Compressor curve

USING EXPANDER-COMPRESSOR CURVES TO PREDICT OFF-DESIGN PEFORMANCE... 

Figure 8-41. Unstable flow region of a centrifugal compressor curve.

When the data have been screened and performance calculated, don t be surprised if all the points plot like a tai et in a shooting range, perhaps with a tighter circle rather than describing the complete compressor curve. 

Figure 10-14. Typical centrifugal compressor curve showing surge.

The balance condition between the evaporator and the compressor can be visualized in a graphic solution, superimposing the basic rating of the cooler on the compressor curves (see Figure 10.1). 

There will also be a gain in usage of the evaporator coil and a corresponding rise in the evaporator temperature, giving a further increase in compressor capacity. This would need to be evaluated from the compressor curves, but might be a further 1%. 

This first estimate for the evaporator coil performance must now he corrected for the change in compressor duty if it is a direct expansion coil, or of water temperature change if using chilled water. Another method is to re-calculate the basic rating figures at the new air flows and plot these against compressor curves. 

The centrifugal compressor, unless it is dirty or mechanically defective, has to operate on its curve. As the compressor discharge pressure increases, then Hp, the feet of polytropic head required, must also increase. Also, as can be seen from the compressor curve, the volume of gas compressed (ACFM) must decrease. When the volume of gas drops below a critical flow, the compressor will be backed up to its surge point. 

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. 

According to the compressor curve shown in Fig. 28.3, the feet of polytropic head Hp increases as the ACFM decreases. 

The operating point is located where the centrifugal compressor curves cross the system curve of the process. The system curve can be a constant pressure one (horizontal line), a mostly friction one, or any other. 

Figure 2.18). The compressor operates at the point where the compressor curve crosses the system curve of the process. In the illustrated case the design point (1) is located in the maximum efficiency region (78%) at design flow and pressure.

There are three basic ways to control throughput in a centifugal compressor spillback (bypassing), suction throttling, and variable speed. Figure 7.2 shows each of these structures and illustrates how they work in terms of their compressor curves. 

A special word of caution is required in relation to the application of turbo-compressors. Although the same general considerations apply as for pump curves of head against pressure, the compressor curve almost invariably exhibits a reduction in head at low flows. In this region, flow... 

Dirt in the compressor rotor. This can be a big problem in dusty environments. Compare the observed head-capacity characteristics of the compressor against the performance predicted by the compressor curve. 

In order to adjust the flow, we have to cause either the process curve or the compressor curve to move. Alternatively we have to remove the condition that discharge and process inlet pressures are equal, or that the flows are equal. 

As might be expected, because we are expending energy to raise the pressure of the gas only to partially reduce it again across the valve, the scheme is not energy efficient. Its range is limited since, as we close the valve, the process curve approaches the end of the compressor curve and surge occurs. 

If the compressor is of the variable speed type then we can use this to control the flow, as shown in Figure 11.6. By changing the speed we move from one compressor curve to another. This is energy efficient and, because the surge point moves, can operate over a wide range. However variable frequency drives (VFD) for large electric motors are costly. Variable speed steam turbine drivers have a mixed reputation. Most success has been had with gas turbine drivers. 

Figure 11.12 shows the compressor curves now redrawn by plotting against F instead of Fg. The surge line is now approximately straight. A second line has been added, building in a safety margin of 15 % over the minimum flow. 

Special consideration must be given to compressors operating in parallel. Even so-called identical machines will have slightly different compressor curves. The machines must operate with the same inlet pressures and the same outlet pressures. If operating on a flat section of the curves, equalising the pressure rise may result in very different flows through... 

Problems requiring reading compressor curves are given at the end of the chapter. 

It is necessary to have pump and compressor curves in order to do the required calculations. In this example, we will use equations for the pump curves. These equations can be obtained by fitting a polynomial to the curves provided by pump manufacturers. As discussed in Chapter 18. pump curves are usually expressed as pressure head versus volumetric flowrate. This is so that they can be used for a liquid of any density. In this section, pressure head and volumetric flowrate have been converted to absolute pressure and mass flowrate using the density of the fluids involved. Pump P-201 operates at only one speed, and an equation for the pump curve is... 

There are two unknowns in the compressor curve equation the conpressor outiet pressure and the mass flowrate. Therefore, a second equation is needed. The second equation is obtained from a base-case ratio for the pressure drop across the heat exchanger. The two equations are... 

Other pertinent information is appended, including pump and compressor curves, Figure C.6 and Figure C.7. a flow summary table. Table C.8. and an equipment list. Table C.9. 

General characteristics cmve of engine should be between choke (right side of compressor curve in Figure 3.5) and surge (left side of compressor curve in Figure 3.5) lines at the upper level of compressor efficiency ciuve. 

In agreement with pervious parts, in order to achieve a proper adoption for engine, all performance points of engine which is equipped to turbocharger should be obtained. As mentioned above, performance points include constant speed lines and constant load which are overlapped on compressor curve. If engine performance condition be at safe region and upper level of compressor efficient curve, it ensures suitable adaptation of engine and compressor. 

As we normally operate on the flat portion of the compressor curve, suction throttling will reduce the amp load on the driver. If, however, we were operating on the steep portion of the compressor curve, suction throttling could increase the amp load on the motor driver. 

Figure 41.9 Centrifugal compressor curve showing effect of stone walling.

Each of the methods of control affects the compressor curve to produce a set of curves called the compressor map (Figure 7.28). The various curves show the compressor characteristic at different values of the parameter being varied, such as inlet valve setting or speed. At each crossing of a compressor curve and a system curve, specific operating points occur which, collectively, estabhsh the controllable range. 

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