Compressors discharge temperature, increase

Two-stage systems should be seriously considered when the evaporating temperature is below—20°C. Such designs will save on power and reduce compressor discharge temperatures, but will increase initial cost. 

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. 

Figure 2-21 show the effect of 5% by weight of steam injection at a turbine inlet temperature of 2400 °F (1316 °C) on the system. With about 5% injection at 2400°F (1316 °C) and a pressure ratio of 17 1, an 8.3% increase in work output is noted with an increase of about 19% in cycle efficiency over that experienced in the simple cycle. The assumption here is that steam is injected at a pressure of about 60 psi (4 Bar) above the air from the compressor discharge and that all the steam is created by heat from the turbine exhaust. Calculations indicate that there is more than enough waste heat to achieve these goals. 

When leakage develops, the output of compressed air is reduced, and overheating occurs due to the recompression of hot air and the inefficient operation of the compressor. This leads to abnormally high discharge temperatures. Higher temperature leads to increased oxidation and hence increased formation of deposits, so adequate cooling of compressors is very important. 

Spillback bypasses the compressor discharge gas, back to the compressor suction. This is a relatively energy-inefficient way to increase the evaporator temperature. Spillback is discussed in Chap. 28, Centrifugal Compressors and Surge. ... 

The tubes in the condenser required for subcooling steal heat-transfer surface area required for condensation. In effect, the condenser shrinks. This makes it more difficult to liquefy the refrigerant vapor. The vapor is then forced to condense at a higher temperature and pressure. Of course, this raises the compressor discharge pressure. And, as we have seen in the pressure section, this increase in compressor discharge pressure invariably reduces the compressor s capacity and may also increase the horsepower needed to drive the compressor. 

There was a problem after the compressor started up. When a newly regenerated gas dryer was put on cycle after the system was running, the temperature at the dryer s outlet increased rapidly, which caused the compressor s second-stage suction and discharge temperatures to correspondingly increase. This resulted in alarm and tripout conditions in the compressor. There were unexpected shutdowns, production outages and poor overall compressor efficiency. 

Table 25-1 shows that for the compressors discussed, the temperature rise for the individual cylinders varied from 28°F for the No. 1 cylinder crank end to 42°F for the No. 2 cylinder crank end. The key point of this table is that compression efficiency varies inversely with temperature rise. As both the suction and discharge pressures were the same for all cylinder ends, the reason for the variable temperature rise was different efficiencies of compression. Because the work performed by the piston at each cylinder end was about the same (except for No. 2 cylinder head end which had a bad unloader), the observed temperature increases were inversely proportional to the gas flows. This means that if the No. 1 cylinder crank end was moving 30 MMscfd of gas, then the No. 2 cylinder crank end was moving only 20 MMscfd and the No. 1 cylinder head end was moving 23 MMscfd. 

Air intake filters plugged Pressure restriction of blower discharge Increased ambient temperature Compressor rotor fouled Blower turbine running slow Wet gas compressor problems Fractionator condensers plugged... 

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