ТЕКСТЫ ДЛЯ ЧТЕНИЯ И УСТНОГО ПЕРЕВОДА

Раздел 1. Тексты для ответа на первом зачёте

One of the most important problems of the engineer is the efficient controlled transfer of fluids from one point to another. This transfer may be opposed by gravitational force or by friction. Under certain conditions the gravitational force and other forces may aid the transfer, but friction always opposes motion.

Fluid is a substance in a liquid, gaseous or vapour state which offers little resistance to deformation. Common examples of the three states of a fluid are water as a liquid, air as a gas, and steam as a vapour. All these types of fluids have a tendency to move because of natural forces acting on them. A city may be supplied with water flowing by gravity from high ground. Air may circulate because of its own temperature difference. Steam rises through the water in a boiler owing to the difference in density or specific weight of steam and water. In many cases, however, the circulation is inadequate and mechanical equipment must be built to supplement the natural circulation. Often mechanical circulation is the only means of obtaining the desired fluid flow. The equipment for producing this fluid flow is divided into two major classes: pumps for handling liquids and fans, blowers and compressors for handling gases or vapours. Both classes of equipment in various forms may be found in the modern stationary power plant or small mobile power plant such as the aircraft engine, diesel locomotive or automobile engine.

The conditions under which liquids are to be transported vary widely and require a careful analysis before the proper selection of a pump can be made.

The conditions that will influence the selection of the type of pump are: 1) the type of liquid to be handled, that is, its viscosity, cleanliness, temperature and so on, 2) the amount of liquid to be handled, 3) the total pressure against which the liquid is to be moved, 4) the type of power to be used to drive pumps.

4) Fluid-impellent (жидкостный) pumps which are not mechanically operated but are fluid-pressure operated.

The centrifugal pump consists of an impeller or rotating section to produce the flow and a casing to enclose the liquid and to direct it properly as it leaves the impeller at its center and parallel to the shaft. The velocity of the liquid with respect to the impeller is in a direction opposite to the impeller motion. The impeller blades are curved backward to permit the liquid to flow to the rim (край) of the impeller with minimum friction. As the liquid leaves the impeller, it is thrown in a spiral motion forward with a certain velocity.

The water is graded away from the impeller by two basic types of casing: the volute and the turbine or diffuser. Liquid enters the impeller in the center, is thrown to the outside, and leaves the pump through the expanding spiral or volute casing. The casing has the volute shape to permit flow with a minimum of friction and to convert a part of the velocity head into static head. The static head is the head that overcomes resistance to flow.

The turbine or diffuser pump has the same type of impeller as the volute pump. The casing has a circular shape, and within the casing is a diffuser ring on which are placed vanes (лопасть). The vanes direct the flow of liquid and a decrease in the velocity of the liquid occurs because of an increase in the area through which the liquid flows. Thus, part of the velocity head is converted into static head as in the volute pump. For multistage pump the diffuser pump has a more compact casing than the volute pump. Generally, the volute pump will be used for low-head high capacity flow requirement and diffuser pump for high-head requirement.

Both volute and diffuser pumps are classified by the type of impeller, the number of stages and the type of suction used.

When two or more impellers are mounted on the same shaft and act in series, the pump is called a multistage pump, the number of stages corresponding to the number of impellers. Usually each stage produces the same head, and the total head developed is the number of heads produced per stage.

The types of impellers installed in centrifugal pumps are as numerous as the uses to which the pumps are put. Each of the impeller types has a specific purpose.

The axial-flow type is used to pump large quantities of fluid against a relatively small static head. It is not a true centrifugal pump but is designed on the principles of airfoil shapes. The radial pump is used for handling smaller quantities of fluid against a high head, because the centrifugal force is high but the flow path is small and restrictive. The open impeller is designed to handle dirty liquids such as sewage, where the flow path must be less restrictive. The partially radial impeller covers (отвечает) intermediate pumping conditions.

In power plant engineering the fan plays an important part. Generally, in small furnace installations a stack can produce a draft sufficiently high to supply air adequately to the fuel bed and to remove the flue gases. But the present day capacities of boilers and furnaces require mechanical draft to supplement the natural draft produced by the stack. Mechanical draft is divided into 2 systems: forced draft and induced draft. In the forced draft system the fan is located on the air-intake (всасывание) side of the furnace. A positive pressure, a pressure above atmospheric pressure, is produced under the fuel bed and act to force air through the bed. The forced draft system is necessary in installations where the pressure drop in the intake system and fuel bed is high.

Generally, the pressure in a furnace should be slightly less than atmospheric pressure. If it is too high, there will be leakage of asphyxiating gases into the boiler room. If the pressure in the furnace is too low, there will be the air leakage to the furnace with reduction in the furnace temperature. Because of these restrictions on the desirable pressure within the furnace, the force draft system is accompanied by a natural draft system. When a forced-draft and induced draft fans are used in combination, the system is called balanced draft.

The present trend is to construct more furnaces with gas-tight casings in order that they may be operated under pressure well above atmospheric pressure. Combustion efficiency is improved at elevated pressures and the induced-draft fan with its high maintenance cost can be eliminated completely.

Fans are used extensively in the heating and ventilating industry and in most power plants. Their basic design principles fall into two cases: axial-flow fans and centrifugal or radial fans. Axial flow fans are basically rotating air-foil fan similar to the propeller of an airplane.

The simplest axial flow fan is the small electric fan used for circulating air in rooms against very little resistance. Axial-flow fans for industrial purposes are the two blades or multiblade propeller type, and the multiblade airfoil type. Air enters the fan section from the left and flows over the rotor with a minimum of turbulence owing to the streamline form of the rotor and drive mechanism. The air stream is straightened by guide vane located on the discharge side, thus decreasing the rotational energy of the air by converting it to energy of translation.

The axial-flow fan operates best under conditions where the resistance of the system is low, as in the ventilating field. The axial-flow fan occupies a small space, is light in weight, is easy to install, and handles large volumes of air.

Centrifugal fan may be divided into two major classes: 1) the long-blade or plate type fan and 2) the short-blade multiblade fan. The blades of either type may be pitched towards the direction of motion of the fan, radially, or away from the direction of motion of the fan.

A plate-type radial blade rotor with double inlet is best suited for handling dirty gases since there are no pockets in the blades to catch and collect the dirt. The fan is designed for induced-draft service.

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