流体力学与 及传热课件Pump priming.ppt

2.2.6 Pump priming ; The increase in pressure, however, is the product of the developed head and the fluid density. If a pump develops, say, a head of 30.5m and is full of water, the increase in pressure is 2.95 atm. ; If the pump is full of air at ordinary density, the pressure increase is about 0.0035atm. It is called air bound and pump can accomplish nothing until the air has been replaced by a liquid. ;2.2.7 pump selection ;In chemical industry, the task of pumps election is frequently further complicated by the presence of solids in the liquid and liquid corrosion characteristics requiring special materials of construction. ;Pumps can generally be ordered with a variety of impeller sizes. Each impeller has a separate performance curve (see Figure ). To select a midrange impeller that can be trimmed or replaced to meet higher or lower flow rate requirements. ;To minimize pumping system energy consumption, select a pump so the system curve intersects the pump curve within 20% of its BEP. ;Select a pump with high efficiency contours over your range of expected operating points. A few points of efficiency improvement can save significant energy over the life of the pump.;2.3 Gas-Moving Machinery ;Classification: fans for low pressures, discharge heads are low, from about 0.1 m to 1.5 m H2O. blowers for intermediate pressures, compressors for high pressures ;2.3.1 Fans ;In a centrifugal fan, the centrifugal force produced by the rotor causes a compression of the gas, called the static pressure head. Also, since the velocity of the gas is increased, a velocity head is produced. ; Writing mechanical energy balance equation between the inlet and outlet;Customarily u1=0 for the fans, rearranging the equation above;Customarily, individual term of the equation above is imparted by head, as follow;2.3.2 Blowers and compressors ; Turbo-blowers or centrifugal compressors are widely used to move large volumes of gas for pressure rises from about 5 kPa to several