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STEAM TABLES

(Thermodynamics\ Level 1\ Steam-tables.doc)

CONSTANT PRESSURE HEATING OF WATER TO PRODUCE STEAM

Temperature

Saturation temperature or boiling point

Fusion temperature or freezing point

 

Superheated steam

Saturated water

 

 

 

Wet steam

 

Water

Dry saturated steam

 

 

Ice

 

(thermo\water-steam.sdr)

 

 

Heat supplied

∑Water changes into steam at the saturation temperature (boil

∑The saturation temperature increases with increasing pressur

∑Water at its boiling or saturation temperature is known as sa

∑The temperature remains constant while water is being chang

∑Steam at its saturation temperature is known as dry saturate

∑When water is changing to steam, a water/steam mixture kn exists

∑Steam above its saturation temperature is known as superhea

Steam does not obey the Gas Laws

∑Never use the Gas Laws for steam

∑Always use Steam Tables for finding the properties of w

∑The recommended tables are:

∑Thermodynamic & Transport Properties of Fluids (SI & Mayhew

Note that temperature and pressure are not independent at the condition, so a saturated fluid is defined if we know either its te pressure. To put it another way, water and steam can only co-e condition) at definite combinations of temperatures and pressu Once in a while an understanding of this sort of thing comes in accident at the Three Mile Island nuclear plant happened becau thought the water coolant was compressed water when in fact it Because of this misunderstanding they turned off the emergenc system causing the destruction of the reactor core.

Thermodynamic & Transport Properties of Fluids (SI unit by Rogers & Mayhew

These tables use the following notation and units:

Property

Symbol

Units

 

 

 

Pressure

p

bar

 

 

 

Temperature

t

oC

Specific volume

v

m3/kg

Specific internal

u

kJ/kg

 

 

 

Specific enthalpy

h

kJ/kg

 

 

 

Specific entropy

s

kJ/kgoK

SUBSCRIPTS USED IN THE TABLES

frefers to water at its boiling point or saturation temperatu

grefers to dry steam that is still at the saturation temperat steam)

fg refers to the change from boiling water to dry saturated pressure

s refers to the saturation temperature or pressure

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PROPERTIES OF WATER BELOW ITS BOILING POINT

A liquid that is below its boiling point or saturation temperat compressed liquid. (Sometimes also known as subcooled liquid)

Water is highly incompressible so its properties are little affec pressure.

For this reason the actual temperature of the water is taken saturation temperature and the actual pressure is ignored. So:

v = vf (page 10)

u = uf (pages 3 – 5)

h = hf (pag

 

 

 

Do not forget to insert two zeros after the decimal point in each cas For example, at 25o C, vf = 0.0010030 m3 / kg and not 0.10030 m3 /

PROPERTIES OF WATER AT ITS BOILING POINT

This is water at its saturation temperature. It is known as “sat Subscript “f ” applies. So:

v = vf (page 10)

u = uf (pages 3 to 5)

h = hf

 

 

 

Do not forget to insert two zeros after the decimal point in ea For example, at 25o C, vf = 0.0010030 m3 / kg and not 0.1003

PROPERTIES OF DRY SATURATED STEAM

Dry saturated steam is steam that has no particles of water c that is still at the saturation temperature (boiling point) at whi Subscript “g” applies. So:

v = vg (pages 3 to 5) u = ug (pages 3 to 5) h = hg

PROPERTIES OF WET STEAM

Wet steam is a mixture of boiling (saturated) water and dry saturat during the change of phase from boiling water to dry saturated stea The proportion of dry steam that is present in a wet steam mixture i

 

Dryness fraction x

 

 

\Drynessfraction x

the mass of dry steampresent in the wet steamm

the total mass of the wet steammixture

 

 

 

Thus in 1 kg of wet steam with dryness fraction x

Mass of dry steam present = x kg,

Mass of water present = (1 – x) kg

Therefore

v = xvg + (1 – x)vf

u = xug + (1 – x)uf

h = xh

 

 

 

Expanding the expression for h gives

h = xhg + hf – xhf or h = hf + x(hg – hf) or h = hf + xhfg

But why bother? Just understand the first method. Values of uf, hf, ug, hg and hfg are given on pages 3 – 5 of the tables. Values of vf are given on page 10 of the tables.

Do not forget to insert two zeros after the decimal point in each cas For example, at 25o C, vf = 0.0010030 m3 / kg and not 0.10030 m3 /

All the above equations can be re-arranged to make x, the dryness f of the equation.

