Answer MEP Question 5
Q. With the aid of a simple sketch, explain the “trouble spots” in a basic air-conditioning unit with reference to your sketch, explain the following: -
i. How the problem of increase in humidity on cooled air is overcome?
ii. How discomfort caused by the excessive drying effect of heated air is overcome?
With the aid of a simple sketch, explain the “trouble spots” in a basic air-conditioning unit.
Usual Trouble spots
Condenser issues - Seawater supply and return issues due to pump tripped, filter clogged, low flow, insufficient cooling water flow due to increase in seawater temperature, or over
cooling etc.,
Setting or electrical issues: Usually the Setting is altered or the circuit breaker trips, malfunctions etc., and after a voltage surge or a blackout the system needs to be checked.
Filter clogging: The filters are one of the main trouble spots in an AC system Where the air intake and the re-circulation filters are usually clogged due to excessive dirt which causes the poor flow of air leading to icing, poor heat transfer, short cycling etc.
Refrigerant leaks - In the drawing the thicker line connecting compressor, oil reclaimed, condenser, dryer, evaporator and return may leak refrigerant due to a damaged pipe or a worn-out pipe. Compressor shaft sealing may also develop leaks.
Evaporator coil - Issues on the evaporator coils are evaporator coil dirty or damaged, icing, failure of de-frosting due to timer fault or ice sensor’ fault etc.
Basic operation
- The air conditioning system on the ship has a refrigeration cycle, which is the same as the refrigeration system.
- The compressors, condensers, expansion valves, filters, dryers etc. are exactly the same as any other refi-iteration unit. However, the evaporator system for the air conditioning system is called an air-handling unit or AHU for short.
- Usually, the ships are fitted with accommodation air conditioning plant capable of meeting the
following conditions:
- The above conditions are based on a maximum of 70% recirculation of air for cabins and suites, and 50% recirculation for public rooms. The hospital and service spaces are supplied with 100% fresh air. In order to maintain these conditions, it is important that all external doors and windows should not be left open.
- An AHU controls temperature, humidity, dampens sound, cleans the air. Mixes fresh air in a controlled amount to enable air changes and provides for the circulation of air through the accommodation.
- It consists of a blower driven by a motor. These blowers take suction of air either from the atmosphere or from circulation or from the combination. This air is passed over felt filters to remove impurities
like dust dirt carbon particles.
-If the air condition system is used under cooling mode then air passes over the evaporator coil through which liquid ref is circulated by the refrigeration system.
A typical refrigeration system consists of:
- Compressor
- Condenser
- Receiver
- Evaporator
- Solenoid valve
- Expansion valve etc.,
- If the air conditioning system is used on heating mode then the ref system is put off and the air is heated by passing over steam coils. This air is distributed via distribution channels to the different decks.
- Air enters the cabin from the top and cold air being heavier comes down and after cooling room is removed from the cabin door into alleyways. From the alleyways it is sucked back to the recirculation system.
i. How the problem of increase in humidity on cooled air is overcome;
- Relative humidity (RH.) of the air indicates the amount of moisture carried by air at a particular temperature as a percentage of the maximum amount that could be carried at the particular temperature.
- As the temperature of the air is reduced, its capacity for carrying water vapor is also significantly reduced.
-With the aid of a psychrometric chart it can be shown that air, with a temperature of 36°C relative humidity of about 60%, will, when cooled to 27°C, have a relative humidity of 100%.
- The temperature drop reduces the capacity of the air to carry moisture in suspension. Further cooling will cause moisture to be precipitated.
- Air-cooled to a comfortable temperature level of 21°C but having a relative humidity of 100% would not be able to take up further moisture.
- People in an atmosphere at 21°C with 100% relative humidity would be uncomfortable.
The remedy dehumidifying the air is achieved by overcooling to precipitate excess moisture, (removed via the drain) so that when air is brought to the correct temperature, its humidity will be at an acceptable level.
- Thus the air could be overcooled to about 1O°C temperature so that warming to about 21°C, would bring humidity to about 50%. The air is warmed in the trunking or by contact with warmer air in the space; a zone heater could be used.
- An adequate drain is required to remove what can be a considerable flow of water from the dehumidification of the air.
ii. How discomfort caused by the excessive drying effect of heated air is overcome.
- When the temperature of air is increased, its capacity for carrying water vapour also significantly increased.
- Using the psychometric chart, it can be Shown that air, with a very low initial temperature of — 5°C and relative humidity of about 50%, will, when heated to 21°C dry bulb temperature, have a relative humidity of about 10%.
- The temperature rise increases the capacity of the air to carry moistiire in suspension. Air
heated to a comfortable temperature level of 21°C but having a relative humidity of 10% will
readily take up moisture whether from perspiration or from the nasal passages and throat.
- People in an atmosphere at 21°C but 10% relative humidity, would experience discomfort from
dryness in their nose and throat and on the skin.
- The remedy is to humidify the air with a hot water or steam spray. This action increases humidity towards 100% relative humidity and also increases the temperature from — 5°C and 50% relative humidity to say, 4-7°C.
- Straight heating by the zone heater bringing the air to about 21°C will drop relative humidity to 40%. The humidity will be at an acceptable level but is kept low to minimize condensation on any very cold external bulkheads.
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