A study has been made of the critical heat input, Qc
, to liquid H2
boiling in an annular gap, of cross‐section area, A
, between a vertical cylindrical heater of height, Z
, and various concentric plugs. The data are in approximate agreement with calculations based on an assumption of homogeneous and frictionless two phase flow. The pressure drop across the channel is spent primarily to accelerate the vapor being formed and the liquid carried with it. The derived relation does not contain any empirical coefficients or exponents. For ρv
the general equation reduces to
is the latent heat of vaporization, ρv
,2 the exit vapor density, ρL
the liquid density, g
the gravitational acceleration, Ze
the hydrostatic head equivalent to the pressure drop across the heater, and f2
the mass fraction of vapor in the fluid leaving the heater.
The optimum operating vapor pressure is discussed for magnets cooled by ideal natural convection boiling. A simple roll‐type of coil is discussed and is shown to be superior in some respects to more conventional windings.