I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at
If we consider the phase diagram of a pure substance such as that of water, the Saturated Liquid water Line and Saturated Vapor Line divide the whole phase diagram into three distinct regions.
A pure substance undergoes state changes as per the variation in temperature, pressure, and volume of the substance. The solid line which separates compressed or sub-cooled liquid from the saturated mixture of vapor and liquid is known as Saturated Liquid Line.
The saturated liquid line denotes the phase change of a substance from liquid to vapor. Below the saturated liquid line, the substance is in pure liquid form, and above it is in partial vapor form till enough heat is added to convert it to 100% vapor.
For a laymen water boils at 100 °C and hence the water is considered saturated at that temperature. The above statement would hold true when water is being heated at atmospheric pressure at sea level. However, if water is heated at a higher elevation (like in a mountain top) or in an enclosed pressurized enclosure (like in a boiler), it would start boiling at different temperatures. In other words, water attains saturation state at different temperatures at different system pressures. Thus, a combination of various temperature and pressure values at which water exists in its saturated form defines the saturated liquid line for water.
Properties of Saturated Liquid Water
If water exists in its liquid state at its saturation temperature and pressure, it is known as a saturated liquid or saturated liquid water.
Water or, for that matter, any liquid tends to lose its saturation state as its temperature is increased or its pressure decreases.
Saturated liquid water is defined by a thermodynamic state specified by its temperature and pressure sets. Water exists in a saturated liquid state from temperatures ranging from 0 deg C to 374 deg C with corresponding Saturation pressures ranging from 0.006 bar to 200.9 bar. At temperatures beyond 374 deg C or pressures beyond 200.9 bar, water exits in both liquid as well as vapor form. These temperature and pressure numbers define what is called the Critical point of liquid water.
On the other hand, at a temperature below 0 deg C, water exists either as solid ice or coexists as a mixture of liquid water and solid ice.
Saturated water becomes sub-cooled as pressure is increased beyond its saturated state. The different states of water are shown by in the following graph:
If we consider compressible fluid like gases, the density increase with the decrease in temperature and increase in pressure. The liquid is termed an incompressible fluid, so the impact of pressure in densities is minimal, but the temperature does have some impact.
To keep liquid water in a saturated state as the temperature is increased, pressure must be increased. Thus, although minimal, the impact of the decrease in density due to the rise in temperature is partly countered by an increase in respective saturation pressure.
Saturated liquid water at 0.01 deg C has a density of 999.79 kg/m3. The corresponding pressure is 0.006 bar. At a temperature of 100 deg C, the density is 958 kg/m3. The corresponding pressure is 101.42 bar. This indicates as the temperature is increased, saturated liquid water densities too decrease.
The internal energy of a compound is dependent on its temperature and so its Enthalpy.
If the temperature of a substance increases, the kinetic energy of the molecules also increase resulting in an increase in its internal energy. The PV work done with increased temperature is nil. Thus,the overall enthalpy impact with an increase in temperature is the same as the impact on internal energy with increased temperature.
Saturated Liquid Water Table
The saturated liquid water table includes temperature and pressure set points at which water exists in a saturated liquid state.
By saturated liquid state, it implies that the addition of a small amount of heat to it would lead to vaporization of water. A standard saturation table for water represents the density, Enthalpy, entropy, and specific volume at the corresponding temperature and pressure values.
The saturated liquid water table starts with a temperature of 0.01 °C with corresponding saturation pressure of 611 MPa or 0.006 bar. The corresponding Enthalpy is 2500.9 Kj/kg, and entropy is 9.1555 KJ/Kg.K, density is 999.79, and specific volume of 1.00021 m3/Kg.
As the temperature increases in the saturated water table, the corresponding pressure increase to keep water in a saturated liquid state. With the rise in temperature, the density decreases and also there is an increase in its specific volume. The Enthalpy increases due to an increase in internal energy, while on the other hand, the entropy decreases.
If we consider the phase diagram of a pure substance, the Saturated Liquid Line and Saturated Vapor Line divide the whole phase diagram into three regions.
A pure substance undergoes state changes as per the variation in temperature, pressure, and volume of the substance. The solid line which separates compressed or sub-cooled liquid from the saturated mixture is known as Saturated Liquid Line.
