Reading Steam Tables for Saturated Liquid Not Vapor

  • #1

Main Question or Discussion Point

Hello, I have an assignment that centers around the Rankine cycle and requires me to find the enthalpy of water entering the boiler. My professor has told me to take the enthalpy value at boiler conditions for a saturated liquid.

I'm not really sure why I should make this assumption as I was under the impression that the water reached saturation as it heated over time in the boiler. Therefore, it would not be a saturated liquid just yet as it entered the boiler.

Anyhow, I set out to find the enthalpy of the saturated liquid water using steam tables, but all the steam tables I come across so far only give values for the saturated steam, not liquid. I'm not sure if I'm simply not reading it properly. However, I'm unsure of how to get the value for the enthalpy of a saturated liquid when only the enthalpy of saturated steams are given.

Any help would be appreciated.
 

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  • #2
SteamKing
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Hello, I have an assignment that centers around the Rankine cycle and requires me to find the enthalpy of water entering the boiler. My professor has told me to take the enthalpy value at boiler conditions for a saturated liquid.
No need to pick an argument with your professor over this point, unless you have a steam plant operator's license.

I'm not really sure why I should make this assumption as I was under the impression that the water reached saturation as it heated over time in the boiler. Therefore, it would not be a saturated liquid just yet as it entered the boiler.
Actually, when the steam condenses in the condenser, it turns into saturated liquid at that point, just at a low pressure and temperature.

https://en.wikipedia.org/wiki/Rankine_cycle

Anyhow, I set out to find the enthalpy of the saturated liquid water using steam tables, but all the steam tables I come across so far only give values for the saturated steam, not liquid. I'm not sure if I'm simply not reading it properly. However, I'm unsure of how to get the value for the enthalpy of a saturated liquid when only the enthalpy of saturated steams are given.
You're not looking in the right portion of the steam tables then. The tables you are looking for should be the "Saturated Water" or "Saturated Liquid" tables. If your tables say "Superheated Vapor", you're in the wrong location.

The saturated liquid tables can be indexed either by Tsat or Psat, and your tables may contain both. Remember, for a Psat = 1.0 atm., Tsat = 100 °C. The associated properties listed in these tables should give values for the specific volume, enthaply, entropy, etc. for fully saturated liquid and fully saturated vapor.
 
  • #3
Okay. Thank you for explaining why the water enters the boiler as a saturated liquid. It makes much more sense now.

Please help me along here...........

I found a table at this link: http://holbert.faculty.asu.edu/eee463/SteamTable.pdf.

It is called a "Saturated Water Table". The boiler conditions are 187 degrees Celsius and 112 psia. I'm not sure whether to choose the temperature or pressure as a reference point.
 
  • #4
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Okay. Thank you for explaining why the water enters the boiler as a saturated liquid. It makes much more sense now.

Please help me along here...........

I found a table at this link: http://holbert.faculty.asu.edu/eee463/SteamTable.pdf.

It is called a "Saturated Water Table". The boiler conditions are 187 degrees Celsius and 112 psia. I'm not sure whether to choose the temperature or pressure as a reference point.
First of all, I don't recommend mixing units from different systems. Using imperial units for pressure and SI for temperature is asking for trouble. You don't specify if you want the enthalpy of the water in BTU/lbm or kJ/kg. Given the conditions you have listed, it looks like you'll have to interpolate to find the enthalpy of your water.

The tables you have posted are for thermo properties in Imperial Units. You can find similar thermo tables for water in SI units.
One is attached at the bottom of this post. The properties of the saturated liquid indexed by saturation temperature start on p. 890; indexed by saturation pressure begin on p. 892.
 

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  • #5
Thanks for that handy table. I'm supposed to get the enthalpy in kJ/kg, so that table is just right.

Now here is where I am confused. Under 187 degrees Celsius and 112 psia, the water is actually a superheated vapor. Therefore, I'm guessing that I'm supposed to choose only one value for the saturation conditions and then find the corresponding temperature or pressure. Does it matter whether I choose the 187 degrees Celsius or the 112 psig, or won't I get the same answer either way?
 
