Refrigeration Cycle

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For HVAC systems to operate, the refrigeration cycle plays the pivotal role of heat transfer. The refrigeration cycle follows the principles of thermodynamics. Prior to plunging into the world of HVAC and how the refrigeration cycle works, understanding some important principles of refrigeration and pivotal concepts in refrigeration.

Principles of Refrigeration

The following key principles are important in the refrigeration cycle:

• When liquids are changing from liquid to gas, they absorb heat.
• When gases change from gas to liquid, they irradiate heat.

Refrigeration Cycle in the HVAC Context

In any system, sustainability is critical and this is the reason air conditioning systems are designed with economics in mind. The refrigerant must be reused, with the HVAC system having compression, condensation, expansion, and evaporation in the same cycle in a closed loop. The refrigerant in the heat transfer process.

Refrigeration Cycle Steps

• The refrigerant is introduced to the compressor as a low-pressure gas, compressed, and then released as a high-pressure gas.
• After that, the gas is directed to the condenser. The gas condenses into a liquid and releases its heat into the surrounding air.
• The liquid then flows under high pressure to the expansion valve. When the fluid exits the expansion valve, this valve limits the flow and lowers the pressure.
• The low-pressure liquid then travels to the evaporator, where it absorbs heat from the inside air and turns into a gas.
• The refrigerant flows to the compressor as a hot low-pressure gas, where the cycle is repeated.

The four-part cycle is divided into a high side and a low side at the center. This relates to the refrigerant pressures on either side of the hvac system.

Important Refrigeration Cycle Concepts

Four main concepts play a crucial role in the refrigeration cycle. They include the enthalpy of vaporization, pressure changes and how they relate to boiling point of liquids, the fact that energy is neither created nor destroyed, and the removal of heat from spaces.

The overall sum of energy and matter in the Universe does not change; it just takes on various forms. This remains true for heat energy and that is the reason when you use an air conditioner or a refrigerator to cool a room, you should not think of it as bringing in cold air. The refrigeration cycle's goal is to extract heat from a given area and send it outside. When a room is cooler, this means that the room contains less heat.

Impacts of Pressure on the boiling point

Pressure, volume and temperature relate according to the equation shown below:

P₁V₁/T₁=P₂V₂/T₂

Higher pressures make it more difficult for a liquid to boil, while lower pressures make it simpler. The refrigeration cycle is made possible by adjusting the refrigerant's pressure in order to change its boiling point.

HVAC Operation

Compressor

The refrigerant starts as a cool vapor and travels to the compressor, which is the first part. The compressor is commonly regarded as the refrigeration cycle's engine; it draws the most power from the HVAC system's components and pushes the refrigerant via it. The cold, gaseous refrigerant is compressed into a very hot, high-pressure vapor during the compression process.

Condenser

The condenser is the second stage in the refrigeration cycle. The condenser's function is to cool the refrigerant until it condenses, or changes from a gas to a liquid. This occurs when hot, gaseous refrigerant is blown across the condenser coil. This allows heat to migrate from the refrigerant to the colder outside air, where it is expelled into the atmosphere. The condenser coils wind through the condenser to increase the piping's surface area and hence the heat transfer to the air. Owing to the high pressure and temperature drop, the refrigerant converts from a gas to a hot liquid.

Expansion Valve

As a hot, high-pressure liquid, the refrigerant is entering the expansion valve. The expansion unit is in charge of rapidly lowering the refrigerant's pressure so that it can boil (evaporate) more easily in the evaporator — and that's all there is to it! The sole aim of the expansion valve is to lower refrigerant pressure. Since the pressure at the expansion system drops so fast, the refrigerant converts into a mix of cold liquid and vapor.

Evaporator

The refrigerant now moves into the evaporator as a cold mixture of liquid and gas (vapor). By boiling (evaporating) the refrigerant flowing through it, the evaporator cools the air going into the space. When warm air is blown through the evaporator coil as cold refrigerant flows through it, this occurs.

Heat is transferred from the air to the refrigerant, which cools it immediately before being vented into the space. The evaporator coil winds through the evaporator, much like the condenser coil, to optimize heat transfer from the refrigerant to the air. The warm air blown over the evaporator quickly boils the low-pressure liquid refrigerant, which then returns to the compressor as a cold gas/vapor.

