Английский язык для обучающихся по направлению «Криогеника и нефтехимическое производство». Учебное пособие

Read the text, describing the essence and the types of vapor-compression process:

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Vapor-Compression Refrigeration Systems

The idea of cooling systems includes understanding technology of producing low and very low temperatures - refrigeration and cryogenics respectively. The usual thing by this is that heat flows in the direction of decreasing temperature. But, however, the reverse process can take place. The heat transfer from a lowtemperature region to a high temperature requires, therefore, intermediate devices and systems called refrigerators. The difference between a refrigeration and a cryogenic system lies in the achievable temperatures, being set at −100 °F or −74 °C.

Refrigerators are systems that operate on a cyclic principle involving a working fluid called a refrigerant. The objective of the refrigerator is to remove heat from the cold medium, whilst the objective of the heat pump is to supply heat to a warm one. Both of these systems transfer heat from low to high temperature media with the input of work.

For refrigerators, heat pumps and air conditioning systems, the most widely used cycle is the vapor-compression refrigeration cycle. It is resulted by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as an expansion valve or capillary tube.

The ideal vapor-compression refrigeration cycle consists of four processes.

1.Isentropic compression.

2.Heat rejection in a condenser at constant pressure.

3.Throttling in an expansion device.

4.Heat absorption in an evaporator at constant pressure. Moreover, this cycle can be well described in five steps:

Step 1: The refrigerant leaves the evaporator at a low pressure, low temperature

as saturated vapor. The refrigerant enters the compressor without heat gain/loss from the environment.

Step 2: The refrigerant is compressed reversibly (without losses) and stays in a high temperature, high pressure and superheated vapor state.

Step 3: Condensing brings the refrigerant out of superheated state and condenses it at a constant pressure. The refrigerant becomes a high pressure, medium temperature, saturated liquid.

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Step 4: Being condensed, the refrigerant expands adiabatically (without heat transfer) and reversibly (at constant enthalpy) in the expansion valve. The refrigerant leaves the expansion valve at a low pressure, low temperature, low enthalpy vapor.

Step 5: At constant pressure and in an adiabatic fashion, the refrigerant enters the evaporator and evaporates. It is during the evaporation that heat transfer occurs from the refrigerated space to the refrigerant.

In the ideal cycle, the refrigerant leaves the evaporator and enters the compressor as saturated vapor. In this case, however, it is not possible to control the state of the refrigerant precisely. Therefore, the system is designed so that the refrigerant is slightly superheated at the compressor inlet. This ensures that the refrigerant is completely vaporized when it enters the compressor. Also, in the ideal case, the refrigerant is assumed to leave the condenser as saturated liquid at the compressor exit pressure.

Another version of vapor-compression cycle is called actual. An actual vaporcompression refrigeration cycle differs from the ideal one in several ways, owing mostly to the irreversibilities that occur in various components. Two common sources of the irreversibilities are fluid friction (which causes pressure drops) and heat transfer to or from the surroundings.

In actual situations it is unavoidable to have some pressure drop in the condenser as well as in the lines connecting the condenser to the compressor and to the throttling valve. Therefore the refrigerant is subcooled before it enters the throttling valve.

The samples of vapor-compression refrigeration system are heat pumps and air conditioners. The most common energy source for heat pumps is atmospheric air. The COP (coefficient of performance) of heat pumps usually ranges between 1.5 and 4, depending on the particular system used and the temperature of the source. Heat pumps and air conditioners have the same mechanical components.

Water and soil are also used as energy source. When the fluid is used, the heat exchanger can be replaced by a mixing chamber (called flash chamber), since it has better heat transfer characteristics. Such systems are called multistage compression refrigeration systems. In this system, the liquid refrigerant expands in the first expansion valve to the flash chamber pressure. Part of the liquid vaporizes during the process. This saturated vapor is mixed with the superheated vapor from the low-pressure compressor and the mixture enters the high-pressure compressor. This is the essence, a refrigeration process. The saturated liquid expands through the second expansion valve into the evaporator, where it picks up heat from the refrigerated space.

