Section 7 - Flash-Steam

The phenomenon of 'flash-steam' is well known to engineers and its application to the steam engine offers a route to significant improvements in steam generating efficiency.

Water if pressurized remains in the liquid phase at higher temperatures and does not boil and vaporize. If such pressurized and heated water is released through an orifice to a lower pressure, part of the water immediately and explosively turns to steam – a process known as 'flashing'. Flash steam is currently exploited in applications such as geothermal power plants. In these, water is drawn from deep underground where it is at high temperature and pressure. Piped to the surface, this water is ‘flashed’ in a chamber to produce steam under pressure which is then expanded in a more or less conventional turbine coupled to an alternator to produce electricity.

Flash steam, therefore, offers one novel way of making a modern steam engine. In Section - 3 the mono-tube steam generator was described. This was the type of steam generator favored in the last production steam cars and was often found in stationary or ship steam plants. This type is almost always employed in current experimental steam vehicles. In the flash-steam generator, however, the idea of the mono-tube steam generator is taken one step further.

Fig.2 Flash steam generator

The crucial difference between these two types of steam generator is that, as previously explained, the mono-tube generator raises steam within a coiled tube which contains water of increasing temperature in the first section, saturated steam in the next section and superheated steam in the final section. Steam is admitted to the engine cylinder by means of a valve. In the flash steam generator, however, the coiled tube contains water only. The coil is heated in a similar way to that in the mono-tube generator, the difference being that the water is pressurized to a level above its saturation temperature and is injected into the engine cylinder as water (Fig 2).

The following table is an extract from standard steam tables and shows that, for example, the saturation temperature of water at 10 000 kPa is 584ºK. Therefore providing the pressure is always maintained above 10 000 kPa, the water in the coil always remains in the liquid phase and does not turn to steam.

EXTRACT FROM STEAM TABLES

As there is now no steam in the coil, the steam inlet valve is replaced by an injector which admits the pressurized high temperature water directly into the engine cylinder. Employing an injector to deliver liquid rather than admitting steam through an inlet valve is of considerable practical advantage. Firstly the quantity of water injected at each stroke is far smaller than the volume of steam passing the inlet valve in an engine served by a mono-tube generator (steam at atmospheric pressure occupies about 1650 times the volume of water). This tiny quantity of water is simply much easier to dispense through an injector than the much larger volume of steam which an inlet valve must admit in a mono-tube engine. This bestows a secondary advantage – namely speed. Such a small quantity of water may be dispensed at speeds equal to those found in diesel injection systems, whereas steam which has to negotiate an inlet valve is seriously impeded if the engine is pushed to higher speeds by a phenomenon known to steam engineers as ‘wire-drawing’.

As the pressure in the engine cylinder is always much lower than the saturation pressure of the water in the coil, once it is injected there is no possibility for the water to remain in the liquid phase and part of it ‘flashes’ into steam. The flashing process results in ‘wet steam’ because only part of the water is converted to steam whilst the rest is atomized by the flashing process into tiny droplets of water. The proportion of water converted to steam is obtained by subtracting the internal energy of the water at the lower pressure from that at the higher pressure. It may be calculated from the equation:
Uf P1 - Uf P2 x 100 = % Flash Steam
      hfg

Where:
Uf P1 = Internal energy of water at higher pressure (kJ/kg)
Uf P2 = Internal energy of water at lower pressure (kJ/kg)
h fg = Enthalpy of evaporation (kJ/kg)

Applying this equation to circumstances where the water is pressurized to above 20 000 kPa which allows it to be heated to 639ºK gives the following result:

1786.9 – 417.86 x 100 = 60.6% Flash steam
      2257.6

Therefore under the above conditions 60 percent of the injected mass of water is immediately converted to steam in the cylinder but 40% remains as water which is atomized into very tiny droplets by a double mechanism. The first mechanism is a result of the depressurization of the liquid water through the nozzle in much the same way as can be found in industrial liquid- spraying systems. The second mechanism results from the explosion of the flash process itself. The two mechanisms occur simultaneously and support each other during the time that injection of the water takes place. This very effective atomization is crucial to the next phase of the expansion process in the cylinder.

If the water were to be injected into a cold engine cylinder, the flash steam would immediately condense and there would be no pressure rise. To overcome this problem, the cylinder head and walls are heated and supply additional heat to the wet steam entering the cylinder. The atomized water droplets experience extremely high collision rates with the cylinder walls because of the explosive effect of the flash process. The tiny size of the droplets, coupled with high collision rates ensure rapid absorption of heat allowing them to be quickly converted to steam which is then heated further to superheat.

A flash steam generator permits the construction of a steam generating unit which is much more compact and has fewer thermal losses than a traditional steam boiler.

N.B. Monotube steam generators are sometimes confusingly referred to as flash-boilers or flash- steam generators. However the key difference between the monotube generator and the flash generator is that in the former water and steam exist within the heating tube, in the latter only water exists there until it is ‘flashed’ to a lower pressure through a valve or injector.

>> Other Vapor Power Systems (Section 8)

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