1 - Form of Steam

8/18/2019 1 - Form of Steam

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SCT 363-2 Heat Transfer: Forms of Steam

Department of Science Technology – UWU

INTRODUCTION

Steam is the gas formed when water passes from the liquid to the gaseous state. At the

molecular level, this is when H2O molecules manage to break free from the bonds (i.e.

hydrogen bonds) keeping them together.

How steam worksIn liquid water, H2O molecules are constantly being joined together and separated. As the water

molecules are heated, however, the bonds connecting the molecules start breaking more rapidly

than they can form. Eventually, when enough heat is supplied, some molecules will break free.

These 'free' molecules form the transparent gas we know as steam, or more specifically dry

steam.

Dry Steam vs. Wet Steam

Dry steam applies to steam when all its water molecules remain in the gaseous state. It's a

transparent gas.

Wet steam applies to steam when a portion of its water molecules have given up their energy

(latent heat) and condense to form tiny water droplets.

Take the example of a kettle boiling water. Water is first heated using an element. As water

absorbs more and more heat from the element, its molecules become more agitated and it starts

to boil. Once enough energy is absorbed, part of the water vaporizes, which can represent an

increase as much as 1600X in molecular volume.

Sometimes a mist can be seen coming out of the spout. This mist is an example of how dry

steam, when released into the colder atmosphere, loses some of its energy by transferring it to

the ambient air. If enough energy is lost that intermolecular bonds start forming again, tiny

airborne droplets can be seen. This mixture of water in the liquid state (tiny droplets) and

gaseous state (steam) is called wet steam.

Saturated Steam (Dry)

Saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can

coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of

condensation.

Advantages of using saturated steam for heating

Saturated steam has many properties that make it an excellent heat source, particularly at

temperatures of 100 °C (212°F) and higher. Some of these are:


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Property Advantage

Rapid, even heating through latent

heat transfer

Improved product quality and productivity

Pressure can control temperature Temperature can be quickly and precisely established

High heat transfer coefficient Smaller required heat transfer surface area, enabling

reduced initial equipment outlayOriginates from water Safe, clean, and low-cost

Unsaturated Steam (Wet)

This is the most common form of steam actually experienced by most plants. When steam is

generated using a boiler, it usually contains wetness from non-vaporized water molecules that

are carried over into the distributed steam. Even the best boilers may discharge steam

containing 3% to 5% wetness . As the water approaches the saturation state and begins to

vaporize, some water, usually in the form of mist or droplets, is entrained in the rising steam

and distributed downstream. This is one of the key reasons why separation is used to dis-entrain

condensate from distributed steam.

Superheated Steam

Superheated steam is created by further heating wet or saturated steam beyond the saturated

steam point. This yields steam that has a higher temperature and lower density than saturated

steam at the same pressure. Superheated steam is mainly used in propulsion/drive applications

such as turbines, and is not typically used for heat transfer applications.

Advantages of using superheated steam to dr ive tur bines:

To maintain the dryness of the steam for steam-driven equipment, whose performance

is impaired by the presence of condensate To improve thermal efficiency and work capability, e.g. to achieve larger changes in

specific volume from the superheated state to lower pressures, even vacuum.

It is advantageous to both supply and discharge the steam while in the superheated state because

condensate will be generated inside steam-driven equipment during normal operation,

minimizing the risk of damage from erosion or carbonic acid corrosion.

Disadvantages of using superheated steam for heating:

Property Disadvantage

Low heat transfer coefficientReduced productivity

Larger heat transfer surface area neededVariable steam temperature

even at constant pressure

Superheated steam needs to maintain a high velocity,

otherwise the temperature will drop as heat is lost from the

system

Sensible heat used to transfer

heat

Temperature drops can have a negative impact on product

quality

Temperature may be

extremely high

Stronger materials of construction may be needed,

requiring higher initial equipment outlay

For these reasons and others, saturated steam is preferred over superheated steam as the heating

medium in exchangers and other heat transfer equipment. On the other hand, when viewed as

a heat source for direct heating as a high temperature gas, it has an advantage over hot air in


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that it can be used as a heat source for heating under oxygen-free conditions. Research is also

being carried out on the use of superheated steam in food processing applications such as

cooking and drying.

