Improving your boiler efficiency
May 1st 2005

Industrial and commercial boiler manufacturers are following the lead of domestic boiler manufacturers in developing more efficient boilers. Here, Geoff Lockwood, technical manager at leading fan manufacturer ebm-papst, provides an insight into why fans are an integral part of the technology used to improve the efficiency of boilers

Gas boilers fall into two distinct types – standard efficiency noncondensing and high efficiency condensing. Standard efficiency, noncondensing appliances typically have a single heat exchanger and the most common arrangement uses a fixed speed hot gas fan to extract the products of combustion from the heat exchanger and blow them through the flue system, (see FIG. 1).

High efficiency condensing boilers have primary and secondary heat exchangers to enable them to extract the maximum heat from the combustion process. The trend in this type of boiler is towards pre-mix fans where the gas and air mixture is injected up-stream of the fan, which then blows it into the burner, (see FIG. 2).

This arrangement results in a high pressure drop across both the heat exchangers, the burner and through the exhaust flue. This pressure fluctuation is important as it can lead to unwanted changes in the combustion process.

The pressure drop across the system is not constant as there is a significant variation across the burner under various conditions and with different flue lengths.

This change in pressure leads to a change in air volume produced by the fan, and it’s this factor that can adversely affect the combustion process.

In response to this fan manufacturers have developed fans with very high pressure development compared to volume flow. These fans provide a small change in volume flow with respect to pressure change. An example of a small fan typically used on a domestic condensing boiler is the RG128 with a typical duty point of 40m3 p/h at 1800Pa. Similarly, a fan used in an industrial/commercial boiler would be the G3G250 with a typical duty point of 600m3 p/h at 2400Pa.

Fan manufacturers achieve these high pressure/low volume flow characteristics by using a narrow radial impeller run at high rotational speeds, which can be up to 8000RPM. The only practical method of achieving such high speeds is by using an EC motor (DC motor with electronic commutation powered from AC mains).

These EC fans also provide a very important solution to achieving the low emission levels required from the gas boiler. This is achieved by careful control of the air/gas ratio and by controlling the volume of air/gas mixture to the burner dependant on the output required from the boiler. In a pre-mix system, the gas valve is pneumatically controlled by the suction of the fan and maintains a constant gas/air ratio as the fan speed is modulated to meet the demands of the boiler output.

The EC motor provides the accurate control required and provides 10% to 100% speed control of the fan.

The trend towards pre-mix technology and condensing boilers in the commercial and industrial market is set to continue.

ebm-papst has a range of pre-mix fans that are currently available from domestic sized boilers through to large industrial sizes of over 500kW. This range is set to continue with the ebm-papst principle of continual evolution.