IES74694B2 - A control system for gas cooker - Google Patents

Abstract

A gas fired twin burner cooker (1) has a cooking section (22) and a boiler (35) thermally insulated from one another. The cooking section has at least one oven (22) and a hotplate. A gas burner (40) heats the cooking section (22) and a gas burner (30) heats the boiler (35). Each burner (30, 40) has a separate fan and separate flue passages to lead the combustion by-products to a common outlet flue (18). Circuitry is provided to operate continuously the fan of the idle burner during the operation of the cooker burner (40) or the boiler burner (30) thereby maintaining air pressure in the flue passageway of the idle burner.

Description

A CONTROL SYSTEM FOR GAS COOKER The present invention relates to a gas fired twin series cooker which is a multi-functional domestic heat and cooking appliance. It can be used as domestic boiler capable of producing up to 100,000 Btu/h and as a domestic cooker with oven and hotplate facilities while also providing space heating. The appliance utilises two independent forced draught burners, one for heating the boiler and the other providing heat for cooking. The boiler and oven are thermostatically controlled with the boiler also having a fully programmable facility.

As of the 1st January 1996 every gas appliance sold in the EU must be manufactured in accordance with the Gas Appliance Directive. In order to attain this CE approval every gas appliance must under-go type testing carried out by a notified body. This type testing is mainly concerned with issues such as combustion emissions under various conditions, surface temperatures, operation of safety devices and general safety, including a blocked flue test.

A blocked flue test for ’a forced draught appliance involves running the appliance at nominal input until it reaches thermal equilibrium. The flue is then gradually blocked until the pressure switch on the appliance senses a build up of pressure in the appliance and shuts off the burner. As the back pressure increases the level of CO2 in the combustion chamber increases which increases the flame length. As a result the flame starts to impinge on the walls of the combustion chamber and CO starts to form. The CO level at shutoff must not exceed 0.2 ppm in an air free sample. The back pressure at which the pressure switch is set to shut off the appliance is very important.

If the pressure switch is set too low the appliance may be subjected to nuisance shut-down in the field by minor pressure fluctuations. These may be caused by gusts of wind for example. The pressure switch must be set high enough to overcome these pressure fluctuations but low enough to shut off the appliance in a blocked flue situation before CO reaches unacceptable levels. On previous single burner appliances shutoff at 0.8 mbar has proven to work well.

During pre-testing of the 100K gas cooker it was discovered that if the flue was gradually blocked while operating on a single burner, sufficient pressure did not build up inside the appliance to actuate the pressure switch. This was due to the combustion gases travelling back down the flue passage of the opposite burner and venting out to the room through the idle burner.

The present invention provides a twin burner gas cooker of the type described in which the fan of the idle burner is operable to run continuously when one burner is in operation alone.

To allow this continuous idle fan to run a control system has to be specially designed around the burner's own sequential control box. When operating one burner alone this special control system has to be capable of: Allowing the control box to go through it's start up sequence without interrupting unnecessarily.

Running the idle fan when one burner is operating. Recognising a loss in burner pressure or a build up of combustion chamber pressure sending the burner to lockout.

Recognising a loss in pressure in the idle fan sending the burner to lockout.

The system must also allow both burners to operate together. The operation of turning on or off either burner at any time should not in itself send either burner to lockout. If both burners are operating and either pressure switch actuates then this should lead to lockout of both burners.

«· As stated already the control system has to be specially designed around the sequence of the burner control box. The start up sequence of the LANDIS AND GRY (Trade Mark) LGB21.A34027 will now be explained in conjunction with the accompanying drawing which is a wiring installation diagram.

When power is sent to the control box it's first step is to wait for a time during which the air pressure switch and flame relays are tested for their correct contact position. This waiting time lasts approximately 8 seconds. When this time has elapsed a signal is sent from pin three (3) to start the motor. The control box then waits for five (5) seconds before checking the air pressure switch. After this time the set value of air pressure must have built up or else a lockout is initiated. If the correct pressure signal has been received then the control box will run through it's pre-purge cycle. This pre-purge of air will last at least thirty (30) seconds. When the pre-purge is completed the control box sends a signal to the transformer to provide an ignition spark. Two (2) seconds later the gas valve is opened and a low-load of gas is admitted to the burner head. After a maximum of five (5) seconds the control box checks that a flame has been established. To achieve this the ionisation current into pin one (1) of the control box must be a minimum of 7 uAmps . This signal will continue uninterruptedly until controlled shut-down occurs or else the control box will initiate lockout. If a flame has been established the burner will continue to run on low-load for another four (4) seconds before opening the gas valve to full load. The burner will then continue to operate until a controlled shutdown occurs or a fault condition is recognised resulting in lockout.

