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Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 12)


In this part of the full HVAC course, we will learn about the working principle of an Air conditioner or refrigeration system.

Air conditioning equipment works on the same concept that refrigerators do but they are much more advanced than what we have today. There are two primary elements which make up air conditioning equipment; water and electricity. The refrigeration unit works off of electricity and converts it into cold air while the compressor pumps the liquid inside the evaporator coil until it becomes vapor. This vapor then travels through the condenser coil where it changes back into a liquid and releases heat energy. Since this cycle goes on continuously, the temperature of the house stays constant regardless of outside conditions. To avoid damaging your electrical components, there should not be any leaks present in the system.

                Refrigeration works using a similar concept. Instead of absorbing heat from the surrounding environment, refrigerators absorb coldness from the outside. Your refrigerator does so by having shelves filled with various types of food products. These products are placed inside the fridge where their temperature is lowered significantly. The colder air temperatures make these products less appetizing to eat; therefore, consuming them becomes undesirable. Also, colder air can hold more moisture than warmer air. Therefore, if you have a bunch of wet groceries in the back of your fridge, this is probably a good indicator that your unit needs repaired. A simple way to tell whether or not your fridge is operating efficiently is to check the temperature of its interior compartments and freezer. If any compartment is too warm, something might be wrong.

Air conditioning (AC) units are used in homes, offices, and other buildings to control temperature. They work by passing air over heat-absorbing material and then through a condenser coil where the warm air becomes cold. In a refrigeration system, cool water running through the evaporator absorbs the heat from the air being conditioned, cooling it down and causing it to change state. The cooled air is forced out through the vent, and this causes the compressor to spin.

In both systems, the fan moves the cool air around inside the building. AC's can either run continuously or cycle on and off periodically. Fans move air around a room and provide ventilation.

The basic parts of any AC unit include a fan motor, fan blades, blower coils, indoor ductwork, outdoor ductwork, dampers, evaporator coil, condensate pan, condensate drain line, thermostat, humidistat, expansion valve, and compressor. These components interact together to provide proper airflow, heating/cooling capacity, and comfort level.

In general, whether a non-centralized AC system i.e; DX system or centralized AC system works on very common phenomenon or working principle, which is known as Vapor compression system or Vapor Compression Cycle (VCC).

Vapor Compression Refrigeration Cycle (VCC):

        It has four stages or processes and works within these 4 stages.
* Compression
* Condensation
* Expansion
* Evaporation

* Compressor (1st Stage) :

            Here, in this stage compression takes place, where low-pressure, low-temperature refrigerant of evaporating side gets compressed and superheated to high pressure, high-temperature refrigerant of various states. 
                        The aim of the compressor is to increase refrigerant various pressure so that its converting condensation temperature is higher than the cooling medium temperature of the condenser.
Types of compressor :
* Reciprocating Compressor
* Centrifugal Compressor
* Rotary Compressor
* Screw type Compressor
* Hermetic Compressor

* Condenser :

Here in this Condenser, condensation activities take place. Basically, this condenser is of shell and tube type heat exchanger, where heat transfer takes between the superheated vapor refrigerant received from the compressor and the cooling medium of the condenser.  
                            Refrigerant vapor is first cooled to the saturation temperature and then condensed to the liquid state. 
Types of Condenser :
* Air Cooled Condenser
* Water Cooled Condenser
* Shell and Tube Condenser
* Double Pipe Condenser
* Evaporative Condenser

* Expansion Valve or Throttle Valve :

        Here, in this stage, the expansion of liquid refrigerant takes place, which comes from the condenser stage. The expansion valve reduces with respect to temperature and pressure of the liquid refrigerant to low pressure and medium temperature. 
                        Another important function of an expansion valve is to regulate the flow of liquid refrigerant to evaporate and to avoid flooding of refrigerant with the evaporator.
Types of Expansion Valve :
* Capillary Tube
* Constant Pressure Expansion Valve
* Thermostatic Expansion Valve.

* Evaporator :

        This is the final stage of the VCC cycle for cooling or chilling or while cooling is done. Here, in this stage, evaporation takes place and while leaving cooling effects are done. 
                The function of the evaporator is to slightly increase the temperature and pressure of the liquid refrigerant that comes from the expansion valve or pressure reducing valve and to convert it into low-pressure vapor refrigerant, thereby giving the required cooling or chilling effect until 15 - 18°C for the AC space under consideration. 
Types of Evaporators:
* Shell and Tube type of Evaporator.

1. The working principle of an air conditioner is based on the fact that heat is transferred from one object to another through conduction. To keep your room at a constant temperature, the air conditioner needs to transfer heat from the inside of the house to the outside or vice versa. This is done by running a current of air over different surfaces using fans.

2. There are two types of fans used in the air conditioning system, centrifugal fans and axial fans. Centrifugal fans use rotating blades attached to a central shaft. When the fan rotates, the blades push air towards the outside. Axial fans do not have any moving parts; instead they have stationary blades arranged in arrays. They create a strong current of air by pushing the surrounding air outwards.

