GB2206470A - Cooker appliances - Google Patents

Abstract

A cooking appliance utilising radio frequency (R.F.) energy is described in which the energy is applied to food to be cooked or heated by way of a slow wave transmission line arrangement. The arrangement may include a coaxial transmission line with an earthed outer sleeve and a helical inner conductor suitably terminated to create either a resonant or a travelling wave applicator. In an alternative arrangement the transmission line includes a meandering conductor disposed in a plane substantially parallel to, and separated from, a ground plane conductor. <IMAGE>

Description

IMPROVENENTS IN OR RELATING TO COORING APPLIANCES The present invention relates to cooking appliances, and it relates especially, though not exclusively, to such appliances as may be used for the cooking of food in bulk quantities.

Such appliances find application, for example, in commercial and industrial premises and in community establishments such as schools and hospitals.

When food is to be cooked in bulk, it is necessary for the relevant cooking appliance to apply energy efficiently and in roughly equal quantities throughout the food. Speed of operation is another important criterion and, in this latter respect, microwave ovens are an attractive proposition. These appliances, however, do not meet the former criterion, at least with the desired speed, since the energy tends, in the early stages of the cooking process, to be concentrated in peripheral regions of the food, leaving the central region uncooked for a considerable time. Indeed, the microwave energy itself is only capable of penetrating into food to the extent of 1 or 2cm; the remainder of the food being cooked by thermal conduction, which is a slow process.This is illustrated in Figure 1, which shows, in solid line, a plot of core temperature against time of a pork roast heated by microwave energy at a frequency of 2450 MHz and at a power of 500 watts. The dashed line will be referred to hereinafter.

It is known that electromagnetic energy at lower frequencies can penetrate much more deeply than microwave energy into food, and thus can heat the central regions of the food much more rapidly because it is not necessary to rely upon the relatively slow thermal conduction process. The cooking can thus be effected much more rapidly and this is illustrated by the dashed line on Figure 1, which shows a plot of core temperature against time for a pork roast subjected to 500 watts of radiofrequency (RF) energy at 14 Mhz.

This principle is known, but a significant problem arises in determining how to apply the RF energy to the food.

It has been proposed to use eddy current (inductive) heating or dielectric heating, the latter using parallel plate or conformal plate configurations but these application techniques all have significant drawbacks.

It is- an object of this invention to provide an RF cooking appliance with improved means for applying the RF energy to the food to be cooked.

According to the invention there is provided a cooking appliance including a source of RF energy and a slow wave transmission line arrangement coupled to said source and capable of applying energy so coupled to food to be cooked in or by the appliances.

In order that the invention may be clearly understood, some embodiments thereof will now be described, by way of example only, with reference to the accompanying drawing of which: Figure 1 has already been described, Figure 2 shows, in schematic form, one example of a slow wave transmission line suitable for use in an appliance according to the invention, Figure 3 shows the line of Figure 2 in more detail as it may be incorporated in a cooking appliance.

Figures 4a and 4b show, respectively, plan and elevational views of another example of a slow wave transmission line suitable for use in an appliance according to the invent-ion, and Figures 5a and Sb show, in elevational view, two alternative dispositions for the food to be cooked in relation to the line of Figures 4a and 4b.

Referring now to Figure 2, there is shown a coaxial slow wave transmission line with a helical inner conductive 1. The conductor 1 is coupled at one end 2 to a source (not shown) of RF energy and the other end 3 of the helical conductor 1 is suitably terminated. The outer conductor 4, coaxial with the helical inner conductor 1, provides RF screening, and food placed inside the helix described by conductor 1 is heated by leakage fields.

The termination at end 3 of conductor 1 may be either open circuit or short circuit, thus creating a resonator, or resistive, creating a travelling wave applicator. In the case of resonance, the dimensions of the device must be chosen to achieve resonance at the frequency of the RF energy. The travelling wave applicator is less frequency sensitive than the resonating applicator, and exhibits less impedance variation with load (i.e. the food to be cooked) but is also less efficient, due to dissipation of power in the resistive termination.

Figure 3, in which features common to Figure 2 carry the same reference numerals, shows a 21 MHz resonator that has been constructed and used to heat a load of 1 litre of water with an efficiency better than 80%. The dimensions of the applicator were chosen for half wave resonance at 21MHz with short circuit termination and were as follows: Outside diameter of helical conductor 1 (consisting of twenty turns of 6mn copper pipe) -15cm Outside diameter of outer conductor -30cm Overall height of outer conductor -30cm The outer conductor was earthed.

Figures 4a and 4b show, in plan and elevational views respectively, an alternative form of slow wave transmission line, wherein the "live" conductor 5 follows a two-dimensional, meandering path, as shown in Figure 4a, above and parallel to a ground plane conductor 6, shown only in Figure 4b.

As indicated in Figures 5a and 5b, the food to be cooked can be placed either above the meandering "live" conductor 5 or between that conductor and the ground plane conductor 6.

It will be evident that other forms of slow wave (leaky) transmission line and other configurations thereof may be utilised without deporting from the scope of this invention.

Claims (6)

1. A cooking appliance including a source of radio frequency energy and a slow wave transmission line arrangement coupled to said source and capable of applying energy so coupled to food to be cooked in, or by, the appliance.

2. An appliance according to Claim 1 wherein said'slow wave transmission line arrangement comprises a coaxial transmission line with a helical inner conductor coupled to said source at one end and to termination means at the other end

3. An appliance according to Claim 2 wherein said termination means includes an open circuit or a short circuit.

4. An appliance according to Claim 2 wherein said termination means is resistive.

5. An appliance according to Claim 1 wherein said slow weve transmission-line arrangement includes a meandering conductor disposed in a plane substantially parallel to, and separated from, a planar ground plane conductor.

6. A cooking appliance substantially as herein described.