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Chapter 1: Introduction

1.1 History of Refrigeration

As far back as history records the activities of the human, we find that one of the greatest concerns of

mankind has been preservation of foods. As man learned the values of the various meats and vegetables as

foods, he also became interested in their preservation.

The earliest attempts at preserving foods consisted of storing them in caves where the temperature

remained lower than the outside air. The disadvantage of these methods lies in the loss of natural

freshness, moisture and flavor.

Refrigeration was first employed by the Romans, Greeks, and Egyptians. The first methods of cooling

foods for preservation consisted of either placing the foods in vessels immersed in streams of cool water

or storing them in holes in the ground. In 18th century England, servants collected ice in the winter and

put it into icehouses, where the sheets of ice were packed in salt, wrapped in strips of flannel, and stored

underground to keep them frozen until summer.

Natural Ice: The melting of the ice absorbs some of the heat of the foods and reduces the temperature

inside the box. This represented a marked degree of improvement than the previously used more crude

methods of preserving foods. This represented a marked degree of improvement than the previously used

more crude methods of preserving foods.

Manufactured Ice: At the beginning of the 19th century, ice boxes were used in England (e.g. Fig. 1.1)

Natural ice was harvested, distributed and used in both commercial and home applications in the mid-

1800s. Considerable progress was made in the development of machines for producing artificial ice

commercially. The use of these machines made pure ice available in even the warmest of climates and in

dependable quantities. At about the same time, some improvements in the construction of household

refrigerators are made.

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Figure 1.1: Ice Refrigerator

More attention was given to better construction and better insulations. This made it possible to maintain

refrigerator temperature from 20 to 30 degrees below room temperature and preserve food for a

considerable length of time. The domestic or household refrigerating machine has been under

development for many years.

Mechanical domestic refrigeration first appeared about 1910. J.M. Larsen produced a manually operated

household machine in 1913. By 1918 Kelvinator produced the first automatic refrigerator for the

American market. They sold 67 machines that year and between 1918 and 1920, 200 more. Now over 10

million units are sold each year.

The first of the sealed automatic refrigeration units was introduced by General Electric in 1928. Automatic

refrigeration units, for the comfort cooling as a part of air-conditioning appeared in 1927.

Practically, in 1940, all domestic units were of the sealed type. Commercial units had also been

successfully made and used. These units were capable of refrigerating large commercial food storage

systems, comfort cooling of large auditoriums, restaurants, departmental stores, residential buildings and

hospitals. From late 1930s, air-conditioning of automobiles has also grown rapidly.

Starting in the 1960s, the home air conditioning market experienced tremendous growth. Energy was

inexpensive and therefore, simple air conditioning became common in many homes. Solar energy and

other alternative energy sources became additional sources for powering heating and cooling systems.

Due to a tremendous growth in technology, by 1990 all areas of refrigeration and air-conditioning were

using microprocessor control systems. The purpose of these systems is to increase reliability and

efficiency of the heating and cooling units. By 1990, the automobile air-conditioner became as standard as

the automatic transmission.

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1.2 How cold preserves food

As the molecules move slower, there is an important effect on the bacteria that are present in most foods.

Cold or low temperature slows up the growth of these bacteria and foods do not spoil as fast. Slowing the

movement or cooling of the molecules tends to make all organisms more sluggish. Spoiling of food is

actually the growth of bacteria in the food. If the bacteria can be kept from increasing, the food will be

eatable for a longer period of time. Since most foods have considerable water content, the food must be

kept just above freezing temperature.

If food is frozen slowly at or near the freezing temperature of water, the ice crystals formed are large and

their growth ruptures the food tissues. When the food melts or defrosts, it spoils rapidly and its appearance

and taste are ruined. Fast freezing at very low temperatures (0 to -15 oF) forms small crystals and the food

tissues are not injured. Food freezers are maintained at or below 0 oF and food placed in them is frozen

very quickly.

