Marine Engineering Study Materials

What are the safety devices in refrigeration system on ships?

1. L. P cut-out on compressor suction side:  Set at a pressure corresponding to 5 °C below the lowest expected evaporating gauge reading
2. H.P cut-out on compressor discharge side:   Set at a pressure corresponding to 5 °C above the highest expected evaporating gauge reading
3. Lube oil low pressure cut-out:  Oil pressure usually set at 2 bar above crankcase pressure
4. Cooling water L .P cut-out in condenser side
5. Safety  spring loaded liquid shock valve on compressor cylinder head
6. Bursting disc on cylinder head, between inlet and discharge manifold
7. Bursting disc on Condenser, [if fitted]
8. Relief valve on Condenser; air purging valve on condenser
9. Master solenoid valve : to prevent liquid being entered into Compressor, when the plant is standstill, especially in Large Plant

Why Refrigeration Compressor Takes Suction from Crankcase ?

See “Suction From Crank Case”

What is critical temperature ?

• Critical temperature is the temperature above which it is impossible to liquefy the refrigerant regardless of pressure.
• It should be as high as possible and certainly above the maximum expected condenser temperature.

What is the primary refrigerant ?

• It is a medium which is used in a vapour compression cycle to remove heat from a low temperature region and convey it to a high temp: region
• It undergoes change of physical state during its working cycles.
• Mostly volatile liquid and employed inside direct expansion closed system.
• Evaporate at low temperature and at reasonable pressure
• Condense at SW temperature at reasonable pressure

What is secondary refrigerant ?

• It is a substance which is transferred heat by conduction and convection.
• Brine is a secondary refrigerant with density 1250 kg /m3
• It is a non volatile liquid during its working cycle and employed at large, complex installation, to avoid circulation of expensive primary refrigerant in large quantities.
• Cooled inside Refrigeration Machinery Room and pumped around the ship to batteries in each cargo space

What is solenoid valve is fitted in refrigeration system on ships ?

• Solenoid valve is a liquid stop valve.
• The valve is either fully open and close.
• To control the cool room temp: within set limit
• It is a electro magnetic cut in cut out device when the room temperature reaches the desire point, the pressure switch which is connected to the thermo bulb in the room shut the solenoid valve which stop the refrigerant flow into the evaporator.

What is the function of thermostatic expansion valve ?

1. To control the flow amount of liquid refrigerant to the evaporator in such a way as to keep practically the entire coils filled with the evaporating refrigerant.
2. To keep constant super heat on the refrigerant vapour leaving the evaporator.
3. For sudden drop in pressure and temperature.
4. Automatic and prompt responds of refrigerant flow to match the Evaporator Load.
5. Prevention of Liquid Refrigerant being entered into the Compressor.
6. Maintaining appropriate amounts of refrigerant in the high pressure side and low pressure side of the system.

 What is relative humidity ?

• Ratio of amount of water vapour in given volume of air, to maximum amount of water vapour that can be present before precipitation occurs.
• It is the mass of water vapour per unit volume of air compared to the mass of water vapour per unit volume of saturated air at the same temperature.

What is saturated air ?

• The air contains maximum weight of water vapour at the particular temperature.

What is super heating ?

• Keeping refrigerant at a temperature above the saturated temperature.

What are the effects and remedies of under charge in refrigeration ?

Symptoms

1. Low discharge pressure
2. Large bubbles in the liquid sight glass
3. Compressor delivery temperature high.
4. The machine runs for longer periods between cut out.

Remedies

1. To charge the refrigerant until the bubbles disappears in the liquid sight glass and charging will correct the pressure gauge reading.
2. To find out the leaky points by halide torch or soap solution or electronic leak detector.

What are the effects and remedies of over charge in refrigeration ?

 Symptoms

1. It is indicated by high compressor discharge pressure.
2. Compressor runs cold.
3. Excess frost on compressor suction line after evaporator.
4. Full of sight glass.

Remedies

• The excess refrigerant is released to atmosphere through the purging valve fitted on the condenser until discharge pressure return to normal.

What are the effects of air in refrigeration system ?

Symptoms

1. High condenser and compressor temperature.
2. Abnormally high discharge pressure
3. Jerky pointer gauge
4. Very small bubbles in liquid sight glass.

Remedies

To remove the air from the system:

1. Change the compressor to manual position
2. Closed the liquid stop valve after the condenser
3. Pumping down the entire charge into the condenser
4. Until the suction pressure just above zero.
5. Stop the compressor and closed the consider inlet valve
6. Then allowed to cool the consider contents.
7. Air is expelled through the purging valve until the refrigerant gas appears at the valve.

What are the effects of moisture in refrigeration system on ships ?

Symptoms 

1. Starving of evaporator and rapid condenser pressure rise cause compressor short cycling.
2. Frost at expansion valve inlet.

Remedy

1. Clean expansion valve filter.
2. Renew drying agents of dehydrator or drier.

Cause of expansion valve chocking ?

When a valve is functioning correctly frost forms on the outlet side of the valve, and if the inlet side does show frosting then this is a sign of blockage.

• It is caused by dirt or freeze up by water present in the system.
• Starving of evaporator and rapid consider pressure rise cause compressor short cycling.

Remedy

• Clean the expansion valve filter
• Renew the drying agent of dehydrator or drier. (activated alumina or silica gel )

What are the effects of oil in refrigeration system on ships ?

Symptoms

1. Incorrect condenser and evaporator temperature differential.
2. The system running longer than normal.
3. Difficult to cool down room temperature.

Remedy 

1. Pumping down the entire charge to condenser.
2. Along the system must be blown out with compress air by stripping individual unit component.
3. Repair the oil separator.
4. Renew the compressor piston rings.

What are the indications of leaky suction and discharge valves in refrigeration system ?

Defective Suction valve

1. Continuous running of compressor
2. Insufficient cooling effects
3. Noisy operation
4. High suction pressure

Defective Discharge valve

1. Continuous running of compressor
2. Insufficient cooling effects
3. Noisy operation
4. High suction pressure during running
5. Low discharge pressure during running
6. Suction pressure increase faster after compressor is shut down
7. Warm cylinder head

 High pressure (HP) cut out and low pressure (LP) cut out in refrigeration system on ships ?

HP cut out

• Occurs due to condenser coolant failure, low coolant  pressure and malfunction of consider cooler valve operation.
• Also can cause dirty or chocked condenser tubes.
• The refrigerant can not liquefy rapidly .The discharge pressure will abnormally raise and high pressure cut out will take in action.

LP cut out

• Operate the solenoid valve is closed when the cool room temperature reach its desired temperature (setting temperature)
• Can cause when expansion valve is chocked by dirt or freeze up by water present in the system.
• Starving of evaporator and rapid condenser pressure rise.
• Compressor may stop by low pressure cut out.

 

Reference:

Oral Guide by – MIN ZAR TAR