million joules 4 2 million joules release about 1 75 kg of vapor the exact values for p c p and the latent heats of vaporization as functions of various different parameters such as the temperature or the salt concentration of the sea water available to be treated may be calculated or may be drawn from published tables removal of one tonne of water from the surface of the reservoir toward the sea requires only a small amount of energy because in doing so potential energy is transformed into pressure energy which the water will have as it is returned to atmospheric pressure it is only necessary to compensate for the frictional energy due to the viscosity of the water during its transfer from the level at the surface of the reservoir to the sea in order to limit the presence of droplets of sea water which may be entrained with the water vapor one must limit the speed at which the water vapor is removed from the surface of the water in the reservoir an acceptable value for this speed is in general a few meters per second this result is obtained by limiting the pressure decrease due to condensation of the water vapor from.