masterclass air conditioning fundamentals

7/30/2019 Masterclass Air Conditioning Fundamentals

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Published on 1 - January - 2006

Masterclass:Air Conditioning Fundamentals - Part 1

Mike Creamer of Business Edge revisits his Masterclass series of

articles, updating and adding to the information which proved so

useful to readers when the series was first published ten years ago. Inthis reincarnation, the series will cover both air conditioning and

refrigeration and serve as an ongoing source of technical reference forexperienced personnel as well as providing a solid educational

grounding for newcomers to our industry.

What is air conditioning?

Full air conditioning implies the control of temperature and humiditylevels within a conditioned space. Control of pressure is occasionally

required for special applications. Air conditioning will always includethe ability to reduce the temperature and humidity level of the air

being processed. Most equipment will offer the option of heating toraise air temperature and more specialised equipment has an optional

humidification facility to raise air humidity levels. Air conditioning also

includes the control of motion of the air and the regulation of puritylevels. This can be summarised as follows:
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Fig 2: The relationship between force, area and pressure

Temperature

Air temperature is controlled by the removal (cooling) or addition(heating) of sensible heat energy.

Humidity

Air humidity level is determined by the moisture content and iscontrolled by the removal (dehumidification) or addition

(humidification) of latent heat energy.

Purity

Air purity is a measure of cleanliness or air quality and is controlled by

filtration and/or ventilation. Ventilation is the controlled introduction ofoutside (ambient) fresh air into the conditioned space to dilute the

concentration of contaminants.

Motion

Motion or air movement covers the distribution and velocity of air

introduced to the conditioned space. This is controlled by the air

conditioning unit(s) or an air distribution system (ductwork and

grilles).

Sound

Sound or noise control may be required to attenuate (reduce) the

noise generated by the equipment and distribution system.


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Air conditioning is normally applied to maintain the comfort and

working efficiency of people or manufacturing, industrial and scientificprocesses. The combination of the above elements has generated the

term HVAC but an air conditioning system need not include all these

elements. For thousands of years mankind relied upon fires for heatingand cool water for relief in high temperatures.

The Romans conceived an effective form of radiant heating in certain

buildings by circulating heated air through hollow walls and floors. In

the warmer climates of the Middle and Far East, wet mats suspendedin open doorways provided evaporative cooling. The incredible

Leonardo da Vinci designed and built an enormous evaporative coolerin the 15th century. This machine consisted of a large drum rotated by

water power (or by slaves when available) which drew in air and

supplied this to the conditioned space after washing and cooling the airinside the drum.

Only within the last hundred years has air conditioning become

established and during this time the technology has developed to veryadvanced levels. Air conditioning is now a major industry throughout

the world worth billions of pounds annually.

Temperature

The temperature reading for air given by a normal thermometer or

digital thermometer is defined as dry bulb temperature (C). Thehigher the sensible heat energy content of the air, the higher the drybulb temperature.

Percentage saturation

This is the ratio of the actual moisture content of the air in relation to

the maximum moisture content the air could support at the same drybulb temperature. (Please note that the term relative humidity should

not be used in this context.)

Human comfort

The objective of all comfort air conditioning installations and systems

is to ensure the comfort of individuals in the conditioned zone and thisis achieved by control of temperature and percentage saturation levels

within prescribed limits. Following studies by ANSI (the American


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National Standards Institute) and ASHRAE (the American Society ofHeating, Refrigeration and Air Conditioning Engineers), which

measured the effects of temperature, humidity (percentagesaturation), air motion and clothing on human comfort, the

ANSI/ASHRAE Standard 55-1981 was developed. The results are very

extensive but, for our general purposes, the ideal comfort envelope isas defined in Fig1.

Basic definitions and principles

This series of articles is based purely on SI units.

MassThe mass of an object is the quantity of matter it contains. Unit of

measure kg.

Force

Force is the push or pull exerted by one body on another. Unit ofmeasure: N (Newton).

Weight

The weight of a body is equal to the force exerted upon it by the

gravitational attraction of the Earth. At sea level, the Earth exerts aforce of 9.81N on each kg of matter (N/kg). Force and weight are in

fact different although the unit kg is used for both.

Density and specific volume

Density is the mass per unit volume of a substance and is expressed

as kg/m3:

Specific volume defines the volume occupied per unit mass and is

expressed as m3/kg:

Density and specific volume vary with temperature and pressure.

Specific gravity


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Specific gravity (sg) is defined as the ratio of the weight of a

substance in relation to the weight of an equal volume of water. Asthis is a ratio there are no units of expression.

Specific gravity changes with temperature. For non-scientificcalculations this can usually be ignored. The density of water is

l000kg/m3 (at 4C).

Pressure

Pressure is given by unit force divided by area and is normallyexpressed as N/m2:

There are many units for the expression of pressure and these include

kN/m2, Pa or kPa, mbar or bar and, of course, Ib/in2. Numerousfactors exist for conversion from one unit to another.

Example: Fig 2 shows a tank containing 1m3 of water weighing

1000kg. The downward force due to gravity is equal to 9.81N/kg. The

total force exerted on the base of the tank is therefore 9810N.

Alternatively, this could be expressed as:


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Atmospheric pressure

Atmospheric pressure is created by the gravitational force exertedupon the atmosphere throughout the full height of approximately

80km (50 miles). This results in an atmospheric pressure at sea level

of approximately 1.01325bar (14.7psi).

Atmospheric pressure therefore decreases with increasing altitude dueto the reduced weight of air above the measured point.

Atmospheric pressure is directly related to the density of the air andsince this varies with temperature, atmospheric pressure also varies.

This is measured by a barometer to indicate imminent weatherchanges.

Fig 3: pressure-zero, absolute, guage and vacuum

Absolute gauge and vacuum pressure

Absolute pressure (Pabs) is the pressure exerted by a gas or a liquid

above zero pressure. Zero pressure exists when a space is fully

evacuated of any gas or liquid.

Gauge pressure is measured by instruments that indicate a differencebetween the pressure of the atmosphere and the pressure of the gas

or liquid. It is therefore necessary to add atmospheric pressure to

gauge readings to establish absolute pressure.Vacuum pressure is measured by instruments that indicate the

difference between atmospheric pressure and zero pressure (see Fig3).

