process control and instrumentaitonHeat Heat may be defined as transfer of energy from a hightemperatureobject to a lower temperature object.TemperatureIt is a measure of the average translationalkinetic energyassociated with the disordered microscopic motion of atoms and molecules.

Difference between heat and temperatureWe have all noticed that when you heat up something, its temperature rises. Often we think that heat and temperature are the same thing. However, this is not the case. Heat and temperature are related to each other, but are different concepts.Heat is the total energy of molecular motion in a substance while temperature is a measure of the average energy of molecular motion in a substance. Heat energy depends on the speed of the particles, the number of particles (the size or mass), and the type of particles in an object. Temperature does not depend on the size or type of object. For example, the temperature of a small cup of water might be the same as the temperature of a large tub of water, but the tub of water has more heat because it has more water and thus more total thermal energy.It is heat that will increase or decrease the temperature. If we add heat, the temperature will become higher. If we remove heat the temperature will become lower. Higher temperatures mean that the molecules are moving, vibrating and rotating with more energy.If we take two objects which have the same temperature and bring them into contact, there will be no overall transfer of energy between them because the average energies of the particles in each object are the same. But if the temperature of one object is higher than that of the other object, there will be a transfer of energy from the hotter to the colder object until both objects reach the same temperature.Temperature is not energy, but a measure of it. Heat is energy.How does heat travel?Heat can be transferred from one place to another by three methods: 1. Conduction in solids2. Convection of fluids (liquids or gases)3. Radiation through anything that will allow radiation to passThe method used to transfer heat is usually the one that is the most efficient. If there is a temperature difference in a system, heat will always move from higher to lower temperatures as described by the second law of thermodynamics or the Clausius statement.CONDUCTIONConduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water. Conduction takes place in all forms of matter, viz. solids, liquids, gases and plasmas, but does not require any bulk motion of matter. In solids, it is due to the combination of vibrations of the molecules in alatticeorphononswith the energy transported byfree electrons. In gases and liquids, conduction is due to the collisions anddiffusionof the molecules during their random motion Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point conductivity increases only slightly with increasing pressure and densityThe rate of conduction heat transfer is:= heat transferred in time =

= thickness of barrier

= thermal conductivity of the barrier

= area

= temperature

Thermal conductivitykis defined as "the quantity of heat, Q, transmitted in time (t) through a thickness (d), in a direction normal to a surface of area (A), due to a temperature difference (T) [...]." Thermal conductivity is a material propertythat is primarily dependent on the medium'sphase, temperature, density, and molecular bonding.

Rearranging the above equation gives thermal conductivity

The SI unit of thermal conductivity is Wm1K1 and English unit is Btu/[hr.ft.0F]. Its dimension is MLT-3K-11 Btu/[hr.ft.0F] = 1.730735 Wm1K1 The law of Heat Conduction, also known asFourier's law, states that the time rate ofheat transferthrough a material isproportionalto the negativegradientin the temperature and to the area, at right angles to that gradient, through which the heat is flowing.Alternatively it can be thought of as a flux of heat [energy per unit area per unit time] divided by a temperature gradient [temperature per unit length]The better the conductor, the more rapidly heat will transfer.Good ConductorsPoor Conductors

Iron Wood

SteelStyrofoam

CopperPaper

SilverAir

Conduction may be steady-state or transientSteady-conductionSteady state conductionis a form of conduction that happens when the temperature difference driving the conduction is constant, so that after an equilibration time, the distribution of temperatures in the conducting object does not change any further.In steady state conduction, the amount of heat entering a section is equal to amount of heat coming out.For example, a bar may be cold at one end and hot at the other, but after a state of steady state conduction is reached, the special gradient of temperatures along the bar does not change any further, as time proceeds. Instead, the temperature at any given section of the rod remains constant, and this temperature varies linearly in space, along the direction of heat transfer.Transient conductionTransient conductionoccurs when the temperature within an object changes as a function of time. Analysis of transient systems is more complex and often calls for the application of approximation theories or numerical analysis by computer.Material

Thermal conductivity

W/(mK)

Silica Aerogel0.004 - 0.04

Air0.025

Hollow Fill Fibre InsulationPolartherm0.04 - 0.4

Alcoholsandoils0.042

Polypropylene0.1 - 0.21

Mineral oil0.25

0.138

Rubber0.16

LPG0.23 - 0.26

Cement, Portland0.29

Epoxy(silica-filled)0.3

Epoxy (unfilled)0.59

Water(liquid)0.6

Thermal grease0.7 - 3

Thermal epoxy1 to 7

Glass1.1

Soil1.5

Concrete, stone1.7

Ice2

Sandstone2.4

Stainless steel12.11 ~ 45.0

Lead35.3

Aluminium237 (pure)

120180 (alloys)

Gold318

Copper401

Silver429

Diamond900 - 2320

Graphene(4840440) - (5300480)

This is a list of approximate values of thermal conductivity,k, for some common materials.

