As compressed air is increasingly used in the industry and the range of working pressures used simultaneously in mining operations is expanding, the traditional air supply systems stand in need of modernization.Generally, simultaneous supply of pneumatic power with working pressures at different levels can be accomplished by combinations of three basic types of compressed air generators: a stationary compressor station located on the earth's surface (CS), mobile compressor stations (MCS), and pressure boosters (PB) installed near the consumption sites (Table i). Proceeding from these combinations, six basic configurations of simultaneous air supply to pneumatic drives of mining machines with working pressures of various levels are conceivable. Each of these is discussed in the present paper. Scheme I (traditional scheme):Air supply is provided at the surface with the top-level pressure followed by a reduction of a portion of the compressed air to lower pressures, as needed by the users.Scheme II: A part of the users operating at low-pressure levels are supplied from the surface, while other users operating at higher pressures are supplied by MCS.Scheme III:The latter group of users receive compressed air as under scheme II, while the former group operating on low pressure is supplied from the surface.In contrast to the preceding pneumatic supply scheme, compressed air from the surface is fed at an even lower pressure and then boosted to a higher pressure by the BC near the consumption site.Scheme IV: Compressed air is fed from the surface to BC at low pressure; some of the BC units raise the pressure to the first working level, while other BC units raise it to the second, higher working level.Scheme V: Air is supplied by MCS, raising the pressure to different levels, as needed by the consumers. Scheme VI:Contains the same generators as scheme IV. Compressed air is fed from the surface under pressure, corresponding to the working pressure of the first (low) level; a portion of this air is then boosted by BC to the high-level pressure.In these schemes the different combinations of compressed air generators lead to different current and basic costs in the production of compressed air and its transportation.The effectiveness of each of the schemes can be evaluated objectively by cost-benefit analysis, primarily in terms of reduced cost [i]:where E is the annual cost of the production and transport of compressed air (operating costs} K is the capital investment in the production and transportation of compressed air; and EH is the normative efficiency ratio.The cost components for each of the schemes are introduced by Table 2.For the schemes, the following expressions of reduced cost can be written:Polytechical Institute, Omsk.
The effLctency of pLstou compressor and pneumatLc hnpactor mechanism operation is ratsed when utilizing a gas-liquid mixture as the working body [1,2]. A mixture of air and water is used most extensLvely. This is because this working body is not toxic, is cheap, and Lts components are widespread In nature. Data available in the literature on the theoretical and experimental lnvestigatLon of compression and expansion processes in ptstou compressors [3] and pneumatic lmpactors show that water [n the form of medhtm and rough dispersion of drops in air will evaporate negligibly little, which does not permit utilization of known methods of analyzing these processes on the basis of total evaporation [4,5]. Methods used to compute the compression and expansion processes with the absence of phase transitions taken into account are characterized, Ln the majority of cases, by making assumptions that do not fully correspond to reality. For instance, this concerns the assumption of thermal equilibrium of the phases during compression [6,7] and a linear law of temperature variation in the gas phase with time [8]. Moreover, all existing analytical methods of analyzing these processes in volume machines have the aim of determining the polytropic index, which, as Ls known, cannot assure equality of expended work and equality of the thermodyuamic parameters of the gas phase at the beginning and ending of the processes simultaneously [9, 10].We execute the solution of the problem posed by taking the following constraints and assumptions: the liquid phase is a system of equal spherical drops, distributed uniformly over the volume, for which there is no slip, fragmentatLon, and coagulation; the mass concentration of the lkluid phase is constant in the working volume in the processes being investigated; there are no leakages and overflows of the working body because of looseness; there is no thermal and mechanical interaction of the liquid phase with the control volume surfaces; the gas phase is subject to ideal gas laws; we neglect changes in the physical properties of the working body as a function of the temperature and pressure.Taking account os the assumptions and constraints made, the first law of thermodynamics for the gas phase is written in the form dQ =. dr+ PdV,where dQ is the element of heat transfer between the gas and the 1LquLd; dU, change in Lnternsl energy of air; dV, change in volume in the processes being LnvestLgated; and P, pressure.The value of the heat transfer element is determined on the basis of the Newton-R[khman lawwhere T d is the temperature of the water drops; ~, heateliminatiou coefficient; and F, surface of heat transfer between the drops and air.Taking account of the assumption made about no phase slip, the heat elimination coefflcLent can be determined from the criterisi equation Nu =2. Hencewhere ~ is the heat conduction ooeffLcient of the gas phase, and r d is the radLns of the drop.Omsk Polytechnic Institute.
Results of numerical and experimental investigations of pneumatic piston engines with self-acting valves and various air-distributing systems are reported.Work safety in fire-and explosion-prone chemical, petrochemical, gas, and mining enterprises is ensured by using power pneumatic actuator (instead of electric) which does not produce sparks.Use of pneumatic power, for example, in mining machines and systems for mining deeply lying commercial minerals is associated not only with increased danger of gas and dust explosion during the use of electric power, but also with cooling effect stemming from expansion of compressed air. This effect increases significantly with increase in depth of mining and with rise in temperature of air in underground mine workings.The cooling effect produced during operation of pneumatic machines and mechanisms influences the microclimate not only because of temperature drop, but also because of reduced humidity of the mine air. Regardless of the type of operation performed, the moisture content of the air at pneumatic machine outlet is low because the drop in moisture content occurs in the section of the pipeline 500-900 m away from the compressor station located on the ground. Since the microclimate in deep mines is characterized not only by high temperature, but also by high air humidity, mixing of used dry air with moist air of the workings makes a positive contribution to improvement in microclimate [1]. Use of pneumatic power helped broaden the area of application of machines and mechanisms, especially in coal mines.In mining enterprises, pneumatic piston engines (PPE) are used most widely for drives of combines, cutting machines, hoists (winches), maneuvering, rock and mineral loading, loading and transporting machines, etc., because of the following merits: the operating chamber of the PPE ensures a high degree of sealing thanks to piston rings; compressed air leakage is negligible, so the efficiency of these engines is higher than that of other types of engine; good start-up characteristics; allow overloading; simplicity of design and control. Currently, compressed air with 0.4-0.63 MPa inlet pressure is used for the PPE.Production and use of compressed air is associated with loss of power in mine pneumatic systems (compressor unit, air network, and engine), as a result of which the overall efficiency of the pneumatic system, as a rule, does not exceed 10% [2]. Considering that as much as 30% of the electric power generated at the enterprises is consumed for driving compressors, enhancing operational efficiency of the elements of the pneumatic system is a pressing task.The main producer in Russia of PPE with a crank mechanism is Rodniki Machine Building Plant (in Rodniki, Ivanovo Region).The salient comparative characteristics of the pneumatic piston engines built by this plant and foreign companies are noted in Table 1. As evident from the table, in terms of the specific parameters, namely, specific compressed air flow rate and specific metal content, the domestic pneumatic en...
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