Giáo trình Tiếng Anh chuyên ngành - Kỹ thuật máy lạnh và điều hòa không khí

TIẾNG ANH CHUYÊN NGÀNH

KỸ THUẬT MÁY LẠNH VÀ ĐIỀU HÒA KHÔNG KHÍ

UNIT 1: FUNDAMENTALS

PART 1: READING COMPREHENSION:

1.1. Reading text: Fundamentals:

A. FUNDAMENTAL OF AIR CONDITIONER:

1. What is air conditioning?

An air conditioning is defined as the process of treating air so as to control

simultaneously its humidity, cleanness, distribution as well as temperature to meet the

requirement or the conditioned space. It is a part of refrigeration in a wide sense.

2. Basic principles of air - conditioning

Most air conditioner uses the evaporation of a refrigerant, like Freon, to provide

cooling. The mechanics of the Freon evaporation cycle are the same as in an air conditioner.

Air conditioning systems are based on four principles. First, the compressor compresses

cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram

above). Second, this hot gas runs through a set of coils so it can dissipate its heat, and it

condenses into a liquid. Third, the Freon liquid runs through an expansion valve, and in the

process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram

above). Fourth, this cold gas runs through a set of coils that allow the gas to absorb heat and

cool down the air inside the building.

Figure 1 Diagram of a typical air conditioner

Most air conditioning systems have five mechanical components such as a

compressor, a condensor, an evaporator coil, a blower and a chemical refrigerant.

B. FUNDAMENTAL OF REFRIGERATOR:

1. What is refrigeration?

Refrigeration is defined as the process of reducing and remaining the temperature of

a space or material below the temperature of a space or material below the temperature of

the surroundings.

2. Refrigerants

Refrigerants are fluids that change their state upon the application or removal of heat

within a system and, in this act of change, absorb or release heat to or from an area or

substance. Fluid which is suitable for as a refrigerant need to have the following properties

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including: inexpressive, non explosive, non poisonous, non flammable, non corrosive, stable

(inert), high latent heat of vaporization, easy to vaporize and condense and easy to detect

leaks. Today fluorinated hydrocarbon refrigerants are being used almost exclusively in air-

conditioning system.

3. Basic principles of refrigeration

The principle of refrigeration is to remove heat from one area (i.e inside your fridge)

and locate it to another area (i.e. outside of your fridge). Air is not brought in from the

outside of the fridge the heat is absorbed by the evaporator inside the fridge which has

refrigerant inside it, this refrigerant at low pressure is at low temperature inside the

evaporator so the heat from the product inside the fridge is absorbed by the evaporator

which has a fan to circulate the air around the fridge. Then the refrigerant is pushed around

the pipe work by the compressor to the condenser where the refrigerant is hot from the heat

out of the fridge. The outside air is absorbed by the ambient air so when coming back into

the evaporator, it can absorb more heat and expel it into the ambient air. Normally a

refrigeration system consists of 5 main components: a compressor, a condenser, an

expansion device or capillary tube and an evaporator.

The mechanics of the Freon evaporation cycle are the same as in an air conditioner.

Air conditioning systems are based on four principles. First, the compressor compresses

cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram

above). Second, this hot gas runs through a set of coils so it can dissipate its heat, and it

condenses into a liquid. Third, the Freon liquid runs through an expansion valve, and in the

process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram

above). Fourth, this cold gas runs through a set of coils that allow the gas to absorb heat and

cool down the air inside the building.

1.2. Comprehension check:

* Task 1: Choose A, B, C or D which best completes each sentence:

1. Refrigerants are being used almost exclusively in today air - conditioning system such as

A. water

B. fluorinated hydrocarbon

C. methyl chloride

D. air

2. In the third principle of air conditioning systems, the hot gas runs through….. before

evaporating to become cold, low-pressure Freon gas.

A. a set of coils

B. evaporator coil

C. capillary tube

D. expansion device

* Task 2: Indicate whether the sentence or statement is true or false. WRITE (T) OR ( F):

1. When two substances of different temperatures are brought in contact with each other,

heat will flow from the colder substance to the warmer substance.

