Pipe sizing of district cooling distribution network including in energy transfer station

Abstract

A district cooling system (DCS) distributes chilled water to different buildings connected to the main plant. The benefits of a DCS include greatly increasing energy efficiency and decreasing life cycle operation costs. The installation, maintenance, and design of a DCS can be expensive, however. Therefore, means of decreasing costs without cutting quality is necessary. Given this, a hypothetical DCS is presented with a constant temperature difference set-point of 9°C. The DCS was set to have a system diversity of about 80%. Hydraulic calculation will be used in the study in order to determine the smallest possible pipe sizes that would permit normal operation below allowable limits. The network is divided into 3 parts; namely the main line (ML), the plot take-off (PTO), and the plate-type heat exchangers (PHEs). The DCS will have a piping network connected to 12 energy transfer stations, each with 3 sets of PHEs. Initial guess values of pipe dimensions and friction factors will first be calculated using continuity and the Von Karman equation. A single-variable Newton-Raphson method is then used on the implicit Colebrook-White equation to estimate the actual friction factor for each pipe. Then, the calibrated pipe sizes will be determined by cross-referencing theoretical values with industry-standard sizes as specified in ASHRAE. In calculating the friction factor, it was found that the values increase as the given tonnage decreases, thus implying a decrease in pipe size as the chilled water flows toward the last building. The entire study was conducted on MATLAB. It must be noted that the study is only limited to the selection and design of a simple DCS.