Question

In thermodynamics for steam table questions, how do you identify what equation to use to get property value ?

(I know how to use steam tables just don’t know the correct equation to get missing property value)

Answer 1

Dear student, there is no direct equation to get desired property from steam table. Depending upon question, desired value of properties are found from steam table. I shall provide you with some common conditions encountered in thermodynamics problem. Before that basic equations associated with processes are attached below:

1. PROCESS IS ISOCHORIC: In such problems, it is specified that vessel is rigid. From rigid vessel we can conclude that volume of vessel is constant. For such questions, from initial conditions find specific volume of stem-water mixture. Now final specific volume shall also be same. Generally you will be provided with either pressure or temperature at final conditions. At final condition provided (temperature or pressure), check specific volume of saturated vapour and liquid. If specific volume falls in between this value, it means final state of water steam exists. Find the value of quality for this mixture. Now since we have gotten saturated mixture, final pressure or temperature is also known. Now all missing property values at these final condition can be obtained.

In second case if specific volume is greater than final saturated vapour, then state is superheated. Find corresponding temperature or pressure from steam table where desired specific volume is achieved. Thus we have now gotten final temperature and pressure & can easily find out rest of properties.

2. PROCESS IS ISENTROPIC : This is the most commonly encountered process in thermodynamics problem involving calculation of efficiency of various power cycles (like Rankine, Carnot etc.). For isentropic process, entropy of inlet and outlet stream is same. In any given problem, you will normally be provded inlet steam condition to turbine & outlet steam pressure from turbine. First, find entropy of steam at inlet conditions. Now entropy ofoutlet stream shall also be same. At outlet pressure, check specific entropy of saturated steam. If this is greater than outlet entropy (same as explained above for isochoric process where entropy is replaced by volume), outlet state is saturated liquid vapour mixture. Find value of quality of outlet steam as per equation 1 attached on first sheet of this problem. Now since quality isknown, all other desired property values can be find out easily.

On second case, if outlet entropy is greater than saturated steam entropy at given outlet pressure, it means that outlet steam is suoerheated. Find temperature (at given pressure) where valure of specific entropy is same as outlet specific entropy. This way we find out outlet temperature. Now pressure & temperature of outlet steam is known hence all missing property va;ues can be find out.

3. PROCESS IS ISENTHALPIC: This process is encountered in Vapour Refrigeration Cycle. In isenthalpic process, inlet and outlet enthalpy is same. All steps same as isentropic process is repeated with only difference being entropy to be replaced with enthalpy

Additionally, if isentropic efficiency is mentioned for turbine then find power generated by turbine for isentropic process (=m*(h1-h2) where h2 & h1 is outlet enthalpy for isentrpic process & h1 is inlet enthalpy) & multiply it by isentropic efficiency to get actual power generated by turbine.

Similarly, if isentropic efficiency is mentioned for pump then find power required by pump for isentropic process (=m*(h1-h2) where h2 & h1 is outlet enthalpy for isentrpic process & h1 is inlet enthalpy) & divide it by isentropic efficiency to get actual power generated by turbine.

NOTE: DEAR STUDENT, I HOPE I MAY HAVE CLARIFIED YOUR DOUBTS. HOWEVER, IF YOU REQUIRE FURTHER ASSISTANCE PLEASE ASK ME. SINCE THIS IS A PROBLEM RELATED TO USAGE OF STEAM TABLE, I HAVE TRIED TO PROVIDE SOLUTION TO BEST OF MY ABILITY. HOWEVER, IF YOU FEEL SOME MORE EXPLANATION IS NEEDED KINDLY ASK ME. THANK YOU.