1.3 Refrigeration

Refrigeration is defined as the branch of science which deals with the process of reducing and maintaining

the temperature of space below the temperature of the surroundings. OR

Refrigeration deals with the transfer of heat from a low temperature level at the heat source to a high

temperature level at the heat sink by using a low boiling refrigerant. OR

The refrigeration is a continuous process of extraction of heat from a body whose temperature is already

below its surrounding temperature. OR

Process of removing heat from a place where it is not wanted and transferring that heat to a place where it

makes little or no difference.

In the average household, the room temperature from summer to winter is normally between 70F ( and

90F. The temperature inside the refrigerator fresh food section should be about 35F. Heat flows

naturally from a warm level to a cold level. Therefore, heat in the room is trying to flow into the

refrigerator, and it does through the insulated walls, the door when it is opened, and warm food placed in

the refrigerator, Figure 1.2, 1.3, and 1.4. For this reason to increase the efficiency of the unit, it is always

best to allow food to cool down to room temperature before placing it in the refrigerator.

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Figure 1.2: The colder air falls out of the refrigerator because it is heavier than the warmer air located outside the

refrigerator. The cooler air is replaced with warmer air at the top. This is referred to as heat leakage.

Figure 1.3: Heat transfers through the walls into the box by conduction. The walls have insulation, but this does not

stop the heat leakage completely.

Figure 1.4: Warm food that is placed in the refrigerator adds heat to the refrigerator and is also considered heat

leakage. This added heat has to be removed or the temperature inside the refrigerator will rise.

1.4 The Refrigeration Process

The refrigerator has to pump the heat up the temperature scale from the 35F (2 0C) refrigeration

compartments to the 70F (22 oC) room. The components of the refrigerator are used to accomplish this

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task, Figure 1.5. The heat leaking into the refrigerator raises the air temperature but does not normally

raise the temperature of the food an appreciable amount. If it did, the food would spoil. When the

temperature inside the refrigerator rises to a predetermined level, the refrigeration system comes on and

pumps the heat out.

Figure 1.5: Heat that leaks into the refrigerator from any source must be removed by the refrigerators heat-pumping mechanism. The heat has to be pumped from the cool, 35F (2 oC) interior of the refrigerator to the warmer, 70F (21

oC) air in the room in which the refrigerator is located.

1.4.1 Temperature and Pressure Relationship

The boiling point of water can be changed and controlled by controlling the vapor pressure above the

water. Understanding this concept is necessary because water is used as the heat transfer medium in the

following example. The next few paragraphs are important for understanding refrigeration.

The temperature/pressure relationship correlates the vapor pressure and the boiling point of water and is

the basis for controlling the systems temperatures. So, if we are able to control the pressures in

refrigeration or air-conditioning system, we will be able to control the temperatures that the system will

maintain.

Fig. 1.6 shows the container of water boiling at sea level at atmospheric pressure. When this same pan of

water is taken to a mountaintop, the boiling point changes, because the thinner atmosphere causes a

reduction in pressure.

Whenever the pressure is increased, the boiling point increases, and that whenever the pressure is reduced,

the boiling point is reduced. If water were boiled at a temperature low enough to absorb heat out of a

room, we could have comfort cooling (air conditioning).

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Figure 1.6: Water boils at 212F (100 oC) when the atmospheric pressure is 29.92 in. Hg (1 bar) and the same Water

boils at 203F when the atmospheric pressure is 24.92 in. Hg (0.843 bar).

Let us place a thermometer in the pan of pure water, put the pan inside a bell jar with a barometer, and

start the vacuum pump. Suppose the water is at room temperature 21 oC (70F). When the pressure in the

jar reaches the pressure that corresponds to the boiling point of water at 70F, the water will start to boil

and vaporize. This point is 0.739 in. Hg (0.025 bar). If we were to lower the pressure in the jar to

correspond to a temperature of 40F (5 oC), this new pressure of 0.248 in. Hg (0.122 psia) will cause the

water to boil at 40F. The water is not hot even though it is boiling.

Figure 1.7: The pressure in the bell jar is reduced to 0.739 in. Hg. The boiling temperature of the water is reduced to

70F because the pressure is 0.739 in. Hg (0.363 psia).

