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Consider that a liquid such as oil will not flow readily at low temperatures. An ideal gas is assumed to undergo a change of condition without internal friction, that is, without the. Furthermore, since the temperature also remains. The molecules of such a gas are entirely free and independent of each other's attractive forces. Hence, none of the energy transferred either to or from an ideal gas has. The concept of an ideal or perfect gas greatly simplifies the solution of problems concerning.
In working with vapors, it is usually. These tables are included as a part of this textbook.
Processes for Ideal Gases. A gas is said to undergo a process when it passes from some initial state or condition to some final state or condition. A change in the condition of a gas may occur in an infinite number of ways, only five of which are of interest. These are the Constant Pressure Process. If the temperature of a gas is increased by the addition of heat while the gas is allowed to expand so that its pressure is kept constant, the volume of the gas will increase in accordance with Charles'.
In describing an ideal gas, it has been said that the molecules of such a gas are so far apart that they have no attraction for one another, and that none of the energy absorbed by an ideal gas has. Since the change in the internal potential energy, AP,. Specific Heat of Gases. The quantity of heat required to raise the temperature of 1 lb of a gas 1 F while the volume of the gas.
Similarly, the quantity of heat required to raise the temperature of. For any particular gas, the specific heat at a constant pressure is always greater than the specific heat. Since the volume of the gas does not change, no external work is done and is equal to zero. Therefore, for a constant. In order to better understand the energy changes which occur during the various processes, it should be kept in mind that a change in the temperature of the gas indicates a change in the internal kinetic energy of the gas, whereas a change in the volume of the gas indicates work done either by or on the gas.
Equation is a statement that during a constant volume process all of the energy. The quantity of energy required to increase the internal kinetic energy of a gas to the extent that the temperature of the gas is increased 1 F is exactly the same for all processes.
Since, during a constant volume process, no work is done, the only energy required is that which. For example, the specific heat of air at a constant volume is 0. For either that. Twelve pounds of air are cooled from an initial temperature of 95 F to a final temperature of 72 F.
Compute the increase in the internal kinetic energy. Btu per pound. For the constant pressure process, the additional 0. The specific heat of a gas may take any value depending upon the amount of work that the gas does as it expands. The Change in Internal Kinetic Energy. During any process in which the temperature of the gas changes, there will be a change in the internal kinetic energy of the gas.
Regardless of the process, when the temperature of a given weight of gas is increased or either positive or negative,. If, in Example , the gas heated while its volume is kept constant, what is the quantity of heat transferred to the gas during the process? The temperature of 5 lb increased by the addition of heat from an initial temperature of 75 F to a final temperature of F.
If C for air is 0. In Example , AK is negative, indicating is cooled and that the internal kinetic energy is decreased rather than increased. It will now be shown that the work done during a constant pressure process may be evaluated by the equation.
Compute the total heat energy transferred to the air during the constant pressure process described in Example Determine the amount of external work in foot-pounds. Heat Transferred during a Constant Pressure Process. According to Equation the total heat transferred to a gas. Equation is a statement that the thermodynamic state of a gas is adequately described by any two properties of the gas.
Hence, using any two properties of the gas as mathematical coordinates, the thermodynamic state of a gas at any given instant may be shown as a point on a chart.
Furthermore, when the conditions under which a gas passes from some initial state to. Notice that the pressure in psfa is used as the vertical coordinate, whereas the volume in cubic. To establish the initial. Through this point draw a. This area is frequently referred to as "the area under the curve.
According to Example 5, the gas is heated and allowed to expand at a constant pressure until its volume is IS cu ft. Since the pressure remains the same during the process, the state. It has been stated that no work is done during a process unless the volume of the gas changes.
Examination of the PV diagram in Fig. Line 1 to 2, then, represents the path that the process will follow as the thermodynamic state of the gas changes from 1 to 2, and is the PV diagram of the process described. The area of a rectangle is the product of its. Hence, no work is done. Constant Temperature Process. According to Boyle's law, when a gas is compressed or expanded at a constant temperature, the pressure will vary inversely with the volume.
The path followed by an isothermal expansion is indicated by line 1 to 2 and the work of the process in foot-. Since the gas will do work as it expands, if the temperature is to remain constant, energy with which to do the work must be absorbed from an external source Fig.
However, since. Since there. Therefore, in Equation , AK is equal to zero. Pressure-volume diagram of constant temperature process. Crosshatched area represents. What is the quantity of heat transferred to the gas during the constant temperature process described in Example But since the change of condition takes place between the same limits in both cases, the amount of work done in each case is the same. Example is an expansion, the process in Example is a compression.