Heat TransferThe most common processes found in a food processing plant involve heating and cooling of

foods. The study of heat transfer is important because it provides a basis for understanding how

various food processes operate. There are two types of Heat Transfer occurs, Steady and

Unsteady heat transfer.

Steady State: The heat transfer rate or the temperature remained unchanged over time.

Unsteady state :The temperature normally varies with time as well as position

Modes of Heat Transfer

Conduction

Transfer of thermal energy from more energetic particles of medium to adjacent less energetic

ones as a result of interaction between the particles. Interaction between particles are due

- Lattice Vibration waves

- Free flow of electrons

Heat transfer is a vector quantity and usually the direction and the magnitude is defined.

Fourier’s law of Heat conduction through a plane layer is proportional to the temperature

difference across the layer and the heat transfer area, But inversely proportional to the thickness

of the layer.

̇ = ∗ ∗ (∆

∆)

K – Thermal conductivity, A – Heat transfer area, T – Temperature difference across and x

– Thickness of the layer. The above equation for heat transfer can be compare with,

=


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In comparison to the above equation It can be defined a thermal resistance, such that

=

This thermal resistance created by the wall of the heat flow and is depends on the geometry of

the medium and thermal property.There fort thermal resistance form of convection heat

transfer,

̇ =∆

Problems

1. The wall of a refrigerator of 4 m2 surface area consists of two metal sheets with insulation

in between. The temperature of the inner wall surface is 5ºC and that of the outer surface is

20 ºC. The thermal conductivity of the metal wall is 16 W/ mºC and that of the insulation

is 0.017 W/mºC. If the thickness of each metal sheet is 2 mm, calculate the thickness of the

insulation that is required so that the heat transferred to the refrigerator through the wall is

10 W/m2.

2. The wall of an oven consists of two metal sheets with insulation in between. The

temperature of the inner wall surface is 200 ºC and that of the outer surface is 50 ºC. The

thickness of each metal sheet is 2 mm, the thickness of the insulation is 5 cm, and the

thermal conductivity is 16 W/m ºC and 0.055 W/mºC respectively. Calculate the total

resistance of the wall to heat transfer and the heat transfer losses through the wall per m 2

of wall area.3.

Hot water is transferred through a stainless steel pipe of 0.04 m inside diameter and 5 m

length. The inside wall temperature is 90 ºC, the outside surface temperature is 88 ºC, the

thermal conductivity of stainless steel is 16 W/mºC, and the wall thickness is 2 mm.

Calculate the heat losses if the system is at steady state.

Convection

Bulk molecular motion is involved in convection heat transfer. Bulk molecular motion is

induced by density changes associated with difference in fluid temperature at different points

in the fluid, condensation, or vaporization (free convection), or when a fluid is forced to flow

past a surface by mechanical means (forced convection).


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Heat transfer by convection is evaluated as the rate of heat exchange at the interface between

a fluid and a solid. The rate of heat transfer is expressed as:

Q = hA(Tm — Ts) = hA T

h - heat transfer coefficient, A - area of the fluid-solid interface where heat is being transferred,Tm – Surface Temperature, Ts – Free stream temperature. As with conduction, above equation

it can be deduce a thermal resistance. Such that,

=

There for thermal resistance form of convection heat transfer,

̇ =∆

Free (Natural) Convection

Occurs due to density differences caused by temperature gradients within the system. Resultsin either laminar or turbulent flow of the fluid will occur.

Forced Convection

By mechanical mean fluid is forced past a solid body and heat is transferred between the fluid

and the body. Examples in the food industry are in the forced-convection ovens for baking

bread, in blast and fluidized freezing, in ice-cream hardening rooms, in agitated retorts, in meat

chillers. In all of these, foodstuffs of various geometrical shapes are heated or cooled by a

surrounding fluid. Higher than for natural convection. Also, as might be expected, the higher

the velocity of the fluid the higher the rate of heat transfer.

