The heat flux limitations of a heat pipe are governed by the five factors. The first is the viscous limit.  At a low temperature, the vapor pressure difference between the evaporator and the condenser may not be enough to overcome viscous forces. Second is the sonic limit. This occurs when the vapor velocity reaches sonic speed at the evaporator and any increase in the pressure difference will not speed up the flow. Next is the entrainment limit. At high vapor velocities, droplets of liquid are torn from the wick and entrained in vapor. The droplets flow to the condenser with the vapor, which results in drying out on the evaporator. Then there is the capillary limit. This is reached when the capillary pressure is too low to transport enough liquid to the evaporator from the condenser. Finally is the boiling limit. The radial heat flux into the heat pipe causes the liquid in the wick to boil and evaporate, which leads to heat pipe dry out.