A conventional pressure gauge reads the positive pressure of a gas or

liquid above atmospheric pressure. A compound gauge is able toindicate both positive pressure and the negative pressure created by

vacuum pressure (see Fig 4).


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Fig 4

Pressure of a column of liquid

The pressure exerted by a column of liquid due to gravitational force isdependant upon the density of the liquid and the height of the column:

(see Fig 5)

Measurement of atmospheric pressure

Atmospheric pressure can be measured using a mercury barometer.The height (h) of the column of mercury (hg) supported by

atmospheric pressure allows a direct pressure reading to be obtainedand is normally indicated as inHg or mmHg (see Fig 6).


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Fig 5: pressure exerted by a column of liquid

Work, power and energyWork is described as the product of a force exerted upon a body

resulting in motion. This is expressed as:

Example: A water chiller weighing 1000kg is lifted from ground level tothe roof of a building. The vertical distance is 15m. The force applied is

exerted against gravity. Weight in kg must therefore be converted toNewtons:

Power defines the rate at which work is applied or absorbed. Power is

determined by:

Using the previous example, if the chiller is lifted to the roof in

5min(300s) the power required will be:-


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Energy is defined as the ability to perform work. A body possesses

energy when it has the capacity to perform work. Energy is measuredin Joules and takes a number of forms:

Kinetic energy is the energy possessed by a body due to motion orvelocity. Kinetic energy is given by:

Potential energy is the energy possessed by a body due to its positionor configuration. The amount of work done by a body when moving

from a given position or configuration to a reference position or

condition is the measure of the body's potential energy and is givenby:

Total external energy is the total of kinetic energy and potential

energy possessed by a body.


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Fig 6: Mercury barometer

Conversion of energy

The First Law of Thermodynamics basically states that the amount of

energy in a thermodynamic system is constant and that none can begained or lost unless it is converted from one form to another. Energy

takes many forms including mechanical energy, electrical energy,chemical energy and heat energy. Energy cannot therefore be

destroyed and is simply transferred from one body to another orconverted in form.

Published on 1 - March - 2006

Masterclass: Refrigeration Cycles - Part 3

THE PRIMARY refrigerant used in air conditioning for decades is R22(HCFC22). Whilst its ozone depletion potential (0.05) is much less

than other less friendly earlier refrigerants (ODP 1.0), it is stillnecessary to phase this refrigerant Accordingly, the majority of new

air conditioning systems now use out. However, there are still manysystems operating on R22 and this will be R410a. the case for someyears to come.

When we originally wrote this article, almost 10 years ago, R22 was apopular and widely used However, we considered revising the content

of the article below, refrigerant. basing this on refrigerant R410A.However, in order to be accurate we would then need to consider thefact that this refrigerant has a glide characteristic due to its mixed

composition. We have therefore decided to use R22 in our work below


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since its single component and non-glide characteristics are moresuited to the fundamentals that we are about to explain. The glide

characteristics of refrigerant such as R407C, R410A and R404A will becovered in due course.

Fig 1 provides a clear visual understanding of the behaviour of waterat a pressure of 1.01325 bar when heat energy is added or removed.

It is important to remember the key values for water shown in Fig 1 asthese can be used in many calculations for air conditioning and

refrigeration design and commissioning work.

Figure 1: Enthalpy (heat energy) values for water at 1.0325 bar

Steam tables

Changes in pressure above any liquid or vapour affect thetemperatures at which a change of state occurs and the amount of

heat energy involved. Steam tables list these different values over awide range of pressures. Table 1 (page 22) lists the values for water ata few selected pressures.

Note the substantial changes that occur in a specific volume and the

rate of change of pressure in relation to saturation temperature.

Observe the substantial amount of latent heat energy transfer involved


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in the change of state from Saturated Liquid to Saturated Vapour (orfrom Saturated Vapour to Saturated Liquid) and how this changes

considerably with pressure. This is far greater than the small amountof sensible heat energy associated with a change in temperature

alone. Clearly, a cooling system for air conditioning or refrigeration

using a circulating fluid would be most effective if the latent heatenergy transfer can be harnessed through a change of state.

The Vapour Compression Cycle and Absorption Cycle use this

characteristic very effectively. The Double Effect Absorption Cycle

actually uses water as the refrigerant. The Single Effect AbsorptionCycle uses ammonia as the refrigerant. A refrigerant can be defined as

a circulating fluid transferring heat energy from one part of the systemto another. The vapour compression cycle uses a wide range of

refrigerants according to application requirements including operating

temperatures, pressures, ambient temperatures and efficiency.Compare the characteristics of R22 (dichloroflouromethane in Table 2

with those of water in Table 1.

Pressure-enthalpy diagram

The figures in Table 2 have been taken from Tables of Refrigerant

Properties which fully define all the thermodynamic characteristics ofR22. These figures can also be obtained from software packages or

from the pressure-enthalpy diagram shown in Fig 2. The detailedoperating characteristics of a working vapour compression system can

be plotted on this diagram.

The vapour compression cycle

The primary purpose of an air conditioning or refrigeration system is to

remove heat energy at a low temperature from a conditioned space or

body and transfer (reject) this heat energy into another medium at ahigher temperature. Heat energy may be rejected into air, water or

soil.

This process is very often continuous since heat energy will alwayscontinue to flow from higher temperature surroundings into theconditioned space being maintained at lower temperature. Insulation

plays a major part in minimising this heat energy flow in lowtemperature applications.

Air conditioning and refrigeration heat load Heat energy flows from

higher temperature surroundings through the fabric of the conditioned


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space are termed Transmission or Conducted gains. There are manyother sources of heat gain to the conditioned space and these include

heat energy from:

Solar radiation energy striking the surfaces of the building or cold

store (sensible)

Warm, moist air entering the conditioned space through infiltrationor ventilation (sensible and latent)

Products or processes (sensible and latent)

Lighting, motors, machinery and computers (sensible)

Occupants (sensible and latent)

These are carefully calculated and the total is popularly known as the

Heat Load or Cooling Load. The heat load normally comprises SensibleHeat and Latent Heat and the air conditioning or refrigeration system

must be capable of removing these continuously. The sum of sensibleand latent heat energy is know as Total Heat or Total Enthalpy. Air

conditioning and refrigeration loads will be discussed later in the

series.

