sign in sign up
<<
Home , Superheated steam, Superheated water

r--

CHEE 221 Tutorial: Saturation and Tables Upon reaching and enthalpy of 2676 only saturated vapour will remain. Further increases in enthalpy will now cause further increases in above 100°C and the vapour What do they really mean by Saturation? is a superheated vapour or superheated steam, meaning that the vapour has been heated above its saturation temperature. 1. Saturation conditions are those under which two or more phases of a pure substance Important note: 419+2257=2676 can exist together in equilibrium. However, note that a second need not actually be Question: Why is this important? present. A phase is considered saturated so long as it is at conditions where another phase could exist in equilibrium. In the case where a single phase is found, it is in a (this is the change in enthalpy between saturated and saturated vapour at 100°C) condition where any system changes (temperature, , enthalpy) will cause some material to change phase. Question: If, at I atm and 100°C with no vapour present, 1000 KJ of heat is added to I kg of water, what percent is converted to vapour? Another way of looking at saturation conditions is that a change of phase can occur (no vapour means that the enthalpy is that of liquid water at 419 kJlkg, then we add without a change in pressure or temperature (what occurs is a change in enthalpy). Yet 1000 kJ which is entirely used to convert a quantity of liquid water into steam. another approach is to consider a saturation state to be the conditions at which a phase change begins, takes place, or ends. .. Since it takes a total of 2257 kJ to convert I kg of water, 1000 kJ will convert 1000/2257. 100% = 44.3%) Here are some examples. Question: What would you call I kg of water at I atm with the following enthalpies: 2. Liquid water (a single phase) can exist in equilibrium at various a) 4 KJ (cold liquid water) (between slightly above 0 and 100°C) while under a pressure Qf I atm. However, liquid water cannot exist at temperatures higher than 100°C while at I atm. Water vapour can b) 419.1 KJ (slightly above saturated liquid water) c) 1500 KJ (a mix ofliquid water and water vapour) exist at this pressure only at temperature of 100°C and higher. Therefore at I atm, three d) 2675 KJ (almost saturated water vapour) possible phase compositions can exist: (I) only water; (2) only water vapour; (3) water and water vapour in equilibrium. e) 2677 KJ (just above saturation, superheated water vapour) I) 3000KJ (superheated water vapour) Lets take a closer look at these situations. Consider I kg of water with an enthalpy of 3000 KJ at I atm. It is a superheated vapour. Suppose we have a system at a temperature below 100°C while at I atm, with only I kg Question: What is its temperature? (262°C, by interpolation from table B.7) ofliquid water. There is no phase (we are not considering the presence of air... yet). At this point the water is below its saturation temperature and is called a subcooled As energy is removed and the temperature decreases it will eventually reach its saturation liquid. Heat can be added and the temperature will increase. At 100°C the following point (100°C with an enthalpy of2676KJ). 11is then a saturated vapour. enthalpies are reported (relative to water at its O°C, where the relative enthalpy is taken to be 0). As more energy is removed, the temperature will remain constant and saturated vapour and saturated liquid will exist in equilibrium until the enthalpy drops to 419.1 KJ. Water 419.1kJlkg OK this is really great and I'm happy that the water went from superheated to saturated Evaporation 2257kJlkg and went through vapour to liquid phase... but what about those steam tables? Steam (Vapour) :2676kJlkg

All right, here we go. Upon arriving at 100°C with an enthalpy of 419.1 kJ,the liquid water is said to be (turn the page) saturated water (there is no water vapour at this point). Additional heat ( of ) will cause no further increase in temperature, but water vapour will begin to form (saturated vapour) and will be in equilibrium with the liquid water so long as the enthalpy is between 419.1 and 2676 kJ. Conversely you could look up the pressure on B.6. Saturated Steam tables - If the temperature is higher the steam is superheated. - If it is the same, you have saturated steam. Two types: Pressure based and Temperature based. - If it is lower somebody lied to you and you don't have steam at Table B.5: TemDerature based Saturated Steam: all but a sub cooled liquid. For the temperatures listed, we are at saturation conditions (along the Vapour-Liquid The temperature must exactly match the given pressure in order to have Equilibrium Curve). The corresponding saturation pressure is also listed here. Any saturated steam. change in T, P, or H will cause material to change phases.

Information on this table: Look at the PT phase diagram on page 327 to convince yourself that this is true. Saturation pressure at the listed temperature Specific Volume (inverse of density) You can also look on B. 7. If the T and P you are interested in, intersect in Enthalpy: Saturatedwater the boxed region, you have a liquid not a vapour and someone lied to you Evaporation/ again. If you are right on the line then it's a saturated vapour, and if you Saturated Steam are outside the box, you've got superheated steam.

Table B6: Pressure based Saturated Steam: 4. You are told that you have water at a certain temperature and no pressure is given. The only difference is that the pressure is the index by which you are looking up Enthalpy is a very weak function of pressure so values from B5 can be used. Even if the water is "subcooled" the values from B5 are close conditions. For given , you can look up saturation temperatures. (the range of enough. Table B.6 is more extensive, which can be useful depending on the information sought).

Information on the table: 4. You are told that you have a subcooled liquid. You cannot use the values from B.5 and B.6 because we are interested in Saturation temperature at the listed pressure the enthalpy difference between the subcooled and the saturation state Specifie Volume (inverse of density) (this won't happen very often in CHEE22I ). You may just be better off Internal Energy Enthalpy: Saturated water using the Cp integrals. However, you could use the Cp integrals to find the heat needed to get you to saturation conditions and then use the tables. Evaporation/condensation Saturated Steam 5. You have superheated steam and the pressure and temperature values are not in Table Uses: B.7. I. For a situation where you have saturated steam at a certain T. See the next Section on how B.5 can help in this situation. All properties can be found on B.S. Superheated Steam Tables 2. For a situation where you have saturated steam at a certain P. All properties. can be found on B.6. TableB.7 To be superheated, you are at a temperature in excess of the saturation temperature. This 3. May have steam ~t a certain temperature and pressure and need to know if it is table is a little more complicated, so I'll go into more detail on each column. saturated. Look up the temperature on B.5 and compare your pressure with the one ColumnI from the table. This is where you locate the system pressure. The temperature at which the steam will - If your pressure is lower, you have superheated steam. become saturated is listed in brackets. This temperature is also the dew point for the - If it is the same, you have saturated steam. system. - If it is higher, somebody lied and you have sub cooled liquid. Columns 2 and 3 The saturated properties are listed. This is the same information you would find on B.5 and B6.. It is here for convenience. You will frequently run into questions where superheated steam is being cooled past saturation. This information saves you having to look on other charts and table.

The rest of the table The system temperature is located across the top row. Intersect it with the system pressure and there you find the properties at the system T & P. If the T and P you are at intersect wihtin the boxed region, it's a liquid not a vapour. If you are exactly on the line then it's saturated vapour (check column 2 and 3 for the properties) and if you are outside the box, it's superheated steam! And then you can determine the degrees of superheat (which is the difference between the temperature of your vapour and its dew point... The dew point is the temperature at which this vapour becomes saturated).

Oh NO! the temperature and Pressure I want are not in Table B.7! What ever will I do!

Don't jump just yet, there is still hope. Table BS can save you. If you are at less than 10 bar or if the pressure is not given then use the saturated enthalpy values from Table B5. If the pressure is greater than 10 bar then use the formula H = U+ PV and the values from B5 to get the enthalpy.

Terms to Remember, and be able to differentiate between:

Subcooled water Saturated water Saturated stearn Superheated steam Degrees of superheat Superheated water

See definitions and further details in your textbook.