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Exercise 3. Translate word-combinations into English:

система охлаждения, снижение температуры, передача тепла, рабочая жидкость, удаление тепла, тепловой насос, подавать (поставлять) тепло, испарение охладителя, регулирующее устройство, расширительный клапан, постоянное давление, испаряющее устройство, насыщенный пар, потеря тепла, конденсированный охладитель, падение давления, регулирующий клапан, механические детали, компрессионная камера.

Speaking

Case 1

Find in the text the sentences that correspond to the following statements:

1.The refrigeration system involves creating low temperatures.

2.Definite heat streams are the basis of creating cold temperatures.

3.Special apparatuses are needed to transfer heat to low temperature.

4.A special substance is used in refrigeration process.

5.Refrigeration process is based on vaporizing cooling substance before being compressed.

6.Refrigeration system is based on four stages.

7.In the ideal cycle the cooling substance transforms into the saturated vapor.

8.There is another type of refrigeration system.

9.The differences between the ideal and actual systems are in indispensability (необходимость) of various parts.

10.The device called chamber transmits heat better than heat exchanger.

Case 2

Speak English on the following information:

1.Система охлаждения предполагает создание низких температур.

2.В системе охлаждения применяется газ, воздух или жидкость.

3.Указанные ингредиенты проходят через систему, называемую охладителем.

4.Ключевым моментом в системе охлаждения является сжатие.

5.Система испарения-сжатия может быть представлена двумя типами – идеальная и актуальная.

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6.Процесс испарения-сжатия состоит из нескольких стадий и шагов.

7.В системе идеального цикла охлаждающее вещество в виде насыщенного пара переходит из испарителя в компрессор.

8.Актуальный цикл испарения-сжатия отличается от идеального необратимостью некоторых компонентов.

9.Холодильники, тепловые насосы и кондиционеры используют систему идеального цикла испарения-сжатия.

10.При использовании жидкости, теплообменник может быть заменен на смесительную камеру.

Case 3

Speak on refrigeration systems based on the vapor-compression cycle:

1.What is the essence of a cooling system? Are you acquainted with such technology?

2.What is the difference between refrigeration and cryogenic systems?

3.What are the principles of working refrigeration system?

4.What are the most widespread refrigeration appliances?

5.What stages does the ideal refrigeration cycle consist of?

6.What technological steps are realized by refrigerant working?

7.What is the peculiarity of actual vapor-compression cycle?

8.What substance can serve as refrigerants?

SECTION IV

Active Refrigeration Systems and Cryocooler Cycle Types

Grammar: Infinitive (the function of adverbial modifier of purpose).

Word-formation: Conversion.

Speaking: The types of the basic cooling systems.

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Learn the pronunciation and the meaning of the words:

Grammar study: Infinitive as adverbial modifier of purpose

Инфинитив в английском языке может выполнять функции всех членов предложения. Одной из часто встречающихся является обстоятельство цели, отвечающее на вопрос “зачем”, “для чего”, и переводится с союзом “чтобы”, “для того, чтобы”.

Sample: Cryogenic substances are used to create low temperature – криогенные вещества используются, чтобы создать низкую температуру.

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Vocabulary development: word-formation: conversion

Конверсия – это способ образования новых значений слов без помощи словообразовательных элементов (суффиксов или префиксов). При этом однокоренные слова относятся к различным частям речи.

Samples: experience (n) опыт – to experience (v) испытывать; engineer (n) инженер, механик – to engineer (v) сооружать,

проектировать.

Exercise 1. Translate the sentences, paying attention to the function of infinitive:

1.If there are no available cryogens or they are inconvenient to use, mechanical refrigerators are preferred.

2.To achieve the cold temperature, expanding a gas from a high pressure to a low one is applied.