Both processes occur between the same two conditions, except that the initial and final are. Furthermore, the pressure, volume, and temperature of the gas all vary during an adiabatic process, none of them remaining constant. The energy of the gas is increased in an amount equal to the amount of work done, and since no heat energy is given up by the gas to an external body during the compression, the heat energy equivalent of the work done on the gas is set up as an increase in the internal energy, and the.
If the final pressure of the air is psfa, how much work is done in heat energy units? If the final pressure is psfa, determine the external work done in heat energy units. Therefore, the internal energy of the gas is always diminished by an. Comparison of the Isothermal and Adiabatic Processes. A comparison of the isothermal and adiabatic processes is of interest. Whenever a gas expands, work is done by the gas, and energy from some source is required to do the work.
In an isothermal expansion, all of the energy to do the work is supplied to the gas as heat from an external source. Since the energy is supplied to the gas from an external source at exactly the same rate that the gas is. During an isothermal compression process, is transferred as heat from the gas to an external sink at exactly the same rate that work is being done on the gas.
Therefore, the internal energy. Then, any expansion process in which the energy to do the work of expansion is supplied partly from an external source and partly from the gas. Such a process is. On the other hand, when the greater part of the energy to do the external work comes from the gas itself, the process more nearly approaches the adiabatic.
Since the temperature, pressure, all change during an adiabatic process, they will not vary in accordance with. The greater the loss of heat, the closer the polytropic process thermal.
Of course, with no heat loss, the process. Water to the surroundings. Usually, the value of must be determined by actual test of the machine in which the expansion or compression occurs.
In some instances average '. If the exponent of polytropic expansion is 1. If the initial temperature is R, what is the final temperature? Since the specific heat may take any value, it follows that theoretically. In actual machines, however, will nearly always have some value between 1. Too, the work of a polytropic process can be determined by Equation if tuted for k.
By this definition, all five processes discussed in this chapter are polytropic processes. It is general practice today to restrict the term polytropic to mean only those processes which follow. Hence, the value of n for the polytropic process must fall between 1 and k. The closer the polytropic process approaches the adiabatic, the closer n will approach k. The volume of a certain weight of air is kept constant while the temperature of the air is increased from 55 F to F.
If the initial pressure is 25 psig, what is the final pressure of the air in psig? One pound of air at atmospheric pressure has a volume of If the air is passed across a heat exchanger and is heated to a temperature of 4.
F while its pressure is kept constant, what the final volume of the air? A cylinder of oxygen has a volume of 5 cu A gage on the cylinder reads psi. Three pounds of air occupy a volume of 24 cu ft. Notice in Example that the work done air in the polytropic expansion is equivalent to Of this amount, If the air enters the cylinder at standard atmospheric pressure and is com-.
If the initial pressure of the air is Assuming that the air in Problem 7 is compressed polytropically rather than isothermally. If n equals 1. The final temperature of the air in degrees Ans. The work of compression is Btu. The decrease in the internal kinetic energy.
Since condensation occurs at a constant temperature, the water resulting from the condensing vapor is also at F. The latent heat of vapor-. If, after vaporization, a vapor. Saturation Temperature. When the temperature of a liquid is raised to a point such that any additional heat added to the liquid will cause a part of the liquid to vaporize, the liquid is said to be saturated.
Such a liquid is known as a saturated liquid and the tempera Saturated Vapor. The vapor ensuing from a vaporizing liquid is called a saturated vapor as long as the temperature and pressure of the vapor are the same as those of the saturated liquid from which it came.
A saturated vapor. Before a superheated vapor can be condensed, the vapor must be de-superheated, that is, the vapor must first be cooled to its saturation. If, after condensaa liquid is cooled so that its temperature is reduced below the saturation temperature, the. Thus, a liquid at any temperature below the saturation temperature and above the fusion point is a subcooled. However, this is a very unstable condition and cannot be maintained except momen-. Figure is a pressure over the water.
At this. To illustrate the effect of pressure on the saturation temperature of a liquid, assume that water is confined in a closed vessel which is equipped with a throttling valve at the top compound gage is used to deter Fig.
With the throttling valve wide open, the pressure exerted over the water is atmospheric 0 psig or Some of the vapor molecules will fall back into the water to become liquid molecules again, whereas others will escape through the opening to the outside and be carried away by air currents. If the opening at the top of the vessel is of sufficient size to allow the vapor to escape freely, the vapor will leave the vessel at the same rate that the liquid is vaporizing.