2. Freon in the compressor remains in the gas state.

3. When a gas is compressed, it heats up, but when a gas expands, it cools down.

4. The warm gas passed through the condenser, where it heats the surroundings and cools

the down.

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5. There are 4 major components in a refrigerator such as a compressor, a condenser, an

expansion device.

* Task 3: Answer the questions:

1. How does air - conditioning work?

2. What are four basic principles of refrigeration?

3. Which properties does a refrigerant need to have?

4. How many principles are air - conditioning system based on?

5. What refrigerants are widely used in today air - conditioning system?

PART 2: LANGUAGE WORK:

Grammar: Present simple

Form: Present Simple Tense

(+) Positive: S + V- infinitive + O

S + V(s/es)+ O

(-) Negative: S + don’t + V-infinitive + O

S + doesn’t + V-infinitive + O

(?) Question:

+ Yes/No question:

Do/Does + S + V-infinitive + O?

+ Wh-question:

Wh- + do/does + S + V-infinitive +O?

Practice:

* Task 1: Describing shapes

+ Study these nouns and adjectives for describing the shapes of objects:

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+ Describe the shapes of the following objects as examples:

A square wave

An H - shaped antenna

a saw - tooth wave

1. A ceramic capacitor

2. Transformer laminations

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* Task 2: Describing position and connection:

When describing the position of a component or how it is connected in a circuit, phrases of

this pattern are used:

Be + past participle + preposition

Example:

1. The tuning capacitor is connected across the coil

2. The semiconductor rectifier is mounted on the heat sink

Be+ past participle + preposition

Complete each sentence using an appropriate phrase from this list:

Wound round

Connected across

Mounted on

Wired to

located within

applied to

connected to

connected between

1. The bulbs are……the battery

2. The core is …....the pole pieces

4. The antenna is ……….the coil

3. The 27pF capacitor is …...the

collector and the base

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5. Feedback voltage is …….the base

6. The rotor is ……....the shaft

of the transistor through C₁

7. The coil is …….an iron core

8. The negative pole of the

battery……..earth

* Task 3: Put the set of words and phrases given into the correct order to make

meaningful sentences:

1. Most/ uses /air /conditioner/ the /refrigerant/ like /evaporation/Freon.

2. Freon/ liquid/ the/ an/ through/ valve/ expansion.

3. system/ of/ 5/ consists/ refrigeration/ a/ normally.

4. cold/ runs/ a/ set/ the/ of/ coils/ through/ that/ gas/ the/ allow/ absorb/ heat/ to.

* Task 4: Translate the sentences into Vietnamese:

1. A refrigerant is a heat carrier to move heat from a room to be cooled to the outside.

2. When the temperature outside begins to climb, many people seek the cool comfort of

indoor air conditioning.

3. Refrigerant is drawn from the evaporator and pumped to the condenser by the

compressor.

4. Refrigeration is the process of removing heat from an area or a substance and is usually

done by an artificial means of lowering the temperature, such as the use of ice or mechanical

refrigeration.

5. Mechanical refrigeration is defined as a mechanical system or apparatus so designed and

constructed that, through its function, heat is transferred from one substance to another.

* Task 5:Translate the sentences into English:

1. Làm lạnh và điều hòa không khí được sử dụng để làm mát sản phẩm hoặc môi trường của

tòa nhà. Hệ thống làm lạnh và điều hòa không khí hấp thụ nhiệt từ nơi cần làm mát và

truyền nhiệt hấp thụ ra khu vực khác, có nhiệt độ cao hơn.

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2. Sự làm lạnh thực hiện theo 3 nguyên lý: Thứ nhất, nếu chất lỏng được làm nóng, nó sẽ

chuyển thành khí hoặc hơi. Khi khí này được làm lạnh, nó lại trở lại trạng thái lỏng. Thứ

hai, nếu một khí có thể giãn nở được, nó sẽ lạnh đi. Nếu một chất khí được nén, nhiệt độ của

nó tăng lên. Thứ ba, chất lỏng giữ ở áp suất thấp hơn áp suất xung quanh, nó có thể sôi.

