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HEAT TRANSFER AND HEAT EXCHANGERS

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HEAT TRANSFER AND

HEAT EXCHANGERS

UNITS USED IN HEAT TRANSFER TEMPERATURE: Temperature is the measurement of HEAT INTENSITY, how hot or how cold, and can be measured by using a thermometer.Temperature is generally measured in units of oC (degrees Celsius or Centigrade), or in o F (degrees Fahrenheit). For scientific uses, temperature is measured in DEGREES ABSOLUTE. These are Kelvin for the Celsius scale and degrees Rankine for the Fahrenheit scale.To convert degrees Celsius to Kelvin, just add 273.

0K = oC + 273To convert degrees Fahrenheit to degrees Rankine, just add 460.

o R = o F + 460

oF oR oC oK

212 672 100 373

100 560 37.8 310.8

32 492 0 273

- 40 420 - 40 233

- 460 ZERO - 273 ZERO

HEAT.Heat is a form of energy and is measured in units called Joules symbol J. Fuels contain heat energy. For example, when 1 cubic meter of natural gas is burned in air, the chemical reaction we call combustion produces 37,000,000 joules of heat energy.The joule is a small unit of energy. 1 kJ = 1000 J or 103 J 1 MJ = 1,000,000 J or 106 J

Energy can also be expressed in Btu, British thermal unit, or in cal, Calorie. The calorie, abbreviated cal, is defined as the amount of heat needed to heat 1 gram of water 1.0 oC. Also, 1 kcal, kilocalorie = 1000 cal. The Btu is defined as the amount of heat needed to raise 1.0 pound of water 1oF. Hence,

1 Btu = 252.16 cal 1 cal = 4.184 J 1 Btu = 1055.06 J

HEAT FLOW

Heat will flow from a hot to a cold area. The speed at which the heat flows will be the number of joules that pass from the hot area to the cold area in one second, i.e. joules per second..In the SI system of units, 1 W = 1 JS-1

If a temperature difference exists between two points, then heat will flow, JS-1 or W, from the hotter to the colder point. The temperature difference between the two points is often

thought as the DRIVING FORCE in heat transfer. The temperature gradient between two points is : Temp. gradient = (higher temp. – lower temp) / Distance

between points

0.3 m

Hot side900oC

(1173 K)

Cold side60oC

(333 K)

Heat flow 1400 J s -1

Temperature 900 – 60 = 840 oCdifference or 1173 – 333 = 840 K

Temperature 840 / 0.3 = 2800 oC m –1

gradient or 840 / 0.3 = 2800 K m –1

Heat flow 1400 J s –1 or 1.4 kJ s-1 1400 W or 1.4 kW

TYPES OF HEATIf heat energy is added to a substance the motion of the molecules increases creating more heat and thereby increasing the temperature. nearly all modern chemical processes, heat is either:Needed to make the reaction between the substances happen, orIs produced during a chemical reaction.

Heat is required to boil water to make steam or to change any liquid into vapor. Heat is required to change ice into water or any solid into its liquid state.

Latent Heat of Fusion changes solid to liquid.

Latent Heat of Vaporization changes liquid to vapor.

Latent Heat of Condensation changes vapor to liquid,

Latent Heat of Solidification changes liquid to solid.

Latent Heat of Sublimation changes solid to vapor.

If heat is given to a substance, we say the substance becomes hotter. Conversely, if heat is taken away we say the substance becomes colder. Heat can also cause a change in the state of a substance without changing its temperature. This is called LATENT HEAT.

HEAT CAPACITY ( specific heat capacity of a substance)

The amount of heat necessary to increase the temperature by 1 degree. It can be expressed for 1g, 1Ib, 1 gmol, 1 kg mol or 1lb mol of the substance. For example, a heat capacity is expressed in SI units as J/kg mol.K; in other units as cal/g.oC, cal/g mol.oC, kcal/kg mol . oC, Btu/lbm. oF or Btu/lb mol.oF.

Gases have different specific heat capacity values: Cp = the specific heat capacity at constant pressure. Cv = the specific heat capacity at constant volume.