A simple saturated liquid line for water at constant pressure is shown in the Temperature-Volume diagram as below:
The saturated liquid line denotes the phase change of a substance from liquid to vapor. Below the saturated liquid line, the substance is in pure liquid form, and above it is in partial vapor form till enough heat is added to convert it to 100% vapor.
A substance occupies a higher specific volume above its saturated liquid line due to vaporization.
What is a Saturation Line
The saturation line is a point in the Temperature Pressure equilibrium diagram beyond which a substance or component in a system is either changing from liquid to vapor or vapor to liquid.
In general parlance, the Saturation line is synonymous with the concepts of Boiling and Condensation. In fact, both boiling and condensation begin with the saturation line as the starting point.
Linguistically speaking, Saturation refers to a state beyond which a particular system cannot accept more of a component: – Like a saturated solution of sugar in water or a sponge saturated with a liquid. If we add more sugar to a saturated solution of sugar in water, no more sugar would be dissolved if we kept the temperature and pressure of water the same. Under such a state, the sugar solution is said to be saturated.
Similarly, water in atmospheric conditions would start to boil at 100 °C. If the pressure of the system is maintained constant, more water will keep boiling off if more heat is added until all water has transformed into vapor, without any change in the temperature of the system. The temperature at which this phenomenon happens is called the boiling point.
On the other hand, if heat is removed from the water-vapor mixture, keeping the pressure constant, the vapor would start condensing at 100 °C, and it is called condensation point.
A substance would exist in liquid form below its saturation line and as a mixture of vapor-liquid above it.
A saturated liquid line identifies the different temperature points in a PV diagram and different pressure points in a TV diagram of any substance beyond which it will cease to exist in pure liquid form.
At any point denoted by the saturated liquid line in the TV and or diagram above, if delta heat is added to the liquid at constant pressure, there will be no change in temperature but its gradual expansion and formation of vapor.
Saturated Liquid Line Temperature
Saturated liquid line temperature varies with system Pressure.
The saturation temperature of a substance shall increase with the increase in pressure, and hence the same is shown by a point higher up in the saturated liquid line of the PV/TV phase diagram.
Saturation liquid line temperature is the point where boiling starts when external heat is added, and the temperature remains the same until all the liquid has vaporized to vapor. This is also the temperature where condensation starts and continues till all the vapors are transformed into liquid. If heat is further removed from the system beyond the point of total vapor condensation, the liquid becomes sub-cooled. Thus the temperature of a sub-cooled liquid lies below the saturated liquid line temperature.
Frequently Asked Questions
Q. What is the definition of a Critical point?
Ans: Critical point signifies the point of a pressure-temperature curve at which liquid and its vapor can coexist.
The top limit of the liquid-vapor equilibrium curve is known as a critical point beyond this point liquid and gas are indistinguishable and form a super critical fluid. The pressure and temperature at this point are known as critical pressure and critical temperature, respectively.
Ans: A point where a small change in pressure and temperature may lead to phase change of a substance.
The point in the pressure-temperature phase diagram, where solid, liquid and vapor phases of a pure substance co exist in equilibrium is known as Triple point.
Saturated Liquid VS Subcooled Liquid: Need to know Critical Facts
Saturated liquid and subcooled liquid are the different stages during the phase change process of a pure liquid. The principal phases of a pure substance are Solid, Liquid, and Gas.
Subcooled liquid refers to that phase of a substance where it exists in liquid form at a temperature below its boiling point at the system pressure. The saturated liquid is a liquid that is about to vaporize, which means any decrease in pressure without changing its temperature causes it to boil.
Any liquid, whether in its pure form or in a mixture exerts a specific pressure over the surface of the liquid at a particular temperature, which is called the vapor pressure of the substance at that temperature. If the temperature of the liquid increases, its vapor pressure also increases. As this vapor pressure equals the pressure of the surrounding atmosphere, the liquid starts to boil. The liquid at its boiling point is called a saturated liquid.
For example, water is a saturated liquid at its boiling point of 100°C under the atmospheric condition at Sea level or 1-atmosphere pressure. As water is cooled below 100°C, it becomes sub-cooled. As the pressure is increased beyond 1 atmosphere, the boiling point of water increases, or it becomes a sub-cooled liquid at 100 °C.
Subcooled liquid example
Any liquid below its boiling temperature at a given pressure can be considered as a Subcooled liquid.
Water boils at 373 °K(100° C). Now water at room temperature293°K(25°C) andat normal atmospheric pressure is an example of a Subcooled liquid.
