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.
The performance curve of centrifugal pump shows the pump’s capacity and total head. It also shows required power, and NPSH requirement over a range of flow rates.
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
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.
Positive-displacement pumps are not subject to air binding.
2.2.7 pump selection
Before a centrifugal pump is selected, its application must be clearly understood. It is necessary to know the liquid to be handled, the total head, capacity and, in most cases, the temperature, viscosity, vapor pressure, and specific gravity.
2.3.1 Fans
The commonest method for moving small volumes of gas at low pressures is by means of a fan. Large fans are usually centrifugal and their operating principle is similar to that of centrifugal pumps.
It also shows the pump’s best efficiency point (BEP). The pump operates most cost effectively when the operating point is close to the BEP.
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.
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.
2.2.6 Pump priming
If the impeller speed, the radius of the impeller, and the velocity of the fluid leaving the impeller are constant, the developed head is the same for fluids of all densities and is the same for liquids and gases
2.3 Gas-Moving Machinery
Gas-moving machinery comprises mechanical devices used for compressing and moving gases. They are often classified from the standpoint of the pressure heads produced
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.