Dr. Albrecht Kaupp Page 1 Steam 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 properties Estimating inaccuracies in steam calculation Identifying opportunities to reduce specific energy costs for steam generation Appreciating the complexity of steam properties as applied to energy conservation 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 temperature. 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 NOTES 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 temperature 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 volume. 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 email@example.com) Three handy equations for saturated steam are Tsat 100 Psat 0.25 in oC (1) 4 T Psat = sat in bar (2) 100 1 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 hSH. Consequently the energy content of steam hg equals hg = hf + hfg + hSH in kJ/kg Moreover 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 steam”. 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 o 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 pressure. 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 possible. 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 stops. 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 boiler. 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 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 NOTES 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 benefication. 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 sense?