For instance, h= xhg+(1-x)hf can be re-arranged to give:

((

)) so if we know either the temperature or the pressure o

and one of the properties v, u, h or s of the wet steam, we can find x fraction. This is often very useful.

APPROXIMATE EXPRESSION FOR SPECIFIC VOLUME O

The volume of water present in wet steam is normally negligible volume of dry steam that is present. The volume of the water p ignored and thus

The approximate specific volume of wet steam v

Do not attempt any similar approximation for any of the other prop

PROPERTIES OF SUPERHEATED STEAM

This is steam that has been heated to temperatures in excess temperature corresponding to the steam pressure.

v, u and h are given in the tables at various pressures and tem 6 – 8.

To determine values of v, u and h at temperatures that are at diff temperatures to those given in the tables, linear interpolation may

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STEAM TABLES - EXERCISE

Use steam tables to determine the specific enthalpy (h), the specific and the specific volume (v) for the following cases.

Remember that the pressures in the tables are expressed in bars.

Note

100 kN/m2 = 105 N/m2 = 1 bar

Also

1 MN/m2 = 106 N/m2 = 10bar

 

 

Answers in kJ / kg or m3 / kg

1

Water at 88o C and 1 bar

[369, 369, 0.001034]

2

Water at it saturation temperature at 2.7 bar[546, 546, 0.00107]

3

Dry saturated steam at 5 bar

[2749, 2562, 0.3748]

4Wet steam with dryness faction 0.9 at 10 bar[2576.5, 2401.8, 0.1

5Superheated steam at 6 bar and 250o C[2958, 2722, 0.394]

6Superheated steam at 6 bar and 260o C[2978.8, 2737.8, 0.4021]

7Water at normal atmospheric pressure and 80o C[334.9, 334.9, 0.

8Water at its saturation temperature at 200 kN/m2[505, 505, 0.001

9 Saturated steam at 400 kN/m2 [2739, 2554, 0.4623]

 

10

Wet steam with dryness fraction 0.8 at 2 MN/m2

[2421, 2

11

Steam at 1 MN/m2 and 225o C

[2886.5,

12

Water at 165o C and 600 kN/m2

[697, 69

13

Boiling water at 2.6 MN/m2

[972, 96

14

Saturated steam at 39 bar

[2801.5,

15

Wet steam with dryness fraction 0.92 at 16.5 bar

 

Use the approximate method to determine v[2640.18, 2457.86, 0. 16 Steam at 550 kN/m2 and 320o C [3104.9,

NON-FLOW PROCESSES – STEAM

(Thermodynamics\ Level 1\ Non-flow steam.doc)

Simple Steam Engine

CLOSED SYSTEMS

Non-flow processes take place in closed systems

Real continuous boundary

Work output from system W

when valves are closed

 

 

 

 

 

CLOSED SYSTEM

P

i

 

 

s

CYLINDER & PISTON

t

o

 

 

n

(thermo\cylinder.sdr)

 

 

Heat transfer into system Q

 

--------------------------------------------------------------------------------

THE NON-FLOW ENERGY EQUATION – applies to all non-flow processes

Q - W = (U2 - U1)

or

q - w = (u2 - u1) per kg

(U2 - U1) is the change in internal energy of the system

STEAM TABLES

∑Steam does not obey the Gas Laws

∑Do not use the Gas Laws for steam

∑Always use Steam Tables for finding values of internal energy, specific volume, ent steam.

∑The recommended tables are:

∑Thermodynamic & Transport Properties of Fluids (SI units) by Rogers & Mayhew

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WORK DONE IN NON-FLOW PROCESSES W

Process

Process law

Work done W

 

 

 

 

 

Constant volume

V1 = V2 =constant

Zero

 

 

 

 

Constant pressure

p1

= p2 = constant

p(V2 – V1)

 

 

 

 

Hyperbolic

p1V1

= p2V2 = constant

p1V1ln(V2 / V

 

 

 

 

Polytropic

p1V1n

= p2V2n = constant

(p1V1 – p2V2) / (n

 

 

 

 

∑A hyperbolic process following the law pV=constant is NOT an isothermal process (con when the working fluid is steam, (although it is when the working fluid is a gas)

∑When steam undergoes an adiabatic process (zero heat transfer, Q = 0), it does NOT fo constant (this only applies to gases undergoing an adiabatic process)