# PRINCIPLES OF REFRIGERATION 5TH EDITION PDF

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dossat, thomas j. horan principles of refrigeration (5th edition) in pdf form, in that free pdf download now!!! source #2: principles of refrigeration roy j cittadelmonte.info 6 days ago Principles Of Refrigeration 5th Edition - [Free] Principles Of Electricity for Refrigeration Heating and Air Conditioning by Russell cittadelmonte.info Pdf. PDF Principles of Refrigeration (5th Edition); 2. DESCRIPTION This book provides a detailed, applications-oriented treatment of the mechanical.

This textbook has been written especially for use in programs where a full curriculum in refrigeration is offered. However, the material covered and the method of presentation are such that the text is also suitable for adult evening classes. Despite a rigorous treatment of the thermodynamics of the cycle, application. The first four chapters deal with the fundamental principles of physics and thermodynamics upon which the refrigeration cycle is based. For those who are already familiar with these fundamentals,.

When heat energy flows into the solid, the molecules Of the solid begin to move slowly and the temperature of the solid begins to climb. Sensible Heat. When heat either absorbed or rejected by a material causes or. The division of heat into several classifications is made only to facilitate and or rejects.

In this type of problem, where the direction of heat flow is obvious, the negative sign can be ignored and the answer assumed to be positive. As previously shown, the quantity of heat which must be transferred in order to bring about a specified change in the temperature of any given mass of any material can be calculated by applying Equation The Melting or Fusion Temperature.

Further, the quantity of heat that must be rejected by a certain is. Hence, the material cannot exist in the solid state at any temperature above its melting or. In general, the melting temperature decreases as the pressure increases except for noncrystalline solids,. As the molecules flow over and about one. The attraction which exists between the molecules of a solid. The latent heat of fusion, along with other values such as specific heat, fusion temperature, etc.

Calculate the quantity of heat required to melt 12 lb of ice at 32 F into water at 32 F. The latent heat of fusion of water under atmospheric pressure is Btu per pound. Since 12 lb of ice absorb Btu in melting into water, it follows that 12 lb of water at 32 F will reject Btu in returning to the solid state.

If SO lb of ice at 32 F absorb Btu, what part of the ice will be melted? The amount of energy required to do the internal work necessary to overcome these restraining forces is very great.

For this reason, the capacity including the force of gravity. It both the latent heat value and the saturation temperature of any particular liquid vary with the pressure over the.

Iron, for example, vaporizes at F, copper at F, and perature. Water, of course, boils at F, and alcohol at F. Some liquids boil at. One gallon of F in an open container absorbs How much water is vaporized? Since the saturation temperature of water at atmospheric pressure is F, the entire mass of the water must be raised to this temperature before any water will vaporize gal of water. Through the use of a temperature-heat diagram, the solution to Example is shown graphically in Fig. When the temperature of a vapor has been so increased above the saturation temperature, the vapor is said to be superheated and is called a superheated vapor.

Superheated vapors are discussed at length in another chapter. Total Heat. The total heat of a called superheat. Latent heat of fusion Sensible heat of the liquid Latent heat of vaporization Total heat of 1 lb of steam. The total heat of 1 lb of saturated steam is the sum of the following heat quantities a To raise the temperature of 1 lb of ice from. The fact that internal energy is usually expressed in heat energy units gives rise to the definition of heat as molecular or internal energy.

As the molecules of the metal is. If a wire is bent rapidly back and forth, the bent portion of the wire becomes hot because of the agitation of the. Often the external energy of a body is converted to internal energy and vice versa.

For example, a bullet speeding toward a target has. Once the energy flows into a body it becomes "stored" thermal energy. Hence, thermodynamically speaking, internal energy is not heat but thermal energy in storage. Not all the heat energy flowing into a body is. In many some or all of the energy flowing into the body passes through or leaves the body as work mechanical energy. This is made clear. Furthermore, up to this point it has been assumed that the internal energy of a body is increased only by the addition of heat energy directly, as from a flame or some other heat source.

However, this is not the case. The internal or molecular energy of a body may also be increased when work is done on the body. That is, the mechanical energy of the work done on a body may be converted to the internal energy of the body.

For example, the head of a nail struck by a hammer will become warm as a. To convert energy in Btu into energy in foot-pounds, the energy in Btu is multiplied by of work.

This quantity. Expressed as equations, these. Convert 36, ft-lb mechanical energy into heat energy units. Applying Equa-. Calculate the quantity. What is the temperature of the vapor in degrees 4. In a certain industrial process, gal of water are cooled from 90 F to 55 F each hour.

Determine the quantity of heat which must be removed each hour to produce the required. Assuming that there is no loss of heat to the surroundings, if the initial temperature of the water is 80 F, to what temperature will the water be cooled Ans.

Determine the quantity of heat required to do the work. It is the relatively large spaces between the crystals of the solid,. This is true also for crystalline solids other than. The amount of expansion which a material experiences with each degree of temperature rise is known as its are based. Since solids and liquids are not readily com-.

This effect is in strict accord-. That is, its volume will increase or decrease with the addition or removal of heat. One of the few exceptions to this rule is water.

If water is cooled, its volume will decrease is. At this point, water attains its maximum density and, if further cooled, its volume will again increase. Furthermore, after being cooled to 32 F, it will solidify and the.

To provide for the normal expansion and contraction occurring with temperature changes, expansion joints are built into highways, bridges, pipelines, etc. The peculiar behavior of water as it solidifies appears to contradict the general laws governing molecular activity as described previously.

