1.1 Introduction
Energy – an increasingly valuable resource – is used in a wide range of industries for both heating and cooling.
The heat transfer process can be:
- Direct – Where heat is transferred directly from its source to the substance to be heated. A metal pipe heated by means of a gas flame for the purpose of bending represents a good example. The heat from the gas flame acts directly on the product to be heated.
- Indirect – Where heat energy is transferred to a substance through an intermediate wall. An example is an old fashioned kettle that absorbs heat on a gas cooker. The heat from the gas flame is transferred to the water through the bottom of the kettle and then into the water. In industry, steam may be generated and used to heat water or thermal oil for the manufacturing process.
In industry, steam may be generated and used to heat water or other process fluids for industrial and HVAC applications including sterilisation, blanching, air heating and distilling. This guide describes the principles of heat transfer by means of steam. The starting point is a boiler installed at a central location and used for steam generation. The steam is supplied to the various heat users and the water returned to the boiler house in the form of condensate
The term 'enthalpy' is used in the following to refer to 'heat' and 'heat content'. Enthalpy is generally defined as the total amount of energy contained in a system. This energy can be present in the form of heat in gases, liquids or steam. However, it also includes chemically bound energy, for example in fuels. Enthalpy additionally describes the amount of energy that is required for a phase change from a liquid to a gaseous state or from solid to liquid.
1.2 What is steam?
Let us imagine an old-fashioned kettle whistling away in the kitchen. We fill it with water, stand it on the cooker and switch on the heat source. After a few minutes, the water begins to boil and steam emerges from the spout. Yet what happens exactly in the water-filled kettle – equivalent to a boiler – from the moment heat is applied to the time when the steam emerges from the spout?
The heat transfer process – brought about, for example, by a gas flame underneath the kettle – begins as soon as the latter is placed on the cooker. Small gas bubbles form on the kettle walls as the temperature of the water rises. After a while, these bubbles drift upwards to the surface and the gases dissolved in the water – one of which is air – are expelled.
Observation 1
The solubility of the gases contained in the water decreases with an increase in water temperature. We will turn our attention to this phenomenon later when we discuss the principle of a deaerator (refer to Chapter 3.14 Deaerator).
Small bubbles now also form on the bottom of the kettle as the water heats up. Although these bubbles likewise rise, they disappear before they reach the water surface. These are not gas bubbles but small steam bubbles that condense as they progress through the colder layers of water. Whereas the temperature on the bottom of the kettle has already reached 100 ºC, in the higher water layers it has not. Moreover, we can observe that after a while the steam bubbles start to rise higher and higher. At the same time, it is clear that the water level rises with the temperature increase in the kettle. It has risen significantly since the moment the steam bubbles started to form.
Observation 2
Water expands as it heats up.
The total volume of the steam bubbles is up to 1700 times larger than that of the water from which they are formed.
The temperature of the complete contents of the kettle reaches 100 ºC at a time „X". From this point on, steam bubbles regularly rise to the surface of the water and escape. The water begins to boil and steam emerges from the spout. If we now measure the temperature of the water in the kettle and that of the escaping steam, the value for both will be 100 ºC.
If we subsequently increase the supply of heat (e.g. by turning up the gas), the quantity of steam bubbles will likewise increase. However, regardless of how much we turn up the heat, the temperature of the water and that of the escaping steam will always remain a constant at 100 ºC.
Observation 3
Under atmospheric conditions, in other words at an absolute pressure of 1013 mbar, water boils at 100 °C. Water with a temperature higher than 100 °C cannot exist under atmospheric conditions. The steam that is formed from water at 100 °C has the same temperature as the water itself.
If the boiling water is heated further, it begins to evaporate. In addition to the sensible heat required for heating, a significantly larger amount of energy must be supplied in order to evaporate the liquid. This 'enthalpy of evaporation' is represented by the symbol hfg. The total enthalpy of the steam that is formed is comprised of the sum of the liquid enthalpy and the enthalpy of evaporation, and is represented by the symbol hg:
hg = hf + hfg
As mentioned earlier, water boils at 100 °C under atmospheric conditions (1013 mbar). At a pressure of 1000 mbar (1 bar), water starts to boil at 99.6 ºC. From now on, for the sake of simplicity, we will adhere to the popular definition and assume that water boils at a pressure of 1 bar and 100 °C.