Now let us circulate this water boiling at 40F through a cooling coil. If room air were passed over it, it

would absorb heat from the room air. Because this air is giving up heat to the coil, the air leaving the coil

is cold. Figure 1.8 illustrates the cooling coil. When water is used in this way, it is called a refrigerant. A

refrigerant is a substance that can be changed readily to a vapor by boiling it and then changed to a liquid

by condensing it. The refrigerant must be able to make this change repeatedly without altering its

characteristics.

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Figure 1.8: The water is boiling at 40F because the pressure is 0.122 psia or 0.248 in. Hg. The room air is 75F and

gives up heat to the 40F coil.

1.5 Refrigerant

The working substance used in a refrigeration system to produce cold is called refrigerant.

Theoretically, any gas that can be alternately liquefied and vaporized within mechanical equipment can

serve as a refrigerant. Practical considerations have led to the use of several refrigerants that can be safely

handled at moderate pressures by equipment having reasonable mechanical strength and with lines of

normal size and wall thickness.

While no substance possesses all the properties of an ideal refrigerant, the hydrocarbon (Freon)

refrigerants come quite close. Refrigerant 12 is made of carbon (C), chlorine (Cl), and fluorine (F). Its

formula is CCl2F2. It is made of a combination of elements. Refrigerant 22 is made of carbon (C),

hydrogen (H), chlorine (Cl), and fluorine (F). Its formula CHClF2 is slightly different from that of R-12.

Each of these manufactured refrigerants has its own characteristics, such as odor and boiling pressure.

Refrigerants are the vital working fluids in refrigeration systems. They transfer heat from one place to

another for cooling air or water in air-conditioning installations.

1.6 Elementary Refrigeration

In Fig. 1.9 a refrigerant cylinder, A, is shown with the valve closed. The pressure inside is 72 psig (87

psia, 603 kPa) and the temperature is 72 oF (22 oC). Ail conditions inside the cylinder are balanced. The

number of molecules leaving the vapor state by diving back into the liquid, and the liquid molecules flying

out of the liquid into the vapor state are equal.

In cylinder B, the valve has been opened slightly some of the vapor is escaping. The results are twofold:

1. The pressure over the liquid refrigerant in the cylinder is reduced to 48 psig (63 psia). This causes

change. There is now more liquid changing to a vapor than there is vapor changing back into a

liquid.

2. With more liquid turning into vapor than vapor returning to a liquid, heat is absorbed. The liquid

refrigerant and cylinder will be cooled. The temperature of the refrigerant and cylinder is now 50 oF (10oC). Some heat from the surrounding area which is at 72 oF (22 oC), will now flow into the

cylinder and the refrigerant.

24oC 13oC

5oC

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Figure 1.9: Cooling effect of different pressures operating on surface of liquid refrigerant R-12.

In cylinder C, the valve has been opened more than in B. Refrigerant vapor now flows out more rapidly.

This further lowers the pressure on the liquid refrigerant, which evaporates even faster. The increase in the

rate of evaporation lowers the temperature of the refrigerant and the cylinder. Now heat will flow even

more rapidly from 72 oF (22 oC) air into the cold cylinder.

In cylinder A there is a state of equilibrium (Balance), with all temperatures and pressures in balance. In

cylinder B, there is a slight imbalance due to the vapor escaping through the valve. If this condition were

to continue for a considerable time, a condition of balance would again exist. In this new condition, its

balance would not be static as in A. Instead, it would be a balance between the rate of heat flow into the

cylinder, the evaporation of refrigerant, and the flow of refrigerant vapor out of the cylinder valve. In this

condition of balance, the refrigerant is cooling the cylinder and its surroundings. As long as the valve is

open and vapor can escape, the temperature will stay low. More molecules are escaping from the liquid

into the vapor. Only a few vapor molecules are diving back into the liquid.

This vapor bombardment is called vapor pressure. If pressure can be reduced, the temperature of the liquid

can be reduced since evaporation will be increased. If the vapor molecules can be removed fast enough,

the vapor pressure may be low enough to create refrigerant boiling temperatures in refrigerating range

(low temperature). Vapor molecules are usually removed with a compressor or by using chemicals which

absorb the molecules.

The operation of the mechanical refrigerator relies on the heat absorption property of a fluid passing from

the liquid to the vapor state. Fig.1.10 displays an elementary refrigerator. A cylinder of refrigerant is

replaced in a box. Its vapor is vented to the outside. The components inside the box act as a heat absorber.

This is the same method that is used in mechanical refrigeration. Thus, the liquid can boil only at or above

its evaporation temperature. The liquid will remain at this temperature until it has completely evaporated.

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Figure 1.10: Components of an elementary refrigerator

Since the liquid is at this low temperature, there is a heat transfer to it from the surrounding objects. This

heat increases the evaporation. The heat itself is carried away as the evaporator passes off. Thus, the

evaporator gets the heat energy for doing this from the objects surrounding it. That same heat is removed

with the vapor to the outside of the box.

1.7 How a Mechanical Refrigerator operates

In a mechanical refrigerator, the vaporized refrigerant is not discarded. Instead, it is captured, compressed,

and cooled to a liquid state again. There are four basic parts in a mechanical refrigeration System.

Refrigerant enters the evaporator in the form of a cool, low-pressure mixture of liquid and vapor (A). Heat

is transferred to the refrigerant from the relatively warm air that is being cooled, causing the liquid

refrigerant to boil. The resulting refrigerant vapor (B) is then pumped from the evaporator by the

compressor, which increases the pressure and temperature of the vapor.

The resulting hot, high-pressure refrigerant vapor (C) enters the condenser where heat is transferred to

ambient air, which is at a lower temperature than the refrigerant. Inside the condenser, the refrigerant

vapor condenses into a liquid and is subcooled. This liquid refrigerant (D) then flows from the condenser

to the expansion device. This device creates a pressure drop that reduces the pressure of the refrigerant to

that of the evaporator. At this low pressure, a small portion of the refrigerant boils (or flashes), cooling the

remaining liquid refrigerant to the desired evaporator temperature. The cool mixture of liquid and vapor

refrigerant (A) enters the evaporator to repeat the cycle.

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Removing heat from inside a refrigerator is somewhat like removing water from a leaking canoe. A

sponge may be used to soak up the water in the canoe. The sponge is held over the side, squeezed, and the

water is released overboard. The operation maybe repeated as often as necessary. This transfers the water

from the canoe into the lake.

In a refrigerator, heat instead of water is transferred. Inside the refrigerating mechanism, heat is absorbed.

It is "soaked up" by evaporating the liquid refrigerant in the evaporator (cooling unit). This occurs as the

refrigerant changes from a liquid to a vapor (gas), as shown in Figure 1.11.

Figure 1.11: Schematic of a mechanical refrigerator

The refrigerant, which has absorbed heat, has now turned into a vapor. It is pumped into the condensing

unit located outside the refrigerated space. The condenser works the opposite of the evaporator. In the

evaporator, the refrigerant enters as a liquid, absorbs heat, and flows out the other end as a vapor. By the

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time it reaches the end of the evaporator, it is all a vapor. Now this vapor flows into the condenser under a

high pressure and high temperature. The vapor gives up its heat to the surrounding air. As it reaches the

end of the condenser, the refrigerant is now cooled. It has become a liquid again. We say that, in the

condenser, the heat is "squeezed out." This cycle repeats until the desired temperature is reached.

Heat enters a refrigerator in many ways. It leaks through the insulated walls or enters when the door is

opened. Still more heat is introduced when warm substances are placed in the refrigerator. Heat is not

destroyed to make the refrigerator cold. It is simply removed from the refrigerated space and released

outside.

1.8 Refrigeration Effect

The quantity of heat which unit mass of refrigerant absorbs from the refrigeration space is known as the

refrigeration effect. E.g. when 1 lb of ice melts, it will absorb heat from the surrounding air and from

adjacent objects and amount of heat is equal to its latent heat of fusion. For example, the specific latent

heat of fusion of ice at 0 oC, is 334 kJ.kg-1. This means that to convert 1kg of ice at 0 oC to 1 kg of water at

0oC, 334 kJ of heat must be absorbed by the ice. Conversely, when 1 kg of water at 0 oC freezes to give

1kg of ice at 0 oC, 334 kJ of heat will be released to the surroundings.

