating elements  76 , the pump may be programmed to operate so that air is delivered to the heating zone  74  in pulses of, say, 10 seconds duration at intervals of say 3 seconds.  
     [0034] The heating elements  76  can be arranged to heat the air to the desired temperature and a thermostat arrangement may be provided for heat control. The air may be heated to temperatures in the range of 80 to 120 degrees Centigrade.  
     [0035] A display may be provided on or adjacent the top of the front panel  20  if desired.  
     [0036] Reference is now made to FIG. 3. In many respects the cabinet shown in FIG. 3 is similar to the cabinet described with reference to FIGS. 1 and 2 and only the differences will be described. In FIG. 3, parts corresponding to parts in FIGS. 1 and 2 carry the same reference numerals.  
     [0037] In FIG. 3, the rear wall  16  is formed with an opening  110  which gives access to the rear of the shelves  46  in the chamber. A door  111  is mounted on upper and lower side hinges  112  carried by the rear wall  16  and can be opened and closed to open and close the opening  110 .  
     [0038] The door  111  is double skinned. A front skin of the door  111  is formed by a perforated panel  116  and a rear skin is imperforate. A space between the perforated panel  116  and the rear skin defines part  117  of the duct  41  which is open at the top and bottom. The front skin carries baffles which incline downwardly as in FIG. 2 but the angle of inclination increases the nearer the baffles are to the top of the perforated panel  116 .  
     [0039] A floor  58  extends between an upstanding perforated rear metal sheet  118  and a perforated front metal sheet  120  extending over a collection zone  106 . The rear metal sheet  118  extends from one side wall  14  to the other upwardly from the bottom wall  12  and is spaced from the rear wall  16  so as to define an open-topped further part  122  of the duct  41  in alignment with the part  117  of the duct  41  in the door  111 . Suitably mounted in the cabinet  10  beneath the floor  58  is an air pump  66  having an inlet  68  and an outlet  70 . The inlet  68  is positioned in front of a divider  72  extending between the side walls  14  and between the bottom wall  12  and floor  58 . The outlet  70  directs air into a heating zone  74  to the rear of the divider  72 . Electrical heating elements  76  are suitable mounted in the heating zone  74 .  
     [0040] Looking at the upper end of the cabinet  10 , a short double skinned section  124  is mounted on a cross member  126  extending between the side walls  14 , the cross member  126  closing the section  124  at its upper end. A perforated metal sheet  127  extends from one side wall  14  to the other and from the cross member  126  to a front cross member  82 . A panel  129  of insulating material is also mounted on the underside of the top wall  18 . An area between the perforated metal sheet  127  and the plate  129  is divided into two compartments  128 ,  128   a  by a metal sheet  130  which carries a fan  92 . A baffle  94  extends between the cross member  82  and panel  129  at an angle of approximately 45 degrees and is intended to direct air towards an outlet  96  formed by one or more slots in the metal sheet  80 . A guide fin  97  projects downwardly from the sheet  127  adjacent the outlet  96 . A portion  132  of the metal sheet  127  between the cross member  78  and the divider sheet  90  is perforated.  
     [0041] A strip light fitting  100  is mounted between the canopy wall  22  and the front cross-member  82  and a front guide fin  134  is provided at the back of the light fitting  100  which is inclined to the vertical to impart an inward movement to the air which passes through the outlet  96  similar to that described with respect to FIGS. 1 and 2.  
     [0042] The operation of the cabinet shown in FIG. 3 will now be described.  
     [0043] With electric power switched on to the pump  66 , the heating elements  76  and the fan  92 , the pump  68  draws air through the front perforated sheet  120  and into the inlet  68 . The air is then blown by the pump  66  through the outlet  70 , over the heating elements  76 , through the perforated sheet  118  and into the part  122  of duct  41 . The heated air is driven upwards through the duct part and into the part  117  of the duct  41  defined in the closed door  111 . The heated air is forced through the perforations  38  in the perforated panel  116  forming the front skin of the door  111  and into the chamber  34  as indicated by the arrows A. The baffles  42  help to deflect the heated air downwardly and encourage flow of air around the food packs  57  on the shelves  46 . The operation of the fan  92  draws some of the air at the top of the chamber  34  through the perforations in portion  132  of the sheet  128  and into the compartment  127 . The fan  92  forces that air into the compartment  128  and through the outlet  96 . The air driven through the outlet  96  and past the fin  98  is forced downwards and slightly inwards similar to the cabinet shown in FIGS. 1 and 2 and as shown by arrows to form an air curtain  104  across the open front  35  of the chamber  34 . Persons requiring a pack  57  of hot food can simply reach through the air curtain  104 , pick up a pack of food and then remove the pack from the cabinet  10 .  
     [0044] The front wall  20  does not carry a see through panel as in FIGS. 1 and 2 but carries a display panel  108  at its upper end. The operation of the cabinet  10  will now be described.  
