The History of Steam
like English, there is the language of steam which needs to be
learnt before we dive into the basics of understanding steam.
- Terms and Definitions
-. Unit converter
Geography of a process plant
11 Steam Table
of Temperature. The degree of hotness with no implication of
the amount of heat energy available. The temperature scale is used
as an indicator of thermal equilibrium between two systems in
contact with each other.
Temperature difference, as used
in many heat transfer calculations, may be expressed in either °C
or K. Since both scales have the same increments, a temperature
difference of 1°C has the same value as a temperature
difference of 1 K.
The Celsius (°C) scale. This
is the scale most commonly used by the engineer, as it has a
convenient (but arbitrary) zero temperature, corresponding to the
temperature at which water will freeze.
absolute or K (kelvin) scale. This scale has the same
increments as the Celsius scale, but has a zero corresponding to
the minimum possible temperature when all molecular and atomic
motion has ceased. This temperature is often referred to as
absolute zero (0 K) and is equivalent to -273.16°C.
Fahrenheit (°F) scale. This scale is used in the FPS
system(US and Canada), but not much elsewhere. To convert °F
to °C, use the formula:
is a comparision of the various scales of temperature, shown
of Pressure is defined as 1 newton of force per square metre
(1 N/m²). The SI unit of pressure is the pascal (Pa), but as
Pa is such a small unit the kPa (1 kilonewton/m²) or MPa (1
Meganewton/m²) tend to be more appropriate to steam
However, probably the most commonly used
metric unit for pressure measurement in steam engineering is the
bar. This is equal to 105 N/m², and approximates to 1
Absolute pressure (bar a)
This is the
pressure measured from the datum of a perfect vacuum. So, a
perfect vacuum has a pressure of 0 bar a.
pressureless state of a perfect vacuum is "absolute zero".
Absolute pressure is, therefore, the pressure above absolute
At mean sea level, for instance, the pressure exerted
by the atmosphere is 1.033 kg/cm2 absolute, when measured as
kilograms per square centimeter. This is always assumed to be 1
kg/cm2 a for calculations.
At sea level, the pressure can
also be stated as 1.013 25 bar a (1 atm) , when measured in bars.
This is always assumed to be 1 bar a for calculations.
pressure is also commonly measured in millimeters of mercury, or
Gauge pressure (bar g)
gauge is the pressure as measured by the pressure gauge measured
from the datum of the atmospheric pressure.Gauge pressure
indication is shown as kg/cm2g.
The pressure gauge –
bourdon tube type - measures pressure relative to the outside
atmospheric pressure. This is rounded off to 1 bar a or 1 kg/cm2a
– at (MSL). Therefore, to convert bar g to bar a, we add 1
bar, and to convert kg/cm2g to kg/cm2a, we again add 1 kg/cm2.
pressure + Atmospheric pressure = Absolute pressure
6 bar g + 1
bar = 7 bar a
10 kg/cm2g + 1 kg/cm2 = 11 kg/cm2a
above atmospheric will therefore, always yield a positive gauge
pressure. Conversely a vacuum or negative pressure is the pressure
below that of the atmosphere. A pressure of -1 bar g corresponds
closely to a perfect vacuum. In the other units, a vacuum exists
below zero kg/cm2g .
bar g = 11 bar a = 10.2 kg/cm2g = 11.2 kg/cm2a = 145 psig = 1 MPa
= 106 N/m2
the data given in the steam table has pressure in kg/cm2gauge and
abs . You can check the steam tables and see that in the lower
values for pressure, the enthalpy values vary a lot more than the
higher pressure readings. Therefore it is very important you
convert bar g to bar a, when the steam tables have pressures
mentioned in bar terms.
Differential pressure ΔT
is simply the difference between two pressures. When calculating
difference in pressure, the reference point becomes meaningless.
