By definition, a refrigerant is a fluid utilized in a refrigeration unit to extract heat at low pressure and temperature during evaporation and reject heat at high pressure and temperature during the condensation process. The fluids ideal for refrigeration can be categorized into primary and secondary refrigerants. The former are fluids that are utilized directly as working fluids, and they enable refrigeration by undergoing a change in phases in the evaporator. The latter are fluids that are used to transport thermal energy from one place to the other. Unlike the primary liquids, they don’t undergo a phase change. The most commonly used secondary refrigerants are often the solutions of ethylene glycol and water, calcium chloride and water or propylene glycol. These fluids are generally referred to as brines.
In this article, we are going to focus on the criteria that you should use during AC refrigerant selection and the TDX 20 refrigerant for the HVAC industry.
AC Refrigerant Selection Criteria
Generally, the selection of a refrigerant for a certain use is dependent on either thermodynamic and thermophysical properties as well as environmental and safety properties.
Thermo-physical and Thermodynamic Properties
Latent Heat of Vaporization
This is the heat needed to change 1 mole of a liquid at its boiling point (BP) under the normal atmospheric pressure. The ideal refrigerant should have a high latent heat of vaporization so that the needed mass flow per unit cooling capacity will be relatively small.
The saturation pressure, at a given temperature of the evaporator, needs to be above atmospheric pressure to prevent moisture or air ingress into the AC system as well as ease leak detection. Simply put, the higher the suction pressure, the better.
This needs to be a small as possible for lower power consumption and large volumetric efficiency.
The discharge pressure at a given temperature of the condenser needs to be as small as possible so that the compressor, condenser and other units can be lightweight.
These are the main properties, but it is also advisable to consider the following:
This should be high in both liquid and vapor stages in order to allow higher heat transfer.
Liquid Specific Heat
This property needs to be small so that the subcooling degree will be high, which leads to a smaller amount of flash gas at the inlet of the evaporator.
Isentropic index of compression
This one ideally needs to be small so that the temperature rise during the compression stage will be as meager as possible.
Viscosity should also be small in the liquid and vapor phase in order to reduce the frictional pressure.
Vapor Specific Heat
Ought to be large in order to reduce the degree of superheating.
The thermodynamic features are interrelated and usually depend on molecular structure and weight, standard boiling point, as well as critical temperature.
Environmental and Safety Properties
These are equally important as thermo-physical and thermodynamic properties. As a matter of fact, the environmental friendliness of a refrigerant is a crucial factor in deciding whether it is useful in this day and age or not. Some of the important environmental & safety properties include:
Global Warming Potential
The GWP is an index utilized to compare the effect of a refrigerant on global warming with the effect of Refrigerant 11 or R-11. Coolants need to have the lowest possible global warming potential in order to reduce the impact of this devastating phenomenon.
Ozone Depletion Potential
The ODP is also an index that is used to compare the relative ozone depletion of different refrigerants. According to the Montreal Protocol, the ozone depletion potential of refrigerants being used today should be 0, meaning that they should be non-ozone depleting. Since the ODP usually depends on the presence of bromine or chlorine in molecules, substances that contain either of these elements can’t be used under the current regulations. Refrigerants such as R-11 and R-12 have a non-zero ODP and have already been phased out.
As you may have guessed, refrigerants used in an air conditioning system should also be non-toxic. However, all liquids other than air and moisture can be referred to as toxic as they cause suffocation when in large concentration. That makes the term toxicity quite relative and becomes meaningful when the concentration, as well as exposure time to produce dangerous effects, are specified. Some fluids are very toxic even in small amounts; others are mildly toxic which means they are only harmful when the concentration and exposure time is long. Others like HCFCs and CFCs are not toxic even when mixed with air in normal conditions. However, most of these are flammable when they come into contact with an open flame or any other heating element.
Generally, the degree of harmfulness depends on:
-Type of occupancy
-The amount of refrigerant substance used vs. the total space
-The odor of the substance
-The presence of heating elements and open flames
As such, the usefulness of a certain refrigerant depends on the application in regards to toxicity. A Class A refrigerant is of low toxicity, and a Class B substance is of higher toxicity.
Miscibility with Lubricating Oils
A refrigeration substance also needs to be miscible with lubricating oils because the mixture helps lubricate bearings, pistons, discharge valves and other moving parts of the compressor. Oil should ideally be from the evaporator and condenser for continued use. The substances that are entirely miscible with oils are generally easier to handle such as R-12. R-22 is partially miscible with mineral oil while R-134a is barely miscible. As such, certain precautions need to be taken when designing AC systems to ascertain that the oil will return to the compressor. An option is to add a polyol ester synthetic lubricant to the refrigerant.
The refrigeration substance should be ideally chemically stable when inside the AC system.
This is another essential feature for units that utilize hermetic compressors. For these units, The refrigerant needs to have a high dielectric strength as possible.
This is an obvious one but one worth mentioning especially when it comes to retrofitting AC systems. The refrigerant needs to be compatible with all materials of the system, both metals, and non-metals.