3. Một hệ thống làm lạnh thông thường gồm 4 thành phần chính: máy nén, dàn ngưng, van

giãn nở hoặc ống mao dẫn và dàn bay hơi.

PART 3: FURTHER READING

Domestic Refrigerators:

Most domestic refrigerators are of two types - either a single door fresh food

refrigerator or a two-door refrigerator - freezer combination. Most refrigerators use R - 22

refrigerant, normally maintaining temperatures of 0°F in the freezer compartment and about

35°F to 45°F in the refrigerator compartment.

Single Door Fresh Food Refrigerator:

A single door fresh food refrigerator consists of an evaporator placed either across

the top or in one of the upper corners of the cabinet. The condenser is on the back of the

cabinet or in the bottom of the cabinet below the hermetic compressor. During operation, the

cold air from the evaporator flows by natural circulation through the refrigerated space. The

shelves inside the cabinet are constructed so air can circulate freely past the ends and sides,

eliminating the need for a fan. This refrigerator has a manual defrost, which requires that the

refrigerator be turned off periodically (usually overnight) to enable the buildup of frost on

the evaporator to melt.

Two - Door Refrigerator-Freezer Combination:

The two - door refrigerator-freezer combination is the most popular type of

refrigerator. It is similar to the fresh food refrigerators in construction and the location of

components except it sometimes has an evaporator for both the freezer compartment and the

refrigerator compartment. Also, if it is a frost - free unit, the evaporators are on the outside

of the cabinet. Because of the two separate compartments (refrigerator - freezer) and the

larger capacity, these types of refrigerators use forced air (fans) to circulate the air through

the inside of both compartments. The two - door refrigerator also has one of the following

three types of evaporator defrost systems: manual defrost, automatic defrost, or frost -free.

Today, the two - door refrigerator-freezer combination is widely used in almost houses.

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UNIT 2: COMPRESSORS

PART 1: READING COMPREHENSION:

1.1. Reading text: Compressors:

The refrigeration system consists of four basic components including the compressor,

the condenser, the liquid receiver, the evaporator, and the control devices. The compressor

is one of indispensable components for any refrigeration system to operate.

The main purpose of refrigeration compressors is to withdraw the heat-laden

refrigerant vapor from the evaporator and compress the gas to a pressure that will liquefy in

the condenser. Compressors are designed depending upon the application and type of

refrigerant. According to the principle of operation, there are three types of compressors:

reciprocating, rotary, and centrifugal. Many refrigerator compressors have components

besides those normally found on compressors, such as unloaders, oil pumps, mufflers, and

so on.

External Drive Compressor:

An external drive or open-type compressor is bolted together. Its crankshaft extends

through the crankcase and is driven by a flywheel (pulley) and belt, or it can be driven

directly by an electric motor. A leak proof seal must be maintained where the crankshaft

extends out of the crankcase of an open - type compressor. The seal which holds the

pressure developed inside of the compressor prevents refrigerant and oil from leaking out

and air and moisture from entering the compressor. Two types of seals are used such as the

stationary bellows seal and the rotating bellows seal while an internal stationary crankshaft

seal consists of a corrugated thin brass tube (seal bellows) fastened to a bronze ring (seal

guide) at one end and to the flange plate at the other.

Hermetic Compressor:

In the hermetically sealed compressor, the electric motor and compressor are both in

the same airtight (hermetic) housing and share the same shaft. After assembly, the two

halves of the case are welded together to form an airtight cover. The compressor, in this

case, is a double - piston reciprocating type, whereas, other compressors may be of the

centrifugal or rotary types. Cooling and lubrication are provided by the circulating oil and

the movement of the refrigerant vapor throughout the case. The advantages of the

hermetically sealed unit (elimination of pulleys, belts and other coupling methods,

elimination of a source of refrigerant leaks) are offset somewhat by the inaccessibility for

repair and generally lower capacity.

1.2. Comprehension check:

* Task 1: Choose A, B, C or D which best completes each sentence:

1. The main function of a compressor is to:

A. condense the high - pressure refrigerant vapor into liquid refrigerant and make ir easier

to vaporize.

