"PURPOSE AND SCOPE OF THIS DATA ITEM This is the second of a series of Data Items on heat transfer in boiling inside tubes. The first@ Data Item 85041 @ summarised the problems and gave a procedure for estimating heat transfer rates in saturated wet-wall boiling. It should be read before reading this Item. Here the emphasis is on determining the conditions when the boiling liquid ceases to wet the heated wall of the tube through which the liquid and vapour are flowing. This is known as ""dryout"" and has been discussed in Section 4.5 of Data Item 85041 . This Item is concerned with the estimation of the ""critical heat flux"" defined as the heat flux above which the wall would become dry@ given the amounts of vapour and liquid flowing through the tube@ the dimensions of the tube and the physical properties of the liquid and its vapour in equilibrium. Many publications contain measured values of the critical heat flux for the upward flow of water and steam and of other single-component two-phase fluids in electrically heated vertical tubes. This Item is limited to flow up uniformly heated vertical tubes. Horizontal tubes will be dealt with in a future Data Item. There are many vaporisers in the process industries@ operating at a high heat flux@ where it is desirable to be able to estimate the critical heat flux. The subject is of special interest in the design and operation of the large steam generators used in the generation of electricity@ but these are highly specialised items and are outside the scope of this Data Item@ which is mainly intended for dealing with the following types of equipment. 1) Vaporisers with recirculation@ such as evaporators and vertical thermosyphon reboilers. Here the amount of liquid entering the tubes considerably exceeds the amount of vapour to be generated. After passing through the tubes@ most of the liquid is returned to the inlet@ after the vapour generated has been separated from it. It is desirable that the wall of the tubes should be wetted by the liquid at all points. It is therefore necessary to check that the heat flux is always significantly less than the critical. Dryout is most likely to occur at the outlet. 2) Once-through vaporisers. Here the intention is that all the liquid should be vaporised in the tubes@ so dryout is inevitable@ and the heat flux falls considerably in the post-dryout region. Hence it is desirable to be able to determine how much vapour has been generated at the level where dryout begins. The estimation of post-dryout heat transfer coefficients will be dealt with in a future Data Item."