Lza

Fig. 30-9 Compression Cycle Illustration for a Single-State Helical Screw Compressor. Source: Compressed Air and Gas Institute

Fig. 30-9 Compression Cycle Illustration for a Single-State Helical Screw Compressor. Source: Compressed Air and Gas Institute diffuser, and scroll volute of a centrifugal compressor.

diffuser, and scroll volute of a centrifugal compressor.

Fig. 30-11 Centrifugal Compressor. Source: Compressed Air and Gas Institute

Fig. 30-10a Impeller, Diffuser, and Scroll Volute of Centrifugal Compressor. Source: The Elliot Company

Fig. 30-11 Centrifugal Compressor. Source: Compressed Air and Gas Institute

Fig. 30-10b Impeller, Diffuser, and Scroll Volute of Centrifugal Compressor. Source: The Elliot Company

Centrifugal compressors range in capacity from about 125 hp (93 kW) to more than 10,000 hp (7,500 kW). Figure 30-11 shows a three-stage centrifugal compressor rated at 7,250 cfm (205 m3/m). This unit operates at a discharge pressure of 124 psia (8.6 bar) and requires a 1,700 hp (1,270 kW) driver. In a multi-stage compressor, as shown in Figures 30-12 and 30-13, air is ducted from the scroll volute through interstage piping to the first intercooler, then to the second stage impeller, again through a diffuser and scroll volute to the second intercooler. Air from the second intercooler moves through a third impeller, diffuser, and volute where it reaches the final discharge pressure.

Centrifugal compressor capacity, or mass flow rate, is a function of volume flow and air density. Because air density is inversely proportional to absolute temperature, mass flow is reduced at higher temperatures. Impellers must be selected to deliver the required flow at the highest anticipated operating temperature. Altitude and the

Fig. 30-12 Cutaway Drawing of Multi-stage Centrifugal Compressor. Source: The Elliot Company
Centrifugal Compressor Source
Fig. 30-13 Vertical Section Drawing Showing Typical Multi-stage Centrifugal Compressor. Source: Compressed Air and Gas Institute

humidity of inlet air are also important variables. As discussed later in the chapter, moisture is condensed out of compressed air in intercoolers and aftercoolers. When inlet air is very humid, more moisture is condensed and mass flow rate is reduced.

Figure 30-14 is a representative centrifugal compressor performance curve in discharge pressure versus flow. Air delivery rises with decreasing pressure until the air velocity in the compressor reaches the speed of sound. At this point, flow is said to be choked, or stonewalled, because any further reduction in system pressure will not result in increased air delivery. This is shown on the lower right region of the curve. Figure 30-15 shows the effect of inlet air temperature on centrifugal compressor performance. Notice that mass flow and power requirement rise with decreasing temperature.

All dynamic compressors have a surge limit or minimum flow point below which the performance of the compressor is unstable. Operation below this point results in pulsations in pressure flow, which could become severe enough to cause damage. The intersection of the surge line and the sloping performance curve represents the maximum discharge pressure of the compressor. As system pressure increases, the compressor delivers less air until the system resistance is matched. This continues until the compressor is unable to maintain a steady flow of air and backflow, or surge occurs from the system through the compressor. The surge limit in a given application is a function of the compressor type, pressure ratio, inlet temperature, gas properties, blade angle, and operating speed.

Since centrifugal compressors are dynamic machines, flow is much more sensitive than with positive displacement machines. Therefore, more sophisticated control logic is required. Capacity modulation is achieved via the following:

• Inlet throttling is often used, particularly in smaller capacities.

• Adjustable guide vanes allow for precise control of air inlet flow and, therefore, reduce the risk of surge. Adjustable vanes also reduce power requirement under partial loads.

• Speed control, achieved with a variable speed prime mover or electric driver, also provides some degree of output capacity control. Unlike the positive displacement compressors, however, centrifugal compressors achieve a more limited benefit through variable speed operation due to the minimum flow rate that is required to prevent surge. Figure 30-16 shows typical performance under variable speed control.

A major advantage of centrifugal compressors is that they provide oil-free air and use oil only in the gearbox. They offer simplicity of design, few moving parts, large clearances, and minimal vibration. This results in high reliability and low maintenance requirements. Their efficiency increases with size and surpasses even reciprocating compressors at full-load operation in the larger capacities. However, like screw compressors, centrifugal compressors exhibit poor part-load performance and are best suited for baseload duty. In smaller capacities, they are less efficient and more costly than screw compressors and offer lower pressure rise per stage.

Discharge

percent

Surge

Power at i

Coupling, 100--------------------I Increase in Air percent ^^ \ Temperature i Reduces Power

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment