Liquefaction

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Unit P-14-02

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Liquefaction

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Liquefaction - Unit P-14-02

UNIT P-14-02

LIQUEFACTION

TABLE OF CONTENTS

1.0. OBJECTIVES/INTRODUCTION..........................................................................3

2.0 PROPERTIES OF GASES.................................................................4

3.0 BASIC CONCEPTS OF LIQUEFACTION..............................................5

3.1 The Selection of a Refrigerant......................................................9

3.2 Liquefaction Cycles....................................................................12

3.2.1 Pure Refrigerant Cascade Process............................................................13

3.2.2 Mixed Refrigerant Cycle............................................................................15

3.2.3 Combination of Pure Refrigerant Cascade and Mixed Refrigerant Cycles.18

3.3 End Flash System.........................................................................................19

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1.0 OBJECTIVES/INTRODUCTION

Objectives

At the end of this Chapter, the trainee must understand:

The basic concept of cooling and liquefaction of a natural gas stream,

The selection of a refrigerant and cooling cycle,

The liquefaction cycle.

Introduction

Transportation is a key aspect of the gas business as gas reserves are often remote from the main markets. Gas can be transported by pipeline, but in its gaseous state, is extremely bulky. A high-pressure gas pipeline can transport only about a fifth of the amount of energy as an oil pipeline, even though gas travels much faster than oil. This means that transportation of gas over large distances is not economic.

If Natural Gas is cooled to -162C, it becomes liquid and the volume is reduced by a factor of 600. Transportation of liquefied gas is economic and transportation after liquefaction is often the only means of transporting gas across oceans to the customers.


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2.0. PROPERTIES OF GASES

Gases and liquids differ from solids in that they have no definite shape and will assume the shape of the container in which they are placed.

Gases are made up of molecules that are constantly and actively moving. The pressure exerted by the gas in a vessel is due to these molecules hitting the vessel walls.

If the volume of the vessel is decreased, pressure will increase as the molecules will hit the walls more frequently in the more confined space. If the molecules are close together, they will exert attractive forces between each other and form liquid. Thus, when the temperature is kept constant, increasing pressure can liquefy the gas.

If the temperature of the gas is reduced, the molecules will have less energy, move slower and hit the vessel walls less frequently. This reduces the pressure. To maintain the pressure more and more molecules of the gas have to be added.

This reduces the distance between them, so that the gas finally liquefies. Hence at constant pressure, reducing the temperature will liquefy the gas.

At higher pressures, gases liquefy at higher temperatures. For example, ethane liquefies (dew point) at -88°C and 1 bar whereas at 4 bar, it will liquefy at -58°C (see Figure 2-1). When liquefying the gas, a great deal of latent heat needs to be removed to reduce the energy of the molecules and allow intermolecular bonds to form. The temperature of the pure component remains constant during this process. At higher pressures, relatively less energy needs to be removed from the gas.


-160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140

5

10

15

20

25

30

35

40

45

50

Pre

ssure

(bar

)

Condensation/Boiling

Methane

Ethane

Propane

Butane

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Figure 2-1

3.0 BASIC CONCEPTS OF LIQUEFACTION

To liquefy natural gas (mainly methane plus some heavier hydrocarbons and nitrogen), heat needs to be removed over a wide temperature range, from ambient to the boiling point of methane (-160°C). The quantities of heat to be removed at the various temperature levels differ significantly (see Figure 3-1). This type of graph is referred to as a cooIing curve and shows the cooling curves of pure methane and of a typical natural gas.

Figure 3-1


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On removing heat from methane vapour at constant pressure, the temperature decreases gradually until it reaches the level at which the methane starts to condense. During condensation, the temperature remains constant. After complete condensation, the methane is further cooled to its atmospheric boiling point. The cooling curve of methane shows that the heat of condensation is a substantial part of the total heat to be removed. The temperature level at which methane condenses depends strongly on pressure. Higher pressures increase the condensation temperature, as illustrated in the Table below.

METHANE PRESSURE (bara) CONDENSATION TEMP (C)

1.0 - 161.5

5.0 - 138.2

10.0 - 123.2

19.6 - 106.2

43.1 - 83.2

The cooling curve for natural gas shows a similar curve to that for pure methane. The major difference is that in the case of a mixture, the temperature changes as the mixture condenses. The heavier components will condense first at the higher temperature, followed by the lighter components at lower temperature. A higher natural gas pressure increases condensation temperature in the same way that has been shown for pure methane.

The removal of heat and the subsequent liquefaction of natural gas require refrigeration at very low temperatures. A mechanism is needed to absorb heat at the lower temperatures and to reject this heat at the temperature of the environment. The process by which heat is removed and rejected is called a Cooling Cycle.

Lower temperatures can be obtained using the following thermodynamic principle.

The temperature at which a pure substance boils and condenses depends on its pressure.

The pressure and condensing/ boiling relationship for a number of light hydrocarbons is shown in Figure 2-1.

These curves show that the temperature at which a substance boils or condenses is reduced when its pressure is reduced. The upper limits are the critical temperatures and pressures above which it is impossible to condense a gas. The lower limit is atmospheric pressure because reducing the pressure to less than atmospheric is undesirable for safety reasons.


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When a liquid boils, it takes heat from its surroundings and when a vapour condenses heat is released to the surroundings. Thus refrigeration at a certain temperature can be achieved by selecting a suitable refrigerant and causing it to evaporate at an appropriate pressure. For example, Figure 2-1 shows that refrigeration at -100°C can be obtained by evaporating liquid methane at a pressure of 26 bar.


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Continuous refrigeration can be obtained by the process cycle shown in Figure 3-2.

A gaseous refrigerant at a low-pressure (P1) is compressed to a higher pressure (P2). Process energy has to be supplied for this. The high pressure/high temperature gas is then cooled to its condensation temperature and the liberated heat removed by a stream of cooling water.

By reducing the liquid refrigerant pressure back to (P1) through an expansion valve, it evaporates drawing heat from the Natural Gas stream. In this way, heat is absorbed at the lowest temperature level and transferred to the environment at a high temperature.

Figure 3-2


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This arrangement is called a cooling cycle and is characterised by four basic elements:

compression

condensation

expansion

evaporation.

The compressor energy required in such a cycle depends on the temperature level at which heat has to be removed. Lower temperatures require lower pressure levels at which evaporation takes place, so more energy is needed to return the refrigerant to the pressure at which it can be condensed.

The temperature range that can be achieved using such a simple cooling cycle is limited. For example, ethane can be condensed at an ambient temperature of 20°C. However, the evaporating temperature is limited to -90°C near atmospheric pressure.

To obtain refrigeration at lower temperatures, a second cooling cycle is required in which heat is rejected at -90°C. Using methane, the condensing pressure must be as high as 45 bar and after pressure reduction, temperatures as low as -160°C can be achieved.

In practise, it is more efficient to apply three cooling cycles with methane, ethane and propane as refrigerants (these are discussed later).

Heat removal at low temperatures requires more cooling cycles and consequently more compression energy. The drastic increase in energy required to remove a quantity of heat as the temperature at which heat is removed is decreased. Therefore, it is beneficial to remove heat at the highest possible temperature.

A higher natural gas pressure increases the condensation temperature. Most of the heat can then be removed at higher temperatures and as a result, less energy is needed for liquefaction.


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3.1 The Selection of a Refrigerant

So, heat must be transferred from natural gas to a refrigerant at each temperature level. The driving force for this heat transfer is the difference in temperature between the stream being cooled and the evaporating refrigerant. A second important parameter is the surface area required for heat transfer. More heat can be transferred between refrigerant and cooling streams as the heat transfer surface area increases.

Larger temperature differences make it possible to transfer the same quantity of heat through a smaller surface area. In such a case however, the temperature of the refrigerant is much lower than that of the natural gas and in consequence, more energy is required to provide refrigeration at lower temperatures. For example, an extra 10°C in temperature difference may result in an increase of more than 20% in power requirement (see Figure 3-3).

Smaller temperature differences reduce the power requirement but require larger heat exchange surfaces. In the design of refrigeration systems, a balance must therefore be found between the efficiency of the process and the size and cost of the heat exchangers.

A further important consideration in the design of refrigeration systems is the choice of refrigerant.

Evaporation of a pure refrigerant occurs at constant temperature while the temperature of natural gas changes during heat transfer. In the same way, temperature changes for other refrigerants vary from large to small. This is shown in Figure 3-4a for three common refrigerants.


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Figure 3-3

To obtain a better temperature approach, it is better to evaporate each refrigerant at a number of different pressure levels, each corresponding to a different temperature level. To realise this in an actual liquefaction cycle, each pressure level should be connected to a separate evaporator, which is connected (through a knock out drum) to a part of the compressor casing at the appropriate pressure.

Another alternative that is used for obtaining a close temperature approach, is the application of mixed refrigerants. A mixed refrigerant consists of a mixture of hydrocarbons such as methane, ethane, propane, butane and pentane and has the advantage that during evaporation at constant pressure, the temperature gradually changes. By selecting a suitable mixed refrigerant composition, it is possible to obtain evaporative cooling over the entire temperature range from ambient to LNG temperature. This is illustrated in Figure 3-4b.

As with the cascade arrangement of pure refrigerants, there are limits to the efficiency that can be obtained. A straightforward mixed refrigerant, however well selected, requires a power consumption that is too high to be feasible.


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Figure 3-4

By the addition of extra cooling stages and separate refrigerant circuits, a large number of more complex and more efficient systems have been developed. Some of those systems are based on pure refrigerants, some on mixed refrigerants and others on a combination of both. Each system varies in efficiency and complexity. A number of different refrigerant systems are illustrated below:


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3.2 Liquefaction Cycles

At present there are three different liquefaction processes in operation in LNG plants.

These three systems are:

The pure refrigerant cascade process

The mixed refrigerant process

A combination of (a) and (b).

The power requirement of a liquefaction cycle is a major factor when evaluating alternative processes. This is underlined by the fact that the capital cost of the power-related equipment is about double the cost of the process heat exchangers and vessels. In addition, the fuel consumption of the power-related equipment contributes to the cost of LNG produced.

The power required for the liquefaction of natural gas (expressed as kW/h per ton of LNG or HP per million SCF liquefied gas) is a frequently used performance figure indicating the efficiency of a liquefaction process.


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3.2.1 Pure Refrigerant Cascade Process

A simplified cascade scheme is illustrated in Figure 3-5. This process consists of a combination of three cooling cycles, each containing four basic elements: compressor, condenser/ cooler, expansion valve and heat exchanger (evaporator). Propane, ethane and methane are used in the cooling cycles to provide cooling at progressively lower temperature levels.

In the propane cycle, cooling water is used to condense the propane. After pressure reduction, the propane condenses the ethane and cools the methane and the natural gas to -30°C in a series of evaporators and is re-compressed. The ethane is then compressed to a pressure at which it will condense at a temperature of -30°C.

After expansion, a temperature of -100°C is reached. Both methane and natural gas are condensed, by cooling with ethane at this temperature. In the last cycle, the methane pressure is reduced and this sub-cools the LNG to -145°C

The advantage of a cascade process is that, providing each refrigerant is arranged to cool in three or more stages, a high efficiency can be achieved.


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Figure 3-5

The simplification of the scheme shown in Figure 3-5, lies in the fact that each evaporator (shown as a single unit) is in fact a combination of three units at different pressure levels corresponding to different temperatures. Each unit is connected to a different suction stream of the appropriate refrigerant compressor.

The disadvantage of this cycle is the high capital cost of the complex refrigerant piping and numerous evaporators.


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3.2.2 Mixed Refrigerant Cycle

By using mixed refrigerants, it is possible to produce refrigeration over the entire temperature range down to LNG temperature. If pure refrigerants are used, only a series of separate temperature levels can be achieved. The simplest arrangement, the full-flow mixed refrigerant cycle, is shown in Figure 3-6.

Figure 3-6

Again, there is an arrangement comprising of a compressor, a condenser/cooler, an expansion valve and a heat exchanger/evaporator.

The refrigerant now consists of a mixture of methane, ethane, propane, butane, pentane and nitrogen so both condensation and evaporation take place over a wide temperature range.

After compression, the mixed refrigerant is partially condensed against cooling water and sent to a gas - liquid separator. The liquid and vapour are distributed over the pipes through the heat exchanger and are condensed completely.


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After reduction of pressure, the gradual evaporation in the mixed refrigerant cycle provides the refrigeration for the liquefaction of the natural gas. This process is extremely simple, but the required power consumption is substantially higher than for the cascade process.