That is, the number of molecules which are leaving the liquid to become vapor molecules will be exactly equal to the number of vapor molecules which are leaving the space, either by escaping to the outside or by falling back into the liquid. Thus, the number of vapor molecules and the density of the vapor above the liquid will velocities. The density of the water vapor at that tempera-. Regardless of the rate at which the liquid is. This condition is illustrated in. The increase in saturation temperature.
Likewise, any decrease in the rate of vaporization will have the opposite effect. The pressure and density of the vapor over the liquid will decrease and the saturation temperature will. Since the saturation temperature of water at atmospheric pressure is F and since a liquid.
On the other hand, in Fig. Since the size of the vapor outlet is. To accomplish this cooling, a portion of the liquid will "flash" into a vapor. The pressure. Enough of the liquid will vaporize to provide the required amount of cooling.
The vaporization of a liquid may occur in two ways: On the other hand, ebullition both at the free surface place or boiling takes and within the body of the liquid and can occur. Up to this point, only ebullition or boiling has been.
Liquids havingthelowest "boiling" points, that is, the lowest saturation temperature for a given pressure, evaporate at the highest. In general, the rate of vaporization as the temperature of the liquid increases and as the pressure over the liquid decreases. Evaporation increases also with the. Furthermore, it will be shown later that the rate of evaporation is dependent on the degree of saturation of the vapor which is always adjacent to and above.
The molecules of a. In the course of their movements the molecules are continually colliding with one another and, as a result of these impacts, some of the molecules of the liquid momentarily attain velocities much higher than the average velocity of the other molecules of the mass.
Thus, their energy is much greater than the average energy of the mass. If this occurs within the body of the liquid, the high velocity molecules quickly lose their extra energy in subsequent collisions with other molecules. However, if the molecules attaining the higher than normal velocities are near the surface, they may project themselves from the surface of the liquid.
Whenever any portion of a liquid. Thus, the energy and temperature of the mass are reduced as it supplies the latent heat of vaporization to that portion of the liquid which vaporizes. The temperature of the objects. They occupy the relatively large spaces which exist between the molecules of the air and.
Rate of Vaporization. For any given temperature, some liquids will evaporate faster. The vapor resulting from evaporation is diffused into and carried away by the air. Confined Liquid-Vapor Mixtures. When a vapor is confined in a container with a portion of its. The water will be evaporating at 70 F and, as described in the previous section, the vapor molecules leaving. Soon the space above the liquid will be so filled with vapor molecules that there will be as many molecules falling back into the liquid as there.
Since no further cooling will take place by evaporation, the liquid will assume the temperature of the surrounding air and' heat transfer. The temperature and average molecular and evaporation will be resumed. The number of liquid. As the density and pressure of the vapor increase, the saturation.
Eventually, the saturation temperature reaches 80 F and is equal to the ambient temperature, no. Condensation of a vapor may be accomplished in several ways: The density and pressure of the vapor will be diminished and the saturation temperature of the mixture will be reduced.
When the saturation temperature of the mixture falls to 60 F will be the same as the ambient temperature and no further heat flow will occur. Equilibrium will have been established and the number of it.
This is because a vapor cannot exist as a vapor at any temperature below its saturation temperature. When the vapor is cooled, the vapor molecules cannot maintain sufficient energy and velocity to over-. Some of the molecules, overcome by the attractive forces, will revert to the molecular structure of the liquid state. If, as in a vapor condenser Fig. Condensing by Increasing the Pressure at a Constant Temperature. When a vapor is compressed at a constant temperature, its volume diminishes and the density of the vapor increases as the molecules of the vapor.
It is possible for a substance to go directly from the solid state to the vapor state without apparently passing.
Any solid substance will sublime at any temperature below its fusion temperature. Sublimation takes place in a manner similar to evaporation, although. When this occurs, the density of the vapor will be at a. One of the most familiar examples of sublimation is that of solid C0 2 dry ice , which, at normal temperatures and pressures, sublimes directly from the solid to the vapor.
Damp wash frozen on the line in the winter time will sublime dry. During freezing state. Section If heat. Furthermore, the vapor is saturated in each case only when the saturation temperature and the actual temperature of the vapor are the. Pressure, temperature, volume, and internal energy have already been discussed to some discussion of enthalpy and entropy extent. In Section , the pressure of the vapor is increased while the temperature of the vapor remains constant until the saturation tempera-.
In both cases, since the vapor must give up the latent heat of vaporization in order to condense, heat must be. The critical temperature is different for every gas. The enthalpy of 1 lb of water at 60 F then is the total amount of heat which must be transferred. For example, the critical temperature of water vapor is F, whereas the critical temperature of air is approximately F. Critical Pressure. Critical pressure is the lowest pressure at which a substance can exist in the liquid state at its critical temperature; ihit is, it is the saturation pressure at the.
