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Air conditioning basic standards for cargo ships and general guideline

Air conditioning: A very significant factor affecting an air conditioning system is the rapidly changing climatic conditions. The equipment has to perform within these variations and has to meet the differing requirements of the occupied spaces of the ship.

The early air conditioning systems were rather bulky because designs were based on low air velocities in the distribution ducts, with velocities in the order of 10 m/s or less. In later years there was extensive standardization with very substantial increases in air velocities, reaching a maximum of about 22.5 m/s in the ducts and producing a large reduction in the space occupied by the equipment. Increased operating costs as a result of higher velocities have to be set against reduced installation costs and the value of space saving, but the owner is usually disposed favourably towards the high velocity system with its lower initial cost.

Basic standards

The designer and user of air conditioning plant must study the physiological factors involved. The terms used to define the atmospheric conditions are fairly well known, but are reviewed here since it is essential to know exactly what they mean before proceeding further.

Dry bulb (d.b.) temperature is the temperature as measured by an ordinary thermometer which is not affected by radiated heat.



Wet bulb (w.b.) temperature is the temperature registered by a thermometer with wetted fabric around the bulb. (When moisture evaporates from a surface, i.e. the skin, the latent heat required, is drawn from the surface causing it to be cooled. If a thermometer bulb is covered by a wetted fabric and exposed to the air, the rate of evaporation will depend upon the humidity of the surrounding air. As the heat required must come from the bulb, this results in a lower temperature reading than if the bulb was dry.)

Psychometric chart or table is used to find relative humidity from dry bulb and wet bulb readings taken at the same location in a space. (The thermometers may be in a fixed position or in a football rattle type device.)

Relative humidity (r.h.) of the air indicates the amount of moisture carried by the air at a particular temperature as a percentage of the maximum amount that could be carried at the particular temperature. (The capacity of the atmosphere to hold water vapour is dependent upon its temperature. At higher temperatures this is much greater than at the lower temperatures. When the maximum is reached at a given temperature, the air is said to be saturated. Saturated air has 100% relative humidity.)

Dewpoint (d,p.) is the temperature to which unsaturated air must be cooled to bring it to saturation point and to cause moisture to precipitate. (If an unsaturated mixture of air and water vapour is cooled at constant pressure, the temperature at which condensation of water vapour begins is known as the dewpoint. Moisture from air starts to condense on a cold window or glass when the air near the cold surface reaches its dewpoint. As the air is further cooled, more moisture is deposited.)

Ideal conditions

The condition of the air in a space depends on its temperature, humidity and movement. The effect of the air on people in a space, is dictated by their metabolism, state of health, acclimatization, degree of exertion and the amount of clothing being worn.

The ideal conditions for comfort vary considerably between one person and another, so it is only possible to stipulate a fairly wide zone. In this connection it would obviously be of great value if a single index could be used to define the physiological reaction to the various combinations of factors involved. Among other suggestions the most satisfactory has been the effective temperature index, It is the temperature of still, saturated air which would produce the same feeling of warmth.

ld obviously be of great value if a single index could be used to define the physiological reaction to the various combinations of factors involved. Among other suggestions the most satisfactory has been the effective temperature index, It is the temperature of still, saturated air which would produce the same feeling of warmth. The American Society of Heating and Air Conditioning Engineers carried out a comprehensive series of tests on a large number of people, from which they were able to draw up an effective temperature chart.

The comfort chart in Figure 12.1 is based on the results. Taking as an example 27UC d.b. and 50% r.k, for zero air speed, the effective temperature is found to be 23.4°C, which is within the summer comfort zone. With an air speed of 1.0 m/s, the effective temperature is reduced to 22.2°C The higher limit of comfort is more critical than the lower as it is closely associated with the essential process of getting rid of body heat. Also it is of considerable significance to the air conditioning engineer, who has to fix the capacity of the refrigerating plant so that it can provide conditions within this limit when the outside conditions are at their most onerous.

Protracted investigations have been made by Hall-Thermotank Ltd., to determine the effective temperature which could be said to represent the upper threshold of the comfort zone. The reactions of a large number of persons, mostly mariners, some of whom had just completed a voyage, were analysed. For a person dressed in tropical clothing and at rest, the threshold value was found to be about 25.6°C effective temperature.



Figure : Comfort chart compiled from tests carried out by the American Society of Heating and Air Conditioning Engineers

Tests showed that most persons tended to sweat when the temperature rose a degree or so above this value, and to cease sweating at the same value as the temperature fell again. The conclusion was therefore reached that there could be a relationship between the threshold of comfort and the onset of sweating, This does not take into account differences in the thermal sensations of men and women due to metabolism, general health and other factors mentioned previously. Conclusions from the study indicated that in the main, the comfortable level of warmth for acclimatized persons of all races is very similar in spite of certain differences in the reaction to heat stress.

At room temperatures above 21°C an air velocity of 0.15-0.2 m/s is desirable, to avoid any feeling of stuffiness, and to provide proof that the space is being ventilated. On the other hand, velocities higher than 0.35 m/s are usually classed as draughts — to be avoided particularly for the person at rest. When a space is heated or cooled, it is impossible to ensure an absolutely uniform distribution of the effect throughout the space. Because warm air rises, the air can be appreciably warmer at the higher levels than at the deck level, giving rise to discomfort unless the air terminals are designed to counteract this effect.

Radiation, planned or otherwise, can contribute to the effective temperature of a space, but in ships' accommodation it is not significant, even in cabins adjacent to engine room bulkheads given high standards of insulation,



Summarized below some of the basic procedure of marine air conditioning system :

1. Types of air conditioning system

Air conditioning systems may be divided into two main classes — the central unit type in which the air is distributed to a group of spaces through ducting, and the self-contained type, installed in the space it is to serve. .....


2. Marine air conditioner

A very significant factor affecting an air conditioning system is the rapidly changing climatic conditions. The equipment has to perform within these variations and has to meet the differing requirements of the occupied spaces of the ship......


3. Air conditioning central unit

The elements of a central unit are fan, filter, cooler, heaters and plenum chamber. Normally these are all housed within a single casing, with the possible exception of the fan. It is possible to carry this further by including the refrigerating plant in a single assembly thus providing a complete package. ....


4. Air conditioning air terminals

The best designed air conditioning system is only as good as the means of delivering the air to the spaces. The main function of the air terminal is to distribute the air uniformly throughout the spaces without draughts......


5. Heating and cooling loads and air quantities

Outside air must be introduced to all living spaces, although the amount of fresh air necessary to sustain life is very small indeed. Space conditions can vary greatly in a short time. They are governed by factors such as body odours and smoking, which may require a fresh air supply of 12 litre/s per person or more. ...


6. Air conditioning unit typical specification

To prevent excessive leakage of conditioned air, all doors leading from the conditioned spaces to the outside atmosphere, machinery casing, etc., should be of the self-closing type and reasonably airtight......

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