When selecting refrigerants for shipboard use, the following properties are considered ideal:
Low Specific Volume: Reduces the size requirements for compressors and piping.
High Critical Temperature: Allows condensation at higher temperatures, which is beneficial in varying marine climate.
High Latent Heat of Vaporization: Ensures efficient heat absorption during the evaporation process.
Chemical Stability: Resists decomposition under operating conditions.
Environmental Friendliness: Low ODP and GWP values are preferred to minimize ecological impact.
Non-Corrosive: Prevents damage to system components.
Non-Flammable and Non-Explosive: Enhances safety aboard ships.
Compatibility with Lubricants and Materials: Ensures smooth operation and longevity of the system.
Easy Leak Detection: Facilitates maintenance and prevents environmental harm.
Key Refrigerant Properties Comparison
Refrigerant
ODP
GWP
Pressure Level
Flammability
Typical Use
R-134a
0
1,430
Medium
Non-flammable
Marine AC
R-404A
0
3,922
High
Non-flammable
Freezers
R-407C
0
1,774
Medium-High
Non-flammable
AC/Refrigeration
R-410A
0
2,088
Very High
Non-flammable
High-efficiency AC
-744 (CO2)
0
1
Extremely High
Non-flammable
Eco-friendly systems
Note: The critical temperature of a substance is the highest temperature at which it can exist as a liquid, regardless of the pressure applied. Above this temperature, the substance cannot be liquefied by pressure alone and will exhibit properties of both gases and liquids.
Temp – com outlet- 70-90, condenser outlet- 38-45, TXV out- 0-10, evaporator out-7-15, Superheat- evaporator outlet 5-6 and compressor inlet around 10 degree Figure 4. T-S diagram for basic vapor compression cycle.
The cycle processes can be described as follows: 7-1 Evaporation of the liquefied refrigerant at constant temperature T1 = T7. 1-2 Superheating of the vapor from temperature T1 to T2 at constant pressure PL. 2-3 Compression (not necessarily adiabatic) from temperature T2 and pressure PL to temperature T3 and pressure PH. 3-4 Cooling of the super-heated vapor to the saturation temperature T4. 4-5 Condensation of the vapor at temperature T4 = T5 and pressure PH. 5-6 Subcooling of the liquid from T5 to T6 at pressure PH. 6-7 Expansion from pressure PH to pressure PL at constant enthalpy. A further difference between the real cycle and the ideal is that temperature T1 at which evaporation takes place is lower than the temperature TL of the cold region so heat transfer can take place. Similarly the temperature T4 of the heat rejection must be higher than the hot region temperature TH to bring about heat transfer in the condenser. It is usual for the vapor-compression cycle to be plotted on a pressure-enthalpy (p-h) diagram as shown in
Figure 5. p-h representation of vapor compression cycles.
Operational Leakages due to malfunction of the mechanical seals resulting :
a) Loss of oil.
b) Loss of refrigerant – the end clearance of the piston/scraper rings allows a small amount of refrigerant gas to reach the crankcase
1.2) Foaming can happen when –
a) Pressure drops rapidly in the evaporator.
b) Compressor high capacity to pull down pressure rapidly.
c) Crankcase space developing a low pressure condition.
With the formation of Low Pressure within the crankcase space :
– lubricating oil is unable to hold the small bubbles in the oil.
– small bubbles enlarge , attain buoyancy
– bubbles raise to the oil surface
Excessive amount of oil in the crankcase
Do not top up the oil level in the crankcase to excessive high level. It may cause :
Overloading of the OIL SEPARATOR
Oil passing to the condenser and the rest of system – hampering optimum heat transfer.
Always maintain the oil level at the recommended level as indicated on the sight glass.
