Piping Network Design for Effective Air Conditioning System Performance

Pipes provide channels for transfer of thermal fluids from chilled/hot water generating equipment to air side equipments. Piping runs are installed vertically in service shafts. These pipes pass close to public areas crossing multiple floors. Proper layout and velocity of fluid inside pipes are important considerations for successful system operation.
Excessive velocities above 10fps in main lines cause noise and vibration in the pipe lines. This noise creates uncomfortable conditions in public areas. Vibration in water pipe lines is transmitted through structure and can be felt at a distance from piping routes, when isolation is not proper.
Pipe line velocity is kept within limits by selecting appropriate size of water pipe lines.
Required volume flows of water through any branch circuit versus desirable velocity provide pipe size. Pipe size selection charts and software programmers are available which are used to determine pipe diameter.
Other parameters of concern are pressure drop through piping system and type of pipe materials. Higher pressure drop means more pumping energy and low initial cost of piping. Low pressure drop results in higher pipe sizes and energy saving during operation. For economical selection, maximum pressure drop is kept less then 3ft/ 100ft in any branch circuit. Velocity is kept within 4-10fps. Selection of pipe lines on these general rules is used to arrive at optimum size.
Water hammer is a common phenomenon which is observed in badly designed piping and fitting arrangement. This is not only a nuisance for building occupants but may also generate shock waves in working fluids. These waves produce high instant pressures. This pressure acts on pipe line joints and weak spots in pipes. Non slammed type non- return valves are available. These are wafer spring operated type. These are used to control noise generated during hammering. Hammering phenomenon itself is controlled by pipe line pressures. For high rise buildings, pumps are located at a level, where high pressure in lines is avoided. For super high rise buildings, intermediate heat exchanger is used to separate plant equipment from excessive water head due to building height.
For maintenance purposes, isolating valves are used. These are installed on inlet and outlet of equipment. Equipment is easily isolated from operating circuit, in case of failure. These may be gate valves or butterfly type valves. Gate valves have better water sealing features as disk is removed from flowing fluid. In case of butterfly valves, disk or wafer remain suspended in the flowing stream of water. Continuous exposure to water chemicals and impurities may affect seal on valve wafer. Due to these reasons butterfly type valves are avoided where tight seal is required during shut off.
Strainers are used for equipment and control valves inlet lines. Their filtration media is selected according to requirement of filtration.
Excessively high velocities damage pipe line internal surfaces. At high velocity solid matters in the fluid strike pipe line bends and cause erosion. These become weak spots and are prone to failure after some operation.
Air in the working fluid is damaging for pipe lines and heat transfer surfaces. Air vents are used at high points in the system. Air separator in plant room at low pressure point removes air from circulating water. Cavitation is another issue which damages pipe surfaces. This is caused at reduced inlet suction pressure of pumps and at very low velocity. Net positive suction head is maintained to avoid this problem.