#### 2.1.2 Computation

The problems to be solved can be classified into three basic types.

• The first type of calculation is based on a single set of parameters to determine the predicted performance of the system. This can be used to compare the overall costs of a small number of discrete options.
• The second type of calculation is a true optimisation. The single calculation described above is repeated many times for a grid of values defining possible CW flows and configurations of condensing surface (i.e., number and length of tubes). The program identifies the lowest overall (capital and operating) cost at each stage and, after a number of iterations, homes in on the optimum within the applied constraints. This type of calculation is important for determining the most appropriate CW flow and condenser size for a new site. Figure 4.4 shows typical graphical presentation of the results.
• The third type of calculation is the sensitivity study. This involves repeating the optimisation process for a range of values of one particular parameters, such as power cost, to examine how this affects the optimum design (i.e., optimum CW flow, condensing surface, etc.). It is also possible to carry out a sensitivity study to show the effect of simultanious variations of two independent variables. This type of calculation is particularly useful where one or two parameters are not known precisely, for carrying out broad-range generic studies.

Referring again to Fig 4.3, the final stage of iessment is covered under the heading 'Engineering Judgement and Practical Constraints'. It is important at this stage in the study to identify the limitations of various parameters, before recommendations can be made for condensing plant design, CW system design, and cooling tower design. For example, with aluminium brass tubes it is not considered advisable for the cooling water velocity to exceed approximately 2.5 m/s or to be less than 1.5 m/s in order to avoid undue tube fouling under favourable conditions. Another limitation is the lowest value of absolute CW system pressure which can be permitted at any point in the hydraulic circuit. It is usual to avoid pressures of less than about 3 m head of water under normal conditions, and pressures higher than normal may be necessary if other studies indicate that water hammer may prevail under unstable conditions such as pump tripping, or valve closure.

Sometimes special external limitations exist; for example, the temperature of cooling water discharge may be of particular concern in rivers with important fishing interests, especially of migratory species such as salmon.

On completion of a successful study, data is avail¬able which can be used in the assessment of a particular station layout, and also as the basis for Enquiry Specifications.