Therefore, the difference between two pressures will have the same
value whether these pressures are measured in gauge pressure or
absolute pressure, as long as the two pressures are measured from
the same reference.
is the study of energy changes accompanying physical and chemical
changes. The term itself clearly suggests what is happening --
"thermo", from temperature, meaning energy, and
"dynamics", which means the change over time.
Thermodynamics can be roughly encapsulated with these
Heat and Work / Energy / Enthalpy / Entropy / Free
Energy is the capacity to do work
(a translation from Greek-"work within"). Therefore work
and energy are one and the same. The SI unit for work and energy
is the joule, defined as 1 Nm.
The total energy of a system
is composed of the internal, potential and kinetic energy. The
temperature of a substance is directly related to its internal
energy. The internal energy is associated with the motion,
interaction and bonding of the molecules within a substance. The
external energy of a substance is associated with its velocity and
location, and is the sum of its potential and kinetic energy.
physics, the amount of mechanical work done can be determined by
an equation derived from Newtonian mechanics
= opposing force x displacement
is in joules (N*m) (or calories, but we are using primarily SI
opposing force is in newtons (kg*m/s2)
is in meters
In chemical reactions, work is primarily
related with expansion. It is generally defined as :
= (area X applied pressure) X displacement
value of displacement X area is actually the change in
volume. If we imagine a reaction taking place in a container of
some volume, we measure work by pressure times the change in
= dV x P
is the change in volume, in litres
Heat and Work
and work are both forms of energy. They are also related forms, in
that one can be transformed into the other. Heat energy (such as
steam engines) can be used to do work (such as pushing a train
down the track). Work can be transformed into heat, such as might
be experienced by rubbing your hands together to warm them up.
Work and heat can both be described using the same unit of
measure. Units of heat energy used may be calorie (cal), Joule (J,
SI unit) or Btu.
Typically, we use the SI units of
Joules (J) and kilojoules (kJ). But sometimes, the calorie is the
unit of measure (MKS unit). Heat energy is measured in
kilocalories, or 1000 calories.
is defined as an amount of heat required to change temperature of
one gram of liquid water by one degree Celsius.
1 cal =
1 kcal = 1000 cal = 4186.8 J
joule is a derived unit defined as the work done or energy
required, to exert a force of one newton for a distance of one
metre, so the same quantity may be referred to as a newton metre
or newton-metre with the symbol N·m.
One Joule is
the mechanical energy which must be expended to raise the
temperature of a unit weight (2 kg) of water from 0°C to 1°C,
or from 32°F to 33°F.
1 J (Joule) = 2.389 X 10-4
Btu. A Btu - British thermal unit - is the
amount of heat energy required to raise the temperature of one
pound of cold water by 1º F. Or, a Btu is the amount of heat
energy given off by one pound of water in cooling, say, from 70º
F to 69º F.
Heat. Heat is energy transferred as
a result of a temperature differences. Energy as heat passes from
a warm body (with higher temperature) to a cold body (with lower
temperature). Heat is a form of energy and as such is part of the
enthalpy of a liquid or gas.
It is a measure of energy
available with no implication of temperature. To illustrate , the
one kcal that raises one kg of water from 18ºC to 19ºC
could come from the surrounding air at a temperature of 35ºC
or from a flame at a temperature of 1,500ºC.
flow. The transfer of energy as a result of the difference in
temperature alone is referred to as heat flow.
What is the
difference btween temperature and Heat?
Temperature is the
cause. Heat is the effect.
Watt is the SI unit of
power and can be defined as 1 J/s of heat flow.
Capacity. Heat Capacity of a system is the amount of heat
required to change temperature of the whole system by one
Specific Heat Capacity
A measure of the
ability of a substance to absorb heat. It is the amount of energy
(kcal) required to raise 1 kg of water 1° C. Thus specific
heat capacity is expressed in kcal/kg/°C.
The specific heat
capacity of water is 1 kcal/kg/° C. This means that an
increase in enthalpy of 1 kcal will raise the temperature of 1 kg
of water by 1° C.
Specific heat capacity.
