CHAPTER-03

THERMODYNAMICS:

What is Thermodynamics?

Thermodynamics is a branch of physics which deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. Small scale gas interactions are described by the kinetic theory of gases. The methods complement each other; some principles are more easily understood in terms of thermodynamics and some principles are more easily explained by kinetic theory.

LAWS OF THERMODYNAMICS:

The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems. The laws describe how these quantities behave under various conditions.

The four laws of thermodynamics are:

Zeroth law of thermodynamics:

If two systems are both in thermal equilibrium with a third systems then they are in thermal equilibrium with each other.

Suppose we have three different bodies A, B and C. Let A is in thermal equilibrium with C and A is also

in thermal equilibrium with B then, as per zeroth law B is in thermal equilibrium with C.

EXAMPLES:

1. It is very similar to say that if A=C and A=B then B=C.

2. Thermometer.

First law of thermodynamics:

Heat and work are forms of energy transfer. OR Energy can be neither created nor destroyed. However, energy can change forms, and energy can flow from one place to another . The total energy

of an isolated system remains the sameEnergy is invariably conserved, however the internal energy of a

closed system may change as heat is transferred into or out of the system or work is done on or by the

system. It is a convention to say that the work that is done by the system has a positive sign and connotes

a transfer of energy from the system to its surroundings, while work done on the system has a negative

sign. For example, changes in molecular energy (potential energy),are generally considered to remain

within the system. Similarly, the rotational and vibrational energies of polyatomic molecules remain

within the system.

From the above, all the energy associated with a system must be accounted for as heat, work, chemical

energy etc., thus perpetual motion machines of the first kind, which would do work without using the

energy resources of a system, are impossible.

Examples:

1. Power wind

2. Generator

3. Solar plate

Second law of Thermodynamics:The second law of thermodynamics asserts the existence of a quantity called the entropy of a system and further states that:

Entropy of a system increases with increase in temperature of system.When two initially isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself but not necessarily with each other, are then allowed to interact, they will eventually reach a mutual thermodynamic equilibrium. The sum of the entropies of the initially isolated systems is less than or equal to the total entropy of the final combination.

This statement of the law recognizes that in classical thermodynamics, the entropy of a system is defined only when it has reached its own internal thermodynamic equilibrium.

The second law of thermodynamics is an expression of the universal principle of decay observable in nature. The second law is an observation of the fact that over time, differences in temperature, pressure, and chemical potential tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how much this process has progressed. The entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

Example :

When we give heat to ice cube is convert to liquid phase from solid body this is example of second law of thermodynamics.

Third law of thermodynamics:

This law says: “The entropy of a system approaches a constant value as the temperature approaches zero.” The entropy of a system at absolute zero is typically zero, and in all cases is determined only by

the number of different ground states it has. Specifically, the entropy of a pure crystalline substance

at absolute zero temperature is zero. At zero temperature the system must be in a state with the minimum

thermal energy. This statement holds true if the perfect crystal has only one state with minimum energy.

Graph of explanation:

Applications of Thermodynamic Laws:

Thermometer is a good example which uses zeroth law of thermodynamics". A body is said to be

in thermal equilibrium if its temperature does not change over time. If two single bodies are in contact

then they evolve to an equilibrium temperature where they will have the same temperature.

The thermal machines

A thermal machine can respond to 2 main functions:

- Transforming the Work in the heat (to move an object from heat): steam engine, heat engine (internal combustion engine, gasoline, diesel, alcohol or gas in cars, jet engines on airplanes) and so on.- Transforming work into heat (from hot to cold or from a moving object): heat pump, refrigerator, air conditioner, etc.

The thermal machines are based on a set of thermodynamic transformations that one comes to subject a fluid and cyclical.

In a steam engine, the fluid in is the water that we usually just heat with a coal boiler to evaporate water. The steam then activates a piston that will lead to a wheel through a crankshaft and cause a rotational movement

Fig: Steam engine

For a motor car explosion, the fluid used is of the diesel, alcohol or gas (LPG) by the motors. In a petrol engine 4 stroke classic, it comes to blow up an air / fuel mixture in a combustion chamber with a small spark found by candles (heat). This explosion allows the movement of a Piston which will generate a rotary motion via a crankshaft to rotate the wheels.

