Understanding the critical point and its implications in refrigeration design.

Last month, we celebrated the genius contribution of Richard Mollier to refrigeration design with his formulation of the pressure-enthalpy chart. This is a good introduction to a few key concepts related to the chart that have become topical recently.

Everything to the left of the saturated liquid line is liquid cooled to a temperature below its boiling point. This is “subcooled liquid,” which simply means it has to be warmed up before it will start boiling. Conversely, everything to the right of the saturated vapor line is gas heated above its condensing temperature. This is “superheated gas,” which means it needs to be cooled a bit before it starts turning to liquid. In general, if a mixture of liquid and gas is well mixed, the liquid can’t be subcooled because the gas warms it up, and the gas can’t be superheated because the liquid cools it down. The two states of the fluid converge on the saturation temperature at the prevailing pressure and everything is stable.

However, in some cases, particularly if the gas and liquid phases are not well mixed, a temperature difference between gas and liquid can persist. This might arise when there is stratified flow in a tube, or in a receiver vessel with cold liquid at the bottom. This can be very confusing. Another example, which can also cause equipment damage, is when small droplets of liquid are carried in streamline flow in the suction line of a compressor. The droplets are so small that the surface tension of the liquid creates a pressurizing effect, which produces an apparent subcooling within the droplet, preventing it from evaporating and enabling it to persist over long distances from the evaporator to the compressor.

These droplets can cause mechanical damage by washing oil from the bearings, or erosion on valves, or can agglomerate and cause excessive pressure when compressed. The only way to deal with the small droplets is to cause them to group together into larger drops that can then be separated from the gas flow.

The place on the chart where the two saturation lines meet is called the “critical point.” This used to be a “don’t even go there” zone on the Mollier chart. For low-pressure refrigerants such as R-11 and R-123, the critical point was usually off the top of the chart. Recent interest in CO2 refrigeration systems that pressurize the gas to a level well above the critical pressure has created new interest in what happens up there.

To really appreciate this, it is necessary to understand what defines the critical point. In general, at a given pressure, liquid is more dense than gas. As saturated gas is pressurized, its density increases; however, if liquid is pressurized and kept at its saturation point, its density reduces because heat has to be added to it. The critical point is simply the point at which the two densities become the same. It is then not possible to differentiate between gas and liquid. The fluid above the critical point is known as “supercritical fluid”—this is neither liquid nor gas. The more heat it contains the more gas-like it becomes, and at lower heat content it becomes almost incompressible, more like liquid.

Many other weird things happen at the critical point on the Mollier chart. As well as the latent heat diminishing to zero, the line of constant temperature (isotherm) that touches the top of the dome is, at that instant, horizontal; its gradient is zero. The specific heat capacity of the gas, which is proportional to the reciprocal of the gradient of the isotherm, therefore becomes infinite and the speed of sound in the fluid drops to zero. All this weirdness suggests that a process that passes through the critical point would be problematic, whether an expansion from high to low pressure or the rejection of heat from high to low enthalpy. In fact, no drama occurs and since the process is just transiting through the critical point from one condition to another, life goes on as normal.