According to. Enthalpy, internal energy, and entropy cannot be measured.
Igriportant Properties of Gases and Vapors. Although a gas or vapor has many properties, only six are of particular importance. These are pressure, temperature, volume, enthalpy, internal energy,. Equation , this is 28 Btu 1 x 1 x Mathematically, enthalpy.
Pressure-volume diagram showing the external work done by fluid expansion as lb of water is vaporized. In Equation , that part of the transferred energy which is stored in the fluid as an increase in the internal energy is represented by the term u, whereas that part of the transferred heat which leaves the fluid as work is represented by the term PvjJ. Notice that, although the energy represented by the term PvjJ, does not increase the internal energy of the fluid and is not stored in the fluid, it nevertheless represents energy which must be transspecified.
The volume of 1 lb of water at F is 0. Hence, for water and its vapor, steam, the zero point of entropy. Again, as in the case of enthalpy, it is the specific entropy s rather than the total entropy.
Therefore, in this book, the term entropy shall be used to mean specific entropy s rather than the total entropy S. It has been shown Section that the mechanical energy or work of a process can be expressed as the product of the change in volume and the average absolute pressure.
Hence, the fluid expands from a volume of 0. The concept of entropy. However, the entropy of a fluid is not affected by external work done either by or on the fluid.
Thus in a as a result of internal friction. Vapor Tables. It has been stated previously that a vapor does not approach the in. On a pressure-volume diagram Fig. The properties of vapors at various conditions have been determined by experiment for all common vapors and these data are published in the form of tables.
Separate tables are used for and superheated vapors.
Saturated Vapor Tables. The average absolute temperature not merely the mean of the initial and final temperatures of the process, but is the average of all of the absolute is. Likewise, when the specific volume is given and the density is wanted, the density is found by dividing the specific volume into one. Three values for enthalpy h are usually given in the saturated vapor tables: For example, the enthalpy of the liquid hf for water at F under atmospheric is Btu 1 x 1 x , whereas the enthalpy of the saturated water vapor at F under atmospheric pressure is Btu, which is.
Pressure Gage Pressure Howa saturated liquid or vapor at any one is only one temperature that the can have and still satisfy the conditions of. One common form of the superheated vapor table is. This is true also for the other of a saturated liquid or vapor. Therefore, if any one property of a saturated liquid or vapor is known, the value of the other saturation.
By locating 20 psia encircled in the second column of the abbreviated table in. This does not mean that the properties of a superheated vapor are entirely independent of the pressure of the vapor but.
As a matter of fact, superheated vapor tables are based on the pressure of the vapor, and before the properties of a superheated vapor can be determined from a table, the. When one of the properties of the vapor at saturation. In addition to the properties of the superheated vapor at various temperatures above the saturation temperature corresponding to the pressure, superheated vapor tables usually list some or all of the properties of the vapor at the saturation temperature.
For example, in Fig. Notice that the temperature of the superheated vapor, given in the extreme left-hand column, is listed in 10 F increments.
Refrigerant vapor is at a temperature of 50 F and its pressure is 40 psia. From the abbreviated table in Fig. The degree of superheat of the vapor in degrees Fahrenheit rf The enthalpy of superheated the. The volume occupied by any given weight of depends upon the pressure and temperature.
Air very nearly approaches the condition of an ideal gas and will follow the gas laws with sufficient accuracy for all practical. Therefore, the volume occupied by any given weight of air at any given pressure and temperature can be determined by applying Equation Determine the volume occupied by 1 lb of air having a temperature of 70 F at standard sea level pressure Air is a mechaand water vapor.
Air Quantities. Air quantities may be stated either in units of volume cubic feet or in units of weight pounds so that the need for converting air. On the other hand, the amount of water vapor in the air varies greatly with the.
With regard to these dry air components, the com-. If the specific. Air, being a mechanical mixture of gases and water vapor, obeys Dalton's law. Therefore, the total barometric pressure is always equal to the sum of the partial pressures of the dry gases.
Because of the difference the individual dry gases are unimportant and, for all practical in the volume of any given weight of air at -purposes, the total barometric pressure may be various temperatures and pressures, an air. Dry air having a specific volume of 1 3. Air at a temperature of 70 F and at standard sea level pressure has this specific volume and density see.
A given volume of air at any condition can be converted to an equivalent volume of standard air by applying the following equation Since all of the components in a gaseous mixture are at the same temperature, it follows that when air is at any temperature above the saturation temperature. On the other hand, when air is at a temperature equal to the saturation temperature corresponding to the partial pressure of the water vapor, the water vapor in the air is saturated and the air is said to be saturated actually it is only the water vapor which is saturated.
