To the uninitiated, Liquid Refrigerant and Coolant sound like two names for the same automobile fluid.
However, both these fluids serve completely different purpose in your car. Refrigerants are the primary working fluid in a refrigeration or Air conditioning system. Coolant on the other hand is a blend of water and an antifreeze.
Is liquid coolant the same as antifreeze?
Liquid coolant and antifreeze are sometimes used interchangeably.
They are not the same. Antifreeze is the chemical ingredient that lowers the freezing point and increases a water-based liquid’s boiling point. Coolant is the mixture of antifreeze agents and water which regulates the engine’s temperature.
The coolant primarily maintains the temperature of a system and prevents it from overheating. It acts as a heat transfer fluid in manufacturing applications, automobile and as a cutting fluid in metalworking, machining processes and industrial rotary machinery.
Coolant is a 50-50 split of antifreeze and water, which means antifreeze is nothing but a coolant component.
So why do we add antifreeze?
Water-cooled engines must be protected from freezing, heating, and corrosion.
However, water absorbs a larger amount of heat in comparison to most other liquids. But it freezes at a relatively high temperature, and also it is corrosive.
A mixture of antifreeze and water gives an adequate coolant solution with :
- Improved anticorrosive properties
- lower freezing point
- higher boiling point of water
Ethylene glycol is a chemical that performs very well as antifreeze. It mixes properly with water and due to having a low viscosity, allows it to circulate simply through the cooling system.
Which liquid is used as refrigerant?
For a fluid to be used as refrigerant it must have few properties that are difficult to find in a liquid at room temperature.
The only refrigerant that is found in liquid form under normal atmospheric conditions is water (R718). However, commercial use of water as a refrigerant is minimal.
In order to delve into further details we must understand…
What Refrigerants do?
Refrigerants are the primary heat transfer agents in an HVAC system.
They absorb heat during evaporation, causing the refrigeration effect at low temperature and pressure, and release heat to cooling media, which is normally water or ambient air during condensation at high temperature and pressure. A schematic diagram of a refrigeration system is shown below:
In a refrigeration system, it is desired that during the evaporation cycle (which sees the lowest pressure), the refrigeration system pressure is maintained above atmospheric so that no non-condensing gas (read air) ingresses into the system and render the system inefficient.
The evaporating pressures (40°F) and condensing pressures (100°F) of all the commonly used refrigerants are above atmospheric (Source: p410, Handbook of air conditioning and refrigeration, Auth Shan K. Wang, Mcgraw-Hill pub). It implies all the refrigerants that are usually being used in the industry are gases at normal atmospheric pressure and temperature.
Types of Refrigerants
The earliest refrigerants used were air and ammonia. Then came the CFCs (Chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) and were extensively used till the 1980s. Due to the environmental concerns of CFCs and HCFC, they are gradually phased out and replaced with new formulations, which can be classified as follows:
- Hydrofluorocarbons: HFCs are a combination of hydrogen, fluorine, and carbon atoms. Due to the absence of chlorine atoms, they are environmentally safe, and there is no chance of ozone depletion. They are chosen by the prefix HFC.
- Azeotropic: Azeotropes are mixtures or blends characterized by constant boiling points. The blends of refrigerants are called azeotropic if there is no change in composition at any point in the vapor-liquid mixture similar to that of a single component. They evaporate and condensate at a fixed temperature under constant pressure conditions.
- Near Azeotropic and Zeotropic: These blends of refrigerants behave as a single component while phase change is taking place. The phase change, however, doesn’t take place at a single temperature, and it happens over a range. This range is lower for near azeotropic mixtures and higher for Zeotropic blends.
Selection of proper refrigerant is important for efficient and safe operation of a HVAC system.
Criteria for selection of Refrigerants
A good refrigerant must fulfill specific properties to be commercially and environmentally viable and safe for use in an inhibited place. Factors that are considered for the selection of a refrigerant are:
- Safety requirements: Leakage of refrigerants may occur from pipe joints, seals, or different parts during the installation period, operations, or accident. Hence, refrigerants must be adequately safe for humans and manufacturing processes, without toxicity or flammability. Ammonia is an example of toxic refrigerant.
- Refrigeration Capacity: Refrigeration capacity is defined as the volume (measured in cfm) of refrigerant required to produce 1 ton of refrigeration. Depending upon the properties of refrigerant, such as its latent heat and its specific volume, the volume of refrigerant would be different, effecting the size of the compressor required and thus affecting both fixed as well as operating cost.
