Basics of Refrigerants in HVAC
A simple cooling system
A refrigerant is a fluid that is used to transfer heat from one point to another in a refrigeration cycle. It works on the principle of latent heat. Your skin feels cool after applying after shave lotion because it evaporates absorbing heat from your body. The refrigerant is the backbone of a refrigeration system. This fact becomes obvious when you picture the simple cooling system. A simple cooling system does not require electricity to run; it just needs a refrigerant with a boiling point less than regular room temperature. A cylinder of pressurized ammonia can be connected to a cooling coil with a valve to act as a simple refrigeration system. Once the valve is opened, ammonia flows through the coil and subsequently outdoors. Indoor air flowing across the coil will give up heat to allow ammonia to vaporize. As long as there’s NH3 in the cylinder, air flowing across the coil will get cooled. Of course, this model is not viable in the real world as it isn’t financially feasible to keep replacing cylinders of ammonia. Hence, we add a compressor to recompress the refrigerant and keep it in a closed cycle. Here’s an overview of refrigerants in HVAC.
Working principle
Refrigeration as a science is based on the fact that a liquid can be vapourised at any temperature by altering its pressure. Large quantities of heat are absorbed when liquids change phase. Refrigerants typically vaporize in the evaporator to absorb heat from the surrounding medium. The air flowing across the evaporator loses energy and hence gets cooled.
Characteristic features of a refrigerant
A good refrigerant in HVAC must
· Not be toxic
· Not be flammable
· Not be corrosive
· Have a low boiling point
· Have high critical temperature
· Have high latent heat
· Low specific heat
A multitude of refrigerants has been used in the HVAC-R industry. Ammonia, carbon dioxide, air, sulphur dioxide, propane, HFCs, and HCFCs have all been used in refrigeration systems the world over. Refrigerants passing through evaporators in conditioned spaces must never be toxic or flammable. There is always a chance of leaks arising in the evaporator and this may lead to refrigerant leaking into conditioned spaces which can prove lethal. Modern refrigerants are based on fluorinated hydrocarbons. The chemistry of these fluids can be altered to achieve desired characteristics such as optimal boiling temperatures, pressures, etc.
Nomenclature- How refrigerants are named
ASHRAE Standard 34- provides a naming system for common refrigerants and assigns safety classifications to them:
Refrigerants-Broadly classified
· 000 - Methane Based
· 100 - Ethane Based
· 200 - Propane Based
· 300 - Cyclic Organic Compounds
· 400 - Zeotropes
· 500 - Azeotropes
· 600 - Organic Compounds
· 700 - Inorganic Compounds
· 1000 - Unsaturated Organic Compounds
How refrigerants are named-Refrigerant nomenclature
Consider a refrigerant compound
1. Denote its chemical formula in terms of carbon, hydrogen, fluorine, and chlorine:
Ca Hb Fc Cl
2. Calculate the values of a, b, and c.
3. The above compound is designated as:
R(a-1)(b+1)(c)
Click here for a detailed list of refrigerant names and their designations
GWP
With all industries set to decarbonize, the pressure is on to reduce the global warming potential of refrigerants. Refrigerants are highly polluting substances. Common refrigerants can be up to 12000 times worse than CO2 for climate change. Each refrigerant is assigned a GWP (Global Warming Potential) and ODP (Ozone Depletion Potential) to denote its polluting capabilities. The higher the value, the worse it is for the environment. CO2 is used as a reference gas by the IPCC and has a GWP of 1. The Montreal Protocol was signed in 1987 to phase out the use of harmful refrigerants after it was found that CFCs in the atmosphere had caused a depletion of the Ozone layer. Refrigerants with high ODPs were phased out and replaced by 0 ODP refrigerants such as R134A. Recent developments show that the treaty has been successful in its endeavor as the ozone layer is showing signs of improving. Several countries are now taking steps to phase out refrigerants with high GWP.
Some common refrigerants and their polluting potentials
|
Refrigerant |
ODP |
GWP |
|
HFC -Hydrofluorocarbons |
||
|
R134a |
0 |
1430 |
|
R404a |
0 |
3922 |
|
R407C |
0 |
1774 |
|
R410a- |
0 |
2088 |
|
HCFC -Hydrochlorofluorocarbons |
||
|
R22 |
0.05 |
1780 |
|
R123 |
0.02 |
79 |
|
CFC -Chlorofluorocarbons |
||
|
R11 |
1 |
4000 |
|
R12 |
1 |
10910 |
|
R113 |
0.8 |
4800 |
|
R114 |
1 |
3.9 |
|
Natural refrigerants |
||
|
R717-Ammonia (NH3) |
0 |
0 |
|
R290-Propane |
0 |
5 |
|
R718-Water (H2O) |
0 |
- |
|
R744-Carbon dioxide (CO2) |
0 |
1 |
Click here for a detailed list of refrigerant names and their designations
Effects of constituent combinations in Refrigerants
HFOs, HFCs, HCFCs, and CFCs have limited combinations. Their properties can be altered by varying the formulation as shown below:
1. Adding bromine or chlorine leads to an increase in ODP.
2. Adding hydrogen leads to increased inflammability.
3. Adding hydrogen leads to a decrease in atmospheric lifetime.
4. Adding fluorine leads to an increase in GWP.
Future of refrigerants
If the world must reach net-zero by 2050, adoption of low-GWP refrigerants isn’t an option but a necessity. Adopting new refrigerants points to inevitable policy changes and more investments. As in any other industry, we need technological innovation along with governmental support to produce a new class of safe, cheap, and environment-friendly refrigerants. In October 2016, measures to phase out HFCs were added to the Montreal protocol-what is now referred to as the Kigali amendment. The EU has already begun phasing out HFCs and the rest of the world will soon follow suit. Countries such as Spain, Denmark, Norway, and Sweden are also experimenting with the taxation of HFCs.
In the long term, sustainability will be the key parameter deciding which refrigerants come out on top. Technologies that can incorporate natural refrigerants such as ammonia, carbon dioxide, or water as refrigerants are garnering more attention now due to increased focus on fighting climate change. The real challenge is to select a suitable refrigerant for each application; one that is sustainable, cost-effective, safe, and energy-efficient.
Click here for a detailed list of refrigerant names and their designations