Better efficiencies can be achieved with mixed refrigerant using a more complicated arrangement. A simplified scheme of this staged mixed refrigerant cycle is shown in Figure 3-7.

Figure 3-7


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Instead of recombining the vapour and the liquid in one stream, use is made of what is called a "leap-frog" arrangement, the liquid providing refrigerant at an intermediate level in the lower part of the main heat exchanger to cool the natural gas and partially condense the vapour refrigerant. This procedure can be repeated in many steps to provide the refrigeration for sub-cooling the LNG.

A slightly more complex arrangement is used in a two-pressure mixed refrigerant cycle, a simplified scheme for which is given in Figure 3-8. The power consumption of this cycle is of the same order as the "staged" mixed refrigerant cycle but two separate heat exchangers operating at two different pressures are used.

Figure 3-8


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3.2.3 Combination of Pure Refrigerant Cascade and Mixed Refrigerant

Cycles

A schematic representation of this type of cycle is shown in Figure 3-9, which shows the propane cycle in detail, including the three evaporators.

Figure 3-9

The propane is condensed by cooling water and expanded to three pressure levels at corresponding temperatures. It cools the natural gas and the mixed refrigerant to -33°C. Mixed refrigerant then cools the natural gas further in a two-stage mixed refrigerant cycle.


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3.3 End Flash System

It is also possible to produce LNG without cooling the natural gas to -160°C. In some plants, natural gas is cooled to only -145°C. The final cooling is then achieved by reducing the LNG to atmospheric pressure, which flashes off part of the LNG. This pressure reduction downstream of the cryogenic exchanger is called end flash. By applying end flash to LNG, a reduction in power requirement of about 5-10% can be achieved.

It may seem unattractive to re-evaporate part of the LNG that has just been produced. However, the reduction in power consumption for cooling to a less cold temperature is so large that with the same power an increase in LNG throughput can be obtained. This increase is larger than the amount of vapour flashed off during the end flash.

The extent to which end flash can be applied depends on the amount of natural gas that is formed. The flash gas is used as fuel in the liquefaction plant and for a good balance it should be less then the total fuel gas requirements.


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GLOSSARY

Note: These definitions are for guidance. The descriptions are designed to help non-refrigeration engineers to understand the terminology without getting too technical or theoretical.

absolute as in absolute freezing point which is minus 273 degrees

Kelvin - the Kelvin temperature scale developed by Lord Kelvin

in the 1800's is also known as the absolute temperature scale.

It is a scale mainly used by scientists and the zero point is

considered the lowest temperature of anything in the

universe. 0ºK is equivalent to -273.16º Celsius.

air we all need it to breathe, sometimes it is dirty and polluted,

sometimes it is clean and fresh

air conditioner term used to describe a packaged (complete) air conditioning

unit in one box, as in mobile, portable, window, console or

rooftop

air flow as in volume air flow rate needed to transfer heat

ammoniaa very toxic, slightly flammable substance that can be used as

a refrigerant.

Btu a measurement of heat, the British thermal unit, as heat flow

rate = Btu/hr. Put simply, one Btu is the amount of heat

required to raise the temperature of 1 lb. of water 1 degree

Fahrenheit. Heat flow rates are now measured in watts or

kilowatts.

Celsius the correct metric unit of temperature measurement 0

degrees Celsius is freezing point and 100 degrees Celsius is

boiling point for water

centigrade see Celsius, it's effectively the same

cfc a chlorofluorocarbon that is now not allowed to be used as a

refrigerant, typically R11 or R12

cfman imperial measurement of air flow rate - cubic feet per

minute

chiller a machine that uses refrigeration to produce chilled water

compressor at the heart of most air conditioning systems there has to be a

refrigeration compressor. With current technology without a

compressor, there would not be enough cooling for most

applications


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condenser a device used to condense a refrigerant thereby rejecting the

heat to another source, typically an air cooled or water cooled

condenser

condensing unit a machine comprising a compressor, condenser and if air

cooled, a fan, used to remove heat

condensatewater, or the moisture removed from the air when the air is

cooled

conductivity the property or power of conducting heat.

cooler a machine, commonly using water, a fan and a coil or mesh to

provide evaporative cooling - very effective and cheap to run

in hot dry climates, but not so effective in humid climates,

such as the UK. It is now illegal to refer to them as air

conditioners.

cooling a process of which there are many to remove either sensible

or latent heat, or both, which is then referred to as total heat

cooling tower a device, commonly used in larger buildings to reject heat

from a chiller to the atmosphere via water and forced air. They

have to be regularly cleaned and maintained, particularly to

prevent legionella bacteria forming within the water. It is

probably preferable to use air cooled condensers now.

cop the coefficient of performance, used to determine how

efficiently a heat pump operates. It is now common for

modern systems to exceed 3 to 1, whereby for every 1kw of

electrical power used by the unit 3kw of heat will be provided.