- Physical Properties: Physical properties of a refrigerant, such as its heat capacity, thermal conductivity, dielectric properties etc., also play an essential role.
Why is gas line larger in size than liquid size in AC
The design of any component can be done based on the phase of matter used in it.
Gasses occupy more volume for the same mass compared to liquid by virtue of their lower density. Liquid state needs to be pumped through a smaller pipe diameter to maintain the same velocities.
In other words, for the same mass flow rates, in order to maintain the same velocities, fluid in its liquid state needs to be circulated through an area lower than that compared to the same fluid in its vapor state.
That is exactly what is happening inside an AC or refrigeration system. Hence, to maintain system pressure drop and velocity across the refrigeration system, gas pipelines are sized larger than liquid.
How line sizing is decided?
The line sizing is decided based on pressure drop, velocity and phase changes of the refrigerants taking place.
As the fluid changes from liquid to vapor phase the velocity increases. As the velocity increases the pressure drop increases. Hence, in order to maintain pressure drop as well as velocity the line sizes are different for liquid and vapor phase.
Let us look at the refrigeration system and see how the refrigerant travels through the four sections of an Air conditioning system.
- Evaporator to Compressor: Low-pressure Saturated Vapor
- Compressor to Condenser: High-pressure Superheated Vapor
- Condenser to Expansion device: High-Pressure Sub-cooled liquid.
- Expansion valve to evaporator: a low-pressure liquid-vapor mixture
A figure of the refrigeration system is shown below:
As shown in the figure above, the refrigerant enters the evaporator from the expansion device in the form of cold, low-pressure liquid with some amount of vapor as a result of expansion cooling or the Joules-Thompson effect. Due to heat transfer from the refrigerant to the warm air outside, the refrigerant turns into a vapor by boiling.
The cold low-pressure vapor is then compressed by the compressor, increasing its temperature and pressure. This hot, high-pressure vapor condenses in the condenser.
The outlet of the condenser is sub-cooled liquid. This sub-cooled liquid refrigerant then flows from the condenser to the expansion valve and the cycle continues.
What are the Design Goals of Piping system?
The main design goals of refrigeration piping are to maximize system reliability and reduce installation costs.
To accomplish the same, the refrigerant must be transferred at proper velocity across the system to maintain the design aspects and also at minimum capital and operating cost.
The primary design goals are as follows:
- Returning of the lubricating oil to the compressor at the proper rate.„
- There is no flashing of liquid taking place before the refrigerant enters the expansion device „
- System pressure drops are within acceptable limits, and no capacity loss is taking place.
- Total refrigerant charge in the system is economical.„
Lubricating oil is required to lubricate and seal the moving parts of a compressor. Since the refrigeration process is a closed system, the oil is present along with the refrigerant and is pumped along with the refrigerant throughout the system. Thus it is important that the refrigerant, whether in liquid or vapor form, should have sufficient velocity to carry the oil along with it.
Let’s start with the Suction line or the line connecting the Evaporator to Compressor. This gas line must have sufficient velocity to carry the entrained oil droplet to the compressor.
Next is the compressor discharge line, which operates at high pressure and high temperature and delivers vapor to the condenser. Thus maintaining the mass flow rates across the system to maintain similar velocities, the discharge line operating at higher vapor densities (because of higher pressure) is comparatively smaller than the suction line.
The most critical piping in the refrigeration system is the liquid line which connects the condenser to the expansion device. Out of the three pipes, the liquid line has the most significant impact on the quantity of refrigerant required to charge the system, and hence its proper sizing becomes critical.
A Larger pipe size would call for a higher refrigerant flow requirement to fill up the pipe. On the other hand, lowing the size of the pipe would cause pressure drop issues. The pressure drop in the line must be small enough so that no vaporization occurs in the pipe before the entry of refrigerant into the expansion device.
Thus to sump-up, the gas-liquid piping in a refrigeration system is designed to minimize the pressure drop and thus reduce compression power cost. Appropriate velocities are to be maintained mainly in the gas phase to carry the entrained oil droplets required for lubrication along with the refrigerant.
Gas being lighter and having low densities need a larger pipe size than liquid for the same mass flow of refrigerant. Finally, liquid line size is minimized to reduce the refrigeration requirement. However, its size is limited by the pressure drop allowed in the pipe to prevent it from flushing before reaching the expansion device.