Текст 18. Packaged boilers

Packaged boilers are small self-contained boiler units. Packaged boilers are used as hot water boilers, aiding utility boilers and process steam producers. Packaged boilers can be both water tube and fire tube boilers. Packaged boilers can only be used with oil and gas as fuel without separate preparation devices. A packaged boiler can also be rented if there is a need for a temporary boiler solution.

The benefits of packaged boilers over common utility boilers are:

- Short installation time and low installation costs

- Small space usage

- Lower acquisition cost

- Better quality surveillance in work

- Standardized units

The drawbacks of packaged boilers are:

- Higher power consumption

- Cleaning periods more frequent

IV. ТЕКСТЫ ДЛЯ ЧТЕНИЯ И УСТНОГО ПЕРЕВОДА

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Текст 1. Pump types

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.

Pumps may be divided into four major classifications:

1) Piston pumps or reciprocating pumps driven by engine or electric motors.

2) Centrifugal pumps driven by steam turbines or electric motors.

3) Rotary pumps driven by steam turbines or electric motors.

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

Centrifugal pumps

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.

Текст 2. Fans

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.

Текст 3. Blowers

Blowers may be divided into 2 types: 1) rotary and 2) centrifugal.

A common type of rotary blower is the Roots (рутсевский) two-lobe blower (вентилятор с двумя зубчатыми колесами). Two double lobe impellers mounted on parallel shaft connected by gears rotate in opposite directions and at the same speed. The impellers are machined to afford only a small clearance between them and between the casing and impellers. As the lobes revolve, air is drawn into the space between the impellers and the casing, where it is trapped (задерживается) and discharged in volumes equal to the space between the impellers and casing, and the operation is repeated four times for each rotation of the shaft.

In order to change the volume rate (обменный расход) of flow, the blower speed is changed. The pressure developed by the blower can force the air through the piping system. The volume of air delivered by the blower will not change. Thus the blower is called a positive-displacement blower (вентилятор с положительной подачей).

Care should be taken in operating any positive-displacement blower. A safety valve should be placed on the discharge line to prevent the discharge pressure becoming excessive. This valve will prevent overloading the discharge line and the driving motor. The advantages of the rotary blower are: 1) simple construction, 2) positive air movement, 3) economy of operation and low maintenance.

Centrifugal blowers and compressors operate on the same principle as centrifugal pumps and resemble the closed-impeller centrifugal pumps. The casing or housing is constructed of heavy steel plate, and the impeller is an aluminum-alloy casting. If care is taken in providing the proper drive motor, the overload characteristics of the centrifugal blowers will cause no trouble.

Текст 4. Centrifugal compressors

Multistage centrifugal blowers are generally named compressors. They resemble multistage centrifugal pumps and many of the problems of their designs are similar to those in pumps designs.

The impellers of a complete centrifugal compressor unit are of the single-suction type and passages lead the air or gas from the discharge of one impeller to the suction side of the next impeller.

Because of an increase in temperature of the gas or air as the pressure is increased, cooling is generally necessary. If the pressures are not high, cooling water circulated in labyrinths between impellers may be sufficient. When high pressures are encountered, the gas may be cooled in interstage coolers.

Axial-flow compressors are designed on the principles of the airfoil section (вентиляторы лопастного типа), and the blade shapes will be similar to the axial-flow fan. These compressors are an essential part of the gas-turbine cycle. The advantages of centrifugal and axial-flow blowers and compressors are:1) non pulsating discharge of the gas, 2) no possibility of building up excessive discharge pressures, 3) a minimum of parts subject to mechanical wear, 4) no valve necessary, 5) minimum of vibration and noise, 6) high speed, low cost and small size or high capacity.

Текст № 5. Power plant cycles

A cycle is a series of operations, which regularly repeat themselves for the purpose of converting a portion of the stored energy of a fuel into a work. There are two general types of power cycles: the closed cycle and the open cycle.

In the closed cycle the working fluid begins at some initial condition, undergoes certain changes and returns to the initial condition.

The Rankin cycle

The simplest ideal of theoretical power plant steam cycle is called the Rankin cycle. The system contains: 1) a steam generating unit by which energy is added to the fluid in the form of heat transfer from a burning fuel; 2) a prime mover or steam turbine; 3) a condenser by which energy is rejected to the surroundings by the heat transfer, and 4) a boiler feed water pump.

The following assumptions are made for the Rankin cycle:

1) The wording fluid, usually water, is pumped into the bailer evaporated info steam in the boiler, expanded in the prime mover, condensed in the condenser and returned to the boiler feed pump to be recirculated through the equipment again and again in a closed circuit under steady flow conditions, that is at any given point in the system, the conditions of pressure, temperature, flow rate etc are constant.

2) All the heat is added in the steam-generated unit, all the heat that is rejected is transferred in the condenser. And there is no heat transfer between the working fluid and the surroundings at any place except in the steam-generating unit and the condenser.

3) There is no pressure drop in the piping system, there is a constant high pressure, p1, from the discharge side of the boiler feed pump to the prime mover, and a constant low pressure, p2, from the exhaust flange of the prime mover to the inlet of the boiler feed pump.

4) Expansion in the prime mover and compression in the pump occur without friction or heat transfer.

5) The working fluid leaves the condenser as liquid at the highest possible temperature which is the saturation temperature corresponding to the exhaust pressure, p2.

If the steam-generating unit is a boiler only, the steam that it delivers will be wet, and its quality and enthalpy can be determined by throttling calorimeter. If a super heater is included in the steam-generating unit, the steam that is delivered will be superheated and its enthalpy can be determined from its pressure and temperature by use of the superheated steam table.

The condensate leaving the condenser and entering the boiler feed pump is always saturated water at the condenser pressure, and its enthalpy can be found from the steam tables at the given condenser pressure.

The Rankin-cycle efficiency is the best that is theoretically possible with the equipment. Better theoretical efficiencies are possible by using more equipment in the more complex cycles.

If should be noted that only a small part of the energy supplied in the boiler as heat is converted into work and the rest is lost in the condenser.

The loss resulting from the heat-transferred to the condenser cooling water is, to a large extent, inescapable. The temperature of the cooling water varies only with the atmospheric conditions, thus, it remains almost constant. To lower it by artificial means would require additional energy.

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