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bed plate of the pump and connecting it with the pump through the standard coupling provided. The steam must be connected with standard steel pipe, to the steam main through a mechanicai se arator to remove entrgned water, through a suitable throttle vafive, and then through a governor to the turbine. The turbine is equi ped with a safety speed-limit regulator. After passing throug! the turbine, the steam may be exhausted to the atmosphere without recovery, in which case the turbine will take steam a t the service main pressure and discard the steam substantially at atmospheric pressure. In the more im ortant applications the turbine will be designed to deliver the exxaust steam a t some relatively low pressure-for example, 20 pounds per square inch. In this case there must be a low-pressure-steam ipe system in the plant, in order to collect the steam from this t u g i n e and other drives to make it available for process use. We must connect the steam discharge of the turbine to this low- ressure-steam main and heat insulate all piping installed before t t e job is complete, While the main for low-pressure exhaust steam is rovided with a trap to dischar e the condensed water, it probabyy will not be necessary to inst31 such a trap at each separate steam turbine if several are located on one section of low-pressure main. We will therefore count upon only a art of a steam trap allocated t o this particular turbine. Somewzat more time of a skilled mechanic is required in setting and connecting the turbine than the motor. Also, somewhat more labor is required from skilled steam fitters to make connections to the turbine and exhaust connection to the low-pressure plant main. As indicated above, this will undoubtedly run t o more money than is required to install the electric motor. But when operating the steam turbine with steam recovery the cost of steam charged to the turbine will always, or should always carry a substantial credit for the exhaust steam delivered to the low-pressure mains which is available to the plant for process uses. On a thermal basis the turbine has taken only a relatively small art of the total heat in the steam in performing its function opdriving the pump. A large part of the total heat still remains in the low-pressure steam, which is all available for process US-. The origmal cost of the high-pressure steam may v more, proportionally, than the cost of electric power, but u s s y where fuel is cheap enough to produce low-cost steam electric power is also low in cost. I n some localities no electrik power is available, and steam must be used. When there is no use for the exhaust steam, if recovered, the exhaust is connected to a water-cooled condenser for steam economy. The labor required to operate either the electric motor connected to the pum or the steam turbine connected to the pump is a relatively s m a i item. It is undoubtedly true that an en@neer would not be re uired to operate the electric motor drive; one of the chemical pyant opbrators would be permitted to start and stop the motor as required. I n the case of the steam turbine it might be necessary to have at least one licensed steam engineer in the plant. Such an engineermight then permit chemical plant operatom to start and stop the steam turbine; but nevertheless it is believed that the steam turbine will reqwre somewhat more labor in-the form of supervision and maintenance than the electric motor, over the useful life of each. The efficiency of the electric motor is slightly higher than that of the steam turbine, and the steam turbine is somewhat enalieed whenever reduction must be used between the t u r f h e and the equipment being riven. The amount of lubricating oil m11 be approximately the same for the electric motor and the steam turbine, but if reduction pars are used, an additional quantity of lubricating oil must be.c arged again& the-turbine. In general, we believe it is a queation of local conditions as to whether the electric motor or steam turbine is preferable. When a steam turbine is well insulated, the room in whch it operates will probably not feel the effect of radiated heat much more than it will that of heat radiated by the electric motor. Both machines are relatively noiseless and there is probably no choice upon this score between one‘ or the other. (Continued on page 70)
question often arises in chemical plant layout as to whether small equipment, such as pumps, blowers, compressors, and refrigerating machines should be driven by electric motors or by steam turbines. $he electric motor has been the popular choice for individual equipment drives in a-majority of cases. On the other hand the modern steam turbine of small capacity has become an adaptable and convenient unit for this same urpose. Because of the great differencesin the. characteristics ofany electric motor and small steam turbine, it is difficult to compare these two power machines. Furthermore, the decision to use electric motors is often guided by such local conditions as lack of steam for driving steam turbines. On the other hand, the decision to use steam turbines is usually made only because of some advantageous erformances which cannot be supphed by en electric motor. many chemcial plants there is a large important use of low-pressure steam for processes; in such plants the electric motor cannot supply this requirement, and a steam turbine is thus more or less necessary. In other applications one wishes to avoid the slightest possible fire risk and may select the steam turbine because of the absence of sparks and static electricity around them. To meet this condition the modern electric motor has been made fully enc!osed and e Iosion-proof. In plants where the decision is easily made a n T t h e choice automatically goes to the electric motor because of local conditions or to the steam turbine .because the motor is who11 unpitable, there is little room for discussion or comparison. $ere :1 is proosed to develop the roblem of choosing between electric motor &wes and steam turiine drives for those conditions where each would be substantially satisfactory. In such cases the final decision mieht rest on the cost of installation. It might be influenced by the time in which electric motors could be installed, connected, and made ready t o run as compared to the time required for the installation of steam turbines. It will be greatly influenced by the possibilities for the use of low-pressure process steam and the necessity for controlled variab!e speed. Let us first look at the cost of installing an electric motor to drive a centrifu a1 ump. We may assume that the plant has power circuits o8228or 440 volts within a reasonable distance of the pump’s location. The job would consist in lacing a motor of the proper speed and power upon the