In refrigerators use a fluid called "refrigerant" as a heat transfer fluid (which carries the heat). Freon was used at the beginning but now complex fluids are used. (Ex. dichlorodifluoromethane, the tetrafluoroethane or methylpropan) those are less harmful to the environment. The general principle of the refrigerator is to compress a fluid through a compressor and then perform a relaxation (decrease fluid pressure suddenly) to the cold.Air conditioners are based on the same principle and heat pumps are just air conditioners that work in reverse.

Fig : Automobile engine.

HEAT TRANSFER REACTION:

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

Endothermic reaction:

In thermodynamics, the word endothermic describes a process or reaction in which the system

absorbs energy from its surroundings in the form of heat.

Examples:

Photosynthesis. Melting ice. Vaporizing Rubbing Alcohol Forming a cation from an atom in the gas phase Thermal decomposition reactions Forming a cation from an atom in the gas phase

Figure only for understanding:

Exothermicreaction:

In thermodynamics, the term exothermic ("outside heating") describes a process or reaction that

releases energy from the system, usually in the form of heat, but also in a form of light (e.g. a spark,

flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen).

Examples:

Condensation of rain from water vapor Combustion of fuels such as wood, coal and oil petroleum Mixing water and strong acids Mixing alkalis and acids The setting of cement and concrete

Figure only for understanding:

HEAT EXCHANGER:

A heat exchanger is a piece of equipment built for efficient heat transfer between two medium (hot to cold.)

Transfer of heat from one fluid to another is an important operation for most of the chemical industries. The most common application of heat transfer is in designing of heat transfer equipment for exchanging heat from one fluid to another fluid. Such devices for efficient transfer of heat are generally called Heat Exchanger. Heat exchangers are normally classified depending on the transfer process occurring in them. Amongst of all type of exchangers, shell and tube exchangers are most commonly used heat exchange equipment.

List of various types of heat exchanger is shown below:

Shell and tube type heat exchanger Plate type heat exchanger Plate and cell type heat exchanger Plate fin type heat exchanger Fluid type heat exchanger Direct contact heat exchanger Spiral heat exchanger

Out of this all type of heat exchanger shell and tube type heat exchanger is most widely used in industries.

Shell and tube heat exchanger

A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat

exchanger in oil refineries and other large chemical processes, and is suited for higher-

pressureapplications. As its name implies, this type of heat exchanger consists of a shell (a large pressure

vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the

tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle,

and may be composed by several types of tubes: plain, longitudinally finned, etc.

Fig: Shell and tube type heat exchanger.

Working:Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side.In order to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be put to use. This is an efficient way to conserve energy. Description of various parts of shell and tube type heat exchanger is given below:

Shell:Shell is the container for the shell fluid and the tube bundle is placed inside the shell. Shell diameter should be selected in such a way to give a close fit of the tube bundle. Shells are usually fabricated from standard steel pipe. The shell thickness of 12-24 inch can be satisfactorily used up to 300 psi of operating pressure.

Tube:Tube of ¾ and 1” are very common to design a compact heat exchanger. The most efficient condition for heat transfer is to have the maximum number of tubes in the shell to increase turbulence. The tube thickness should be enough to withstand the internal pressure along with the adequate corrosion allowance. The tube length of 6, 8, 12, 16, 20 and 24 ft are preferably used. Longer tube reduces shell diameter at the expense of higher shell pressure drop. Finned tubes are also used when fluid with low heat transfer coefficient flows in the shell side. Stainless steel, admiralty brass, copper, bronze and alloys of copper-nickel are the commonly used tube materials.

