Kebersihan Bengkel Praktek yaitu
1. melestarikan lingkungan bengkel
2. meningkatkan kesadaran dalam lingkungan bengkel
3. menghidari pencemaran limbah bengkel
Pemahaman kebersihan bengkel: Pendidik dan siswa yang menggunakan bengkel praktek agar lebih peduli memperhatikan lingkungan bengkel agar merubah sikap memperhatikan pentingnya pemeliharaan serta kebersihan bengkel kerja. Dilingkungan bengkel sendiri adalah gabungan berbagai faktor-faktor fisik, kimiawi,hayati dan sosial yang dapat mempengaruhi kegiatan di dalam bengkel. Oleh karena itu seluruh kegiatan yang ada di dalam bengkel, wajib untuk melestarikan kebersihan lingkuran tersebut agar tidak merugikan kehidupan yang berada di bengkel. Agar terlaksananya kebersihan bengkel harus melibatkan guru serta siswa yang sering mnggunakan fasilitas bengkel.
Rabu, 20 Mei 2009
Selasa, 19 Mei 2009
Transfer of Heat
Basic Methods of Heat Transfer:
The prime purpose of refrigeration is to produce desired temperatures within a spesific area by transferring unwanted heat to a location where it is not objectionable. To understand how these processes are carried on,it is necessery to have a working knowledge of heat flow, how heat may be transferred, and how heat enters a refrigerated space.
There are three basic methods of heat transfer: conduction,,convection and radiation. Each of these methods is described and illustrated in this unit.
Next, the materials and operating mechanisms that are either protection against or part of control devices for heat transfer are reviewed in terms of each basic method. A few formulas and typical problems are included. These show some of the places where the servicman, designer or manufacturer must be able to use simle mathematical calculations in the testing, servicing, design or production of refrigerating equipment.
Consept of Heat flow and Heat Transfer :
The natural tendency of heat is always to flow from a warm body to a cooler one. Heat is thus said to flow "down hill". In this respect, heat flow may be compared to water flow. When water in two separate containers is at the same level, no water flow from one the other. When the water level is either raised or lowered in one of the containers, the water flow from the higher level to the lower one.
In a like manner, if two materials or substances are at the same temperature, no heat flows betwen them. However, if there is a temperature difference between them, heat flows from the warm body to the colder one. In the following illustration, (A) shows two liquids at the same temperature. If the temperature of one liquid is lowered to 32 degrees F., (B).The temperaure of the liquid at (C) has been increased to 212 degrees F. The heat now flows from this higher temperature of 212 degrees to 70 degrees F.
The heat that must be transfered enters a refrigerated space in three main ways:
1. The heat leaks through the walls surrounding the space to be cooled.
2. The heat rushes into the refrigerated area as a door is opened.
3. The heat is introduced into the refrigerated area in the material that is to be
cooled, frozen or stored.
Several steps are involved in the transfer of heat energy from the time it is picked up in the evaporator until it is rejected at the condenser. The schematic drawing shows that in the transfer of heat, different temperatures are involved as the heat travels through diffrent material. In addition, the refrigerant must be moved through the system, water or air must be circulated in the condenser , and air or brine must be moved in the evaporator. All of these steps or processes depend on an understanding of the three basic ways that heat moves or is transferred.
The flow of heat energy is always from hot to cold or from a high intensity to a low intensity. This flow continues until the speed of the molecules at the cold end increases with the absorption of heat and it reaches the same temperature.
At the same temperature, the molecules in all parts have the same average activity.
This method of trnsferring heat from molecule is known as "conduction".
The conduction process may be illustrated with a heated steel part. As such a part is placed in cold water, the fast-moving molecules on the outer surface of the steel transmit the heat to the slower moving molecules of water and increase their speed. In turn, as the steel cools to the temperature of water, the speed of its moleciles decreases.
While the outer surface of the steel is cooled, the center is still hot. The rapid motion of the molecules is transmitted to the outer surface by conduction. The collision of a fast-moving molecule at the center with a slow moving one from the outer surface causes the to lose speed while the other gains speed. This process continues until a uniform temperature is reached throughout the steel part.
The prime purpose of refrigeration is to produce desired temperatures within a spesific area by transferring unwanted heat to a location where it is not objectionable. To understand how these processes are carried on,it is necessery to have a working knowledge of heat flow, how heat may be transferred, and how heat enters a refrigerated space.
There are three basic methods of heat transfer: conduction,,convection and radiation. Each of these methods is described and illustrated in this unit.
Next, the materials and operating mechanisms that are either protection against or part of control devices for heat transfer are reviewed in terms of each basic method. A few formulas and typical problems are included. These show some of the places where the servicman, designer or manufacturer must be able to use simle mathematical calculations in the testing, servicing, design or production of refrigerating equipment.
Consept of Heat flow and Heat Transfer :
The natural tendency of heat is always to flow from a warm body to a cooler one. Heat is thus said to flow "down hill". In this respect, heat flow may be compared to water flow. When water in two separate containers is at the same level, no water flow from one the other. When the water level is either raised or lowered in one of the containers, the water flow from the higher level to the lower one.
In a like manner, if two materials or substances are at the same temperature, no heat flows betwen them. However, if there is a temperature difference between them, heat flows from the warm body to the colder one. In the following illustration, (A) shows two liquids at the same temperature. If the temperature of one liquid is lowered to 32 degrees F., (B).The temperaure of the liquid at (C) has been increased to 212 degrees F. The heat now flows from this higher temperature of 212 degrees to 70 degrees F.