1.9 Units of refrigeration capacity

The capacity of any refrigeration system is the rate at which it will remove heat from the refrigeration

space. It is rated in kJ/s, or Btu/h or in terms of ice melting equivalent.

For commercial and industrial refrigeration systems, most of the world uses the kilowatt (kW) as the basic

unit of refrigeration. Typically, commercial and industrial refrigeration systems in North America are

rated in tons of refrigeration (TR).

1.9.1 Tons of refrigeration

Due to the fact that refrigeration was first produced by ice, with the development of mechanical

refrigerator, it was only natural that the cooling capacity of mechanical refrigerators should be compare

with an ice melting equivalent. Hence a refrigerating system having a capacity of one ton is the one that

has a cooling capacity equivalent to the melting of one ton of ice in 24 hours.

If 288,000 Btu are required to make one ton of ice, divide this by 24 hours to get 12,000 Btu/h required to

make one ton of ice in one day. This is the requirement for the phase change from liquid to solid to

convert water at 0C (+32F) into ice at 0C (+32F).

To be specific, one ton of refrigeration capacity can freeze one short ton of water at 0C (32F) in 24

hours.

So, a ton of refrigeration is 3.517 kW. This is derived as follows:

The latent heat of ice (also the heat of fusion) = 333.55 kJ/kg = 144 Btu/lb

One short ton = 2000 lb

Heat extracted = 2000 x 144/24 hr = 288000 Btu/24 hr = 12000 Btu/hr = 200 Btu/min

1 ton refrigeration = 200 Btu/min = 3.517 kJ/s = 3.517 kW = 4.713 HP

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The capacity of a mechanical refrigeration system, i.e. the rate at which the system will remove heat from

the refrigerated space depend on two factors: the mass of refrigerant circulated per unit time and the

refrigeration effect per unit mass circulated. It can be expressed as an equation:

Q = q 1.1

Where; Q = refrigerating capacity in BTU/min or Kj/sec or KW

= mass flow rate in lb/min or kg/sec

q = Refrigerent effect in Btu/lb or KJ/kg

1.10 Applications of Refrigeration

In the old days, the main purpose of refrigeration was to produce ice, which was used for cooling

beverages, food preservation and refrigerated transport etc. Now-a-days refrigeration and air conditioning

find so many applications that they have become very essential for mankind, and without refrigeration and

air conditioning the basic fabric of the society will be adversely affected. Refrigeration and air

conditioning are generally treated in a single subject due to the fact that one of the most important

applications of refrigeration is in cooling and dehumidification as required for summer air conditioning.

Of course, refrigeration is required for many applications other than air conditioning, and air conditioning

also involves processes other than cooling and dehumidification. Figure 1.12 shows the relation between

refrigeration and air conditioning in a pictorial form.

Figure 1.12: Relation between refrigeration and air conditioning

The temperature range of interest in refrigeration extends down to about 100o

C. Now-a-days

refrigeration has become an essential part of food chain- from post-harvest heat removal to processing,

distribution and storage. Refrigeration has become essential for many chemical and processing industries

to improve the standard, quality, precision and efficiency of many manufacturing processes. Ever-new

applications of refrigeration arise all the time. Some special applications require small capacities but are

technically intriguing and challenging.

Air-conditioning is one of the major applications of refrigeration. Air-conditioning has made the

living conditions more comfortable, hygienic and healthy in offices, work places and homes. Air-

conditioning involves control of temperature, humidity, cleanliness of air and its distribution to meet

the comfort requirements of human beings and/or some industrial requirements. Air-conditioning

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involves cooling and dehumidification in summer months; this is essentially done by refrigeration. It

also involves heating and humidification in cold climates, which is conventionally done by a boiler

unless a heat pump is used.

The major applications of refrigeration can be grouped into following four major equally

important areas.

1. Food processing, preservation and distribution

2. Chemical and process industries

3. Special Applications

4. Comfort air-conditioning

1.7 Important Definitions/Concepts

Heat

Heat is a form of energy. It has a relationship to the atom, the smallest indivisible part of an element. As

the temperature of a substance increases, the atoms move more rapidly. As the temperature drops, they

slow down. If all heat is removed from a substance (absolute zero), all molecular motion stops.