The following relations apply to ideal gases:

Cp – Cv = R = 8314.34 J/kg mol. 0K

For mono atomic gases: Cv = 3/2R and Cp =5/2R For bi-atomic gases: Cv = 5/2R and Cp =7/2R For tri atomic gases: Cv = 6/2R and Cp =8/2R

The following is a general empirical formula applies to all multi-atomic gases: Cp = a + bT + cT2 + dT3

Where: T = temperature in K a,b,c, and d are constants for the substance concerned and can be obtained from tables.

The specific heat capacity of liquids is usually between 1.6 and 2.1 kJ/ kg.K, the specific heat will increase with rising temperature.

MIXTURES.Mixtures frequently have to be heated or cooled in the process industry. We have to calculate an average specific heat capacity from the composition by mass, weight percentages.EFFECT OF PRESSURE.

Pressure has little or no effect on solids The Cp of liquids increases with rising pressure. The Cp of gases increases as the pressure increases. The quantity of heat which is absorbed or released can be calculated by the following way: Quantity of heat = mass x temperature difference × specific heat The equation is: Q == m × Δ t × c Where: m = mass Δt = temperature difference c = specific heat capacity

Example: 2 kg of water are heated from 30oC to 60oC specific

heat capacity of water = 1 kcal/ kg.°C = 4.1868 kJ/ kg.0K

Q = 2 × 30 × 1 = 60 kcal Q = 2 × 30 × 4.18 = 251.4 kJ

HEAT TRANSFER Heat transfer is the flow of heat energy from a hot

substance to a colder substance.

METHODS OF HEAT TRANSFER. Heat can be transferred by THREE methods: By CONDUCTION By CONVECTION By RADIATION

HEAT TRANSFER BY CONDUCTIONHeat has traveled through the metal rod by a process known as CONDUCTION.In metals the process is usually rapid, whereas in non-metals it is slow.Metals contain FREE ELECTRONS in their structure. These electrons move about the crystal structure of the metal in a fairly haphazard and random way. When a metal is heated, the free electrons begin to move faster, i.e. their kinetic energy increases. The hot electrons drift towards the colder parts of the metal taking with them the energy they have picked up. At the same time the slower moving, cooler, electrons drift towards the heated end, pick up energy and in turn move towards the colder parts. This movement of the electrons thus mixes and passes energy through the metal rod.

Energy is also transmitted through the metal rod by VIBRATION OF THE ATOMS/MOLECULES themselves. The atoms/molecules at the hot end pick up energy and vibrate more rapidly. Under these conditions they collide with the colder atoms/molecules adjacent to them and in turn cause them to vibrate at a new, faster rate. In this way heat energy is transferred along the metal rod.

Movement of free electrons, which have acquired more energy.

Heated layers of molecules colliding with other colder molecules adjacent to them.

The first of the two processes is the more effective.Non-metals do not have free electrons in their structure, and only the second process is available for heat transfer by conduction. This explains, to some extent, why metals are good conductors and non-metals are usually poor conductors of heat. Copper and aluminum are good conductors of heat, whilst substances such as cork, wood and polystyrene are poor conductors.

THERMAL CONDUCTIVITY

Thermal conductivity can be defined as: The number of heat units (Joules) flowing per unit of time (1 second) through a cross-sectional area (1 square metre) when the temperature falls by one degree (1 Kelvin or 1 degree Celsius) per unit length (1 metre) of its path.

The units of thermal conductivity are, therefore, joules per second per metre squared (area) per meter (thickness) per degree Celsius, or Kelvin.

hot face(say 25oC)

Area (A) 1 m2

heat flowJ s-1

cold face

heat flow J s-1thickness through which heat flows d = 1 m

units of thermal conductivity are: Watts per meter per oC or K (W m-1 K-1 )

K VALUESThe k values for most materials have been determined in the laboratory. The thermal conductivities of some materials used in industry are shown below.

