7. THERMAL DESIGN CALCULATION

Single effect evaporator calculations are pretty straight forward. The latent heat of condensation of the steam is transferred through the heating surface to vaporize water from a boiling solution. Therefore two enthalpy balance equations are required to in order to calculate the rate of solvent vaporization and the rate of required input heat.

Generally it is possible to solve the energy and the material balance equations analytically by a sequential approach. The following assumptions are made to develop the mass and energy balance equations:-

• there is no leakage or entrainment

• the flow of non-condensable is negligible

• heat loss from the evaporator system is negligible

From the enthalpy data of the solutions, steam and condensate, the rate of heat input or the rate of steam flow can be calculated. The overall heat transfer coefficient U_D (including dirt factor) is should be either known from the performance data of an operating evaporator of the same type and processing the same solution or a reasonable value can be selected from the standard text books ([1] Table 16.1 page 475; [2] Table 9.2 page 388) . With this information, the required area of heat transfer can be estimated.

Calculate the tube-side and shell-side pressure drop using the method discussed during design of shell & tube exchanger from specified values of the tube length, diameter and the tube layout (refer module #1 for detail calculations). If the pressure drop value is more than the corresponding allowable pressure drop, further adjustments in the heat exchanger configuration will be required.

7.2. Multiple effect calculations

Typically, multiple effect evaporator calculations require a trial-and-error approach as many of the necessary properties depend on unknown intermediate temperatures. Often the heat transfer areas in all effects are considered to be equal. Use of equal size evaporator in all effects, reduces the cost of equipment significantly. In a typical evaporator problem, you are provided with the supply pressure and temperature of steam, the operating pressure of the final effect, the feed and concentrations. The designer is often required to have trial estimates of overall heat transfer coefficients.

The overall strategy is to estimate intermediate temperatures. The energy and material balance equations are solved sequentially to determine the heat transferred in each effect and the heat transfer area. If the areas are not equal, the calculation is repeated to revise the intermediate temperatures and the procedure is repeated till the heat transfer area in all effects are equal.