Space must be allowed for the normal expansion. Otherwise the tremendous expansive forces generated by a temperature increase will cause the con-. In fact, 1 cu ft of water will freeze into approximately 1. This accounts for the tremendous expansive force created during solidification which is solidification.

Because of its loose molecular structure, the change in the volume of a gas as the gas is heated or cooled is much greater than that. The volume of many common materials can be found in various. It should be noted at the outset that in applying the fundamental gas laws it is always necessary to use absolute pressures and. If the density of the water is In order to better visualize a constant pressure change in condition, assume that a gas is confitting,. If the specific volume of dry saturated steam at F is Since the piston.

Gas confined In a cylinder with a perfectly fitting, frictionless piston, Fig. Constant pressure process, is b As gas is heated, both the temperature and the volume of the gas Increase.

The increase in volume exactly proportional to the increase in absolute temperature, c As gas is cooled, both the temperature and the volume of the gas decrease. The decrease in volume is exactly proportional to the decrease in absolute a. Charles' Law for a Constant Pressure Charles' law for a constant pressure. Thus, if the absolute temperature of a gas is doubled while its pressure is kept constant, its volume will also be doubled.

Likewise, if the absolute temperature of a gas is reduced by one-half while the pressure. A gas, whose initial temand whose initial volume is 5 cu ft, is allowed to expand at a constant pressure until its volume is 10 cu ft. Determine Example. By rearranging and applying Equation , the final temperature of the gas T2.

When any three of the preceding values are known, the fourth may be calculated by applying. If the initial volume of the gas is 8 cu ft, what is its final. Since the temperatures are given must be converted to degrees Rankine before being substituted in. When the volume. Thus, when a gas is compressed volume decreased while its tem-. That is, the greater the number of molecules the greater the quantity of gas and the higher the velocity of the molecules the.

The force with which the mole-. It is this incessant molecular bombardment which produces the pressure that a gas exerts upon the walls of its container. The magnitude of the pressure exerted depends upon the force and frequency of the molecular impacts upon a given area.

The greater the force. The increase in pressure which occurs is accounted for by the fact that the volume of the gas is diminished and a given number of gas molecules are confined in a smaller space so that the frequency of impact is greater. Any thermodynamic process which occurs in such a way that the temperature of the working substance does not change during the process is called an isothermal constant temperature the gas.

Constant volume process, a Initial condition, b The absolute pressure increases in direct proportion to the increase in absolute temperature, c The absolute pressure decreases in direct proportion to the decrease in absolute temperature.

Five pounds of air are expanded at a constant temperature from an initial volume of 5 cu ft to a final volume of 10 cu ft. If the initial pressure of the air is 20 psia, what is the final pressure in psia? Relationship Pressure-Temperature at a Constant Volume.

Assume that a gas is confined in a closed cylinder so that its volume cannot change as it is heated or cooled Fig. When the temperature of a gas is If the gas is cooled, the absolute pressure of the.

If the. The reduction in the force and the frequency of molecular impacts is accounted. The gas constant R is different for each gas. The gas constant for most common gases can be found in in Table Since the value of R for most gases can be found in tables, if any three of the four properties, P, V, M, and T, are known, the fourth property can be determined by Equation Notice that the pressure must be in.

If a gage on the tank reads If the gas is heated until the final gage pressure is 50 psi, what is the final temperature in degrees.

Charles' Law for a Constant Volume Process. Charles' law states in effect that when a gas is heated or cooled under such conditions that the volume of the gas remains unchanged or constant, the absolute pressure varies directly with the absolute temperature. However, if, for any one gas, the weight of 1 lb.

The constant, of course, will be different for different gases and, for any. Two pounds of air have a volume of 3 cu ft.

If the pressure of the air is External Work. Whenever a material undergoes a change in volume, work is done. If For example, consider a certain weight of gas confined in a cylinder equipped with a movable decreases,. It is possible, however, for a gas to do external work without the addition of energy from an external source. In such cases, the gas does the work at the energy. When a gas is compressed its volume decreased , a certain amount of work must be done on the gas in order to compress it.

And, an amount of energy equal to the amount of work done will be imparted to the molecules of the temperature decreases in an the. That is, the mechanical energy of the piston motion will be transformed into the internal kinetic energy of the gas molecular motion and, unless the gas is cooled during the compression, the temperature of the gas will increase in proportion to the.

Some work is done, also, in overcoming friction overcoming the pressure of the atmosphere. The General Energy Equation. The law of conservation of energy clearly indicates that the energy transferred to a body must be accounted for in its entirety.

It has been shown that some part or all of the energy taken in by a material may leave the material as work, and that only that portion of the transferred energy which is not utilized to do external work remains. Depending upon the particular process or change in condition that the material undergoes, any of the terms in Equation may have any value either positive or negative, or any may be equal to zero. This will be made clear later. This is not true, however, when a liquid changes into the vapor phase.

The change in. For example, when 1 lb of water at atmospheric pressure changes into a vapor, its volume increases from 0. Of the Vaporization of the liquid, of course, causes a greater separation of the molecules and brings about a substantial reduction in internal friction, but some interaction between the molecules of the vapor still exists. In the gaseous state, intermolecular forces are greatest when the gas is near the liquid phase and diminish rapidly as the gas. Although no such thing as an ideal or perfect.

However, as the liquid is heated and the mole-. The idea of internal friction is not difficult to comprehend. 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.

## Roy J. Dossat - Principles Of Refrigeration.pdf

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.

## [ePub] Principles of Refrigeration (5th Edition) by Roy J. Dossat Ful…

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.

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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: Show related SlideShares at end. WordPress Shortcode. Published in: Full Name Comment goes here. Are you sure you want to Yes No. Be the first to like this. No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide.

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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