Compressors speed-limited

Speed-limited compressors Horsepower-limited compressors... 

Synchronous speeds are calculated by Eq. (29-10). Speeds above the limits given are obtained through step-up gears large high-speed centrifugal compressors are examples. Two-pole (3600 r/min at 60 Hz) synchronous motors can be built but are uneconomical in comparison with geared drives. 

The basis for evaluation of piston speed varies throughout industry. This indicates that the subject is spiced with as much emotion as technical basics. An attempt to sort out the fundamentals will be made. First, because there are so many configurations and forms of the reciprocating compressor, it would appear logical that there is no one piston speed limit that will apply across the board to all machines. The manufacturer is at odds with the user because he would like to keep the speed up to keep the size of the compressor down, while the user would like to keep the speed down for reliability purposes. As is true for so many other cases, the referee is the economics. An obvious reason to limit the speed is maintenance... 

Late in the field life it is desirable to compress the 100 MMscfd for the example field downstream of the separator from 800 psig at 100°F to l.(XX) psig. An engine-driven separable compressor is available from surplus. The engine is rated for 1,600 hp at 900 rpm. Horsepower is proportional to speed. The compressor frame has six 7-in, bore by 6.0-in. stroke double-acting cylinders with a minimum clearance of 17.92%, a rod load limit of 25,000 lb, and rod diameter of 1.75 in. Assume k = 1.26, Z, -0.88, and Zd = 0.85. 

Because of their cantilever design and pinion rotating speeds, bullgear compressors are extremely sensitive to variations in demand or down-stream pressure changes. Because of this sensitivity, their use should be limited to base load applications. 

If the compressor is suction-volume-limited, we say that it is speed-limited. If the compressor is driven by an AC motor, its speed is determined by the motor speed and any connecting gears. If the compressor is driven by a turbine, then the rated speed of the turbine limits the compressor speed. 

Another way to reduce the compressor discharge pressure is to render the refrigerant heavier, or less volatile. Adding isobutane to a propane refrigerant is one common example. Naturally, this will lower the suction pressure to the refrigerant compressor shown in Fig. 22.1, and increase the actual volume of vapor flow. If the compressor is speed-limited, this will not be practical. 

Refrigerant composition (refrigeration systems), 231-234, 241 speed-limited, 232-233 horsepower-limited, 233 accumulator relief valve, 233-234 minimum suction pressure, 234 Refrigerant condenser problems (evaporating problems), 240 Refrigeration systems, 227-241 efficiency, 227-229 diagnosing compressor problems, 229—231 refrigerant composition, 231—234 ... 

When a compressor is running at its maximum rated speed, it is speed-limited. The maximum rated speed is stamped on the compressor nameplate. There is no physical restraint to prevent operation of the compressor above its nameplate speed. However, compressors are equipped with overspeed trips, which automatically shut the compressor driver down when the compressor reaches a preset speed. Certainly, compressors that run well above their nameplate speed can self-destruct in a dramatic and tragic fashion. 

When a compressor is reported to be speed-limited, one should take a tachometer and compare the actual speed of the machine against the nameplate speed. The machinist who set the overspeed trip may have set it too low. The operators are careful not to let the compressor trip off. They avoid this by running at speeds 100 rpm to 200 rpm below the set trip speed. 

This technique was used daily on an alkylation unit that was refrigeration-compressor limited. During periods of cold weather, the compressor would change from horsepower-limited to speed-limited. Propane was then added to the refrigerant accumulator drum. The turbine governor control valve (i.e., the driver steam inlet valve) would open to maintain compressor speed. When the governor valve was fully open, the compressor was using maximum available horsepower. The optimum propane concentration had been reached. 

In a V/f control generally, only the frequency is varied to obtain the required speed control. Based on this frequency, the switching logistics of the inverter control circuit control the inverter s output voltage using the PWM technique to maintain the same ratio of V/f. A W/control is, however, not suitable at lower speeds. Their application is limited to fan, pump and compressor-type loads only, where speed regulation need not be accurate, and their low-spccd performance or transient response is not critical and they are also not required to operate at very low speeds. They arc primarily used for soft starts and to conserve energy... 

Compressors driven by steam mrbines, gas turbines, hot gas expanders, or variable speed electric motors usually employ variable rotation speed for capacity control. Generally, the surge limit slope varies witli rotation... 

Most of the drivers used with the sliding vane compressor are electric motors. Variable speed operation is possible within the limits of vane speed requirements. The vanes must travel fast enough to seal against the cylinder wall but not so fast that they cause excessive wear. For the smaller units, under 100 hp, V-belts are widely used. Direct connection to a motor, however, is possible for most compressors and is used through out the size range. 

I or biualkr compressors, where the speed is relatively high and space is limited, a solid rotor construction is used. This is similar to the disc type of construction, except that the discs are an integral part of the rotor Blade attachment slots are cut into the rotor, similar to the slots cut into the discs. Rotor blades are rarely, if ever, shrouded. 

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