Thermodynamics
Page Summary
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Thermodynamics, heat transfer and fluid mechanics are the basic principles underlying heating, refrigeration and air conditioning. This section is devoted to basic properties and principles of thermodynamics.
Cyclic Process
If we start with a particular set of thermodynamic co-ordinates of a system describing its state point and then carry the process through a complete cycle ending up with the initial state point, the system is said to undergo a cyclic process. Thus all the thermodynamic properties are the same at the start and the end of a cyclic process. |
Thermodynamic ProcessesWhen a substance, in any of the physical states - solid, liquid or gas - is heated, it expands i.e. its volume increases and thus its density decreases. Similarly, when a substance is cooled, it contracts or its volume decreases. Water, however, behaves differently between the temperatures 0 *C (32 *F) and 4 *C (39.2 F). When water at 0 *C is heated, instead of expanding, it contracts. This contraction continues until the temperature of water reaches 4 *C (39.2 *F). Thereafter, further heating will result in expansion. Similarly, water at 5 *C when cooled contracts, but on attaining 4 *C, any further cooling will make the water expand and not contract until the temperature touches 0* C (32 *F).
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An Iconic creation of Thermodynamics - Internal Combustion Engine
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Thus, water has maximum density at 4 *C (39.2 *F). When it is cooled below 0 *C (32 *F), solidification (formation of ice) occurs accompanied by further expansion, reducing the density of ice to a level below that of water. This is why ice floats on water.
As the temperature of water on the surface in a lake or ocean reaches 4 *C, it becomes denser, and hence drops down, pushing the warm water from below. This process goes on till the whole mass of water is at 4 *C. When the surface temperature goes below 4 *C, the surface layer becomes lighter because of expansion and thus does not go below, while the top layers gradually freeze as the temperature falls to 0 *C. Therefore, water in a lake or ocean freezes at the surface while the water below remains as liquid at 4 *C. This property of water enables the aquatic animals to live comfortably even in the severest of winters.
The property of water to expand on solidification creates a tremendous expansive force, sufficient to burst water pipes in winter and in refrigeration water chillers.
Like solids and liquids, gases also expand on heating. However, there is a difference in the case of gas, because of its pressure. In the case of gas, there are three variables: (1) pressure, (2) volume, and (3) temperature.
Before proceeding further on the properties of gases, it is necessary to understand the difference between gas and vapor. There is a certain temperature for every liquid/gas which is called its critical temperature. When a gas is above its critical temperature, any amount of increase in pressure cannot liquefy it. When the temperature is below its critical point, the gas can be liquefied without lowering its temperature by merely increasing the pressure. Vapor is defined as that which can be liquefied by only increasing its pressure, while to liquefy gases, not only an increase in its pressure, but also a lowering of its temperature is required. For example, alcohol, gasoline, refrigerants, etc. are vapors; hydrogen, oxygen, helium, etc. are gases. Thus, vapor behaves as gas above its critical temperature, and gas behaves as vapor below its critical temperature. In the following sections, the gas laws are described. In mechanical refrigeration, our concern is with vapor and not with gases.
Like solids and liquids, gases also expand on heating. However, there is a difference in the case of gas, because of its pressure. In the case of gas, there are three variables: (1) pressure, (2) volume, and (3) temperature.
Before proceeding further on the properties of gases, it is necessary to understand the difference between gas and vapor. There is a certain temperature for every liquid/gas which is called its critical temperature. When a gas is above its critical temperature, any amount of increase in pressure cannot liquefy it. When the temperature is below its critical point, the gas can be liquefied without lowering its temperature by merely increasing the pressure. Vapor is defined as that which can be liquefied by only increasing its pressure, while to liquefy gases, not only an increase in its pressure, but also a lowering of its temperature is required. For example, alcohol, gasoline, refrigerants, etc. are vapors; hydrogen, oxygen, helium, etc. are gases. Thus, vapor behaves as gas above its critical temperature, and gas behaves as vapor below its critical temperature. In the following sections, the gas laws are described. In mechanical refrigeration, our concern is with vapor and not with gases.
