Rit (Vísindafélag Íslendinga) - 01.06.1976, Side 30

can lose its heat either due to cooling by heat conduction or also
because of self-evaporation when the pressure of the water drops
below its vapour pressure.
Einarsson (1942) and BöSvarsson (1951) have discussed this
matter in detail. They concluded as follows: Heat loss through
conduction is only important when a small amount of water flows
up through restricted fractures. Hot springs discharging such small
amounts of water usually have a temperature lower than 100°C.
In cases where a hot spring discharges a considerahle amount of
water — and boiling springs are usually of this type — the heat
loss due to conduction need not he considered. Heat loss through
conduction can also be ignored in drill holes.
Therefore practically the only way for water in boiling springs
and drill holes to lose heat on its upwards movement is due to
self-evaporation.
The relative amount of water evaporating during the adiabatic
cooling in a borehole or hot spring channel can therefore easily be
calculated if the base temperature of the water is known.
As an example consider a borehole which receives water with base
temperature to °C, where t0 is higher than 100°C and lower than
the critical temperature, 374°C. The borehole is assumed to receive
the water in the liquid phase, that is at a depth where the stationary
pressure in the hole is higher than the vapour pressure of the water
at the temperature t0. When the water flows upward in the bore-
hole its temperature remains constant until the stationary pressure
becomes equal to the saturated vapour pressure. At that point the
water begins to cool through self-evaporation and from now on
there exists a mixture of water and steam.
As the water moves further upward the temperature at each depth
in the borehole will be the same as the boiling temperature of the
water at the corresponding pressure. Thus the temperature of the
steam-water mixture becomes lower and the steam-to-water ratio
greater as the mixture moves further upwards. At the surface,
where the pressure is one atmosphere, we obtain a mixture of water
and steam at a temperature of 100°C.
The same consideration must hold for a hot spring channel.
The hydrogen isotope ratio of the vapour phase relative to that
of the water phase is given by the fractionation factor
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