Friction loss in piping systems is caused by fluid interacting with pipe walls and fittings, resulting in pressure and flow rate reduction. Pipe material, length, and liquid velocity affect frictional loss. Friction loss tables show expected values per 100 feet of pipe, which can be used to determine pump power requirements. Friction loss increases with system length and the number of fittings and bends. Higher resistance means higher pumping costs, making friction loss important in all commercial piping systems. Friction loss also occurs in air conditioning systems, where improper sizing of fans and ducts can cause poor airflow.
Pipe friction loss is a measure or calculation of the loss of flow or pressure due to the interaction of the fluid with the pipe walls. These losses must be determined for piping systems, as pumps must be specified with sufficient power to overcome losses and provide adequate flow rates. Frictional loss varies depending on pipe materials, length, and liquid flow rate or velocity.
Piping systems consist of straight or curved sections of pipe, connections, valves, and other fittings. As a system increases in length and complexity, the fluids flowing through it interact with the pipe walls and various fittings, creating resistance. Excessive amounts of resistance will eventually cause a loss of flow to the customer or destination if not overcome by increased pumping power.
The materials used in pipes vary widely, from cast iron to plastic, and each type has a particular roughness on the inside walls. Pipe manufacturers publish tables showing the expected friction loss per pipe length for different pipe materials and diameters, with the diameter measuring the distance across the internal area of the pipe. A measured or estimated roughness factor is used in the friction loss calculation, with unfinished iron pipe having a higher roughness factor than plain plastic pipe.
The friction loss tables show values in feet of water head per 100 feet of pipe (the value is the same in meters) because pump capacity and power requirements are normally rated in feet of water. This value estimates how many vertical feet a pump can push water with a given motor size, which allows designers to connect pump power directly to a piping system. The effect of friction loss increases with the length of the system and the number of fittings and bends, and can only be reduced by using smoother pipe, but cannot be eliminated.
Losses also increase as the velocity, or velocity, of the fluid in the pipe increases. Increasing system pressure is a better solution than increasing flow rates, which is why municipal water systems use elevated water towers. Maintaining a water supply in an overhead tank not only provides extra water for times of high demand, but also maintains a more constant pressure to overcome system losses.
Friction loss is important in all commercial piping systems, because higher resistance means higher pumping costs. There are some applications where adequate pumping capacity is critical, such as firefighting. A fire engine must deliver large quantities of water through long stretches of hose, often to the upper floors of a building. As water demand increases, the fire engine must provide adequate pressure to the nozzles to maintain firefighting capability. Hose nozzles are designed as long, thin tubes because this converts the hose pressure into a high flow at the nozzle without the need for high velocities in the hose.
Air is also a fluid and friction loss occurs in heating and air conditioning systems as the air moves through the ducts that connect the various rooms. System designers use friction loss calculations to determine the fan power required for adequate airflow in buildings. Improper sizing of fans and ducts can cause poor airflow in certain areas of a home or building, or inadequate heating or cooling on the upper floors of a multi-story building.
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