Compressors power calculations

Calculation of the compressor power and energy requirements (see Chapter 3). 

The compressor efficiency (in this case, isentropic efficiency) can now be calculated and in turn the polytropic coefficient. This allows the outlet temperature to be calculated, together with the compressor power. 

An actual split-shaft Brayton cycle receives air at 14.7 psia and 70° F. The upper pressure and temperature limit of the cycle are 60 psia and 1500°F, respectively. The turbine efficiency is 85% for both turbines. The compressor efficiency is 80%. Find the temperature and pressure of all states of the cycle. The mass flow rate of air is 1 Ibm/sec. Calculate the input compressor power, the output power turbine power, rate of heat... 

Gas expanders are used to recover energy from high pressure process gas streams in a plant when the lower pressure is adequate for further processing. Power calculations are made in the same way as those for compressors. Usually several hundred horsepower must be involved for economic justification of an expander. In smaller plants, pressures are simply let down with throttling valves (Joule-Thomson) without attempt at recovery of energy. 

The first two effects are calculated directly from the overall process design. The frictional losses are obtained from the compressor power requirements. They amount to a total exergy destruction of 10 MW. Heat exchanger losses due to heat transfer in Units 1-6 (excluding the reformer) can be evaluated from the exergy exchange calculations they amount to 49 MW for a 20K temperature difference. 

Use Eq. (24) in Chap. 14 of this text to calculate the theoretical compressor power. The brake horsepower efficiency of the electric motor... 

To size a conq)ressor requires calculating the power needed for compression. This can be done by assuming an isentropic compression and then correcting the result by dividing by an isentropic efficiency. The power can also be calculated by assuming a polytropic compression, and then correcting the result by dividing by a polytropic efficiency. Both methods will be considered. The isentropic method is also used for blowers and vacuum pumps, but the polytropic method could also be used if data were available. First, we need to derive relationships to calculate the compressor power. 

Finally, the compressor power (W) is calculated using the following formula ... 

Example 5. Determine the compressor power to pass reactant gas into the plenum of the fluid bed system. Also calculate the temperature rise due to heat of compression. The system parameters are... 

The minus sign indicates that power is given out. This is a useful addition to the 100-kW electrical power output of the fuel cell, but note that it provides less than half of the power needed to drive the compressor, as calculated with the worked example of Section 9.5. Furthermore, this example is the best possible result turbine efficiencies will usually be somewhat lower than the 0.7 we obtained here. As can be seen from the chart, much of the operating region is at greatly lower efficiencies. 

The values of compressibility factor and isentropic exponent (k or heat capacity latio) arc usually decreased as gas is compressed. It is usually use average compressibility factor to caJcuIaie compressor head and power. Compressor head calculation is not sensitive to isentropic or polytropic exponent. Therefore, isentropic or polytropic exponent at compressor inlet or its average ean be used for compressor head and power calculation. 

Published analyses of cascade cycles by means of energy balances under conditions comparable to those used in analyzing expander cycles are very scarce. Longwell and Kruse tabulated computer-calculated compressor and expander powers for a C3Hg-C2Fl4-CF[4 cascade. For a feed of 1,566 Ib-moles/hr at 515 psia and 60°F, 490 lb-moles of LNG and two gaseous products were produced. 

The compressor of a gas turbine is one of the most important components of the gas turbine. It consumes between 50-65% of the energy produced in a gas turbine. Thus fouling of the compressor can cause large losses in power and efficiency for the gas turbine. Furthermore, the fouling of the compressor also creates surge problems, which not only affects the performance of the compressor but also creates bearing problems and flame-outs. The following are some of the major characteristics that need to be calculated ... 

The calculation of the turbine expander module depends whether or not this is a single shaft gas turbine or a multiple shaft gas turbine. In aero-derivative turbines, there are usually two or more shafts. In the latest aero-derivative turbines, there are usually two compressor sections, the LP compressor section, and the HP compressor section. This means that the turbine has three shafts the third shaft is the power shaft. The turbines that drive the compressor section are known as the gasifier turbines, and the turbine, which drives the generator, is the power turbine. The gasifier turbine produces the work to drive the compressor. 

Power requirements and discharge temperatures are calculated using the same relationships as used with the other rotary compressors already discussed. The efficiency is. 80 for air service and pressure in the 30 psig range. The mechanical losses are higher than the other rotaries. The mechanical loss is variable and dependent on gas, lubrication, and other factors. For an estimate, use. 15 of the gas horsepower. This approxuna tion should be close enough for an estimate. 

See the cooling curves in Figure 33.1. The evaporating temperature will now fall to about 0.2°C. Compressor manufacturers tables show 10.3% loss in duty for 1.5% less power at the new condition - an overall power increase of 9%. A more accurate estimate can be obtained by calculating a new basic rating for the reduced air flow. This shows about 8% extra power. 

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 electrical power required to drive a compressor (or pump) can be calculated from a knowledge of the motor efficiency ... 

For a centrifugal or axial compressor, the refrigeration power can be calculated directly from (see Appendix B) ... 

The capital costs for an ES-300H unit tested by the U.S. Navy at Naval Air Station North Island were estimated to be 50,000. For the test, electric power costs were calculated to be 6.78 per day, which would equal 2475 per year. This estimate includes the cost of operating the unit as well as the cost of running an air compressor required for treatment. Based on demonstration results, monitoring costs for the unit would add an additional 6300 per year (180 worker-hours per year at 35 per worker-hour) (D21558H, pp. 4-1, 4.2). 

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