Power Plant Engineering

ELECTRICAL SYSTEM 391

Air

side

Axial

seal

Shaft

H side 2

Spring

Oil Feed

H side 2

Spring

Radial

seal

Air

side

Fig. 12.10. Hydrogen Shaft-seals.
With integral fans mounted on the rotor, the air is fed to the space surrounding the stator over-
hang, and pipes and channels convey a proportion towards the centre of the stator core. There- from it
flows readily inward to the air gap, then axially to the end outlet compartments. With separate fans,
however, air can be fed directly to the middle as well as to the ends, as shown in Fig. 12.10.

(b) Hydrogen Cooling. Compared with
air, hydrogen has 1/14 of the density, reducing
windage loss and noise; 14 times the specific
heat; 1.5 times the heat-transfer, so more
readily taking up and giving up heat; 7 times
the thermal conductivity, reducing temperature
gradients; reduces insulation corona; and will
not support combustion so long as the
hydrogen/air mixture exceeds 3/1. As a result,
hydrogen cooling at 1, 2 and 3 atmospheres
absolute can raise the rating of a machine by
15, 30 and 40 per cent respectively.

The stator frame must be gas-tight and
explosion-proof. Oil- film gas-seals at the rotor
shaft ends are necessary. Two forms are shown
in Fig. 12.10, each must accommodate axial
expansion of the rotor shaft and stator frame.
Oil is fed to the shaft and the flow is split, part
towards the interior (gas) side and part to the
airside. The latter mingles with the bearing oil,
while the former is collected and degassed.

Fans mounted on the rotor circulate hydrogen through the ventilating ducts and internally mounted
gas-coolers. The gas pressure is maintained above atmospheric by an automatic regulating and reducing
valve controlling the supply from normal gas cylinders. When filling or emptying the casing of the
machine, an explosive hydrogen-air mixture must be avoided, so that air is first displaced by carbon
dioxide gas before hydrogen is admitted: the process is reversed for emptying. It is usual to provide a
drier to take up water vapour entering through seals. The hydrogen purity is monitored by measurement
of its thermal conductivity.

Turbo-alternators operating at hydrogen pressures just above atmospheric (so that leaks will be
outwards) require about 0.03 m^3 per mW of rating per day. This rises to about 0.1 m3 for hydrogen
pressure of 2 atm. abs. The gas consumption of synchronous capacitors, which do not need shaft seals,
is very much less.

Gas cooler

Seal

H manifold 2

CO manifold 2

Oil from

H side 2

Oil

to

shaft

seal

From coupling-

end seals

Oil

fromair

side

Emergency

oil supply

Oil pump

Vacuum

tank Vacuum

pump

Diffe re ntialpressure

regulator

Gasdryer atmosphereVent to

Pressure

regulator

CO 2 H 2

Liquid

detector

Fig. 12.11. Arrangement of Hydrogen-Cooling

System.