load they would take their current from the main bus. The exhaust from the auxiliary generator would thus be available for heating the feed water, and as this machine could be run merely as a standby at light loads there would be no exhaust steam wasted. This system has been used to advantage at the Connors Creek plant of the Detroit Edison Company and other stations in the east, but has not yet been introduced on the Pacific Coast. The modern tendency is to eliminate as far as possible all reciprocating pumps and at the present time centrifugal pumps are used in all cases except for fuel oil and lubricating pumps where the reciprocating type is still used. Steam turbines are used in preference to reciprocating engines even in cases where slow speed is desired such as for operating circulating pumps for condensers. In such cases it is customary to provide reduction gears so as to be able to operate the turbine at high speed and the pump at low speed thus enabling each machine to operate at the speed best suited to highest efficiency. AUTOMATIC CONTROL As economical operation of a plant is obtained by the careful watching of all details, it is a growing conviction that personal control under trained supervisors is the one way to produce high economy. This is especially true of the boiler room, and it is common practice in the best plants to employ a combustion engineer to make flue-gas analyses and to keep continual check on the boiler efficiency. There is a tendency toward introducing automatic control into the fire room. Automatic oil-fire regulators have been placed in service in a number of plants on the Pacific Coast and increases in efficiency as high as 3 or 4 per cent. are reported. Another tendency at the present time is to equip the plant with recording meters which register automatically all important elements of operation throughout the full twentyfour hours of the day. It was formerly the custom to operate boilers with no instruments except a steam gage, but it is now customary to install steam-flow meters which register the quantity of steam produced by the boiler and the quantity of steam uesd by the burners, and air meters which indicate the amount of air passing through the boiler settings. These instruments are of great value in assisting the combustion engineer to secure maximum efficiency from the boiler plant. APPENDIX I ILLUSTRATIVE PROBLEMS Problem No. 1.-The mean effective pressure of a single-acting oil engine cylinder under test is found from an indicator card to be 43.9 lb. per sq. in.; the cylinder has 47.5 working strokes per minute; the diameter of the cylinder is 30 in.; and the length of stroke is 30 in. What is its horsepower? Solution. By reference to formula for horsepower computation, we find for Problem No. 2.—In a turbine test the atmospheric barometer reduced to the 32°F. standard of measurement, read 29.93 in. If the condenser vacuum reduced to the same standard read 28.23 in. of vacuum, what was the absolute pressure in the condenser? X 1.70 = 0.835 lb. per sq. in. absolute pressure in condenser. Problem No. 3.-A 10,000 kw. turbine under test operated with a gage reading of 171.5 lb. per sq. in. The gage, however, read one pound too low. The computed absolute pressure was found to be 187.2 lb. per sq. in. What was the barometer reading for the day? Problem No. 4.—A corrected atmospheric barometer reading is found to be 29.942 in. of mercury on the 32°F. standard. How many lb. per sq. in. does this represent? Solution. To convert to lb. per sq. in. by formula in the chapter on pressures: Problem No. 5.-A corrected barometer reading is 29.937 in. of mercury on the 30-inch vacuum standard. What is the pressure in lb. per sq. in.? Solution. To convert to lb. per sq. in. from formula in the chapter on pressures: Problem No. 6.—(a) At what temperature do the Fahrenheit and Centigrade scales read the same? Fahrenheit and Reamur? Centigrade and Reamur? (b) Assuming the absolute zero of the Fahrenheit scale to be 459.6°F. compute the absolute zero on the Centigrade and Reamur scales. When the scales have identical numerical readings, then F = C = X Let us substitute this value of F in the general relationship, R=218.049 = absolute zero on Reamur scale. Problem No. 7.—The temperature of the steam entering a turbine during a test was found to be 521.2°F.; the correction for stem exposure of the thermometer was 5.6°F.; the corrected steam gage reading 172.5 lb. gage; and the atmospheric barometer read 14.7 lb. per sq. in. What was the superheat of the steam? Problem No. 8.-The temperature of the superheated steam entering a turbine during a test was found to be 544.8°F. The pressure of the steam in the main was 182 lb. abs. What was the superheat of the steam? Solution. By reference to Table 2 of the steam tables the temperature of saturated steam corresponding to 182 lb. pressure is found to be 374.0°F. Substract this value from the temperature of the steam entering the turbine and the result will be the degrees of superheat, or Problem No. 9.-Regnault's classic formula for total heat of saturated steam is: Compute the total heat of saturated steam at the boiler pressure corresponding to 382.3°F. Problem No. 10.—Compute the total heat of saturated steam at 382.3°F. by the formula: H = 1150.3 + 0.3745 (t 212) 0.000550 (t − 212)2 Solution. Substituting the value of temperature, we have = 0.00585 per cent. Problem No. 11.—The specific volume of saturated steam is represented on page 104 of Marks and Davis Steam Tables by the formula: N. B.-This formula evidently does not check up at all for this temperature, since the specific volume for a temperature of 382.3°F. is 2.279 from the steam tables. Problem No. 12.—The mean specific heat of steam is represented mathematically on page 92 of Marks & Davis Steam Tables by the formula: = · 0.9983 0.0000288(t - 32)- 0.0002133(t − 32)2 What is the mean specific heat of steam for t Cm Solution.-Substituting, we have €382.3°F.? Cm 0.9983 - 0.0000288 (382.3 32) 0.0002133(382.3 = 0.9983 0.0000288(350.3) + 0.0002133(350.3)2 = = 0.9983 0.0101 + 26.17 =27.1582 Mean specific heat. 32) 2 Evidently a mistake is made in translating the last term of this formula from its original source, for it should be .0000002133 (t - 32). On this basis, we have that In the steam tables the heat of liquid for 382° is 355.0 B.t.u. and for 383° is 356.1 B.t.u. Hence the mean specific heat Cm is approximately 1.1, which indicates that had the decimal points been carried further the specific heat approaches that set forth in the above correction. Problem No. 13.-At a certain central station there are four 773 boiler horsepower Parker boilers. These boilers were used to give a 10,000 kw. load at the terminals of a turbine which has an over-all efficiency of 21 per cent. What was the percentage of overload on the boilers? |