The flow rate through the circulation loop of an evaporator determines the heat transfer coefficient in the calandria and also the quality (weight fraction vapour) of the two-phase mixture leaving the calandria, and is a key parameter in the design. In order to determine the circulation, it is necessary to be able to estimate the pressure drop through the circulation loop. This is especially important in a natural circulation evaporator, where the circulation rate must be determined by trial and error so that the head available from the differences between the densities in the riser and downcomer equals the loss due to friction and acceleration around the circulation loop. Computer programs are normally used for this. With assisted and forced circulation it is also necessary to be able to estimate pressure drop so that the head required from the impeller can be estimated.

The estimation of pressure drop in the regions of single-phase flow presents no major problems, and the methods can be obtained from Section 144 and Section 146. The pressure drop in the boiling zone is more important because it is usually greater and always much more difficult to estimate, there being big discrepancies between the various published and proprietary correlations. It is first necessary to be able to estimate the density of a two-phase mixture, for the estimation both of the pressure drop due to acceleration and of the pressure rise or drop due to gravity. Second, the pressure loss due to friction must be estimated. Correlations for two-phase density and friction are given in Section 154.

Two phase fluid flows are liable to be unstable. There are several different forms of instability, and it is important to ensure that the flow in an evaporator will be steady. More information on the subject may be found in Hewitt et al. (1994). Unsteady flow in an evaporator can upset other items of equipment on the plant, and it has a very serious effect in a crystallizing evaporator on the size of the crystals.

It is important when estimating the pressure drop through an evaporator to allow for the effects of the deposition of dirt or scale — see also Section 275.

For single phase flow in clean heat exchanger tubes, the estimated pressure drop is likely to be accurate to within ± 2–3%, assuming the physical properties are known. The pressure drop in a fouled tube may be significantly higher, and may not be estimated correctly by the computer programs used for exchanger design. There are two factors resulting from fouling which are important here.