System and Equipment Performance Guidelines

In designing a system, an engineer accounts for the physical performance and limitations of equipment to be utilized in the system. For example, practical heat exchangers are limited in how close the temperature of the cold fluid can come to the temperature of the hot fluid at any point in the heat exchanger. This minimum temperature difference is known as the "approach." For a gas to gas heat exchanger, a reasonable approach design value is 100°F. An engineer who employs a gas to gas heat exchanger with only a 50°F approach will have implied the use of a very large and expensive heat exchanger, and is likely to find the process economics are not practical.

This section documents reasonable equipment performance assumptions that can be used in a first pass conceptual design effort. The reader should be aware that the list includes many assumptions and simplifications that may not be suitable for detailed design. The documentation of equipment guidelines at a significant level of detail is the subject for entire books [e.g., several excellent books have already been written concerning conceptual design and equipment performance (34,35,36)]. The list presented here simply illustrates the more important equipment performance considerations and their common performance ranges, which may be useful to the novice system designer for incorporating a level of realism. Detailed conceptual design efforts need to address many factors not addressed by the list below, such as the effects of flow rates, temperatures, pressures, corrosive elements, the impact of the equipment on the cycle itself, and, of course, the specific performance of the actual equipment.

The list of equipment performance assumptions is presented in Table 9-5.

Table 9-5 Equipment Performance Assumptions

Parameter

Common Range

Notes

Pump Efficiency

10 to 90%

Flow rate dependent.

100 gpm

35 to 60%

Pump efficiencies do not include the

1000 gpm

60 to 80%

motor or driver efficiency.

10,000 gpm

78 to 90%

Compressor Efficiency

Flow rate and PR dependent.

Reciprocating

65 to 90%

Compressor efficiency only. Motor or

Industrial quality- Centrifugal

76 to 85%

driver efficiency not included.

High quality- Centrifugal

82 to 90%

Compressor Intercooling

PRi=(PRtotal)1/n StaSeS

Optimal per stage pressure ratio

For a two-stage system, PR-i=PR2.

Intercooled temperature

130°F

Assumes 100°F cooling water.

Intercooling recommended

Prtotai > 5.0

Turbine Efficiency (isentropic)

Flow rate and condition dependent.

Steam Turbine

75 to 90%

Best to refer to a heat balance or

Gas Turbine

80 to 90%

specific model information.

Gas Expander

80 to 85%

Pressure Drops

Heat exchanger - gas side

1-2%

Gas phase pressure drop.

Heat exchanger - water side

5-10 psi

Water side pressure drop.

Fuel cell

2%

Fuel processor

2%

Steam superheater/reheater

5-10%

Temperature Approaches

Gas to Gas

100°F

Air to water coolers

30°F

Gas to steam (superheater)

30°F

Water to water

20°F

Economizer

20°F

Evaporator

20°F

Heat Recovery Boiler

Radiant heat loss

0.5 to 1.0%

Fuel Cell

Fuel utilization

-

See Technology specific sections.

Oxidant utilization

-

See Technology specific sections.

Heat loss

Inverter Efficiency

94 to 98%

96.5% is common for sizes ~ 1 MW.

Turbine Generator Efficiency

96 to 98.5%

1 to 10 MW

98.0%

Transformer Loss

0.5 to 0.8%

Stepping up or down.

Motor Efficiency

1 to 10 kW

<90%

10 to 100 kW

90 to 92%

100 to 1000 kW

92 to 95%

1 to 10 MW

95 to 97%

Auxiliary Power

Dependent upon auxiliary systems

Steam turbine auxiliaries

0.5%

Gas turbine auxiliaries

0.5%

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