The U.S. conventional unit of heat is the British thermal unit (Btu). The metric unit of heat is the joule (j).

If a substance is warmed, heat is added; if cooled, heat is removed. The amount of heat in a substance

equals the mass of the substance multiplied by its temperature.

Heat always flows from a warmer to a cooler substance. The faster moving atoms give up some of their

energy to slower moving atoms. Therefore, each fast atom slows down a little and the slower one moves a

little faster.

Heat is often confused with temperature. Temperature is the measurement of heat intensity. It is not a

direct measure of heat content. Heat content is not dependent on temperature. Heat content depends on the

type of material, the volume of the material, and the amount of heat that has been put into or taken from

the material. For example, one cup of coffee at 200F (93.3C) contains less heat than one gallon of coffee

at 200F (93.3C). The cup at 200F (93.3C) can also contain less heat than the gallon at a lower

temperature of 180F (82.2C).

Cold

Cold means low temperature or lack of heat. Cold is the result of removing heat. A refrigerator produces

"cold" by removing heat from the inside of the refrigerator- The refrigerator does not destroy the heat. It

pumps heat from the inside of the cabinet to the outside.

Sensible heat

The heat which causes a change in temperature of a substance is called sensible heat. If a substance is

heated and the temperature rises as the heat is added. This increase in heat is called sensible heat.

Likewise heat may be removed from a substance. If the heat is removed and the temperature falls, again

this heat removed is called sensible heat.

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Latent (Hidden) heat

A heat which brings about a change in state with no change in temperature is called latent heat. All pure

substances are able to change their state. Solids become liquids, liquids become gas. These changes of

state occur at the same temperature and Pressure combinations for any given substance. It takes the

addition of heat or the removal of heat to produce these changes.

In Figure 1.13, note that considerable heat (144 Btu/Ib., 335 kJ/kg) was added between Points B and C.

Even so, the temperature did not change. This heat was required to change the ice to water. This heat is

called "latent heat of melting" or "latent heat fusion," which means the same thing. Likewise, between

points D and E, 970 Btu/lb (2257 kJ /kg) were added and the temperature did not change. This heat was

required to change the water to steam. This heat is called "latent heat of vaporization". When cooling the

steam to water, the latent heat removed is called the "latent heat of condensation." There are two latent

heats for each substance, solid to liquid (melting and freezing) and liquid to gaseous (Vaporizing and

condensing).

Figure 1.13: Temperature-heat diagram for 1 kg of water at atmospheric pressure (100kPa)

Application of latent heat

In refrigeration work, the physics of latent heat is especially important. Applications of this principle give

the cold or freezing temperature desired. As ice melts, its temperature remains constant. Nevertheless, it

absorbs a considerable amount of heat in changing from ice to water. To melt one kg of ice, 335 kJ of heat

are required. When a substance passes from a liquid to a vapor its ability to absorb heat is very high. This

principle is useful in the operation of the mechanical refrigerator.

The temperature at which a substance changes its state depends on the pressure. The higher the pressure,

the higher the temperature needed to bring about the change. The opposite is also true. If the pressure is

lowered, the temperature at which the change of state will take place is also lowered. This principle is

shown in Figure 1.14.

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Figure 1.14: Temperature-pressure curve for water at atmospheric pressure

A liquid under low pressure will boil at a lower temperature. If the vapor resulting from this boiling is

then compressed, it will condense back into a liquid at a higher temperature.

Effect of pressure on boiling temperatures

The evaporating (boiling) temperature for any liquid is controlled by the pressure placed upon it. Water at

atmospheric pressure (15 psia or 100 kPa) normally boils at 212 oF ( 100"C). If the pressure is increased to

45 psia (311 kPa), the boiling temperature is raised to 271oF (133oC). If the pressure is lowered to 3 psia

(20k Pa), the boiling temperature will be lowered to 142oF (62f), as shown in Figure 1.15.