MATERIAL THERMAL CONDUCTIVITY k (Wm-1 K-1) Solids – metals Copper 390 Aluminum 202 Steel 55 Solids – nonmetals Magnesia 0.07 Cork 0.043 Glass 1.09 Asbestos 0.16 Brick (alumina) 3.1 Fluids Water 0.62 Benzene 0.16 Air 0.024

CONDUCTION OF HEAT THROUGH SINGLE WALLSThe rate of flow Φ (J S-1) from the hot side to the cold side will depend on the following factors:-The thermal conductivity k (W m-1 K-1) The area A (m2), The difference in temperature T1 – T2 (K) (T1 – T2 is referred to as ΔT). The thickness d (metres)

Refractory wall,thermal conductivity, k,

(W m-1 K-1)

Thickness of walld (m)

cold side temperatureT2 K

hot side temperatureT1 K

area of wall overwhich heat flows

A (m2)

heat flow by conduction through wall (J/s)

If Φ represents the heat flow in J S-1 (or watts), then

Φ = k A (T1 – T2) / d

Where: Φ rate of heat flow (in J S-1)k thermal conductivity (in W m-1 K-1)A area over which heat is passing (in m2)T1 hot face temperature (in K)T2 cold face temperature (in K)d thickness or distance between hot face and cold face (in m)

Example 1. The outer surface of a boiler is covered with insulating

material of thermal conductivity 0.04 W m-1 K-1. It is 125 mm thick and has a surface area of 50 m2. The inside edge of the insulating material has an average temperature of 423 K and the temperature of the outside surface is 303 K. Calculate the heat loss through the insulation per hour.

Φ =kA (T1 – T2) / d

List the information given in the question: Φ= this is the heat flow in JS-1, we need to calculate this

value. K= 0.04 W m-1 K-1

A= 50 m2

T1= (423) K T2= (303) K d = 0.125 m

All of these values must be expressed in SI units before they are substituted in the formula:

Φ =0.04 × 50 × (423 – 303) 0.125 Φ = 1920 J S-1 (or Watts)

1920 represents the number of joules of heat which pass through the insulation in one second. To find the quantity which will pass through the insulation per hour, we must multiply this figure by 3600, the number of seconds in 1 hour.

Thus heat loss through insulation per hour= 1920 × 3600 = 6192000 J h-1

= 6912 kJ h-1

= 6.912 MJ h-1

Example 2A horizontal steam pipe of 50 mm outside diameter is covered with 10 mm thickness of lagging of thermal conductivity 0.12 W m-1 K-1. If the outer surface temperature of the lagging is 308 K and the outer surface of the pipe has a temperature of 423 K. Estimate the heat loss through the lagging per metre length of the pipe.Take π = 3.142. Assume the inside temperature of the lagging is the same temperature as the outside surface of the pipe.

Heat will travel from the inside area of the lagging through to the outside area as indicated by Φ, but in all directions along the 1 metre length asked for in the question. The inside area of the lagging is calculated by multiplying the inside circumference by its length.

Inside area of lagging = π × 0.05 × 1 = 0.157 m2

(N.B. The circumference of a circle is π x diameter)A similar calculation will give us the outside area of the lagging.Outside area of lagging = π × 0.07 × 1 = 0.220 m2

Thus the heat from the hot side of the lagging will travel from an area of 0.157 m2 through a thickness of 0.01 m to an outside area of 0.220 m2.

Which area do we use in our formula?One solution to this problem, which is accepted at this level, is to calculate the average area and use this as A in the conductivity equation.Average area across which heat flows = (0.157 + 0.220) / 2 = 0.189 m2

Φ= This is the heat flow in J s-1 which we are asked to estimate. k = 0.12 W m-1 K-1

A = 0.189 m2 (average) T1 = 423 K T2 = 308 K d = 0.01 mNow substitute these figures into the standard formula: Φ = k A (T1 – T2) / d Φ = 0.12 × 0.189 × (423 – 308) / 0.01 = 261 J s-1 =261W

SUMMARYThe rate at which heat travels by conduction through solids is indicated by a physical property called thermal conductivity.Thermal conductivities are listed in standard reference books. A high value indicates a good conductor, a low value a poor conductor.The rate of heat flow Φ (in J s-1) through a solid, by conduction, can be calculated using the standard formula:

Φ =kA(T1 – T2) / d

This is the way in which heat energy travels through liquids and gases. (FLUIDS)

CONVECTION IN LIQUIDS When the vessel is heated at the bottom, a current of hot liquid moves upwards and its place is taken by a cold current, moving downwards. As heating proceeds, well defined hot upward and cold downward moving currents can be seen. These currents are called CONVECTION CURRENTS.