1.3 Boiling and evaporation point of water
Fig. 1-1 shows the gas consumption that is needed to boil and evaporate water.
Fig. 1-1: Kettle with gas meter
The gas meter is set to zero at the start of the experiment. After the water has been heated to 100 °C, the meter shows '1 unit of gas' has been consumed. A further six units of gas are required to evaporate all the water, in other words it takes approximately six times as much energy to evaporate water as it does to boil it. Conversely, this means that 1 kg of steam at 100 °C contains about six times as much heat as 1 kg of water at 100 °C. If heat is removed from the steam at a temperature of 100 ºC, condensate with a temperature of 100 ºC is formed. Put another way, the condensing process results in around six times as much heat from 1 kg of steam at 100 °C as is contained in water at the same temperature.
Fig. 1-2 a shows the amount of energy that is needed to heat and evaporate 1 kg of water under atmospheric conditions. The specific heat of water is 4.18 or approx. 4.2 kJ/kg K. 100 x 4.2 = 420 kJ of sensible heat (hf ) is required to heat 1 kg of water from 0 ºC to 100 ºC. The enthalpy of evaporation (hfg) necessary to completely evaporate 1 kg of water at 100 ºC is 2258 kJ/kg and can be taken from the steam tables (refer to Fig. 1-3).
Fig. 1-2 a: Evaporation
Fig. 1-2 b: Condensation
The total heat required to completely evaporate 1 kg of water from a temperature of 0 °C under atmospheric conditions is (hg = hf + hgf) 420 + 2258 = 2678 kJ. When heat is transferred from steam to the product or in a heat exchanger, the steam condenses. The heat that is released in the process is known as the latent heat of condensation. If heat is extracted from steam under atmospheric conditions, 2258 kJ/kg of the latent heat of condensation will be consumed while 420 kJ/kg will remain in the 100 ºC condensate as sensible heat (refer to Fig. 1-2 b).
The boiling point is higher in a closed vessel.
The enthalpy of evaporation is equal to the amount of heat that is released when steam condensates. The residual condensate has the same temperature as the steam from which it was formed
Processes with a steam pressure of 1 bar are rare because steam pressure varies according to the required process temperature.
- Every steam pressure has a saturation temperature.
- Saturated steam has a saturation temperature of 180 ºC at 10 bar, 152 ºC at 5 bar, 234 ºC at 30 bar, etc.
Several other properties of steam are also dependent on the pressure in addition to the saturation temperature, such as the liquid enthalpy, enthalpy of evaporation, and specific volume. These values are listed in the saturated steam tables.
1.4 Steam table for saturated steam
In Chapter 10.0 Appendix of this book contains steam tables for saturated and superheated steam. Fig. 1-3 shows an excerpt from the table for saturated steam.
You can find the corresponding state variables for each steam pressure in this table. The meanings of the table columns are explained below.
Fig. 1-3: Excerpt from the steam table
Column 1: Steam pressure (P)
The steam pressure specified in the steam table is the absolute pressure in bar. This is extremely important. When we speak of pressure, we are usually referring to gauge pressure, in other words the value that is read off on a pressure gauge. However, process engineers prefer to use absolute pressure for calculation purposes. This discrepancy often leads to misunderstandings in practice, sometimes with undesirable consequences.
When the SI system was first introduced, it was agreed that only absolute pressure should be used. All pressure measuring devices (other than barometers) in a process plant show gauge pressure. When reference is made in this book to "pressure", we always mean absolute pressure. Under standard conditions (sea level), the barometric pressure is 1 bar. Gauge pressure is normally indicated in barg. This guide uses bar a, or bar absolute
Column 2: Saturated steam temperature (Ts)
The saturated steam temperature is shown next to the steam pressure. The temperature at this pressure is identical to the boiling point of the water and is thus the temperature of the saturated steam. This same temperature column is also used to determine the temperature of the condensed steam.
Saturated steam at 8 bar has a temperature of 170.4 ºC. The condensate from steam at a pressure of 5 bar therefore has a temperature of 152 ºC.
Column 3: Specific volume (vg)
The specific volume is indicated in m3/kg. It decreases as the steam pressure increases. Steam at a pressure of 1 bar has a specific volume of 1.694 m3/kg. Put another way, 1 dm3 (1 litre or 1 kg) of evaporated water expands to a volume 1694 times its original form. The specific volume at a pressure of 10 bar is (1/ =) 0.194 m3/kg, in other words 194 times the volume of water. The specific volume is required to determine the diameter of steam and condensate pipes.
