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lecture14 Powered By Docstoc
					Dr. Albrecht Kaupp                                            Page 1


Issue                Steam is a widely used secondary energy
                     source in industrial production. Its
                     generation requires considerably fuel energy.

Learning              Understanding the nature of steam
Objectives            Knowing how to calculate steam
                      Estimating inaccuracies in steam
                      Identifying opportunities to reduce
                       specific energy costs for steam generation
                      Appreciating the complexity of steam
                       properties as applied to energy
Steam                                                                              Page 2

                                                                          NOTES

1. The nature of steam generation
It may be trivial to discuss the nature of steam and its generation.
However as a consultant advising on energy efficient steam
generation, one is certainly better off to know the terminology and
understand the basics of steam generation.

Steam is the “product” of a boiler, like cars are the product of a car
manufacturing plant. The skill is to generate steam at a desired
quality as cost effective as possible.

For a client steam is a source of energy that he needs at a certain
temperature and pressure.

Boilers are therefore selected on the basis of how much energy in
form of steam they can provide at a specific pressure and

This lecture follows the terminology recommended by the
American Boiler Manufacturers Association published in the
“Handbook of Power, Utility and Boiler, Terms and Phrases”.

As every engineer knows, if water is heated at a pressure of 1.013
bar, which is about the atmospheric pressure, it will change its state
(or phase) and become a vapor at 100 oC.

Generally if water is heated at a constant pressure, P, it will
increase its temperature to a so-called saturation temperature, at
which evaporation occurs.

As soon as evaporation occurs, the temperature of the water will
remain the same and any additional energy input is used to
evaporate more water until all liquid water has been evaporated and
became saturated steam.

2. How many types of steam?
We distinguish among different types of steam.

   Dry steam            = Steam containing no moisture
   Wet steam            = Steam containing moisture
Steam                                                                             Page 3

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   Saturated steam     = Steam at the pressure corresponding to
                          its saturation temperature
   Superheated steam = Steam at a higher temperature than its
                        saturation temperature
   Steam quality       = The percent of weight of vapor in a
                          steam and water mixture
   Life steam          = Steam which has not performed any of
                          the work for which it was generated
   Process steam       = Steam used for industrial purposes
                          other than for producing power or for
                          space heating
   Saturated water     = Liquid water at the pressure
                          corresponding to its saturation

Explaining the different types of steam imagine the boiler drum is
at 10 bar (absolute), which corresponds to a saturation temperature
of 179.88 oC. The steam generated in the drum will leave the boiler
through the steam dome as saturated steam. However because not
only steam but as well entrained moisture in the steam will leave
the boiler, we rarely have dry steam, but rather wet steam leaving
the steam dome. The steam is life steam because it has so far not
been used. In case the steam is used for drying purposes, we would
also call it process steam.

In case the steam is used for power generation it is usually not
sufficient to generate saturated steam. Rather the saturated and wet
steam is further heated to become superheated steam. Superheated
steam is usually dry steam with no entrained moisture.

3. Implications for energy audits
As mentioned steam and water is separated in a steam-and-water-
drum and the steam is either further heated to become superheated
steam or released as saturated steam through a steam scrubber, a
series of screens, wires, or plates to remove the entrained moisture.
The final product may be steam that closely resembles dry steam.
Steam                                                                              Page 4

                                                                          NOTES

It is important to have some idea whether the steam is very wet, dry
or superheated. The two most common measurement devices, an
orfice plate or turbine flow meter measure only the dry steam
fraction (= gas phase) and will not detect any moisture (liquid water
droplets) in the steam. Most reliable measurements are achieved
with superheated dry steam.

4. Steam properties
Dry steam is like a gas, and is invisible. Consequently the ideal gas
law applies to a certain extend. The state of steam is therefore fully
described by its pressure and temperature. In the case of saturated
steam we only need to know either the temperature or the pressure
to calculate other important parameters such as the energy content
of the steam (= enthalpy) and its specific volume.

In the case of superheated steam we need to know the steam
temperature and pressure to calculate the enthalpy and the specific

Equations to calculate precisely the physical properties of steam are
very complicated and most practitioners either use the International
Steam Tables or appropriate software. We use the TAFTAN steam
calculator software (e-mail 100131.2557@compuserve.com)

Three handy equations for saturated steam are

                 Tsat  100   Psat 
                                                in oC        (1)

                        T 
                 Psat =  sat                  in bar       (2)
                         100 

                 v =    P
                                                in m3/kg     (3)
                            / 2 + 0.1

Accuracy is about 2 % and therefore sufficient for our applications.
However, do not use these equations for superheated steam.

