The hydraulic calculation and system simulation of two (2) district cooling distribution network models with and without secondary lines are presented in which the theoretical system pressure drop, system flow rate and flow rate requirements in each energy transfer station were determined considering a temperature difference set-point of 9°C. Variable primary flow of chilled water pumping arrangement was used to improve energy usage and to eliminate the need for a separate distribution pump in the network.
Pipe sizing and friction factor identification were necessary to determine the frictional coefficients of distribution network components. Implicit Colebrook-White equation was used to determine the pipe friction factor. The method of least squares and Cholesky decomposition method were also adopted to derive new set of pump characteristic curve considering that pumps are modulated at its best efficiency points. The governing equations consisted of mass conservation and energy equations in the form of pump characteristic curve and distribution network characteristics. The system of nonlinear equations was solved using multivariable Newton-Raphson method. The linearized equations revealed that coefficient matrices formed between the two networks were different from each other which suggested that different decomposition algorithms must be used to ensure that solution vectors are properly determined. Distribution networks with and without secondary lines showed that Jacobian matrix can be solved using singular-value decomposition and LU decomposition methods, respectively. The results of system simulation defined the coordinates of system characteristic curve which indicated the best efficiency points during selected part load and full load conditions.
An optimization technique such as exhaustive search method was used to determine the piping network design criteria that could give minimum overall costs of construction and maintenance of piping system. Numerical results show that distribution network with secondary lines yields minimum overall costs as compared with piping network without secondary line considering that nominated loads, pipe lengths and fittings, normalized annual demand factor and costs parameters associated with components of objective function are held constant.