The temperature at which the water vapor in the air as the. Hence, when the partial pressure exerted by the water vapor is known, the dew point temperature of the air can be determined. Dalton's law of partial pressures states in effect that in any mechanical mixture of gases and. Assume that a certain quanhas a temperature of 80 F and that the partial pressure exerted by the water vapor in the air is 0.
Determine the dew point temperature of the air. From Table , the saturation temperature of steam corresponding to a pressure of 0. Therefore, 50 F is the dew point temperature of the air. Determine the partial pressure exerted by the water vapor in the air. From Table , the saturation pressure corresponding to 40 F is 0. Since the weight of water vapor contained in the air is relatively small, it is often measured in grains rather than in pounds grains equal. Since the dew point temperature of the air depends only on the partial pressure exerted by the water vapor, it follows that, for any given volume of air, the dew point temperature of the air depends only upon the weight of water vapor in the air.
As long as the weight of water vapor in the air. If the amount of water vapor in the air is increased or. Increasing the amount of water vapor in the air will increase the pressure exerted by the water decreased, the will also.
Likewise, reducing the amount of water vapor in the air will reduce the pressure of the water vapor and lower the dew point temperature. Since the amount of water vapor in the air determines the partial pressure exerted by the water vapor, it is.
Because of this fixed relationship between the dew point temperature and the absolute humidity of the air, when the value of one. Too, the partial pressure saturation pressure of the vapor corresponding to each dew point temperature is given in inches of mercury in column 2 and in pounds per square inch in.
Relative Humidity. Relative humidity RH , expressed in percent, is the ratio of the actual weight of water vapor per cubic foot of air relative to the weight of water vapor contained in a cubic foot of saturated air at the. Air at a temperature of 80 F has a dew point temperature of 50 F. Determine the relative humidity. The specific humidity of air at various dew point temperathe.
Columns 6 and 7 of Tables and In Column 6, the specific humidity is given in pounds of water vapor per pound of dry air, whereas in Column 7 the specific humidity is given in grains of water vapor per pound of in. Air at standard sea level pressure has a temperature of 80 F and a dew point temperature of 50 F. Determine the specific humidity and percentage humidity of the air.
Since the density. Note the dew point temperature of the air does not change because the moisture content does not change. Table , absolute humidity corresponding to dew point temperature of 50 F of saturated. When measuring the dry bulb temperature of the air, the bulb of the ther-. The wet bulb WB temperature of the air is the temperature as measured by a wet bulb ther-.
A wet bulb thermometer is an ordinary thermometer whose bulb is enclosed in a wetted cloth sac or wick. To obtain an accurate reading with a wet bulb thermometer, the wick should be wetted with clean water at approximately the dry bulb temperature of the air and the air velocity around the wick should be maintained between and ft per minute.
As a practical matter, this velocity can be simulated in still air by whirling the thermometer about on the end of a chain. An instrument especially designed for this purpose is the sling psychrometer Fig. The sling psychrometer is made up of two thermometers, one dry bulb and one wet bulb, mounted side by side in a protective case which is attached to a handle by a swivel connection so that the case can be easily rotated about the hand.
After saturating the wick with clean water, the instrument. The process should be repeated several times to assure that the lowest possible wet bulb temperature has been recorded. Unless the air is saturated, in which case the dry bulb, wet bulb, and dew point tempera-. The amount by which the wet bulb temperature is reduced below the dry bulb temperature depends upon the relative humidity of the air and is called the wet bulb tures of the air will. The reason for this.
When unsaturated air is brought into contact with water, water will evaporate into the air at a rate proportional to the difference in pressure. Hence, when a wet bulb thermometer is whirled rapidly about in unsaturated air, water will evaporate from the wick, thereby cooling the water remaining in the wick and the thermometer bulb to some temperature below the dry bulb temperature of the.
It is important to recognize the fact that the wet bulb temperature of the air is a measure of the relationship between the dry bulb and dew point temperatures of the air, and as such it provides a convenient means of determining the dew point temperature of the air when the dry. In general, for. In order to understand why the wet bulb tem-. Air sensible heat at various. When water evaporates from the wick of a wet bulb thermometer, heat must be is.
Column 10 of Tables and With regard to Column 10, the temperatures listed in Column. Under this condition, a part of the vaporization heat is being supplied by the air while the other part is supplied by the water in the wick. As the temperature of the wick continues to decrease, the temperature.