The generation of steam is accomplished in three stages:
Steam                                                                              Page 5

                                                                          NOTES

Stage 1:   The liquid feedwater is heated to the saturation
           temperature at a given boiler drum pressure. The energy
           needed for this stage equals hf.

Stage 2:   At the saturation temperature more energy is needed to
           evaporate the water to steam. This heat of evaporation
           equals hfg.

Stage 3:   In some cases the saturated steam is further heated to
           temperatures above the saturation temperature and
           becomes superheated steam. The energy input equals

Consequently the energy content of steam hg equals
                   hg = hf + hfg + hSH    in kJ/kg

                   hg = hf + hfg          for saturated steam
                   hg = hf + hfg + hSH    for superheated steam

5. Interpretation of steam enthalpy
The enthalpy of steam is shown in graphics form as a function of its
pressure in the folders “P-hf steam”, “P-hg steam”, and “P-hfg

In addition a P-T diagram of saturated steam is given. For our
purposes it is sufficient to qualitatively understand the relationship
between pressure of steam and its other properties such as
temperature, energy content and specific volume.

Any boiler has a pressure indicator at the exit of the steam dome,
where steam leaves the boiler. Steam indicators either measure the
absolute or the gauge pressure of steam. By gauge pressure we
mean the absolute pressure minus the atmospheric pressure. For
instance steam at zero gauge pressure is at atmospheric pressure. A
brief discussion of the various graphics follows.
Steam                                                                            Page 6

                                                                        NOTES

5.1 P-T diagram
Note that the saturation temperature of steam increases steeply with
pressure up to about 15 bar and than levels off and approaches
asymptotically the critical temperature of 374.15 oC.

Generating dry steam at high temperatures is therefore best done by
superheating the steam. From a technical point of view it is easier
to generate saturated dry steam at 15 bar and superheat it from 200
 C to 300 oC instead of generating saturated steam at 85 bar, that
also has a temperature of 300 oC.

5.2 P-hf diagram
This diagram shows the energy content of liquid water under
pressure. At 0 bar, gauge pressure, the saturated water has an
enthalpy of 419.07 kJ/kg, corresponding to a temperature of 100oC.
Observe that the diagram refers to liquid water at saturated

One of the challenges of an energy manager is to ensure that the
feedwater of a boiler is entering the boiler at the highest
temperature possible for any given pressure. This is usually
accomplished by returning as much hot condensate as technically

5.3 P-hfg diagram
The diagram shows how much energy is needed to evaporate
saturated water to saturated steam. Notice that the higher the
pressure, the less energy is needed. From a high value of 2257.9
kJ/kg to evaporate water at 100 oC and 1 atm this value drops to 0
kJ/kg to evaporate water at the critical pressure of 221.2 bar.

5.4 P-hg diagram
This diagram shows the total energy content of saturated steam
which is the sum of hf and hfg. Note that the energy content steeply
increases with pressure in the range of 1 to 20 bar, than remains
about the same for the pressure range of 20 to 40 bar and drops
Steam                                                                               Page 7

                                                                           NOTES

slowly to a final value of 2107.4 kJ/kg at the critical pressure of
221.2 bar.

6. Energy efficiency and steam pressure
It is general not true that high temperature and high pressure steam
is more efficiently to generate than low pressure steam.

Steam should be always generated at a pressure and temperature
where it is needed to avoid unnecessary throttling of steam to lower
its pressure for the envisioned task. One should check for
mismatches of steam pressure at the boiler outlet and required
steam pressure for the process or steam turbine.

However it is true that high pressure and temperature steam is more
efficiently to convert to power than low pressure steam.

The steam exit pressure in a boiler is rarely in equilibrium because
of the cycling behavior of demand. In reality we see low and high
pressure set points at the boiler, meaning at the low pressure point
the burner starts firing, and at the high pressure point the burner

Very fast cycling (2-5 minute intervals) indicates a rather narrow
pressure range setting.

Cycling periods with short firing periods and extended no fire
intervals indicate a sluggish steam demand and/or an oversized

Long cycling periods or no cycling at all with the burner at “high
fire” happens with overloaded boilers.

A cycling boiler is always less efficient than a boiler in equilibrium.
Steam                                                                              Page 8

                                                                          NOTES


Exercises will deepen the understanding of steam properties, and
familiarize with the use of an electronic steam table calculator.

Task 1
An interesting point is the split up of the total enthalpy between the
two stages of generating saturated steam. Consider saturated steam
at different pressures. Complete the table.

   bar      hf (kJ/kg) and %    hfg (kJ/kg) and %       hg (kJ/kg)
    1      417.51       16.6   2,257.92       84.4 2,675.43      100

    50                    %                     %                100

   100                    %                     %                100

   220                    %                     %                100

Task 2
One cubic meter of saturated water at 50 bar is converted to steam.
How many cubic meters of steam are generated?