The heat that must be transfered enters a refrigerated space in three main ways:
1. The heat leaks through the walls surrounding the space to be cooled.
2. The heat rushes into the refrigerated area as a door is opened.
3. The heat is introduced into the refrigerated area in the material that is to be
cooled, frozen or stored.
Several steps are involved in the transfer of heat energy from the time it is picked up in the evaporator until it is rejected at the condenser. The schematic drawing shows that in the transfer of heat, different temperatures are involved as the heat travels through diffrent material. In addition, the refrigerant must be moved through the system, water or air must be circulated in the condenser , and air or brine must be moved in the evaporator. All of these steps or processes depend on an understanding of the three basic ways that heat moves or is transferred.
The flow of heat energy is always from hot to cold or from a high intensity to a low intensity. This flow continues until the speed of the molecules at the cold end increases with the absorption of heat and it reaches the same temperature.
At the same temperature, the molecules in all parts have the same average activity.
This method of trnsferring heat from molecule is known as "conduction".
The conduction process may be illustrated with a heated steel part. As such a part is placed in cold water, the fast-moving molecules on the outer surface of the steel transmit the heat to the slower moving molecules of water and increase their speed. In turn, as the steel cools to the temperature of water, the speed of its moleciles decreases.
While the outer surface of the steel is cooled, the center is still hot. The rapid motion of the molecules is transmitted to the outer surface by conduction. The collision of a fast-moving molecule at the center with a slow moving one from the outer surface causes the to lose speed while the other gains speed. This process continues until a uniform temperature is reached throughout the steel part.
Jumat, 15 Mei 2009
Absolute Temperature Scale
The Centigrade and the Fahrenheit scales indicate relative temperature "Absolute" temperature may be measured only when the reading scale starts at a true zero temperature where, because there is no heat, there is no degree of heat.
This "absolute zero" is the basis of two other temperature scales. These are called the "Kelvin" and the "Rankine" scales.
The Kelvin (K) scale is referred to as the "absolute Centigrade scale" and is used in scientific work.
The Rankine (R) scale is the "absolute Fahrenheit scale"
These are the scales whichare applied in refrigeration and air conditioning.
A comparison between the normal scales and the absolute scales for boiling and freezing points is illustrated.
The Kelvin Temperature Scale :
Scientifically, it has been estabilished that a ga decreases 1/273 of its 0 degree Centigrade pressure for each degree that it is cooled. This condition exists for all gases. Thus, the temperature which no pressure is exerted is at the same point as the temperature at which there is no heat, or -273 degrees Centigrade.
Before gases change state, it is also true that a constant pressure there is an expansion (or contraction) of 1/273 of their volume for each degree Centigrade increase (or decrease). Referring to the illustration on which the temperature scales are compared, note that the unit degree on the Kelvin absolute scale is equal in value to the Centigrade degree.
To convert from the Centigrade to the Kelvin scale when the reading is 0 degree Centigrade, or higher, merely add 273 degrees. This is so because the zero reading on the Kelvin scale is located 273 degrees below 0 degree. For example, 35 degrees Centigrade, is equal to 273 + 35, or 308 degrees Kelvin.
The 273 represents the number of Centigrade degrees between absolute zero and the melting point of ice. Since the reading is 35 degrees above 0 degree Centigrade.,35 added to 273 to get the 308 degrees Kelvin value.
On readings which are blow 0 degree Centigrade., subtract the reading from 273 in order to get the Kelvin scale reading. For instance, -15 degrees Centigrade is equal to 273-15, or 258 degrees Kelvin.
The Rankine Temperature Scale:
On the F.scale,absolute zero is 460 Fahrenheit degrees below 0 degree Fahrenheit or -460 degrees Fahrenheit.This is place at which there would be no heat whatsoever and no motion of the molecules. Since the size of the units on the F. and R. scales is the same, ordinary thermometer temperatures in Fahrenheit may be easily changed to R, temperatures.
If the temperature reading is above 0 degree Fahrenheit.,merely add 460 to the reading. When a temperature reading is below 0 degree Fahrenheit., subtarct the reading from 460. The answer will then be in degrees on the R.scale.
EXAMPLE 1 : Change a 50 degrees Fahrenheit reading to R. absolute temperature.
460+50 = 510
Thus,the 50 degrees Fahrenheit. reading is 510 degrees R (Rankine
Absolute)
EXAMPLE 2 : Convert - 20 degrees F. to its equivalent R. value.
460-20 = 440
Thus, the -20 degrees F. reading is 440 degrees R.
The absolute scales must be used when calculating the changes in prossure thar a refrierant ga experiences when being superheated in the evaporator or in a cylinder left out in the hot sun.
This "absolute zero" is the basis of two other temperature scales. These are called the "Kelvin" and the "Rankine" scales.
The Kelvin (K) scale is referred to as the "absolute Centigrade scale" and is used in scientific work.
The Rankine (R) scale is the "absolute Fahrenheit scale"
These are the scales whichare applied in refrigeration and air conditioning.
A comparison between the normal scales and the absolute scales for boiling and freezing points is illustrated.
The Kelvin Temperature Scale :
Scientifically, it has been estabilished that a ga decreases 1/273 of its 0 degree Centigrade pressure for each degree that it is cooled. This condition exists for all gases. Thus, the temperature which no pressure is exerted is at the same point as the temperature at which there is no heat, or -273 degrees Centigrade.