Indication of refrigerant leakages:
Low refrigerant level in sight glass
Large bubbles in sight glass
Oil weeping at joint and connection
Relative lower pressure readings across the system (LP,OP & HP)
High superheat at compressor suction
Compressor running continuously – room temperature not reducing
Fall off in refrigerating effect (over a short period)
Refrigerant loss through valve stem gland packing, pipes, fittings compressor etc
Broken suction, discharge valves of compressor
Belt slipping – motor to compressor
Icing of expansion valve
HP cut out
Insufficient or intermittent water flow for condenser cooling
Relatively higher temperature of cooling water
Scaled or fouled condenser
Overcharging of refrigerant
Air in the system
Short cycling on LP cut out
Malfunction LP pressure switch
Evaporator coils heavily frosted
Strainer for TEV chocked
Leaky discharge valves
Deflective expansion valve
Refrigerant undercharge
Refrigerant undercharged
Large bubbles noted in sight glass
Lower LP , OP , HP pressure
Continuous running of compressor- room temperature not reducing
Relative less frosting on compressor suction line/valve
System performance drops
Refrigerant overcharged
Sight glass refrigerant level higher than normal
Higher LP/OP/HP pressure
Compressor stopping on HP cutout
Severe frosting on compressor suction line/valve – Malfunction of TEV
Moisture in the system
Frosting on inlet side of expansion valve
Low LP pressure
Corrective Actions : Renew the filter/drier.
Water (not removed by the filter/Drier) if present with the refrigerant at the
TEV will become ice – restricting proper refrigerant flow.
In a good working order refrigeration system, a thin layer of ice of about 2-4mm will be formed on the TEV (external body). The ice formation is due to the unavoidable “flash off” of liquid refrigeration when it passes through the orifice. The presence of an extra volume of ice formation on the TEV indicates that Excessive flash off is taking place.
Frost on evaporator coils
Compressor runs longer
Short cycling on LP switch
Performance drops
Low suction pressure – Refrigerant temperature drops to 0 degree celsius or lower
Frost coming back in evaporator coil continuously because of relative lower pressure existing in the coils. The low pressure could be due to refrigerant leakages, dirty filter/drier, dirty strainer of the TEV – any reasons that results in a lower than normal pressure within the coils. The pressure of the refrigerant directly affects the temperature of the refrigerant in the coil. The lower the pressure – the lower will be the temperature of the refrigerant. If the temperature of the refrigerant is near to or lower than or at the freezing point of water, high relative humidity air flowing pass the evaporator coil will cause ice to be built up persistently.
Remedy : Restore the working pressure of the LP side to the marker’s recommended value – bring the operating temperature of the refrigerant away from the freezing point of water.
Compressor drawing in refrigerant liquid
Usually is due to malfunction of TEV resulting :
Excessive frosting at inlet valve body of the compressor and/or icing on the cylinder head.
Oil level at compressor sump reduced
High suction pressure
Noisy compressor operation
Noisy compressor
Liquid knock / hammering*
Lack of lubrication
Internal components damaged
Air in the system
High condenser pressure (HP pressure)
Jumping of pressure gauges pointer
Compressor noisy
Small bubbles at sight glass
Relative small difference in cooling water in & out temperature – Less heat transfer
Air (or moisture) can be accidentally introduced into the refrigeration system during topping up of refrigerant into system or during topping up of lubrication oil for the compressor.
Air will finally be accumulated in the condenser.
1) Mainly nitrogen(78%) and oxygen(21%) – it is not possible to condense air with the cooling water.
2) The volume of air will occupy the space at the top of the condenser –
a) prevents refrigerant gas entering the condenser.
b) reduces the total cooling surface area for the refrigerant.
3) Result in
(a) higher pressure reading in the HP side of the system and
(b) reduction in temperature differential of the cooling water.Cooling water inlet temperature minus outlet temperature.
Removal of Air :
1) Connect a refrigerant recovery bottle to the purging cock (of the condenser) via a flexible hose.
2) Remove the air until the flexible hose is cold or/and the cooling water difference temperature of about 8-10 degree Celsius is achieved.
Note : The refrigerant used although do not contribute to ozone depletion , it is still a greenhouse gas Therefore , it should not be released directly into the atmosphere
Master solenoid valve
The master solenoid is installed after the receiver, which is controlled by the control unit. In case of sudden stoppage of the compressor, the master solenoid also closes, avoiding the flooding of the evaporator with refrigerant liquid
