Tech Tips: Super-Heat in a Fixed Orifice Refrigeration Circuit

Super-Heat is defined as any heat added to a substance beyond its saturation (boiling) temperature. Simple right? Actually, this is usually where people begin to get confused …. after all, doesn’t boiling indicate hot and aren’t we talking about air conditioning?

Example 1

Water can exist in one of three states: Solid (Ice), Liquid (Water), Gas (Steam). The temperature range at which water can exist in a liquid form is 32-212 degrees Fahrenheit. If we drop water below 32 degrees, then it will change physical state from a liquid into a solid as it freezes. Likewise, if we heat water to 212 degrees or above then it will change state to a gas as it boils. Now as we are talking about super-heat lets look deeper at this boiling water. At 212 degrees water may exist as either a liquid or a gas. This is the temperature at which water boils or becomes fully saturated. So if I heat boiling water and begin to create steam in excess of 212 degrees, I am creating super-heat. Steam at a temperature of 215 degrees in water with 3 degrees of super-heat.

Example 2

Refrigerant will follow this same example with one major difference: refrigerant boils off at a much lower temperature than water! Refrigerant has a very strong pressure-temperature relationship. As we increase the pressure in a system the refrigerant gets warmer and its boiling point increases. Similarly, as we decrease the pressure in a system the refrigerant loses heat and its boiling point decreases.

 For R410-A refrigerant at 120-psig the saturation point or the boiling temperature of that refrigerant is 40 degrees Fahrenheit. In an operating refrigeration circuit with a low side pressure of 120-psig and a low side line temperature of 50 degrees we have created 10 degrees of super-heat.

This is important because super-heat is a measurement of metering. Without the ability to measure the metering of refrigerant in a circuit or the amount of heat being absorbed we would have no accurate way of knowing if the refrigerant charge was correct. Speaking of a correct refrigerant charge, and to further illustrate my point: what should a correct refrigerant charge look like?

There is no correct answer to that question in terms of actual pressures because the refrigerant pressures will vary based on load conditions (indoor temperature, outdoor temperature, etc.). Measuring the super-heat is what will allow us to have a specific value by which to judge the accuracy of our refrigerant charge. Notice I used the term specific value … the number will be specific, but it will not be constant. Super-heat in a fixed orifice refrigeration circuit will always have a constant relationship to the load conditions, but as the load conditions change the specific value of super-heat your system should have will change with them.

To properly determine the super-heat in any fixed orifice refrigeration circuit you will need to know two key pieces of information. The first is the return duct wet bulb temperature. The second is the outdoor dry bulb temperature. With this information you can do a simple mathematic formula to determine your target super-heat. Let’s look at this in an example scenario:

Conditions

  • 79-degree indoor temperature
  • 71-degree indoor wet bulb
  • 93-degree outdoor temperature

Target Super-Heat

Indoor Wet Bulb multiplied by 3 | Subtract 80 | subtract outdoor temperature | divide by 2

71 x 3 = 213 | 213 – 80 = 133 | 133 – 93 = 40 | 40 / 2 = 20 | Target Super-Heat = 20 degrees

When looking to explain this formula we are looking for several key pieces of information. The first is the indoor wet bulb temperature. By looking at the wet bulb instead of just the dry bulb temperature it allows us to factor in both sensible and latent heat. This is very important because your super-heat will vary greatly based on the latent load (humidity level) inside a home. The second piece of information we are looking at is the outdoor temperature; as the outdoor temperature changes it will affect our refrigerant pressures throughout the entire refrigerant circuit. Higher ambient temperatures result in higher condensing coil pressures which in turn will affect heat transfer through the entire system as the unit must work harder to achieve heat transfer through the outdoor coil.

We begin by taking our wet bulb temperature and multiplying it by three. Next, we will subtract 80 from our multiplied wet bulb temperature; think of the number 80 as a mathematical value that is part of the formula … think of it like “PI”. Once we have subtracted 80 from our multiplied wet bulb temperature we will then subtract the outdoor dry bulb temperature. The last step is to take our final number and divide it by 2; this is your target super-heat.

By utilizing target super-heat which may be done manually with the above formula or calculated automatically by many manufacturer’s digital manifolds we are able to properly identify whether a system is charged adequately with refrigerant. This will remove a large amount of  guess work from a technician on a service call and allow for the technician to have confidence in knowing the system is properly charged.

Author: Matt Rutland

Matt is our Director of Technical Services and has been with East Coast Metal Distributors since March 2016. He is devoted husband and father to three boys in the Charlotte, NC area. He stumbled into HVAC completely by accident and is now climbing within the industry to be the best he can be.