The temperature at which the wick stabilizes is called the temperature of adiabatic saturation and is the wet bulb temperature of the air. For example, the lower the relative humidity of the the greater. The mean air. Compute the quantity of sensible heat required to raise the temperature of 10 lb of air from 0 F to 80 F.
Table , the sensible heat of 1 lb of air at 80 F Sensible heat of air at 0 F. Obviously, the greater the need for heat, the greater is the wet bulb. Since all the components of dry air are noncondensable at normal temperatures and pressures, for all practical purposes the only latent heat in the air is the latent heat of the water vapor in the air.
Therefore, the amount of. Since the saturation temperature of the water vapor is the dew point temperature of the air, point temperature determines not only the weight of water vapor in the air but also the value of the latent heat of vaporization. The total heat content of water vapor at various temperatures as computed from 32 F is given in Btu per pound in Column 1 1 of Tables.
Compute the latent heat content of the air in Example , if the dew point temperature of the air is 50 F. Table , the actual weight of water vapor per pound of dry air specific humidity at Latent heat of water vapor mixed with 10 lb of dry air at 50 F DP, from Example It has been shown in preceding sections that the sensible heat of the air the heat content of the dry air is a function of the dry bulb temperature and that the latent heat of the air the heat content of the water vapor mixed with the dry air is a function of the dew point temperature.
Since, for any given combination of dry bulb and dew point temperatures, the wet bulb temperature of the air can have only. However, it is important to recognize that although there is only one wet bulb temperature that will satisfy any given combination of dry bulb and dew point temperatures, there are many combinations of dry bulb and dew point temperatures which will have the same wet bulb temperature see Fig.
Howboth of these values are very small, the error incurred by neglecting them has no practical. The Psychrometric Chart. Psychrometric charts Fig. The use of psychrometric charts permits graphical analysis of psychrometric data and thereby facilitates the air which would otherwise require tedious mathe-. When any two of these four properties are known, the other two can be. The skeleton chart in Fig. The volume. The lines of wet bulb temperature run diagonally across the chart.
The curved line bounding the chart on the left side is. Values of specific humidity and vapor pressure are given on the right and left margins of the chart. For any given air condi-. The following example will use of the psychrometric chart. From the psychrometric chart determine all of the following values: Therefore, the specific humidity, dew point temperature, and latent heat of the air remain unchanged.
Hence, the initial dew point temperature and the new dry bulb temperature can be used as coordinates to locate the new condition of the air on the psychrometric chart point in Fig.
The following properties of the air at the new condition are taken from the psychrometric chart as indicated in Fig.
Since the air is not cooled. With respect to Fig. Example , compute: The total heat removed per pound of air b The sensible heat removed per pound of.
Once this point has been established, the other properties of the air at this condition can be read directly. Values for dry bulb, wet bulb, and dew. Example is cooled to 40 F and determine: Since the air is cooled below the point temperature, moisture will be condensed out of the air and the air at the final condition will be saturated. On the psychrometric chart, the condition of the air falls on the saturation curve at 40 F point B in Fig.
Compute the quantity of sensible heat required to raise the temperature of 10 lb of air from a temperature of 35 F to a temperature of F. Compute the volume of the air in Problem 1 the barometric pressure is Air at a temperature of 90 F is circulated over a cooling coil at the rate of cu ft per.
Determine the volume occupied by 1 lb of air having a temperature of 80 F at standard sea Ans. Heat given off by people working in the re-. More specifically, refrigeration is denned as that branch of science which deals with the process of reducing and maintaining the temperature of a space or material below the temperature of the.
With either process, if the refrigeratingeffectisto be continuous, the temperature of the refrigerating agent must be maintained continuously. To accomplish this, heat must be removed from the body being refrigerated and transferred to another body whose temperature is below that of the refrigerated body. Since the heat removed from the refrigerated body is transferred to another body, it is evident that refrigerating and heating are actually opposite ends of the same process.
Howeach one Btu of heat that the water absorbs from the space, the temperature of the water will increase 1 F, so that as the tempera-.
Since heat will always travel from a region of high temperature to a region of lower temperature, there is always a continuous flow of heat into. Now assume that 1 lb of ice, also at 32 F, is substituted for the water Fig. This time. The Heat Load. The rate at which heat must be removed from the refrigerated space or In passing over the ice the air is cooled as heat is conducted from the air to the ice. On cooling, the air becomes more dense and falls back into the storage space, whereupon it absorbs more heat and the cycling continues.
Water temperature rises as space temperature Fig. Heat flows. The ice merely changes from the solid to the liquid state while its temperature remains constant at 32 F. The heat absorbed by the ice leaves the. Latent cooling may be accomplished with either solid or liquid refrigerants. The solid. Ice, of course, melts into the liquid phase at 32 F, whereas solid carbon dioxide sublimes directly into the vapor phase at a temperature of F under standard atmospheric pressure.