Dimensionless Numbers

Dimensionless quantities are used to explain characteristics of a fluid and the conditions that

exist in an experiment. So that fluids with the similar characteristics are explained with

dimensionless quantities.

Raynolds Number This is an indication of ratio of inertia forces to viscous forces in fluid flow

defined by

=

=

Nusselt number represents the enhancement of the heat transfer through a fluid layer as result

of convection relative to conduction across the same fluid layer. It’s defined as

=

=

ℎ


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Plate 1

Plate 2

Fluid

Prandtl number

Relative thickness if the velocity and the thermal B.L. is best described by the Prandtl number

and Defined as

= = =

Grashof number

Ration between the buoyancy force and the viscous force

=

Thermal Resistance Networks

Since the heat transfer coefficients are modelled as a resistance, the heat transfers through

multilayer wall can also be studied easily use of resistor network theories

=

⁄

3 = 1 ℎ⁄

Total Resistance,

= + 2 + 3 Above equation valid for radiation as well

1. Water flows in a pipe of 0.0475 m inside diameter at a velocity of 1.5 m/s. Calculate the

heat transfer coefficient if the temperature of the water is 60 ºC and 40 ºC at the inlet and

the outlet of the pipe respectively, and the inside wall temperature of the pipe is 35 ºC.

2. Sucrose syrup flows in a pipe of 0.023 m inside diameter at a rate of 40 lt/min, while steam

is condensing on the outside surface of the pipe. The syrup is heated from 50 to 70 ºC,

while the inside wall temperature is at 80 ºC. Calculate 1) the heat transfer coefficient and

2) the required length of the pipe.

2 =2

⁄


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Cross-flow Heat exchanger (Different Flow configurations

Parallel Flow Counter Flow

Radiation

Radiation heat transfer occurs between two surfaces by the emission and later absorption of

electromagnetic waves. For the transmission of energy no any media is required. Radiation

heat transfer is expressed as,

̇ = ∗ ∗ ( −

) - missivity of the surface, - Stefan Boltzmann Constant, A – Heat transfer area, T1 –

Absolute temperature of surface, T2 - absolute temperature of the surrounding

Heat Exchangers

In a food processing plant, heating and cooling of foods is conducted in equipment called heat

exchangers. Heat exchangers can be broadly classified into noncontact and contact types. Heat

exchangers are classified according to the flow type as parallel flow, counter flow, and cross-flow arrangement. In parallel flow, both the hot and cold fluids enter the heat exchanger at

the same end and move in the same direction. In counter-flow, the hot and cold fluids enter the

heat exchanger at opposite ends and flow in opposite direction. In cross-flow, the hot and cold

fluid streams move perpendicular to each other


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Shell and tube

In terms of construction type, heat exchangers are classified as compact, shell and tube and

regenerative heat exchangers. Compact heat exchangers are specifically designed to obtain

large heat transfer surface areas per unit volume. The large surface area in compact heat

exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls

separating the two fluids. Shell and tube heat exchangers contain a large number of tubes

packed in a shell with their axes parallel to that of the shell. Regenerative heat exchangers

involve the alternate passage of the hot and cold fluid streams through the same flow area. In

compact heat exchangers, the two fluids usually move perpendicular to each other.

Compact heat exchanger

A heat exchanger is classified as being compact if > 700 m2/m3 or (200 ft2/ft3) where is the

ratio of the heat transfer surface area to its volume which is called the area density. The area

density for double-pipe heat exchanger cannot be in the order of 700. Therefore, it cannot be

classified as a compact heat exchanger.

Counter-flow & Cross FlowIn counter-flow heat exchangers, the hot and the cold fluids move parallel to each other but

both enter the heat exchanger at opposite ends and flow in opposite direction. In cross-flow

heat exchangers, the two fluids usually move perpendicular to each other. The cross-flow is

said to be unmixed when the plate fins force the fluid to flow through a particular inter fin

spacing and prevent it from moving in the transverse direction. When the fluid is free to move

in the transverse direction, the cross-flow is said to be mixed.

Shell and Tube exchangersIn the shell and tube exchangers, baffles are commonly placed in the shell to force the shell

side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing

between the tubes. Baffles disrupt the flow of fluid, and an increased pumping power will be


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needed to maintain flow. On the other hand, baffles eliminate dead spots and increase heat

transfer rate.

