GB2251988A

GB2251988A - Rapid warm-up control system for process ovens

Info

Publication number: GB2251988A
Application number: GB9100904A
Authority: GB
Inventors: Alfred Alexander Staden
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-01-16
Filing date: 1991-01-16
Publication date: 1992-07-22
Also published as: GB9100904D0

Abstract

Rapid heating to full radiating power is achieved by applying to the heating elements a voltage which is at least 30% higher than their rated voltage for a predetermined period, eg. up to 3 minutes, or until a predetermined temperature is reached. Two 220 volt elements, which are normally connected in series across two phases of a 440 volt 3 phase supply, may initially have their series connection point coupled to the third phase of the supply. In alternative arrangements, three 80 volt elements initially each receive 240 volts and are then connected in series across the 240 volt supply (Fig 2), or reduction in effective applied voltage following warm up is achieved by burst-firing or phase-angle control of a thyristor. The oven may be a static oven for heating an adhesively-coated shoe sole, or an on-line oven through which motor car bodies are conveyed. <IMAGE>

Description

IMPROVED CONTROL SYSTEM FOR ON-LINE PROCESS OVENS This invention relates to an improved control system for electric on-line process ovens.

'On-Line' or 'Corridor' Ovens through which the product is conveyed in course of manufacture are widely used in Industry. In a simple form the oven comprises a tunnel in which the temperature is maintained at the desired level so that articles or the product passing through it are heated. More sophisticated forms of oven can embody groups of Infra-Red radiating elements which for the sake of improved control and economy of energy may be arranged to switch on and off automatically according to the presence or absence of workpieces in or approaching the oven or individual zones of an oven.

This facility becomes increasingly valuable in large and long ovens, such as are used in the production of motor cars, but the effectiveness (and resulting control and economy) can be seriously reduced because some types of electric heating element, preferred for other reasons, do not produce full radiated power until they reach full heat which can take up to several minutes after being switched on.

This invention provides an improved control system with means to reduce dramatically the heating up time of slow response electric heating elements such as metal sheathed, ceramic, and spiral wound elements open or enclosed, and describes inexpensive ways in which the invention may be applied.

I have determined that within certain constraints, it is practical to energise a cold or warm electric element for a short period of time with a voltage much higher than it's normal running voltage. This causes the element to reach running temperature very much quicker, at which point the voltage is reduced to normal to sustain a full output.

A very simple and inexpensive way to achieye this is illustrated by way of example in Diagram 1. in which R1 and R2 are resistance heating elements designed to operate normally on a 220/250V A.C. supply but which are connected in series between two phases of a 440V 3 phase A.C. sup ply. During the rapid heatingup period, the third phase is connected to the junction of the two elements so that each is subjected to 415 Volts when SW1 is closed. SW1 is controlled either by a timer or a thermostat device which causes it to open after a timed period or when the elements reach a pre-determined temperature. When a plurality of heating elements is employed, the phases are distributed to provide a balanced load.

An alternative scheme is shown for example in Diagram 2 in which three resistance elements R1, R2 and R3 each designed to run normally on 80 Volts are connected in series across 240V A.C. and which are brought rapidly to full heat from cold when switches SW1 and SW2 (conveniently contacts in a relay) are closed thereby applying 240V to each element for the heating-up period.

A further alternative, but much more expensive, method is to connect the elements permanently to a higher than rated voltage and to reduce the applied power when full heat is reached by switching the current repeatedly on and off, for example by use of a 'burst-fire' thyristor device to sustain the element at full temperature.

In practice, these control systems could be applied to successive zones of a tunnel or corridor oven operating for example as follows: a) The oven is totally off but the conveyor is running through it but carrying nothing.

b) A sensor detects the arrival of an article (e.g. a motor-car body) on the conveyor a short distance ahead of the oven entrance.

c) The first zone of the oven is energised by this Quick-Response circuit and is at full heat by the time the product has arrived.

d) Second and subsequent zones are switched on automatically according to conveyor speed. Thus the extra power demanded for the 'heating-up' period is phased along the length of the oven and could be limited to that required for only one zone of an oven comprising many zones.

In a further application of this invention, this rapid heat-up system is applied to the heating elements of a static device which is designed to heat in a few seconds by a timed period of Infra-Red radiation, an article placed in it, for example, a shoe sole coated with adhesive which is thereby activated and in which the power to the heating element or elements is removed or substantially reduced between periods of full activation.

Claims

1.

An electric Infra-Red oven having means to convey a product through it, in which the heating elements are initially energised by electricity at a voltage at least 30% higher than their rated voltage for a period of up to 3 minutes each time they are switched on.

2.

An electric Infra-Red oven as in Claim 1 in which the heating elements are operated by electricity at a voltage substantially higher than their rated voltage but which are maintained at operating temperature by cyclicly reducing or interrupting the supply voltage after they have reached full operating temperature.

3.

An electric Infra-Red oven as in Claim 2 in which the heating elements are operated by an alternating current of electricity which if maintained would seriously overheat or destroy the elements but which are maintained at safe operating temperatures after the initial heating up period by reducing the effective power applied by use of an electronic phase-angle controller which cuts out part of each cycle of alternating current.

4.

A control system for electric heating elements which applies a voltage at least 30% higher than the normal running voltage for a period of up to 3 minutes each time they are switched on.