3. The compressor first compresses the gas (air) and then sends it to the condenser coil where it becomes liquid. Once it has passed through the condenser coil, it turns back into gas and passes through the expansion valve. After that, the refrigerant enters the evaporator coil and turns into a low-pressure vapor before returning to the compressor.

4. Refrigerants can be grouped together according to their chemical properties. These include simple hydrocarbons, halogenated compounds, and mixtures of these two groups. In some cases, non-chlorinated fluorocarbons (CFCs) were previously used as coolants in air conditioners. However, chlorine is considered harmful to the environment and should be avoided, especially since CFCs are known greenhouse gases. Hydrofluorocarbons (HFCs), however, are environmentally friendly alternatives. HFCs are composed of hydrogen and carbon, but have a higher concentration of fluorine than other refrigerants. They can be used safely alongside each other in the same system.

5. Most air conditioners operate on electricity supplied from the electrical grid. Electricity consumption varies depending on the time of year and season, how many people live in a home, and whether the home is occupied. Some homes may have their own power generator.

6. A typical air conditioner is capable of cooling a small to moderate sized space. If you want to cool a large area, you need multiple units working together to form a multi-zone system. You can even find systems for commercial buildings, industrial spaces, boats, and cars.

Condenser (Compressor): The compressor compresses refrigerant gas to a high pressure that turns into liquid state. As we know air conditioning is based on compression of the evaporator coil. This is done by using the compressor. Compression reduces the temperature of the refrigerant gas entering the compressor, and condenses the vapor phase back into liquid form. Since the pressure inside the system stays constant, the lower the initial gaseous phase temperature, the higher the final liquid phase temperature. In addition, this causes the amount of energy used by the compressor to decrease because the heat transferred from the gas to the liquid phase has been reduced substantially.

In addition, the condensing coils reduce humidity in the room air, thus reducing the need to use dehumidifier. Dehumidifiers work by using the evaporation of water from the surface of objects in order to remove moisture from the air.

 Evaporator (Expander): The evaporator expands the refrigerant gas to low pressure. This helps to absorb the latent heat released by the refrigerant gas as it changes its phase from liquid to gas. The expansion of the refrigerant gas absorbs heat from the surrounding environment, thereby cooling the space.

On the other hand, the compressor puts out the heat absorbed by the refrigerant gas. Thus, the compressor functions as a heater. However, since the compressor is usually placed outside, it can cause a buildup of heat if it's not properly vented. This means that the compressor needs to be vented to prevent overheating.

 Expansion Valve: When the refrigerant passes through the expansion valve, it undergoes a further reduction in pressure. This allows the refrigerant gas to change back into a liquid. At this point, the change in volume due to the change of the phases of the refrigerant gas is taken up by the liquid refrigerant. This causes a sudden drop in temperature of the refrigerant fluid and hence the air surrounding it.

 Refrigerant Chamber: After passing through the expansion valve, the liquid refrigerant flows through the suction line into the refrigerant chamber. The refrigerant chamber consists of metal fins attached to a copper housing that increases the rate at which the refrigerant gas cools the surrounding air. Also, it is here where the compressor sucks the refrigerant gas and pushes it into the evaporator.
                    Air conditioning systems are used to remove heat from air inside buildings. These units may use different techniques to accomplish this task. Some common methods include refrigeration, adiabatic cooling, absorption cooling, desiccant cooling, and evaporative cooling.

Refrigeration uses compression-refrigeration cycles to transfer thermal energy from the interior air to other objects. This technique requires a constant supply of electricity in order to function, and it cannot cool the air below 32 °F (0 °C). Refrigerators can create ice using cold water coils to condense moisture into water vapor, thereby absorbing heat from the surrounding environment. When the refrigerator's compressor stops running, the temperature inside will rise until equilibrium is reached once again. As the humidity increases, the system will not work efficiently.

Adiabatic cooling refers to a method where air moves through a chamber filled with some type of absorbent material (such as activated carbon) that absorbs heat from the air passing through it. In this way, the air remains relatively dry and does not require much power to operate. On average, about 50% of the total electrical load in an air conditioning unit is used to run fans and compressors. However, these devices do not need to be running continuously if the heating coil is not being used.

Absorption cooling works similarly to adiabatic cooling but instead uses a liquid solution to absorb heat. Cooling becomes effective only after a steady source of water is provided. Absorption cooling systems are typically less efficient than air conditioning systems because they rely on chemicals to perform the job of removing heat.

Desiccant cooling is a form of absorption cooling. Instead of using water or other liquids, desiccants use chemical compounds that absorb water from the air. By replacing the absorbed water molecules with those containing a lower level of water, the molecules themselves become cooler. Desiccant absorption cooling systems are more sensitive to temperature variations than traditional air conditioning systems because heat is transferred between two separate materials.

Evaporative cooling utilizes a fan to blow air over a surface that contains a series of small tubes. Water is sprayed onto the tubes and the resulting evaporation creates a cool spray to keep the room at a comfortable temperature. Evaporative cooling is effective because it allows a large space to remain comfortably cool without requiring the entire space to be occupied by the unit. It is a low cost option compared to the other types of air conditioning technology.

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