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B. reduce and maintain temperature of a space or material below the temperature of a

space or material below the temperature of the surroundings.

C. withdraw the heat-laden refrigerant vapor and compress the gas to a pressure that will

liquefy.

D. change cool refrigerant back into liquid form.

2. Select the appropriate type of compressor to handle oil or water aerosol contamination.

There are three types of compressors classified according to the principle of operation—

reciprocating, rotary, and centrifugal.

A. Separator filter

B. Particulate filter C. Coalescing filter D. Adsorption filter

3. A bronze ring is also called:

A. seal bellows

B. stationary bellows C. rotating bellows

D. seal guide

4. The hermetically sealed unit remains some drawbacks:

A. elimination of pulleys, belts, other coupling methods and a source of refrigerant leaks

B. inaccessibility for repair and

C. lower capacity

D. B and C are all correct

5. A component which has the functions as holding the pressure increased inside of the

compressor and preventing refrigerant and oil from leaking out and air and moisture from

entering the compressor is:

A. the seal

B. the oil pump

C. the muffler

D. the crankshaft

* Task 2: Indicate whether the sentence or statement is true or false. WRITE ( T) OR ( F):

1. There are three types of compressors used in refrigeration systems.

2. One of drawbacks of a hermetic compressor is difficult to repair.

3. A refrigeration compressor has one purpose to compress the gas to a pressure that will

liquefy in the condenser.

4. Seals in an external drive compressor has four types.

5. In a hermetic compressor, a piston is often a centrifugal or a rotary type.

* Task 3: Answer the questions

1. How many types of seals does an External Drive Compressor has?

2. What are the two drawbacks of a hermetic compressor?

3. What are main components in the refrigeration system?

4. How are cooling and lubrication provided for the Hermetic Compressor?

5. What are differences between the External Drive Compressor and the Hermetic

Compressor?

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PART 2: LANGUAGE WORK

Relative Clause (1):

We use relative clauses to give additional information about something without

starting another sentence. By combining sentences with a relative clause, your text becomes

more fluent and you can avoid repeating certain words.

How to Form Relative Clauses:

Imagine, a girl is talking to Tom. You want to know who she is and ask a friend

whether he knows her. You could say:

A girl is talking to Tom. Do you know the girl?

That sounds rather complicated, doesn't it? It would be easier with a relative clause:

you put both pieces of information into one sentence. Start with the most important thing –

you want to know who the girl is.

Do you know the girl …

As your friend cannot know which girl you are talking about, you need to put in the

additional information – the girl is talking to Tom. Use „the girl― only in the first part of the

sentence, in the second part replace it with the relative pronoun (for people, use the relative

pronoun „who―). So the final sentence is:

Do you know the girl who is talking to Tom?

Relative Pronouns:

relative

use

example

pronoun

Who

subject or object pronoun for people

I told you about the woman

who lives next door.

Which

Whose

Whom

subject or object pronoun for animals and

things

Do you see the cat which is

lying on the roof?

Referring to a whole sentence

He couldn’t read which

surprised me.

possession for people animals and things

Do you know the boy whose

mother is a nurse?

object pronoun for people, especially in non- I was invited by the professor

defining relative clauses (in defining relative whom I met at the conference.

clauses we colloquially prefer who)

that

subject or object pronoun for people, animals I don’t like the table that

and things in defining relative clauses (who

or which are also possible)

stands in the kitchen.

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Subject Pronoun or Object Pronoun?

Subject and object pronouns cannot be distinguished by their forms - who, which,

that are used for subject and object pronouns. You can, however, distinguish them as

follows:

If the relative pronoun is followed by a verb, the relative pronoun is a subject

pronoun. Subject pronouns must always be used.

the apple which is lying on the table

If the relative pronoun is not followed by a verb (but by a noun or pronoun), the

relative pronoun is an object pronoun. Object pronouns can be dropped in defining relative

clauses, which are then called Contact Clauses.

the apple (which) George lay on the table

Relative Adverbs:

A relative adverb can be used instead of a relative pronoun plus preposition. This

often makes the sentence easier to understand.

This is the shop in which I bought my bike.

