General Description and Definition
Air conditioning systems serve to modify the outside fresh air to improve the ship's interior environment. This is accomplished by heating, cooling, dehumidifying, and contaminant removal processes. These processes may be used singularly or in combination to achieve the required environmental conditions.
Air conditioning is used almost exclusively for living spaces such as staterooms, messrooms, offices, lounges, and other public areas. However, many items of electronic equipment must also be maintained at a controlled temperature and humidity. Conditioning of the air is accomplished by a cooling medium of chilled water or freon and a heating medium of steam, hot water or electricity and is designed to permit simultaneous heating and cooling as may be necessary to satisfy specified design conditions.
De-humidification or humidity control may be accomplished by cooling or through the use of desiccant dryers. Contaminant removal is accomplished by means of filtering, adsorption, electrostatic charging or absorption. The conditioned air is distributed to the spaces served by the same network of ducts used for space ventilation systems.
Centralized air conditioning systems may be generally categorized by function such as: Systems which provide a combination of heating and cooling, systems which provide cooling only, systems which provide heating only, and systems which provide dehumidification only.
Various components are included within the ductwork system for the transmission of air to and from the interior compartments of the ship.
Types of Fans
Axial flow fans are used widely because of compactness and high efficiency and are well adapted for ventilation of cargo spaces, machinery spaces, and other places where noise is not a significant consideration. Centrifugal fans are used for ventilation where quiet operation is desired and also for galleys, battery room exhaust, and areas where explosive vapors arc removed, where the motor is not to be located in the air stream. Propeller fans are used in bulkhead installations and sometimes in a cowl for machinery space supply and exhaust systems where the pressure required is small.
For maintenance, fans must be located for easy accessibility to the motors. Many motors are provided with two-speed controls to permit reduction in supply air during cold weather. Motors are selected for 40 °C ambient temperature except when located where high temperatures prevail, in which case they may be selected for 50 °C or 65 °C ambient temperatures. Axial and propeller fans generally are provided with waterproof or totally enclosed motors.
The interior environment aboard ship is controlled to provide an atmosphere which is agreeable to the operating personnel, machinery and equipment, cargo and ship stores, and passengers. The environment may be modified by means of ventilation, heating, cooling, and dehumidification or by any combination of these means. Additionally, the elements of noise and vibration must be controlled. The means of controlling these environmental factors must be as reliable, simple, and maintenance free as practical, consistent with the desired results.
Ventilation is the process used to provide fresh outside air to various spaces within the ship. The air is distributed by means of a duct network and suitable weather openings in the ship's envelope. The type of ventilation used depends upon the nature of the space and the service of the ship. The fresh air may be supplied by natural draft or mechanical means and is provided for the removal of heat, noxious or explosive vapors, and to assure an adequate supply of oxygen to personnel. The quantity of air required for each space ventilated is determined by heat transfer or empirical calculations.
This article continues to discuss the shipboard alarms and protection devices. And now, let us look at sensors starting with level indicators. These monitor liquid level in fuel tanks, lube oil tanks, boilers and the like. The simplest systems are activated by the liquid itself. The device is activated by the level of fluid rising or sinking above or below the set safety levels. When this occurs, a sensor sends a signal to the warning devices, coupled usually with the indicator. One of the most important level alarms on board ship monitors boiler water level. It is critical to maintain right water level in boilers, especially in high-pressure water tube boilers to ensure safe operation.
A very dangerous situation can rapidly arise if the water quantity inside the boiler is not maintained at the correct level. There are visual level indicators as well as alarm systems monitoring the water level to ensure that the level is within set safety limits. Once we are dealing with boilers, we must mention a unique sensor in the alarm circuit known as the flame-out alarm system. A sudden loss of flame inside the boiler can lead to many undesirable consequences. Therefore, optical systems, or flame eyes, provide continuous monitoring of presence of fire and will give instant warning should the flame go out...
We all understand that the failure of the shipboard machinery may and will result in avoidable and expensive disasters. Do they occur due to bad design, inadequate maintenance or incorrect operational procedures? Experience shows that a large number of machinery breakdowns occur due to a mistake in operation and maintenance of the alarm systems and protection devices.
Yes, engines, generators, boilers, compressors do break down, sometimes fatally damaged because their alarm systems and protection devices monitoring them have been prevented from working, sometimes deliberately disconnected, sometimes clogged up with paint and forgotten, abandoned without checking for long periods of time.
This, of course, is asking for trouble, when only a little attention by duty engineers could avoid an unforeseen incident resulting in possible injuries to people, machinery breakdowns, upsetting the commercial operation of the vessel, leading to extra work load for ship's personnel. Things can go wrong at any time.
If a pump draws in air while in operation the noise is produced and the amount of sea water discharged decreases, significantly lowering the pump's operating efficiency. Also, the amount of sea water being discharged will drop to zero in an instant once a pump is drawing a large amount of air. Why? When a small amount of air is drawn into the pump, the air passes out of the pump together with the sea water.
But, if a large amount of air has entered a pump, it gathers and gradually spreads to the pump periphery, decreasing the centrifugal force the sea water receives from the impeller and disabling the pump from discharging. If the discharge amount is extremely small, there are cases where unnoticeably small amounts of air gather within the pump and build up to a level that could disable the pump. If left as it is, sea water in the pump will run out resulting in burn.
There are several ways to prevent air drawing. Purge air thoroughly before starting the pump. Decrease the suction flow from the bell mouth by reducing the discharge valve opening to decrease the discharge amount. Note, beforehand, the water level at which pump begins to draw air and stand on the alert as the water approaches the critical level. You may use other tanks with a suitable water level to continue discharging. With some ships, cargo planning is based on the hull with trim at the stern. It is the suitable condition to discharge all the remaining ballast.
First, let us look at what ballast operations are and their purpose. Since the ship's draft is shallow while in ballast condition, it is susceptible to wind and wave impact - as a result, rolling increases and hull stability deteriorates. To improve hull stability, the ship's ballast tanks are filled with sea water. When loading the ship with cargo, sea water is discharged. Such sea water loading and discharging are called ballast operations which are conducted either by gravity or ballast pumps. Improper handling of pumps and/or valves may result in various accidents.
Valves are provided where the pipeline is connected to ballast tanks. The pipeline is fitted with many such valves. Improper valve opening or closing can cause accidents. Improper valve opening or closing can cause accidents. This is an example of an accident caused by valve rupture. In this accident, corrosion of the valve body combined with the sudden pressure change due to pumped stuck ruptured the valve body during the stroke. To prevent such accidents, you must have good understanding of the procedures for proper handling of pumps and valves.