A basic understanding of air conditioning and refrigeration

17 November 2010 05:39 am - 0     - {{hitsCtrl.values.hits}}

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By T. L. B. Hurulle
In warm countries such as Sri Lanka, the very mention of the word air conditioning brings to mind a cool and comfortable environment. However, in cold climates, air conditioning could mean the heating of the environment to a comfortable level.

Both processes employ electro-mechanical cooling and heating equipment. People are more comfortable and perform better in cooled and de-humidified environments. In addition, most machines, materials and components perform and preserve better in air conditioned environments.

In this article, for ease of understanding, the air conditioning applied to the cooling of buildings, motor vehicles and other spaces in tropical climates is discussed. In a nutshell, air conditioning is the process of an air conditioner drawing in warm humid air, cooling and de-humidifying the same and releasing this air to the space requiring air conditioning.

The basis of the process of air conditioning discussed in this article is the refrigeration cycle. This creates the 'refrigeration effect' in which components, materials and controls are used to cool buildings and other spaces.

The refrigeration process
The refrigeration process above could be broadly described by the steps below which produce the refrigeration effect for air conditioning:

The mechanical compressor compresses the low pressure refrigerant vapour within the interconnected copper tube to a high pressure and temperature.

The compressed high pressure refrigerant vapour enters the condenser, gives up its heat within and the refrigerant is converted to a high pressure, low temperature liquid (Condensed liquid refrigerant)
The high pressure refrigerant liquid is sent through an expansion valve (or capillary tube) thus dropping its pressure when it enters the evaporator. The sudden drop in refrigerant liquid pressure causes the refrigerant to 'boil' thus absorbing heat from the surfaces of the evaporator which causes cooling to occur. The refrigerant thereafter changes from liquid state to low pressure vapour within the evaporator.
The low pressure, low temperature refrigerant is thereafter 'pulled in' to the mechanical compressor and the process repeated as a continuous process thus giving the refrigeration effect to deliver air conditioning.

Applications of air conditioning and refrigeration
Although air conditioning and refrigeration have a multitude of applications, the under mentioned applications fall within the main categories and would be adequate for this article.

Air conditioning: motor vehicles, offices, shops, residences, hotels, research laboratories, test chambers, hospital operating theatres, computer rooms, electronic control rooms, etc.
Refrigeration: Domestic fridge-freezers, deep freezers, commercial cold rooms, commercial freezer rooms, mortuary coolers, process coolers, etc.

Temperature control within air conditioned buildings and spaces
It has been observed that in most comfort air conditioning applications the temperature within a conditioned space should be 5.5 deg. C (10 deg. F) lower than the air temperature outside. It is also a known fact that setting the temperature of air conditioned spaces below the specified limits causes the mechanical compressor of the air conditioners to function for longer periods. This will result in increased power consumption and reduced life of equipment.

Types of air conditioning equipment
Although there are many types and combinations of air conditioning equipment, the following have been identified through use and experience and would suffice in this instance:

Window type air conditioner
Window type air conditioners are the lowest priced and most commonly used. Such equipment is in a single piece which makes it convenient for installation through an aperture in the wall. However, its main disadvantage is that noise of the operation of mechanical compressor enters the air conditioned space, making it unsuitable when quietness of operation is necessary (0.75 to 3.0 tons R capacities).

Split type air conditioner
Split type air conditioners come in two separate components (i.e. outside condensing unit and inside evaporator-cooler). Split type air conditioners are more suitable for quiet operation since their mechanical compressors are located outside the conditioned spaces.

However, these types of equipment are more cumbersome to install, cost more and there is a requirement to interconnect inside and outside units with refrigerant copper tubing (0.75 to 4.0 tonnes R capacities).

Packaged type air  conditioner
Packaged air conditioners are commonly used to air condition large offices, shops and public areas where large cooling capacities are required. These units may be connected to air distribution ducting when even distribution of cooling is required (3.0 - 30.0 tonnes R capacities). However, such equipment need to be connected to remotely located water cooling towers/air cooled condensers, circulation water pumps for the installation to be complete.

Liquid chiller unit
Liquid chiller units are employed to air condition large hotels, offices, shopping centres, reception halls and auditoriums where reliability, continuous operation and accurate cooling of individual spaces are required (50-5,000 tonnes R capacities).

These liquid chillers too require to be connected to water cooling towers, air cooled condensers, air handling units, fan coil units, circulation water pumps and electrical control panels.

The above would suffice to obtain a basic understanding of air conditioning while not going into additional areas such as control of relative humidity, ventilation, air filtration, 100% fresh air systems, domestic and commercial refrigerators and freezers, cold rooms, freezer rooms, blast freezers, proper installation and maintenance of equipment, brazing, safe pressure testing, vacuuming, temperature control and electrical power saving devices, close control air conditioning systems, absorption type air conditioning and refrigeration systems, heat pump/recovery systems, multi-stage refrigeration compression, air cycle refrigeration and properties of refrigerants and their effects on the environment.
(T.L.B. Hurulle, Engineer, AC&R, can be reached via e-mail: themiyah@gmail.com.)

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