Compressors Constant speed

Motor-driven compressors usually operate at constant speed, and other methods of controlhng the capacity are necessary. On reciprocating compressors discharging into receivers, up to about 75 kW (100 np), two types of control are usually available. These are auto-matic-start-and-stop control and constant-speed control. 

Constant-speed control should be used when gas demand is fairly constant. With this type of control, the compressor runs continuously but compresses only when gas is needed. Tiiree methods of unloading the compressor with this type of control are in common use (1) closed suction unloaders, (2) open inlet-valve unloaders, and (3) clearance unloaders. The closed suc tion unloader consists of a pressure-actuated valve which shuts off the compressor intake. Open inlet-valve unloaders (see Fig. 10-89) operate to nold the compressor inlet valves open and thereby prevent compression. Clearance unloaders (see Fig. 10-90) consist of pockets or small reservoirs which are opened when unloading is desired. The gas is compressed into... 

It is sometimes desirable to have a compressor equipped with both constant-speed and automatic-start-and-stop control. When this is done, a switch allows immediate selection of either type. 

Motor-driven reciprocating compressors above about 75 kW (100 hp) in size are usually equipped with a step control. This is in reality a variation of constant-speed control in which unloading is accomplished in a series of steps, varying from full load down to no load. Three-step eontrol (full load, one-half load, and no load) is usually accomplished with inlet-valve unloaders. Five-step eontrol (fuU load, three-fourths load, one-half load, one-fourth load, and no load) is accomphshed by means of clearance pockets (see Fig. 10-91). On some machines, inlet-valve and clearance-control unloading are used in combination. 

The surge protection described above is for a simple, single section, constant geometry compressor with constant inlet conditions and constant speed of rotation. Many compressor installations involve more complex configurations and applications. 

In the following discussion on TTE degradation, it is assumed that for constant speed cases the turboexpander drives an induction generator and for variable speed cases it drives a compressor with normal design conditions as given in Table 7-8. 

Compressor performance can be represented in various ways. The commonly accepted practice is to plot the speed lines as a function of the pressure delivered and the flow. Figure 3-9 is a performance map for a centrifugal compressor. The constant speed lines shown in Figure 3-9 are constant aero-dynamic speed lines, not constant mechanical speed lines. 

The turboprop engine has a power turbine instead of the nozzle as seen in Figure 4-2. The power turbine drives the propeller. The unit shown schematically is a two-shaft unit, this enables the speed of the propeller to be better controlled, as the gasifier turbine can then operate at a nearly constant speed. Similar engines are used in helicopter drive applications and many have axial flow compressors with a last stage as a centrifugal compressor as shown in Figure 1-14. 

Generally, there are no vanes in the inlet of an axial entry compressor (see Figure 5-19). Normally there is no more than the plenum divider vane in the inlet section of the typical multistage compressor, although there are designs that use vanes in this area. These are externally movable and are used to provide flow control for constant speed machines. The use of these vanes will be explored further in the section on capacity control. 

For centrifugal and axial compressors, some form of override control is recommended for constant speed motor drivers to sense motor over load and override the process control until the cause of overload has... 

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed ... 

Usually a constant speed, motor-driven compressor will be equipped with inlet guide vanes to the first wheel to allow suction volume control. If the guide vanes are not used, some other throtding device should be available. Figure 12-62 presents the constant speed performance of a centrifugal machine with inlet guide vanes. 

A constant speed air compressor has been designed for the following conditions ... 

Figure 12-111. Typical constant-speed performance of spiral screw rotor compressor. (Used by permission 1961. Roots Division, Dresser-Rand Industries, Inc.)...

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

Control Devices In many installations the use of gas is intermittent, and some means of controlling the output of the compressor is therefore necessary. In other cases constant output is required despite variations in discharge pressure, and the control device must operate to maintain a constant compressor speed. Compressor capacity, speed, or pressure may be varied in accordance with requirements. The nature of the control device will depend on the function to be regulated. Regulation of pressure, volume, temperature, or some other factor determines the type of regulation required and the type of the compressor driver. 

Motor-driven reciprocating compressors above about 75 kW (100 hp) in size are usually equipped with a step control. This is in reality a variation of constant-speed control in which unloading is accomplished in a series of steps, varying from full load down to no load. 

Throughout our discussion of motor-driven, constant-speed centrifugal compressors, I have assumed that the suction temperature was constant. But let s refer back to Fig. 28.1. Assume that the temperature in the drum increases. How will this affect the pressure in the drum ... 

Induction motors are the most frequent in use because of their simple and rugged construction, and simple installation and control. They are constant speed devices available as 3600 (two-pole), 1800, 1200, and 900 rpm (eight-pole). Two speed models with special windings with 2 1 speed ratios are sometimes used with agitators, centrifugal pumps and compressors and fans for air coolers and cooling towers. Capacities up to 20,000 HP are made. With speed... 

Synchronous motors are made in speeds from 1800 (two-pole) to 150 rpm (48-pole). They operate at constant speed without slip, an important characteristic in some applications. Their efficiencies are 1-2.5% higher than that of induction motors, the higher value at the lower speeds. They are the obvious choice to drive large low speed reciprocating compressors requiring speeds below 600 rpm. They are not suitable when severe fluctuations in torque are encountered. Direct current excitation must be provided, and the costs of control equipment are higher than for the induction types. Consequently, synchronous motors are not used under 50 HP or so. 

The book Gas Turbine Performance by Walsh and Fletcher [10] has excellent treatment on turbomachineiy maps. An example of a compressor map is shown in Figure 8.10. This map fully defines the pressure-flow-efficiency-rotational speed relationship of the compressor. Employing the Beta-line or R-line method, maps can be digitized into tabular form as described by Walsh and Fletcher [10], The Betaline method is helpful to ensure numeric stability with such maps that would be otherwise problematic because of the near zero and infinite slope at the ends of the constant speed lines. Note that the compressor model can either be a flow element or a pressure element. That is, from speed and pressure it is possible to obtain flow and efficiency from the map, or from speed and flow it is possible to obtain pressure and efficiency from the map. The choice depends on what is more convenient, that is, what type of elements are modeled upstream and downstream. 

Table 8.7 shows some data characteristics of the experimental system and parameters used for the transient calculations. The system was operated as a conventional constant speed machine, with extensive piping between compressor and turbine. 

There is no hard requirement for constant speed operation of the compressor in the primary system. If one chooses to run at constant speed, then inventory control is used to maintain high efficiency. If variable speed is used, then inventory is maintained constant to remain near the peak efficiency point of the compressor. 

In the PCU and primary system, however, there is only one compressor to manage mass flow rate while there are several different circuits. To achieve the desired control of helium mass flow rate compressor control provides little flexibility. Rather inventory control is used to obtain a flow rate proportional to heat exchanger power. Because density is proportional to pressure for fixed temperature, by varying pressure and maintaining constant speed turbo-machinery, gas velocity remains constant and mass flow rate (proportional to the product of density and velocity) is linear with pressure. Thus, pressure is manipulated through coolant mass inventory so that it is proportional to heat exchanger power so that in turn mass flow rate is proportional to heat exchanger power. Results obtained for this control scheme are described below. 

The centrifugal compressor is a machine that converts the momentum of gas into a pressure head. The compressor pressure ratio (PD/P,) varies inversely with mass flow (W). For a compressor running at constant speed (co), constant inlet temperature (Tj), and constant molecular weight, the discharge pressure may be plotted against mass flow (Curve I in... 

---