     [0045] In order to fill the shelves  46  with food packs  57  and to replenish the shelves without having to do so from the front of the cabinet, the door  111  can be swung open rearwardly about the hinges  112  and closed again afterwards. An audible and/or visible alarm may be provided to indicate to an operative that the door has been opened. The use of the alarm acts as a reminder to the operative to close the door  111  again.  
     [0046] On opening the door  111  as the door  111  is opened, a switch or switches (not shown) may be operated automatically so as to switch off the supply of electricity to the heating elements  76  and to the pump  66 . The parts  117  and  122  of the duct  41  will no longer be in alignment and air pumped in to the part  122  of duct  41  as the pump  66  slows down and the heating elements cool could escape through its open upper end and blow on to an operative. To prevent that a cover  136  may be provided which can be used to cover the upper end of the duct  64  when the door  26  is swung open. Such a cover  136  is shown in FIGS. 4 and 5. In FIG. 5 the cover  136  is shown broken away. The cabinet  10  of FIG. 3 is modified by repositioning the hinges  112  on the inside of the cabinet as shown in FIG. 5. The cover  136  comprises a rectangular metal sheet fixed to a shaft  137  which is rotatably mounted on spaced apart brackets  138  (one only of which is shown) on the inside of the cabinet  10 . The metal sheet forming the cover  136  has a sealing strip  136   a  along its free edge. An end of the shaft  137  adjacent the lower hinge  112  is drivably connected to a bevel gear  139 . The bevel gear  139  which meshes with a bevel gear  140  fixed to a hinge shaft  112   a  which is drivably connected to the door  111 .  
     [0047] As the door is moved towards its open position as shown in FIG. 5, the bevel gear  140  moves with the door and transmits drive to the shaft  137  so as to move the cover  136  downwardly from the FIG. 4 position to the FIG. 5 position in which it covers the complete upper end of part  122  of the duct  41 . Once the door  111  is closed again, the supply of electricity is switched on again to drive the pump  66  and heat the elements  76 . Preferably, the bevel gears  139 ,  140  will be suitably encased to render them inaccessible to persons taking food packs  57  via the open front of the cabinet  10 . The use of the cover  136  also prevents objects accidentally being dropped into the part  122  of the duct  41  when the cabinet  10  is being refilled.  
     [0048] Although a single door  111  is shown, a double door arrangement may be provided so that each door is substantially half the width of the door  111 . In such a case, a bevel gear arrangement can be provided adjacent the lower hinge  112  for each door  111 .  
     [0049] It should be appreciated that the cabinet shown in FIGS. 1 and 2 can be modified so as to have an open rear door or doors  112  similar to that shown in FIGS.  3  or FIGS. 4 and 5.  
     [0050] As shown in FIG. 3, the cabinet  10  may be arranged back-to-back with a refrigeration unit  150  indicated diagrammatically in broken lines and shown partly broken away. As is well known, a refrigeration unit  150  gives off heat extracted from within the refrigerator itself. In the present case, the heat given off is transferred to air in a heat exchanger  152  or other suitable means and ducting  154  is provided to enable to heated air to enter the cabinet  10  upstream of the pump  66 . The air from the refrigeration unit  150  supplements or provides the air which is circulated through the cabinet  10 . The flow of air from the refrigeration unit  150  can be controlled by a flow controller  156 .  
     [0051] Referring now to FIGS.  6  to  9  there is shown an alternative embodiment of a door, generally designated  160 , for use with a cabinet  168  according to the present invention. The door comprises a frame  162  defining an aperture  164 , the aperture being fitted with a double glazed panel  166 . The provision of such a panel  166  enables staff stationed behind the cabinet and charged with ensuring that it is adequately stocked with produce at all times to monitor the contents of the cabinet  168  without the need to either open the door  160  or walk round to the front of the cabinet  168 . The provision of double glazing reduces heat loss and prevents condensation.  
     [0052] The door is further provided with louvres  170  instead of the perforated panel  116  described above. The rear face  172  of the door  160  is provided with opposed channels  174  which are slotted to receive individual louvres  170 . The channels  174  are formed from the same material as the door frame  162 , for example stainless steel, while the louvres  170  are manufactured from a plastics material, for example Lexan having a thickness of around 5 mm. As in the previously described embodiment, the door  160 , channels  174  and louvres  170  define a portion of the duct  41  and, in use, heated air is forced between the louvres  170  and into the chamber  34 .  
     [0053] In FIG. 9, a heating element  180  is provided in addition to the heating element  76 . The provision of the additional heating element  180  is desirable in circumstances where the voltage of electricity supplied to the cabinet is variable. In the United Kingdom, mains electricity is supplied at a nominal voltage of 240 volts. However depending upon such factors as location and time of day, the actual voltage of electricity supplied may drop as low as 230 volts. In such circumstances the existing heating elements  76  may be unable to maintain a desired temperature in the cabinet  168 . Any such drop in the chamber temperature is sensed and, via an appropriate control, the additional heating element  180  is activated to bring the temperature to the desired level. Additional heating elements  180  may also be used in the cabinets described with reference to FIGS.  1  to  5 .