Refrigeration Principles: Understanding Vapor Compression System

4 Basic Parts of Vapor Compression Refrigeration System

One of the most important aspects that must be understood in the study of refrigeration and air conditioning is the principles of vapor compression system.  This is the foundation for the creation of mechanical refrigeration that we have today.  According to the history, the first refrigerating machine using vapor compression system that was commercially available was made in 1834 by Jacob Perkins. This was the first refrigeration unit that uses compressor.  The purpose of compressing highly volatile fluid is to release the heat that it contained. The release of heat makes the vapor liquefy and ready to make another cycle of heat absorption when subjected to low pressure.

Vapor compression is just one task done by the mechanical parts to complete the heat extraction process.  Heat absorption has nothing to do with it, but the exact opposite. To successfully absorb and remove heat, there are 4 major processes that should occur in the system.  This could be expressed in terms of what happen on the refrigerant, and what mechanical processes that causes it.  To better understand this concept, let us study the processes that occur in the system as the refrigerant circulates.

Compression or Decreasing the Volume of Vapor

When the heat absorbing agent called refrigerant is injected into the system, the first process that it undergoes is compression.   This happens inside the compressor when refrigerant that must be in vapor form is being induced by the suction pressure of the compressor. As vapor refrigerant enters into the compression chamber, it is about to be subjected to sudden decrease in volume (compression action). This occurs when the piston moves up and compresses the vapor within the cylinder.

Compression is basically the act of decreasing the volume of a vapor in an abrupt manner to increase its pressure.  The quick increase in pressure of a vapor refrigerant as a result of compression also increases its temperature.   To understand the effect of vapor compression, it is required to understand the 2 Gas Laws that  serve as a foundation of vapor compression system.

Boyle's Law

This law introduces the relationship of pressure to the volume of fluid. It states that the pressure of vapor changes if the volume also changes at constant temperature. This is the perfect description of the condition of vapor refrigerant inside the compression chamber before piston moves upward to accomplish compression stroke (in the case of reciprocating compressors).   According to this law, change in pressure is inversely proportional to the change in volume  or the space of a  container to which the gas is being held. When the volume   increases, the pressure decreases. So, to increase the pressure of that vapor refrigerant, volume must be reduced. Boyle's law is expressed in the following formula.

 

Gay Lussacs's Law

This  gas law  tells us the relationship between  pressure  and temperature if the volume of  gas is held constant. It says that, temperature and pressure  of vapor or any fluid are directly proportional to each other at constant volume.  This is based from  the idea that when we increase the amount of heat on a fluid, its molecular activity increases. The increase in molecular movement can be expressed as a pressure of fluid. Gay Lussac's law can be expressed in the following formula.

Vapor compression is  based on the idea that has been brought by these two gas laws.  The ultimate purpose is to release the heat that vapor holds so that vapor will liquefy or condense and can be used again. So, to increase the temperature of fluid without  burning anything or without producing a flame, it can be done by increasing the pressure or reducing the cylinder of fluid.

The increase in pressure is required in order to release the heat into the outside air.  From the second principles of thermodynamics that states “heat always travels from warmer body to colder body.” In order for heat to travel into the air (colder body) from the vapor refrigerant inside the system ( warmer body) the temperature of vapor refrigerant must be higher than the temperature of the outside air.

Condensation or Heat Extraction

Condensation is just an after effect of compression. In short, if compression is not properly done due to mechanical failure, condensation will never take place. When temperature of compressed vapor is not high enough against the ambient temperature, heat will not move effectively from the compressed vapor. Condensation is a result of   increasing the temperature of vapor refrigerant to create temperature difference from the air surrounding the condenser coil.

As heat transfer occurs and leaves the vapor refrigerant within the condenser tube, vapor turns into liquid at high temperature.  With this idea, it is important to note the fact that, when refrigerant is in liquid state, it means it does not hold heat in that body.  This is the reason why heat can be felt when touching condenser coil of any refrigerating equipment.  The heat that we can sense at the condenser is the proof that heat is being rejected outside in that specific point of the system. From there, liquid refrigerant continues to move to the next stage.

Expansion or Increasing the Volume of Liquid

Heat absorption that creates cooling effect is due to the expansion of liquid refrigerant.   If compression is a process if decreasing the volume of vapor to increase its pressure and temperature, to create a temperature different between the vapor refrigerant and the surrounding air, expansion is the perfect reverse of the process.