Before gases change state, it is also true that a constant pressure there is an expansion (or contraction) of 1/273 of their volume for each degree Centigrade increase (or decrease). Referring to the illustration on which the temperature scales are compared, note that the unit degree on the Kelvin absolute scale is equal in value to the Centigrade degree.
To convert from the Centigrade to the Kelvin scale when the reading is 0 degree Centigrade, or higher, merely add 273 degrees. This is so because the zero reading on the Kelvin scale is located 273 degrees below 0 degree. For example, 35 degrees Centigrade, is equal to 273 + 35, or 308 degrees Kelvin.
The 273 represents the number of Centigrade degrees between absolute zero and the melting point of ice. Since the reading is 35 degrees above 0 degree Centigrade.,35 added to 273 to get the 308 degrees Kelvin value.
On readings which are blow 0 degree Centigrade., subtract the reading from 273 in order to get the Kelvin scale reading. For instance, -15 degrees Centigrade is equal to 273-15, or 258 degrees Kelvin.
The Rankine Temperature Scale:
On the F.scale,absolute zero is 460 Fahrenheit degrees below 0 degree Fahrenheit or -460 degrees Fahrenheit.This is place at which there would be no heat whatsoever and no motion of the molecules. Since the size of the units on the F. and R. scales is the same, ordinary thermometer temperatures in Fahrenheit may be easily changed to R, temperatures.
If the temperature reading is above 0 degree Fahrenheit.,merely add 460 to the reading. When a temperature reading is below 0 degree Fahrenheit., subtarct the reading from 460. The answer will then be in degrees on the R.scale.
EXAMPLE 1 : Change a 50 degrees Fahrenheit reading to R. absolute temperature.
460+50 = 510
Thus,the 50 degrees Fahrenheit. reading is 510 degrees R (Rankine
Absolute)
EXAMPLE 2 : Convert - 20 degrees F. to its equivalent R. value.
460-20 = 440
Thus, the -20 degrees F. reading is 440 degrees R.
The absolute scales must be used when calculating the changes in prossure thar a refrierant ga experiences when being superheated in the evaporator or in a cylinder left out in the hot sun.
Kamis, 14 Mei 2009
Temperature Measuring System
The word "scale" is used with temperature measuring devices to identify a system of measurement. The scale must have definite "fixed point" or standards that always have the same value and may be reproduced easily.
Fixed reference points on temperature scales :
Certain standard temperatures are used that depend upon the physical conditions of a material. For example, the temperature at which water freezes was selected as one of the standard temperatures that can always be reproduced. This freezing temperature is influenced by extreme changes in pressure and only minutely by ordinary changes in atmospheric pressure. So, the freezing point of water at atmospheric pressure is the first of the standard temperatures which is used as a fixed reference point.
Another common temperature is the boiling point of water under normal conditions. Unlike freezing, the temperature at which water boils is greatly affected by atmospheric pressure. For this reason it is important to know what the pressure conditions are when boiling takes place.
The third fixed reference point is known as "absolute zero". This is the temperature at which scientists believe all movement of molecules ceases. Since their movement causes heat energy, it follows that if there were no movement there would no longer be any heat. Exprimenters have been able to produce temperatures within a few hundredths of a degree of this absolute zero.
Ambient and operating temperature :
The temperature of the surrounding air is knownts
in refrigeration work as "ambient temperature". This ambient temperature is usually expressed in degrees Fahrenheit, and is used whenever an "operating temperature" is required. The operating temperature is equal to the sum of the ambient temperature and the rise in temperature of the unit itself.
Suppose for instance that the temperature-rise as market on an electric motor spesification plate reads "55 degrees centigrated". Under normal operating conditions the motor can be expected to run hot to within 55 degree centigrade above the temperature of the surrounding air. If the ambient temperature is 30 degree centigrade the operating temperature is:
55 + 30 = 80 degrees centigrade
Temperatur Measuring Scales:
Changes of temperatur are measured on either the "Farhenheit" or "Centigrade"scale. These two basic scales are known as the "normal scales". There are two fixed points on both scales which indicate:
1. The temperature at which ice melt
2. The temperature at which pure water boils under a standard atmospheric pressure.
These fixed reference point may be duplicated exactly in any part of the world.
Measurements are made on either the Fahrenheit or the Centigrade
scales in units called "degrees". The degree is written and is follewed by a letter to show to what scale it applies. A reading like 25 degrees Farhenheit, means that the temperature is 25 degrees as measured on the Farhenheit scale. The letter "C" is used for the Centigrade scale. It is becoming standard practice to-omit the symbol and state simply "25 F".
The point at which ice mrlts is known as zero degrees on the Centigeade (C) scale and 32 degrees on the Fahrenheit (F) scale. The boiling point of pure water at standard pressure is 100 degrees centigrade and 22 degrees Fahrenheit. The locations of the freezing and the boiling points are shown in the illustration.
There are 180 Fahrenheit degrees between the freezing and boiling points of water, but only 100 Centigrade degrees. Thus, the diffrence between the melting point of ice and the boiling point of water ia 100 degrees in the Centigrade system and 180 degrees in Fahrenheit system. This means that each degree change in temperature on the Fahrenheit scale is equal ti five-ninths of a degree on the Centigrade scala.
