# chee 221 tutorial: saturation and steam tables what … · r--chee 221 tutorial: saturation and...

Post on 09-Sep-2018

266 views

Embed Size (px)

TRANSCRIPT

r--

CHEE 221 Tutorial: Saturation and Steam Tables

What do they really mean by Saturation?

Upon reaching and enthalpy of 2676 only saturated vapour will remain. Further increases

in enthalpy will now cause further increases in temperature above 100C and the vapour

is a superheated vapour or superheated steam, meaning that the vapour has been heatedabove its saturation temperature.

1. Saturation conditions are those under which two or more phases of a pure substancecan exist together in equilibrium. However, note that a second phase need not actually bepresent. A phase is considered saturated so long as it is at conditions where anotherphase could exist in equilibrium. In the case where a single phase is found, it is in acondition where any system changes (temperature, pressure, enthalpy) will cause somematerial to change phase.

Important note: 419+2257=2676

Question: Why is this important?

(this is the change in enthalpy between saturated liquid waterand saturated vapour at 100C)

Here are some examples.

Question: If, at I atm and 100C with no vapour present, 1000 KJ of heat is added to I

kg of water, what percent is converted to vapour?

(no vapour means that the enthalpy is that of liquid water at 419 kJlkg, then we add

1000 kJ which is entirely used to convert a quantity of liquid water into steam.Since it takes a total of 2257 kJ to convert I kg of water,1000 kJ will convert 1000/2257. 100% = 44.3%)

Question: What would you call I kg of water at I atm with the following enthalpies:

Another way of looking at saturation conditions is that a change of phase can occurwithout a change in pressure or temperature (what occurs is a change in enthalpy). Yetanother approach is to consider a saturation state to be the conditions at which a phasechange begins, takes place, or ends. ..

2. Liquid water (a single phase) can exist in equilibrium at various temperatures(between slightly above 0 and 100C) while under a pressure Qf I atm. However, liquidwater cannot exist at temperatures higher than 100C while at I atm. Water vapour canexist at this pressure only at temperature of 100C and higher. Therefore at I atm, threepossible phase compositions can exist: (I) only water; (2) only water vapour; (3) waterand water vapour in equilibrium.

Lets take a closer look at these situations.

a) 4 KJ (cold liquid water)b) 419.1 KJ (slightly above saturated liquid water)c) 1500 KJ (a mix ofliquid water and water vapour)d) 2675 KJ (almost saturated water vapour)e) 2677 KJ (just above saturation, superheated water vapour)I) 3000KJ (superheated water vapour)

Consider I kg of water with an enthalpy of 3000 KJ at I atm. It is a superheated vapour.Question: What is its temperature? (262C, by interpolation from table B.7)Suppose we have a system at a temperature below 100C while at I atm, with only I kg

ofliquid water. There is no gas phase (we are not considering the presence of air... yet).At this point the water is below its saturation temperature and is called a subcooledliquid. Heat can be added and the temperature will increase. At 100C the followingenthalpies are reported (relative to water at its triple point OC, where the relativeenthalpy is taken to be 0).

As energy is removed and the temperature decreases it will eventually reach its saturationpoint (100C with an enthalpy of2676KJ). 11is then a saturated vapour.

WaterEvaporation

Steam (Vapour)

419.1kJlkg2257kJlkg:2676kJlkg

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.

OK this is really great and I'm happy that the water went from superheated to saturatedand went through vapour to liquid phase... but what about those steam tables?

Upon arriving at 100C with an enthalpy of 419.1 kJ,the liquid water is said to besaturated water (there is no water vapour at this point). Additional heat (latent heat ofvaporization) will cause no further increase in temperature, but water vapour will begin toform (saturated vapour) and will be in equilibrium with the liquid water so long as theenthalpy is between 419.1 and 2676 kJ.

All right, here we go.(turn the page)

Saturated Steam tablesTwo types: Pressure based and Temperature based.

Conversely you could look up the pressure on B.6.

- If the temperature is higher the steam is superheated.- If it is the same, you have saturated steam.

- If it is lower somebody lied to you and you don't have steam atall but a sub cooled liquid.Table B.5: TemDerature based Saturated Steam:

For the temperatures listed, we are at saturation conditions (along the Vapour-Liquid

Equilibrium Curve). The corresponding saturation pressure is also listed here. Anychange in T, P, or H will cause material to change phases.

The temperature must exactly match the given pressure in order to havesaturated steam.

Information on this table:

Saturation pressure at the listed temperature

Specific Volume (inverse of density)

Internal Energy

Enthalpy: SaturatedwaterEvaporation/condensationSaturated Steam

Look at the PT phase diagram on page 327 to convince yourself that this istrue.

You can also look on B. 7. If the T and P you are interested in, intersect in

the boxed region, you have a liquid not a vapour and someone lied to youagain. If you are right on the line then it's a saturated vapour, and if youare outside the box, you've got superheated steam.

Table B6: Pressure based Saturated Steam:

The only difference is that the pressure is the index by which you are looking upconditions. For given pressures, you can look up saturation temperatures. (the range ofTable B.6 is more extensive, which can be useful depending on the information sought).

4. You are told that you have water at a certain temperature and no pressure is given.Enthalpy is a very weak function of pressure so values from B5 can beused. Even if the water is "subcooled" the values from B5 are closeenough.

Information on the table:

Saturation temperature at the listed pressureSpecifie Volume (inverse of density)Internal EnergyEnthalpy: Saturated water

Evaporation/condensationSaturated Steam

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

the enthalpy difference between the subcooled and the saturation state

(this won't happen very often in CHEE22I ). You may just be better off

using the Cp integrals. However, you could use the Cp integrals to findthe heat needed to get you to saturation conditions and then use the tables.

Uses:I. For a situation where you have saturated steam at a certain T.

All properties can be found on B.S.

5. You have superheated steam and the pressure and temperature values are not in TableB.7.

See the next Section on how B.5 can help in this situation.

Superheated Steam Tables2. For a situation where you have saturated steam at a certain P.

All properties. can be found on B.6.

3. May have steam ~t a certain temperature and pressure and need to know if it issaturated.

Look up the temperature on B.5 and compare your pressure with the onefrom the table.

- If your pressure is lower, you have superheated steam.

- If it is the same, you have saturated steam.

- If it is higher, somebody lied and you have sub cooled liquid.

TableB.7To be superheated, you are at a temperature in excess of the saturation temperature. Thistable is a little more complicated, so I'll go into more detail on each column.

ColumnIThis is where you locate the system pressure. The temperature at which the steam willbecome saturated is listed in brackets. This temperature is also the dew point for thesystem.

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 tolook 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 areoutside 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 dewpoint... 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 10bar 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 fromB5 to get the enthalpy.

Terms to Remember, and be able to differentiate between:

Subcooled water

Saturated waterSaturated stearn

Superheated steamDegrees of superheat

Superheated water

See definitions and further details in your textbook.