→ This is the shop where I bought my bike.

relative

meaning

use

example

adverb

When

in/on which refers to a time expression

in/at which refers to a place

the day when we met him

Where

Why

the place where we met

him

for which

refers to a reason

the reason why we met

him

* Task 1: Put the set of words and phrases given into the correct order to make

meaningful sentences.

1. system/ of/ four/ the/ basic/ including/ refrigeration/ the/ compressor/ condenser/ the/

liquid receiver/ the/ evaporator/ the/ devices/ the/ control.

2. in/ external/ seals/ compressor/ four/ has/ types/ an.

3. types/ are/ types/ used/ as/ the/ two/ bellows/ seal/ stationary/ and/ seal/ bellows/ the/

rotating.

* Task 2: Combine the following sentences, using relative pronouns.

Examples:

1. Starter motor brushes are made of carbon.

2. The carbon contains copper

1 + 2: Starter motor brushes are made of carbon which contains copper

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3. Industrial consumers are supplied at higher voltages than domestic consumers

4. These consumers use large quantities of energy

3+4. Consumers who use large quantities of energy are supplied are higher voltages

than domestic consumers.

5. 33kV lines are fed to intermediate substations

6. In the intermediate substations the voltage is stepped down to 11kV

5+6: 33kV lines are fed to intermediate substations where the voltage is stepped

down to 11 kV

Now link these sentences. Make the second sentence in each pair a relative clause.

1. The coil is connected in series with a resistor

The resistor has a value of 240 ohms

2. The supply is fed to a distribution substation

The supply is reduced to 415 V in the distribution substation

3. Workers require a high degree of illumination

The workers assemble very small precision instruments

4. Manganin is a metal

This metal has a comparatively high resistance

5. The signal passes to the detector

The signal is rectified by the detector

6. A milliammeter is an instrument

The instrument is used for measuring small currents

7. Workers require illumination of 300 lux

The workers assemble heavy machinery

8. Armoured cables are used in places

There is a risk of mechanical damage in these places

* Task 3: Reason and result connectives 1

Study the examples:

1. Copper is used for cables

2. Copper is a good conductor

1+2. Copper is used for cables because it is a good conductor

3. The flow of electrons through an insulator is very small

4. The flow can be ignored

3 + 4: The flow of electrons through an insulator is very small, therefore it can be ignored.

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Use because or therefore to link these sentences into one as the above examples:

Soft iron is used in electromagnets. Soft iron can be magnetized easily.

1. The voltage is 250V and the current is 5A. The resistance is 50 ohms.

2. PVC is used to cover cables. PVC is a good insulator.

3. Transistors can be damaged by heat. Care must be taken when soldering transistors.

4. Capacitance is usually measured in microfarads or picofarads. he farad is too large a unit

5. Output transistors are mounted on a heat sink. Output transistors generate heat.

6. It is easy to control the speed of dc motors. Dc motors are used when variable

are required.

speeds

7. A cathode ray tube screen glows when an electron beam strikes it. The screen is coated

with a phosphor.

* Task 4: Use the suggested words to join the following groups of sentences:

1. or

Circuits can be protected from excessive currents by a fuse.

Circuits can be protected from excessive currents by a circuit breaker.

2. however

A fuse is the simplest and cheapest protection.

For accurate and repetitive operation a circuit breaker is used.

3. which

The simplest circuit breaker consists of a solenoid and a switch with contacts.

The contacts are held closed by a latch.

4. thus energizing

The current from the supply line flows through the switch and solenoid coil.

This energizes the solenoid.

5. which, therefore

At normal currents the pull of the solenoid on the latch will not overcome the tension of the

spring.

The spring holds the latch in place.

The switch remains closed.

6. if

The current rises to a dangerous level.

The pull of the solenoid on the latch increases.

7. and

The increased pull overcomes the latch spring tension.

The increased pull pulls the latch towards the solenoid.

8. which

This releases the switch contacts.

The switch contacts are pulled apart by a spring.

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9. as

The circuit is now broken.

The unit is protected.

10. when

The fault in the supply or unit is put right.

The latch can be reset.

