Subject: Modulating Burners
Modulating burners are designed to control the burner output (size of flame) to match the boilers variable load requirements, during this process the burner is designed to stay at the correct fuel air ratios across the complete firing range ensuring maximum combustion and boiler efficiencies.
There are 3 basic types of modulating burner control:
This will independently control the fuel and air volume to the burner via separate fuel and air valves, normally this type of control will have an integral PID load control to ensure that the boiler’s set point is being maintained to the boiler’s load requirements. This type of system offers high levels of combustion control and consistency.
This controls the non linear relationship between the fuel and air ratios, a characterisation cam is used (sometimes call a compound control). This is linked directly to the fuel valve and air damper of the burner; this has a limited control resolution due to the hysteresis and lost motion of mechanical systems i.e. inertia, friction, wear and tear.
- This type of system is very commonly used by burner manufacturers as a low cost solution for gas fired burners only. The burner has a gas control valve that is operated by a diaphragm and impulse line from the burner’s air supply to the burner.
- The burner will have a PID load control (such as RWF32/40) this directly controls the burner air damper to give the required air volume, the gas valve will operate to give a directly proportional gas volume according to the correct fuel/air ratio.
- With this type of system, it is limited to the air control of the burner and suffers from hysteresis and limited turn down ratio.
- Modulating burners are designed to constantly match the firing rate according to the boiler load demands and in a perfect case, the burner would remain firing constantly whilst heat is required. This however, is rarely achieved for the following reasons:
Limited Turn Down Ratio:
- The “turn down ratio” is a function of the burner’s capacity to match the current base load of the boiler i.e. a 400kW boiler may have a burner that is correctly sized for the boiler application, for example the burner at high fire will give a output of 400 kW (100%) and at low fire will give an output of 100 kW (25%) this would be referred to a “turn down ratio” of 4:1.
- If the above example is used and that the base load remains at above 100 kW the burner will modulate without turning off and “standby cycling ” will not occur. However, if the base load is below 100 kW the burner will reach and exceed the set point and the burner will turn off and “standby cycle”.
Other factors that will cause the burner not to modulate:
- The boiler manufacturers do not normally manufacture the burner to be fitted to the boiler; this is left to the application and the customer’s requirements to remain flexible and not to limit the boiler market. In this case the specified burner and type of burner will be deemed suitable.
- The burner manufacturers offer a range of burner types and capacities, in most cases the burners output is not perfectly matched to the boiler output and as a consequence the burner is not able to fire below the minimal base load. This will in most cases cause the burner/boiler to cycle on and off – wasting energy.
- Modulating burners with poor “turn down ratios” i.e. 2:1 will act as a high / low or on/off burner again causing excessive boiler cycling – wasting energy.
- Incorrectly sized boilers for their applications will also cause modulating burner to operate as on/off burners.
- Incorrectly commissioned modulating burners with poor combustion and, or incorrectly commissioned PID load controller, will also cause the burner not to modulate.
Modulating burners rarely operate as designed due to the above, further with the intervention of BMS control; the boiler will turn off before the burner has had time to fully modulate to low flame conditions or indeed at high fire. Historically 97% of modulating burners do not have sufficient turn down or control to give constant burner operation and cause boilers to standby cycle, and therefore waste energy. Installing M2G to boiler with modulating burners can remove the unnecessary energy costs and CO2 emissions caused by standby cycling.