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Having both the pressure and temperature for saturated-liquid in a particular state of a cycle, I was interested in its enthalpy. When looking at steam tables I am not sure if I should use the pressure table or the temperature table to retrieve the value as both tables have enthalpy for water as a fluid at either T or P. But the value is not quite the same.

Thank you guys, kindly.

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You are mis-reading the tables or trying to get a value you cannot get via temperature and pressure. Your question didn't make sense to me, so I consulted my dad's Thermodynamics text book just top make sure (Emswiler & Schwartz, McGraw-Hill, 1943, $3.50 USD, used by him for his BSME degree, class of 45). This book is fine for our use because nothing has changed about how we look at steam since it was written.

Let's start with basics.

"Saturated water" cannot give you an enthalpy value based on temperature and pressure alone. Saturated means basically "at the boiling point temperature for its pressure." But water can be saturated liquid or saturated steam (gas) or a combination of the two. The act of going from liquid to steam is the process of boiling, and to do it we must put in enough heat to change the phase.

Saturated liquid (100% liquid, no change to steam at all) has an enthalpy defined solely by temperature and pressure. For any given temperature , there is a single pressure that will give you saturated liquid (100C at 1 atmosphere for example). For any given pressure, there is a single temperature. As such, there is not a "pressure table" nor a "temperature table." Both are defined by each other for saturated water. You might have a table where pressure is given in round numbers and the temp in decimals or the other way around, but the two are mated to each other for saturated steam.

For any given temperature/pressure combination of saturated water (typically called the saturated steam table) we have an enthalpy of saturated liquid, and enthalpy of saturated steam, and usually the enthalpy change required to get from one to the other. US engineers who deal with steam usually remember that the enthalpy of a pound of saturated steam is about 1200 btu/lb at normal boiler pressures and the enthalpy of saturated liquid goes up by about one btu per degree F. Sorry, I did learn SI in college, but the Navy & industry in the US aren't interested in megapascals, etc.

Now, if you are given saturated water, a temperature and a pressure, you can determine if it is saturated, but not how much steam or liquid it has. You have to know the quality of the steam, which is the percent team it is. You can measure this in a steam pipe, but not from only temperature & pressure.

So, if you want to know the enthalpy of saturated liquid, it's an easy read from the steam table, and it matters not whether you look up temp or pressure, the answer will be the same. If it's anything other than pure liquid, you need more information.

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  • $\begingroup$ I was wondering because, depending on the problem i have seen the following: Sat-liquid at 6kPa so h = hf at 6kPa. But in other times, I have seen that we have sat-water, at 12MPa, and T = 210 ºC, thus h = hf at 210ºC, I know it is not superheated as T is below the sat-temperature for superheated vapor, but I am not sure how is it that they know you can take h = hf in this case and why do they choose to use the temperature table over the pressure one. $\endgroup$
    – RSM
    Aug 10 at 14:25

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