Liquid piston compressors perform compression with the use of a liquid ring acting as a piston.

ADVANTAGE

-operation is not affected by the intake of liquid

-The liquid-piston type of compressor has been of particular advantage when hazardous gases are being handled.

DISADVANTAGE

-low efficiency

Reciprocating compressors are the most widely used type of compressor. They are positive displacement, intermittent compression device analogous to a typical bicycle pump.*

ADVANTAGES -especially useful for low to moderate flow rates, low molecular weight gases, and high required discharge pressure -easily multi-staged DISADVANTAGES -low capacity -low reliability due to many wearing parts -high maintenance required -little flexibility in material choices -pulsation and unbalanced forces created by reciprocating motion

Pumps exist in a variety of shapes and sizes, depending on their intended function.

When selecting pumps for any service, it is necessary to know:

>the liquid to be handled

>the total dynamic head

>the suction and discharge heads and

>in most cases, the temperature, viscosity, vapor pressure, and specific gravity.

>range of operation

>Materials of construction

>presence of solids

Centrifugal pumps use impellers to move fluid. They can have more than one stage. The number of stages determines how many impellers are present in the pump. They add pressure to a liquid by increasing its kinetic energy and then converting that energy into pressure energy.

ADVANTAGES

-steady delivery; uniform flow

-can handle all types of fluids

-can be mounted horizontally or vertically

-relatively low cost

-high clean ability

DISADVANTAGES

-reduced performance when handling viscous fluids

-can only handle small amounts of gases in liquids

-not effective for high pressure applications; single stage pumps require high velocities but multi stage configurations are expensive

-priming often needed before start up

In positive displacement reciprocating pumps, fluid is pumped by a plunger traveling back and forth in a chamber. Valves for intake and discharge operate in conjunction with the plunger to transfer the fluid.

ADVANTAGES

-can handle volatile and very viscous liquids

-adjustable capacities for improved metering

-able to produce very high pressures

-diaphragm pumps are leak free and do not expose packing or seals to the process liquid

-no priming typically needed

DISADVANTAGES

-produces a pulsating flow and operates at low speeds

-limited range of capabilities

-high net positive suction head requirements

-much maintenance required

Rotary gear pump

In rotary gear pumps, lobes rotate inside the casing with very little clearance. The fluid enters the spaces between the lobes as they pass the inlet and then it is carried around and forced out of the outlet.

Screw rotor pump

A helical rotor rotates inside a double helix stator without any clearance. As it rotates, cavities form that channel the flow to the outlet

ADVANTAGES

-handles highly viscous fluids well

-able to handle fluids that are corrosive, edible or contain entrained gases

-quick and easy cleaning

-works well for liquids with low vapor pressures

-compatible with many materials

DISADVANTAGES

-does not produce high pressures as well as reciprocating pumps

-practical sizes are more restricted than in centrifugal pumps

ADVANTAGES

-transport high volumes in short time

-only one moving part

-quiet

DISADVANTAGES

-power intake increases rapidly with decreasing flowrate

-sensitive to dirt