Ideal gas

An ideal gas is a gas whose pressure P, volume V, and temperature T are related by the ideal gas law

PV = nRT .

where:
P is the pressure of the gas
V is the volume of the gas
n is the amount of substance of gas (also known as number of moles)
R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant = 8.3145 J/mol K.
T is the temperature of the gas

At normal conditions such as standard temperature and pressure, most real gases behave qualitatively like an ideal gas. Many gases such as nitrogen, oxygen, hydrogen, noble gases, and some heavier gases like carbon dioxide can be treated like ideal gases within reasonable tolerances. Generally, a gas behaves more like an ideal gas at higher temperature and lower pressure, as the work which is against intermolecular forces becomes less significant compared with the particles' kinetic energy, and the size of the molecules becomes less significant compared to the empty space between them.

Read More

Ideal gas law

The ideal gas law is the equation of state of a hypothetical ideal gas. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law.

Equation

The state of an amount of gas is determined by its pressure, volume, and temperature. The modern form of the equation relates these simply in two main forms. The temperature used in the equation of state is an absolute temperature: in the SI system of units, Kelvin.

Common form:

The ideal gas law is often introduced in its common form:

PV=nRT 

where:

P is the pressure of the gas
V is the volume of the gas
n is the amount of substance of gas (also known as number of moles)
R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant = 8.3145 J/mol K.
T is the temperature of the gas

In SI units, P is measured in pascals, V is measured in cubic metres, n is measured in moles, and T in kelvin (273.15 kelvin = 0.00 degrees Celsius). R has the value 8.314 J·K−1·mol−1 or 0.08206 L·atm·mol−1·K−1or ≈2 calories if using pressure in standard atmospheres (atm) instead of pascals, and volume in liters instead of cubic metres.

Molar form:

How much gas is present could be specified by giving the mass instead of the chemical amount of gas. Therefore, an alternative form of the ideal gas law may be useful. The chemical amount (n) (in moles) is equal to the mass (m) (in grams) divided by the molar mass (M) (in grams per mole):

By replacing n with m / M, and subsequently introducing density ρ = m/V, we get:

Defining the specific gas constant R_specific as the ratio R/M,



This form of the ideal gas law is very useful because it links pressure, density, and temperature in a unique formula independent of the quantity of the considered gas. Alternatively, the law may be written in terms of the specific volume v, the reciprocal of density, as



It is common, especially in engineering applications, to represent the specific gas constant by the symbol R. In such cases, the universal gas constant is usually given a different symbol such as R to distinguish it. In any case, the context and/or units of the gas constant should make it clear as to whether the universal or specific gas constant is being referred to.

Applications to thermodynamic processes

The table below essentially simplifies the ideal gas equation for a particular processes, thus making this equation easier to solve using numerical methods.

Process Constant Known ratio P2 V2 T2 Isobaric process Pressure V2/V1 P2 = P1 V2 = V1(V2/V1) T2 = T1(V2/V1) T2/T1 P2 = P1 V2 = V1(T2/T1) T2 = T1(T2/T1) Isochoric process (Isovolumetric process) (Isometric process) Volume P2/P1 P2 = P1(P2/P1) V2 = V1 T2 = T1(P2/P1) T2/T1 P2 = P1(T2/T1) V2 = V1 T2 = T1(T2/T1) Isothermal process  Temperature  P2/P1 P2 = P1(P2/P1) V2 = V1/(P2/P1) T2 = T1 V2/V1 P2 = P1/(V2/V1) V2 = V1(V2/V1) T2 = T1 Isentropic process (Reversible adiabatic process) Entropy P2/P1 P2 = P1(P2/P1) V2 = V1(P2/P1)(−1/γ) T2 = T1(P2/P1)(γ − 1)/γ V2/V1 P2 = P1(V2/V1)−γ V2 = V1(V2/V1) T2 = T1(V2/V1)(1 − γ) T2/T1 P2 = P1(T2/T1)γ/(γ − 1) V2 = V1(T2/T1)1/(1 − γ) T2 = T1(T2/T1) Polytropic process P Vn P2/P1 P2 = P1(P2/P1) V2 = V1(P2/P1)(-1/n) T2 = T1(P2/P1)(n - 1)/n V2/V1 P2 = P1(V2/V1)−n V2 = V1(V2/V1) T2 = T1(V2/V1)(1−n) T2/T1 P2 = P1(T2/T1)n/(n − 1) V2 = V1(T2/T1)1/(1 − n) T2 = T1(T2/T1)