Figure 2: Pressure-enthalpy diagram containing all thermodynamic properties of R22


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The evaporator

Since the capacity of a refrigerant to absorb heat energy is greatest

when changing state from liquid to vapour, the heater exchanger(Evaporator) within the conditioned space is continuously supplied with

liquid refrigerant which vapourises in order to absorb heat energy fromthe conditioned space. Air is used to transport sensible and latent heat

energy from products, lights, machinery and occupants to the

evaporator. In order for this to be effective and efficient, anevaporator fan is used to pass return air over the evaporator coil and

to distribute conditioned air throughout the space. If liquid refrigerantR22 is allowed to vapourise at a pressure of 1.01325 bar, the

Saturation Temperature (or Evaporating Temperature) will be -48C.

As the refrigerant vaporises, heat energy is absorbed and is termedLatent Heat of Vaporisation. The latent heat of vaporisation (approx

140kj/kg) required to vapourise the liquid is taken from theconditioned space thus providing cooling. The heat energy from the

space is transferred to the vapour.

However, in order to maintain the conditioned space at a suitable

temperature for comfort air conditioning (22C), it is not necessary forthe refrigerant to change state at such a low temperature (evaporating

temperature). The design of the evaporator and the amount of airflowwill determine what evaporating temperature is required for a given

leaving air temperature from the evaporator coil. If an evaporatingtemperature of 5C were required, it would be necessary to raise theevaporating pressure to 5bar.


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Figure 3

Superheat

Note that some of the heat energy from the room is also raising thetemperature of the refrigerant above the saturation temperature

within the evaporator. This superheating of the refrigerant is essentialto protect the compressor from taking in liquid refrigerant which would

otherwise result in mechanical damage and failure. However, assuperheating the vapour does not absorb much heat energy from the

conditioned space and also makes poor use of the evaporator, itshould be maintained at a minimum level of 5-7K. The process taking

place is shown on the pressure/enthalpy diagram in Fig 3. The amount

of energy absorbed by the refrigerant during vaporisation is 167kJ/kg(latent heat of vaporisation) and a further 3kJ/kg has been absorbed

during superheating of the vapour. The total of these (170k)/kg isequal to the amount of cooling performed in the evaporator (and the

space) and is termed the Net Refrigerating Effect.

Figure 4

The compressor

The superheated refrigerant vapour leaving the evaporator must be

recycled and returned to liquid form for use at the evaporator. In order

for the refrigerant to be returned to a liquid state, it is necessary toremove heat energy by bringing the refrigerant into contact with a

medium (sink) at a lower temperature. If the system is to reject this


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heat energy to outdoor air during peak summer conditions where theair temperature may be as high as 30C, the saturation temperature

of the refrigerant must be raised from 5C to a higher temperaturethan 30C, say 40C. This is achieved by raising the pressure of the

saturated vapour leaving the evaporator by passing the vapour

through a compressor. This higher saturation temperature is known asthe Condensing Temperature. The process taking place within the

compressor is shown on the pressure/enthalpy diagram in Fig 4. Notethat the compression process follows the lines of Constant Entropy.

The refrigerant has taken up 23kJ/kg of energy during this process and

this is termed the Heat of Compression. The resulting leavingtemperature is now much higher than the entering temperature (70C)

and the condensing pressure has been raised to 15bar.

The condenser

The condenser must remove heat energy from the refrigerant and

reject this to a lower temperature medium (sink), usually outdoor(Ambient) air. Assuming an ambient temperature of 30C and a

condensing temperature of 40C, a temperature difference of 10K

exists. This is often known as the Approach Temperature.

Figure 5

Total heat of rejection


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The energy that must be rejected by the condenser comprises the heatenergy removed by each kg of refrigerant passing through the

evaporator (170kJ/kg) and the heat energy added to each kg ofrefrigerant passing through the compressor (23kJ/kg). The total heat

that must be rejected therefore equals 193kJ/kg and is termed the

Total Heat of Rejection (THR). The Condenser Coil is thereforenormally larger than the evaporator coil. The gas leaving the

compressor and entering the condenser is considerably superheated.The condenser must therefore Desuperheat the refrigerant first by 30K

until the saturation temperature of 40C is reached (70-40=30K).

When the refrigerant has reached the saturation temperature, thecondenser will then remove substantial latent heat energy as the

refrigerant changes state from saturated vapour to saturated liquid.This energy is known as the Latent Heat of Condensation (417-

250=167kJ/kg).

The condenser normally holds a small amount of liquid refrigerant at

the base of the coil. If the system is fitted with a Liquid Receiver, thecondenser coil holds very little liquid refrigerant as this is stored in the

liquid receiver. This liquid, at a starting temperature of 40C, losesfurther heat energy to the air passing over the coil at 30C (and whilst

residing in a liquid receiver). This causes the refrigerant to be Sub-

cooled to a temperature below the saturation temperature. Fig 5shows a loss of 10kJ/kg due to sub-cooling in the condenser coil and

the liquid receiver (250 minus 240kJ/kg). The process through thecondenser and liquid receiver on the pressure/enthalpy diagram is

illustrated in Fig 5.===========================================Published on 1 - April - 2006

Masterclass: Refrigeration Cycles - Part 4

In the last article we looked at the vapour compression cycle and the

behaviour of the recirculating refrigerant (R22) as it passes throughthe evaporator, compressor and condenser. In this month s article we

continue with the expansion device and move on to describe theabsorption refrigeration cycle.

The expansion device

The refrigerant is now available as a high pressure, sub-cooled liquidfor return to the evaporator coil via the expansion device. This is

normally a regulating valve (TEV thermostatic expansion valve) witha sensor attached to the external surface of the refrigerant pipe

leaving the evaporator coil. The TEV regulates the flow of liquid


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refrigerant into the evaporator coil by maintaining a pre-set superheatvalue at the evaporator coil outlet. This ensures that the evaporator is

adequately supplied with liquid refrigerant to meet the instantaneouscooling load and that the superheat is correctly maintained at a level

which ensures the best utilisation of evaporator capacity, whilst

ensuring liquid refrigerant does not return to the compressor. The TEVwill be studied in greater detail later in the series.