Note from the drawing that the freezing point on the Centigrade scale starts at zero while the Fahrenheit scale begina with the number 32. These two numbers must be remembered when a value on one scale is changed to its equivalent value on the other scale.
Knowing that a degree on the Fahrenheit scale is smaller (5/9's)than a degree on the Centigrade scale, and that the freezing point of water is 0 degree Centigrade and 32 degrees Fahrenheit, it becomes a single matter to change readings from one scale to the other.
To change a Centigrade reading to Fahrenheit:
1. Multiply the reading by 9/5
2. Add 32 degrees
To change a Fahrenheit reading to Centigrade :
1. Subtract 32 degrees (or add a - 32 degrees)
2. Multiply by 5/9
Three examples are used to show how temperatures are converted (changed)from one normal scale to the other. The reason for saying "add a -32 degrees" will be shown when below zero readings are to be converted.
EXAMPLE 1 : Find the Fahrenheit temperature equal to 35 degrees Centigrade.
1. Multiply the reading by 9/5
35 degrees x 9/5 = 63 degrees
2. Add 32 degrees
63 degrees + 32 degrees = 95 degrees Fahrenheit. Ans.
EXAMPLE 2 : Find the Centigrade equivalent of 77 degrees Fahrenheit.
1. Subtract 32 degrees from the reading
77 - 32 = 45 degrees
2. Multiply the remainder by 5/9
45 x 5/9 = 25 degrees Centigrade. Ans
EXAMPLE 3 : Find the C temperature equal to -58 degrees Fahrenheit
1. Subtract 32 degrees from the reading.
Note: Because this is a minus reading, subtraction actually
increases the minus quantity because a minus 32 is added.
-58 + (-32) = - 90 degrees
2. Multiply by 5/9
-90 x5/9 = -50 degrees Centigrade Ans
Fixed reference points on temperature scales :
Certain standard temperatures are used that depend upon the physical conditions of a material. For example, the temperature at which water freezes was selected as one of the standard temperatures that can always be reproduced. This freezing temperature is influenced by extreme changes in pressure and only minutely by ordinary changes in atmospheric pressure. So, the freezing point of water at atmospheric pressure is the first of the standard temperatures which is used as a fixed reference point.
Another common temperature is the boiling point of water under normal conditions. Unlike freezing, the temperature at which water boils is greatly affected by atmospheric pressure. For this reason it is important to know what the pressure conditions are when boiling takes place.
The third fixed reference point is known as "absolute zero". This is the temperature at which scientists believe all movement of molecules ceases. Since their movement causes heat energy, it follows that if there were no movement there would no longer be any heat. Exprimenters have been able to produce temperatures within a few hundredths of a degree of this absolute zero.
Ambient and operating temperature :
The temperature of the surrounding air is knownts
in refrigeration work as "ambient temperature". This ambient temperature is usually expressed in degrees Fahrenheit, and is used whenever an "operating temperature" is required. The operating temperature is equal to the sum of the ambient temperature and the rise in temperature of the unit itself.
Suppose for instance that the temperature-rise as market on an electric motor spesification plate reads "55 degrees centigrated". Under normal operating conditions the motor can be expected to run hot to within 55 degree centigrade above the temperature of the surrounding air. If the ambient temperature is 30 degree centigrade the operating temperature is:
55 + 30 = 80 degrees centigrade
Temperatur Measuring Scales:
Changes of temperatur are measured on either the "Farhenheit" or "Centigrade"scale. These two basic scales are known as the "normal scales". There are two fixed points on both scales which indicate:
1. The temperature at which ice melt
2. The temperature at which pure water boils under a standard atmospheric pressure.
These fixed reference point may be duplicated exactly in any part of the world.
Measurements are made on either the Fahrenheit or the Centigrade
scales in units called "degrees". The degree is written and is follewed by a letter to show to what scale it applies. A reading like 25 degrees Farhenheit, means that the temperature is 25 degrees as measured on the Farhenheit scale. The letter "C" is used for the Centigrade scale. It is becoming standard practice to-omit the symbol and state simply "25 F".
The point at which ice mrlts is known as zero degrees on the Centigeade (C) scale and 32 degrees on the Fahrenheit (F) scale. The boiling point of pure water at standard pressure is 100 degrees centigrade and 22 degrees Fahrenheit. The locations of the freezing and the boiling points are shown in the illustration.
There are 180 Fahrenheit degrees between the freezing and boiling points of water, but only 100 Centigrade degrees. Thus, the diffrence between the melting point of ice and the boiling point of water ia 100 degrees in the Centigrade system and 180 degrees in Fahrenheit system. This means that each degree change in temperature on the Fahrenheit scale is equal ti five-ninths of a degree on the Centigrade scala.
Note from the drawing that the freezing point on the Centigrade scale starts at zero while the Fahrenheit scale begina with the number 32. These two numbers must be remembered when a value on one scale is changed to its equivalent value on the other scale.
Knowing that a degree on the Fahrenheit scale is smaller (5/9's)than a degree on the Centigrade scale, and that the freezing point of water is 0 degree Centigrade and 32 degrees Fahrenheit, it becomes a single matter to change readings from one scale to the other.
To change a Centigrade reading to Fahrenheit:
1. Multiply the reading by 9/5
2. Add 32 degrees
To change a Fahrenheit reading to Centigrade :
1. Subtract 32 degrees (or add a - 32 degrees)
2. Multiply by 5/9
Three examples are used to show how temperatures are converted (changed)from one normal scale to the other. The reason for saying "add a -32 degrees" will be shown when below zero readings are to be converted.