Baffles:Baffles are used to increase the fluid velocity by diverting the flow across the tube bundle to obtain higher transfer co-efficient. The distance between adjacent baffles is called baffle-spacing. The baffle spacing of 0.2 to 1 times of the inside shell diameter is commonly used. Baffles are held in positioned by means of baffle spacers. Closer baffle spacing gives greater transfer co-efficient.

REFRIGERATOIN SYSTEM AND AIR CONDITIONING SYSTEM:

Meaning of refrigeration is the production of cold confinement relative to its surroundings. In this, temperature of the space under consideration is maintained at a temperature lower than the surrounding atmosphere. To achieve this, the mechanical device extracts heat from the space that has to be maintained at a lower temperature and rejects it to the surrounding atmosphere that is at a relatively higher temperature. Since the volume of the space which has to be maintained at a lower temperature is always much lower than the environment, the space under consideration experiences relatively higher change in temperature than the environment where it is rejected.

The precise meaning of the refrigeration is thus the following: Refrigeration is a process of removal of heat from a space where it is unwanted and transferring the same to the surrounding environment where it makes little or no difference. To understand the above definition, let us consider example from the daily life.

It is a well-known fact that the spoilage of food and many other items reduces at a lower temperature. At a lower temperature, molecular motion slows down and the growth of bacteria that causes food spoilage also reduces. Thus to preserve many types of perishable food products for a longer duration, we use refrigerators in our homes, canteens, hotels, etc.

Air Conditioning:Merely lowering or raising the temperature does not provide comfort in general to the machines or its components and living beings in particular. In case of the machine components, along with temperature, humidity (moisture content in the air) also has to be controlled and for the comfort of human beings along with these two important parameters, air motion and cleanliness also play a vital role. Air conditioning, therefore, is a broader aspect which looks into the simultaneous control all mechanical parameters which are essential for the comfort of human beings or animals or for the proper performance of some industrial or scientific process. The precise meaning of air conditioning can be given as the process of simultaneous control of temperature, humidity, cleanliness and air motion. In some applications, even the control of air pressure falls under the purview of air conditioning. It is to be noted that refrigeration that is control of temperature is the most important aspect of air conditioning.

For comfort, both temperature and humidity have to be in the specified range. This is true for both human beings and scientific processes. Apart from the above two, from intuition one can also say that purity or cleanliness of the air is an essential item for the comfort and it has been established that the air motion is also required for the comfort condition.

It has been mentioned above that the refrigeration and air conditioning are related. Even when a space has to be heated, it can be done so by changing the direction of flow of the refrigerant in the refrigeration system, i.e., the refrigeration system can be used as a heat pump (how this is possible will be explained later). However, some section of the people, treat refrigeration exclusively the process that deals with the cooling of the space. They treat heating operation associated with the heat pump. The relationship between air conditioning and refrigeration fields can be understood from the Figure given below.

Fig: Relationship between the Refrigeration and Air Conditioning

Basic Refrigeration Cycle:

Mechanical refrigeration is accomplished by continuously circulating, evaporating, and condensing a fixed supply of refrigerant in a closed system. Evaporation occurs at a low temperature and low pressure while condensation occurs at a high temperature and high pressure. Thus, it is possible to transfer heat from an area of low temperature (i.e., refrigerator cabinet) to an area of high temperature (i.e., kitchen).

In refrigeration, heat is pumped out from a lower temperature space to a higher temperature environment. We know from our experience in daily life that water flows from a higher level to a lower level and heat flows from a body at a higher temperature to a body at a lower temperature. The reverse, i.e., flow of water from a lower level to a higher level and flow of heat from a body at a lower temperature to a body at a higher temperature do not occur naturally. In practice these are achieved at the cost of external work (power) done on the water and the carrier of heat (here the refrigerant) with help of a mechanical device.

Fig: Block diagram of refrigeration cycle Fig: Basic refrigeration cycle

Principles of Refrigeration:

Liquids absorb heat when changed from liquid to gas

Gases give off heat when changed from gas to liquid.

For an air conditioning system to operate with economy, the refrigerant must be used repeatedly. For this reason, all air conditioners use the same cycle of compression, condensation, expansion, and evaporation in a closed circuit. The same refrigerant is used to move the heat from one area, to cool this area, and to expel this heat in another area.