HEAT TRANSFER BY CONVECTION

So how do convection currents work?When a portion of the liquid near the bottom of the vessel is heated, it expands. Since its mass remains the same it must become less dense, and therefore it rises to the surface of the liquid. Thus a warm convection current moves upwards. Colder denser liquid falls to the base of the vessel and in its turn is heated, becomes less dense and circulates, taking heat with it.

CONVECTION IN AIR.

Hot convection currents can often be seen and felt above a domestic water radiator. Hot air rises from the radiator and circulates into the room thus warming it. At the same time colder air circulates towards the base of the radiator. In its turn it becomes heated and circulates upwards taking and mixing the heat it contains with the rest of the air in the room.

Heat transfer by convection occurs when a fluid, liquid or gas, comes into contact with a surface hotter, or cooler, than itself.

CALCULATION OF THE HEAT TRANSFER RATE BY CONVECTION

When the fluid outside the solid surface is in forced or natural convective motion, the expression of the rate of heat transfer from the solid to the fluid, or vice versa, is as follows: qc = hc A (Ts – Tf)

where: qc =rate of heat transfer convection in J/s or W

A =Area of heat transfer, m2

Ts = The temperature of the solid surface, K

Tf =The average temperature of the fluid, K

hc =The convection heat transfer coefficient, W/m2.K

The coefficient hc is a function of the system geometry, fluid properties, flow velocity, and temperature difference. In many cases, empirical correlation are available to predict this coefficient, since it often cannot be predicted theoretically.

FACTORS AFFECTING THE RATE OF HEAT TRANSFER BY CONVECTION.

1. DENSITY and GRAVITY.Thus the rate of change of density with change of temperature is an important factor to consider when predicting the speed of heat transfer by convection.2. COEFFICIENT OF THERMAL EXPANSION.A fluid, which has a high coefficient of thermal expansion, will consequently have a high change of density with change of temperature. This, in turn, will tend to speed up the formation and movement of convection currents.3. VISCOSITY.Convection currents will move slowly within fluids of high viscosity and quickly within fluids of low viscosity. Thus the rate of heat transfer by convection will depend upon the viscosity of the fluid being heated.

4. THERMAL CONDUCTIVITYMost fluids have very low thermal conductivity values. Accordingly, heat transfer by conduction is slow and far less important than heat transfer by convection. The fluid would need to have an uncharacteristically high thermal conductivity for this factor to be of any importance.

5. SPECIFIC HEAT CAPACITYA fluid with a high specific heat capacity will carry more heat with it, as it circulates, than one with a low value. Thus the specific heat capacity is an important factor to consider when evaluating heat transfer by convection

6. VESSEL DIMENSIONS.The distance traveled by the convection currents and the area of the vessel, which is heated will both affect the rate of heat transfer by convection. So the fluid in a tall narrow vessel may take longer to heat up than the same mass of the same fluid in a shorter, wider vessel.7. TEMPERATURE DISTRIBUTION. The rate of heat transfer will depend upon the difference in temperature between the hottest and coldest parts of the system. For a vessel, the hottest part will be at the base and the coldest part near the surface of the fluid. As heating proceeds, this temperature difference gets smaller. It reaches a minimum when the liquid begins to boil.

NATURAL AND FORCED CONVECTIONConvection currents occur when fluids come into contact with hot surfaces. The convection currents produced give rise to a MIXING ACTION, which helps to transfer heat throughout the body of the fluid. The process of heat transfer by NATURAL CIRCULATION is called NATURAL CONVECTION. Movement of the fluid is caused only by the differences in density, The rate of heat transfer by convection can be improved by agitating the fluid as it is heated.