EXAMPLE 1 : Find the Fahrenheit temperature equal to 35 degrees Centigrade.
1. Multiply the reading by 9/5
35 degrees x 9/5 = 63 degrees
2. Add 32 degrees
63 degrees + 32 degrees = 95 degrees Fahrenheit. Ans.
EXAMPLE 2 : Find the Centigrade equivalent of 77 degrees Fahrenheit.
1. Subtract 32 degrees from the reading
77 - 32 = 45 degrees
2. Multiply the remainder by 5/9
45 x 5/9 = 25 degrees Centigrade. Ans
EXAMPLE 3 : Find the C temperature equal to -58 degrees Fahrenheit
1. Subtract 32 degrees from the reading.
Note: Because this is a minus reading, subtraction actually
increases the minus quantity because a minus 32 is added.
-58 + (-32) = - 90 degrees
2. Multiply by 5/9
-90 x5/9 = -50 degrees Centigrade Ans
The effect of heat
The effect of heat energy on pressure and volume : since there are millions of molecules of a refrigerant gas in even the smallest container, the molecules move or bump against each other and hit into the walls of the container. Any rise in the temperature of the gas is accompanied by an increase in the speed of the molecules. As the bombardment against the walls of a container increases, so does the pressure increase.
The cylinders or containers of a gas must be built strong enough to resist the greatest forse or pressure that may be exerted against the walls. Such a pressure builds up according to the highest temperature to be met. This highest temperature must take into consideration that for some operations, where tanks of a liquid or gas must be transported, there is direct exposure to the sun's rays. Two of the safety precautions that must be taken are :
1. Cylinders should be protected against exposure to these rays.
2. Care must be taken in disposing of used throw-away containers to be certain they are not thrown on trash fires.
Pressure used for refrigerant controls : the change of pressure which accompanies change of temperature may be put to good use in operating refrigerant controls. For instance, the "thermostatic valve" which is one of many metering devices, responds to pressure change. This change is caused by the change insuperheat of the refrigerant gas.
The cylinders or containers of a gas must be built strong enough to resist the greatest forse or pressure that may be exerted against the walls. Such a pressure builds up according to the highest temperature to be met. This highest temperature must take into consideration that for some operations, where tanks of a liquid or gas must be transported, there is direct exposure to the sun's rays. Two of the safety precautions that must be taken are :
1. Cylinders should be protected against exposure to these rays.
2. Care must be taken in disposing of used throw-away containers to be certain they are not thrown on trash fires.
Pressure used for refrigerant controls : the change of pressure which accompanies change of temperature may be put to good use in operating refrigerant controls. For instance, the "thermostatic valve" which is one of many metering devices, responds to pressure change. This change is caused by the change insuperheat of the refrigerant gas.
Energy in refrigeration
Any one or combination of several chanes may take place when heat, which is a form of energy, is added to a material or a substance:
1. A rise in temperatur may occur
2. The substance may melt or vaporize
3. The substance may change size or color
4. The substance may be caused to exert a greater pressure
When the heat energy is removed, the reverse of these effects takes place.
Sensible Heat: refers to that heat which produces a temperature rise as it is added to a material. The temperature rises because the molecules absorb heat energy and move faster when heated. As sensible heat is removed, the temperature drops. The molecules are said to move slower when there is a loss of heat energy.
Latent Heat of Vaporization: this heat-carrying ability of the refrigerant is called the "refrigerating effect". The refrigerating effect is found by determining the amount of heat that one pound of the refrigerant is capable of absorbing as it passes through the evaporator. The refrigerant, as it boils in the evaporator and changes state from a liquid to a gas, must absorb its "heat of evaporizaion". Thus, the refrigerant vapor (gas) contains a great deal more heat energy than does the refrigerant in liquid form.
This in true even through the temperature of both the vapor and the liquid is the same.
This mount of heat is the "latent heat of vaporization". It differs in the amount per pound for each refrigerant. The "net refrigerating effect" is only a little less than the heat of vaporization. This loss is represented by:
1. the emaount of heat required to precool the liquid refrigerant, and
2. the heat added to the gaseous refrigerant when it is compressed.
Heat of Condensation : the refrigerant which is a gas at the condenser is changed back into the liquid state by the removal (rejection)of heat. The emount of heat to be removed per pound of refrigerant is called the "heat of condensation". The heat of condensation is equal to the heat of evaporation at a given temperature.
Latent heat of fusion : another kind of latent heat which has only an indirect relation to refrigeration is "latent heat of fusion" Because mechanical refrigeration system followed refrigeration where natural ice was used, it is understandable why the capacity of a cooling ability compares with that of ice.
The standard of measurement which is used is known as the "cooling effect". The cooling effect refers to the effect produced by the melting of one ton of ice as it absorbs heat energy over a 24 hour period. A machine with capacity to produce this same cooling effect is classed as a "one ton machine". Thir does not mean that the machine can produce one ton of ice in a similar period.
The ability to produce ice is referred to as "ice making capacity". A refrigeration system may produce ice by removing latent heat to bring about a required change of state. However, there must first be the removal of a large amount of sensible heat in order to cool water down to the temperature at which it freezes (solidifies).
