Pressure, Temperature and Fluid phase change

In summary, when temperature increases, isn't that the fluid will evaporate? This has been bugging for awhile. Please help!Perhaps you should stipulate, or re-examine the conditions under which the fluid is condensing as it is being compressed.
  • #1
Nicky_Boy02
2
0
Some please clear my confusion here, when you compress a fluid the temperature increases. Since the temperature increases (fluid gets warmer), why is the fluid condensing from gas to fluid?

when temperature increases, isn't that the fluid will evaporate? This has been bugging for awhile. Please help!
 
Engineering news on Phys.org
  • #2
Perhaps you should stipulate, or re-examine the conditions under which the fluid is condensing as it is being compressed.
For example, I have never had had the chance to observe air turning into a liquid as it is being compressed to pump up a car tire.

In other words, where on a phase diagram are you referring.

An example PT phase diagram for water:
http://upload.wikimedia.org/wikipedia/commons/3/34/Phase-diag2.svg
http://upload.wikimedia.org/wikipedia/commons/3/34/Phase-diag2.svg
If we start with the gaseous phase of water somewhere in between temperatures Ttp and Tcr, any compression of suitable magnitude will eventually cause a phase change from gaseous to liquid. Below Ttp, a compression will turn the gas into a solid ( and then a liquid with more compression ). Above Tcr, any amount of compression does not cause a phase change, but the fluid changes into what is called a supercritical fluid above a pressure of Pcr.

Of course, I am using a constant temperature for the fluid throughout, assuming that the heat of compression is not being retained by the fluid to cause a temperature increase.

If the fluid is insulated so no heat can flow out, the temperature will, as you say, increase during compression. It may be that the fluid may cross a phase change boundary during the process, or it may not. A calculation would be in order to determine the state of the fluid.
 
  • #3
Maybe I should put it in this way. When you are compressing gas into liquid in a processing equipment (eg. compressor), the pressure and temperature increases due to PV/T.

My confusion is, if temperature increases, why is the compressed gas turning into liquid (condensing) and not evaporating/vaporizing?
 
  • #4
You are thinking about a phase in the traditional sense. A liquid is defined by density, so is a gas. This definition comes from statistical mechanics and is one reason why we do not have formulas for liquids that work as nicely for gasses (PV = rho RT is not applicable, for example, for liquids). Temperature is due to multiple modes of energy in an ensemble of molecules.

You have translational (mostly dealt
with in kinetic theory), vibrational, electrical, rotational, and sometimes magnetic modes of energy that comprise temperature we perceive.

A cool example is ice (nice pun??). Ice has 17 different forms according to Wikipedia. These forms are defined by their microscopic crystal structure and are affect by temperature, pressure and a few other variables.
 
  • #5
If the vapor is being compressed adiabatically, you need to consider the slope of the adiabatic temperature-pressure relationship compared to the slope of the saturation temperature vs saturation pressure relationship. If the slopes are such that the adiabatic temperature-pressure graph intersects the saturation temperature vs saturation pressure relationship as the pressure on the vapor is increased, then even though the gas temperature is increasing along the adiabat, condensation will occur.

chet
 

Related to Pressure, Temperature and Fluid phase change

1. What is pressure and how does it affect fluid phase change?

Pressure is defined as the amount of force exerted on a given area. In the context of fluid phase change, pressure plays a crucial role in determining whether a substance will exist as a solid, liquid, or gas. Generally, an increase in pressure will result in a higher likelihood of a substance remaining in its liquid or solid state, while a decrease in pressure will encourage the substance to change into a gas.

2. How does temperature impact fluid phase change?

Temperature is a measure of the average kinetic energy of the particles in a substance. When temperature increases, the particles gain more energy and move more rapidly, which can lead to a change in phase. For example, when water is heated, the added energy causes the bonds between molecules to break, resulting in a phase change from a liquid to a gas (water vapor).

3. What is the difference between boiling and evaporation?

Boiling and evaporation are both processes by which a substance changes from a liquid to a gas. However, boiling occurs when the temperature of a liquid reaches its boiling point, which is a specific temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure. On the other hand, evaporation can occur at any temperature and involves the gradual escape of molecules from the surface of a liquid.

4. Can pressure and temperature both affect fluid phase change simultaneously?

Yes, pressure and temperature can both impact fluid phase change at the same time. In fact, the phase diagram of a substance shows the relationship between pressure and temperature and how they affect the state of a substance. For example, at a certain combination of pressure and temperature, a substance may exist in a liquid state, but at a higher pressure or lower temperature, it may exist in a solid state.

5. How do we determine the critical point of a substance?

The critical point of a substance is the highest temperature and pressure at which the substance can exist as a liquid. This point is determined by observing the phase changes of the substance at varying pressures and temperatures and identifying the point at which the liquid and gas phases become indistinguishable. The critical point is important in understanding the behavior of substances under extreme conditions, such as in outer space or inside a high-pressure vessel.

Similar threads

Low pressure, high velocity flow vs static pressure
    • Mechanical Engineering
Replies
5
Views
1K
Could somebody please explain the vapor compression cycle?
    • Mechanical Engineering
Replies
3
Views
2K
Why is air in atmospheric pressure and room temperature a gas?
    • Mechanical Engineering
Replies
3
Views
2K
What does the T-S diagram look like for an ammonia heat pump?
    • Mechanical Engineering
Replies
1
Views
2K
Bernoulli's Principle and Fans [split]
    • Mechanical Engineering
Replies
12
Views
4K
Thermodynamic Challenge Problem
    • Mechanical Engineering
Replies
5
Views
1K
Discharge of a Pressure Vessel Containing Liquid Nitrous Oxide
    • Mechanical Engineering
Replies
14
Views
2K
Tubing Pressure drops with Compressible Fluids
    • Mechanical Engineering
Replies
3
Views
858
A Why are the Navier-Stokes equations inconsistent in this case?
    • Classical Physics
Replies
18
Views
1K
Expansion in non-adiabatic Otto / Diesel cycles
    • Mechanical Engineering
Replies
13
Views
1K

Hot Threads

Recent Insights

Back
Top