Mechanical and absorption refrigerators use the effect of reduced pressure to lower the boiling

temperature. Consider the refrigerant, R-12, It boils under atmospheric pressure (15 psia or 100 kPa) at 20 oF ( 29 oC). If the pressure is lowered to 9 psia (62 kPa), the boiling temperature is 42 oF (41oC).

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Figure 1.15: Temperature-pressure curve for water.

Saturation temperature

The temperature at which a fluid will change its state from liquid to vapor or conversely from vapor to

liquid is called saturation temperature. A liquid at saturation temperature is called saturated liquid, while a

vapor at saturation temperature is called a saturated vapor.

OR Saturation Temperature can be defined as the temperature of a liquid, vapor, or a solid, where if

any heat is added or removed, a change of state takes place.

A change of state transfers a large amount of energy.

At saturation temperature, materials are sensitive to additions or removal of heat.

Water is an example of how saturation property of a material, can transfer a large amount of heat.

If the pressure is low, the saturation temperature is low. If pressure is high, saturation temperature

is high.

Sub cooled liquid

If after condensation, the resulting liquid is cooled further so that its temperature is reduced below the

saturated temperature is called sub-cooled liquid.

Super-heated vapor

A vapor at any temperature above saturated temperature corresponding to its pressure is called a super-

heated vapor.

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Once a liquid has been vaporized, its temperature can be still further increased by the addition of heat.

When the temperature of vapor has been so increased above the saturation temperature, the vapor is said

to be super-heated and the energy supplied to super-heated vapor is called super heat. Before a saturated

vapor can be converted into superheated vapor it must be removed from the contact with vaporizing

liquid.

Brine Water

Some refrigeration and air conditioning applications require that water be kept from freezing at

temperatures considerably below the normal freezing temperature of 32 oF (0 oC). Other applications

require that water at atmospheric Pressure be kept from boiling at temperatures above 212 oF (100 oc).

Refrigeration by ice alone will not provide temperatures below 32 oF (0oC). Therefore, to get the lower

temperatures required in some instances ice and salt mixtures are used. Salt, sodium chloride de NaCI), or

calcium chloride (CaCl2), added to water, raises the temperature at which the water will boil. It also

lowers the temperature at which it will freeze.

Enthalpy

Enthalpy is the measure of the heat content of a substance with respect to an accepted reference point. The

amount of enthalpy is determined by both the temperature and the pressure of the substance. Enthalpy is

all the heat in one pound or one kg of a substance calculated form an accepted reference temperature, for

example 32 oF or 0 oC. This reference temperature can be used for water and water vapor calculations.

There is a property a body has that is a combination of its energy due to temperature, pressure, and

volume; it is called enthalpy.

In SI, the zero enthalpies for water, refrigerants, and air are also taken at a convenient temperature

(reference temperature or Tr) and pressure:

For water, 0 enthalpy is at 0 oC and 100 kPa

For refrigerants, -40 oC and 100 kPa

For air, 25 oC and 100 kPa

The enthalpy is measured in joules (j) or kilojoules (kJ),

H = M sp. Heat t

Specific enthalpy is enthalpy per unit mass. It is measured in J/kg, i.e. h = H/M.

Cryogenics

Cryogenics refers to creating and using temperatures in the range of 115 K down to 0 K (or -157 oC down

to -273 oC). The term is applied to the low-temperature liquefaction of gases and their handling and

storage. It includes insulation of containers, instrumentation, and techniques used in such work.

Chapter 1: Introduction1.1 History of Refrigeration1.2 How cold preserves food1.3 Refrigeration1.4 The Refrigeration Process1.4.1 Temperature and Pressure Relationship

1.5 Refrigerant1.6 Elementary Refrigeration1.7 How a Mechanical Refrigerator operates1.8 Refrigeration Effect1.9 Units of refrigeration capacity1.9.1 Tons of refrigeration

1.10 Applications of Refrigeration1.7 Important Definitions/ConceptsHeatColdSensible heatLatent (Hidden) heatApplication of latent heat

Effect of pressure on boiling temperaturesSaturation temperatureSub cooled liquidSuper-heated vaporBrine WaterEnthalpyCryogenics