The water freezes as a result of realising the potential energy of the molecules. Thus, one pound of ice has less energy than one pound of water even though they are both at the same temperature. This difference in energy is the "heat of fusion" or the "heat of solidification".
Latent heat of fusion means the amount of heat which must be added to a pound of material to cause it to melt (change from a solid to a liquid state without any change of temperature). The "latent heat of solidification" is just the reverse and refers to the liquid to become a solid.
In review, it may be said that the heat energy is a combination of the kinetic energy and the potential energy continuous motion.
1. A rise in temperatur may occur
2. The substance may melt or vaporize
3. The substance may change size or color
4. The substance may be caused to exert a greater pressure
When the heat energy is removed, the reverse of these effects takes place.
Sensible Heat: refers to that heat which produces a temperature rise as it is added to a material. The temperature rises because the molecules absorb heat energy and move faster when heated. As sensible heat is removed, the temperature drops. The molecules are said to move slower when there is a loss of heat energy.
Latent Heat of Vaporization: this heat-carrying ability of the refrigerant is called the "refrigerating effect". The refrigerating effect is found by determining the amount of heat that one pound of the refrigerant is capable of absorbing as it passes through the evaporator. The refrigerant, as it boils in the evaporator and changes state from a liquid to a gas, must absorb its "heat of evaporizaion". Thus, the refrigerant vapor (gas) contains a great deal more heat energy than does the refrigerant in liquid form.
This in true even through the temperature of both the vapor and the liquid is the same.
This mount of heat is the "latent heat of vaporization". It differs in the amount per pound for each refrigerant. The "net refrigerating effect" is only a little less than the heat of vaporization. This loss is represented by:
1. the emaount of heat required to precool the liquid refrigerant, and
2. the heat added to the gaseous refrigerant when it is compressed.
Heat of Condensation : the refrigerant which is a gas at the condenser is changed back into the liquid state by the removal (rejection)of heat. The emount of heat to be removed per pound of refrigerant is called the "heat of condensation". The heat of condensation is equal to the heat of evaporation at a given temperature.
Latent heat of fusion : another kind of latent heat which has only an indirect relation to refrigeration is "latent heat of fusion" Because mechanical refrigeration system followed refrigeration where natural ice was used, it is understandable why the capacity of a cooling ability compares with that of ice.
The standard of measurement which is used is known as the "cooling effect". The cooling effect refers to the effect produced by the melting of one ton of ice as it absorbs heat energy over a 24 hour period. A machine with capacity to produce this same cooling effect is classed as a "one ton machine". Thir does not mean that the machine can produce one ton of ice in a similar period.
The ability to produce ice is referred to as "ice making capacity". A refrigeration system may produce ice by removing latent heat to bring about a required change of state. However, there must first be the removal of a large amount of sensible heat in order to cool water down to the temperature at which it freezes (solidifies).
The water freezes as a result of realising the potential energy of the molecules. Thus, one pound of ice has less energy than one pound of water even though they are both at the same temperature. This difference in energy is the "heat of fusion" or the "heat of solidification".
Latent heat of fusion means the amount of heat which must be added to a pound of material to cause it to melt (change from a solid to a liquid state without any change of temperature). The "latent heat of solidification" is just the reverse and refers to the liquid to become a solid.
In review, it may be said that the heat energy is a combination of the kinetic energy and the potential energy continuous motion.
Refrigeration cycles in commercial and Industrial Units
A typical commercial mechanical refrigeration system is illustrated. This system is made possible by the proper use of temperature, pressure and latent heat of vaporization. Instead of ammonia, other substances having certain advantages are widely used. In the system swown, water is circulated in the condenser. The water removes heat from the hot, compressed refrigerant to condense it. Thus, the water carries away the heat that is picked up by the evaporator as the refrigerant boils. It can now again carry out its function of picking up heat.
Note that at the top of the drawing an expansion (metering device) valve is placed in the line between the condenser and the evaporator. This expansion valve provides a restrction (reduced sized opening) so that there is a steady flow of refrigerant.
The valve also maintains the diffrence of pressure required to change the state of the efrigerant, that is, fromm a liquid to a gas.
The accompanying schematic diagram shows a refrigeration cycle simplified terms. Since this is a basic cycle, the following facts must be known:
1. Heat is picked up by the boiling refrigerant regardless of the shape of the
evaporator.
2. Heat which is rejected at the condenser may be carried away by air, water, the
evaporation of water, or other means.
3. The function of a metering device or expansion valve is to permit the compressor
to maintain a pressure difference.
4. The heart of the system provides the energy for its operation whether it be by
mechanical, heat, or other energy.
The diffrent components (main parts) which are identified and described in detail in succeeding units are related to this basic refrigeration cycle.
Note that at the top of the drawing an expansion (metering device) valve is placed in the line between the condenser and the evaporator. This expansion valve provides a restrction (reduced sized opening) so that there is a steady flow of refrigerant.
The valve also maintains the diffrence of pressure required to change the state of the efrigerant, that is, fromm a liquid to a gas.
The accompanying schematic diagram shows a refrigeration cycle simplified terms. Since this is a basic cycle, the following facts must be known:
1. Heat is picked up by the boiling refrigerant regardless of the shape of the
evaporator.
2. Heat which is rejected at the condenser may be carried away by air, water, the
evaporation of water, or other means.
3. The function of a metering device or expansion valve is to permit the compressor
to maintain a pressure difference.
