Basics of Steam
Why do we use steam?
Steam is used as a carrier of heat and pressure.
It is produced by the a relatively inexpensive and Its temperature can be It carries a large amount of
evaporation of water. plentiful commodity which adjusted very accurately by energy in a small mass.
is environmentally friendly. the control of its pressure.
Absolute Pressure and Gauge Pressure
• Absolute pressure
The theoretical pressure-less state of a perfect vacuum is
“absolute zero”. Absolute pressure is therefore a pressure
above vacuum. The pressure exerted by the atmosphere is
1.013 absolute at sea level.
• Gauge pressure
as shown on a standard pressure gauge, is the pressure above
atmospheric pressure. Therefore 0 bar gauge (bar g) is the
equivalent of 1.013 bar absolute (bar a).
Pressure -Temperature Ratings
• It is not possible to give direct conversions between DIN and ANSI ratings, and
space does not permit a detailed comparison.
From Ice to Water…then Steam
At atmospheric pressure, water cannot
exist as a liquid above100°C, so any
additional heat energy added after it
Steam reaches this temperature will cause some
of the liquid to boil off as steam.
100°C Saturation point
If heat energy is added to water, its
temperature rises again to a point at which
Water it can no longer exist as water.
0°C Melting point
If heat energy is added to ice, its
Ice temperature rises to a point at which it can
no longer exist as ice.
Enthalpy
This is the term given to the total energy with regards to both
the pressure and the temperature of a fluid or vapour (such
as water or steam) at any given time and in any given
condition.
The basic unit of measurement is the joule (J).
4.18 Joule (1 BTU) is the amount of heat required to raise 1g
(1 lbs) of water from 14.5 °C ( 60°F) to 15.5°C ( 61°F)
Formation of Steam
Specific enthalpy of steam
2676 The total heat energy in each kilogram of
kJ/kg steam at 1 bar abs
h‘‘ ( hg ) = h‘ + r (Total heat)
2257 Specific enthalpy of evaporation
kJ/kg
The extra heat energy required to trans-
form each kilogram of liquid water into
steam.
r (hfg ) (latent heat)
419 Specific enthalpy of saturated water
kJ/kg The total amount of heat energy held by each
kilogram of liquid water at its boiling
temperature.
‚
h (hf ) (sensible heat)
Steam Generation
• Steam is generated by adding heat energy to water,
causing it to evaporate. This operation is carried out in a
boiler.
• It is generated at pressure from below 1 bar (abs), for
steam heating applications, up to supercritical conditions
above 220 bar (a) and 374°C for power generation.
• The most common steam boiler type in general industrial
use for steam pressures up to 30 bar g is the packaged
shell & tube boiler.
• With this type of boiler it is unlikely that the steam
produced will be 100% dry (normally 96 to 98%).
Evaporation of water at p= 1 barabs
Temperature (°C)
Sensible Latent heat Sensible
heat of heat
of vaporization of
water dry steam
100°C
Saturated
Boiling
Steam
0°C Water
418 kJ/kg 2675 kJ/kg
Enthalpy
Ice Water Wet steam Super-
heated Steam
Saturated Steam Curve
240
230
Superheated steam area
220
210
200
190 Saturation curve (t s)
180
Temperature [ °C ]
170
160
150
140
130
Hot water area
120
110
100
90
80
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Pressure [ barg ]
Enthalpy - Pressure Chart for Steam
Mass, Volume and Specific-Volume of Steam
• If one 1 Lit mass of water is all converted into steam, the
result will be exactly one 1 kg mass of steam.
• However, the volume occupied by the steam will be far
greater.
• At atmospheric pressure, 1 kg of steam occupies nearly 1.7
m3
• As steam pressure increases, its specific volume decreases.
Example: Steam and Condensate behavior in Heat
Exchanger
Point 1 Point 2 Point 3
Mass
1000 1000 848
(Kg)
Pressure
10 10 1
(bar a)
Temperature
(⁰C) 179.89 179.89 100
Specific Volume
(m3/Kg) 0.1943 0.0011274 0.0010434
Enthalpy
2776.2 762.61 417.51
(KJ/Kg)
Heat of Evaporation (r) : 2013.6 KJ/Kg