Ice Refrigeration. Melting ice has been used successfully for many years as a refrigerant. Not too many years ago ice was the only cooling agent available for use in domestic and small commercial refrigerators. In a typical ice refrigerator Fig. In some cases, the melting temperature of the ice can be lowered to approximately 0 F by adding sodium chloride or calcium chloride to produce a freezing mixture.
Some of the other more obvious disadvantages of ice are the necessity of frequently replenishing the supply, a practice which is neither convenient nor economical, and the problem of disposing of the water resulting from the melting. Heat leaking through insulation Fig. Heat flows from warm space to cold ice.
Temperature of space decreases as ice melts. Temperature of ice remains at 32 F. Heat absorbed by ice leaves space in water going out the drain. Another less obvious, but more important, disadvantage of employing ice as a refrigerant is. Fresh vegetables, fish, and poultry are often packed and shipped in cracked ice to prevent dehydration and to preserve appearance.
Too, ice has tremendous eye appeal and can be used to considerable advantage in the displaying and serving of certain foods such as salads, cocktails, in chilling beverages. Moreover, the vapor can be readily collected and condensed back into the liquid state so that the same liquid can be used over and over again to provide a continuous supply of liquid for vaporization.
Until now, in discussing the various properties. In order to vaporize at temperatures low enough to. Table is a tabulation of the thermodynamic properties of R saturated liquid and. Tables through There are numerous other fluids which have lower saturation temperatures than water at the same pressure. However, many of these fluids. Actually, only a relatively. There is no one refrigerant which is best suited all. Vaporizing at this low temperature, the R readily absorbs heat from the 40 F space through the walls of the containing vessel.
The heat absorbed by the vaporizing liquid leaves the space in the vapor escaping through the open vent. Since the temperature of the liquid remains constant during the vaporizing process, An in Vaporizing the Refrigerant. It is one of a group of refrigerants introduced to the industry under the trade name of "Freon," but is now. Refrigerant R has a saturation temperature of The Refrigerant liquid vaporizes as it takes in heat from the 40 F space.
The heat taken in. Controlling the Vaporizing Temperature. The temperature at which the liquid vaporizes in the evaporator can be controlled by controlling the pressure of the vapor over the liquid, which in turn is governed by regulating the rate at which the vapor escapes from the evaporator Section For example, if a hand valve is installed in the vent line and the vent is partially closed off so that the vapor cannot escape freely from the evaporator, vapor will When this occurs, there will be.
When vaporizing temperatures below This can be accomplished through the use of a vapor pump as shown in Fig. By this method, vaporization of the liquid R can be brought about at very low temperatures in accordance with the pressuretemperature relationships given in Table Maintaining a Constant Amount of Liquid in the Evaporator.
The action of the float assembly is to maintain a constant. Salvaging the Refrigerant. As a matter of convenience and economy it is not practical to permit the refrigerant vapor to escape to the outside and be lost by diffusion into the air.
The vapor must be collected continuously and condensed back into the liquid state so that the same refrigerant is used over and over again, thereby eliminating the need for ever replenish-. The liquid refrigerant does not vaporize in the storage cylinder and feed line because the pres-. In passing through the float valve, the high pressure refrigerant undergoes a pres-.
The body of material employed to absorb the latent heat from the vapor, thereby causing the vapor to condense, is called the condensing medium. The most common condensing media are air and water.
The water used as a condensing in order to. However, since the pressure and temperature of the saturated vapor leaving the evaporator are the same as those of the vaporizing liquid, the temperature.
Therefore, heat will not flow out of the refrigerant vapor into the air or water used. The heat given off by the vapor in is carried away by the condensing medium. The resulting condensed liquid, whose temperature and pressure will be the same as temperature. Notice that the refrigerant, sometimes called the working fluid, is merely a heat transfer. The refrigerant absorbs heat from the refrigerated space in the evaporator, carries it out of the space, and rejects. The high pressure side or "high side" of the system consists of the compressor, the discharge or "hot gas" line, the condenser, the receiver tank, and the liquid line.
The pressure exerted by the refrigerant in this part of the system is the high pressure under which the refrigerant is. Flow diagram of simple vapor compression system showing the principal parts. The assembly consists of a direct-driven compressor mounted on a common shaft with the motor rotor and the whole assembly hermetically sealed in a. Condensing units equipped with hermetically sealed motor-compressor assemblies are known as "hermetic condensing units" and are employed on a number of small commercial refrigerators and on almost all household refrigerators, home freezers, and window air conditioners.
