Compressors running

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... 

The applicable code procedures should be followed, particularly in regard to instrumentation. On an existing plant, this will require planning in advance of a scheduled shutdown. The pressure and temperature tap locations and installation details can all be developed with the compressor running. All details must be followed. When a drawing states, deburr, do not chamfer that is exactly what it means (see Figure 10-7). 

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

Reciprocating compressors are classified as either high speed or slow speed. Typically, high-speed compressors run at a speed of 900 to 1200 rpm and slow-speed units at speeds of 200 to 600 rpm. 

Variable Speed Compressor. Evei-y time the compressor cycles on during the first few minutes of running, the compressor works on building up the pressure difference between the evaporator and the condenser, and is extremely inefficient. By letting the compressor run all the time, and modulating its speed according to the refrigeration needs, these periods of inefficiency can be eliminated. 

Number and stages of compressors running Timitation of motor start frequency Timitation of maximum electrical demand Number of fans or condensers running Ean speeds... 

HA A single-stage double-acting compressor running at 3 Hz is used to compress air from 110 kN/irr and 382 K to 1150 kN/nr. If the internal diameter of the cylinder is 20 cm, the length of stroke 25 cm and the piston clearance 5%, calculate (a) the maximum capacity of the machine, referred to air at the initial temperature and pressure, ami power requirements under isentropic conditions. 

The axial compressor running in the fouled condition is operating at only 82.5 percent of its normal condition efficiency. This means that the fouling problem is really rather severe. 

The adiabatic compression efficiency of the air compressor, running in its normal condition, is only 79.4 percent of its design condition. The manufacturer s design adiabatic compression efficiency is quoted as 81 percent. Therefore, the normal condition adiabatic compression efficiency is approximately... 

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... 

During the SD of a freeze-drying process less and less ice is condensed. The compressor runs a long time at the minimum pressure. Thereby the cooling of the motor and/or the lubrication can be reduced. To avoid this the minimum suction pressure of the compressor has to be limited, with the consequence that the lowest temperature to be reached is also limited. 

Avoid steam systems. All compressors run on electricity rather than steam turbines. 

Figure 11a shows a schematic flow sheet for the process. The air compressor runs intermittently during this RPSA cycle. Very small adsorbent particles (20-120 mesh) are used to facilitate near-equilibrium operation (rapid kinetics) of the process. Extensive experimental evaluations of this process using 5A zeolites have been published [15, 16]. The process can produce a low purity (<30%) 02-enriched product gas at a very high rate of production (>100 VA /Hr) and at a decent O2 recovery (>40%) but the O2 recovery rapidly decreases at higher O2 product purity. The total cycle time for the process can be between 3-6 seconds. 

If steam conditions are normal, loss of turbine horsepower is likely due to fouling of the turbine wheels. This fouling may be removed by condensate washing. With the compressor running, pump clean steam con-... 

For compressors driven by an ordinary (nonvariable speed) motor, the speed of the compressor is fixed. However, there is a method one can employ to overcome capacity limitations imposed by fixed speed compressors (and compressors running at their maximum rated speed). This method is to change the composition of the circulating refrigerant. Details are discussed later in this chapter. 

The best way to unload a reciprocating compressor is to increase the cylinder clearance. When the piston completes the end of its stroke, the space between the piston face and the cylinder head is called the clearance. Increasing the clearance reduces the scfd flow of gas to a compressor running at a constant speed and a fixed suction pressure. The effect of adjusting the cylinder clearance is proportional to the suction stroke volumetric efficiency (Evs). Evs is calculated as follows ... 

The adiabatic compression efficiency of the air compressor, running in its normal condition, is only 79.4 percent of its... 

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