4. The heart of the system provides the energy for its operation whether it be by
mechanical, heat, or other energy.
The diffrent components (main parts) which are identified and described in detail in succeeding units are related to this basic refrigeration cycle.
Rabu, 13 Mei 2009
Industrial applications of refrigeration
Refrigeration is needed to produce correct climatic conditions for certain manufacturing processes. For example, cool cutting oil helps in machining operations by lowering the temperature of the work peace to prevent overheating. Quenching baths for heat treating operations may also be controlled through refrigeration processes.
In the pharmaceutical field, refrigerating units are used to store, process and test penicillin, aureomycin, and many other chemical and biological materials.
Refrigeration, as a quick cooling process, speeds production, cuts moisture losses in foods and reduces mold. The large frozen food industries, and others engaged in the preparation, marketing and purchasing of foods all depend on refrigeration. Impotent studies of the exact nature of electron movement are being undertaken through a process which demants that the material being studied be subjected to the lowest possible temperature at which electron movement slows down to the point where it may be observed.
Steels that must aged to retain shape and dimensional accuracy are now refrigerated under new and rapid deep- freezing treatments. In other ways, aluminium is kept from aging too rapidly.
In these and other industral applications, refrigeration units capable of reducing temperatures to as low as - 150 degree Farhenheid are used in metal-working plant,tool shops and in metallurgical laboratories for heat treating and hardening operations.
The list of applications is without end for the principle of refrigeration has progressed as far since the crude experiment of Francis Bacon as the principle of heat in several thousand years.
As scientists, technicians and craftmen experiment at still lower and colder temperatures approaching -273 degree Celcius (-460 degree Farhenheid), the new science of "cryogenics" (refrigerant) will reveal materials in a state that is neither a solid, liquid or a gas.
Refrigeration systems cycles and classification : Refrigeration systems consist of a series of main units (components), each of which performs a special function or job. The engeneering, design, and planning that goes into the manufacture of this equipment is very elaborate and costly. However, regardless of how complex the refrigeration equipment or the operations are, each of the separate units as well as the entire system depends on simple scientific principles.
The work of early scientists to establish some of these principles is briefly described in this units. Later, these principles are brought together in mechanical refrigeration systems. Such systems are then classified according to use. Finally a comparison is made between refrigeration and reverse refrigeration.
In the pharmaceutical field, refrigerating units are used to store, process and test penicillin, aureomycin, and many other chemical and biological materials.
Refrigeration, as a quick cooling process, speeds production, cuts moisture losses in foods and reduces mold. The large frozen food industries, and others engaged in the preparation, marketing and purchasing of foods all depend on refrigeration. Impotent studies of the exact nature of electron movement are being undertaken through a process which demants that the material being studied be subjected to the lowest possible temperature at which electron movement slows down to the point where it may be observed.
Steels that must aged to retain shape and dimensional accuracy are now refrigerated under new and rapid deep- freezing treatments. In other ways, aluminium is kept from aging too rapidly.
In these and other industral applications, refrigeration units capable of reducing temperatures to as low as - 150 degree Farhenheid are used in metal-working plant,tool shops and in metallurgical laboratories for heat treating and hardening operations.
The list of applications is without end for the principle of refrigeration has progressed as far since the crude experiment of Francis Bacon as the principle of heat in several thousand years.
As scientists, technicians and craftmen experiment at still lower and colder temperatures approaching -273 degree Celcius (-460 degree Farhenheid), the new science of "cryogenics" (refrigerant) will reveal materials in a state that is neither a solid, liquid or a gas.
Refrigeration systems cycles and classification : Refrigeration systems consist of a series of main units (components), each of which performs a special function or job. The engeneering, design, and planning that goes into the manufacture of this equipment is very elaborate and costly. However, regardless of how complex the refrigeration equipment or the operations are, each of the separate units as well as the entire system depends on simple scientific principles.
The work of early scientists to establish some of these principles is briefly described in this units. Later, these principles are brought together in mechanical refrigeration systems. Such systems are then classified according to use. Finally a comparison is made between refrigeration and reverse refrigeration.
Air conditioning applications
Air conditioning units provide safety and comport in nearly all forms of transportation including automobile, bus, trucks, ship, submarine, and aircraft. They are also used to provide comfort conditions in the home, office, business, hotel, apartment building, hospital and industry.
Air conditioning equipment is capable of performing the following functions:
1. Filtering out soot, dirt, and dust
2. Humidifying and dehumidifying the air (adding or removing moisture)
3. Cooling the air
4. Circulating the air within a prescrbed space
Air conditioning thus makes it possible to do something about "the weather".
Proper air conditioning during and after aperations is important in communicable disease wards of hospitals. The rapid removal of infected air helps speed recovery.
Refrigeration of blood plasma makes blood banks possible, to meet emergency needs.
Air conditioning equipment is capable of performing the following functions:
1. Filtering out soot, dirt, and dust
2. Humidifying and dehumidifying the air (adding or removing moisture)
3. Cooling the air
4. Circulating the air within a prescrbed space
Air conditioning thus makes it possible to do something about "the weather".
Proper air conditioning during and after aperations is important in communicable disease wards of hospitals. The rapid removal of infected air helps speed recovery.
Refrigeration of blood plasma makes blood banks possible, to meet emergency needs.
