THE EFFECT OF DIFFERENT PRESSURES UPON SMOKE-BOX TEMPERATURES
- The smoke-box temperature falls between the limits of 590 degrees F. and 860 degrees F., the lower limit agreeing with the rate of evaporation of 4 pounds per foot of heating surface per hour and the higher with a rate of evaporation of 14 pounds per square foot of heating surface per hour.
- The smoke-box temperature is so slightly affected by changes in steam pressure as to make negligible the influence of such changes in pressure for all ordinary ranges.
CONCLUSIONS
- The steam consumption under normal conditions of running has been established as follows:
BOILER PRESSURE...............STEAM per HORSEPOWER HOUR
120...........................................29.1
140...........................................27.7
160...........................................26.6
180...........................................26.0
200...........................................25.5
220...........................................25.1
240...........................................24.7
- The results show that the higher the pressure, the smaller the possible gain resulting from a given increment of pressure. An increase of pres- sure from 160 to 200 pounds results in a saving of 1.1 pounds of steam per horse-power per hour while a similar change from 200 pounds to 240 pounds improves the performance only to the extent of .8 of a pound per horse-power hour.
- The coal consumption under normal conditions of running has been established as follows:
BOILER PRESSURE...............COAL per HORSEPOWER HOUR
120...........................................3.84
140...........................................3.67
160...........................................3.53
180...........................................3.46
200...........................................3.40
220...........................................3.35
240...........................................3.31
- An increase of pressure from 160 to 200 pounds results in a saving of 0.13 pounds of coal per horse-power hour while a similar change from 200 to 240 results in a saving of but 0.09 pounds.
- Under service conditions, the improvement in performance with increase of pressure will depend upon the degree of perfection attending the maintenance of the locomotive. The values quoted in the preceding paragraphs assume a high order of maintenance. If this is lacking, it may easily happen that the saving which is anticipated through the adoption of higher pressures will entirely disappear.
- The difficulties to be met in the maintenance both of boiler and cylinders increase with increase of pressure.
- The results supply an accurate measure by which to determine the advantage of increasing the capacity of a boiler. For the development of a given power, any increase in boiler capacity brings its return in improved performance without adding to the cost of maintenance or opening any new avenues for incidental losses. As a means of improvement it is more certain than that which is offered by increase of pressure.
- As the scale of pressure is ascended an opportunity to further increase the weight of a locomotive should in many cases find expression in the design of a boiler of increased capacity rather than in one of higher pressures.
- Assuming 180 pounds pressure to have been accepted as standard and assuming the maintenance to be of the highest order, it will be found good practice to utilize any allowable increase in weight by providing a larger boiler rather than by providing a stronger boiler to permit higher pressures.
- Whenever the maintenance is not of the highest order, the standard running pressures should be below 180 pounds.
- Where the water which must be used in boilers contains foaming or scale-making admixtures, best results are likely to be secured by fixing the pressure below the limit of 180 pounds.
- A simple locomotive using saturated steam will render good and efficient service when the running pressure is as low as 160 pounds. Under most favorable conditions, no argument is to be found in the economical performance of a machine which can justify the use of pressures greater than 200 pounds.
Table of Contents; Page 59; Page 63; Index
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