Air Conditioning

The Refrigeration Cycle

Refrigeration cycles are the model used in refrigerators and air conditioning units and heat pumps. The man differences between the two is, heat pumps change from cooling to heating and refrigerators will cool only. Technically in the heat pump cycle it incorporates the refrigerator cycle. To put it in another way, a heat pump chooses which side of the system rejects the heat that was accepted by the evaporator to give the user the required temperature.

A heat pump removes heat from a low-temperature source, and expels it to a high-temperature sink, with a bit of help from external mechanical works.

The opposite of the heat pump cycle is called the thermodynamic power cycle. In this cycle, heat is supplied from a high-temperature source, to the heat engine, also a part of the heat is used to produce mechanical work and the rest being rejected to a lower temperature sink. Having this here satisfies the second law of thermodynamics.

Air is passed over the condenser coil, re-warming the dehumidified air. The terms condenser coil and evaporator coil do not refer to the behaviour of water in the air as it passes through each coil, instead it refers to the phases in the refrigeration cycle. Having the condenser coil in the main air path rather than in a separate outdoor air path ( just like a regular air conditioner) tends to result in two different consequences; the output air is warm as opposed to cold, and the unit is then able to be placed anywhere in the environment that is to be conditioned, without a need to have the condenser outdoors.

Ironically, unlike an air conditioner, a dehumidifier will actually heat a room lust like an electric heater that draws the same amount of electricity as a dehumidifier. A regular air conditioner transfers energy from the room by means of the condenser coil, which is outdoors. This is a thermodynamic system where the room acts as a system and energy is transferred out of the system. Conversely with a dehumidifier, no energy is transferred out of the thermodynamic system because the air conditioning unit is entirely inside the room. Therefore all of the electricity that’s consumed by the dehumidifier is energy that is input to the thermodynamic system, and tends to remain in the room.

Dehumidifiers are used in the cold, whereas cold, more damp climates to prevent mould growth indoors, especially in basements. They also are sometimes used in hot, humid climates for comfort because they reduce the humidity which tends to cause discomfort. (Just as a regular air conditioner, but without cooling the room in which its contained.)  

Comfort air conditioning is specially designed to create a 40%-60% humidity rating in the given space. In food retailers large open chiller cabinets are used as highly effective air dehumidifying units.

A specific type of air conditioner that is only used to take humidity out of the air, this is called a dehumidifier. Dehumidifiers are different from regular air conditioning units, whereas both the condenser and evaporator coils are placed in the same air path, the entire units are placed in the air that wants to be conditioned (in this case dehumidified), rather than the condenser coil being outside. Having both the condenser coil and the evaporator coil in the same air path produces warm, dehumidified air. The evaporator coil is placed in the first air path, dehumidifying the air just like a regular air conditioner does.

Cleanrooms also use them for the production of integrated circuits, pharmaceuticals etc, in which very high levels of air cleanliness and humidity control is needed to successfully complete the process. There are many other areas that use process air conditioning, for example, Breeding laboratories, aircraft air conditioning, data processing centres, Textile factories, physical testing facilities, plant nurseries, nuclear facilities, laboratories, mines and food processing areas.

In all of the above mentioned circumstances the air around will not only need to be temperature controlled but also to control the humidity, the air quality, air motion and air movement.

Humidity Control

Air conditioning equipment reduces the humidity of the air controlled by the given system. The relatively cold evaporator coil condenses the water vapour from the processed air sending the water to a drain and removing water vapour from the cooled space thus lowering the humidity in the given room. Since humans perspire, to provide natural bodily cooling by the evaporation of perspiration from the skin, dry air provides a more comfortable surrounding.

The Process applications are the secondary use of air conditioning, when using it as a process, air conditioning aims to provide a suitable environment for a process that is being carried out in regardless of internal heat and humidity and the external weather conditions. The condition of the air is often within the comfort range, but it must be remembered the process determines the condition whereas comfort application is the user who chooses the temperature. Some buildings, such as hospitals need conditioned air to reduce the spread of infection, also the humidity needs to be controlled to ensure the patients dehydration is limited. Some applications such as open heart surgery needs lowered to approximately 18ºC, whereas neonatal needs a relatively high temperature of around 28 ºC.

Comfort air conditioning makes the process of deep plan Buildings feasible (a deep plan building is where the horizontal distance from an external wall is far greater than the floor to floor height, deep plan building is a method that makes more efficient use of the given area.). A building without air conditioning must be built narrower or with light wells so the inner space will receive sufficient outdoor air, otherwise known as natural ventilation. With the use of air conditioning, buildings can be taller as wind speeds increase significantly, making natural ventilation unsafe and impractical for very tall buildings. Comfort applications for various buildings are different and may be categorized as one of the following:

1)      A Low-Rise Residential building, including family houses, duplexes and small apartment buildings.

2)      A High-Rise Residential building, such as dormitories and apartment blocks.

3)      A Commercial building, built for commerce, such as offices, malls, shopping centres and restaurants etc.

4)      An Institutional building, such as hospitals, governmental buildings, academic buildings etc.

5)      Industrial buildings/ spaces where the thermal comfort of workers is desired.

Air conditioning is not only used in buildings, it is also used widely in a variety of transportation such as Vehicles, Ships, Trains, Aircraft and Spacecraft.