Column 4: Specific mass (ρ = rho)
The specific mass (also referred to as density) is expressed in kg/m3. It indicates the amount of steam that will fit into 1 m3. The specific mass increases as the pressure increases. At a steam pressure of 6 bar, 1 m3 has a mass of 3.17 kg. This value increases to 5.15 kg at 10 bar and to more than 12.5 kg at 25 bar.
Column 5: Sensible heat or Liquid Enthalpy (hf)
The sensible heat is expressed in kJ/kg. This column shows the amount of heat that is needed to boil 1 kg of water at a defined pressure or the amount of heat remaining in the condensate after 1 kg of steam has condensed at the same pressure. The sensible heat is 417.5 kJ/kg at a pressure of 1 bar, 762.6 kJ/kg at 10 bar and 1087 kJ/kg at 40 bar. The sensible heat increases as the steam pressure increases and accounts for an ever larger proportion of the steam's total enthalpy. The higher the steam pressure, in other words, the more residual heat there is in the condensate.
Column 6: Total enthalpy (hg)
The enthalpy is expressed in kJ/kg. This column shows the steam's total enthalpy. It is additionally clear that the enthalpy increases up to a pressure of 31 bar and decreases thereafter. The enthalpy is 2801 kJ/kg at a pressure of 25 bar but only 2767 kJ/kg at 75 bar.
Column 7: Enthalpy of evaporation (condensation) (hfg)
The enthalpy of evaporation (condensation) is expressed in kJ/kg. This column shows the amount of heat that is needed to completely evaporate 1 kg of boiling water at a defined pressure. Conversely, it also specifies the amount of heat that is released if the (saturated) steam is completely condensed at a defined pressure.
hfg is 2258 kJ/kg at a pressure of 1 bar, 1984 kJ/kg at 12 bar and only 1443 kJ/kg at 80 bar. The enthalpy of evaporation (condensation) decreases as the pressure increases.
Examples from the steam table
Example 1: Boiling and evaporation (Fig. 1-4 a)
To evaporate water at a pressure of 7 bar, the water must be boiled at a temperature of 165 °C. The enthalpy (hf ) in this case is 697 kJ/kg. The amount of heat (hfg) that must be supplied in order to evaporate the water is 2065 kJ/kg. The total enthalpy (hg) of the steam that is produced is thus: 697 + 2065 = 2762 kJ/kg
Fig. 1-4 a: Boiling and evaporation
Fig. 1-4 b: Condensation
Example 2: Condensation
The bar graph for condensation (Fig. 1-4 b) reveals that 2065 kJ/kg of heat per kg of steam is released when steam condensates at a pressure of 5 bar (165 ºC). The residual condensate has a temperature of 165 ºC and an enthalpy hf = 697 kJ/kg.
Example 3: Heat exchanger
How much heat is required to heat 8 m3 (8000 kg) of water from 10 ºC to 85 ºC? How much steam is needed for this purpose if the steam pressure in the heat exchanger is 7 bar? Water has a specific heat of 4.2 kJ/kg K. The amount of heat required is calculated as follows: mass x temperature rise x specific heat = 8000 x (85 - 10) x 4.2 = 2520000 kJ. At a steam pressure of 7 bar the latent heat of condensation is 2065 kJ/kg. The amount of steam necessary to heat the water is 2520000/2065 = 1220 kg. The temperature of the condensate that must be drained off from the steam system by the steam trap is 165 ºC.
Example 4: Steam boiler
If feedwater with a temperature of 105 ºC (hf = 440 kJ/kg) is supplied to a steam boiler operating at 12 bar (T = 188 ºC and hf = 798 kJ/kg), this water must first be preheated from 105 ºC to 188 ºC before it begins to evaporate. The following heat is required to preheat the feedwater: 798 - 440 = 358 kJ/kg. A further hfg = 1984 kJ/kg (from steam tables) of heat must then be supplied in order to evaporate the water. The steam produced in this way has a total enthalpy of 798 + 1984 = 2782 kJ/kg.
1.5 Superheated steam
Superheated steam is steam with a temperature and enthalpy higher than the values specified for this pressure in the saturated steam table. Example: Steam at 10 bar with a temperature of 200 ºC is superheated steam (by comparison, the saturated steam temperature is 180 °C).
