Patent GB-785033-A (GB)
Title: Improvements in or relating to electric control devices for furnaces
IPC: G05D23/27
Filed: 1954-07-16
Abstract: 785,033. Controlling burners. MEIKLE, O., FIANDER, L. P., and ANDREWS, D. J. G. July 18, 1955 [July 16, 1954], No. 20805/54. Class 75(1) [Also in Group XL(c)] An electric control device for an oil or gas burning furnace comprises two electric relays K, L energized from the anode circuits of two electronic discharge devices each having a control grid F<SP>1</SP>, G\ a capacitance-resistance timing circuit H, H<SP>1</SP> or J, J' connected to each control grid and means whereby each relay controls the application of potential to the timing circuit of the electronic device energizing the other relay, the arrangement being that each relay can be energized only after expiration of the time delay of the timing circuit controlled by the other relay. The device is used in conjuction with an electric motor A controlling the fuel and air supplies to a burner, an ignition transformer B supplying a spark igniter for the burner, a flame detector C such as a thermostat or a photocell or a contact device responsive to flame conductivity and thermostats D responsive to temperatures determined by the furnace or steam pressure responsive devices. The electronic discharge devices consist of two triodes FF<SP>1</SP> F<SP>2</SP>, GG<SP>1</SP> G<SP>2</SP> which may be combined in a single envelope as shown. The first relay K operates the contacts K<SP>2</SP>...K<SP>6</SP> two of which K<SP>2</SP>, K<SP>5</SP> are change-over contacts. The second relay L operates the contacts L<SP>2</SP>, L<SP>3</SP>, L<SP>4</SP> two of which L<SP>2</SP>, L<SP>3</SP> are changeover contacts. When the burner is out of action and the furnace is gradually cooling, the thermostats D are open, the cold contact C<SP>2</SP> of the flame detector C, is closed and both relays K, L are de-energized, their contacts being in the positions shown in the drawing. The second timing circuit J J<SP>1</SP> is connected through resistance J<SP>2</SP> to the positive pole of the supply source M so that its condenser J is charged by grid current and the positive voltage on the grid G<SP>1</SP> ensures that the anode circuit of triode G G<SP>1</SP> G<SP>2</SP> when connected supplies insufficient current to actuate the second relay L. The first timing circuit H H<SP>1</SP> however, is connected to the cathode F<SP>2</SP> of the first triode F F<SP>1</SP> F<SP>2</SP> and provided that two minutes have expired since the last energization of the second relay L to allow the condenser H to discharge the grid of the first triode F F<SP>1</SP> F<SP>2</SP> is at zero potential so that the valve is in a condition to permit immediate energization of the first relay K. When, therefore, one of the thermostats D closes its contacts the first relay K is energized from the positive pole of the source M, the cold contact C<SP>2</SP> of the flame detector, the change-over contact L<SP>3</SP> and the first triode F, F<SP>1</SP>, F<SP>2</SP>. Contact K<SP>3</SP> then contacts the first relay K to the hot contact C<SP>1</SP> of the flame detector to form the main holding circuit for the first relay K; contact K<SP>4</SP> completes a temporary holding circuit for the first relay K to keep it energized when the flame detector breaks the connection to its cold contact C<SP>2</SP>; contact K<SP>6</SP> connects the positive pole. of the source M to the second relay L and to the ignition transformer B; contacts K<SP>6</SP> and K<SP>5</SP> energize the motor A; and the change-over contact K<SP>2</SP> disconnects the second timing circuit J, J<SP>1</SP> from the positive pole of the source M and connects it to the cathode G<SP>2</SP> so that the condenser J of the second timing circuit discharges and the grid potential after a delay permits anode current to flow through the second valve to the second relay L, the delay being sufficient to permit ignition of the burner. On energization of the second relay L, the contact L4 opens to de-energize the igintion transformer, the changeover contact L<SP>3</SP> opens the circuit to the first relay K (which, however, remains energized through hot contact C<SP>1</SP> of the flame detector) and closes a circuit connecting the second relay L to the cold contact C<SP>2</SP> of the flame detector, and the change-over contact L<SP>2</SP> disconnects the first timing circuit H, H<SP>1</SP> from the cathode F<SP>2</SP> of the first triode and connects it to the positive pole of the source M through a resistance H<SP>2</SP>. The grid F<SP>1</SP> of the first triode is thus given a positive potential causing the flow of grid current and a reduction in anode current -which, however, is not sufficient to de-energize the first relay K but is sufficient to prevent re-energization after deenergization thereof. When the controlling thermostat D opens on a predetermined temperature being attained both relays are de-energized, the condenser J of the second timing circuit becomes charged to prevent re-energization of the second relay L and the condenser H of the first timing circuit discharges to remove the negative potential it applies to the grid F<SP>1</SP> of the first triode so that the first triode is'again in a condition to energize the first relay K after a two minute delay. If the burner fails to ignite the flame detector C does not open its cold contact C<SP>2</SP> and therefore when the second relay L is energized it de-energizes at change-over contact L<SP>3</SP> the first relay K. The second relay L however remains energized through the cold contact C<SP>2</SP> of the flame detector and the change-over contact L<SP>3</SP> and a lamp N and a warning device are energized through the change-over contact L<SP>3</SP> and the changeover contact K<SP>5</SP>. A manual switch O is then operated to de-energize the second relay L. If contacts L<SP>4</SP> for de-energizing the ignition transformer fails to open after the time delay determined by the second timing circuit J J<SP>1</SP> a relay P is energized to open contacts P<SP>1</SP> in the supply line from the source M.