3.One of the Stirling cycles is using a mechanical displacer to effect the thermodynamic cycle.

4.The Pulse Tube replaces the mechanical displacer with a pneumatic expander to achieve the desired gas pressure.

5.One of the main elements of the cooler is electronics used to drive the compressor.

Exercise 2. Read and remember the meanings of the words formed by convertion:

cycle (n) цикл — to cycle (v) совершать цикл, развитие; cost (n) цена, стоимость — to cost (v) стоить, назначать цену;

design (n) проект, конструкция — to design (v) проектировать, конструировать, изображать;

phase (n) период, стадия, фаза — to phase (v) фазировать, вводить;

feature (n) особенность, характерная черта, признак, свойство — to feature (v) изображать, быть характерной чертой;

flow (n) течение, поток — to flow (v) течь, литься; heat (n) теплота, нагрев — to heat (v) нагревать;

stream (n) поток, течение — to stream (v) течь, вытекать;

speed (n) скорость — to speed (v) (sped, sped) спешить, ускорять; exchange (n) обмен — to exchange (v) обменивать, меняться;

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key (n) ключ, клавиша, переключатель — to key (v) запирать, закреплять; measure (n) мера, мероприятие — to measure (v) измерять;

use (n) [ju:s] употребление, использование — to use (v) [ju:z] использовать; range (n) ряд, линия, сфера, область, пространство — to range (v) выстраи-

вать в ряд, классифицировать, колебаться (в пределах);

throttle (n) регулятор, дроссель — to throttle (v) дросселировать, уменьшать; influence (n) влияние, воздействие — to influence (v) влиять.

Exercise 3. Find in the text the sentences with the words, formed by conversion, define their grammatical status (part of speech), function and translate:

cycle, design, pressure, feature, heat, flow, stream, exchange, key, measure, phase, use, cost, range, throttle, speed, influence.

Read the text, telling of various types of refrigeration systems:

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Active Refrigeration Systems and Cryocooler Cycle Types

For cryogenic application where stored cryogens like liquid nitrogen and liquid helium are not readily available or are inconvenient to use, mechanical refrigerators or cryocoolers are often the preferred design solution. All mechanical refrigerators generate cooling by basically expanding a gas from a high pressure to a low pressure. The primary distinguishing features between cycles are: how the compression is accomplished, what pressure ratio is used, what method of expansion is used to achieve the cold temperature, how well and where heat is rejected, how well thermodynamic efficiency is maintained using heat exchangers, regenerators and recuperators. The most fundamental distinction between cryocooler types is the nature of the refrigerant flow inside: either alternating flow (AC systems) or continuous flow (DC systems).

In an AC-type cooler system, a regenerative heat exchanger stores and releases energy to the alternating refrigerant stream. In a DC-type system, a recuperative heat exchanger exchanges energy between two opposing streams of flowing gas or liquid.

The most common mechanical refrigeration cycles include: Stirling, Pulse Tube, Gifford-McMahon (GM), Joule-Thomson (JT) and reverse-Brayton cycle. Such cooler types as Stirling, Pulse tube and Gifford-McMahon use regenerative (AC flow) cycles, whilst Joule-Thomson and turbo-Brayton systems use recuperative (DC flow). A key distinguishing feature of these systems is that the compressor must be quite

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close to the cold expander in a regenerative AC flow cooler, and can be remoted (many meters away) for a recuperative DC flow cooler. One exception is the GiffordMcMahon cooler: it uses a regenerative refrigeration cycle, but uses a constant flow DC compressor that can be remotedly located.

Before brief description of the above mentioned cycles, it is worth mentioning the Carnot cycles, the background and the standard measure of cryocooler efficiency, the Carnot Coefficient of Performance.

The coefficient of refrigeration performance (COP) for any refrigerator is defined as the ratio of the extracted heat to the applied work:

COPCooler = cooling power/inpute power.