The refrigerant comes into the compressor as a low-pressure gas, it is compressed and then moves out of the compressor as a high-pressure gas.

The gas then flows to the condenser. Here the gas condenses to a liquid, and gives off its heat to the outside air.

The liquid then moves to the expansion valve under high pressure. This valve restricts the flow of the fluid, and lowers its pressure as it leaves the expansion valve.

The low-pressure liquid then moves to the evaporator, where heat from the inside air is absorbed and changes it from a liquid to a gas.

As a hot low-pressure gas, the refrigerant moves to the compressor where the entire cycle is repeated.

Note that the four-part cycle is divided at the center into a high side and a low side. This refers to the pressures of the refrigerant in each side of the system.

Applications of Refrigeration and Air Conditioning:Air Conditioning of Residential and Official Buildings

Industrial Air Conditioning

Spot Heating

Spot Cooling

Environmental Laboratories

Printing and Textiles

Precision Parts and Clean Rooms

Computer Rooms

Air Conditioning of Vehicles

Food Storage and Distribution

Difference between refrigeration & air conditioning system 

What is refrigeration

Refrigeration is the removal of heat from a place that needs to be cooled, such as a reefer container, a reefer cargo hold, a cold room, an air conditioned room or a domestic fridge, and its

transfer to, for example, the outside atmosphere .Refrigeration is used in the carriage of some liquefied gases and bulk chemicals, in air conditioning systems, to cool bulk CO 2 for firefighting systems and to preserve perishable foodstuffs during transport or storage. 

Heat naturally rises, so refrigeration works against the natural flow of heat. A confined area is cooled by removing the heat within it to the outside atmosphere, where it is warmer than the

space being cooled.

In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases - oxygen, nitrogen, propane and methane, for example. In compressed air

purification, it is used to condense water vapor from compressed air to reduce its moisture content. In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to

maintain certain processes at their required low temperatures (for example, in the alkylation of butenes and butane to produce a high octane gasoline component). Metal workers use

refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and sea-going vessels, refrigeration is a necessity.

Fig:Integrated reefer container air circulation process

Refrigeration and Air Conditioning

While the principles behind refrigeration and air conditioning are similar, there are certain basic differences:

Temperature: For human comfort, air conditioning needs a standard temperature of 22 deg C. In refrigeration the temperature can vary from minus 30 deg C to +25 deg C, depending on the cargo

Cargo: Humans and live animals use air conditioning, but refrigeration is used for non- living cargoes or live plants.

Humidity: For human comfort, air conditioning requires a certain humidity range, but in refrigeration, the level of humidity required depends entirely on the cargo. For some

cargoes the presence of humidity will even be harmful and they must therefore be maintained in a humidity-free condition.

Air Purification: An air conditioning system requires that the air is filtered and purified continually to remove odors and dust particles, using enzyme, mechanical and bio-filters. Refrigeration systems only require the same volume of air to be cooled and passed through the cargo compartment in order to maintain the desired temperature.

Fresh Air Requirement: An air conditioning system demands the circulation of a certain volume of fresh air containing 20.9% O2. If fresh air is not provided, after a short time the O2 in the room's atmosphere will be depleted and replaced with CO2. This makes it initially uncomfortable and then unsafe for human occupancy.

Although the air conditioning is controlled with the correct temperature and humidity, at the end of the day the room will feel stuffy as the amount of CO2 will have increased significantly. A certain degree of fresh air (and air movement, see below) is needed to replenish the consumed O2. In refrigeration, the timing and the amount of fresh air required strictly depends upon the cargo, as some need fresh air and others do not.

Air Movement: Air movement is required for air conditioning in a closed room, as to keep the occupants cool the air needs to be circulated continually. In refrigeration, air movement is controlled by the temperature that is needed to be maintained (ie the demand temperature or thermostat set-point). Once the desired temperature is reached, the air movement simply stops.