Expansion is a method of increasing the volume of metered liquid refrigerant, to decrease its pressure and temperature. This is to create temperature difference between the surrounding air (outside the evaporator tube) against the partially vapor refrigerant inside. Again, from the second principle of thermodynamics mentioned above, heat always travels from warmer body to colder body. In this particular spot in the system, the colder body is the refrigerant that has been subjected to low pressure. 

As being said, pressure and temperature are directly proportional to each other, and in inverse proportion to the change in volume. When increasing the volume (expansion), pressure of the liquid refrigerant decreases as well as the temperature.  This makes the heat transfers from outside surface of the tube into the refrigerant, which is partially in vapor state. When heat leaves a certain space, it produces cooling effect. 

This makes the refrigerator cabinet becomes cooled. The point is, the heat that must be removed must be properly isolated and no external heat leakage should occur.

Evaporation or Absorption and Handling of Heat

If condensation of vapor refrigerant is an after effect of compression, similarly, evaporation of liquid is an after effect of expansion. By subjecting liquid refrigerant at high temperature into sudden increase in volume, its pressure and temperature decreases.  As the temperature of refrigerant decreases lower than the ambient temperature, heat moves from warmer body (air outside) to the colder body (refrigerant inside the tube).  

As heat penetrates into the inner part of the evaporator to find that colder body inside, partially liquid refrigerant at low pressure and temperature completely evaporates. With this in mind, it is important to note that, when refrigerant is in vapor state, it means it holds a certain amount of heat. Just like what happen when water evaporates into the atmosphere when heated up to 100 degrees Celsius.

One thing that must be understood here is that, heat moves into the refrigerant body because refrigerant itself is a substance that is highly reactive to heat.  It evaporates at negative temperatures lower than freezing point of water.  This makes the liquid refrigerant evaporates or boil even at lower temperatures or even when cold is already produced sensed by our bare hands. This characteristic of refrigerant leaves no heat behind from its surrounding area.

When liquid refrigerant completely turns into vapor, it then moves into the suction line of the compressor at low pressure and temperature. Another factor that makes the vapor refrigerant stays in low pressure aside from the expansion of liquid is the suction pressure by the compressor.  At this stage, heat that has been absorbed is being properly handled by the refrigerant. It then transported into the compressor to be extracted outside when it reaches condenser, where heat can  do no harm. After the condensation process the liquid refrigerant is now again ready for another cycle.

To know the specific spots of the refrigeration system where this processes took place, it is necessary to mention again the 4 parts of mechanical refrigeration system.

Compressor – Where compression or sudden decrease in volume of   vapor occurs to increase its pressure and temperature.

Condenser – Where heat leaves the vapor body of refrigerant and move into the outside air due to mandatory heat transfer when there is a temperature difference between 2 bodies. Heat transfer is also aided by used of forced air or induced air by the used of fans and blowers. On larger units heat transfer is sometimes aided with water.

Expansion Valve –Where expansion or sudden increase in volume of metered liquid refrigerant occurs.

Evaporator – Where heat absorption occurs and where cooling effect is produced. In actual unit, since refrigeration system is nothing but just a loop of tubes separated by the compressor and the expansion valve, expansion process occurs at the inlet of evaporator. It is the point in the system where refrigeration cycle starts, where heat absorption started to transpire. 

Refrigeration Principles: 2 Kinds of Heat

In the study of refrigeration and air conditioning, it is required to understand the two kinds of heat that affects the behavior of a liquid substance when heat is added. This is to understand how the refrigerant works during the heat absorption and extraction process in all refrigeration and air conditioning units.

Before discussing the 2 kinds of heat, it is necessary to recall the effect of heat on water.  As presented on page I labeled "principles" in this website, water evaporates when heat is added and when the temperature reaches  to 100⁰C  (boiling point). When heat is removed, it solidifies into ice at temperature of 0⁰C.

Before evaporation occurs when water is heated, the only thing that would change is the water temperature.  It increases from its initial state, which must be above 0⁰C until it reaches its boiling point.  In the same way, when water is put in a freezer, its temperature decreases until it reaches to its freezing point. Beyond this level, water must undergo change in state.  The 2 kinds of heat are responsible when changes in temperature and state occurs. These are the latent heat and sensible heat.

