# Specific volume and superheated gas

• I
I was assigned an example problem today and i can do the math, look at my tables and get the right answer no problem. What i want to understand, is why when specific volume increases for r134a, that it means its supper heated?

My given value for sv was .1384m3/kg which is above the .09 in the table. I get that if its above, then it means superheated. Looking at the saturation dome, i see why, but scientifically, i dont. I asked a couple of professors and i didnt understand their answer. They were trying to inform me about how the gas takes up more space... i get that, but how does taking up more space mean superheated?

Thanks

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Chestermiller
Mentor
Are you saying that the saturation specific volume is 0.09, but at the same pressure and a higher temperature, the specific volume is 0.1384?

russ_watters
Here is the problem statment:
R-134a at 400psia has a specific volume of
0.1384ft3/lbm. Determine the temp of the refridgerant based on the gerealized compresabilty chart, ideal gas law, and the steam tables.

The professor was doing this problem step by step:

Ok, so i answered all 3 parts correctly, but on part (c) using the tables, i go to table A-12E, find 400psi and it says that at pressure, the satliq is 0.017 and satvap is 0.096. The value given to me is 0.1384 which is above the 0.09. My professor said since its above that satvap number, it means its a superheated gas. So use those tables to find temp, which yields 240F or 700R

What im confused about is how does a larger specific volume, such as the 0.01384, mean that its a superheated gas?

Thanks

Chestermiller
Mentor
Here is the problem statment:
R-134a at 400psia has a specific volume of
0.1384ft3/lbm. Determine the temp of the refridgerant based on the gerealized compresabilty chart, ideal gas law, and the steam tables.

The professor was doing this problem step by step:

Ok, so i answered all 3 parts correctly, but on part (c) using the tables, i go to table A-12E, find 400psi and it says that at pressure, the satliq is 0.017 and satvap is 0.096. The value given to me is 0.1384 which is above the 0.09. My professor said since its above that satvap number, it means its a superheated gas. So use those tables to find temp, which yields 240F or 700R

What im confused about is how does a larger specific volume, such as the 0.01384, mean that its a superheated gas?

Thanks
At the same pressure, it has a higher specific volume than the saturated vapor. From the ideal gas law, the specific volume is given by $$v=\frac{RT}{PM}$$where M is the molecular weight. So, according to this, at a given pressure, the specific volume is proportional to the absolute temperature. That means that, in your case, the temperature must be above the saturation temperature. This is the definition of superheated.

russ_watters and LT72884
Hmm, ill have to do some more thinking to understand it better haha. Its all new to me

Thanks much:)