In an isentropic process, system entropy (S) is constant. Under these conditions, P1 V1γ = P2 V2γ, where γ is defined as the heat capacity ratio, which is constant for an ideal gas. The value used for γ is typically 1.4 for diatomic gases like nitrogen (N2) and oxygen (O2), (and air, which is 99% diatomic). Also γ is typically 1.6 for monatomic gases like the noble gases helium (He), and argon (Ar). In internal combustion engines γ varies between 1.35 and 1.15, depending on constitution gases and temperature

Read More

Energy

Energy

In physics, energy is the ability of a body or system to do work or produce a change; or is a property of objects that is transferable among them via fundamental interactions, which can be converted in form but not created or destroyed and expressed usually in joules or kilowatt hours (kWh).

The joule is the SI unit of energy, based on the amount transferred to an object by the mechanical work of moving it 1 metre against a force of 1 newton.

Read More

Heat

Heat

Heat is the condition or quality of being hot or cold. In other words, the degree of hotness or coldness of a body or environment is termed as heat.

In physics, heat is a form of energy associated with the motion of atoms or molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation. The higher the temperature of a material, the faster the atoms are moving, and hence the greater the amount of energy present as heat.

See also Temperature 

Read More

Isolated system

Isolated system

An isolated system is one devoid of interactions of any kind with the surroundings (including mass exchange, heat, and work interactions).

An isolated system cannot exchange any heat, work, or matter with the surroundings, while an open system can exchange all heat, work and matter; a closed system can exchange only energy(as heat or work) but not matter, with its surroundings.

Open-Close-Isolated System

See also Open and Close System.

Read More

Open system and Close system

Close system

In thermodynamics, a closed system can exchange energy (as heat or work) but not matter, with its surroundings.
A closed system is one in which no mass crosses the system boundaries, but energy can cross the system boundary in form of heat and work.

Open-Close-Isolated System

Open system

An open system can exchange all heat, work and matter, with its surroundings.
An open system is one in which mass crosses the system boundaries. The system may gain or lose mass or simply have some mass pass through it.

See also Isolated system.

Read More

SHELL & TUBE HEAT EXCHANGER

SHELL & TUBE HEAT EXCHANGER

Shell and tube heat exchangers are the most widely used type of heat exchanger.

GENERAL INFORMATION

The inside of the exchanger contains many tubes and baffles, as shown in the picture below. These tubes and baffles help direct the two streams flowing through the exchanger.

STRAIGHT TUBE SHELL & TUBE HEAT EXCHENGER

STRAIGHT TUBE SHELL & TUBE HEAT EXCHENGER

A shell and tube heat exchanger consists of several tubes enclosed in a shell. One fluid flows through the tubes while the other fluid is conducted through the shell. Flow through the shell and tubes can be countercurrent, cocurrent, or cross flow. In countercurrent flow, the shell fluid flows in the opposite direction of the tube fluid. In cocurrent flow the shell fluid flows in the same direction as the tube fluid. Lastly, in cross flow the shell fluid flows perpendicular to the flow of the tube fluid. In general, countercurrent flow results in the most efficient heat transfer.

U TUBE SHELL & TUBE HEAT EXCHENGER

The baffles serve two functions: Their strategic positioning supports the tubes, preventing vibration and sagging, and they also channel the fluid in the shell side, resulting in more efficient heat transfer.

U TUBE SHELL & TUBE HEAT EXCHENGER

Static mixers are sometimes placed in the tubes of the shell and tube exchangers to help dissipate heat. Mixing of the fluids in the tube removes temperature, velocity, and material composition gradients. Static mixers also allow fluids to be cooled near their freezing temperature without plugging the tubes.

Static mixer

The diagram below shows the mixing that occurs as the flow in the tube encounters the static mixer. The mixer itself does not move.

Static mixer

ADVANTAGES	DISADVANTAGES
Can handle fluids at high temperatures and pressures. Can handle fluids of all states. Easy to dismantle for cleaning or repairs. Design can be adapted to meet operating conditions.	One unit can only be used for one duty. High amounts of heat loss occur, so insulation is required. Larger space requirements and more expensive than plate and frame. Over time vibrations may damage the heat exchanger. Baffle placement may be optimized to reduce vibrations and help the heat exchanger last longer.

