The VMC gas heating system works based on the measurement in both air and gas lines. Metering orifice units are installed in the air and gas supply lines to monitor the respective flow. Both signals are fed to the VMC gas control valve in the gas supply line, which adjusts the gas flow to give the required air/gas ratio. Any change in the airflow results in an immediate equivalent adjustment of the gas flow.
The air/gas ratio maintained by the system can be varied by adjusting the size of the metering orifice in the gas line. A simple mixer is installed to provide adequate mixing of the gas and air. The VMC system conforms to the strict and current EN746-2 regulations.
The air control valves with optional bypass, air/gas mixers, gas control valves, non-return valves in the gas supply line to each zone, gas filter and two gas safety valves are typically installed together in an air/gas control station. The equipment for all the heating zones of one forehearth can be combined in a single station or for each heating zone.
On a standard forehearth, the heating of the left and right sides are coupled together and controlled simultaneously as one control loop, usually based on a temperature reading made in the centre of the channel.
However, on some installations, the glass temperature may be quite different on the two sides of the channel, possibly due to a corner in the channel.
If this is a possibility, the operator can separate the left and right heating within a zone and measure and control the temperature individually. The separated heating can be implemented in any number of zones in the distributor and forehearths as needed.
The VMC gas heating system used on all SORG® forehearths will maintain an accurate air/gas ratio at all times. However, there are situations when being able to check and adjust the radio manually is better.
Some heavily-reduced glasses are more susceptible to thermal reboil, and it may be necessary to keep the oxidation level of the gas heating as low as possible.
With many colouring forehearths, precise control of the atmosphere is necessary in the melting zone. The patented SORG® OMT oxygen trim system is ideal for this application. The air/gas mixture supplied to the burners is burnt in a special combustion chamber on the forehearth, and the waste gases from the combustion are passed over a zircon oxide oxygen sensor.
The sensor signal is passed to a transmitter and a ratio controller. An air bleed line and a control valve add a small amount of air to the air/gas premix downstream of the central air/gas mixer.
The control valve varies this bleed air supply to modify the air/gas ratio according to the output of the ratio controller. The system provides additional security in terms of accurate maintenance of the required air/gas ratio. It also permits easy and precise adjustment if regular changes in the ratio are needed.