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Versa­tile system consisting of a fan, gas valve and venturi mixer

Opti­mised compo­nents for a wide modu­la­tion range for gas condensing boilers

As an envi­ron­men­tally friendly heat source, gas has multiple advan­tages. In conjunc­tion with modern gas condensing boilers, the fuel is opti­mally converted into useful heat. Gas condensing boilers can be made very small, are long-lasting and very quiet. For prac­ti­cally every appli­ca­tion, there­fore, you can find opti­mally adapted heating units on the market. In order to build these devices as compactly as possible and still do justice to the exten­sive vari­ance, there is now a new, highly compact unit consisting of a fan, venturi and gas valve. It covers the large power range from 2 to 35 kW and can be easily adapted to different instal­la­tion condi­tions in the gas condensing boiler. That saves time and money in design engi­neering and stock-keeping.
State-of-the-art gas condensing boilers are avail­able in a large number of vari­ants. The heart of such a device, however, is always the fuel manage­ment system, in other words, the optimum dosing and treat­ment of the constant fuel-air mixture. With the NRV 77, ebm-papst Land­shut, the specialist for state-of-the-art condensing boiler system tech­nology is now offering a new system for this, which allows for versa­tile adap­ta­tion to the different output and instal­la­tion require­ments. The objec­tive of the inno­va­tion was to cover a large power range with the smallest, most compact dimen­sions possible, and simul­ta­ne­ously be able to vary the arrange­ment of the compo­nents when inte­grating them into the gas condensing boiler. Further­more, the NRV 77 takes into account the devel­op­ment of heat outputs which are becoming ever smaller.

The whole is more than the sum of its indi­vidual parts

The fuel manage­ment system is composed of multiple single compo­nents. Along with the fan for the combus­tion air and the gas valve, the newly devel­oped venturi is the compo­nent that is the key to success. All three compo­nents have to be opti­mally matched to each other for the respec­tively required heat output. This is the only way to guar­antee effi­cient, low-emis­sion combus­tion.

The fan: always the correct air flow

Figure 1: Typical CO<sub>2</sub> curve

For conveying fresh air, this means that the fan used can always reli­ably move the air flow required for heat outputs from 2 to 35 kW. The reason for this is that a fan with a rela­tively small impeller diam­eter is used. This enables low air flows to be achieved with minimum speeds that can still be controlled well. To be able to cover the upper power range as well, speeds of up to 11,500 rpm are required.
However, the high speed and the long service life desired for use in heating require a special bearing design. There­fore the bearing geom­etry and above all the lubri­ca­tion have to be adapted to these condi­tions. Care­fully selected greases with ideal viscosity, temper­a­ture behav­iour and ageing stability are the deci­sive factor here. Intri­cate (long-term) test bench trials in conjunc­tion with long-time expe­ri­ence helped in the search for the optimum lubri­cant for a speed-proof bearing with life­time lubri­ca­tion.

The gas valve: large modu­la­tion range

Figure 2: NRV 77 with gas outlet

For the smallest heat outputs the required fuel gas quan­ti­ties are down­right minus­cule. NRV systems have a modu­la­tion range from 1 to 7, over which the air-fuel ratio of the combus­tion has to be controlled. What is called the main injec­tion throttle of the gas valve is an impor­tant element here. It consists of a rotating plastic part, which is fastened in place on a sheet. The parts have to be as flush as possible with one another to ensure that the entire gas quan­tity is fed through the main injec­tion throttle and no leaks past the throttle can develop. A leak would have a nega­tive impact on the minimum heat output. More­over, the choke (plastic part) has to move easily, so that the air-fuel ratio of the combus­tion is config­ured at the rated load.
The objec­tive here was to find the best possible compro­mise, which was achieved by tempering the installed plastic part in the sheet metal and by opti­mising the sheet metal part. Thus it was possible to achieve the goal of creating a largely leakproof main injec­tion throttle that can be moved ideally.

The venturi mixer

Figure 3: Power range of the NRV 77 multi-venturi

Venturi nozzles are frequently used to generate the required vacuum, through which the correct gas quan­tity is dispensed through the gas valve. A single venturi is not able to cover the entire power range of the fan and gas valve. But each new venturi nozzle means a new mold.
In order to find a flex­ible and cost-effec­tive solu­tion for our customers, we created a venturi with an inte­grated displace­ment body. Its diam­eter can easily be adapted in the overall tool by using an insert, but this does not have any nega­tive impact on the venturi´s perfor­mance. Addi­tion­ally, the entire venturi can be flex­ibly attached to the fan with various mounting options. We call this patented solu­tion “multi-venturi”.

Tech­nology in numbers

Thanks to the multi-venturi solu­tion, system solu­tions (gas-air mixture) can also be cost-effec­tively provided in new tools in customer-specific, small annual produc­tion volume without invest­ment costs. The product launch of the NRV 77 includes three perfor­mance classes, which cover the power range from 2 to 35 kW. Addi­tion­ally, depending on the instal­la­tion condi­tions, the gas valve can be turned in up to 15 different posi­tions. A liquid seal is inserted between the multi-venturi and fan housing. This sealing point is 100 % tested in the plant.
Due to its wide output-control range, the new NRV 77 gas-air mixture permits use in a wide variety of different gas condensing boilers. Stock-keeping is reduced as a result of the large output vari­ability; the design can rely on uniform stan­dard compo­nents as a result of the flex­i­bility. This not only saves money and time in devel­op­ment and produc­tion, but also improves the market oppor­tu­ni­ties by providing better quality at a favourable price.

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