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CHE315 3.3 Pumps and gas moving equipment For the fluid flow from point to another, a driving force is needed. The driving force may be supplied by gravity, or a mechanical device that increase the mechanical energy of the fluid. The most common methods of adding energy are positive displacement and centrifugal action. Pumps: To move incompressible liquids. Fans, blowers and compressors: to move gasses (usually air).

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CHE315 Fans discharge large volumes of gas at low pressures. Blowers and compressors discharge gas as at higher pressures. In pumps and fans, the fluid density doesn’t change considerably, therefore the fluid is assumed incompressible.

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CHE315 As discussed in sec. 2.7, a mechanical energy balance can be applied on a piping system and the mechanical energy added by the pump, W s, is calculated (See Example 2.7-5). The work delivered to the pump can, then, be calculated by knowing the pump efficiency, , as follows: Pumps Power and work required

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CHE315 The actual, or brake power of a pump is: The theoretical or fluid power is :

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CHE315 Power and work required The mechanical energy added to the fluid, Ws, is often expressed as the developed head of the pump, H (in m of fluid being pumped) :

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CHE315 Pumps are usually driven by electric motors. Therefore, the electric motor efficiency, must be taking into account to determine the total electric power input to the motor. Electric motor efficiency

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CHE315 The power calculated from mechanical energy balance depends on the pressure differences and not on the actual pressures. In pumps, the lower limit of the absolute pressure in the suction (inlet) line is fixed by the vapor pressure of the liquid at the inlet temperature.

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CHE315 inlet pressure ≤ vapor pressure liquid flashes into vapor no liquid can be drawn to the pump (Cavitations problem). inlet pressure > vapor pressure and exceed it by a value termed the Net Positive Suction Head Required or (NPSH) R. ZsZs ZpZp To calculate the NPSH) A, we can use:

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CHE315 Minimum (Required) NPSH R To ensure that cavitation will not occur, a minimum NPSH is established for a pump. Minimum NPSH (required NPSH) is the smallest amount of net positive suction head a pump must have at the inlet in order to prevent cavitations anywhere in the pump. The available NPSH A must be maintained at a value greater than or equal to the minimum NPSH R allowable to guarantee no cavitations.

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CHE315 They are common in process industries. A centrifugal pump consists mainly of an impeller rotating inside a casing. How fluid is pumped? Pumps Centrifugal pumps The performance of a pump is usually expressed by means of the characteristic curves, usually available for water. An example of characteristics curves is given in Figure 3.3-2.

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CHE315 Pumps Centrifugal pumps The pressure produced, p = h g, will be proportional to the density. As approximation (Affinity laws): the capacity q 1 (m 3 /s) is proportional to the rpm: (rpm means revolutions per minute)

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CHE315 The head, H 1, is proportional to q 1 2 : The power consumed, W 1 is proportional to the product of H 1 q 1 : Pumps are usually rated based on the head and capacity at the point of peak efficiency (see figure 3.3-3).

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CHE315 Example 3.3.1 Example 3.3.2

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CHE315 An engineer, a physicist and a mathematician are staying in a hotel. The engineer wakes up and smells smoke. He goes out into the hallway and sees a fire, so he fills a trash can from his room with water and douses the fire. He goes back to bed. Later, the physicist wakes up and smells smoke. He opens his door and sees a fire in the hallway. He walks down the hall to a fire hose and after calculating the flame velocity, distance, water pressure, trajectory, etc. extinguishes the fire with the minimum amount of water and energy needed. Later, the mathematician wakes up and smells smoke. He goes to the hall, sees the fire and then the fire hose. He thinks for a moment and then exclaims, "Ah, a solution exists!" and then goes back to bed.

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PUMPS AND GAS-MOVING EQUIPMENT

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