Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- HIgh

Central Air Conditioning Systems

Central air conditioning systems serve multiple spaces from one base location. These typically use chilled water as a cooling medium and use extensive ductwork for air distribution.

The principal advantages of central air conditioning systems are better control of comfort conditions, higher energy efficiency and greater load-management potential.

The main drawback is that these systems are more expensive to install and are usually more sophisticated to operate and maintain. Centralized systems are mostly used in mid to high rise buildings, which are structures with 5-7+ floors.

Unitary or Decentralized Air Conditioning Systems

Decentralized air conditioning systems commonly known as by various generic names viz. local systems, individual systems, unitary systems or packaged systems provide cooling to single room/spaces rather than the building.

Decentralized air conditioning systems typically serve single or small spaces from a location within or directly adjacent to the space.

These include:

Packaged thru-the-wall and window air conditioners;

 Interconnected room by room systems;

 Residential and light commercial split systems;

 Self-contained (floor by floor) systems;

 Commercial outdoor packaged systems

The principle advantages of decentralized air conditioning systems is lower initial costs, simplified installation, no ductwork or pipes, independent zone control, and less floor space requirements for mechanical room, ducts and pipes. A great benefit of decentralized systems is that they can be individually metered at the unit.

Disadvantages are short equipment life (10 years), higher noise, higher energy consumption (kW/ton) and are not fit where precise environmental conditions need to be maintained.

Decentralized systems are used in most classes of buildings, particularly where low initial cost and simplified installation are important, and performance requirements are less demanding

District Air Conditioning Systems A District Cooling System (DCS) is essentially a centralised energy plant generating thermal media (chilled water) for air-conditioning requirements of several buildings within a district.

The thermal energy may be in the form of steam, hot water, or hot air, or any combination of the three. The consumer system would usually comprise of air handling units and chilled water piping in the building.

DCS applies in most areas with appreciable concentration of cooling loads, such as industrial complexes, densely populated urban areas and high density building clusters, and can offer economic and environmental benefits. A district cooling system can offer significant benefits to a community. The three groups most likely to benefit from district energy in a community are property/building owners, the municipality, and society at large.

As the central plant of a DCS is large, there will be economies of scale and higher thermal efficiency as compared to that of many isolated small systems. District cooling is up to 10 times more efficient than conventional cooling technology.

Centrally produced, and often based on natural cooling from the cold bottom water of lakes, ground water, rivers, , district cooling is delivered to buildings via water at 6°C in a closed, in-ground pipe system. After performing the cooling service, the energy content of the now 16°C water can be recycled for use in a district heating system to provide heat and domestic hot water.

The main idea behind district cooling is to use local sources for cooling that would otherwise be wasted or not used. District cooling projects of any size, from housing projects to, airports and industrial campuses can be worthwhile in reducing global warming.