Liquid refrigerant enters the TEV at 15bar, a saturation temperature of

40C and at a temperature of 32.5C. As the liquid refrigerant passes

through the valve from the high pressure region to the low pressureregion, the pressure drops top 5bar causing the saturation

temperature to fall immediately to 5C at 5bar.

Figure 1

Flash gas

The drop in pressure forces the refrigerant to immediately drop in

temperature from 32.5C to 5C and a portion of the refrigerant isvaporised (flash gas) as the remainder rejects heat energy. This is

effectively a loss of refrigeration capacity since less liquid refrigerant is

now available for subsequent cooling within the evaporator. Thebalance of liquid refrigerant remaining vaporises within the evaporator

and is superheated by approximately 5-7K. The cooling effect resulting


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from this process is known as the net refrigerating effect. The flashgas process does not result in any change of energy since the

remaining liquid has lost an equal amount of energy to that gained bythe vapour produced.

The absorption refrigeration cycle

The absorption refrigeration cycle was conceived by Carl Munters, aSwedish chemist. He was also responsible for the development of the

lithium chloride desiccant dehumidifier, a remarkable product with

which the writer was extensively involved whist working at RotaireDriers Ltd (now Munters Ltd) over several years. This technology will

be described later in the series.

The absorption refrigeration cycle is mainly directed at air conditioning

applications. There are two basic variants: single-effect and double-effect. The cycle utilises two fluids, one termed the refrigerant, the

other being referred to as the absorbent. These fluids are:

Refrigerant / Absorbent

Single-effect absorption machine: ammonia / water

Double-effect absorption machine: water / lithium bromide

The presence of either ammonia or lithium bromide within a confined

space occupied by people cannot be allowed from a safety viewpointand the cooling equipment must therefore be sited outside the buildingto be conditioned. As it is not possible to pipe ammonia into the

building, the DX split system approach cannot be employed and allabsorption cycle equipment is therefore designed to chill water (water

chiller) which is then piped to the conditioned space(s). The chilled

water is supplied to fan coil units sited in each of the conditionedspaces. The application of this technology is therefore well suited to

multiple room systems or large spaces requiring air conditioning.Absorption chillers are a well-established technology that can offer

considerable advantages over conventional, mechanically driven,vapour compression chillers.

Rather than using a mechanically driven, electrically poweredcompressor and conventional refrigerants, the absorption cycle uses

the fluid pair described above and a heat source. The refrigerant has ahigh affinity for the absorbent, which essentially means that the

absorbent attracts the refrigerant vapour and absorbs the refrigerant.


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This creates an attraction, which can be likened to the suction action ofa mechanical compressor in a vapour compression cycle.

The cooling cycle is driven by the heat source, which is typically a gas

burner, steam or hot water. The heat source is used to boil and drive

the refrigerant out of the refrigerant/absorbent mixture. This mixtureis normally termed solution.

The refrigerant is then passed through a condenser to remove the

latent heat of condensation, thus returning the refrigerant to liquid

form. It is then passed to the evaporator where evaporation drawsheat from the recirculating chilled water. The resulting vapour from the

evaporator is attracted to and absorbed by the absorbent solution tocomplete the cycle. In effect, the mechanical compressor of a

conventional vapour compression cycle has been substituted with a

chemical pump driven by heat.

Figure 2: Single effect absorption cooling cycle

The generator

Referring to Fig 2, the refrigerant/absorbent solution in the singleeffect cycle is heated directly by a gas burner or by hot water or steam

from a combined heat and power (CHP) system or boiler. The boiling


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point (saturation temperature) of the refrigerant is lower than that ofthe absorbent and the solution therefore separates as the refrigerant

evaporates and passes to the condenser. The pure concentratedabsorbent passes to the absorber vessel. The heat applied to the

generator also generates the high side pressure of the system.

The condenser

The air-cooled condenser rejects heat from the refrigerant to

atmosphere, causing the refrigerant to desuperheat and condense to

liquid form. The liquid refrigerant then passes through an orificehaving the same action as an expansion device in the vapour

compression cycle, which results in a reduction in pressure andsaturation temperature. The refrigerant then passes to the evaporator.

The evaporator

Water returning from all fan coil units contains heat energy from the

conditioned spaces and is passed through the evaporator where theheat energy is absorbed by the refrigerant vaporising at low pressure

and saturation temperature. The low-pressure refrigerant vapour is

drawn to the absorber by the absorbent. The chilled water returns tothe building to perform further cooling.


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Figure 3: Double effect absorption cooling cycle

The absorber

The concentrated absorbent is cooled and mixed with the low-pressure

refrigerant resulting in a weak solution. This weak solution is pumpedback to the generator to complete the cycle.

The pump

The pump is the only moving part in the absorption cycle and isusually a diaphragm pump running on a single-phase power supply

drawing very low current.

Double-effect absorption cycle


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This is a variation on the single-effect cycle and has two generatorstages. Heat energy is recovered from the first stage and is utilised in

the second stage. Consequently, these units are more efficient thansingle effect units. The double-effect absorption cycle normally

employs water as the refrigerant and lithium bromide as the absorbent

as stated above. This cycle operates at very low pressures within theevaporator in order to produce a low saturation temperature for water.

Construction

As ammonia reacts adversely with copper and brass, the entire

refrigeration circuit is constructed from steel.

Capacity range

Absorption chillers are available in the following capacity ranges:

Direct-fired absorption cycle chillers: 10 kW to 5MW Indirect-fired

absorption cycle chillers: 20 kW to 5MW

Most direct fired units are able to offer an optional gas-fired heating

capability.

Energy efficiency

The performance efficiency of all heating and cooling equipment can bedefined by the coefficient of performance (COP). Chilling output isdivided by energy input to arrive at COP. Single effect absorption

chillers have a COP of approximately 0.6 and double effect units canboost this to 0.95. The COP of vapour compression chillers is often

quoted between 2.5 and 3.0. However, the generation of electricity

results in losses that do not occur with the burning of gas. If thegeneration efficiency of electricity at the power station were assumed

to be 33%, the true overall COP of vapour compression chillers wouldfall between 0.83 and 1.0.

