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What is Net Refrigeration Effect (NRE)?
Net refrigeration effect (NRE) is the quantity of heat that each pound of the refrigerant absorbs in the refrigerated space to produce useful cooling.
That’s a pretty vague definition. We know that it’s an amount of heat in processes that take place within the evaporator. Still, the phrase “useful cooling” seems rather broad. Even though it may seem a bit undefined right now, “useful cooling” is the key to understanding what NRE is and how it applies to HVAC techs in their everyday operations.
What is “useful cooling,” anyway?
Useful cooling occurs in the evaporator when the refrigerant absorbs heat from the conditioned environment, cooling the space. The total quantity of heat absorbed in the evaporator is the NRE. In essence, the NRE is all the difference between the total energy absorbed in the evaporator and the cooling that occurred anywhere outside of the evaporator (in BTUs/lb).
When a refrigerant evaporates or condenses, it undergoes a phase change. When it undergoes a phase change from liquid to vapor or vice versa, the temperature stays the same. All of the heat added or removed (in BTUs/lb) contributes to the phase change. The heat energy required to complete the phase change is called the latent heat of vaporization.
In almost all cases, the condensation temperature is higher than the evaporation temperature. That’s because the vapor’s temperature rapidly increases in the compressor before going to the condenser. The condenser rejects the heat to another medium, usually outdoors.
Some heat must be removed to reduce the refrigerant from the condensing temperature to the evaporation temperature. However, that heat removal occurs in the liquid line and the end of the condenser and does not immediately contribute to refrigeration. We must drop the liquid line temperature to achieve subcooling and limit flash gas in the evaporator inlet. However, the refrigerant doesn’t absorb heat from the conditioned environment in the liquid line, so this process is not useful cooling.
Enthalpy and NRE
Enthalpy is the total internal energy contained in the refrigerant, including sensible and latent energy.
Changes in enthalpy correspond with the phases of the refrigeration cycle. Enthalpy significantly increases in the evaporator. This occurs because the refrigerant absorbs heat in the evaporator.
On a pressure-enthalpy diagram of the ideal refrigeration cycle, you’ll notice that the bottom edge of the figure will have a horizontal line. This line represents the evaporation phase. The pressure remains low and constant while the enthalpy rises. The refrigerant’s enthalpy at the end of evaporation is significantly higher than its enthalpy when it entered the evaporator.
How do we determine NRE?
To find the NRE, you subtract the enthalpy of the liquid entering the evaporator (He) from the vapor leaving the evaporator (Hl). All units in the equation will be BTUs/lb.
NRE = Hl – He
Enthalpy heavily increases during the evaporation phase. So, you will have a positive answer. Your answer will tell you how many BTUs per pound are absorbing external heat. Thus, the NRE is a system’s ability to cool its environment by extracting heat from it.
Why should HVAC/R techs care about NRE?
A unit’s NRE is its measure of cooling performance in both the HVAC and refrigeration industries. The NRE of an A/C unit will give you an idea of its comfort cooling capabilities. The same idea applies to product cooling in the refrigeration industry.
How can we improve a cooling system’s NRE?
You can improve a system’s NRE by facilitating greater subcooling in the liquid line.
Flash gas is reduced at the evaporator inlet when the subcooled refrigerant enters the evaporator at a lower temperature. Flash gas is the percentage of refrigerant that immediately boils upon entering the evaporator. Boiling is necessary for cooling to occur, and some flashing is required to drop the temperature from the liquid line to the evaporator inlet. Too much flash gas in the liquid line hinders the cooling process in the evaporator coil, so you can improve NRE by limiting flash gas at the evaporator inlet and liquid line.
Liquid line/suction line heat exchangers and mechanical subcoolers help promote subcooling when inserted into an HVAC system. You can also reduce flash gas in the evaporator inlet by keeping the liquid line insulated and inspecting it for kinks or an inappropriate length.
When you optimize your NRE, you aim to get the most out of your evaporator. A lot of this comes down to keeping your system well-maintained and looking after the suction and liquid lines that surround the evaporator. You can also keep your evaporator outlet superheat as low as safely possible to optimize your NRE. A lower superheat indicates that the saturated refrigerant feeds more of the evaporator coil. Coils that are better-fed with saturated refrigerant are more efficient.
Superheat can also indicate problems with your metering device at the evaporator inlet or the evaporator load. When you safely limit your superheat by checking and fixing all the causes of high superheat, you can make your evaporator more efficient and increase the system’s NRE. Just don’t overdo the low superheat. You can easily lose control of the superheat and run liquid into the compressor, resulting in flooding or even slugging.