Air Conditioning

Absolute temperature - Entropy

The basic principals of air conditioning

Air conditioning is the control of humidity, temperature, cleanliness and air motion. Winter conditioning relates to increasing temperature and humidity whilst summer conditioning relates to decreasing temperature and increasing humidity

Specific humidity-Is the ratio of the mass of water vapour to the mass of dry air in a given volume of mixture.

Per cent relative humidity-is the mass of water vapour per m3 of air compared to the mass of water vapour per m3 of saturated air at the same temperature. This also equals the ratio of the partial pressure of actual air compared to the partial pressure of the air if it was saturates at the same temperature. i.e.

m/mg = p/pg

Partial pressure, Dalton's Laws

Barometer pressure = partial pressure of N2 + p.p.O2 + p.p.H2O,

from Daltons Law viz:

The pressure exerted by a mixture of a gas and a vapour, of two vapours, or of two gasses, or a number of same, is the sum of the pressure which each would exert if it occupied the same space alone, assuming no interaction of constituents.

Dew point

When a mixture of dry air and water vapour has a saturation temperature corresponding to the partial pressure of the water vapour it is said to be saturated. Any further reduction of temperature (at constant pressure) will result in some vapour condensing. This temperature is called the dew point, air at dew point contains all the moisture it can hold at that temperature, as the amount of water vapour varies in air then the partial pressure varies, so the dew point varies.

It can be seen that cooling a superheated vapour at constant pressure will bring it to the saturated vapour line, or Dew point. It can also be seen that cooling at constant temperature raises the partial pressure until the dew point is reached.

Therefore from the above equation for determining the relative humidity,

%R.H. = m/mg x 100 = p/pg x 100

= pdew/pg point x 100

where g refers to the sat condition. This means dry air contains the maximum moisture content (100% R.H.) at the saturation conditions.

Psychrometric chart

This chart is used for finding the relative humidity of air which has been measured using a 'wet and dry bulb' thermometer. This is a pair of thermometers, one of which has its bulb wrapped in a damp cloth. The drier the air,the greater the evaporation of water off the cloth and therefore the lower the reading on the 'wet bulb' thermometer.

Typical system

The core components of the system such as the oil seperator, filter drier and condenser are dealt with on the Fridge system page, instead described are those components which are generally unique to air conditioning plant.


May be reciprocating or rotary. In nearly all cases a method of varying the quantity of delivery is incorporated. For reciprocating compressors this may take the form of an unloader and for rotary variable speed drive.


The compressors have protection systems similar to their fridge counterparts with High Pressure and Low Pressure cut outs that require manual resets. In addition to this an interlock is fitted so that the compressor cannot be started if the air handling unit fan is not running. Should the fan be stopped the compressor will cut out.

An alternative to this is to fit solenoid valves before the compressor, as in the diagram above, which open only when the fan is running. The compressor will trip on Low suction pressure.

The purpose of both these systems is to prevent liquid returning to the compressor.

Air Handling Unit

One or more is fitted. In the diagram above a single unit contains two individual evaporators which are independently supplied by a compressor. A belt driven fan delivers air to the evaporators via a fine mesh air filter. This filter is removed on a regular basis and washed in a soapy solution containing disinfectant.

The air passes over the evaporator where it is cooled and releases water vapour. The water condenses and is fed away via a drip tray and pipework, the water is quite clean and can be used for domestic purposes after treatment although this practice is not common. On the above design a catcher has been fitted to remove water droplets entrained in the air, these are not always fitted.

A perforated pipe is fitted after the evaporator allowing low quality steam to be fed into the air improving its humidity when too dry.

Contamination of ships air conditioning systems by legionnella bacteria

Legionnaires disease is caused by bacteria which flourishes in stagnant water or sludge . It can also be found in wet matrix filters, which may be found in the ships filtration system for the air conditioning plant.

Main danger areas

Air inlet arrangements-This may be direct or indirect from the air conditioning room via jalousies, which, when incorrectly designed may lead rain water onto the filters. It may also be allowed to accumulate in the space where drainage is not efficient

Filters-These filters made of a 25mm thick synthetic material can trap water as well as insects and soot and provide a rapid growth area for the bacteria. Regular washing is essential

Cooler unit (dehumidifier)-Ineffective drainage can allow water to stagnate in the catchment sumps. Also, where air velocities are high over the block, air can become entrained and carry moisture into the air stream. An efficient moisture eliminator is required.

Humidifier-Steam humidifiers, where fitted, do not appear to be a problem. However, adiabatic humidifiers of which the water spray type appears to offer a special hazard. The enclosed tank and matrix elements provide an ideal breeding ground for the bacteria which may then be carried into the air stream when sprayed.

Plenum Insualtion-Where the PVC GRP facing of the rock wool insulation and sound deadening breaks down the considerable levels of water may be present.

Recommended countermeasures.

Filters-Should be washed in 50ppm solution on a regular basis

Coolers-Special attention to drainage arrangements as well as superchlorinating the condensate sump every 3months.

Plenum insulation-Insulation to be examined at refit and damaged areas resealed.