TEWI The total equivalent warming impact (TEWI) is a measuredeveloped by the air conditioning and refrigeration industry to assess

the total contribution to global warming emissions for a specific item ofair conditioning or refrigeration equipment. These emissions include

the energy consumption of the equipment throughout its working lifeand the energy efficiency of the equipment plays a major part in

determining the TEWI rating for a particular item of equipment. It


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must be remembered that each model in every manufacturers range ofelectrically driven and absorption cycle products will have different

efficiency levels and TEWI ratings. Other systems exist that willtransfer heat energy from one part of the building to another thus

attaining incredibly high COPs and very low TEWI ratings.

==========================================

Published on 1 - May - 2006

Masterclass: Compressors - Part 5

In the last article we looked at refrigeration cycles. We shall now move

on to the key components of the vapour compression cycle startingwith the heart of the system the compressor.

Refrigerant vapour compressors fall into five principal types:

1. Reciprocating

2. Scroll

3. Screw

4. Rotary vane 5. Centrifugal

Reciprocating piston compressors

The reciprocating piston compressor is still by far the most widely usedbeing employed in all fields of commercial refrigeration, process

cooling, industrial refrigeration, close control and comfort airconditioning.

Early models of refrigeration compressors were of the so-called open-drive type, with the pistons and cylinders sealed within a crankcase,

the crankshaft extending through the body for connection to anexternal power source. Open compressors are widely used for many

applications. Open-drive compressors can be connected to a motor in

direct-drive arrangement using a flexible coupling or side-by-side forbelt-drive configuration.

The semi-hermetic compressor was pioneered by Copeland to

overcome various difficulties including shaft alignment, seal failures,the short life of belts and direct drive components. The semi-hermetic

compressor is driven by an electric motor mounted directly on the


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compressor crankshaft, with both the motor and the compressorworking parts hermetically sealed within a common enclosure. The

shaft seal is thereby eliminated and the motors can be specificallysized for the load to be handled. The resulting design is compact,

economical, efficient and basically maintenance free. Removable

heads, stator covers, bottom plates and housing covers allow easyaccess for field repairs.

A small penalty in energy consumption will occur as the gas absorbing

motor heat energy is expanded and the mass flow rate through the

compressor is therefore reduced. Waste motor heat energy must alsobe rejected at the condenser and this component must be increased in

size and capacity to allow for this.

Figure 1: Open drive compressor cutaway

Figure 2: Open drive compressor


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Figure 3: Semi hermetic compressor cutaway

The welded hermetic compressor represents a further decrease in sizeand cost, and is widely used in small horsepower unitary equipment.

Again the motor is mounted on the compressor crankshaft, but thebody is formed from a metal shell hermetically sealed by welding. No

field repairs can be performed on this type of compressor.

The compression process

Before attempting to analyse the performance of compressors it is

necessary to become familiar with the series of processes, which make

up the compression cycle of a reciprocating piston compressor. Acompressor, with the piston shown as four points in its travel in the

cylinder is illustrated in Fig 4. As the piston moves downward on thesuction stroke, low-pressure vapour from the suction line is drawn into

the cylinder through the suction valves. On the upstroke of the piston,

the low-pressure vapour is first compressed and then discharged as ahigh-pressure vapour through the discharge valves into the head of

the compressor.


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Figure 4: The compression cycle

To prevent the piston from striking the valve plate, all reciprocatingcompressors are designed with a small amount of clearance between

the top of the piston and the valve plate when the piston is at the top

of its stroke. The volume of this clearance space is called the clearancevolume and is the volume of the cylinder when the piston is at top

dead centre.

This means that not all the high-pressure vapour will pass out through

the discharge valves at the end of the compression stroke. A certainamount will remain in the cylinder in the clearance volume region.

Reference to Fig 4, 5 and 6 will help to clarify the operation of the

compressor. Fig 5 is a time-pressure diagram in which cylinderpressure is plotted against crank position and Fig 6 is a pressure-

volume diagram.


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Figure 5: Time pressure diagram plotting cylinder pressure against crank position

Figure 6: Pressure volume diagram

At point A, the piston is at the top of its stroke, which is known as topdead centre. When the piston is at this position, both the suction and

discharge valves are closed. The high pressure of the vapour trappedin the clearance space acts upward on the suction valves and holds

them closed against the pressure of the suction vapour in the suctionline.

Because the pressure of the vapour in the head of the compressor is


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approximately the same as that of the vapour in the clearance volume,the discharge valves are held closed either by their own weight or by

light spring loading.

As the piston moves downward on the suction stroke, the high-

pressure vapour trapped in the clearance space is allowed to expand.The expansion takes place along line A-B so that the pressure in the

cylinder decreases as the volume of the clearance vapour increases.When the piston reaches point B, the pressure of the re-expanded

clearance vapour in the cylinder becomes slightly less than the

pressure of the vapour in the suction line; whereupon the suctionvalves are forced open by the higher pressure in the suction line and

vapour from the suction line flows into the cylinder.

The flow of suction vapour into the cylinder begins when the suction

valves open at point B and continues until the piston reaches thebottom of its stroke at point C. During the time that the piston is

moving from B to C, the cylinder is filled with suction vapour and thepressure in the cylinder remains constant at the suction pressure. At

point C, the suction valves close, usually by spring action, and thecompression stroke begins.

The pressure of the vapour in the cylinder increases along line C-D asthe piston moves upward on the compression stroke. By the time the

piston reaches point D the pressure of the vapour in the cylinder hasbeen increased until it is higher than the pressure of the vapour in the

head of the compressor and the discharge valves are forced open;whereupon the high-pressure vapour passes from the cylinder into thehot gas line through the discharge valves. The flow of the vapour

through the discharge valves continues as the piston moves from D toA while the pressure in the cylinder remains constant at the discharge

pressure. When the piston returns to point A, the compression cycle is

completed and the crankshaft of the compressor has rotated onecomplete revolution.

Efficiency parameters

There are two basic efficiency parameters used to quantify theperformance of reciprocating and the other positive displacement

types. These are volumetric efficiency and isentropic efficiency. It isconvenient to plot these values in the way shown in Fig 7 and 8

because they are primarily dependent on pressure ratio.