Certain conditions that cause a liquid to be Subcooled are:
At a given system pressure when the liquid attains a temperature lower than its saturation temperature. When the liquid is at a pressure higher than its saturation pressure at the given temperature. Thermodynamically the liquid has lower enthalpy and Specific volume than that of a saturated liquid.
Liquid ammonia at a temperature below -33.3°C and pressure 1 bar or higher.
Liquid ammonia at a temperature below -50°C and pressure 0.41 bar or higher.
Ethylene glycol at a temperature below 197 C and pressure 1 bar or higher
Ethyl alcohol at a temperature below 77.8 C and pressure 1 bar or higher.
As can be seen above, in the example of liquid ammonia, it can exit in sub-cooled conditions at different conditions of pressure and temperature. -33.3°C is the saturation temperature of ammonia at 1 bar pressure. Similarly, for a temperature of -50°C, the corresponding saturation pressure for ammonia is 0.41 bar (approx).
Subcooled liquid pressure
A liquid with a pressure higher than its saturation pressure at the given temperature is said to be a Sub-cooled or Compressed liquid.
Sub-cooled liquid means that the temperature of the liquid is lower than the saturation temperature for that particular pressure and a compressed liquid means that the pressure of the liquid is superior than the saturation pressure for the given temperature. Both the terms can be used alternatively.
Since liquids are incompressible in nature, their properties are relatively independent of pressure. Subcooled liquids are defined by:
Higher pressure than a saturated liquid(P>Psatat a given T)
Lower temperature than a saturated liquid(T<Tsat at a given P)
Lower enthalpy than a saturated liquid(h<hfat a given T or P)
Lower internal energy than a saturated liquid(u<ufat a given T or P)
Lower specific volume than a saturated liquid (v<vf at a given T or P)
Enthalpy is mostly affected by pressure, a more accurate relationship for h
Q. What is the difference between Saturated and Subcooled liquid?
Ans:Thoughboth Saturated liquid and Subcooled liquid are the two phases of the same liquid, they are quite different from each other.
Subcooled is the condition where the liquid is colder than the minimum temperature (saturation temperature), which is required to keep it away fromboiling. On the contrary saturated liquid is the condition when the liquid is almost at its boiling point.
In the case of saturated liquid, if the pressure is further lowered andkeeping the temperature constant, the saturatedliquid will start to boil. On the other hand, the liquid will besub-cooled if pressure is increased beyond its saturated pressure at its boiling point.
Q. Why is sub-cooling desirable in a refrigeration system?
Ans:A sub-cooled liquid increases the energy efficiency of a refrigeration system as it has lower specific volume
The refrigerant is sub cooled in the condenser to avoid the early vaporization of the refrigerant before heading towards the expansion device.
If proper condenser surface area is not provided to ensure proper subcooling, the refrigerant may partially evaporate in the piping as it moves to the expansion device. If the refrigerant partially evaporates before entering the expansion device, it will require higher refrigerant volume to be pumped to achieve the same cooling. This increases the pumping cost and hence the energy consumption.
We all know that the heart pumps blood throughout the human body; in the same way, a Compressorenables the flow of the refrigerant throughout the refrigeration cycle.
In general, low pressure and low-temperature gaseous refrigerant from the evaporator enters the Compressor and gets compressed inside it to high pressure and temperature gas.
A piston moves inside a cylinder to help in and out of gas in the Compressor. The high-pressure hot refrigerant gas from the Compressor is then pushed into the condenser. The refrigerant comes out from the condenser as a high-pressure liquid that enters the evaporator through the expansion valve.
What happens if liquid refrigerant enters the Compressor
Compressors are meant to compress gasses as gas is a compressible fluid. On the contrary, liquids are incompressible. If an incompressible fluid enters the Compressor, it can potentially damage the Compressor internals.
When in an HVAC system, the refrigerant is not entirely vaporized inside the evaporator, then liquid refrigerant directly touches the crankcase of the Compressor. This situation mainly occurs in an operating condition frequently faced by the servicing technicians, popularly known as Flooding.
Liquid refrigerant in Compressor dilutes the lube oil resulting in wear and tear of different parts
Entering the cylinder liquid refrigerant may damage the reed valve, piston connecting rods, crankshaft, etc. It even leads to total Compressor failure.