  • #6
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Thanks for that handy table. I'm supposed to get the enthalpy in kJ/kg, so that table is just right.

Now here is where I am confused. Under 187 degrees Celsius and 112 psia, the water is actually a superheated vapor. Therefore, I'm guessing that I'm supposed to choose only one value for the saturation conditions and then find the corresponding temperature or pressure. Does it matter whether I choose the 187 degrees Celsius or the 112 psig, or won't I get the same answer either way?
I think you may still be having problems with your units. 112 psia is equivalent to 772.2 kPa. SI units for pressure are Pascals (Pa), not pounds per square inch (psi).
The tables I have attached are abbreviated. If your temperature and pressure combination falls too far off the saturation line, expanded tables may be required to find the enthalpy you are looking for.
 
  • #7
I was under the impression that the temperature and pressure of saturated steam are mutually dependent. Therefore, the water can not be a saturated liquid/vapor at those conditions, but it is rather a super-heated vapor.

I think what I am supposed to do is perhaps choose the 112 psia as the reference temperature because in the boiler heat addition occurs at constant pressure.
Both the 112 psia and 187 Celsius represent average pressure and temperature values in the boiler respectively. Do you think that this is a correct assumption?

I don't think that converting between units will really be a problem with all the online conversion calculators available.
 
  • #8
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I was under the impression that the temperature and pressure of saturated steam are mutually dependent. Therefore, the water can not be a saturated liquid/vapor at those conditions, but it is rather a super-heated vapor.

I think what I am supposed to do is perhaps choose the 112 psia as the reference temperature because in the boiler heat addition occurs at constant pressure.
Both the 112 psia and 187 Celsius represent average pressure and temperature values in the boiler respectively. Do you think that this is a correct assumption?

I don't think that converting between units will really be a problem with all the online conversion calculators available.
psia is not used to measure temperature.

It's not clear what you mean by "average pressure and temperature values in the boiler". Boilers are designed to produce steam at a given temperature and pressure. The feed water may be taken into the boiler at a lower temperature than the outlet conditions specify, but it must be pumped into the boiler at a greater pressure than what is present in the steam drum, otherwise water would flow out of the boiler back into the feed water system.

In order to avoid pointless back and forth here, please post the particulars of the boiler or the problem statement of the assignment you're working on.
 
  • #9
Sorry, I meant to say reference 'pressure', not temperature.

I will post the assignment PDF here as an upload.

We used something called a Rankine Cycler which has an attached laptop that records the steady state temperature and pressure of the various equipment(boiler, turbine, etc.) over very small time increments, and then imports the data into an Excel spreadsheet.

To find boiler inlet and outlet temperature/pressure, I was told to take the average of the reported values at steady state. This what I meant by average.
 

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  • #10
Here is the actual assignment spreadsheet where I am asked to perform certain calculations. It was already graded, but I'm making corrections in order to do my short report. Hopefully, this will make my questions make more sense.
 

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  • #11
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Here is the actual assignment spreadsheet where I am asked to perform certain calculations. It was already graded, but I'm making corrections in order to do my short report. Hopefully, this will make my questions make more sense.
Well, for the feed inlet conditions, you appear to need compressed liquid tables, rather than saturated liquid or steam tables.

Compressed liquid tables and superheated vapor tables from the NIST can be found here:

http://www.nist.gov/srd/upload/NISTIR5078-Tab3.pdf

The horizontal bar in these tables represents the saturation line.

The rest of these tables and notes are here:

http://www.nist.gov/srd/upload/NISTIR5078.htm

You can use Table 3 to check the enthalpy, h1, of the feed water at the inlet to the boiler. Remember to use the pressure of the feed water in MPa rather than psia for these tables. Some interpolation will be required.

As far as the rest of the data is concerned, there is quite a drop in steam pressure temperature between the outlet of the boiler and the inlet to the turbine. You want to use superheated steam in a turbine to maximize power production and minimize erosion of the turbine blades due to moisture impingement.
 

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