Ensuring optimum efficiency
Published: 04 February, 2016
Steve O’Neill, technical specifications engineer at EOGB Energy Products Ltd, looks at how the implementation of new heating technology can have a significant effect on the efficiency of industrial heating systems.
Heating and hot water possibly accounts for the largest annual consumption of energy in domestic and non-domestic buildings in the UK and produces almost half the CO2 emissions (approximately 37 Mt of CO2 per annum (CIBSE knowledge series KS14)).
It is estimated that the heating and hot water requirements for a typical 4000m˛ naturally ventilated building can result in the production of approximately 115 tonnes of CO2 per annum alone.
The first step in achieving an energy efficient heating system is to minimise demand for heat. This can be achieved by improving the insulation value of the structure, which will inevitably reduce the required heating load thus allowing a reduction in the size of the plant required to heat the space. Further improvements can be made by introducing a well-designed and correctly installed heating and ventilation system into the building, which will ensure the correct temperature rises to match the design criteria.
Guidance can be sought on this subject from numerous sources such as Building Regulations Approved Documents L1 and L2, which provide guidance upon the minimum requirements of the regulations.
Significant focus has been placed upon the emission of CO2 and other harmful gases such as nitric oxide (NO) and nitrogen dioxide (NO2) into the atmosphere. The aforementioned gases are the result of the combustion process utilised to heat the building and produce the hot water needed on a daily basis. The quantity of these, and other gases, is dictated by how complete the combustion process is and by the method in which the fuel has been burned off in the first instance.
It can therefore be assumed that the more efficient the combustion process, the fewer emissions should be introduced into the atmosphere.
The first step in ensuring optimum efficiency is the confirmation that the burner is correctly matched up to the boiler/heat exchanger. This will ensure that the burner can provide an optimum turn down ratio, which will keep burner cycling to a minimum and will ensure that there are no excessive heat losses in the stack or undesired condensing if the burner is too small.
Energy saving measures
Usually the options for upgraded burner controls are not taken up at the specification stage of a new installation and this is often because initial extra burner controls may seem very expensive compared to a standard package burner. However, the significant energy savings and carbon reduction in the long term should also be a major factor in burner selection.
It is hoped that in the future, with more people looking to save costs, the industry will accept many of the controls available now to reduce operating costs and it will become standard practice.
The airflow of a typical blown burner is usually regulated by the graduation of an air shutter or damper via servomotors and cams. The fact that the fan is running at a constant speed inevitably incurs higher load losses, which in turn dissipates some of the electrical power generated by the fan motor. The incorporation of an inverter drive can vary the RPM of the fan in correlation with the specific burner load, thus delivering energy savings and reduced noise levels.
The level of combustion efficiency can be predicted by comparing the quantity of O2 in the stack gases to that of theoretical or stoichiometric combustion conditions (assuming complete combustion). The lower the amount of O2 (or excess air), the higher the combustion efficiency.
Under normal operating conditions it is typical that the combustion engineer will set a burner up with a degree of excess air to ensure that it will not become starved of combustion air and begin to cause incomplete combustion. Several factors can affect the combustion process over time such as:
• Barometric conditions - atmospheric temperature affects the density of air and ultimately can have a bearing upon the fuel/air mixture
• Calorific value - the calorific value of a fuel can change from time to time and have a bearing on the fuel /air ratio
• Mechanical hysteresis - depending upon the type of burner in question, it can be possible for the burner to drift away from its original settings due to continuous operation
It can be observed why it is necessary to introduce more air than that which is required for complete combustion but this inevitably incurs a detrimental effect on combustion efficiency.
As it is not cost effective to have constant monitoring from an engineer, it is now possible to incorporate a constant monitoring system into the burner control. O2 Trim systems gather the amount of oxygen present in the stack via a digital or analogue feedback system and then position the burner air damper to the appropriate position to maintain a consistent quantity of O2 throughout the full operating range of the burner.
A good oxygen trim system will ‘learn’ during commissioning what the affect is to the burner for every single trim that it performs. This allows for full time oxygen trim operation without the necessity of a full time plant operator irrespective of how the plant conditions change.
Although O2 trim technology can make significant savings if applied correctly, it can be further improved with the introduction of CO trim.
It should be noted that the sensors employed in this application do not in fact detect CO and that it is more reactive to detect the other elements produced when incomplete combustion is present and is therefore necessary to refer to the sensor as the COe (carbon monoxide equivalent) sensor.
The COe sensor is identical to the O2 sensor apart from the fact that the electro-chemical and catalytic properties in the signal materials are different, thus enabling combustible components such as CO and Hydrogen (H2) to be detected.
In the event of incomplete combustion and in the presence of unburned hydrocarbons, a non-Nernst voltage UCOe forms on the COe sensor and the characteristics for both sensors alter and relay a signal to the control module which adjusts the burner as necessary. This process is identical if an excess of O2 in the atmosphere exists.
Close to the emission edge, the sensor signal for the COe electrode UCO/H2 increases at a disproportionate rate due to the additional non-Nernst COe signal and searches for the optimum working point of the combustion near the emission edge, adjusting this and optimising combustion further.
This procedure is repeated cyclically so that the optimum working points are always maintained even for un-stable barometric conditions, mechanical hysteresis or fluctuating calorific values.