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Superheat and Sub-cooling - In HVAC refrigeration coils and fans are used for air conditioners and heat pumps to transfer heat in one direction or another. A good HVAC technician is well versed in refrigeration. Refrigeration is defined as moving heat from one place to another. Modern air conditioners and heat pumps use a chemical refrigerant used in vapor compression refrigeration.
This chemical refrigerant has great properties for absorbing heat. What makes these chemicals so efficient is the temperature at which they change state or change from a liquid to a vapor and condense back to a liquid again. This is key to understanding superheat and subcooling.
Superheat and Sub-Cooling
Whenever an HVAC technician needs to add refrigerant to a system or adjust the charge the tech needs to know what superheat and/or sub-cooling is to properly ensure the refrigerant charge is correct. It is also important to have a pressure-temperature or PT Chart to properly read the pressures and temperatures.
To understand properly charging an air conditioner or heat pump one must understand the Laws of Refrigeration, Thermodynamics, and heat exchange properties of various substances including air. Many manufacturers include charts with their equipment. The refrigerant charging charts are usually located inside the panel near the condensing unit. Other charts can be found in the installation instructions.
The charts include a graph. The graph includes a line that will intersect on the proper charge. This is based on outside air temperatures, indoor air wet bulb temperature, superheat, and the pressure of the suction line.
It is important to note that the proper refrigerant charge is necessary for any air conditioner or heat pump to work efficiently. With too much refrigerant the unit will use more energy to run. Additionally, it will not properly move the necessary amount of heat to be efficient. Also, an overcharged system is at risk of flooding the compressor with liquid refrigerant. Referred to as liquid slugging the compressor. An air conditioner or heat pump system with not enough refrigerant charge will freeze the evaporator coil. Additionally, the undercharged system will also not remove enough heat to be efficient.
Refrigerant Charging without Superheat or Sub-cooling | Superheat and Sub-Cooling
To make things simple some manufacturers of air conditioners and heat pumps offer a pressure-temperature chart. The chart to follow for charging, An HVAC technician uses the chart to charge a system with the proper refrigerant charge. These charts can often be found at the condensing unit and base the amount of refrigerant charge on outside ambient temperature and a few other factors.
Most condensing units come pre-charged with refrigerant and depending on the manufacturer the charge in the condenser will take into account the evaporator coil and a line-set run of 25 to 50 feet. This makes it easy for the HVAC installation technician when installing new HVAC equipment as it is easy to make the refrigerant charge adjustment after installation based on the length of the line set. Another charging method sans the superheat and sub-cooling methods is to weigh the charge is based on what the manufacturer recommends for the proper charge.
Methods and Benefits | Superheat and Sub-Cooling
This method works especially well on package units where the refrigeration loop needed a repair and the refrigerant charge was recovered. The new refrigerant charge is weighed in by the technician to the exact ounce as recommended by the manufacturer. So there are methods to properly charge an air conditioner or heat pump without using the superheat method or the sub-cooling method of refrigerant charging.
What is Superheat? | Superheat and Sub-Cooling
Superheat is the amount of heat the refrigerant holds above the temperature at which the refrigerant changed state from a liquid to vapor or gas. A good relational example would be steam. Water will change state to steam (vapor) when it is boiled at 212 degrees Fahrenheit at sea level.
It can be stated that the steam is 212 degrees Fahrenheit at the time it changes state. But what if we left the steam in a pressure vessel and didn’t let it go anywhere while continuing to add heat to it? Of course, the steam would continue to get hotter and hotter.
The pressure would also climb as the temperature climbed because we are adding more steam as more water evaporates into steam. The same is true with refrigerants. As the refrigerant enters the evaporator it begins to boil as it absorbs heat. As it boils into vapor it does not stop absorbing heat but continues the heat exchange process by absorbing more heat. The amount of heat it absorbs over the temperature at which it changed its state is called superheat.
Charging the AC or Heat Pump
When charging an air conditioner or heat pump it is important that the tech get the superheat to the proper temperature difference from where the refrigerant changed state. Too little superheat and the system could be overcharged and too much superheat and the system could be undercharged. This is called charging the system using the superheat method and the superheat method is not always used when charging an air conditioner or a heat pump. How would a technician charge an air conditioner or heat pump if the system was equipped with a TXV or thermostatic expansion valve? They would have to use one of the methods above or use the sub-cooling method of charging the system.
What is sub-cooling?
Sub-cooling is the opposite of superheat. In reference to refrigeration for air conditioners or heat pumps, sub-cooling will be the measured amount of temperature of the refrigerant after it has changed state from a vapor to a liquid in the condensing unit. For example, if the temperature of the refrigerant was 110 degrees Fahrenheit when it condensed as the refrigerant moved through the condenser more heat was removed from the liquid than the temperature at which the refrigerant was when it condensed from a vapor into a liquid. So when the refrigerant leaves the condenser if it is 95 degrees Fahrenheit it can be said that there are 15 degrees of sub-cooling.
Conclusion | Superheat and Sub-cooling
It is very important that a technician understand superheat and sub-cooling as it relates to the function of refrigeration. For the apprentice or the up-and-coming student technician, it is important to study these concepts and practice them in the real world. In the real world as it relates to air conditioners and heat pumps. Other areas of study for learning how to properly charge an air conditioner or heat pump would be Boyles Law. Latent Heat, Sensible Heat, and the Laws of Refrigeration. Grasping these concepts and applying them to how refrigeration works in HVAC will open more areas of learning and cement your knowledge in HVAC and refrigeration as applied to HVAC.
Superheat and Sub-cooling – HVAC Refrigeration
Technical Resource: Refrigeration and Air Conditioning Technology