The rate of heat transfer by convection will be greater if propeller is used to mix or agitate the fluids. Heat transfer produced in this way, i.e. by FORCING a fluid to move over a heating surface, is referred to as FORCED CONVECTION.Generally speaking, with forced convection there is:

more rapid circulation. more turbulence and, more rapid heat transfer.

Baffles fitted to the heating vessel will produce more turbulence, improved mixing and hence more rapid heat transfer by convection.

Convection is the transfer of heat within a fluid, or from a heated surface to a fluid. It occurs by movement of the fluid itself and can be either:

Natural convection, where movement of the fluid is caused by temperature differences (followed by density differences).

Forced convection, where movement is caused mainly, or entirely, by mechanical means (e.g. a propeller).

SUMMARY

HEAT TRANSFER IN FLUIDS FLOWING THROUGH PIPES AND TUBES

Heat passes through the tube wall by conduction and into the fluid flowing through the tube by convection. If the fluid flows through the tube in a TURBULENT way, then a mixing action will be produced. This mixing action will improve the rate of heat transfer. Greater turbulence will produce faster heat transfer by convection within the fluid.

Fouling deposits on tubes of water tube boilers

Usually heat transfer equipment is operated with both the process fluid and the thermal fluid flowing in a TURBULENT manner. This procedure results in the following benefits Good heat transfer within the fluids, i.e. forced convection.Static layers of fluid on the heat transfer surface are reduced

to a minimum.

Static layers of fluid on each side of heat exchanger tube

r

Resistances to the flow of heat through pipe walls

NATURAL AND FORCED CONVECTOR HEATERS (EXAMPLE)These are similar in principle to the domestic heater.

(a) shows cold air circulating over the tubes of a heat exchanger. Heated air then moves upwards and away from the heating surfaces. As this happens, cold air is drawn in near the base of the exchanger and, in its turn, becomes heated.

(b) shows a fan forcing cold air over the tubes of the heat exchanger. This enables very much larger volumes of air to be heated than can be heated by natural circulation, i.e. natural convection, alone.

Fan assisted convector heaters can be fitted with a number of smaller diameter tubes so increasing the area of the hot surface available for heat transfer for a given unit size. This, in turn, will increase the volume of cold air that can be heated in a given time. The extra force required to push the air through the bank of tubes is provided by the fan.

SUMMARY

Heat transfer through fluids takes place by convection. It occurs when cold fluids come into contact with hot surfaces.The rate of heat transfer by convection is influenced by the physical properties of the fluid. Some of the important ones are viscosity, density and coefficient of expansion.Forced convection occurs when the fluids are forced over hot surfaces, usually by means of pumps or agitators.Heat transfer by forced convection is more effective than heat transfer by natural convection .Fluids are forced through tubes of heat exchange equipment at high velocity to produce turbulence which results in heat transfer by forced convection.

RADIATION consists of invisible energy waves, which are able to pass across a space. Unlike heat transfer by conduction and convection, heat transfer by radiation does not require any material to be present between the hot part and the cold part of the system. Heat can travel by radiation across a vacuum. For example, heat energy from the sun travels across the empty space beyond the earth’s atmosphere. Scientists call these invisible heat waves ELECTROMAGNETIC WAVES. Electromagnetic waves travel across a space very rapidly, 3 × 108m s-1.

If you hold your hand near to an electric light bulb when the current is switched on, you will feel radiant heat immediately. Bulbs of this type are evacuated during manufacture so that heat from the glowing element cannot have traveled by conduction or convection. Energy radiated from hot surfaces travels by means of electromagnetic waves. These have similar properties as LIGHT WAVES and travel at the same speed, i.e. 300,000 km s-1.

When electromagnetic waves from a hot surface fall on a cooler surface, they are partially absorbed and the cooler surface heats up, i.e. heat energy is transferred from the hotter surface to the cooler.