* Task 5: Translate the sentences into Vietnamese:

1. The designs of compressors vary, depending upon the application and type of refrigerant.

2. Oil from the crankcase lubricates the surfaces of the seal guide and seal ring.

3. Two types of seals are used—the stationary bellows seal and the rotating bellows seal.

4. The refrigeration system consists of four basic components such as the compressor, the

condenser, the liquid receiver, the evaporator, and the control devices.

5. If there is liquid in the vapor as it enters the compressor, this can be harmful to the

compressor.

* Task 6: Translate the sentences into English:

1. Thiết bị làm lạnh hấp thụ là một máy tạo ra nước lạnh bằng cách sử dụng nhiệt như hơi,

nước nóng, khí, dầu, vv… Nước lạnh được tạo ra dựa trên nguyên tắc là dung dịch (tức là

môi chất lạnh, bốc hơi ở nhiệt độ thấp) hấp thụ nhiệt từ xung quanh khi bốc hơi. Nước tinh

khiết được sử dụng làm môi chất lạnh và dung dịch lithi bromua (LiBrH2O) được sử dụng

làm chất hấp thụ.

2. Môi chất lạnh (nước) bay hơi ở nhiệt độ khoảng 4oC trong điều kiện chân không 754 mm

Hg ở thiết bị bay hơi. Nước lạnh đi qua ống của bộ trao đổi nhiệt trong thiết bị bay hơi và

truyền nhiệt cho môi chất lạnh đã hoá hơi.

3. Khi đã mua máy nén, bảo trì hiệu quả là yếu tố then chốt giúp tối ưu hoá mức tiêu thụ

năng lượng.

4. Để máy nén hoạt động hiệu quả, tỷ suất nén phải thấp, để giảm áp suất và nhiệt độ đẩy.

5. Với những thiết bị ứng dụng nhiệt độ thấp có tỷ suất nén cao, và cần dải nhiệt độ rộng,

sử dụng máy nén pittông đa cấp hoặc máy nén ly tâm/trục vít thường được ưa chuộng hơn

và mang tính kinh tế hơn (do hạn chế trong thiết kế thiết bị).

PART 3: FURTHER READING

Open Types of Compressors

Some of the duties you may perform in maintaining this and other open-type

compressors are discussed in this lesson.

Shaft Bellows Seal

Refrigerant leakage often occurs at the shaft bellows seal with consequent loss of

charge. Install a test gauge in the line leading from the drum to the compressor. Attach a

refrigerant drum to the suction end of the shutoff valve outlet port. Apply the proper amount

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of pressure, as recommended in the manufacturer's instructions. Test for leaks with a halide

leak detector around the compressor shaft, seal gasket, and seal nut. Slowly turn the shaft by

hand. When a leak is located at the seal nut, replace the seal plate, gasket, and seal

assembly; when the leak is at the gasket, replace the gasket only. Retest the seal after

reassembly.

Valve Obstructions

Obstructions, such as dirt or corrosion, may be formed under seats of suction or

discharge valves.

When the suction side is obstructed, the unit tends to run continuously or over long

periods. Connect the gauge manifold and start the unit. This pressure gauge (HI) will not

indicate an increase in pressure.

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UNIT 3: COMMISSIONING AND MAINTENANCE

PART 1: READING COMPREHENSION:

1.1 Reading text: COMMISSIONING AND MAINTENANCE:

The commissioning of a refrigeration or air - conditioning plant starts from the stage

of static completion and progresses through the setting – to - work procedure and regulation

to a state of full working order to specified requirements. Commissioning is the completion

stage of a contract, when the contractor considers that the plant is in a correct state to hand

over to the purchaser for acceptance and payment

Maintenance is the effort required to ensure that a commissioned plant continues to

deliver correct performance, and includes inspections designed to detect signs of

deterioration prior to any noticeable effects.

The work of commissioning must be under the control of a single competent

authority, whether it be the main contractor, a consultant or the user. It can happen that

when building specifications are written, they split the responsibilities to sub - contractors

with the presumption that everything will be exactly as specified after commissioning,

which it rarely is. With no one responsibility for the total operation of the cooling system it

can be found that it operates, but in an unsatisfactory manner - for example, frequent

cycling instead of steadily operating at reduced capacity.