Representing Boyle's Law
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Boyle's LawThe Boyle's law gives us the relation between pressure (P) and volume (V) when temperature (T) is kept constant. The law states that at constant temperature, pressure varies inversely with the volume of the gas. In other words, if volume increases two times, pressure comes down by half. This means that:
Pressure X Volume = constant -------- PV = C, where P = absolute pressure T = absolute temperature V = volume |
Charles' Law(1). Charles' Law gives us the relation between volume and temperature, with the pressure kept constant. The law states that at constant pressure, volume variation is directly proportional to the temperature variation of the gas.
---- V/T = constant ----- (2). Charles' Law also gives the relation between pressure and temperature. The law states that pressure variation is directly proportional to temperature variation, provide that the volume is kept constant. |
Representing Charles' Law - Volume and Temperature Relationship
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---- P/T = constant ----
Combining these two laws, we have the general gas law, giving the equation,
---- PV/T = constant ---- where, P = absolute pressure, T = absolute temperature.
The gas laws are applicable only to gases (i.e. superheated condition) and not to vapor (i.e. under saturated condition or in other words, where liquid and its vapor exist together such as in a closed vessel).
Combining these two laws, we have the general gas law, giving the equation,
---- PV/T = constant ---- where, P = absolute pressure, T = absolute temperature.
The gas laws are applicable only to gases (i.e. superheated condition) and not to vapor (i.e. under saturated condition or in other words, where liquid and its vapor exist together such as in a closed vessel).
Specific Heat of Gases
The quantity of heat required to raise the temperature of unit mass of a gas through one degree, with the volume of gas kept constant, is known as the specific heat at constant volume. Again, the heat required to raise the temperature of unit mass of a gas by one degree with the pressure remaining constant is defined as the specific heat at constant pressure.
When a gas passes from an initial condition to some final condition, it is said to have undergone a process, These changes can occur in many ways and two of them are of interest to us, namely, isothermal and adiabatic. When during process, there is no change in the temperature of gas, it is called an isothermal process.
Examples of isothermal processes are:
During the process, if there is no absorption or rejection of heat, it is called an adiabatic process. The compression of a gas in a compressor is very nearly an adiabatic process.
When a gas passes from an initial condition to some final condition, it is said to have undergone a process, These changes can occur in many ways and two of them are of interest to us, namely, isothermal and adiabatic. When during process, there is no change in the temperature of gas, it is called an isothermal process.
Examples of isothermal processes are:
- The evaporation of a refrigerant liquid in the evaporator at constant temperature.
- The condensation of gas in a condenser at constant temperature.
During the process, if there is no absorption or rejection of heat, it is called an adiabatic process. The compression of a gas in a compressor is very nearly an adiabatic process.
Critical Pressure and Critical Temperature
As explained under Boiling Point previously, at saturation, any refrigerant (or for that matter any liquefiable gas) has a definite 'pressure-temperature' relationship and so its change of state from liquid to gas (or gas to liquid) can be achieved at that pressure and temperature, by adding (or removing) its latent heat of vaporization by heating (or cooling it).
However, beyond a particular pressure, known as its critical pressure, a gas cannot be liquefied at all by cooling it to any extent. Again, above a particular temperature, known as its critical temperature, it cannot be liquefied at all by increasing its pressure.
However, beyond a particular pressure, known as its critical pressure, a gas cannot be liquefied at all by cooling it to any extent. Again, above a particular temperature, known as its critical temperature, it cannot be liquefied at all by increasing its pressure.
pH Value - Acidity/AlkalinityEvery liquid, because of impurities dissolved in it, becomes acidic or alkaline according to the impurities in it. pH is an arbitrary symbol in the form of numerals (0 - 14) to identify the degree of acidity or alkalinity of the liquid. pH values below 7.0 and moving toward '0' denote an increasingly acidic condition, while values from 7.0 to 14.0 are increasingly alkaline, and the value 7.0 is taken as a 'neutral' condition. Most natural waters have a pH value of 6 to 8.