The bolted construction permits the assemblies to be. Care should be taken not to confuse the suction and discharge valves in the compressor with the suction and discharge service valves. The suction and discharge valves in a reciprocating compressor perform the same function as the intake and exhaust valves in an automobile engine and are vital to the.
Definition of a Cycle. As the refrigerant circulates through the system, it passes through a number of changes in state or condition, each of which is called a process. Note separate fan to circulate over condenser. Courtesy Tecumseh Products Company.
The pressure of the liquid is reduced to the evaporator pressure as the liquid passes through the refrigerant flow control so that the saturation refrigerant. In the evaporator, the liquid vaporizes at a constant pressure and temperature as heat to supply the latent heat of vaporization passes from the refrigerated space through the walls of the evaporator to the vaporizing liquid.
By the action of the compressor, the vapor resulting from the vaporization is drawn from the. The vapor leaving. While flowing through the suction line from the evaporator to the compressor, the vapor usually absorbs heat from the air surrounding the suction line and. Although no heat as such is transferred either from the refrigerant during the compression, the temperature and enthalpy of the vapor to or.
Whenever a vapor is compressed, unless the vapor is cooled. Therefore, when a vapor is compressed adiabatically, as in a refrigeration compressor, wherein no heat is removed from the vapor during the compression, the tempera-.
The Compression Process. In modern, high speed compressors, compression takes place very rapidly and the vapor is in contact with the compressor cylinder for only a short time. Because the time of compression The energy equivalent of the work done is called the heat of compression. The energy to do the work of compression, which is transferred to the vapor during the compression process,. It will be shown horsepower required to drive the compressor can be calculated from the heat of compression.
Since the condensing temperature is always equal to the temperature of the condensing. The discharge temperature is that at which the vapor is discharged from the compressor, whereas the condensing temperature is that at which the vapor condenses in the condenser and is the saturation temperature of the vapor corresponding to the pressure in the condenser. Because the vapor is usually superheated as it enters the compressor and because it contains the heat of compression, the vapor discharged from the compressor is highly superheated and its temperature is considerably above the satura-.
The discharge vapor is cooled to the condensing temperature as it flows through the hot-gas line and through the upper part of the condenser,. Condensing Pressure. The condensing is always the saturation pressure corresponding to the temperature of the liquid-. When the compressor is not running, the temperature of the refrigerant mixture in the condenser will be the same as that of the surrounding air, and the corresponding satura-.
For any given condenser,. For example, when 1 lb of ice melts it will absorb from the surrounding air and from adjacent objects an amount of heat. If the ice melts at 32 F it will absorb Btu per pound, so that the refrigerating effect of. However, as the liquid passes through the refrigerant control its pressure is reduced from For this reason, only a part of each pound of Hquid actually vaporizes in the evaporator and.
Therefore, the refrigerating effect per pound of liquid circulated is always less than the total latent heat of vapori-. Since condensation occurs at a constant temperature, the temperature of the liquid resulting from the condensation is also F. After condensation, as the liquid flows through the lower part of the condenser it continues to give up heat to the cooler condensing medium, so that before the liquid leaves the condenser its temperature is usually reduced somewhat below the temperature at which it condensed.
The liquid is then The temperature at said to be subcooled. In any case, because of the heat exchange between the refrigerant in the liquid line and the surrounding receiver tank. Because the hquid expands through the refrigerant control so rapidly, the liquid.
In this instance, enough of each pound of liquid vaporizes while passing. Obviously, only the liquid. That portion of each pound of hquid circulated which vaporizes in the refrigerant control produces no useful cooling and represents a loss of in the evaporator. In this. If, in Example , the temperature of the liquid entering the refrigerant control is 60 F rather than 86 F, determine the refrigerating effect. If, in Example , the pressure in the evaporator is That is, since there is no heat transfer between the refrigerant and the control, the enthalpy of the liquid-vapor.
Therefore, the difference between the enthalpy of the re-. Therefore, it is evident that the refrigerating effect per pound of liquid. Hence, for any given conditions the refrigerating effect per.
The higher the evaporating temperature and the lower the temperature of the liquid entering the refrigerant control,. System Capacity. The capacity of any. Start on. Show related SlideShares at end. WordPress Shortcode. Published in: Full Name Comment goes here. Are you sure you want to Yes No.
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A four-part organization covers mechanical refrigeration and food preservation, the thermodynamic processes of refrigeration systems, ideal and real refrigeration 3. This book provides a detailed, applications- oriented treatment of the mechanical refrigeration cycle, associated equipment, component design, and system operation.
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