Complex systems need to have follow - up on the initial commissioning at low,

medium and high load (warm, medium and cold ambient) to ensure proper and energy

efficient operation. To include this as a responsibility in the contract is very cost - effective.

The commissioning engineer will require details and ratings of all major items of the

plant and copies of any manufacturer’s instructions on setting to work and operating their

products. If this information is not to hand, the work will be delayed. The following stages

may be identified in a commissioning process.

The installation should be initially checked to ensure that it is in accordance with the

specified design. The next stage is to preset as many controls and protection devices as pos-

sible

The refrigerant charge and system operation should be checked and set for the

complete range of load and ambient conditions. Follow - up tests may be required to cover a

range of ambient conditions. The refrigerant charge should have been added according to

the weight specified, and additional charge should not be required. When the system has

been operating for a sufficiently long period, conditions in the cooled space can be checked

against specification, and other typical points to watch for include evaporator superheat and

refrigerant distribution, excessive pressure drops, compressor oil levels, correct condensing

pressure. The operation of pressure controls such as evaporator pressure regulator and

condensing pressure controls together with defrost systems can be checked.

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In the final commissioning stage, readings are taken and recorded and compared with the

specification and design figures. Some final adjustments to air flows, secondary fluid flows,

etc. may be necessary

Grammar: Relative Clauses 2

Defining Relative Clauses

Defining relative clauses (also called identifying relative clauses or restrictive

relative clauses) give detailed information defining a general term or expression. Defining

relative clauses are not put in commas.

Imagine, Tom is in a room with five girls. One girl is talking to Tom and you ask

somebody whether he knows this girl. Here the relative clause defines which of the five girls

you mean.

Do you know the girl who is talking to Tom?

Defining relative clauses are often used in definitions

A seaman is someone who works on a ship.

Object pronouns in defining relative clauses can be dropped. (Sentences with a

relative clause without the relative pronoun are called Contact Clauses.)

The boy (who/whom) we met yesterday is very nice.

Non-Defining Relative Clauses

Non-defining relative clauses (also called non-identifying relative clauses or non-

restrictive relative clauses) give additional information on something, but do not define it.

Non-defining relative clauses are put in commas.

Imagine, Tom is in a room with only one girl. The two are talking to each other and

you ask somebody whether he knows this girl. Here the relative clause is non-defining

because in this situation it is obvious which girl you mean.

Do you know the girl, who is talking to Tom?

Note: In non-defining relative clauses, who/which may not be replaced with that.

Object pronouns in non-defining relative clauses must be used.

Jim, who/whom we met yesterday, is very nice.

How to Shorten Relative Clauses?

Relative clauses with who, which, that as subject pronoun can be replaced with a

participle. This makes the sentence shorter and easier to understand.

I told you about the woman who lives next door. – I told you about the woman living

next door.

Do you see the cat which is lying on the roof? – Do you see the cat lying on the roof?

* Task 1: Making definitions.

One use of defining relative clauses is to make definitions. Study this diagram.

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is

(a) a solar cell

which

c) converts sunlight

(b) an electric cell

into electrical

We can make a definition of a solar cell by joining (a), (b) and (c)

A solar cell is an electric cell which converts sunlight into electrical energy.

Combine (a), (b) and (c) to make correct sentences as the above Examples:

(a)

1. A generator

(b)

A material

An instrument

A current

(c)

- Measures light

2. An insulator

- Readily releases electrons

- Flows first in one direction, then

in the other

3. An alternating current

4. A direct current

5. A resistor

A device

- Does not readily release electrons

Impedes the flow of current in a

circuit

6. A conductor

7. A light meter

8. An ammeter

- Measures current

- Converts mechanical energy into

electrical energy

- Flows in one direction only

* Task 2: Adding information to a passage:

Use non-defining relative clauses to add extra information to this paragraph about a

relay. This extra information is given bellow the paragraph. Then translate it into

Vietnamese:

Electric relays (1).............utilize the magnetic effect of a current in a solenoid. One

of the most common types (2) ............consists of a solenoid with a soft-iron core. When

energized by a suitable dc current, the solenoid attracts an armature (3) ............ The

armature is pivoted in such a way that it pushed together or pulls apart a set of contacts (4)

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.............. These contacts (5) ............ control one or more circuits. Normally these circuits

draw a much heavier current than the relay coil itself.

