Flue gas analysers have gone through quite a transformation in recent years. In the past they normally allowed you to measure the Oxygen (O2) and Carbon Monoxide (CO) content of tested flue gas, as well as the flue, ambient, inlet and net temperatures. They would then calculate the Carbon Dioxide (CO2) and the net or gross percentage boiler efficiency for a wide range of different fuels.

Today's analysers can also measure fuel or flue draught pressures, Nitric Oxide (No) and either calculate or measure total Nox content. Some may also measure Sulphur Dioxide (SO2), Hyrdrogen Sulphide (H2S) and unburnt Hydrocarbons (HC) to indicate total boiler performance. In addition, they might also make direct measurements of CO2 instead of calculating them from the fuel used and the Oxygen content measured.

Flue gas analysers will often measure, use a built-in or remote printer to record readings, have backlit displays for easy viewing of up to eight parameters simultaneously and use a memory to record and transfer multiple readings to a PC with "Windows" compatible software. Modern analysers are portable and easy to use; in some cases the analyser can be fully operated with just three buttons. Whilst most analysers already comply with the requirements of BS7927 : 1998, some have been additionally tested by TUV to meet a different performance criteria. Either way, check with the manufacturer what standard applies to their analysers.

Given all these possibilities, analyser manufacturers are well used to offering help and advice on the most appropriate product for your needs.

In this article we will look at testing non-domestic forced draught boilers using flue gas analysis equipment.

A flue gas analysers has fuel types and their chemical compositions programmed in its' memory. Therefore, when you use the analyser to measure flue gas temperatures and either the O2 or CO2 levels, it instantly and automatically calculates and displays boiler efficiency. On many boilers the setting can be adjusted to maximum efficiency.

In an ideal world, the maximum efficiency would be achieved by measuring zero percent O2 (excess air) in the flue and the lowest flue gas temperature. It is however accepted that an allowance needs to be made for variations and uncertainties and therefore, zero per cent O2 (Perfect or "stoichemetric combustion") is not practical. The settings on a gas boiler need to take into consideration differences in atmospheric pressure, wind direction and boiler demands etc.

A typical natural gas boiler oxygen reading would normally be in the range 3 to 5 percent.

Commissioning procedures

Prior to first firing the boiler, it must be commissioned by a competent person, following the requirements of IGE/UP/4 - Commissioning of Gas Fired Plant on Industrial and Commercial Premises - published the Institution of Gas Engineers and Managers.

Commissioning checks include:

• Planning and programming period - with the fuel gas isolated - Verification that the equipment/design complies to current requirements
• Inspection period - with the fuel gas isolated - Visual and where appropriate a physical examination of the installation and its individual components
• Activation period - with the fuel gas isolated - Systematic search for and elimination of any faults - including a 'dry run' to check the operation of safety interlocks and electrical controls. Once satisfied and with the fuel gas available, a live run is carried out to prove safe operation
• Operation period - to establish satisfactory operation/performance and setting of the controls
• Completion period - handing over the plant to its operators

USING THE FLUE GAS ANALYSER

As part of the commissioning process, the burner combustion characteristics need to be checked and set up by using combustion analysis equipment in accordance with the combustion analyser manufacturer's instructions. This is achieved by setting up the air/gas ratio controls as detailed in the appliance manufacturer's instructions and checking the results of the adjustments on the combustion analyser.
Many boilers incorporate a suitable test point on the boiler itself. If there were no suitable test point the sampling would normally be carried out in the primary flue. The flue stack temperature would then also be checked.

Here are some practical tips when using the analyser:

Allow the flue gas analyser to calibrate itself automatically in fresh, outdoor air. It usually only takes 1 or 2 minutes and ensures more accurate and stable readings.

Don't immediately put the flue probe in the flue before firing up the boiler but wait until the boiler has "settled down". Very large volumes of CO go up the chimney and will affect the sensors within the analyser. If you need to measure very high CO levels, make sure the analyser is equipped with a sensor able to measure from 10,000 ppm CO to 100,000 ppm as most usually only measure up to 10,000 ppm.

Analysers typically react within 30 seconds to boiler performance changes so you'll soon see the impact of your changes.

During a routine boiler service, make a "before" and "after" record of boiler performance so you can see the impact of your work - It make it easier to show your customer what you achieved during your visit and by how much you improved their boiler's performance and it also ensures you leave the boiler operating as safely and efficiently as possible.

Use the analyser's printer to leave a copy on site - It's a simple way to know how the boiler operated when last serviced and, as most printouts are dated and timed, it also shows when this happened.

Some analysers can be set up to measure automatically and record for up to several days - Very useful when trouble shooting "difficult" boilers as it frees you for other jobs whilst collecting useful information for later analysis.

Finally, when you have finished using the analyser, remove its' probe from the chimney and let it cool down in fresh air before switching off the analyser. It will only take two minutes and allows the analyser's sensors to purge themselves clean of any flue gases.

If you need further information on flue gas analysers, a useful place to start is CoGDEM, (Council of Gas Detection and Environmental Monitoring), the UK's leading trade association for gas analysis and detection companies. You can contact them on 01462 434322 or email This email address is being protected from spam bots, you need Javascript enabled to view it or see their web site, www.cogdem.org.uk.

Jonathan Kane
Kane International Ltd

Gas Installer Magazine, 2002