Two PCB mounted relay safety systems are specially provided to allow an idle fan to operate without interfering unnecessarily with the sequence of the control box. In the diagram BRT and ORT are time delayed relays whose contacts make 1.5 seconds after they are energised.

The relays PSO and PSB are energised from the normally open (NO) contact of the oven and boiler air pressure switches respectively. When the oven thermostat is closed OR is energised instantly and ORT 1.5 seconds later. This results in the boiler fan starting which subsequently energises PSB. The control box checks that the relay PSO is in it's correct position by testing the continuity between pin three (3) and pin six (6). When satisfied a signal is sent from pin three (3) to start the oven motor. After the oven motor starts the PSO is energised and when the control box checks for air it finds PSO in it's correct position. The control box will then go through the remainder of it's start sequence as described earlier. The boiler burner will operate in a similar manner.

If the boiler burner thermostat is turned on while the oven burner is running BR and BRT are energised. The boiler motor will stop and PSB will de-energise. The oven burner will continue to run however as BR is bridged across the PSB contact connected to pin eleven (11) on the oven control box. With PSB in it's correct starting position and PSO and OR closed the boiler control box will go through it's starting sequence as normal. If one burner is operating alone and there is a fault with either the burner's own fan or the idle fan, PSO or PSB will de-energise and the burner goes to lockout the 0R2 or BR2 is energised and power is cut off to the opposite burner.

It is seen that relays BR2 and BR3 are energised from the boiler pressure switch and 0R2 and OR3 from the oven pressure switch. When the oven thermostat is closed both oven and boiler fans start. BR3 and 0R3 are energised. The oven control box checks and finds continuity between pin three (3) and pin six (6). It then sends a signal from pin three (3) which energises relay 0R1. This in turn energises 0R2 giving continuity between pin three (3) and pin eleven (11) which satisfies the control box when it checks for air. The control box will then continue through it's start sequence as described earlier. The boiler burner will operate in a similar manner.

If the boiler thermostat is turned on while the oven burner is operating, relay R1 is energised. Power continues to be supplied to pin twelve (12) of the oven control box and both motors continue to run with power supplied from the normally open contact of R1 . As a result the oven burner continues to operate. The boiler burner will go through it’s sequence as described above with BR2 remaining de-energised until BR1 is energised by a signal from pin three (3) of the boiler control box. If either pressure switch is actuated due to a pressure build up in the combustion chamber of fan failure BR2 and BR3 or 0R2 and 0R3 will de-energise and the control box go to lockout.

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention.

MACLACHLAN & DONALDSON, Applicants' Agents, Merrion Square, DUBLIN 2.

Claims (5)

CLAIMS :

1 . A gas fired twin burner cooker having a cooking section and a boiler thermally insulated from one another, 5 the cooking section having at least one oven and a hotplate, the cooker comprising a gas burner to heat the cooking section and a gas burner to heat the boiler, each burner having a separate fan and separate flue passages to lead the combustion by-products from the cooker burner and 10 the boiler burner to a common outlet flue characterised in that means are provided to operate continuously the fan of the idle burner during the operation of the cooker burner or the boiler burner thereby maintaining air pressure in the flue passageway of the idle burner.

2. A gas fired twin burner cooker as claimed in Claim 1, including a first control apparatus for overriding burner control means provided on each burner, the first control apparatus having means for allowing the burner control 20 means to go through its start-up sequence, and means for running the fan of the idle burner when the other burner is operating.

3. A gas fired twin burner cooker as claimed in Claim 2, 25 in which the first control apparatus includes means for detecting a loss of pressure in the operating burner or for detecting a build up of combustion chamber pressure and means for disabling the operation burner on detection of a loss or build-up of pressure.

4. A gas fired twin burner cooker as claimed in Claim 2 or Claim 3 in which the first control apparatus includes means for detecting a loss of pressure in the fan of the idle burner and means for disabling the operational burner 35 on detection of such a loss of pressure; and optionally includes means disabling both in which the first control apparatus for operating both burners together and burners on detection of a change of pressure in relation to either one of the burners,

5. A gas fired twin burner cooker substantially as herein described with reference to and as shown in the accompanying drawing.