1. Electric relays are widely used in telecommunications

2. The most common type of relay is known as the hinged armature relay

3. The armature is also made of soft iron

4. The sets of contacts are mounted on the body of the relay

5. The contacts are often made of platinum.

* Task 3: Combine the following sentences, using relative clauses or reduced relative

clauses:

1. In Britain electrical energy is fed to the National Grid. The energy is generated in power

stations.

2. The energy passes though a transformer. The transformer steps up the voltage to 132, 25,

or 400kV.

3. The transmission lines are usually arranged in groups of three overhead conductors. The

lines carry the supply. The overhead conductors are suspended from porcelain insulators.

4. Energy from power stations may be fed to the Grid by underground cables. The power

stations are located in built-up areas.

5. For voltages up to 400 kV cables are used. These cables contain oil under low pressure.

* Task 4: Reason and result connective 2:

We learned that BECAUSE links a statement and a reason and that THEREFORE

links a statement and a result. The following connectives can also be used:

Statement

Example:

reason

Since /As/ For the reason that

result

Hence/Consequently/For this reason

Dc motors are used for cranes, for the reason that their speed can be finely

controlled.

The current rose above the maximum. Consequently the circuit breaker opened.

Copper is often used for cables since it is a good conductor.

Now link these ideals with either reason, result or qualification connectives:

1. Conventional current flow is from positive to negative. In fact electrons flow from

negative to positive.

2. Alternators are preferred to dynamos for cars. Alternators give higher outputs at low

speeds.

3. Dirt and dust reduce effective light. Lamps must be kept clean.

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4. Squirrel cage motors are simple, cheap and strong. Squirrel cage motors are used for

many general duties.

5. It is convenient to describe magnetic lines of force. In reality magnetic lines of force do

not exist.

6. Transistorized equipment is easily portable. Transistors can operate from battery voltages.

7. Ultrasonic welding is better than heat welding. The materials are not distorted.

8. Watchmakers work with very small parts. Watchmakers require a lot of light.

* Task 5: Join the following group of sentences to make 10 longer sentences. You may

omit words and make whatever changes you think are necessary in the word order and

punctuation of the sentences.

1. A zinc case is used as a container for the cell. The zinc case is used as the negative

electrode.

2. A carbon rod forms the negative electrode. The carbon rod is in the centre of the cell.

3. The space between the zinc case and the carbon rod is filled with a paste of ammonium

chloride. The paste is used as an electrolyte.

4. The electrolyte is a paste and not a liquid. This type of cell is called a dry cell.

5. The paste also contains manganese dioxide. The manganese dioxide prevents gas being

formed.

6. The cell is sealed with a cap. The cap is made of metal or plastic. The cap is to prevent

the paste coming out.

7. A small space is left below the cap. Gas formed by the cell can collect in the space.

8. Dry cells are usually enclosed in a cardboard case. An additional metal jacket may be

added. The jacket makes the cell leakproof.

9. Leakproof cells are often preferred. The electrolyte cannot leak out.

10. Leaking electrolyte may damage the equipment. The cells are installed in the equipment.

PART 3: FURTHER READING:

FAULT - FINDING

System faults fall into two general classes: the sudden catastrophe of a mechanical

breakdown and the slow fall-off of performance which can be detected as a malfunction in

its early stages, but will also lead to a breakdown if not rectified. Identification of the first

will be obvious. To track down the cause of a malfunction is more complicated.

Fault tracing is seen as a multi-step process of deduction, ending in normal operation

again and a record of the incident to inform other operatives. The steps are as follows:

1. Detection, i.e. detection of abnormal operation.

2. Knowledge of the system to track down the cause.

3. Observation of exact operating conditions.

4. I dentification of the fault.

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