Refrigerants

Now that we have covered the refrigeration cycle, the next step is to cover the different refrigerants used in the refrigeration cycle.

What are Refrigerants?

Refrigerants are fluids used in the refrigeration cycle for heat transfer. The thermodynamic phenomenon of phase changes, in which a fluid changes to a gas or vice versa, is used by refrigerants to absorb heat and then release it to another room.

There are 4 major categories of refrigerants. These include chlorofluorocarbons(CFCs), Hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and natural refrigerants.

Natural refrigerants

Natural refrigerants, unlike the other refrigerants are found in nature. Refrigerators and air conditioners may use them as cooling agents. Hydrocarbons (isobutane – R-600A), ammonia, carbon dioxide, and water are all common natural refrigerants.

While hydrocarbons (HC) contain carbon dioxide, this has a GWP of 1, while the HCFCs and HFCs currently on the market have GWPs in the thousands. As a result, using hydrocarbons as a refrigerant is the most environmentally friendly alternative currently available.

Hydrofluorocarbons

Refrigeration manufacturers prefer HFCs because they are a good substitute for CFCs and do not deplete the ozone layer as much as CFCs or HCFCs do. HFCs, on the other hand, are powerful greenhouse gas sources with a strong global warming potential.

A UNEP study makes a variety of suggestions, including the use of HFCs with a limited atmospheric lifetime (days or weeks, instead of the dozens of years for a few HFCs now in use). Others, such as Greenpeace, are calling for them to be phased out due to their role in global warming.

Hydrochlorofluorocarbons (HCFCs)

HCFCs are currently being used to supplement CFCs, but they will eventually be phased out. HCFCs are less harmful to the ozone layer than CFCs, but they also deplete the ozone layer, albeit at a slower pace.

Regrettably, HCFCs are a powerful greenhouse gas, several times stronger than carbon dioxide. HCFCs have also led to an increase in the amount of chlorine in the atmosphere.

Chlorofluorocarbons(CFCs)

CFCs have a low boiling point and are an ideal unreactive refrigerant. They are non-toxic, do not cause fires, are affordable, and are simple to store. CFCs, on the other hand, are ozone-depleting greenhouse gases that contain fluorine, making them extremely harmful to the atmosphere.

In 1994, the majority of countries stopped manufacturing CFCs. By 2020, developed countries will have eliminated all CFC stock, and by 2030, developing countries will have eliminated all CFC stock. A CFC widely used for refrigeration is Freon, a DuPont brand.

Refrigerants have been playing a key role in the heat transfer process and in air conditioning. However, they have their own negative impacts. In fact, since they contribute to global warming and deplete the ozone layer, many refrigerants are considered to have a harmful impact on the climate. Some of the refrigerants used previously in heating and cooling systems have been banned. For instance, R22 refrigerant which was being utilized previously is now being replaced by R410A, which is more environmentally friendly. However, Since 2010, R22 was being used to service existing air conditioning systems. This refrigerant will no longer be in production.

Hydro-fluorocarbon (HFC) zeotropic blends are used in the R-400 series. R-410A is the most common, and although it does not contribute to ozone depletion, it does contribute to global warming, much like R-22. R-410A is the most popular refrigerant for new light commercial air conditioning systems, but its operating pressures are more than 50 percent higher than R-22, necessitating components that can withstand these higher pressures. R-410A is marketed under a number of brand names, including GENETRON AZ-20, SUVA 9100, Forane 410A, and Puron.

The move from ozone-depleting R-22 to replacement refrigerants like R-410A necessitated a renovation of heat pump and air conditioning systems. Compressors and other elements of modern systems are specifically designed for use with these replacement refrigerants.

R-600 series

R-600a, also known as isobutane, is the most commonly used of the R-600 series refrigerants. R-600a, which is produced from natural sources, has no ozone depletion potential and causes very little global warming. Since isobutane is flammable, systems must be constructed with this in mind, and technicians must be adequately qualified to handle it.

R-600a is mainly used in domestic refrigerators in Europe (most notably in Germany), although it has also been used in industrial refrigerators in the United States, although its use is limited.