 Hint: Use equation (3) to calculate the steam volume, and assume
       one cubic meter of water at 50 bar weighs 775 kg at the
       saturation temperature of 263.91 oC. Compare the results with
       the electronic steam calculator.

Approximate value       _________ m3
Steam calculator value _________ m3
Steam                                                                             Page 9

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Task 3
In a simplified way the German DIN 1942 norm defines the boiler
efficiency as
                 =         Adsorbed heat
                       Energy input of fuel and air

The adsorbed heat is the heat added to the steam water-circuit and
is defined as

        (steam flow)  (steam enthalpy - feedwater enthalpy)
+ (blowdown flow)  (blowdown enthalpy - feedwater enthalpy)

The major energy input is fuel and combustion air. The complete
equation accounts as well for other energy inputs such as power of
electrical motors associated with a boiler operation and fuel

As an exercise in using steam tables, calculate the adsorbed heat for
one ton of saturated steam output (10 bar) at 10 % blowdown, and
feedwater temperature of 90 oC.

                   Steps                            Results

 Saturated steam enthalpy at 10 bar (MJ/ton)

 Blowdown (ton)

 Blowdown enthalpy (MJ)

 Feedwater enthalpy (MJ)

 Adsorbed heat (MJ)

Task 4
One of the better electronic steam tables in terms of looks, accuracy
and ease of handling is from TAFTAN. Before one can use the
steam calculator, it is necessary to explain the term “quality” or
“wetness” of steam.
Steam                                                                          Page 10

                                                                      NOTES

The quality, x, of steam is the fraction of dry steam in a steam-
water mixture. Consequently the wetness, 1 - x, is the amount of
water vapor in a steam-water mixture. If steam is totally dry we
have x = 1. If all steam has condensed to water we have x = 0.

Calculate the following:

   The saturation temperature of steam at 1
    bar equals                                      __________ oC

   The saturation temperature of steam at
    1.0138 bar equals                               __________ oC

   Superheated steam at 10 bar and 350 oC
    has an enthalpy of                              ________ kJ/kg

   Liquid feedwater at 25 bar and 120 oC has
    an enthalpy of                                  ________ kJ/kg

   Calculate the saturation pressure of steam
    at 350 °C.                                     __________ bar

   The heat of evaporation of saturated steam
    at 35 bar equals                               _________ kJ/kg

   Saturated steam at 17 bar losses some of its
    energy. The quality drops to 0.9.
    - The temperature of saturated steam at
      x = 1 is                                      __________ oC
    - The temperature of saturated steam at
                                                    __________ oC
      x = 0.9 is

   Test the nature of the fluid and decide
    whether it is liquid water, saturated water,
    saturated steam, dry steam, superheated
    steam, or wet steam.

    - 10 bar and 140 oC                ________________________
    - 10 bar and 300 oC                ________________________
Steam                                                                             Page 11

                                                                         NOTES

    - 10 bar and 179.88 oC             ________________________
    - 10 bar and 2,172.1 kJ/kg         ________________________
    - 10 bar and x = 0.3               ________________________
    - 300 bar and x = 0.9              ________________________

Task 5
As a practicing engineer one deals with almost all combinations of
input data for steam and has to calculate other relevant steam data.
Steam properties temperature (T), pressure (P), enthalpy (h),
entropy (s), quality (x), and specific volume (v) are the outputs and
inputs one should be familiar with.

A list of functional relations:
h(T,x)       s(T,x)        v(T,x)     h(P,x)      s(P,x)      v(P,x)
h(P,T)       s(P,T)        v(P,T)     T(P,h)      s(P,h)      v(P,h)
x(P,h)       T(P,s),       h(P,s)     v(P,s)      x(P,s)      T(P,v)
h(P,v)       s(P,v)        x(P,v)

In practical field work one is interested in solving the following:

 Superheated steam of 30 bar at 420 oC losses 3 % of its energy
  in the steam distribution system. What is the new steam
  temperature? Can you solve this problem with the steam
  calculator? Which one of the above functional relations would
  apply to this problem?

   The functional relation is       ___________________

   The new steam temperature is     _________________ oC

 Saturated steam of 30 bar at 420 oC losses 3 % of its energy in a
  steam distribution system. The new steam temperature equals
  ____ oC.

 Superheated steam at 30 bar and 420 oC lost 1 bar in the steam
  distribution system.
Steam                                                                       Page 12

                                                                   NOTES

  The temperature changes to ______ oC

  The specific volume changes from _____ m3/kg to ____ m3/kg

  The enthalpy changes from ______ kJ/kg to _____ kJ/kg

Which steam parameter must be constant? Does this exercise make