Can liquid refrigerant in Compressor damage it
Analysis reveals that entry of liquid refrigerant in Compressor during the running cycle is one of the primary reasons for Compressor failure.
The variety and expanse of damage depend on the amount of liquid refrigerant that enters into the Compressor.
Major damages caused by refrigerant flood back or Flooding due to entry of liquid are:
Poor lubrication of Compressor parts
The lower efficiency of the system
Oil Foaming etc.
Since the Compressor motor draws more current, it may lead to compressor burnout.
State of refrigerant entering the Compressor
For the smooth running of an HVAC system, the entry state of the refrigerant must be gaseous.
In normal conditions, the refrigerant enters into the Compressor in a gaseous state from the evaporator. But due to certain factors, the liquid refrigerant returns to the Compressor in large volume through the suction pipe.
The evaporator is responsible for the refrigeration effect in the HVAC system. The compressed hot refrigerant liquid is cooled in the condenser and sent through the expansion valve into the evaporator. At the inlet of the evaporator, the refrigerant is a mixture of gas and liquid at low pressure. The refrigerant takes heat from ambient air (causing the cooling effect) in the evaporator and transforms it to vapor form. It enters into the compressor suction in the vapor form.
The Compressor is the most critical component in the refrigeration system, and its failure becomes the most expensive problem. Compressor Flood back or Flooding is one of the significant reasons for compressor failure.
The continuous flow of liquid refrigerant into the Compressor in oil droplets instead of superheated vapor is known as Compressor Flood back.
Liquid refrigerant in Compressor mixes with the lube oil present in the crankcase of the Compressor and reduces its viscosity. Inefficient lubrication leads to wear and tear of Compressor parts and overheating. In a compressor, Flooding can be detected through the crankcase sight glass, where the oil appears to be foaming during operating conditions.
Main Causes of Compressor Flood back
Technicians should properly aware of the causes which may lead to Compressor Flood back.
To find the root of the problem is essential to prevent Flood back of Compressor from happening again and again. Proper knowledge of the causes and symptoms also help in identifying between Slugging and Flooding.
Main causes of Compressor Flood back are listed below:
Problem with an expansion device. The expansion valve bulb strap is not insulated correctly, or the bulb is in the wrong position on the suction pipe.
Improper adjustment of the expansion valve. The proper adjustment of the expansion valve is necessary to regulate the appropriate quantity of refrigerant to keep the refrigerant in vapor form while entering the Compressor.
Incorrect sized capillary tubes send more refrigerant to the evaporator, a large amount of refrigerant couldn’t reach the boiling point resulting in Compressor Flooding.
Low load situation.
What is liquid Slugging in a compressor?
Liquid Slugging is the term associated with failure of a reciprocating compressor due to carryover of liquid in its suction.
A compressor is designed to pump refrigerant in its vapor form, but if liquid refrigerant returns to the Compressor and passes through the suction valve, it may bend or break the suction valve. A loud knocking sound arises due to Slugging.
As it enters the cylinder, liquid refrigerant dilutes the lubricant oil present in the crankcase, creating an oil and foamy liquid mixture. This slug of liquid (oil droplets+ refrigerant) gets up and reaches the top of the piston. Since the piston fails to compress the slug, high pressure is created inside the cylinder and destroys the piston crown. Significant damages due to Slugging are:
Ans: Oil foaming occurs due to the mixing of lubricating oil with liquid refrigerant, which can be detected through the compressor crankcase sight glass.
If the Compressor is restarted with liquid refrigerant in the crankcase along with lubricating oil, the liquid refrigerant-oil mixture begins to vaporize rapidly due to a rapid decrease in pressure and increase in temperature. This phenomenon causes foaming.
Oil foaming results in carryover of oil with the refrigerant. The carried-over mixture of oil and liquid refrigerant doesn’t have lubricating properties and can cause severe damage to the Compressor.
Q.Does Compressor Flood back effect its efficiency?
Ans: When the liquid refrigerant enters the Compressor, it readily mixes with the lube oil and dilutes it, causing inadequate lubrication. Certain features may overheat and fail.
Since liquid refrigerant is non-compressible, high hydraulic pressure is required inside the cylinder, resulting in excessive stress generation. More than average crankcase pressure is required to pump the refrigerant through the cylinder, resulting in lower system efficiency.
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 :
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:
Refrigeration System with liquid refrigerant coolant Credit tranebelgium
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 theDesign 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.