ConvectionIn convection heat energy is transferred between a solid surface and the nearby moving fluids [gasses, liquids] at different temperatures. Convection is the up and down movements of gasses and liquids caused by heat transfer.In reality this is a combination of diffusion and bulk motion of fluid molecules. Near the solid surface the fluid velocity is low, and diffusion dominates. Away from the surface, bulk motion of fluids increase the influence and dominates. As the fluid motion goes more quickly the convective heat transfer increases.As a gas or liquid is heated, it warms, expands and rises because it is less dense. The presence of bulk motion of fluid enhances the heat transfer between the solid surface and the fluid.Examples of convection Warmer water at the surface of a lake or swimming pool Wind currents Hot air balloon Lower floors of a building being cooler than the top floorConvective heat transfer may take the form of either ForcedOrAssistedConvection NaturalOrFreeConvectionForced or Assisted ConvectionForced convection occurs when a fluid flow is induced by an external force, such as a pump, fan or a mixer. Natural or Free ConvectionNatural convection is caused by buoyancy forces that result from the density variations due to differences of temperature in the fluid. For instance in the absence of an external source, when the mass of the fluid is in contact with a hot surface its molecules separate and scatter causing the mass of fluid to become less dense. When this happens the fluid is displaced vertically or horizontally while the cooler fluid gets denser and the fluid sinks. Thus the hotter volume transfers heat towards the cooler volume of that fluid. This continues phenomenon is called free or natural convection.

Boiling or condensing processes are also referred as a convective heat transfer processes.Internal or external flow can also classify convection. Internal flow occurs when the fluid is enclosed by a solid boundary such as a flow through a pipe. An external flow occurs when the fluid extends indefinitely without encountering a solid surface. Both these convections, either natural or forced, can be internal or external as they are independent of each other.The heat transfer per unit surface through convection was first described by Newton and the relation is known as theNewton's Law of Cooling.The equation for convection can be expressed as:q = h A dTWhereq= heat transferred per unit time A= heat transfer area of the surface h=convective heat transfer coefficientof the process dT= temperature difference between the surface and the bulk fluid

Convective Heat Transfer CoefficientsTheconvection heat transfer coefficient-h -is dependent on the type of media, gas or liquid, the flow properties such as velocity, viscosity and other flow and temperature dependent properties.For laminar flows the heat transfer coefficient is rather low compared to the turbulent flows, this is due to turbulent flows having a thinner stagnant fluid film layer on heat transfer surface.

In general the convective heat transfer coefficient for some common fluids is within the ranges: Free Convection - Air : 5 - 25 (W/m2K) Free Convection - Water: 20 - 100(W/m2K) Forced Convection - Air:10 - 200 (W/m2K) Forced Convection - Water:50 - 10.000 (W/m2K) Boiling Water : 3.000 - 100.000 (W/m2K) Condensing Water Vapor: 5.000 - 100.000 (W/m2K)Example A fluid flows over a plane surface1 m by 1 mwith a bulk temperature of50oC. The temperature of the surface is20oC. The convective heat transfer coefficient is2,000 W/m2oC.Solution:Using formulaq= (2,000 W/m2oC) ((1 m) (1 m)) ((50oC) - (20oC)) =60,000(W) =60(kW)Heat exchanger

Aheatexchangeris a specialized device that is built for efficient heat transferfrom one fluid to the other. In some cases, a solid wall may separate the fluids and prevent them from mixing. In other designs, the fluids may be in direct contact with each other. In the most efficientheatexchangers, the surface area of the wall between the fluids is maximized while simultaneously minimizing the fluid flow resistance. Fins or corrugations are sometimes used with the wall in order to increase the surface area and to induceturbulence.ApplicationsHeat exchangers are widely used in refrigeration, air conditioning, space heating, power generation, and chemical processing. One common example of a heat exchanger is the radiator in a car, in which the hot radiator fluid is cooled by the flow of air over the radiator surface.Fluid flow arrangements in heat exchangerThere are three primary flow arrangements withheatexchangers: Counter-Flow Parallel-Flow Cross-Flow In the counter-flowheatexchanger, the fluids enter theexchangerfrom opposite sides. This is the most efficient design because it transfers the greatest amount ofheat. In the parallel-flowheatexchanger, the fluids come in from the same end and move parallel to each other as they flow to the other side. The cross-flowheatexchangermoves the fluids in a perpendicular fashion.

Designs of heat exchangerThere are also four different designs ofheatexchangers: Shell And Tube Plate, Regenerative, Intermediate Fluid Or SolidThe most typical type ofheatexchangeris the shell and tube design. Thisheatexchangerhas multiple finned tubes. One of the fluids runs through the tubes while the other fluid runs over them, causing it to be heated or cooled. In the plateheatexchanger, the fluid flows through baffles. This causes the fluids to be separated by plates with a large surface area. This type ofheatexchangeris typically more efficient than the shell and tube design.The regenerativeheatexchangertakes advantage of theheatfrom a specific process in order toheatthe fluid used in the same process. Theseheatexchangers can be made with the shell and tube design or the plate design. The intermediate fluid or solidheatexchangeruses the fluids or solids within it to holdheatand move it to the other side in order to be released. This method is commonly used to cool gases while removing impurities at the same time