1. Safety devices on refrigeration system
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
2. What is critical temperature ?
Critical 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.
3. 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.
1) Mostly volatile liquid and employed inside direct expansion closed system.
2) Evaporate at low temperature and at reasonable pressure
3) Condense at SW temperature at reasonable pressure.
4. What is secondary refrigerant ?
It is a substance which is transferred heat by conduction and convection. Brine is a secondary
refrigerant .Its density 1250 kg /m2
1) It is a non volatile liquid during it working cycle and employed at large, complex installation, to avoid circulation of expansive Primary Refrigerant in large quantities.
2) Cooled inside Refrigeration Machinery Room and pumped around the ship to batteries in each cargo space.
5. What is solenoid valve? How to operate? Why fitted ?
@ Solenoid valve is a liquid stop valve.
@ The valve is fully open and close.
@ To control the cool room temp: within set limit
@ It is a electro magnetic cut in cut out devices when the cool room temp reaches the desire point,the pressure switch which is connected to the thermo bulb in the cool room shut the solenoid
valve which stop the refrigerant flow into the evaporator
6. 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) It sudden drop in pressure and temperature
@ Automatic and prompt responds of refrigerant flow to match the Evaporator Load.
@ Prevention of Liquid Refrigerant being entered into the Compressor.
@ Maintaining appropriate amounts of Refrigerant in the High pressure side and Low pressure side of the system.
7. 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.
8. What is saturated air ?
The air contains maximum wt of water vapour at the particular temperature.
9. What is super heating ?
Temperature above which the saturated temp:
10. UNDER CHARGE
Symptoms
@ Low discharge pressure
@ Large bubbles in the liquid sight glass
@ Compressor delivery temp: high.
@ The machine runs for longer periods between cut out.
Remedies
@ To charge the refrigerant until the bubbles disappears in the liquid sight glass and
charging will correct the pressure gauge reading.
@ To find out the leaky points by halide torch or soap solution or electronic leak detector.
11. OVER CHARGE
Symptoms
@ It is indicated by high compressor discharge pressure.
@ Compressor runs cold.
@ Excess frost on compressor suction line after evaporator.
@ 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.
12. AIR IN THE SYSTEM
. Symptoms
@ High condenser and compressor temp:
@ Abnormally high discharge pressure
@ Jerky pointer gauge
@ Very small bubbles in liquid sight glass.
Remedies
To remove the air from the system –
@ Change the compressor to manual position
@ Closed the liquid stop valve after the condenser
@ Pumping down the entire charge into the condenser
@ Until the suction pressure just above zero.
@ Stop the compressor and closed the consider inlet valve
@ Then allowed to cool the consider contents.
@ Air is expelled through the purging valve until the refrigerant gas appears at the valve.
13. MOISTURE IN THE SYSTEM
Symptoms
> Starving of evaporator and rapid condenser pressure rise cause compressor short cycling.
> Frost at expansion valve inlet.
Remedy
> Clean expansion valve filter.
> Renew drying agents of dehydrator.
14. 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. (activated alumina or silica gel )
15. OIL IN THE SYSTEM
Symptoms
> Incorrect condenser and evaporator temperature differential.
> The system running longer than normal.
> Difficult to cold down room temperature.
Remedy
# Pumping down the entire charge to condenser.
# Along the system must be blown out with compress air by stripping individual unit component.
# Repair the oil separator.
# Renew the compressor piston rings.
Defective Suction valve
Indication
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 faster after compressor is shut down
7) Warm cylinder head
16. How do take in action high pressure cut out and low pressure cut out ?
H. P pressure 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.
Low pressure cut out
> Operate the solenoid valve is closed when the cool room temp: reach its desired temp:
(setting temp:)
> 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.
17. Leakage Test for Refrigerant
CO2 – Soap and water solution
NH3 –Wet litmus paper (Red ~ 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)
By halide torch, How to indicate the flame ?
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.
It is a process in which the temperature of a space or its contents is reduced to below that of their surrounding.
19. 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.
20. What is ventilation ?
It is the circulation and refreshing of the air in the space without necessarily a change of temperature.
21. Comfort zone ?
It is between the 40 to 70 % relative humidity at the temperature of 20 to 29C dry bulb temp:,
air motion is 100mm per sec.
22. Advantages of secondary refrigerant ?
- Low initial cost
- Low maintenance cost
- Suitable for large refrigeration plant
- Easily produce onboard by mixing Ca Cl2 and distilled water.
- Easily store as a salt onboard
Defrosting
A method of removal of frost, built-up on Evaporator coils. Defrosting should be done before snow thickness exceeds ¼”.
Reasons for defrosting
> Affecting heat transfer properties
> Affecting air flow and circulation
> Liquid back to compressor
23. 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
Defrost brine System
Hot brine thawing : Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes, collected 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 heat of the air in
circulation .Very slow operation and tends to throw back great deal of moisture into cargo space.
Notes:
Direct expansion grid system : Hot gas defrosting
Battery cooling system : Water spray, electrical or steam heater
Brine cooling : Hot brine thawing
24. Purpose of ventilation for cargo hold ?
@ To remove surplus heat and humidity
@ To prevent the condensing of moisture on cargo or hull
@ To remove gases produced in ripening process of some fruits and vegetables cargos.
25. 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.
26. Why fitted high pressure cut out ?
It is fitted to shut down the compressor in the event of high pressure .After remedy the fault, have to run necessary to reset.
27. 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.
28. Explain about one method of refrigerant charging ?
Normally charging is made through the liquid charging valve at the high pressure side.
01). Firstly, weighting the gas bottle.
02). Connect the gas bottle and charging valve with the connection pipe.
03). Before tightening the cap on charging pipe, open bottle valve to remove air in the pipe.
04). Then tighten the cap and open bottle valve fully, charging valve is still closed.
05). Change compressor to manual running and start it.
06). Close the condenser outlet valve.
07). Pumping down the entire charge to the condenser.
08). Open the charging valve slowly when suction pressure just above zero.
09). 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.
29. Why super heat is required at evaporator outlet ?
@ To prevent the liquid refrigerant entering into the compressor.
@ If no super heat, hammering may happen and the valve will suffer damage and breakage.
30. How to fill fridge plant compressor oil ?
01) Change the compressor to manual running.
02) Pumping down the entire charge to condenser.
03) Connect the L.O hand pump to L. O filling valve after air is purged out.
04) When compressor suction pressure just above zero, open the oil filling valve, inject the L.O into crank case.
05) Then stop the compressor and close compressor discharge valve. Then cool down the refrigerant.
06) Then purged out the air through the purging valve until refrigerant appears at purging valve.
31. Types of expansion valve.
- Thermostatic control
- Electronic control
- Manual control
32. Prevention of Liquid Flow Back
2) Liquid shock valve (on cylinder head0
3) TEV
4) Master solenoid valve (when the plant is standstill, especially in Large plant)
5) Defrosting
6) Bursting disc (on cylinder head, between inlet and discharge manifold)
Back pressure valve at the outlet of vegetable room to prevent under cooling of coargo.
Next Read : Stern Tube Sealing Arrangement.