Latest systems can exceed 4 to 1, but note that the COP is

related to external and internal temperatures.

de-humidifyto remove moisture from the air, achieved by mechanical

cooling

dew point the temperature at which the moisture in air condenses from a

vapour back to a liquid. As in the condensation that forms on a

cold surface relative to air temperature, i.e. glazing or un-

insulated ducts or pipes.

diffuser a device forming part of an air conditioning unit or duct

system to deliver and diffuse air efficiently without causing

discomfort or cold draughts

drip tray a component of most air conditioning units and fan coil units

to collect the condensate water removed from the air whilst


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cooling

dry bulb the dry air temperature - relative to the wet bulb temperature

- see wet bulb.

duct a pipe or tube made from various materials either rectangular,

circular, oval, flexible, vapour sealed or and insulated used to

convey air.

eer energy efficiency ratio - a factor calculated to determine how

efficient a unit/system is.

energy labelling a system commonly used from 2003 to show how energy

efficient a unit or system is.

Enthalpy a term used to determine the amount of moisture held by a

specific volume of air at a certain condition.

Evaporative

cooling

cooling effect caused by air blowing across a moisture laden

surface – see cooler – but also is the effect you feel if air is

blown across your skin. Try it now, you have moisture on your

skin and by blowing on it you will feel the cooling effect.

Evaporator commonly the room unit of an air conditioning unit, that gives

off cooling by typically evaporating refrigerant from a liquid to

a gas. This action causes a refrigerated cooling effect and the

heat within the room air is transferred to the refrigerant. Also

a component part of a chiller.

Fahrenheit the imperial measurement of temperature

filter a device of varying types and qualities used to remove

impurities from air, gases or liquids. Always keep your filters

clean! Dirty air conditioning filters can be the largest cause of

unnecessary breakdowns!

Freezing the process of converting a liquid or gas to a solid state

gas common term used to refer to some refrigerants, which are at

sometimes in a gaseous state – otherwise any substance that

is freely moving but is neither a solid or a liquid.

Gwp global warming potential – is a measurement of how much

effect any given refrigerant will have on global warming in

relation to carbon dioxide. This is usually measured over a 100

year period. The lower the value of GWP the better the

refrigerant is for the environment.


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Hc hydrocarbon

hcfchydrochlorofluorocarbon – a refrigerant, typically R22 – see

refrigerants

heat pump a machine similar to a condensing unit but can reverse it’s

refrigeration cycle and extract heat from an external source,

air or water and then pump it back in to a building to provide

heating. Very efficient because even when it is -15ºC outside

there is still heat in the air and a heat pump can extract this

heat, upgrade it and use it.

Hfchydrofluorocaron – a refrigerant, typically R134A, R407C,

R410A – see refrigerants

humidity or relative humidity, the measurement of how much water

vapour is contained within the air - i.e. 50% is considered

acceptable comfort, 20% would be an uncomfortable dry

condition and 100% means it's probably raining!

hygrometer an instrument used to measure the relative humidity

ice solid water at 0ºC, -32ºF

internal heat

gain

the heat produced by internal rather than external sources,

can be people, lights, machines, computers, electronics,

cooking, TV's, hot meals, etc.

inverter an electrical device that allows the frequency of the electric

supply to be varied from it's standard 50Hz, thereby allowing

motors (compressors) to be run at variable speeds. This in

turn allows more precise delivery of energy to match the

required load at any one time, rather than a constant delivery,

which may not match. The result is an extremely controllable

and energy efficient system that is not only cheaper to run but

also needs a smaller power supply.

