Tag Archives: Prevent Liquid Flow Back

MEO Orals Questions on Air Conditioning System on Ships

What are the objectives of air conditioning on ships ?

  1. To extract excess heat
  2. To raise air temperature when required
  3. To add moisture as required
  4. To reduce moisture content as required
  5. To maintain sufficient oxygen and air flow
  6. To remove dust

How control of temperature is attained ?

  • Comfortable temperature range is about 22 °C and relative humidity (RH) about 60% (usually 40 ~ 70%)

All zone temperature

  1. Controlled by compressor suction pressure, via solenoid valve as step controlling.
  2. Thermostat, placed at some accommodation space actuates the Master Solenoid Valve of the plant, which will stop the Compressor, when preset temperature is reached
  3. Capacity unloader of compressor units, does last step controlling, as required

Particular zone temperature

  1. Controlled by flap valve fitted in each zone loop
  2. Local cabin temperature can be adjusted by volume control at delivery point of air duct controller.

Write a note on Ozone depletion ?

  • Ozone is gas; between 15 & 30 kilometer above the surface of earth.
  • This layer controls our climate & protects us from radiation.
  • The release of industrial waste and other process are now increasing the ozone breakdown to disturb the natural balance.
  • Pollutant (e.g. CFC) gas from ground level travel to atmosphere.
  • In the upper atmosphere Ultra Violet Ray breaks off Chlorine atom from the CFC molecule.
  • The free chlorine atom attack the one ozone molecule. Breaking it form to chloromonoxide. The remainder of the ozone molecule formed into regular oxygen molecule.
  • A free oxygen atom now steels the oxygen from chloromonoxide molecule form an oxygen molecule. Chlorine atom is again free, attack and break up an ozone molecule. In this way single chlorine atom destroy 1 million of ozone molecule.

Ozone depletion leads to:

  • More ultra violet radiation
  • Global warming
  • Increasing skin cancer
  • Immune system affected
  • Reduced forest production and crops
  • Sea ecology disturbed.
  • More green house gas
  • Warmer & more humid climate
  • More desert
  • Less forest
  • Higher sea level
  • Sea ecology system destroy

Refrigerant affected to Ozone layer R11 (CClF ),  R12 (CCl2F2), R22 ( CHClF2)

Refrigerant not affected to Ozone layerNH3, R134A

What are the leakage tests for various refrigerants ?

CO2 – Soap and water solution

NH– Wet litmus paper ( Red to Green ); Sulphur candles, which gives off  white dense smokes when contact with ammonia.

Freon  – Soap and water solution / Halide torch /Electronic leak detector (buzzer sound)

How to indicate the flame by halide torch ?

  • Content is methylated spirits type.
  • Butane can also be used.
  • When the leak is detected the flame will change from blue to green depending on the concentration of gas.

What is refrigeration ?

  • It is a process in which the temperature of a space or its contents is reduced to below that of their surrounding.

What is air conditioning ?

  • It is the control of temperature and humidity in a space together with the circulation, filtering and refreshing of the air.

What is ventilation ?

  • It is the circulation and refreshing of the air in the space without necessarily a change of temperature.

What is comfort zone ?

  • It is between the 40 to 70 % relative humidity at the temperature of 20 to 29 °C dry bulb temperature and air motion is 100mm per sec.

What are the advantages of secondary refrigerant ?

  1. Low initial cost
  2. Low maintenance cost
  3. Suitable for large refrigeration plant
  4. Easily produced on board by mixing CaCl2 and distilled water.
  5. Easily store as a salt on board

What is defrosting ?

  • A method of removal of frost, built-up on evaporator coils. Defrosting should be done before snow thickness exceeds ¼”.

What are the reasons for defrosting ?

  • Affecting heat transfer properties
  • Affecting air flow and circulation
  • Liquid back to compressor

 What are the defrosting methods ?

  • By stopping the system
  • By washing with warm water
  • By means of electric heater coil fitted at the evaporator
  • By hot gas defrosting method

How to defrost brine system ?

Hot brine thawing

  • Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes, collect the dripping water.

Hot air from atmosphere

  • It is important that isolating doors in air trunks are perfectly tight, so as to prevent hot air going into cargo spaces.

By shutting off brine

  • Allow the snows to be melted by the heat of the air in circulation .Very slow operation and tends to throw back great deal of moisture into cargo space.

What is the purpose of ventilation for cargo hold ?

  1. To remove surplus heat and humidity
  2. To prevent the condensing of moisture on cargo or hull
  3. To remove gases produced in ripening process of some fruits and vegetables cargos.

What is short cycling ?

  • It is a condition of a compressor unit repeatedly running for a few second and then cutting out. This is the result of L.P controller.

Why high pressure cut out is fitted ?

  • It is fitted to shut down the compressor in the event of high pressure. After remedy the fault, it must reset manually.

Why fitted master solenoid valve for large plant ?

  • If compressor stops due to a fault, the master solenoid valve will close to prevent flooding by liquid refrigerant and possible compressor damage.

Explain about one method of refrigerant charging ?

Normally charging is made through the liquid charging valve at the high pressure side.

  1. Firstly, weighting the gas bottle.
  2. Connect the gas bottle and charging valve with the connection pipe.
  3. Before tightening the cap on charging pipe, open bottle valve to remove air in the pipe.
  4. Then tighten the cap and open bottle valve fully, charging valve is still closed.
  5. Change compressor to manual running and start it.
  6. Close the condenser outlet valve.
  7. Pumping down the entire charge to the condenser.
  8. Open the charging valve slowly when suction pressure just above zero.
  9. Control the valve opening slowly that no frost formed on the compressor suction pipe.
  10. Check the level in the condenser sight glass.
  11. Close the charging valve and pumping down the entire charge until suction pressure just above zero.
  12. Stop the compressor and close the discharge valve.
  13. Cooling water kept running for some hour.
  14. Then air is purged out through purging valve on condenser until the refrigerant gas appear at the valve.
  15. Calculate the amount of refrigerant (charging) and enter the engine log book.

 Why super heat is required at evaporator outlet ?

  1. To prevent the liquid refrigerant entering into the compressor.
  2. If no super heat, hammering may happen and the valve will suffer damage and breakage.

How to fill fridge plant compressor oil ?

  1. Change the compressor to manual running.
  2. Pumping down the entire charge to condenser.
  3. Connect the L.O hand pump to L. O filling valve after air is purged out.
  4. When compressor suction pressure just above zero, open the oil filling valve, inject the L.O into crank case.
  5. Then stop the compressor and close compressor discharge valve. Then cool down the refrigerant.
  6. Then purged out the air through the purging valve until refrigerant appears at purging valve.

What are the types of expansion valve ?

  • Thermostatic control
  • Electronic control
  • Manual control

How to prevent liquid flow back to compressor ?

  1. Liquid shock valve (on cylinder head)
  2. Thermostatic Expansion Valve
  3. Master solenoid valve (when the plant is standstill, especially in Large plant)
  4. Defrosting
  5. Bursting disc (on cylinder head, between inlet and discharge manifold)

Why back pressure valve is fitted ?

  • It is fitted at the outlet of vegetable room to prevent under cooling of cargo.


Oral Guide by – MIN ZAR TAR