Volumetric efficiency is the ratio of volume of gas actually pumped to


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the theoretical swept volume of the compressor. This will be bore areax stroke x speed for a piston compressor. The effect of the clearance

volume means that the volumetric efficiency is always less than 100%and decreases in a linear manner with pressure ratio. This is primarily

due to the effect of the clearance volume. As will be described later,

scroll and screw compressors can have higher volumetric efficienciesbecause these types have no clearance volume.

Isentropic efficiency is the measure of energy efficiency. It is the ratio

of ideal gas compression power to actual absorbed power. The major

energy losses arising in compressors consist of friction losses, flowlosses, heat losses and electrical motor losses. These will vary to some

extent from one compressor type to another and so the isentropicefficiency may typically be in the range 60%-80%. For open

compressors, shaft power input is used and for semi-hermetic and

hermetic types, electrical power input is used and this must be takeninto account if the efficiencies of the two types are compared.

Figure 7: Volumetric efficiency


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Figure 8: Isentropic efficiency

Piston compressor development

From the beginning of vapour compression refrigeration, pistoncompressors have been the workhorse of the refrigeration, air

conditioning and heat pump markets. Piston compressor technology

has traditionally offered good efficiency levels and, through properdesign and application, piston compressors have become very reliable.

In addition, the design and operating parameters of pistoncompressors are well developed and understood and the technology

presents no particular manufacturing problems.

Industry demands placed on systems are changing and the

requirements of compressors changing accordingly. Competition, highenergy costs and environmental considerations are compelling

manufacturers to develop even more efficient systems for the future.

To do this cost effectively (for example, without inordinate heat

exchanger size) will require compressor efficiencies higher thancurrent reciprocating piston compressor technology can achieve.

System sound levels are of increasing concern, with an increasing

number of local regulations placing tighter restrictions on the sound

levels of systems.

These demands have lead some compressor manufacturers away from

piston compressor technology to more advanced compressor

technologies.

NEXT MONTH: More on compressors


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Published on 1 - June - 2006

Masterclass: Compressors - Part 6

This month we continue our study of compressors and the alternativesto the reciprocating compressor.

Scroll Compressors

The scroll is a simple compression concept centred around the unique

involute spiral shape of the scroll and its inherent properties. Two

identical scrolls are mated together forming concentric spiral shapes.The concept was proposed almost 100 years ago but has only

relatively recently been developed to practical engineering and mass

production levels.

As shown in Fig 1, during compression, one scroll form (fixed scroll)

remains stationary while the other scroll form (orbiting scroll) is

allowed to orbit around it. Note that the orbiting scroll does not rotateor turn but merely orbits the stationary scroll. The orbiting scroll draws

gas into the outer crescent-shaped gas pocket created by the twoscrolls. The centrifugal action of the orbiting scroll seals off the flanks

of the scrolls.


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As the orbiting motion continues, the gas is forced toward the centreof the scroll and the gas pockets become compressed. When the

compressed gas reaches the centre, it is discharged vertically into achamber and discharge port in the top of the compressor. The

discharge pressure, forcing down on the top scroll, helps seal off the

upper and lower edge tips of the scrolls.

During a single orbit, several pockets of gas are compressed

simultaneously, providing smooth, continuous compression. Both thesuction process (outer portion of the scroll members) and thedischarge process (inner portion) are continuous. When compared to

piston compressor technology, the scroll compressor offers several

significant advantages:


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Figure 2: In a scroll compressor, only two components, a fixed scroll and orbiting scroll, arerequired to compress gas

Simplicity Only two components, a fixed scroll and orbiting scroll,

are required to compress gas. These two components replace theapproximately fifteen components in a piston compressor, which are

required to do the same work.


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Figure 1: Operation of the scroll compressor

Efficiency - The Scroll compressor offers three efficiency advantages

over a piston compressor:

1 The suction and discharge processes of a scroll compressor arephysically separated, reducing heat transfer between suction and

discharge gas. In a piston compressor, the cylinder is exposed to bothsuction and discharge gas, resulting in high heat transfer. This reducesthe efficiency of the compressor (see fig 3).

2 The scroll compression and discharge process is very smooth. A

scroll compresses gas in approximately 112 revolutions as compared

to less than half of a revolution for a piston compressor. The dischargeprocess occurs for a full 360 of rotation versus 30-60 of rotation for


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a piston compressor.

3 The scroll compressor has no valves. While a piston compressorrequires suction and discharge valves, the scroll design does not

require a dynamic valve. This eliminates all valve losses. The result is

that the scroll compressor is inherently 10-15% more efficient than apiston compressor.

High Volumetric Efficiency

A scroll compressor has no clearance volume. All gas that is trapped inthe compression process in the outer pocket of the scroll members is

released through the discharge port. This means that the scrollcompressor inherently has a higher capacity than a piston compressor

at extreme operating conditions.

Noise Level

A scroll compressor has extremely limited motion, which, unlike a

piston compressor, can be perfectly balanced. Because suction anddischarge flow is continuous, a scroll compressor has very low gas

pulses. No dynamic valves equals no valve noise, a common problem

in a piston compressor, is not a factor.

Durability

While a piston compressor has been designed to be durable in all typesof systems, significant design effort and system cost is required toprotect the compressor from liquid slugging and debris in the system.

A scroll compressor can be designed to be compliant to both liquid anddebris. This can be done by allowing the scroll forms to separate from

each other in the presence of contaminants or liquid. This feature

allows a compliant scroll compressor to have superior tolerance toliquid and debris. Compliance has the added benefit of allowing the

scrolls to wear-in over time; that is, to increase compressor andsystem efficiency with running time. Compliance refers to the method

in which the two scroll members interact to achieve high efficiency anddurability simultaneously. The unique and patented CopelandCompliant Scroll Compressor, with both radial and axial compliance,

has several advantages:


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Figure 3: In a scroll compressor, the suction and discharge process are physically separated,

reducing heat transfer between suction and discharge gas.