base of &e pump which bas been purchased specifically t o accommodate the motor, directly connected by a standard couplin The motor must be equipped with a starting box which prgably is some few feet away from the motor or erhaps across the room on an instrument or panel board, In ad%ition the starter must b’e connected to the electric service main witLn the buildmg. The cost of installation would include the first cost of the motor, the cpnduit, insulated wires between the motor and the starter, mounting the starter on a panel board or other support at convenient height, and the conduit and wirea necessary to connect the starter t o the plant’s electrical circuit. The labor involved would be tha$ of a mechanic t o u t the motor onto the pump base and adjust the coupling, a n t t h a t of an electrician to lay the conduit, draw the wires make the connections and do the necessary teatmg. This is a simple case and offhand will probably &ways cost less than the steam turbine. After the motor is installed and its useful life begins, one must give major consideration to operating costsgross cost of ower, maintenance, and supervision. The instalgtion of a small steam turbine is more complicated and, therefore, more costly than the installation of an electric motor. Let us take for comparison a steam turbine of the same horsepower as the electric motor used above, connected to, drive the same ump. We may w u m e that the steam turbine is dvectly connectea to the pump through a coupling similar, t o that used with the electric motor. When connected to a refn erating machine the steam turbine must be su plied with rduction gears to reduce the 3500 r.p.m. turbine shag down to the Felatiyely low s p d of a few hundred r.p.m. reqcured by the r e f n p a t i o n mac me. The installation consista in mounting the turbine upon the HE
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The amount of space consumed will be somewhat more for the steam turbine. The electrical conduit serving the motor and starter are relatively inconspicuous and might be said to rcqhire no appreciable planning for space. This is not true of the steam turbine. The high-pressure-steam piping must be provided for and @ill require a certain space across the bays of the plant, and the low-pressure-exhaust piping will require considerably more, usually under the floor. If the number of steam turbines in a given process room is numerous; this piping may amount to enough to require considerable space. Long runs of steam piping, both high- and low-pressure, will require hangers and expansion joints, which require maintenance. Before analyzing the approximate installation costs, one might like to know more about the operating efficiency of each machine. The'electric motors now available are highly efficient. I t is difficult to state the efficiency of an induction motor in one simple percentage figure because it changes somewhat as a result of several factors. In most cases a compromise is made between the maximum possible efficiency and near unity power factor, but let us take 90% as a psable value. The steam turbine IS a more flexible unit; any single machine may be operated from 1000 to 4000 r.p.m. and may also be supplied with steam a t any pressure between 20 and 200 pounds per square inch. The superheat may also vary from 1 to 5. Every set of operating conditions in the above ranges yields a different power output a t a different thermal efficiency. With such flexibility i t is difficult to fix a stated efficiency, but i t is apparent that a turbine may be selected which will serve any operating condition closely. The following installation cost's will vary in different locations for many reasons, but it is believed that both the turbine and motor will be affected in the same way, so that this brief analysis may be helpful in selecting the desired type of drive. The cost of installing electric motors, under the' short specifications given above, has been estimated for various sizes of motors. The 5horsepower steam turbine installation costs two and a quarter times more than does a 5-horsepower motor; most of the difference is due to the price of the turbine. The difference is only 50% more for a 25-horsepower turbine; while above 100 horsepower the installed cost of steam turbines should be lower than motors, reaching 16% less a t 150 horsepower, which is the maximum power used in this comparison. Any comparison of the operating costs of the two machines is difficult, Assume that steam is not recovered from a 50-horsepower turbine driving a centrifugal pump, where steam costs 40 cents per 1000 pounds at 200 pounds per square inch, saturated. The water rate for turbines of three manufacturers averaged 69.6 pounds per brake horsepower per hour or 3480 pounds of steam per hour, costing $1.39 per hour of operation. The cost for electric power for the motor, a t 90% efficiency, to equal this cost of steam would be $0.0335 per kilowatt-hour. Since i t is probable that the cost of electric power for a 50-horsepower motor in the average city would not exceed 2.5-2.75 cents, the motor would be more economical. On the other hand, to equal the cost of power at 2.5 cents per kw.-hr., the cost of steam must drop to 29.8 cents per 1000 pounds. Steam often costs less than 30 cents per 1000 pounds in the coal fields, but a t these locations the cost of electric power is probably below 2.5 cents per kw.-hr. While powkr and steam are the principal costs, there are other minor charges which slightly favor the electric motor. If we assume that process steam is needed in the lant,, at 15 pounds back pressure, an accurate analysis favors t t e use of a turbine. The largest factor is steam balance. At 15 pounds back pressure the average water rate of three makes of turbines amounts to 4520 pounds per hour, of which 4060 pounds could be recovered for process use. Based upon heat in the steam before and after passing through the turbine, only 12.65% has been removed. The real cost of steam to furnish power for the pump should be approximately $0.23 per hour; $1.58 per hour may fairly be charged to the plant process using 4060 pounds per hour of low-pressure steam. Several comparisons may be made. The process used obtains steam for 38.9 cents per 1000 pounds a lower cost than that of generatlng low-pressure steam dlrectly. The cost of power is a small fraction of the cost of electricity, so that one could neglect the 24.5% greater first cost of the turbine. If electric power costs $0.025 per kw.-hr. or $1.04 per hqur total, the saving by the use of the turbine would write off its installed cost in a little over four years. Other problems covering plant conditions may be analyzed in this manner, by referring to hbles giving the performance and water rates of the turbines required.
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