Jensen 95G Model Steam Turbine Power Plant | Live Steam

Описание к видео Jensen 95G Model Steam Turbine Power Plant | Live Steam

Watch the Jensen 95G steam turbine power plant running on steam.

About the steam turbine
The Jensen 95G steam turbine power plant is a rare beast indeed. I would guess that more than 99 percent of model steam engines are piston-type engines, but this one is a turbine. And compared with the other model steam turbines I've seen this is a large one. And unlike most model turbines I've seen, it looks great. Most model turbines hide the turbine wheel, but here you can see the turbine wheel, its blades, toothwheels in the 1:10 transmission, a steampunk-style all-metal housing - and the turbine makes a distinct screaming turbine sound when running. The sound is not for the faint-of-heart.

The Jensen 95G turbine power plant was the result of a cooperation between the family-owned business Jensen Steam Engines in the USA and an expert model engineering machinist, Karsten Gintschel of Germany. Gintschel has for many years made many small, beautiful model steam turbines, and the turbine component with reduction gearbox for the Jensen 95G was designed and made by him.

Specifications
Boiler: 660 watts heater.
Turbine: 2.5 inch diameter impulse-type turbine.
Reduction gear: 1:10 gear ratio

The challenge with small steam turbines
You may be thinking: why are there so few miniature steam turbines on the market? After all, power stations everywhere in the world use steam turbines, not piston steam engines, because of their higher efficiency and power output. I think the answer is two-fold. Firstly, piston steam engines are easier to understand and easier to relate to for hobbyists, and have more moving parts that are more enjoyable to watch. So quite simply: better playability. And secondly, small turbines are difficult to make well, because due to the laws of thermodynamics small turbines tends to be very, very inefficient at converting steam to do useful work.

Given how simple a turbine is, it may come as a surprise that designing and building a miniature turbine capable of driving any meaningful accessories is no small accomplishment. Steam turbines are generally highly inefficient in small sizes. Indeed, the 95G does drive Jensen's model 15 AC generator - but only barely so, despite being powered by a 660 watt heat source. I would estimate that the power output of the 95G turbine is maybe of the order of one-tenth that of the 55G twin cylinder power plant driven by the same boiler and heater.

One of the primary challenges with the efficiency of a small turbine like this is that it consumes a large amount of steam, and the steam consumption remains the same regardless of the turbine's rotational speed - and even if it is not running. If it's not running, the steam will just bounce off the turbine blades and into free air, doing no useful work at all.

Due to the inherent dynamics of steam turbines - which are quite different from classic piston engines - impulse-type steam turbines like the 95G turbine are only reasonably efficient when the turbine blades' velocity is a significant fraction of the steam jet's speed. If it's not running at all, the steam is simply wasted as it bounces off the blades without doing any work, but on the other hand if it's running at the same speed as the steam jet, the steam will transfer none of its momentum to the turbine blades because there is no speed difference for the jet to act on. Somewhere between these two extremes (i.e. being either completely stopped or reaching speed parity between the turbine and the steam jet) - probably maintaining the blades' speed somewhere around 50 percent of the steam jet's speed - is where the optimal rpm range lies.

Then, how fast should the steam jet be? Now the challenge is that steam, in order to convert its potential heat energy efficiently, must be released through a thin nozzle at very high speed (to get the maximum potential amount of kinetic energy before hitting the turbine blades), and consequently the turbine wheel should either be large in diameter or rotate very fast if the turbine blades' speed is to reach a meaningful fraction of the steam jet's speed. This calls for very high rpm and for a reduction gearbox to convert the very low torque at high rpms to meaningful torque at lower rpms. Designing a reliable gearbox for reducing the rpm is not trivial given that the small toothwheel connected to the turbine must run at the turbine's full speed. Who can say friction ?

So where does that leave us? Well, it leaves us with pretty much the same insights that early turbine makers realized: Turbines are hungry for steam and they scale up very well, but the efficiency suffers when they are scaled down, and the mechanical complexity increases.

The Jensen 95G is an amazing-looking model engine, and fun to play with. I got mine around 2017 but it appears that Jensen have discontinued the model since then.

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