 

Latent Heat

Latent heat causes the water to change its phase from liquid to vapor when it is added. Similarly, it causes the water vapor to change its phase into liquid when it is removed without having the temperature to change. The word "latent" means concealed or hidden, or something that our physical senses could not detect.

In the context of refrigeration and air conditioning, air temperature is always measured to identify the efficiency of the equipment with the use of thermometer. But when talking about latent heat, its presence can never be measured by any temperature measuring device, only by its effect.

Sensible Heat

Another kind of heat is called sensible heat.  This is what we always experience in our daily lives when we feel being uncomfortable in the presence of heat. When we touched something hot, we know that  it is hot because we sensed it. Sensible heat causes the change in temperature of any given substance when added or removed, without changing the state.This is the heat that we can measure using thermometer.

Sensible heat is limited with respect to the kind of substance being subjected to it. In the case of water, sensible heat is only limited to 99 ⁰C.  Beyond this point, latent heat  comes into play to break the chemical composition of water and turn into vapor. On lower temperatures, sensible heat for water is only limited to temperature above 0 ⁰C. Beyond this point, latent heat must be taken away from water to turn into ice. In some other liquid substance like oil, this might not be the case.

The amount of sensible heat that a substance can handle identifies its freezing and boiling point. For refrigerants, sensible heat is limited to temperature below the boiling point of water. This makes refrigerants highly reactive to even slightest amount of heat at negative temperatures.

For example, R-12 evaporates at temperature of -29 degrees Celsius while water evaporates at 100 degrees Celsius, which is quite hot for all living organisms.  Boiling point is the maximum temperature of sensible heat to be held upon in that particular liquid substance. This property of the substance called refrigerant is the reason why it is the best medium for heat absorption used in refrigeration and air conditioning.

As being mentioned earlier, rain occurs as a result of condensation of water vapor in the atmosphere. This is due to the effect when latent heat leaves the water vapor collected in the air. When sea water evaporates, latent heat has been absorbed by the body of water.

Sensible heat and latent heat goes together when heat is added or removed in a substance. When sensible heat reaches its equilibrium point, latent heat will become noticeable as it causes chemical change to occur.

Refrigeration Principles: The Second Thermodynamic Principle

In refrigeration, the process of heat absorption and extraction was made possible by the application of the second thermodynamic principle.  The word "Thermodynamics" comes from the Greek word “thermē” which means heat, plus the word “dynamic” which means in motion. Combined together, it forms the word “thermodynamic” which refers to movement of heat.

Scientifically, thermodynamic is a study of heat or energy that affects living organism and all matters on earth. When learning about heat movement and its underlying fact, the best source of knowledge are those that were presented in this field of study.

Before getting into the discussion about the second law of thermodynamics, which directly relates to what happen inside the refrigeration system, it is required to study the characteristics of heat to understand how the process in this principle is taken into action. The following are the characteristics of heat or thermal energy.

1. Heat is Everywhere

Heat is present in all places and in all substances; this makes the earth go round. It gives life to all living organisms in our planet. With the presence of heat, natural processes occur in our environment. Plant grows; animals become alive and multiply including human beings.

For non-living things, heat affects the binding and breaking of its molecules and atomic structures to create chemical change, which may form other substances for some other purposes. For example, in welding or joining of metals, heat must be applied to melt that metal surface to alter its structure and connect other piece of metals.

In quantum theory, all matters are just form of energy that takes its physical form according to the level of vibration of its subatomic particles. This means that, whatever substance it might be, it could be a gas, solid or liquid, regardless of its chemical composition and properties, the one that make it exist is the thermal energy that it carries within.

This thermal energy or presence of heat creates the vibration of its minute particles to attract other particles and form molecules or the chemical composition of that particular matter. So, if heat is completely taken away, all natural processes may also stops including the existence of life.

2. Heat can Never be Destroyed

The theory that heat is present in all places is due to the fact that it can never be destroyed.  It naturally exists in all bodies of matter at varying degree or level. When fire occurs and fire fighters stop the fire, it does not mean that heat has been eliminated or destroyed. It is just being dispersed and transferred into other bodies of matter where its presence creates no harm or any untoward effect.

