Pressure

Why would a website about cryogenic engineering have an article on ‘Pressure’?

The answer is that to the cryogenic process engineer, fluid pressure is like gold.
It is the basic form of energy that is used judiciously to make a low temperature process work. Refrigeration processes almost all use compression to ultimately deliver cooling to either a process fluid or to the food in the refrigerator or freezer. So, for the benefit of the non-specialist here is a brief refresher on pressure.

So what is pressure?

In physics and engineering it’s an important characteristic or ‘property’ of a fluid in the gas state.

Like in so many areas in language we unconsciously adopt analogies from some unrelated field to help us describe something  else.  This makes language very rich (and potentially incomprehensible.)

But, I don’t want to put you (or me) under pressure or cause you any ‘stress’ (stress = a force per unit area on a solid material object causing its deformation), so here’s a simple explanation of pressure.

Compression - Thermodynamicists (scientists who deal with energy) will have to forgive the following.

Compression is increasing pressure - caused by force exerted by the gas molecules bombarding the walls of a container.  It’s actually the total force divided by the inside wall area of the container.  If you consider the cylinder of a bike pump as the container, when the piston is pushed in (when you start to pump up your tyre), the surface area is reduced.  If the pump outlet nozzle is closed (with your finger maybe), and molecules each have the same (bombarding) energy as before, the pressure will be higher because the area within the cylinder will be less.

In practice, forcing the molecules to be closer together, without taking heat away at the same time, raises their temperature as well as the pressure.   In fact, small simple molecules consisting of a single atom like helium or argon have less degrees of freedom to wriggle around – spin and vibrate than do bigger molecules and their temperature rises more when they are compressed.

It’s as if the molecules are getting more energetic to resist being pushed closer together.  A crowded commuter train during rush-hour needs air conditioning to stop commuters getting hot and irritable. (We are pushing the limit of this analogy.)

The minimum energy for increasing the pressure of a gas is achieved by removing heat as it is compressed (this is called iso-thermal = same temperature compression).  In this case the work or energy needed to compress the gas is exactly balanced by the heat removed to keep the temperature constant.  This ideal is difficult to achieve in practice, but can be approached by compressing in several stages to reach the final desired pressure and re-cooling (inter-cooling) between stages.

Compressors come in various forms. Two of the most common are:

• a reciprocating compressor which is a piston moving in a cylinder like the bike pump, A compressor may have several cylinders in series to achieve the desired end pressure. The gas is alternately drawn into the cylinder via inlet valves and then compressed as the piston moves towards the end and leaves via discharge valves.

• a centrifugal compressor has curved blades that rotate at very high speed inside a cylindrical casing. These cause the gas fed to the centre to accelerate towards the perimeter. From here the high velocity of the gas molecules leave the casing in a ‘diffuser’ where their kinetic energy is converted to pressure energy as they slow down.

Reciprocating machines are typically used for lower gas capacities and high pressures and centrigugal machines are mostly used for large capacities and moderate pressures. There is however a considerable overlap.

Other common types include Screw compressors and vacuum pumps.

Final thoughts.

The Exergy of a process stream is strongly dependent upon its Pressure, and specifically is proportional to the natural logrithm of the (Stream pressure / Atmospheric pressure).

E = R.To.ln (P / Po), where R = universal gas constant; To - enviroinment temperature in K.
See article on Exergy.

Future articles are planned that will illustrate how the pressures of the feed and products of a cryogenic process can determine its efficiency and even whether or not it will work.