Stirling coolers (both mechanical displacer and pulse tube-based) are of the most widely used cryorefrigerator types for small remote and aerospace applications. Small size and mass and high thermodynamic efficiency are crucial. Classic examples of Stirling application include remote cell phone towers, military infra-red vision sensors and spacecraft-instrument infra-red and gamma-rey sensors.

Stirling cycle exists in two versions: using a mechanical displacer to effect the thermodynamic cycle, while the second version for the similar purpose is based on a pneumatic Pulse Tube (PT) circuit. In other words, the pulse tube replaces the mechanical displacer of the classic Stirling cycle with a pneumatic (without moving parts) expander to achieve the desired mass flow/gas pressure phase. The benefit of the PT version is lower expander vibration and elimination of complexity and possible mechanical wear associated with the moving displacer.

Gifford-McMahon (GM) cryocoolers (with both mechanical displacer and pulse tube coldhead) are one of the most widely used coolers for commercial and laboratory use where low cost and operational convenience is important and lots of electrical power is widely available. The GM cycle is very similar to the Stirling cycle in that its expander is based on an AC oscillating flow, typically using helium in the 10 to 30 bar range as the refrigerant gas with a working frequency of 1 to 2,4 Hz.

Joule-Thomson (JT) refrigeration systems are the most familiar type to the general public. A variant of this cycle referred to as the vapor compression or throttle cycle is used in nearly all domestic refrigerators and freezers and residential, commercial and automotive air conditioning systems. Another major use of the vapor compression cycle is the liquefaction of oxygen and nitrogen for industrial uses. However, today the use of the JT cycle is not common for general cryogenic cooling appli-

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cations, only for cooling the small tip of cryogenic surgical probes and as a bottoming cycle for cooling focal planes to 4-6 K in vibration sensitive space-viewing instruments and telescopes. Fundamentally, the Joule-Thomson (JT) cycle is a recuperative cycle which is built on a constant DC flow of high-pressure fluid that is expanded isenthalpically (without heat transfer) to a low pressure through a JT expansion valve.

A reverse-Brayton cycle cryocooler is a second type of recuperative or DC flow cryocooler. However, it differs considerably from the JT cycle by using a highflowrate , low pressure-ratio refrigerant stream to produce cooling. As a result, it generally uses an entirely different type of compressor, a high speed gas turbine which operates at the speeds of 100,000 to 600,000 rpm. Positive displacement compressors and expanders can also meet the functional needs of the cycle; however, their mass and vibration characteristics tend to make them less desirable. Because of the use of the turbine compressor and expander, the cycle is also commonly referred to as the turbo-Brayton cycle.

As for engineering aspects of turbo-Brayton cryocooler, three factors influence its efficiency: the speed of the compressor, the thermal effectiveness of the recuperator and the precision of the small-scale turbine blades. A fourth key element of the cooler is the specialized electronics used to drive the compressor and control the cooler’s operation.

Exercise 4. Compose the pairs of synonyms and translate them:

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Speaking

Case 1

Find in the text the sentences corresponding to the following statements:

1.The preferable choice of mechanical refrigerators is realized when availability of the stored cryogens is not enough.

2.The generation of cooling is created by pressing a gas to a low pressure state.

3.There are a lot of different properties concerning various types of cycles.

4.The type of electric current influences the cryocooler design.

5.The mostly widespread mechanical refrigeration systems are based on the alternating current flow.

6.Some of the systems use the direct current flows.

7.For calculation the factor of work special equation has been deducted.

8.Almost in all alternatives for coolers small size and high performance are decisive.

9.Pulse Tube differs from the Stirling cycle by using the expander without moving parts.

10.Gifford-McMahon cryocoolers combine the elements of the Stirling cycle and the Pulse Tube.

11.All household appliances are designed as Joule-Thomson systems.

12.Four main factors are the sources of efficiency of the reverse-Brayton cryocooler.

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