When putting off a fire using water, the heat that burns the flammable material, has just been absorbed by the water and goes together with hydrogen and oxygen in the air. There is no way to destroy heat it can only be transferred from one place to another.

3. Heat can Never be Created

If heat can never be destroyed by any means, in the same way it can never be created. The only source of heat anywhere in this planet is the sun in our solar system. But by means of some mechanical or chemical processes, heat as a form of energy can be generated here on earth for some specific purposes.

Generating heat does not mean of producing it out of nowhere. It is simply allowing heat to travel into a certain medium or material to collect and increase its amount up to the level of intensifying it, where it can be used or converted into other forms of energy.

When creating a fire, heat is just being collected by using a flammable substance where it requires extreme heat in order to create chemical change on that material. The flame that is visible is just the physical appearance of heat collected in an extreme level.

4. Heat always Travels from Warmer Body to Colder Body

As an infinite form of energy that could be anywhere, heat always tries to seek its own way to equalize in all bodies of matter as much as possible.  Like water or any liquid substance, it seeks its way to equalize in uneven surfaces. If we pour water into the ground, it flows first to the lowest possible level before occupying the other surfaces.

In the same way, heat moves in a bit similar manner. Heat also finds its way to equalize in all bodies of matter. Whenever there are two bodies with different temperatures, heat always travel from warmer body to colder body to make its presence equal in both bodies. This makes heat keeps on moving from one body to another every now and then.

The above figure shows the direction of heat movement when two bodies of matter at different temperatures are put together in a place with even more lower temperature. As you can see, heat flows from warmer body to colder body to equalize its presence.

Whenever it completely equalizes in all matters, that is the only time heat stops moving. But it never happens because as heat moves from one place to another, there are also factors that affect its presence to make a change on its level on every spot. This makes the heat constantly in motion as it seeks its way to find its equilibrium point.

The second law of thermodynamics was just mentioned in the above paragraph. It is one of the fundamental principles in the study of refrigeration and air conditioning. It tells that “heat always travel from warmer body to colder body” regardless of the phases of matter and this principle can never be altered or reversed by any means. To understand how heat moves from different bodies of matter, we need to study and understand the three methods of heat transfer.

3 Methods of Heat Transfer

 

Radiation - Heat travels by radiation when it moves from warmer body to colder body without any direct contact between two bodies. In this method, distance between two bodies affects the rate of heat transfer, the closer the bodies are, the faster the rate of transfer.

One of the most evident examples of heat transfer via radiation is how the sun illuminates the earth. Heat travels in a form of light from the solar system through the atmosphere and reaches the surface of the earth it just a matter of seconds. This could result to increase in temperature for all objects here on earth, which are directly exposed to the heat of the sun.

Another example of heat transfer via radiation is when you put your hands near the flame when you cook something. As you do that, heat travels via radiation from the body of flame into the surface of your skin.

Convection - Heat travels by convection when it moves from warmer body to colder body by means of any medium. Any substance gas or liquid could act as a medium for heat to transfer into other bodies.  In our bathroom for example, we need to use a heater during winter when we take bath to neutralize the cold water in the morning that may come out in our faucet.

As we turn on the heater and open the faucet, hot water will start to flow. As it flows, the faucet also will become hot. In this scenario, the heat from the heater coil transfers to the faucet by means of convection. The flowing water which also becomes hot acts as a medium for heat to transfer into the faucet, which is made of metal.

Another example of convection is when you stand outside the building near the operating condensing unit of split ac system.  That unit gives off heat via induced or forced air. When the air hits your body, you can feel that it’s hot.  If you are wearing a gold necklace or a metallic watch, you can feel that it is also heating up.

In that case, the heat given off by the condenser transferred to your necklace or watch via convection. The air that conveys the heat to your body serves as a medium for heat to transfer.

Conduction - Conduction is the most common method of heat transfer. It is where heat moves from warmer body to colder body via direct contact or via movement of atoms in the same body.

When a piece of metal is fixed on a vise or anything that can hold it firmly, then heating up one end, heat will travel into the other end and to the vise.  It means heat moves from point A to point B in the same body and by direct contact between two bodies.

In refrigeration system, these 3 methods of heat transfer occur simultaneously within the system during the heat absorption and extraction process. The following figure shows the three methods of heat transfer occurs simultaneously whenever there is a presence of heat.

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