For specific applications, the efficiencies of both living and non-living beings depend to a great extent on the physical environment. The nature keeps conditions in the physical environment in a dynamic state ranging from one extreme to the other. Temperature, humidity, pressure, and air motion are some of the important environment variables that at any location keep changing throughout the year. Adaptation to these many a times unpredictable variations is not possible and thus working efficiently is not feasible either for the living beings or the non-living ones. So for any specific purpose, control of the environment is essential. Refrigeration and air-conditioning are the subject that deals with the techniques to control the environments of living and non-living subjects and thus provide them comforts to enable them to perform better and have longer lives.
Refrigeration
Literal meaning of refrigeration is the production of cold confinement relative to its surroundings. In this, temperature of the space under consideration is maintained at a temperature lower than the surrounding atmosphere. To achieve this, the mechanical device extracts heat from the space that has to be maintained at a lower temperature and rejects it to the surrounding atmosphere that is at a relatively higher temperature. Since the volume of the space which has to be maintained at a lower temperature is always much lower than the environment, the space under consideration experiences relatively higher change in temperature than the environment where it is rejected.
The precise meaning of the refrigeration is thus the following: Refrigeration is a process of removal of heat from a space where it is unwanted and transferring the same to the surrounding environment where it makes little or no difference. To understand the above definition, let us consider two examples from the daily life.
It is a well-known fact that the spoilage of food and many other items reduces at a lower temperature. At a lower temperature, molecular motion slows down and the growth of bacteria that causes food spoilage also retards. Thus to preserve many types of perishable food products for a longer duration, we use refrigerators (Figure 1.1) in our homes, canteens, hotels, etc. The temperature of the food products has to be maintained at a level below that of surroundings. For this we keep the food products in a refrigerator.
The inside volume of the refrigerator where we store food products or any other items is much less than the volume of the room where the refrigerator is kept. The room in this case is the surrounding environment. Food products in the refrigerator initially were at a higher temperature than desired temperature, meaning that it had some unwanted heat. If its heat is removed, its temperature will decrease.
The refrigerator removes unwanted heat from the food products and throws away that heat to the room – the surrounding environment of the refrigerator. The amount of heat makes a big difference in temperature inside the refrigerator and almost little or no difference in the temperature of the room.
As a second example let us consider travel in a car in an Indian summer of Delhi or Kanpur. Outside temperature is very high. It is highly uncomfortable. For a comfortable drive, now-a- days we have air- conditioned cars. You will come to know later, that refrigeration is an integral component of air- conditioning. To have a comfortable drive in the car, the temperature inside the car has to be lowered from about 40oC to 25 oC. This means that heat of the space inside of the car and its occupants has to
be thrown outside. This is done by the refrigeration unit fitted in the car. The volume of the car is much less than that of the surroundings. With the rejected heat there is an appreciable change in the temperature inside the car, but no change in the temperature of the surroundings.
In many places and situations, environment temperature is lower than the temperature of the space that we desire. As an example consider car driving in the Winter of Delhi or Kanpur.
Temperature outside is about 2-6oC. For a comfortable drive with light clothing, temperature inside the car has to be about 25oC. This means that heat has to be supplied or pumped inside the car and
thus its temperature has to be increased. The machinery that performs this operation is known as heat pump. But in applications such as that of the comfortable driving in a car that depending upon the season requires temperature to be lower or higher than the surroundings, heat has to be pumped to the car or rejected from the car.
Since both the operations are performed by the same unit in the car, in a much broader sense we can say that a refrigeration unit controls the temperature of a space. In the normal refrigeration system, this is done by reversing the operation.
In refrigeration, heat is pumped out from a lower temperature space to a higher temperature environment. We know from our experience in daily life that water flows from a higher level to a lower level and heat flows from a body at a higher temperature to a body at a lower temperature. The reverse, i.e., flow of water from a lower level to a higher level and flow of heat from a body at a lower temperature to a body at a higher temperature do not occur naturally. In practice these are achieved at the cost of external work (power) done on the water and the carrier of heat (here the refrigerant) with help of a mechanical device.
Whether the space under consideration has to be maintained at a temperature lower or higher than the surrounding environment, to pump out or in the heat, external power is always required. In relation to heat pump, this will be explained later.
Air Conditioning
Merely lowering or raising the temperature does not provide comfort in general to the machines or its components and living beings in particular. In case of the machine components, along with temperature, humidity (moisture content in the air) also has to be controlled and for the comfort of human beings along with these two important parameters, air motion and cleanliness also play a vital role. Air conditioning, therefore, is a broader aspect which looks into the simultaneous control all mechanical parameters which are essential for the comfort of human
beings or animals or for the proper performance of some industrial or scientific process. The precise meaning of air conditioning can be given as the process of simultaneous control of temperature, humidity, cleanliness and air motion. In some applications, even the control of air pressure falls under the purview of air conditioning. It is to be noted that refrigeration that is control of temperature is the most important aspect of air conditioning.
To understand the above definition in a better way, let us consider one example. In the summer, the temperature in Delhi is about 10 oC higher than in Kolkata where temperature varies in the range of 32 oC to 35 oC. We feel uncomfortable in both places. Weather in Delhi is hot and dry (moisture content
in the air is low) whereas in Kolkata it is (mild) hot but humid (moisture content in the air is very high). If we go to a hill-station, say Shillong in the summer, we feel comfortable there.
Temperature there remains about 25 oC and relative humidity of the air is also in the comfortable range, say about 65%. In Delhi, temperature is very high and humidity is low, whereas in Kolkota,
temperature is low but humidity is high. In Delhi if there is a rain, we feel more comfortable whereas in Kolkata even with rain, the relative comfort is less. In Delhi temperature falls down and humidity also increases towards the comfortable value. In Koklata, temperature falls down but humidity still remains on the higher side. Thus, for comfort, both temperature and humidity have to be in the specified range. This is true for both human beings and scientific processes. Apart from the above two, from intuition one can also say that purity or cleanliness of the air is an essential item for the comfort and it has been established that the air motion is also required for the comfort condition.
Depending upon the requirement, air conditioning is divided into the summer air conditioning and the winter air conditioning. In the summer air conditioning, apart from cooling the space, in most of the cases, extra moisture from the space is removed, whereas in the winter air conditioning, space is heated and since in the cold places, normally the humidity remains low, moisture is added to the space to be conditioned. The summer air conditioning thus uses a refrigeration system and a dehumidifier.
The winter air conditioning uses a heat pump (refrigeration system operated in the reverse direction) and a humidifier. Depending upon the comfort of the human beings and the control of environment for the industrial products and processes, air conditioning can also be classified as comfort air conditioning and industrial air conditioning. Comfort air conditioning deals with the air conditioning of residential buildings, offices spaces, cars, buses, trains, airplanes, etc. Industrial air conditioning includes air conditioning of the printing plants, textile plants, photographic products, computer rooms, etc.
It has been mentioned above that the refrigeration and air conditioning are related. Even when a space has to be heated, it can be done so by changing the direction of flow of the refrigerant in the refrigeration system, i.e., the refrigeration system can be used as a heat pump (how this is possible will be explained later). However, some section of the people, treat refrigeration exclusively the process that deals with the cooling of the space. They treat heating operation associated with the heat pump.