Superheated steam has a much lower heat transfer coefficient than saturated steam. If superheated steam is used to transfer heat, e.g. in a heat exchanger, part of the heated surface is used to cool the steam down to saturated steam temperature. The amount of heat that is exchanged during this saturation process is small, yet it occupies a relatively large proportion of the heated surface. The most important step in the heat transfer process takes place during condensation. There is a persistent and widespread misconception that a higher product temperature can be achieved with superheated steam than with saturated steam at the same pressure. Superheated steam is principally used to drive steam turbines (for instance for power generation) and is only employed for heating in the production process if no saturated steam is available. The specific volume of superheated steam is considerably higher than that of saturated steam at the same pressure (compare the specific volume vg of superheated steam and saturated steam). In addition to various technical and design modifications, it is therefore necessary to take account of the lower absorptivities of the pipe that is installed when changing over from saturated steam to superheated steam.
Superheating with flue gases
The superheating process takes place in a separate tube bank that is mounted in the steam boiler's flue gas flow. Heat continues to be supplied to the saturated steam at a constant pressure in this tube bank. The temperature and enthalpy of the steam are increased.
Superheating by reducing the steam pressure
Saturated steam with a pressure of less than 31 bar can be superheated by reducing the pressure. No work is done and no heat supplied or dissipated as a result of a reduction in steam pressure and the enthalpy remains constant. This process is termed 'adiabatic' as no work is done.
1.6 The h-p-t diagram
Why is the h-p-t diagram mentioned in this book in addition to the steam table?
The h-p-t diagram (Fig. 1-5) has proven to be an invaluable instrument in practice for determining heat engineering concepts such as evaporation, condensation, flash steam formation, superheating, steam moisture content during pressure reduction, etc. It enables a few of steam's thermal properties that are also included in the steam tables to be read off directly and very simply.
Fig. 1-5: h-p-t diagram
The diagram shows the steam pressure on the horizontal axis, the temperature on the right hand vertical axis and the enthalpy on the left-hand vertical axis. The bottom curve is the water saturation curve. For example, the axis on the right reveals that the saturation point at 50 bar is 264 ºC while the axis on the left indicates the liquid enthalpy as 1155 kJ/kg.
The top curve is the steam saturation curve. The area between the water saturation curve and the steam saturation curve is referred to as the evaporation and condensation or coexistence region, where steam and condensate / water phases coexist. This region also contains several curves (x = 0.1 to x = 0.9) that will be explained later (x = mass ratio of steam / water). The area below the steam saturation curve is the condensate / water region and the area above it the superheated region.
The point at which the saturation curve and the saturation curve meet is referred to as the critical point. The pressure at this point is 221.2 bar and the temperature 374.15 ºC. The enthalpy of evaporation at the critical point is zero. No water can exist above this state and the liquid and vapour phases are not distinguishable.
Examples:
The water saturation curve shows that the liquid has an enthalpy of 760 kJ/kg at a pressure of 10 bar. If 200 kJ of heat is supplied to this liquid, the total enthalpy is 960 kJ. Let us follow a horizontal line from 960 kJ on the left-hand axis to the 10 bar line in the evaporation and condensation region. We discover that this point is located at x = 0.1. After 200 kJ has been supplied to the condensate at 180 ºC, 10 % of this condensate will have evaporated. If 840 kJ were to be supplied instead, the total enthalpy would be 1600 kJ and 42 % of the condensate will have evaporated. Conversely, if 400 kJ of heat were to be extracted from saturated steam at a pressure of 10 bar with an enthalpy of 2776 kJ/kg, the total enthalpy would be 2376 kJ. Let us now follow a horizontal line from 2376 kJ on the left-hand axis to 10 bar. We can see from the result that 20 % of the steam has condensed.
Pressure reduction
What happens in a high-pressure steam system if the steam pressure upstream of a heat exchanger is reduced by means of a control valve or if the pressure in a low-pressure steam system is controlled with pressure reducing valves?
If the pressure of the saturated steam is reduced using a control or pressure reducing valve, the pressure changes but the enthalpy of the steam remains constant because no work has been done and no heat exchanged. This reduction can be represented in the h-p-t diagram by drawing a horizontal line leftward from the point at which the saturation curve intersects the pressure curve up to the point at which this line intersects the required pressure.
