Tip:
Highlight text to annotate it
X
In this screencast we are going to use the property of a steam table to solve an example
problem. So lets say we have 10 kg/min and we have saturated water at 10 bar, and it
is fed to a heat exchanger where it is heated isobarically. In other words at the same pressure
to 300 degree C, and we want to know. First of all what the final state of the water is.
Another worlds at 10 bar and 300 degree C. We want the change in enthalpy in kW, and
finally we want to compare the volumetric flow in to the volumetric flow out. Another
words is it greater then, equal to, or less then the volumetric flow out. Lets draw a
picture here. Here is our little heat exchanger, What we have coming in is saturated water.
It is at 10 kg/min and it is at 10 bar. Coming out we have H2O, at 300 degree C, and 10 bar.
So first lets determine what properties we are going to need. We are certainly going
to need our enthalpy coming in as well as our enthalpy coming out. In addition we are
going to need the specific volumes in order to determine the volumetric flows. So since
we are given the pressure of the saturated water entering the system. We will use the
saturated pressure table. So again we are looking at saturated liquid or saturated water
at 10 bar. So lets see where that is on the table. So we note that our specific enthalpy
of our saturated liquid or saturated water is equal to 762.52 kJ/kg and this is our specific
enthalpy in, and our specific volume, right here. Is equal to 0.0011272 meters cubed per
kg. Now we have to look at our steam or water coming out, and this is at 10 bar and 300
degree C. If you notice however, at 10 bar the saturation temperature is 179.88 and the
temperature we have at 300 degree C is higher then that. So that means we have to use a
superheated steam table. So the values we are going to need are right here. So our specific
enthalpy Hout is going to equal 3051.6 kJ/kg, and our specific volume out is going to equal
0.25799 meters cubed per kg. So lets get back to our questions, and the first question was
what is the final state of the water, and as you can see, because the temperature was
higher then the saturated temperature at that pressure 10 bar. The final state is superheated
steam. Now we need to find the change in enthalpy. So we are looking for delta H, but we are
not looking for specific enthalpy. We are looking for the overall enthalpy change in
the kW. So we figure this out by taking the mass flow rate, 10 kg/min and multiplying
it by the change in specific enthalpy to find our change in enthalpy as 22891 kJ/min. How
ever we were asked for it in kW. Which is kJ per second. So what we have to do is convert
this from minutes to seconds, and what we are left with then is 381.5 kW. Which is the
change in enthalpy of the water. Finally we are asked to compare the entering and exiting
volumetric flow rates. To find our volumetric flow rate we take out mass flow rate, and
that is equal to density times volume, and density is just the inverse of the specific
volume. So if we want to find, and since these are rates we will put the dots on them. The
volumertic flow rate we are going to have the mass flow rate times the specific volume.
So we will state with the volumetric flow rate coming in. Our mass is 10 kg per min.
Our entering specific volume is equal to 0.0011272 meters cubed per kg. So our volumetric flow
rate coming in is 0.011272 meters cubed per min. Now lets look at our volumetric flow
rate coming out. Again that is our mass flow rate multiplied by the specific volume coming
out, and that is 0.25799 meters cubed per kg, which is equal to 2.5799 meters cubed
per minute. Sizable increase. So steam tables can be used to find all sorts of different
quantities and can be used in conjunctions with kinetic and potential energy or work,
or heat to solve different kinds of energy balances.