Specific heat, given by the symbol "C", is generally
The amount of heat required to raise the
temperature of one (1) kilogram of a substance by one (1) degree.
The heat required to raise one (1) gram of a material one
(1) degree. It can be thought of as the ability of a substance to
Water has a
very large specific heat capacity (4.19 kJ/kg°C) or, 1
cal/gram°C compared with many fluids. Water is therefore, a
good heat carrier.
heat may be measured in kJ/kgC,kcal/kg°C, cal/gram°C or
Btu/lb°F. For comparing units, check the unit converter for
more information. Specific heat capacities for different materials
can be found in the Material Properties section.
of Heat Required to Rise Temperature
The amount of heat
needed to heat a subject from one temperature level to an other
can be expressed a
= specific heat capacity (kcal/kg°C)
Q = amount of heat
M = mass (kg)
ΔT = rise in temperature of
the material in degrees Celsius (°C)
Heat transfer is the flow of enthalpy from matter
at a high temperature to matter at a lower temperature when
brought into contact.
This is the term
given to the total energy, due to both pressure and temperature,
of a fluid (such as water or steam) at any given time and
condition. More specifically it is the sum of the internal energy
and the work done by an applied pressure.
The basic unit of
measurement is the SI unit joule (J). Since one joule represents a
very small amount of energy, it is usual to use kilojoules (kJ)
In MKS units, the basic unit of measurement for
all types of energy is kcal/kg.
specific enthalpy is a measure of the total energy (enthalpy) of a
unit mass (1 kg), and the units are usually kJ/kg or
Heat of the Liquid (Enthalpy of Saturated
Water) – hf
Expressed in kcal's, this is the amount
of heat required to raise the temperature of 1 kg of water from 0°
C to the boiling point of a given pressure/temperature
correlation. Also referred to as Sensible Heat.
hf – heat of the fluid.
Latent Heat of
Evaporation (Enthalpy of Evaporation) – hfg
in kcal's, this is the amount of heat required to change 1 kg of
boiling water to 1 kg of steam. This same amount of heat is
released when a kg of steam is condensed back to a kg of water.
The quantity of latent heat will vary with the pressure and/or
temperature of a closed system.
Written as hfg –
heat incurred in change of state from fluid to gas, or
Total Heat of Steam (Enthalpy of Saturated Steam)
The sum of the Heat of the Liquid and Latent
Heat of Evaporation, also expressed in kcal's.
hg – Total enthalpy of saturated
Subscript f = Fluid or liquid state, for
example hf: liquid enthalpy
Subscript fg = Change of state
liquid to gas, for example hfg: enthalpy of evaporation
g = Total, for example hg: total enthalpy
The density ρ of a substance can
be defined as its mass (m) per unit volume (V). The specific
volume (vg) is the volume per unit mass and is therefore the
inverse of density. In fact, the term ‘specific’ is
generally used to denote a property of a unit mass of a substance.
m = Mass (kg)
V = Volume (m3)
= Specific volume (m3/kg)
The SI units of density (ρ)
are kg/m³, whilst conversely the units of specific volume
(Vg) are m³/kg.
Another term used as a measure of density is the
specific gravity. It is a ratio of the density of a substance (ρs)
and the density of pure water (ρw) at
standard temperature and pressure (STP). This reference condition
is usually defined as being at atmospheric pressure and 0°C.
Sometimes it is said to be at 20°C or 25°C and is referred
to as normal temperature and pressure (NTP).
density of water at these conditions is approximately 1 000 kg/m³.
Therefore substances with a density greater than this value will
have a specific gravity greater than 1, whereas substances with a
density less than this will have a specific gravity of less than
Since the specific gravity is a ratio of two densities,
it is a dimensionless variable and has no units. Therefore in this
case the term specific does not indicate it is a property of a
unit mass of a substance. The specific gravity is also sometimes
known as the relative density of a substance.