Modern uses of refrigeration
Refrigeration may be defined very simply as the process of removing heat under controlled conditions. Cold is merely a relative term referring to the absence of heat. Thus, to to produce "cold" heat must be removed. This may be accomplished by using ice or by any mechanical means.
While refrigeration has been discussed in terms of food preservation, thir is only one of many pricipal applications. It is also used to provide comport conditions and to produce raw materials and finished products.
While refrigeration has been discussed in terms of food preservation, thir is only one of many pricipal applications. It is also used to provide comport conditions and to produce raw materials and finished products.
Exprimenters of ice-making machines
One of the first patents 1834 for a practical ice- making machine was granted to Jacob Perkins, an Americans engineer living in London. These machines were used successfully in meat-packing plants. Within the next fifty years ice-makers were produced in the Umited States, France and Germany. In this period about 3,000 patents on refrigeration systems had been applied for in the United States.
While progress was made in producing ice by articial means, nearly everyone favored natural ice, believing that atificial ice was unhealthful. Eventually, this superstition was upercome because: 1. artificial ice was produced from purer water than that usually found in lakes and ponds; 2. it could be made as needed; and 3. it did not need to be stored for long periods of time. Thus, by the end of the 19th century, ice and refrigeration were becoming commonplace in the American home.
One factor that contributed greatly to the further development of dependable refrigeration equipment was the availablity of inexpensive electric power and the development of the small electric motor. These were important mechanical millestones.
Paralleling these developments, scientists kept up the constant search for simple truths about cause and effect upon which all of refrigeration depends.
The units which follow in this book deal with: 1. the scientific laws governing refrigeration; 2.the fundamental theories about the construction and operation of refrigeration equipment and the their maintenance.
While progress was made in producing ice by articial means, nearly everyone favored natural ice, believing that atificial ice was unhealthful. Eventually, this superstition was upercome because: 1. artificial ice was produced from purer water than that usually found in lakes and ponds; 2. it could be made as needed; and 3. it did not need to be stored for long periods of time. Thus, by the end of the 19th century, ice and refrigeration were becoming commonplace in the American home.
One factor that contributed greatly to the further development of dependable refrigeration equipment was the availablity of inexpensive electric power and the development of the small electric motor. These were important mechanical millestones.
Paralleling these developments, scientists kept up the constant search for simple truths about cause and effect upon which all of refrigeration depends.
The units which follow in this book deal with: 1. the scientific laws governing refrigeration; 2.the fundamental theories about the construction and operation of refrigeration equipment and the their maintenance.
Early Experiments with food preservation
Some of the earlyest recorded experiments with food preservation date back to 1626 when Francis Bacon attempted to preserve a chicken by stuffing it with snow. In 1683,Anton van Leeuwenhoek opened up a whole new scientific world. This Dutchman invented a microscope and discovered that a clear crystal of water contains millions of living organisms. Today,these are known as microbes.
Scientists studied these microbes and found that rapid multiplication took place in warm, moist conditions such as provided in food materials. This multiplication of microbes was soon recognized as the major cause of food spoilage. By contast, the same types of microbes in temperatures of 50 degree Farhenheid or less did not multiply at all.
Through these scientific studies, it became apparent that fresh foods could be safely preserved in temperatures of 50 degree Farhenheid or less. It was now possible to preserve food by drying, smoking,spicing,salting or cooling.
Since little was known about how to create temperatures low enough to freeze water into ice, ice was transported from its source by Clipper ships to the principal cities of the world.
Scientists studied these microbes and found that rapid multiplication took place in warm, moist conditions such as provided in food materials. This multiplication of microbes was soon recognized as the major cause of food spoilage. By contast, the same types of microbes in temperatures of 50 degree Farhenheid or less did not multiply at all.
Through these scientific studies, it became apparent that fresh foods could be safely preserved in temperatures of 50 degree Farhenheid or less. It was now possible to preserve food by drying, smoking,spicing,salting or cooling.
Since little was known about how to create temperatures low enough to freeze water into ice, ice was transported from its source by Clipper ships to the principal cities of the world.
Basic Refrigeration
Brief historical sketch of ,man-made, cold.
The teory of ice dates back as recorded history. While the stone Age caveman knew what ice was,there was no thought about using it to preserve food. Thousands of years later the Chinese learned that ice improved the taste of drinks. So they cut ice in winter,packed it with straw and chaff, and sold it during the summer.
The early Egyptians found that water could be cooled by placing it in porous jars on roof tops at sundown. The night breezes evaporated the moisture which seeped through the jars, making the water inside the jars cooler. The Greeks and Romans had snow brought down from mountain tops to cone-shaped pits which were lined with straw and branches and then covered with a thatched roof. As civilization advanced, people learned how to cool beverages and foods for enjoyment. This knowledge increased the use of ice and snow
The teory of ice dates back as recorded history. While the stone Age caveman knew what ice was,there was no thought about using it to preserve food. Thousands of years later the Chinese learned that ice improved the taste of drinks. So they cut ice in winter,packed it with straw and chaff, and sold it during the summer.
The early Egyptians found that water could be cooled by placing it in porous jars on roof tops at sundown. The night breezes evaporated the moisture which seeped through the jars, making the water inside the jars cooler. The Greeks and Romans had snow brought down from mountain tops to cone-shaped pits which were lined with straw and branches and then covered with a thatched roof. As civilization advanced, people learned how to cool beverages and foods for enjoyment. This knowledge increased the use of ice and snow
Langganan:
Postingan (Atom)