Figure 4: the scroll compression and discharge process is very smooth. A scroll compressesgas in approximately 1 1/2 revolutions as compared to less than half of a revolution for apiston compressor. The discharge process occurs for a full 360degrees of rotation versus 30-60degrees for a piston compressor.

Figure 5: The scroll compressor has no valves. While a piston compressor requires suction anddischarge vlves, the scroll design does not require a dynamic valve.

1 Continuous flank contact, maintained by centrifugal force, minimises

gas leakage and maximises efficiency.

2 Radial compliance allows the scroll members to separate in thepresence of liquid refrigerant or debris, eliminating high stress in the

members and substantially improving the durability of the compressor.

3 Axial compliance allows the scroll tips to remain in continuous


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contact in all normal operating conditions, ensuring minimal leakagewithout the use of tip seals. This means the scroll performance will not

degrade over time because there are no seals to wear and cause gasleakage. Because of the radial and axial compliance feature, the

Copeland Compliant Scroll Compressor has unprecedented liquid

handling capability. In addition, it is designed to start under anysystem load. Combined, these characteristics simplify the system

design, reduce the system operating costs and increase the durabilityof the system.

NEXT MONTH: Rotary vane, screw and centrifugal compressors

Published on 1 - July - 2006

Masterclass:Compressors - Part 7

In parts 5 and 6 we studied reciprocating and scroll compressors.Rotary vane, screw and centrifugal compressors are covered here inpart 7.

Rotary vane compressors

The rotary vane compressor employs a series of rotating vanes orblades, which are installed equidistant around the periphery of a

slotted rotor. The rotor is mounted eccentrically in a steel cylinder so

that the rotor nearly touches the cylinder wall on one side, the twobeing separated only by an oil film at this point. Directly opposite this

point the clearance between the rotor and the cylinder wall ismaximum. Heads or end plates are installed on the ends of the

cylinder to seal the cylinder and to secure the rotor shaft. The vanes

move back and forth radially in the rotor slots as they follow thecontour of the cylinder wall when the rotor is rotating. The vanes are

held firmly against the cylinder wall by action of the centrifugal forcedeveloped by the rotating rotor. In some instances the blades are

spring loaded to obtain a more positive seal against the cylinder wall.


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Figure 1: vane type rotary compressor

The suction vapour drawn into the cylinder through suction ports in the

cylinder wall is entrapped between adjacent rotating vanes. Thevapour is compressed as the vanes rotate from the point of maximum

rotor clearance to the point of minimum rotor clearance. The

compressed vapour is discharged from the cylinder through portslocated in the cylinder wall near the point of minimum rotor clearance.

The rotary vane compressor is a rotary positive displacement type,which has the advantage of simplicity where a complex screw or scroll

form need not be manufactured. The high sliding speeds at the contact

of the vanes with the cylinder walls demand careful design andgenerally limit this type of machine to smaller compressors such as

fractional horsepower units. However, quite large displacement

machines of this type have been successfully built and used asboosters. A booster is the first stage of a two-stage compression

process. In such applications the loading is relatively light. Wherehigher compression ratios are required for low temperature

applications, it is quite common for rotary vane compressors to bearranged in two- stage configurations.

The rotary vane compressor does not have a sump to contain the oil

reserve. The oil is therefore extracted from the discharge gas by

means of an oil separator (described later in the series) and

continually delivered in a controlled manner to the internal surface ofthe rotor housing to perform essential lubrication. Some models arenow constructed within a body that has an oil reservoir adjacent to the

compressor to simplify oil management and to improve the security of

lubrication.

Screw compressors


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Screw compressors are extensively used in large air conditioning andindustrial refrigeration applications. The first practical design of screw

compressor was patented by Lysholm in Sweden in 1934 anddeveloped by Svenska Rotor Maskina (SRM). Screw compressors

based on the Lysholm design with twin screw rotors were introduced

into the refrigeration market in 1958 and found their place in therefrigerating capacity gap between reciprocating and centrifugal

machines. Following the use of injected oil for cooling, sealing andlubrication, the versatility, reliability and compactness of screw type

compressors has been increasingly appreciated. This has earned them

a significant share of the market in a capacity range now overlappingthat of the reciprocating and centrifugal machines.

The rotary screw compressor is a positive displacement helical-axial

design and is well suited to high-pressure refrigerants and alternative

gas applications such as propane, helium, CO2, natural gas and air. Inthe twin-screw compressor, compression is achieved by two

intermeshing rotors housed in a close fitting casing (see fig 2). Themale rotor has lobes which are non-symmetrical profile sections

formed vertically along the rotor length and these mesh withcorresponding recesses on the female rotor. As the rotors turn, gas is

drawn through the inlet port to fill the space between adjacent lobes.

When the interlobe space along the rotor length is filled the rotation ofthe rotors moves the end of the lobes past the inlet port so sealing the

interlobe space. As the rotors continue to rotate, the intermeshing ofthe lobes on the discharge side of the compressors progressively

reduces the space occupied by the gas causing compression.Compression continues until the interlobe space becomes exposed tothe outlet port in the casing and the gas is discharged. The machine

has few moving parts (seven): slide valve, two rotors and two sets ofheavy-duty industrial bearings. This construction allows the

compressor to operate at two-pole motor speeds (3600rpm

synchronous) with high efficiency.


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Figure 2: screw rotors that form the heart of the Bitzer screw compressor range

Uniform gas flow, unidirectional compression process, even torque andpositive displacement through rotary motion contribute to vibration-

free operation. The design provides simplicity and the absence of aclearance volume leads to high volumetric efficiency. The screw

compressor can be arranged with vertical or horizontal rotors and an

illustration of the vertical screw compressor is shown in fig 3.

Capacity control

The screw compressor is able to offer infinite capacity modulation to aslow as 10% of full load. This is achieved by means of a hydraulically

actuated slide valve in the compressor housing which creates a gap to

allow suction gas to pass back to the suction inlet manifold thusreducing the compressor pumping rate. As the gas is released prior to

compression, it is assumed that minimal thermodynamic losses occur.The location of the capacity control slide valve is controlled

electronically and is determined by temperature, pressure or power

input signals for the optimum match of compressor capacity to loadvariations.