Two examples are described in the diagram: in the first, steam is reduced from 20 to 10 bar while in the second, it is reduced form 150 to 60 bar (an extreme value is used for the sake of clarity). In the first instance, the point of intersection of the 10 bar line is clearly located in the superheated region. The reduced steam is thus superheated.
In the second example, the horizontal line intersects the 60 bar pressure curve in the evaporation and condensation region. Part of the steam will condensate because the enthalpy of saturated steam at 150 bar is 2615 kJ/kg, which is lower than the enthalpy of steam at 60 bar, namely 2785 kJ/kg. The intersection occurs on the line x = 0.9, in other words 10 % of the steam condenses. The steam is wet.
If saturated steam is reduced from 60 to 10 bar, the situation is slightly more complicated. Initially, the horizontal line passes through the evaporation and condensation region and condensation takes place. Later, however, the line reaches the superheated region and part of the condensate that formed previously now evaporates again.
Fig. 1-6: Example in the h-p-t diagram
Condensate flashing
A point A is specified at an arbitrary point on the saturation curve in Fig. 1-6. If the pressure is reduced starting at A, for instance from 50 to 10 bar, the horizontal line runs leftward through the evaporation and condensation region up to the 10 bar mark, where it intersects the curve x = 0.2, in other words 20 % of the condensate has evaporated again.
Wet steam
Strictly speaking, there is no such thing as wet steam - there is either steam or condensate. The density of condensate is higher than that of steam. In a horizontal pipe, condensate drops directly to the bottom of the pipe. When we speak of wet steam, we mean steam in which condensate is entrained as a result of heat losses during transport. The denser condensate is entrained due to the high steam velocity. Water hammer often occurs as a direct consequence of this entrained condensate (refer to Chapter 5.0 Pipe Drainage).
It is not unusual for boiler water to be entrained in the steam. There are several possible reasons for this: the steam boiler could be overloaded, the water level in the boiler could be too high or the TDS concentration of the boiler water could likewise be too high. In this case, steam formation in the boiler takes place at such a high rate that boiler water is entrained in the steam when it breaks through the water surface.
White deposits at leakage points on pipe components are an indication that the boiler water is entrained with the steam.
1.7 Temperature of a steam-air mixture
Dalton's Law predicts how a steam-air mixture behaves. Steam and air each have their own partial pressure. The sum of the partial pressures is equal to the total pressure of the mixture. Let us assume that the total pressure in a vessel is 4 bar and that the mixture consists of 75 % steam and 25 % air. In this example, the steam would have a partial pressure of 0.75 x 4 = 3 bar, while the partial pressure of the air would be 0.25 x 4 = 1 bar. The mixture temperature corresponds to the saturated steam temperature for 3 bar, specifically 133.5 ºC. Saturated steam without any air would have a temperature of 143.6 ºC at 4 bar. The table in Fig. 1-7 shows the mixture temperature for various mixing ratios and steam pressures. This information is essential for processes such as sterilisers and autoclaves.
Fig. 1-7: Influence of air in steam on the mixture temperature
1.8 Energy costs per tonne of steam
We would now like to estimate the energy costs in the following with the help of an example.
Steam boiler for 10 bar steam (hf = 2776 kJ/kg)
Efficiency of the boiler plant: 90 %
Condensate return: 100 % (T = 100 ºC, hf = 420 kJ/kg)
Calorific value of natural gas: 31.65 MJ/m
The gas consumption for one tonne of steam is therefore calculated as follows:
If only 50 % of the condensate is returned, the consumption is increased to:
The steam costs are thus 8.9 % higher than if the entire condensate is returned. The enthalpy of the cold make-up water is disregarded in this calculation for the sake of simplicity. No account is taken of any of the incidental costs for water (water costs, effluent treatment costs, depreciation, etc.). If no condensate is returned, then 97.6 m3 of natural gas is necessary to produce one tonne of 10 bar steam.
In reality, this calculation should be based on 50 % condensate return and a feedwater temperature of 10 ºC, for example.
In this case, the gas consumption would be:
Condensate
If one tonne of condensate with a temperature of 100 ºC is not returned, the energy loss is as follows, assuming a boiler efficiency of 90 %:
The energy of the condensate from 5 bar steam (152 ºC, hf = 640 kJ/kg) is equivalent to 22.5 m3 of natural gas for a boiler efficiency of 90 %.