FACTORS AFFECTING THE RATE OF HEAT TRANSFER BY RADIATION

The rate at which a hot surface, or object, radiates heat, J s-1,

depends upon:its temperature (T2)the surrounding temperature (T1)its area (A)its nature (ε)

1. RADIATION RATE AND SURFACE TEMPERATURE The hotter a surface or object is, the more energy it will

radiate. The energy radiated from a surface has been shown to be directly proportional to the fourth power of its absolute temperature. This can be expressed mathematically as:

Where T is the temperature in Kelvin (i.e. o C + 273)

This is a difficult relationship to understand. Put in simple terms, the heat radiated from a surface increases very rapidly with increase in temperature. This increase in radiant energy with increase in temperature is very much greater than that for heat transfer by convection. At temperatures over 500oC, most of the heat transferred from a surface will be by radiation. For example, most of the heat transferred from flames to water tubes in boilers is by radiation

2. RADIATION RATE AND SURROUNDING TEMPERATURE

All surfaces, whatever their temperature, radiate energy in the form of electromagnetic waves. A hot surface will radiate to cold surroundings. However, at the same time, those surroundings will radiate back to the hotter surface. The net effect will be the transfer of heat from the hot surface to the colder surfaces or surroundings.

If we take the temperature of the surroundings into consideration, we must modify what we said earlier and say:

The rate of heat transfer by radiation is directly proportional to the DIFFERENCE BETWEEN the fourth power of the surface temperature (T2

4) and the fourth power of the surrounding temperature (T1

4), both temperatures being expressed in degrees absolute (K).This can be expressed mathematically as:

Energy radiated from surface to surroundings α (T24

- T14)

Where T2 is the surface temperature (K) and T1 is the temperature of the surroundings (K)

3. RADIATION RATE AND SURFACE AREA For a given temperature, the larger the area the greater is the amount of energy radiated from it. Expressing it mathematically, we can say: Energy radiated α A Where A is the surface area in m2.

4. RADIATION RATE AND THE NATURE OF THE SURFACE. Not all surfaces radiate the same amount of energy for a given size and temperature. It is found that, for a given temperature, a body radiates MOST HEAT when its surface is DULL BLACK and LEAST HEAT when its surface is HIGHLY POLISHED.

Three surfaces are shown; all have the same area and are held at the same temperature. They radiate to surroundings at constant temperature. Detectors are placed at the same distance in front of each surface and measure the radiant heat transmitted from each of them.

It is found that the dull black surface radiated most energy, 100 J s-1, the polished surface the least, 6 J s-1, with the grey surface somewhere between the two extremes, 45 J s-1. You should note that these figures are illustrative and do not represent any

particular experiment,

The experiment can be repeated using different surfaces with different colour and textures. It will be found that no surface radiates more energy than the dull black surface. Generally speaking, darker surfaces radiate more energy than lighter coloured polished surfaces. Both colour and texture of a surface affect the energy it will radiate at a given temperature. The radiating property of a surface is referred to as its EMISSIVITY of a surface and given the symbol ε. Since a dull black surface radiates the maximum amount of energy at a given temperature it is used as the standard and the emissivity of a surface is defined as:

Emissivity is the ratio of the energy radiated from that surface to the energy radiated from a dull black surface of the same size and held at the same temperature.

The dull black surface is shown to radiate 100 J s-1.

The grey surface has the same size, is held at the same temperature, and is shown to radiate 45 J s-1.

Thus the emissivity (ε) of this surface is: 45/ 100 = 0.45

The polished surface has the same size and temperature as the dull black surface and is shown or radiate 6 Js-1Thus the emissivity (ε) of this surface is: 6 / 100 = 0.06

(NOTE: by definition, no surface can have an emissivity greater than 1.)

Surface Emissivity(ε) Furnace interior 1.00 Iron oxide surface 0.82 Oxidized copper surface 0.79 Refractory bricks 0.78 Aluminium paints 0.50 Polished copper 0.04 Polished Aluminium 0.04 Oxidizd Aluminium 0.15

CALCULATING HEAT TRANSFER BY RADIATIONAs you will recall, the factors which affect the rate of heat transfer by radiation from a body are:

◦ Its temperature T2 (K)◦ Temperature of surroundings T1 (K)◦ Its area A (m2)◦ Its emissivity ε

Scientists have combined these factors and produced a formula from which the rate of heat transfer by radiation, Φ (J s-1 or W), can be calculated. This is given below.

