Economical heating, environmentally friendly combustion technology and convenient operation are the main requirements for heating systems in private homes today, so systems using wood pellets, wood chips and split logs are used in a variety of situations. They can be used within living areas or for central heating in single- or multi-family homes as well as public buildings. Compact dimensions and quiet operation are important considerations. After all, nobody wants environmentally friendly heating to come at the expense of living comfort. Fans for fresh air and exhaust gases play an important role here and need to deliver convincing performance in terms of energy efficiency, noise, reliability and compactness.
Modern biomass heating systems are enjoying increasing popularity. They can be operated with pellets, wood chips or split logs – renewable energy sources with carbon-neutral combustion. Such systems are comparable to oil and gas systems in terms of operation and maintenance. They have low emissions and reach efficiencies of over 90% (even as much as 105% Hi for condensing boilers). To optimise the efficiency while minimising the pollutant content of the exhaust gases, modern systems control the combustion process with either a temperature or combustion chamber sensor or a lambda probe, typically in combination with an induced draft fan (Figure 1). The fan’s speed is controlled based on the measured values or in accordance with a specific set of parameters to ensure an appropriate flue draft, so the fan makes an essential contribution to clean, high-quality combustion.
External rotor design for compact dimensions
The demands placed on induced draft fans are particularly high. They have to withstand high temperatures and be quiet, energy-efficient and easy to integrate. This is particularly the case when they are used in heating systems for living areas, but even when they are installed in central heating systems, induced draft fans should take up as little space as possible.
For the latest generation (Fig. 2) of induced draft fans, the motor and fan specialist ebm-papst Mulfingen employs the proven external rotor design. Here the static part of the motor, the stator, is located on the inside and is surrounded by the rotating part, the rotor. Simply by using this configuration, a higher torque can be achieved than with an internal rotor motor of the same length and using the same magnet system. So for a given output, external rotor motors can be much more compact. In combination with the fan, the result is a compact unit that is also rugged and durable.
AC or EC motor?
For the new induced draft fans, users can choose between conventional asynchronous (AC) motors and electronically commutated (EC) synchronous motors. However, for controlled fans the choice will nearly always be in favour of the energy-saving EC technology. For asynchronous motors, the maximum speed is limited by the mains frequency. The rotor speed adapts to the rotating field frequency via slip depending on the load situation. As an alternative, speed control via frequency change or phase control is also possible. However, this requires external control electronics and an additional Hall sensor in the motor for speed measurement. Since the efficiency of AC motors decreases sharply in closed-loop operation, much more efficient induced draft fans can be produced using EC motors. EC fans consume up to 70% less energy in partial-load operation, resulting in reduced operating costs and environmental impact and making energy-saving fans and environmentally friendly heating an excellent combination.
EC technology: energy-efficient and quiet
In an EC motor, the rotor with its permanent magnets rotates synchronously with the stator’s rotating field (Figure 3). In contrast to the mains-powered asynchronous motor, the rotor speed is not automatically coupled to the frequency of the supply voltage but is pre-determined by the commutation electronics, which determine the angular speed of the stator rotating field with which the rotor synchronously rotates.
The commutation electronics determine the rotor position in part without sensors by using the parameters rotor voltage or motor current. The idle speed depends on the applied voltage and the number of turns in the stator winding, making nearly any operating speed possible without slip (synchronously with the stator’s rotating field) and, in contrast to the mains-powered asynchronous motor, even above the mains frequency. In addition, the minimum speed that can be reached with an EC motor is lower than that for an AC motor. So the speed can always be adjusted over a wider control range to meet current requirements. Furthermore, in partial-load operation in particular but at full load as well, EC motors are considerably more efficient than mains-powered AC motors (Figure 4) and are very quiet in comparison with speed-controlled asynchronous motors, whose triac controllers can cause objectionable noise.
New mechanical design with extra benefits
Mechanically, the new induced draft fans feature a modular design that permits the use of both conventional AC and energy-efficient EC motors with identical external dimensions (Figure 5). The EC motor was designed so that the electronics integrated in the motor are situated in the cool region as far as possible from the hot combustion chamber to protect the electronic components from the unavoidably high temperatures there. The insulated, customer-specific mounting plate with adjacent cooling wheel also helps to protect the electronics. Long-term testing has confirmed that temperatures of 250 °C have no detrimental effect on functionality over time; the induced draft fans are even designed for brief temperature peaks up to 300 °C. The stainless steel impellers are available in different designs with diameters from 120 mm to 250 mm. An optional shaft seal developed for these fans prevents flue gas from leaking through the shaft gap. This is important since ventilation systems in living areas can cause underpressure which results in flue gas being sucked into the room. Sealed stoves with an external air supply are mandatory here.
The mechanical design has further advantages. The induced draft fan’s hood is made of a very tough, temperature-resistant and flame-proof fibreglass-reinforced plastic material. The fan’s torsion-resistant hollow ribs provide a sturdy support structure. If an AC motor is used, the terminal box can simply be fastened to the hood. For applications with an existing external power supply, low-voltage DC motors with the same performance data and an identical design will be available in future.