Understanding Air-Conditioning


WHAT IS AIR-CONDITIONING?

Air-conditioning is the ability to simultaneously and effectively control the temperature humidity, air movement and the quality of air in a given space?.

HOW DOES AN AIR-CONDITIONER WORK?

Heat naturally flows from a warm to a cold substance, so air can therefore be moved over a warm or cold surface for a desired effect. This warm or cold surface can be obtained only through a process called THE REFRIGERATION CYCLE. This refrigeration process provides cooling below ambient temperature by transferring heat from one area (where it is not wasted) to another area (where it is not objectionable).

An air-conditioner consists of four main components for the Cooling Cycle:

Evaporator - The liquid refrigerant comes into contact with warm air causing it to evaporate. When refrigerant evaporates, it absorbs heat from the surrounding air. The warm air is drawn in by a fan and sent back into the room.

Compressor - Here the vapour refrigerant (from evaporator) is ?compressed? to a high pressure so that it can be liquefied more readily (by condenser) to repeat the cooling process. The compressor also assists with the circulation of the refrigerant.

Condenser - The high-pressure vapour refrigerant ( still at a high-pressure) is relieved of its high temperature by being cooled by outdoor air (approx. 30-35 degrees Celsius) and liquefied.

Capillary Tube - Here the pressure of the refrigerant (still at a high-pressure is lowered by being forced through a narrow tube (capillary tube) causing energy loss and thus resulting in a pressure drop the sudden lowering of pressure causes the temperature to lower further. Thus as the liquid refrigerant flows through the capillary tube its pressure and temperature is lowered to create a volatile state.

TYPES OF AIR-CONDITIONING

The structure of an air-conditioner is divided into an indoor and outdoor unit, which are connected by pipes (as per above diagram).

Window Type - The window type air-conditioner is designed to go through a hole in a wall or in a window frame.

This type is known as a SELF-CONTAINED type of air-conditioner due to the indoor and outdoor parts being together in one casing (housing).

Being "all-in-one" it is necessary to install this air-conditioner on an external wall / window for the heat to be disposed of out the back of the unit (condense section).

Split Type - These consist of indoor unit (FAN COIL UNIT) and an outdoor unit (CONDENSER UNIT) (as per diagram on previous page)

There are many different types of indoor units for the split type i.e.:

  • Wall mounted (i.e.: mid-wall split)
  • Ceiling mounted (i.e.: cassette split)
  • Floor mounted type and ducted type

CAPACITIES OF AIRCONDITIONERS

Capacity is a term used to measure the cooling (or heating) time.

The capacity required from an air-conditioner differs according to the region (Samsung in S.A.: Outdoor ? 35 degrees Celsius DB; Indoor - 27 degrees Celsius DB / 19.5 degrees WB) and the amount of air it has to cool. Therefore one must choose the correct unit by calculating the HEAT LOAD.

In South Africa, the cooling / heating capacity of an air-conditioner is measured mainly in one of the following ways:

  • BTU (British Thermal Unit)
  • KW (Kilowatt)

(1 KW = 3412 BTU)

A GUIDE TO UNDERSTANDING HEAT LOADS

PLEASE NOTE: Hereunder follows a GUIDE ONLY. For precise heat-load calculation a HEAT LOAD CALCULATION SHEET should be completed.

BASIC HEAT-LOAD CALCULATIONS

REMEMBER:

ALL the air in a ?given space? needs to pass through the refrigeration cycle (air-conditioner) for there to be ?effective control? of that area. THEREFORE air-conditioning can only be effective if the area calculated for is a SEALED ENVIRONMENT.

In the case of offices, homes and your average area requiring air-conditioning keep the following factors in mind:

  • Size of area (m2)
    Ins the height under 3m (If not add on the increased % of area or use the H/L Calculation Sheet)
  • Exposure to direct sunlight
    Is there excessive heat gain from large of many windows or a non-insulated roof? etc
  • Are there curtains or blinds?
    i.e.: If the area is an office with blinds or glazed windows the heat gain is reduced.
  • Type of building construction
    A brick house would be a lot cooler than a timber or metal one!
  • Amount of people in the area and their level of activity
    i.e.: An office with 2 people can be considered as a ?standard? people heat load, but an office of the same size used as a boardroom with 8 people is not. A person sitting in an office is giving off less heat than one sitting in a restaurant eating and drinking!
  • Extraction / intake of air
    Should one add air to an area, this air will be warmer than the air in the given area and will need to pass through the refrigeration cycle to lower its temperature, therefore the air-conditioner is working harder to ?maintain? the lower temperature so the capacity is affected.
  • Electrical equipment in use (lights etc.)
    i.e.: A small office in the center of a building with no persons in it might not have much of a heat-load, but should it be a computer room the heat given off by the systems will effect the capacity. Should the area being calculated not have any of the above excessive factors and considered to be a standard area, an average heat-load can be used of: 500 BTU per m2.

CONVERTING ADDITIONAL HEAT LOADS TO BTU'S

People = 500 ? 600 BTU / person (average) depending on activity
Electrical Equipment = wattage x 3.4 = BTU (i.e. 100w globe = 340 BTU)



Self-contained type (Window / Wall Unit)

Basic entry level "All-in-one" type of Air-conditioner used for smaller areas such as offices / homes up to approx. 48 square meters & cost-effective to install.


Split type units (Mid-wall split type)

Popular for most offices/homes. Up to approx. 48 square meter. More features than W/W, quieter but more costly.


Split type units (Multi-Mid-wall split)

Same as above midwall with 2 indoor units and 1 outdoor unit. Cost saving on unit, installation and only 1 unit outside. Approx. 18 and 24 square meter areas only.


Split type units (Ducted Systems)

One indoor unit mounted in the ceiling. Cooling 1 or more areas via ducts to a grill in the ceiling/s or wall/s. Cheaper option to air-condition many areas.


Split type units (Cassette)

In ceiling mounted unit with grille exposed beneath ceiling. Popular in line shops, large open plan areas.