Air Conditioning Systems

Introduction

Air conditioning means the maintaining the desired air conditions inside a confined space irrespective of outside atmospheric conditions. Air conditioning is basically provided to:

  1. Maintain the air at the desired temperature
  2. Control moisture content of the air
  3. Hold contamination to an acceptable level.
  4. Circulate the air properly in order to have correct proportion of oxygen.

Human comfort is not the only purpose of air conditioning. It may also be provided to prevent deterioration of equipment and to maintain a specified temp & humidity for electrical & electronic equipment. Air conditioning is an essential requirement for proper functioning of a telephone exchange.

Table of Contents

TYPES OF A.C. SYSTEM

Air conditioning system in use may be categorized in to the following Types :

  1. UNITARY SYSTEM : 

    1. Window Type.
    2. Split Type.
    3. Package Type with:

      • Air Cooled Condenser.
      • Water Cooled Condenser.
  2. CENTRAL AIR CONDITIONING SYSTEM : 

    1. Direct Expansion Type.
    2. Chilled Water Type.

Each of these has its limitations as well as advantages and the most suitable one should be selected taking all relevant factors into account.

UNITARY SYSTEMS

WINDOW TYPE AIR CONDITIONERS

Window type air conditioners are self contained units with the compressor, condenser, evaporators, refrigerant piping and air filters, all assembled in a very compact single unit. The size of typical 2 TR unit is 470 mm × 700 mm.

The window units are usually of 1, 1.5, 2TR capacity. Modern tendency is to employ sealed type motor compressor unit. The condenser is air cooled. The motor usually has shaft extension on both sides for air to draw the return air from the room, pass through evaporator and send the cooled air back into the room.

The units are easy and convenient to handle. One limitation however is that a window type can only be installed in a wall whose outer face is exposed, so that the hot air from the condenser may be discharged in to the atmosphere.

ADVANTAGES: 

  1. These units don't occupy floor space.
  2. Can be mounted on the window side or on a hole cut in the wall.
  3. Absence of ducting and plumbing.
  4. Easy and quick installation.

DISADVANTAGES: 

  1. No Humidity control.
  2. No fresh air control.
  3. Not suitable for continuous operation.
  4. No effective uniform air distribution and week air throw.
  5. No effective dust control.

SPLIT TYPE AIR CONDITIONERS

These units are functionally very similar to the window models. Here the condenser/ compressor unit (outdoor unit) and cooling coil unit (indoor unit) are housed in two separate enclosures and both the indoor and outdoor units are connected through refrigerant pipe line.

ADVANTAGES : 

  1. Opening in wall/window is not required.
  2. Indoor units can be placed any where inside the room.
  3. Effective Dust Control.
  4. No Noise problem

LIMITATIONS : The distance between Indoor and outdoor units should not be more than 30 feet.

PACKAGE TYPE A/C UNITS

These units are also functionally very similar to the window models but are very much bigger in size and therefore installed on the floors. Self contained package units possess several advantages. It can be installed in AC space with or without duct system. The nominal capacity ranges from 5 TR to 15 TR.

The Compressor units may be of hermetically sealed semi sealed or open type. The control panel is conveniently located on the unit itself.

ADVANTAGES: 

  1. Structural alterations required are negligible.
  2. Prompt delivery & immediate A.C. benefits.
  3. Ease of installation and removal.
  4. Simplified field engineering.
  5. Factory assembly of balanced and tested equipment.
  6. More flexibility in operation.

CENTRAL AIR CONDITIONING PLANT

Central air conditioning plant becomes a necessity when areas to be conditioned is large and in multiple zones as the unitary air conditioners will be highly costly proposition. Another aspect is that the nearly infinite combination of coil configuration & evaporator temperature, air handling arrangements and other variations that are available in the control system are rarely possible with unitary systems.

DIRECT EXPANSION TYPES AC PLANT : In this type of system evaporation of refrigerant takes place in the cooling media. Refrigerant cools the air in direct contact with it. The heat released by the air is picked by the refrigerant and the condenser water picks that heat from the refrigerant and reject into atmosphere by means of cooling tower.

CHILLED WATER TYPE A.C. PLANTS : In chilled water type A/C plants water as second intermediate heat transfer medium is used. Water absorbs heat from the air returned from the conditioned space and then transfers the same to the refrigerant in the chiller. Here water circulates in chillers, water pumps and air handling unit coils.

Some of the big installations use more than one chilled water circuits like primary and secondary circuits.

Other items remain similar to that of direct Expansion Type Plants.

COMPONENTS OF A.C. SYSTEM

A.C. system can be broadly sub-divided into three areas :

  1. Refrigerant Circuit.
  2. Water Circuit.
  3. Air Circuit.

REFRIGERANT CIRCUIT

In a vapour compression type refrigerant cycle, a refrigerant is used in conjunction with compressor, evaporator and condenser.

COMPRESSOR : . Compressor only helps to maintain pressure difference in evaporator and condenser so that the refrigerant cycle can work continuously. It expends Freon Gas from the evaporator as fast as it is formed and maintains low pressure 69 p.s.i.g at low temperature of 40 °F.

It compresses and discharges the gas to the condenser at a pressure of about 229 p.s.i.g at a temperature of 110 °F.

Because of rapid compression, the refrigerant becomes hot.

CONDENSER :It is intended for cooling the hot gas and liquidating it under pressure. It may be air-cooled or water-cooled. At the condenser, the refrigerant vapour which is maintained at high pressure releases heat to the condenser water through heat transfer on surface of condenser water tubes and becomes liquid by principle of Latent heat of condensation.

EXPANSION VALVE :Its function is to control and regulate the flow of high pressured liquid Freon and allow it in to the evaporator under low pressure.

EVAPORATOR :This constitutes the cooling unit in which the liquid Freon under a low pressure evaporates and in doing so picks up heat from the air (by principle of latent heat of evaporation) thereby cooling the medium surrounding the cooling coil.

An important point to note is that the compressor and expansion valve constitute the two dividing points between high and low pressure and that the condenser and evaporator contains the refrigerant both in liquid and gas form and that they both act as heat exchanger i. e. evaporator absorbs heat and Condenser releases heat. In other words the refrigerant cycle can be used either for cooling or for heating.

WATER CIRCUIT

The condenser water picks up heat from the refrigerant system and rejects heat into the atmosphere with the help of cooling tower, where the water is sprayed and subject to cooling by atmospheric air and re-circulated to the condenser by pumps.

Reasonably soft water is to be supplied for make up of condenser water. Hard water causes fouling of condenser tubes thereby reducing the heat transfer efficiency.

AIR CIRCUIT

Cooling and dehumidification of air is done at cooling coil to off set sensible and latent heat of the space.

Distribution of conditioned air to the areas in proportion to the load is achieved through supply air ducting with the help of the fan. Collection of return air back to the air handling system takes place through return air space and fan provides the necessary suction force. Dust filtration by filters is provided in return air stream.

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