Read More

Plate and Frame Heat Exchanger

PLATE & FRAME

Typically, plate and frame heat exchangers are used for liquid-liquid exchange at low to medium pressures. However, gasket-free plate and frame heat exchangers can safely operate at high temperatures and pressures. Plate and frame heat exchangers offer flexibility because plates can be either added or compressed for each different situation.

Plate and Frame Heat Exchanger

Plate and Frame Heat Exchanger

GENERAL INFORMATION

The gaps between the plates in plate and frame heat exchangers can be adjusted according to the degree of fouling (deposits, corrosion, etc.) that is expected.

Plates of Plate and Frame Heat Exchanger

The counter-current flow of fluids that occurs in plate and frame heat exchangers allows approach temperatures as low as 1 to 2°F.

Gaskets ensure that the cold fluid (blue) and the hot fluid (red) don't mix. Alternatives to the traditional gasket seal include brazing and laser-welding.

The plates are stacked in an alternating manner to cause the counter current flow. The diagram below shows the flow in a heat exchanger. The design allows for the two media to flow in alternate directions and not be mixed. However, heat can be transferred from one medium to the other through the plates. 

ADVANTAGES

DISADVANTAGES

Require less space and are less expensive than shell and tube heat exchangers. Easy to adjust for different liquids by adding or subtracting plates. Pressure can be varied by compressing the plates. One frame can be used for multiple duties by simply changing plates. Working temperatures up to 550°C and pressures of 780 psi are possible with gasket-free versions. High heat transfer coefficients relative to shell and tube heat exchangers. Up to ten times more resistant to fouling than shell and tube heat exchangers.

Gasketed plate and frame heat exchangers have a maximum operating condition of 149°C and 300 psi. Not good for vaporizing fluids or large amounts of vapor. Gasket-free versions are impossible to open for inspection or cleaning.

Read More

Charle's Law

For a fixed mass of gas at constant pressure, the volume is directly proportional to the kelvin temperature.

this directly proportional relationship can be written as:

or

where:

V is the volume of the gas

T is the temperature of the gas (measured in Kelvin).

k is a constant.

This law describes how a gas expands as the temperature increases; conversely, a decrease in temperature will lead to a decrease in volume. For comparing the same substance under two different sets of conditions, the law can be written as:

An animation demonstrating the relationship between volume and temperature.

Read More

Boyle's law

For a fixed mass of gas at constant temperature, the volume is inversely proportional to the pressure.

Mathematically, Boyle's law can be stated as

or

where P is the pressure of the gas, V is the volume of the gas, and k is a constant.

The equation states that product of pressure and volume is a constant for a given mass of confined gas as long as the temperature is constant. The law can be usefully expressed as

Read More

Thermodynamics

Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work.

Read More

Endothermic reaction

In thermodynamics , the term endothermic describes a process or reaction in which the system absorbs energy from its surroundings in the form of heat.

See

Exothermic reaction.

Read More

Exothermic reaction

An exothermic reaction is a chemical reaction that releases energy in the form of light or heat.

Expressed in a chemical equation: reactants → products + energy

An exothermic reaction is a chemical or physical reaction that releases heat. It gives out energy to its surroundings. The energy needed for the reaction to occur is less than the total energy released.



See Endothermic reaction

Read More

Specific enthalpy

The specific enthalpy of a uniform system is defined as h = H/m where m is the mass of the system.
The SI unit for specific enthalpy is joule per kilogram (J/kg). It can be expressed in other specific quantities by h = u + pv, where u is the specific internal energy, p is the pressure, and v is specific volume, which is equal to 1/ρ, where ρ is the density.

Read More

Adiabatic process

An adiabatic process is a thermodynamic process where a system exchanges no heat with its surroundings (Q = 0). In an adiabatic process, ΔT ≠ 0 but Q = 0.

Read More

Isothermal process

An isothermal process is a thermodynamic process, in which the temperature remains constant: ΔT = 0.The heat transfer into or out of the system typically must happen at such a slow rate that the thermal equilibrium is maintained. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. in an isothermal process, the value ΔT = 0 and therefore ΔU = 0 (only for an ideal gas) but Q ≠ 0.

Read More

Enthalpy

Enthalpy : A thermodynamic property of a system equal to the sum of its internal energy and the product of its pressure and volume.

H = U + pV

Read More

Temperature

Temperature : A measure of the warmth or coldness of a sample or an object with reference to some standard value.

Read More