Gas burners: Selection matters

Adrian Langford, general manager of Lanemark International, comments: "For a typical gas fired, industrial, re-circulating oven or dryer, the first priority is to correctly match the burner model selection to the maximum heating demand, while the selection of the most efficient temperature and burner control system is also key. 

"At the same time, the issue of direct or indirect firing is particularly significant and should be addressed from the outset. In most cases, the direct option should be selected because 100% of the available heat is delivered into the oven. Indirect systems, which incorporate heat exchangers, are ideal where the ‘products of combustion’ could have a direct detrimental effect on the end product, but their operational efficiency is typically up to 20% lower.  The impact of this on energy use is self-evident.”

Langford draws attention to the choice of temperature and burner control systems. Installations which use simple high/low gas only control – switching between pre-set high fire and low fire rates – maintain a constant combustion air pressure/flow that is set only for the high fire rate.  Significantly, there is no reduction when the burner operates at the low fire rate. Modulating (gas only) alternatives, where the gas input is controlled by a motorised gas ball or butterfly valve, will offer superior temperature control, but also see the combustion air pressure/flow remain unchanged when the gas flow rate to the burner is increased or decreased.  

"Neither of these options optimises energy consumption which is why a modulating gas and combustion air alternative is often the preferred solution,” continues Langford.  "In this case, both the gas and combustion air flows respond directly to the process oven temperature requirement.”  However, he highlights disadvantages with ‘traditional’ methods of gas and air modulation which use a linked gas butterfly valve and combustion air damper – 

•  The burner combustion air fan runs continuously at maximum speed.

•  The linkage arrangement between the gas and air supplies is inflexible and can be prone to ‘sticking’, and ‘progressive mis-adjustment’ through wear in typical process heating operating environments.

• The burner does not respond to changes in the oven/dryer process airflows and air pressures.

The latest burner control systems, which benefit from the use of electronic variable speed drives, address these issues head-on. Here, the gas burner receives temperature control signals directly from the oven, causing the burner combustion air fan motor speed to increase or decrease accordingly.  The changes in combustion air pressure are then transmitted to the latest generation of gas/air modulating control valves – such as the VAV series produced by Kromschroder – so that the gas flow is increased or reduced proportionally.

Langford continues: "The energy savings associated with modulating gas/air control methods can be readily quantified. The percentage gas saving that is available with this option rises dramatically as the ‘Burner Utilisation Factor’ – the ratio between the average and maximum firing rate of a burner system – decreases.”

By way of illustration, he highlights a typical installation of an oven operating efficiently at 225°C, where the Burner Utilisation Factor is 75%. "In this case, the gas saving available from a modulating gas and air control system would be around 3%, which increases significantly to 9% if the Burner Utilisation Factor reduces to 50%.  It would be over 20% if the factor is 30% or lower,” he says.

Langford points out that, for an oven fitted with a total burner capacity of 600kW and at a natural gas cost of 3p/kWh, the above gas savings would translate into annual savings of £781, £1562 or £2187 respectively at Burner Utilisation Factors of 75, 50 and 30% – based on an 8h production day, 5 days a week and a 48 week year.

"These are factors that are difficult to ignore in the current energy cost environment. Minimising the negative impact of increasing energy costs by focussing on such issues can help to optimise efficiencies and thus energy consumption – which clearly offers important benefits to everybody involved in the manufacturing and processing sector,” concludes Adrian Langford.