The prominent computer room air conditioning manufacturers are shipping units that use R407c, a blended refrigerant, so, the subject of how to determine superheat and sub-cooling using a chart that refers to “bubble and dew” is coming up again and again. So, let’s break it down…
Looking at a pressure/temperature chart for a blended refrigerant, you are presented with 2 columns, one labeled “Bubble”, and one “Dew”. One might deduce that the refrigerant boils at one temperature and condenses at another. Then we take a look at the range of pressures listed on the chart and it would appear that the labels are swapped; it can be very confusing. Okay, I confess, it happened to me. What I had forgotten was, the component refrigerants that make up the blend, boil and condense at different pressure/temperature points as they pass together through the evaporator, or condenser.
When we first learn about refrigerants and how they change state, a comparison is made to water, because we are familiar with water changing state from ice, to water, to steam. We learn that if water is 31 degrees, it is all ice. If water is 33 degrees it is all water. This same concept of knowing for sure what the state of a substance is, carries over to single component refrigerants, like R22, and we use this knowledge as the basis for determining the state of a refrigerant, in a given part of a system, without being able to see it, inside a pipe.
As a blended refrigerant passes through an evaporator, picking up heat, one of the component refrigerants that make up the blend begins to boil at the “Bubble” pressure/temperature point. As this takes place, the ratio of the component refrigerants that make up the blend in the liquid, changes, because some of one of the components has become a gas. This “new blend” has a different boiling point because it’s composition has changed. This process continues until the liquid reaches the “Dew” pressure/temperature point, where the last of the liquid is changed into a gas. Any temperature rise above this point is superheat.
This change in the saturation temperature, as the blend changes state is called glide. The opposite process occurs in the condenser. As heat is removed from the gas the temperature falls to the “Dew” point, where one component begins to change to a liquid. The ratio of component refrigerants in the remaining gas changes as the saturation temperature falls through the glide range, until it reaches the “Bubble” point. Any temperature drop beyond that is sub-cooling.
With this in mind, the labels on the refrigerant chart make more sense. The “Bubble” point is when the first bubble would appear in a liquid. The “Dew” point is where the first drop of liquid would fall out of the surrounding gas.