Polytropic feet of head

The gas enters the compressor s rotor through the large wheel shown in Fig. 28.2. The purpose of this wheel is to increase the velocity or kinetic energy of the gas. After the high-velocity gas escapes from the vanes in the wheel, the gas enters the stationary elements fixed to the inner wall of the compressor case. This is called the stator. Inside the stator the velocity or kinetic energy of the gas is converted to polytropic feet of head, or potential energy. 

Brave reader, do not be afraid of the term polytropic feet of head. It really has the same simple meaning as described in Chap. 23, except the term polytropic feet of head means feet of head for a compressible fluid. 

Dv = density of the vapor at the suction of the compressor H = polytropic feet of head... 

As the ACFM increases, we run out to the right on the compressor curve, shown in Fig. 28.3. As we move away from the surge point, the polytropic feet of head decreases. As the polytropic feet of head is reduced, the compressor AP comes partially back down to its initial value, until a new equilibrium is established. But because the initial disturbance of the equilibrium—the increased molecular weight— moved us away from surge, the new equilibrium will be established farther away from surge than the initial equilibrium. Not only will the new equilibrium be established farther away from surge, but the pressure in the wet gas drum will also wind up lower than the initial pressure in the drum. 

A centrifugal compressor developing a lot of AP with a low gas density needs to produce a lot of polytropic feet of head. This means that the centrifugal compressor must... 

To convert from polytropic feet of head to AP, which is really what process people are interested in, we use the following very rough approximation ... 

Centrifugal compressors operate on a curve, just like centrifugal pumps. A typical curve is shown in Fig. 41.3. The horizontal axis is actual cubic feet per minute (ACFM). This is analogous to GPM, used on the horizontal (x) axis of centrifugal pump curves. The vertical axis is Hj, (polytropic feet of head). This is analogous to the feet of head used on the vertical (y) axis of the centrifugal pump curve. 

Centrifugal compressors are dynamic machines. They convert velocity, imparted to the gas by a spinning wheel, to polytropic feet of head. The feet of head multiplied by the density of the gas equals the pressure boost produced by the centrifugal... 

Let s now assume that there is a sudden decrease in the molecular weight of the wet gas. This results in a decrease in the gas density. The AP developed by the compressor goes down. As a consequence, the compressor s suction pressure rises. This reduces the ACFM volume of gas flowing into the compressor. As the ACFM decreases, we back up on the compressor curve toward the surge point. As we move closer to surge, the polytropic feet of head developed increases. The compressor AP comes partly back up to its initial value, until a... 

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