Keeping heating on top form
Regular burner maintenance is the key to achieving to achieving optimum efficiency and avoiding safety problems. Steve O’Neill of EOGB Energy Products explains the importance of regular burner maintenance and the correct procedures to ensure optimum efficiency and safety.
Manufacturers of most gas- and oil-burning equipment recommend regular maintenance and safety checks for their products in order to achieve optimum levels of safety and efficiency. This will typically include the necessity to confirm the following.
• Adequate supply of combustion, cooling and dilution air.
• Correct burner, operating pressure and gas rate/heat input.
• The effectiveness of any flue.
• Safe function of the plant and confirmation that it will always fail to a safe situation.
Failure to ensure any of the above factors could have the effect of causing incomplete combustion and/or cause plant failure — with the potential to injure people and/or cause large financial losses.
By implementing a suitable maintenance programme for the plant it is possible to avoid potential breakdowns, as the maintenance will inherently involve the visual inspection and cleaning of the plant components, allowing a good opportunity to replace or adjust accordingly without incurring periods of downtime at a later date. Furthermore, optimum efficiency is maintained throughout the operating time of the plant.
To ensure complete combustion of a fuel, there must be an adequate supply of oxygen, which is usually taken from the atmosphere). Air in the atmosphere consists of about 77% by mass (79% by volume) of nitrogen and 23% by mass (21% by volume) of oxygen.
Depending on the design of the appliance water, formed during combustion can be either in a liquid or vapour state in the combustion products.
The provision of suitable combustion air should be confirmed on every inspection, with all restrictions and debris being removed accordingly.
In addition to the confirmation of a satisfactory supply of combustion air etc, the flue should be visually inspected at regular intervals to confirm all products are being evacuated effectively, that the flue is adequately supported, there is no evidence of mechanical failure and that the flue is terminating satisfactorily.
The gas rate should be checked to confirm the correct volume of fuel is being delivered to the appliance, thus ensuring the design output can be achieved. A gas rate that is outside the scope the plant was designed for it would result in both a loss in efficiency and potentially incomplete combustion, as well as potential damage to the plant and ancillary equipment. The gas rate/burner pressure is checked on almost all routine maintenance visits, and the necessary adjustments made to return the plant to optimum operation.
The ability to interpret the information being provided by an electronic combustion analyser can be a critical factor in ensuring both optimum efficiency and combustion safety. Furthermore the engineer should be able to discern when it is necessary to rely upon the combustion ratio to confirm correct operation and when to read the concentrations of oxygen, carbon dioxide and carbon monoxide.
The theoretical equation for complete (stoichiometric) combustion using air is:
CH4 + 2O2 + 7.52N2 → CO2 + 2H2O + 7.52N2
(CH4 is methane — natural gas)
It is possible to identify from that equation that for a specific fuel complete combustion will yield a fixed percentage of carbon dioxide (CO2) and water (H2O) when a given volume of gas is burned in theoretical air. It is also a fact that any deviation in the quantity of air from that of theoretical value will alter the quantity of carbon dioxide produced, and combustion efficiency will be affected.
An increase in the amount of combustion air will result in a reduction in carbon-dioxide concentration, as the increased volume of air will dilute the carbon dioxide within the products, thus reducing both flame temperature and overall combustion efficiency. The common definition of this additional air above theoretical is known as excess air.
Conversely reducing combustion air below stoichiometric would mean that not all the fuel would be burned, which would allow the formation of dangerous carbon monoxide and other unburned hydrocarbons. It can also be observed that combustion products other than carbon dioxide and water will be formed — such as carbon monoxide, hydrogen and other unburned hydrocarbons.
Although a significant quantity of carbon monoxide can still be produced if there is not enough combustion air, the unburned fuel will compete for any remaining oxygen leading to the formation of differing species in various quantities.
It can be observed from the statement above that the higher the carbon-concentration, the more efficient the combustion.
However, it cannot be assumed that by simply increasing the concentration of carbon dioxide we can confirm optimum efficiency or safety. and indeed due to the higher flame temperature a dissociation of the combustion products would follow resulting in the production of CO and other unburnt hydrocarbons.
It becomes evident that it is necessary to incorporate a quantity of excess air. This must be monitored on a regular basis as the plant equipment can have a propensity to drift over time. Regular maintenance will provide an opportunity for minor adjustment and will ensure sustained periods of optimum operating efficiency, whilst maintaining safety at all times.
Steve O’Neill is technical specifications engineer with EOGB Energy Products.