A liquid with pH value below 7.0 becomes corrosive (being acidic) and attacks the materials it comes in contact with. Alkalinity is the important characteristic of water for determining its scale-forming tendency. |
Illustrating Critical Pressure & Critical Temperature
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For example, when pH of water exceeds 7.5 or 8.0, the calcium carbonate dissolved in it precipitates more readily as scale on the water tubes. pH is measured with an electrometric pH meter. Color indicators are also available. The pH scale is logarithmic in nature; that means, it is not linear in proportion; e.g. a solution having a pH value of 5.0 is 10 times more acidic as compared to one at a pH value of 6.0, i.e. when the pH value rises from 5 to 6, the increase is only 20% (0.2 times), while the decrease in acidity is 10 times.
pH values of various substantances
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Relative Humidity (RH)Relative Humidity is the ratio of the actual amount of moisture present in unit volume of dry air at a certain temperature to the amount of moisture needed to saturate it at that temperature.
In other words, it is the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. Absolute HumidityIt is the actual amount of moisture in the unit volume of dry air (g/cc or gr/cu.ft.)
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Specific Humidity
It is the weight of moisture in the unit mass of dry air (g/kg or gr/lb.). It is also defined as a ratio of the water vapor content of the mixture to the total air content on a mass basis.
Corrosion
It is the destruction of a metal by chemical or electrochemical reaction with its environment. In most cases, the reaction is electrochemical in nature - a flow of electricity between certain areas of the metal surface through a solution capable of conducting an electric current. This electrochemical action causes eating away of metal at certain areas.
The important factors in corrosion are voltage difference between pure and impure areas; temperature, humidity and oxygen in the surrounding air - higher humidity in the surrounding air can cause condensation of water vapor on the cold metal surfaces, and the concentration of the water molecules and its thickness increases with the surrounding relative humidity. Iron may react with the condensed water, and with oxygen form hydroxide, i.e. rust.
The important factors in corrosion are voltage difference between pure and impure areas; temperature, humidity and oxygen in the surrounding air - higher humidity in the surrounding air can cause condensation of water vapor on the cold metal surfaces, and the concentration of the water molecules and its thickness increases with the surrounding relative humidity. Iron may react with the condensed water, and with oxygen form hydroxide, i.e. rust.
Heat Transfer Coefficient
Heat transfer occurs when heat flows from a substance at a higher temperature level to another at a lower temperature. The rate at which heat flows or heat transfer occurs from one substance to another, is the heat transfer coefficient or overall conductance factor or U factor of the heat transfer surface. It is expressed in heat units per unit area (A) of the heat transfer surface, per unit degree of temperature difference (TD) between the substances - in metric scale: kcal/m^2/*C or in fps scale: BTU/ft.^2/*F.
Heat Transfer Capacity Equation
Quasi-Static Process
If a process is carried out in such a way that at every instant the system departs only infinitesimally from the thermodynamic equilibrium state, such a process is defined as Quasi-Static process., i.e. a process closely approximating a succession of equilibrium states is classified as quasi-static. Only a quasi-static process can be represented on a thermodynamic place.
Illustrating a Quasi-Static Process
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Thermodynamic PropertiesA property of a system is any observable characteristic of a system. The properties we shall deal with are measurable in terms of numbers and units of measurements and include such physical quantities as location, speed, pressure, density, etc. A listing of a sufficient number of independent properties constitutes a complete definition of the state of a system. Such characteristics are also called state variables or thermodynamic co-ordinates of the system.
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When all the properties of a system have definite values, the system is said to be in a definite state. Systems in identical states must have identical values of their corresponding properties. Thus, property of the system depends solely upon the state of the system and not upon how that state may have been reached.