Whether a classic boiler in a single-family dwelling, a micro CHP for heat and energy generation, or the deep fryers in restaurants for crispy french fries – as different as the functions of these devices are, they all have one thing in common: They all use energy produced by burning gas. Compared to other fuels, this process is more efficient and produces lower emissions.
Gas can only develop these environmentally friendly characteristics in an optimum combustion process. Achieving the highest energy output is always a challenge for equipment manufacturers. The mix ratio, among other things, is crucial. If the concentration of gas is too high, not enough oxygen will be available. This is referred to as a “rich mixture”. The result: The gas that is not burned escapes with the fumes, the amount of soot increases, and carbon monoxide is released into the atmosphere. On the other hand, if the concentration of gas is too low, the flame will be extinguished in the burner. Furthermore, not all gases are alike. There are many different types of gas, with different compositions and energy contents, depending on where they are extracted. Therefore the gas flowing through the supply grid varies from region to region. For the best mixture, these factors have to be considered.
The mixture preparation plays an important role in devices that use gas as an energy carrier. The unit consists essentially of three components: a blower, a venturi mixing device, and a valve. In order to achieve perfect harmony, ebm-papst in Landshut has combined these three components into a so-called “NRV system”. All versions of the system are built according to the same principle. In the venturi, fresh air flows through a tapered nozzle. This produces a vacuum at the narrowest point that causes the gas to be pulled through the valve. It is attached to the side of the venturi, and – in the so-called pneumatic combustion system – regulates the amount of gas that may actually flow out. The blower is responsible for a homogeneous gas/air mixture and conveys it to the burner. As an option, ebm-papst also offers the appropriate burner control unit that monitors gas safety and controls the heat output for this system. Thus, customers receive a complete system perfectly matched to their requirements.
Currently the NRV family includes four different systems with different blower sizes: NRV 77, 118, 137 and 148. They cover different power ranges, and all have a turn down ratio of at least 1:5. Depending on the model, even a ratio of 1:10 is possible. The specific parameter indicates the ratio between the possible minimum and maximum power output of the heating unit. The greater the modulation range, the better the power can be adapted to the actual need. This enables efficient combustion even in low power ranges. The power ranges in the NRV systems can be modified to allow for the best possible adaptation. This is made possible by the multiventuris developed especially by ebm-papst. Here, a displacer located in the plastic tool is used to vary the cross section of the venturi (figure 1). For example, for the NRV 77 three different venturi elements can realize 2 to 15, 5.5 to 28, and 7 to 35 kilowatts, thus enabling individual adjustments for small quantities without additional costs.
One thing that all NRV systems have in common is their compact design. Due to the modular structure, they can be adapted easily to the space requirement of the respective customer appliance. This allows different installation positions. Further flexibility is given by the possibility to adjust the mounting positions of the multiventuri and the valve. Adjustable mounting clips make it possible to mount the burner control unit directly on the assembly (figure 2) without any further adjustments. But it can also be installed separately from the system.
The electronically controlled combustion system: iNR77
With the iNR77, component integration has been taken to the next level. In this electronic system, a mass flow sensor measures the air flow supplied to the burner. Based on this information, the blower motor is controlled specifically to generate the optimum gas/air mixture. The iNR77 requires significantly less space because the electronic gas valve is integrated directly in the blower housing (figure 3). In the company’s own gas laboratory in Landshut, the engineers are able to adjust the systems to the specific needs of the customers. This means a very efficient procedure in the development process together with the customers, and the production of optimally adjusted, environmentally friendly, and energy-efficient devices (figure 4).
The NRV systems can be used in the following applications:
- Gas-condensing boilers for households or commercial applications
- Air and water heaters
- Restaurant equipment such as gas-operated deep fryers or stoves
- Fuel cells