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Figure 3: a sectional view of the vertical screw compressor

Single screw compressor

An alternative type of screw compressor has been successfullydeveloped and introduced. This is the single screw compressor. It has

a screw rotor in mesh with two rotor seals. An efficient and reliablecompressor was conceived by Bernard Zimmern from this known

principle in the 1960s.

The essential functional elements of today's single screw compressor

are a six-flute driven rotor meshing with two star rotors each havingeleven teeth. The star rotors are made from a special synthetic

material and the dynamically balanced rotor is made from cast iron.

The portion of the casing corresponding to the entry end of thecylindrical main rotor is relieved so that the inlet gas may enter the

flutes both axially and radially. The discharge end of the main rotorextends a short distance beyond the points at which the flutes run out;

the discharge ports comprising essentially triangular openings in themain rotor casing in this region.

During the compression process, gas becomes trapped in the flutes bythe teeth of the stars and is compressed by the face of each tooth until

the flute wall uncovers the discharge port, and the compressed gas iscompletely expelled. The size of the discharge port determines the in-

built compression ratio. The compression process occurring on the face


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of the teeth of one star is exactly duplicated by a series ofcompression processes occurring on the opposite face of the other

star. The existence of two stars thus causes each flute to be usedtwice in a complete revolution of the main rotor. Furthermore, the

symmetry of the compression processes results in zero radial gas

pressure loads on the main bearings. Also, because the flutesterminate on the cylindrical surface of the main rotor at the discharge

end, it is possible to arrange that both ends of the main rotor are atsuction pressure, in which case the thrust load approaches zero. Thus,

apart from the weight of the rotor assemblies, the only loading on the

bearings arises from gas pressure acting on the small-engaged area of2 or 3 star teeth on each star. As the single screw is a positive

displacement compressor, there are three stages to the compressioncycle. The following illustrations in figure 5 will serve to describe

these:

Figure 4: sectional view of Bitzer screw compressor

Suction: fig 5 - A & B

Main rotor flutes a, b & c are open to suction at one end and aresealed at the other end by the star rotor teeth. As the main rotor

turns, the effective length of the flutes increases with a correspondingincrease in the volume open to the suction chamber as shown in fig 5 -

A. As flute a assumes the position of flutes b and c its volumeincreases, inducing suction vapour to enter the flute. Upon further

rotation of the main rotor (Fig 5 - B), the flutes which have been open

to the suction chamber engage with the star rotor teeth. This coincideswith each flute being progressively sealed by the cylindrical annulus

housing the main rotor. Once the flute volume is closed off from thesuction chamber, the suction stage of the compression cycle is

complete.


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Figure 5: single screw compression process

Compression: fig 5 - C

As the main rotor turns, the volume of gas trapped within the flute is

reduced as the length of the flute shortens and compression takes

place.

Discharge: fig 5 - D

As the star rotor tooth approaches the end of a flute, the pressure of

the trapped vapour reaches a maximum value occurring when the

leading edge of the flute begins to overlap the triangular shapeddischarge port. Compression immediately ceases as the gas isdelivered into the discharge port. The star rotor tooth continues to

scavenge the flute until the flute volume is reduced to zero. This

compression process is for each flute/star tooth in turn.

As with the twin-screw compressor, this machine is also designed torun at 2-pole 3600rpm synchronous speed and utilises suction gas to


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cool the 3-phase motor windings. Capacity control is achieved with aslide valve mechanism, which allows infinite modulation between 100

and 25% of full load capacity. Oil is normally injected through thecasing near the discharge end of the compressor to act as a coolant,

lubricant and sealant. Most of this oil leaves with the compressed gas

where it is separated before being cooled and re-injected. Some singlescrew compressors do not require lubricating oil at all thus eliminating

the need for oil control management equipment and leading to higheroverall system efficiency.

An oil separation section incorporating a sound attenuation chamberand a discharge gas non-return valve exists within the compressor.

The separator also acts as the oil reservoir, has an oil level sight glassand a 150 mesh stainless steel oil strainer. An oil heater can also be

incorporated to prevent refrigerant migration and condensation within

the lubricating oil.

Suction strainer

In order to protect such compressors from dirt and particles, whichmay be recirculating with the system refrigerant, an integral suction

strainer is incorporated to trap and retain these particles. This suction

strainer is normally inaccessible. Where a compressor is installed intoa site-built system, an additional suction strainer, which can be easily

serviced, should be installed at the inlet to the compressor.

Figure 6: flow diagram of typical centrifugal water-cooled water chiller

Centrifugal compressors

The operating principles of the centrifugal compressor are similar tothose of the centrifugal fan or pump. Low-pressure, low-velocity

vapour from the suction line is drawn in the inlet cavity or eye of the


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impeller wheel along the axis of the rotor shaft. On entering theimpeller wheel, the vapour is forced radially outward between the

impeller blades by action of the centrifugal force developed by therotating wheel and is discharged from the blade tips into the

compressor housing at high velocity and at increased temperature and

pressure. The high-pressure, high velocity vapour discharged from theperiphery of the wheel is collected in specially designed passages in

the casing which reduce the velocity of the vapour and direct it to theinlet of the next stage impeller or, in the case of the last stage

impeller, to a discharge chamber, from where the vapour passes

through the discharge line to the condenser.

The centrifugal compressor is simple in principle and it is a perfectlybalanced machine with no contacting compression surfaces. However,

because high gas velocities are needed for this process, the centrifugal

machine really only becomes effective in quite large sizes. Moreover, ahigh compression ratio could require many stages of compression. This

increases cost and complexity and at the same time introduces moregas friction losses. The centrifugal compressor is very effectively

applied in air conditioning applications where the pressure ratio ismodest. Even under these conditions, the smaller types utilise speed

step-up drives to attain the required compression ratio. Because the

refrigerant itself generates the pressure, the vapour density of therefrigerant has to be taken into account in the compressor design and

the centrifugal compressor is not nearly as versatile as piston or rotarypositive displacement types. Such machines are normally of very large

cooling capacity and are designed for substantial cooling loads in largebuildings and industrial applications. These are heavily applied in theUSA and other countries featuring large buildings operating in high

ambient temperatures.

NEXT MONTH: Air and Water Cooled Condensers

masterclass air conditioning fundamentals

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