Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

237 °C, which is too high as the minimum temperature in the cycle. Therefore,

the use of mercury as a working fluid is limited to the high-temperature

cycles. Other disadvantages of mercury are its toxicity and high cost. The

mass flow rate of mercury in binary vapor cycles is several times that of water

because of its low enthalpy of vaporization.

It is evident from the T-sdiagram in Fig. 10–26 that the binary vapor cycle

approximates the Carnot cycle more closely than the steam cycle for the

same temperature limits. Therefore, the thermal efficiency of a power plant

can be increased by switching to binary cycles. The use of mercury–water

binary cycles in the United States dates back to 1928. Several such plants

have been built since then in the New England area, where fuel costs are typ-

ically higher. A small (40-MW) mercury–steam power plant that was in ser-

vice in New Hampshire in 1950 had a higher thermal efficiency than most of

the large modern power plants in use at that time.

Studies show that thermal efficiencies of 50 percent or higher are possible

with binary vapor cycles. However, binary vapor cycles are not economically

attractive because of their high initial cost and the competition offered by the

combined gas–steam power plants.

588 | Thermodynamics

4

58

1

T

s

6

3

STEAM

CYCLE

MERCURY

CYCLE

MERCURY

CYCLE

Heat exchanger

STEAM

CYCLE

Steam

turbine

Condenser

Steam

pump

6

5

1

Mercury

turbine

4

Boiler

Mercury

pump

7

Q

2

7

Saturation

dome

(steam)

Saturation dome

(mercury)

23

Superheater

8

FIGURE 10–26

Mercury–water binary vapor cycle.