The design of a power plant is determined largely by the consideration of capital cost and operating cost; the former depends mainly on the plant size and latter is primarily a function of the overall efficiency of the plant. In general the efficiency can usually be improved, but only by increasing the capital cost of the plant, hence a suitable compromise must be reached between capital costs and operating costs.
l. Specific steam consumption (S.S.C.). The plant capital cost is mainly dependent upon the size of the plant components. These sizes will themselves depend on the flow rate of the steam which is passed through them. Hence, an indication of the relative capital cost of different steam plant is provided by the mass flow rate m of the steam required per unit power output, i.e., by the specific steam consumption (s.s.c.) or steam rate.
In M.K.S. system
1 horsepower hour ≈ 632 k cal
1 kilowatt hour ≈ 860 k cal.
∴ s.s.c. = 632/W kg/Hp -hr =860/W
3. Work ratio is defined as the ratio of net plant output to the gross (turbine) output.
