VAULT // SEPTEMBER 1915
TURBINES VS. ENGINES IN UNITS OF SMALL CAPACITIES
The relative advantages of steam turbines versus traditional steam engines were still being determined when this article was first published in September 1915.
Written by J. S. Barstow
A scan of the original journal cover from September 1915.
THE TERM “UNITS OF SMALL CAPACITIES” as used herein is intended to include steam turbines and engines of less than 500 hp capacity. The paper will necessarily deal largely with the prime movers of auxiliary apparatus in power plants, since the tendency of the times in all industries, and particularly in central stations, is toward the concentration of power in a few units of large size and uniform capacity as opposed to a multiplicity of small units of different capacities. However, there is a wide field for power application where the steam-operated prime movers are of relatively small size, and where transmitted or central station energy is not able to successfully compete; and it is intended to discuss the type of apparatus best suited in these cases, as well as the type of apparatus which it is advisable to employ for auxiliary units in large plants or central stations.
There are certain definite fields where the small turbine is of conceded superiority, and other fields where the engine must hold sway. The desirability of the one as compared with the other is largely determined by the following factors, which govern the adaptability, cost, and economy of the equipment to be installed for any given service:
a) Speed conditions and limitations involving consideration of maximum or minimum permissible speed, and whether the driven apparatus is of the constant or variable speed class
b) Steam pressure and temperature conditions involving consideration of initial and final pressures, and superheat, if any
c) Power capacity of apparatus
d) Relative space requirements of turbine and engine units involving consideration of available room, character of powerhouse construction and cost of foundation or other supporting structure
e) Use or application, if any, of the exhaust steam for feed water heating, steam heating, or process purposes
f) Available cooling water supply, if the turbine or engine is to run condensing, involving also consideration of the temperature of the water and whether it must be artificially cooled and re-circulated
g) Operating conditions including consideration of attendance, oiling, starting and stopping, vibration, noise, etc.
h) Relative cost of complete installations including necessary foundations, piping, and condenser equipment, if any
As to the practicability of the small turbine, it may be said that not until about 20 years ago was any really practicable apparatus of this kind developed, and even up to 10 years ago the turbine was looked upon mainly as an experiment. The last few years have witnessed, however, the practical perfection of this type of prime mover in sizes as large as 50,000 hp, with units of 30,000 hp quite common in large central stations. They have also seen quite as much good work done in the perfection of small turbine units as in the development of very large ones, and the turbine in all sizes is quite as well developed today as is the steam engine after more than one hundred years of constant effort and improvement.
The present day builders of reciprocating engines are able to report further progress, however, and as a result of their recent efforts there have appeared the rejuvenated poppet valve engine adapted to the use of highly superheated steam; the unaflow or parallel-flow engine, also with poppet type valves, in which cylinder condensation is reduced by causing the steam to travel in one direction only; and the small self-contained power plant, or “locomobile,” consisting of a steam engine mounted upon a tubular boiler and operating at high superheat. All these engines are adaptations from European practice, where, owing to the high cost of fuel and the relatively larger number of technically trained operators, they have found high favor.
While there is the possibility that the use of a shell type of boiler for high pressures will be considered by many engineers as unwise, especially for installation in densely populated centers, it is still too early to predict what the future of this apparatus is to be. However, as indicating what may be accomplished in fuel economy in a well-designed plant of small size, the results reported are interesting.
Such an arrangement would minimize the possible interference by one prototype plant with the operation of the other and at the same time provide valuable coupling data. Successes or difficulties also could be assessed separately. If each proves successful, the next step can be a combination, dual-purpose plant of the 3,500 MW (thermal) reactor prototype with a large-size prototype desalting plant.
Perhaps the most likely project in which the scale-up of nuclear-desalting technology will begin in the United States is being studied by the Department of the Interior and the Metropolitan Water District of Southern California (MWD). The three organizations entered into a contract for a detailed economic and engineering study of dual-purpose plants in the 150-to-750 MW electrical size range and 50-to-150 million gpd of water production capacity. We are hopeful that such an installation will become a reality by 1970.
Based on the data obtained from our research and development efforts and the prototype construction and operation experience of the program that have been proposed, we believe the technology and economics of nuclear energy sources will be available to meet the power and desalting needs of the United States and the world. We fully expect that by the mid-1970s we will have developed the technology of the large nuclear energy sources required for regional water supply systems, enabling water planning groups to consider nuclear desalting as a proved technical and economic alternative to more conventional water supply schemes.
OPERATING ADVANTAGES
From the operating point of view, the turbine possesses a great advantage in the simplicity of its construction, a factor which tends toward increased reliability and lower cost of maintenance. It can usually be more quickly started and loaded and, in operation, usually requires very much less attention than an engine unit of corresponding capacity. The lubricating arrangements are few in number and of simple design.
APPLICABILITY OF TURBINES
1 Direct connected units, operating condensing.
60-cycle generators in all sizes, also 25-cyele generators above 1000 kw. capacity. (This paper is, however, not intended to deal with units of this size.)
Direct current generators in sizes up to 1000 kw. capacity, including exciter units of all sizes.
Centrifugal pumping machinery operating under substantially constant head and quantity conditions, and at moderately high head, say from 100 ft. up, depending upon the size of the unit.
Fans and blowers for delivering air at pressures from 145 in. water column to 30 Ib. per sq. in.
2 Direct connected units, operating non-condensing
For all the above purposes, in those eases wherein steam economy is not the prime factor or where the exhaust steam can be completely utilized, and, in the latter case, particularly where oil free exhaust steam is desirable or essential.
3 Geared units, operating either condensing or noncondensing
For all the above mentioned applications, and in addition, many others which would otherwise fall in the category of the steam engine, on account of the relatively slow speed of the apparatus to be driven.
APPLICABILITY OF ENGINES
1 Noncondensing units, direct connected or belted and used for driving
Electric generators of all classes excepting exciter sets of small capacity, unless belted from the main engine.
Centrifugal pumping machinery, operating under variable head and quality conditions and at relatively low heads, say up to 100 ft., depending on the capacity of the unit.
Pumps and compressors for delivering water or gases in relatively small quantities and at relatively high pressures—in the case of pumps at pressures above 100 Ib. per sq. in. and in the ease of compressors at pressures from 1 lb. per sq. in. and above.
Fans and blowers (including induced draft fans) for handling air in variable quantities and at relatively low pressures, say not over 5 in. water column.
Line shafts of mills, where the driven apparatus is closely grouped and the load factor is good. All apparatus requiring reversal in direction of rotation, as in hoisting engines and engines for traction purposes.
2 Condensing units, direct-connected or belted
For all the above purposes, particularly where the condensing water supply is limited, and where the water must be re-cooled and re-circulated.
J.S. Barstow was a non-member of ASME from Philadelphia.
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