Φ = 5.7 × 10-8 × ε × A × (T24 – T1

4)Where:

◦ Φ is the heat radiated from the hot surface (W)◦ ε is the emissivity, from reference books.◦ A is the surface area radiating heat (m2)◦ T2 is the surface temperature (K)◦ T1 is the surrounding temperature (K)

The following simplified example will be used to illustrate the use of this formula.

Example 3.Calculate the rate of heat transfer by radiation, and hence the heat loss by radiation, from a flat furnace roof at 90oC to surroundings at 20 oC. The roof measures 7 m by 3 m and has an emissivity of 0.8.Use the formula Φ = 5.7 × 10-8 × ε × A × (T2

4 - T14)

Where:◦ ε = 0.8◦ A = (7 × 3) = 21 m2

◦ T2 = (90 + 273) = 363 K◦ T1 = (20 + 273) = 293 K

Substitute these figures into the formula: Φ = 5.7 × 10-8 × 0.8 × 21 × (3634 – 2934) Φ = 9569.3 J s-1 or WThis heat loss can be reduced by covering the roof with Aluminium sheeting, since this would reduce the emissivity of the surface.Assuming the emissivity of Aluminium sheeting is 0.15; calculate the new rate of heat loss from the furnace roof.Substituting ε = 0.15 for ε = 0.8 in the equation, we have; Φ = 5.7 × 10-8 × 0.15 × 21 × (3634 – 2934) Φ = 1794.2 J s-1 or WWhen we compare the two answers, we can see that the use of Aluminium foil has reduced the heat loss from 9569.3 W to 1794.2 W, a worthwhile saving.

You should not forget that other heat losses would occur by convection from the hot furnace roof.

ABSORPTION OF RADIANT HEAT BY A SURFACE

When radiant heat falls onto a surface, some is absorbed and some is reflected. The portion, which is absorbed heats up the surface. Some surfaces are good absorbers, whilst others are good reflectors.

Two sheets of tin plate have corks held onto their surfaces by thin films of wax. One of the sheets is polished whilst the other is painted dull black. These plates are then set vertically, a short distance apart, with a Bunsen burner midway between them. When the burner is lit, both surfaces receive equal quantities of radiant heat. In a very short time the wax on the dull plate melts and the cork slides off. The polished plate, however, remains cool and the wax unmelted.

waxwax

dark black surface polished surface

wax holding this corksoon melts

The results of such experiments will show that darker, duller surfaces are good absorbers of heat whilst lighter, more polished surfaces are poor absorbers of heat. Lighter, polished surfaces, in fact, reflect heat rather than absorb it.

Vessels used for the storage of volatile liquids are often covered with Aluminium foil, or sheeting, to reduce heat absorption on hot days. Such absorption could result in excessive losses through evaporation, with possible associated safety and toxic hazards.

This experiment shows that a dull black surface is a much better absorber of radiant heat than a polished one.

SUMMARY

Heat transfer by radiation takes place by means of electromagnetic waves. These do not require a material substance for their movement.

The rate of heat transfer by radiation from a hot surface to a cold surface depends upon:

The surface temperature. The surrounding temperature. The surface area. The emissivity of the surface.

Duller, darker surfaces radiate more energy than lighter, smoother surfaces. Emissivity is a measure of the ability of a surface to radiate.

The amount of energy absorbed by a surface also depends upon its colour and texture. Generally speaking, darker, duller surfaces absorb radiant heat more readily than lighter, smoother surfaces.

HEAT TRANSMISSION (EXAMPLE)The figure represents a heating furnace fired by natural gas. It consists of a steel shell lines on the inside with refractory bricks and covered on the outside with insulating material. The furnace is used to preheat cold raw material up to process temperature.

The raw material flows through a series of heat resistant tubes, usually 18/8 stainless steel, which are suspended in the furnace space above the flame.When natural gas burns, an exothermic chemical reaction takes place. Energy is released at the rate of 37 MJ for each cubic metre of gas burned and a very hot flame, about 2000oC, is produced.

Let us follow the progress of this energy.