Kelvin see absolute

kcala measurement of heat used in continental Europe (1000

calories)

kw standard measurement of heat or power, 1kw = 1000 watts =

3412Btu/hr = 860kcal


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kilowatt same as kw above

Kyoto protocol - it covers the recommendations to reduce global

warming. Air conditioning manufacturers and refrigerant

producers are working hard to produce new and energy

efficient products.

joule a measurement of energy

latent heat the heat that does not change the temperature but does

change the state of a substance when added to or removed

from it.

liquid a substance that is neither solid nor a gas

legionella it's full name is Legionnaires' Disease (Legionellosis) and is a

very uncommon form of pneumonia caused by a bacterium

that when inhaled may have serious consequences. It is only

possible to catch this via a water based aerosol and apart

from natural sources the most likely way a single individual

can catch this is from a little used shower head. Cooling

towers that have not been correctly maintained more often

causes larger public outbreaks.

Montreal protocol covers the subject of substances that deplete the

ozone layer.

M3/hra measurement of air volume flow rate – cubic metres per

hour

m3/s a measurement of air volume flow rate – cubic metres per

second

m/s a measurement of air velocity or speed – metres per second

nitrogen a constituent part of air – oxygen free nitrogen (OFN) an inert

and safe gas is used to pressure test refrigeration pipework

and is passed through the pipework when it is being installed.

Odp ozone depleting potential – new refrigerants have zero odp

and therefore, if released do not deplete the ozone layer.

However, they still must not be released to atmosphere.

Ofn oxygen free nitrogen – used in brazing copper refrigeration

pipes to prevent moisture and oxides collecting and remaining

in the pipework during manufacture or on site installation.

Failure to use it can have disastrous effects later, causing


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expensive failure of compressors.

Oil used to lubricate the compressor, can be mineral oil, but

increasingly synthetic with new refrigerants

oxygen we need it to oxygenate our bloodstream and live, therefore

we have to ensure there is enough in the air we breathe.

Ozone as in that layer above earth that protects us from ultraviolet

rays and other nasty things happening to our world. Some

refrigerants are ozone depleting, which if they were released

could possibly damage the ozone layer. New refrigerants are

not able to deplete ozone.

Propane or R290, although a naturally occurring substance that can be

used as a refrigerant it is also a highly flammable gas and

must be carefully used.

Pressure the changing pressure of a substance can affect it’s state and

hence properties. A compressor is used to change the

pressure of a refrigerant and hence it’s ability to absorb or

transfer heat. The pressure a compressor, fan or pump is

capable of producing will also affect how far a gas or liquid can

be moved. The resistance to pressure a pipe or duct causes

will also determine how far a gas or liquid can be moved.

R22an hcfc refrigerant which from 2002 can only be installed new

as small heat pumps

R407C an hfc refrigerant that is non-ozone depleting, zero odp,

commonly used in small air conditioning systems

R410A an hfc blended refrigerant that is non-ozone depleting, zero

odp, but operates at higher pressures than other hfc

refrigerants.

R417A the zero odp replacement, suitable for new equipment and as

a drop-in replacement for existing R22 systems.

refcom the register of companies assessed as competent to handle

refrigerants

refrigerant a substance used to absorb and transfer heat, can be air,

water, gas or chemical.

refrigeration the process of mechanical cooling

sensible heat the heat which affects the temperature of a substance. e.g.


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there is no moisture change, such as heat from machines,

lights or the sun etc.

sling

physchrometer

looks like a football rattle but comprises two thermometers,

one to measure dry bulb, the other wet bulb temperatures.

has to whirled around to get air temperature and a correlation

between the two temperatures gives us the relative humidity

solar gainthe heat gained by a building or room from the suns heat

radiation.

temperature a measurement of how hot or cold a substance is. A

comfortable summer internal temperature would be 24ºC, in

Winter 21ºC

thermometer an instrument for measuring the temperature

thermostat a device for controlling or switching on or off another device

based on a pre-set temperature.

tonan imperial measurement of refrigeration heat - 1 ton =

12000Btu/hr or 3.517kw

total heat the sum total of sensible and latent heat

u value a calculated value used to further calculate the rate of heat

flow through a structure, e.g. the heat conducted through a

wall, roof or window

velocity speed

waterwe drink it, swim in it, pollute it and sometimes use it for air

conditioning

wet bulb the air temperature measured with a wet bulb - a small fabric

sleeve around the bulb of a thermometer and wetted


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