Hydrocarbon Compression

DYNAMIC COMPRESSORS – CENTRIFUGAL COMPRESSORS

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If a body is set in motion, it tends to continue in motion unless some force acts in the opposite direction to stop it. If there is no gravity pull, nor any obstacle to deflect it, a body in motion travels in a straight line.

Suppose a ball attached to a string is set in motion, as shown in Figure 1.9. Assume that there is no gravity and that the string has no effect on the ball. The ball moves in a straight line.

Suppose the string is fastened to a fixed pivot point and then the ball is set in motion, as shown in Figure 1.10. At first, the ball moves in the direction of motion. When the string becomes taut, it deflects the ball. Because of the deflection, the ball actually travels in an arc, or a circle. Assuming it has enough energy, the ball continues to move in a series of arcs. At each instant of its travel, the physical tendency of the ball is to travel in a straight line. But instead, the ball travels in a circle because it is held or deflected by the string.

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The string actually applies centripetal (pulling-in-toward-the center) force, causing the path of the ball to change or curve. If the string breaks, the ball flies out in a straight line. Any object traveling in a circle is kept in that path of travel by a force called centripetal force. The force holding the ball in the circle of motion, that is, from the ball to the pivot point is the centripetal force.

To hold the ball in its position and path, an opposite force is required. That force is called the centrifugal force. If the centripetal force is eliminated, the object then moves in a straight line.

The force pulling an object in a circular path toward the center is centripetal force. The centrifugal tendency of the object is its tendency to pull away from the center of rotation, or to pull against the centripetal force. The centrifugal tendency acts in the direction opposite to the centripetal force.

The centrifugal tendency is actually not a force but is the result of the tendency of the object to move in a straight line while being pulled toward a center of rotation by the centripetal force. Assume a ball bearing is placed close to the center of a disk that has blades, as shown in the figure below. As the disk begins to move, one of the blades forces the ball bearing to move. The ball bearing tends to travel in a straight path. The drawing shows the actual path of the ball bearing as the disk rotates.

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When the disk rotates, the bearing is forced away from the center of the disk, as shown in the figure below.

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Let us consider two points A and B located on the disk. Point A is at the tip of the disk, while point B is closer to the center of the disk. When this disk starts rotating, as shown in the figure above, point A covers a larger distance than point B. When the disk is rotating, point A moves faster than point B. Anything that is being carried along by the rotation of the disk has a greater velocity when it is near the outer rim of the disk. If anything being carried along by the rotation of the disk also travels outward from the center to the outer rim, it gains velocity.

The velocity of point A is proportional to the RPM (revolution per minute), or the rotating speed of the disk. The energy picked up by the material at point A is given by

KE = m × v2/2gc

If D is the diameter of the disk in meters and N is the RPM of the disk, the longitudinal velocity is equal to ? × D × N/60 m/sec (i.e.,3.28 × ? × D × N/60 = 0.17181 × D × N m/sec. The kinetic energy gained will be m × v2/2gc)

To achieve this kinetic energy, work has to be done on the disk or impeller.

The figure given below is a compressor impeller. An impeller is made of two plates separated by blades.

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When the impeller begins to rotate, the blades force the air in the impeller to move. Air molecules tend to travel in a straight line. Because there is no centripetal force, the rotation forces the air molecules outward from the center, or eye, of the impeller. As the air molecules move outward, they gain velocity, or speed. The air also tends to oppose the push of the blades, so the pressure of the air is increased. The impeller adds both pressure and velocity to the air.

The tendency of air or gas to move outward from the center of a rotating impeller is the centrifugal tendency. A compressor that uses centrifugal tendency to impart pressure and velocity to a gas is called a centrifugal compressor.

The part of the centrifugal compressor that moves the gas is the impeller. As the impeller rotates, it moves the gas toward its outer rim. As the gas moves toward the outer rim of the impeller, its velocity increases.

This increase in velocity away from the eye creates a low-pressure area at the eye. This low-pressure area causes a suction, which allows more gas to enter. The impeller does work on the gas. The work is converted into the energy that the gas gains, which is in the form of both pressure and velocity. When the gas is at the tips of the impeller blades, it is at maximum velocity. As the gas leaves the impeller, it is thrust into a passageway called the diffuser (refer figures given below). When the gas enters the diffuser, the impeller is not acting directly on the gas.

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The radius of the diffuser is larger than the radius of the impeller. Due to the larger radius, the flow path of the gas through the diffuser is in a larger spiral. Since the flow path is longer and there is no direct action by the impeller blades, the velocity of the gas decreases. As the velocity of the gas decreases, its pressure increases.

The diffuser converts the velocity of the gas to increased gas pressure. Gas passes from the diffuser into the volute. In the volute, the conversion from velocity to pressure continues.

Gas passes from the diffuser into the volute as shown below (single stage/last stage of a multistage compressor).

In the volute, the conversion from velocity to pressure continues. In a centrifugal compressor, work is done on a gas to impart both pressure and velocity.

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A centrifugal compressor, by doing work on gas, imparts both pressure and velocity to the gas. Then, the velocity of the gas is converted into pressure within the compressor. Look at the compressor below.

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• It has four separate impellers.
• Each impeller and diffuser makes a stage.
• This is a four-stage centrifugal compressor.

As the gas leaves the first impeller, it gains some velocity and pressure. The increased velocity is partially converted into pressure in the diffuser.

As the gas leaves the diffuser, it enters the return passage, which guides it into the eye of the next impeller. When the gas enters the eye of the second impeller, it has greater pressure than when it entered the eye of the first impeller. Each impeller adds to the total energy of the gas. It may be noted that the velocity added by the impeller is converted into pressure energy within the diffuser. When the gas leaves the compressor, its pressure is higher than the inlet pressure. The work done by a compressor is the total energy added to the gas through impellers. A gas leaving the compressor has added energy in the form of pressure and temperature.

Written by Jack

October 1st, 2021 at 1:21 am

Posted in Fundamental

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