Regenerative heat exchangerRegenerative heat exchanger involves the alternate passage of the hot and cold fluid streams

through the same flow area. The static type regenerative heat exchanger is basically a porous

mass which has a large heat storage capacity, such as a ceramic wire mash. Hot and cold fluidsflow through this porous mass alternately. Heat is transferred from the hot fluid to the matrix

of the regenerator during the flow of the hot fluid and from the matrix to the cold fluid. Thus

the matrix serves as a temporary heat storage medium. The dynamic type regenerator involves

a rotating drum and continuous flow of the hot and cold fluid through different portions of the

drum so that any portion of the drum passes periodically through the hot stream, storing heat

and then through the cold stream, rejecting this stored heat. Again the drum serves as the

medium to transport the heat from the hot to the cold fluid stream.

Fouling FactorThe performance of heat exchangers usually deteriorates with time as a result of accumulation

of deposits on heat transfer surfaces. The layer of deposits represents additional resistance toheat transfer and causes the rate of heat transfer in a heat exchanger to decrease. The net effect

of these accumulations on heat transfer is represented by a fouling factor R f , which is a measure

of the thermal resistance introduced by fouling.

Common forms of the fouling are precipitation, chemical fouling, corrosion and biological

fouling. Water treatment prior to feeding and chemical treatment are the solution for the

fouling. When Is the fouling factor, A is the resective surface area

The overall heat transfer coefficientThe thermal resistance network associated with this heat transfer processinvolves two

convection and one conduction resistances, as shown in Figure. Here the subscripts i and o

represent the inner and outer surfaces of the.

Surface areas,

= =

Thermal resistance of wall,

where k is the thermal conductivity of the wall material and L is the length of the tube. Then

the total thermal resistance becomes,

A single resistance R, and to express the rate of heat transfer between the two fluids as

Where U is the overall heat transfer coefficient, whose unit is W/m2 · °C, which is identical to

the unit of the ordinary convection coefficient h. Canceling T,

When the wall thickness of the tube is small and the thermal conductivity of the tube material

is high, as is usually the case, the thermal resistance of the tube is negligible (R wall ≈0) and the

inner and outer surfaces of the tube are almost identical (Ai ≈Ao ≈ As). Then for the overallheat transfer coefficients implies to


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Properties of Water

Properties of oil

When one of the fluids is a gas and the other is a liquid. In such cases, fins are commonly used

on the gas side to enhance the product UA s and thus the heat transfer on that side. Since say, hi

≪ ho we have 1/hi≫ 1/ho, and thus U ≈ hi. When the tube is finned on one side to enhance heat

transfer, the total heat transfer surface area on the finned side becomes

where fin is the fin efficiency. This way, the temperature drop along the fins is accounted for.

Problems

1.

Hot oil is to be cooled in a double-tube counter-flow heat exchanger. The copper inner

tubes have a diameter of 2 cm and negligible thickness. The inner diameter of the outer

tube (the shell) is 3 cm. Water flows through the tube at a rate 0.5 kg/s, and the oil throughthe shell at a rate of 0.8 kg/s. Taking the average temperatures of the water and the oil to

be 45°C and 80°C, respectively, determine the overall heat transfer coefficient of this heat

exchanger.

2. A double-pipe (shell-and-tube) heat exchanger is constructed of a stainless steel(k = 15.1

W/m · °C) inner tube of inner diameter Di =1.5 cm and outer diameter Do = 1.9 cm and an

outer shell of inner diameter 3.2 cm. The convection heat transfer coefficient is given to be

hi = 800 W/m2 · °C on the inner surface of the tube and ho = 1200 W/m2 · °C on the outer

surface. For a fouling factor of Rfi = 0.0004 m2 · °C/ W on the tube side and Rf,o = 0.0001

m2 · °C/ W on the shell side, determine (a) the thermal resistance of the heat exchanger per

unit length and (b) the overall heat transfer coefficients, Ui and Uo based on the inner and

outer surface areas of the tube, respectively.

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