Temperature (boiling point). The temperature for a
corresponding Saturation Pressure at which a liquid boils into its
vapor phase. The liquid can be said to be saturated with thermal
(heat) energy. Any addition of thermal energy results in a phase
Boiling Point. A somewhat clearer (and
perhaps more useful) definition of boiling point is "the
temperature at which the vapor pressure of the liquid equals the
pressure of the surroundings".
The pressure at which vaporization (boiling) starts to occur for a
corresponing Saturation temperature. For water at 100°C, the
saturation pressure is 1 atm and, for water at 1 atm, the
saturation temperature is 100°C.
The term 'saturation'
defines a condition in which a mixture of vapor and liquid can
exist together at a given temperature and pressure.
and other units
– °C or °K
Pressure SI unit – Pascal
Pa = 1 N/m2,
too small a unit
Common unit = Bar
1 Bar =105
Atmospheric Pressure = 1 Bar abs at MSL
0 Bar abs
Bar Gauge + atm pressure = Bar abs
Volume = 1 / Density in m3/kg
Gravity = Density ratio to water
Energy SI unit = 1 Joule =
1 Nm = 4.186 cal
Common unit = kilocalorie
1 kcal = heat
reqd to raise 1 kg water by 1°C
1 kcal = 4186.8 Joules
= sp. heat capacity in kcal/kg °C
SI and other units
mWC = meters water column
1 Bar = 10
1 Bar = 14.23 PSI (Lbs/in2)
psi = 10.54 Kg/cm2g
psi = 3.5 Kg/cm2g
is the pressure exerted by a static head of water column. Remember
that 10 meters of head = 1 bar
pressure + Atmospheric pressure = Absolute pressure
most common pressure for utilization of steam is 3.5 kg/cm2g.
Density is about a thousandth of water.
• Boilers come
from an era when the industrial revolution was at its peak in
Britain. Therefore, the world standards for boilers are British.
ie, the MKS system.
• Specific gravity is dimensionless as
it is a ratio. Density of any lquid relative to water. It is used
mostly for fuel.
• The SI unit of 1 Joule is too small,
therefore the kilocalorie was developed.
base units. The
International System of Units (SI) is founded on seven base units:
Length, Mass, Time, Electrical current, Thermodynamic temperature,
Luminous temperature and Amount of substance. They are defined in
an absolute way without referring to any other units. We will be
working with the following four units only..
derived units. Derived units are algebraic combinations of the
seven base units with some of the combinations being assigned
special names and symbols.
properties of matter are measured at STP - Standard temperature
Temperature: freezing point of pure water, 0°C
Pressure: 760 mm Hg or one atmosphere
1. Heating Water
What is the energy needed to heat a mass
of 1.0 kg of water from 0°C to 100°C when the specific
heat of water is 1kcal/kg°C.
Q = M Cp ΔT
1 kg X 1 (kcal/kg°C) X (100 - 0)(°C)
= 100 kcal
If 200 kgs of a substance at 22 °C with a specific
heat of 0.88 kcal/kg°C is heated with 10,000 kcal of energy,
what is the new temperature of the substance?
Q = M Cp ΔT
= Q / M Cp
= 10,000 / 200 X 0.88 = 56.82
temperature is 22+56.82 = 78.82°C
Assume that water at 50° C water is fed to a boiler
at atm pressure. This begins to boil at 100° C. 1 kcal will be
required to raise each kg of water by 1° C. Therefore, for
each kg of water, the increase in enthalpy required to raise the
temperature from 50° C to 100° C is:
(100 - 50) x 1 =
the boiler holds 10000 kg mass the increase in enthalpy to bring
the total mass of water to it's boiling point is therefore:
kcal/kg x 10000 kg or 5,00,000 kcal.
must be remembered, this figure is not the sensible heat, but
merely the increase in sensible heat required to raise the
temperature from 50° C to 100° C. The datum point of the
steam tables is water at 0° C, which is assumed to have a heat
content of zero for our purposes.
total sensible heat of water at 100° C is therefore:
- 0) x 1 = 100 kcal/kg