Heat is transferred by RADIATION from the flame to the walls of the furnace and from the flame to the tubes through which the process fluid is flowing. This is a very rapid process since electromagnetic waves, energy waves, are radiated across space, i.e. from flame to walls and from flame to tubes, at 3 x 108 m s-1

◦ Heat is transferred by CONVECTION as the hot gases, produced during combustion, circulate over, round and between the tubes and the walls of the furnace before they pass out through the chimney stack.

◦ The heat energy arriving at the tubes passes through the tube walls by CONDUCTION and then by CONVECTION into the process fluid.

◦ The heat arriving at the inside walls of the furnace travels by CONDUCTION through the refractory brick, the steel shell and then, slowly, through the insulating material on the outside. This, unfortunately, represents a loss of energy from the process. We shall have more to say about thermal efficiencies and energy recovery systems in later lessons.

◦ Heat is also lost by CONDUCTION through the base and foundation of the furnace.

◦ The outside walls of industrial furnaces are usually hotter than their surroundings. Under these conditions, the heat, which has passed through the walls, will leave the outside surfaces by CONVECTION, as it heats up the surrounding air, and RADIATION.

Heat exchangers are devices, which transfer heat from a hot fluid to a cold fluid usually across a metal tube wall.The most common heat exchangers used in the chemical industry are the SHELL AND TUBE types. In their simplest form, these consist of a bank, bundle, of small diameter tubes fitted inside a large diameter tube, usually referred to as the SHELL. One fluid flows through the tubes, while the other flows inside the shell, circulating around and between the small diameter tubes. Heat is transferred, from one fluid to the other, by CONDUCTION, across the tube walls, and then by FORCED CONVECTION, through the fluid.There are several different designs of shell and tube exchangers. A number of the more important ones are discussed in the following sections.

HEAT EXCHANGERS

DOUBLE PIPE HEAT EXCHANGEROne of the simplest types of heat exchanger is the double pipe design. It consists of two concentric pipes.

Hot fluid in

One fluid flows through the inner tube whilst the other flows through the annular space between the tubes. Heat passes from one fluid to the other by conduction through the inner pipe wall followed by forced convection through the fluid.The main advantages of this type of heat exchanger are that they are simple and cheap to make. They can often be made from standard diameter piping. Their main disadvantages are that they have low thermal efficiencies, and cannot handle large quantities of process fluid.

The performance of the Double pipe Heat Exchanger type can be improved by connecting two or more units together.

Fluid A in

Double pipe exchangers are sometimes referred to as ANNULAR or CONCENTRIC tube heat exchangers.

SHELL AND TUBE HEAT EXCHANGER

Simple shell and tube heat exchanger

The major features of this type of heat exchanger are:◦ A large number of small diameter TUBES fitted into a TUBE PLATE to form a BANK or BUNDLE of tubes.◦ A large diameter tube called a SHELL into which the bank of tubes fits.◦ End covers or HEADERS, which are fitted over each end of the tube bundle.◦ INLET and OUTLET pipes fitted to the shell to allow fluid into and out of the shell.◦ INLET and OUTLET pipes fitted to each end of the cover to allow fluid to flow through the tubes.◦ An EXPANSION JOINT, fitted to the shell. This is included to relieve stresses due to thermal expansion. It

allows the shell to expand and contract as the tube bundle expands and contracts.◦ A set of BAFFLES, i.e. plates set at right angles to the tube bundle. Their function is to cause shell side fluid

to flow at right angles to the tubes. They also support the tubes within the bundle.

Direction of flow of fluid A

Direction of flow of fluid B

COUNTER CURRENT AND CO-CURRENT FLOW

Co-Current heat exchange

The heat exchanger is operating with CO-CURRENT FLOW since the two fluids flow through the exchanger in the same direction.Generally speaking, counter current flow provides the most efficient heat exchange since the temperature of the cold fluid can be raised to a temperature just below the temperature of the hot fluid. With co-current flow the heated fluid can be no hotter than the outlet temperature of the cooled heating fluid. However, the temperature distribution within a co-current heat exchanger is more even. For this reason co-current is often preferred when heat-sensitive fluids are being heated.

Direction of flow of fluid A

Direction of flow of fluid B