Question

(a) Detail the desirable properties of a refrigerant. (8)

(b) Make a table and compare following refrigerants for use in a provision cooling plant for a 50000 DWT oil tanker: R-22, R-134a. (8)

Answer 1

Part (a)

Desirable Properties of a Refrigerant

A good refrigerant must possess favorable thermodynamic, chemical, and physical properties to ensure efficiency, safety, and environmental compliance in marine refrigeration systems.

1. Thermodynamic Properties

Property	Desirable Feature	Reason
High latent heat of vaporization	Large refrigerating effect per kg	Reduces mass flow rate and compressor size
Moderate evaporating pressure	Above atmospheric pressure	Prevents air or moisture ingress into the system
Moderate condensing pressure	Not excessively high	Reduces compressor work and mechanical stress
Low specific volume of vapor	Small compressor displacement	Improves system compactness
High coefficient of performance (COP)	High efficiency	Lowers power consumption
Suitable boiling point	Below desired evaporator temperature	Ensures effective refrigeration

2. Chemical and Physical Properties

Property	Desirable Feature	Reason
Chemical stability	Stable under operating temperature & pressure	Prevents decomposition and corrosion
Non-corrosive to metals and seals	Safe for Cu, Al, and steel parts	Ensures long service life
Non-toxic and non-flammable	Safe for crew and vessel	Essential for shipboard use
Miscibility with lubricating oil	Uniform oil return	Prevents oil logging in evaporator
Easy leak detection	Detectable by odor or sensors	Enhances safety and maintenance

3. Environmental Properties

Property	Desirable Feature	Reason
Low Ozone Depletion Potential (ODP)	Near zero	To comply with MARPOL Annex VI and Montreal Protocol
Low Global Warming Potential (GWP)	As low as possible	To reduce environmental impact
Readily available and cost-effective	Easy maintenance and spares

An ideal refrigerant should be efficient, safe, non-toxic, non-flammable, stable, non-corrosive, and environmentally acceptable with low ODP and GWP.

Part (b)

Comparison of R-22 and R-134a for Provision Plant on a 50,000 DWT Oil Tanker

Property	R-22 (Chlorodifluoromethane)	R-134a (Tetrafluoroethane)
Chemical Formula	CHClF₂	C₂H₂F₄
Refrigerant Type	HCFC	HFC
Ozone Depletion Potential (ODP)	0.05 (non-zero)	0.0 (zero)
Global Warming Potential (GWP)	≈ 1810	≈ 1430
Boiling Point at 1 atm	–40.8 °C	–26.1 °C
Operating Pressure (approx.)	High (10–15 bar suction)	Moderate (6–10 bar suction)
Latent Heat of Vaporization	High (~233 kJ/kg)	Moderate (~216 kJ/kg)
Volumetric Refrigerating Effect	Higher	Lower
Compressor Displacement	Smaller	Larger (for same capacity)
Lubricant Compatibility	Mineral oils (easy)	Requires polyolester (POE) oil
Toxicity/Flammability	Non-toxic, non-flammable	Non-toxic, non-flammable
Material Compatibility	Good	Good
Environmental Impact	Phase-out under Montreal Protocol	Accepted as replacement for R-12/R-22
Energy Efficiency (COP)	Slightly higher	Slightly lower
Leak Detection	By halide torch or sensors	By electronic sensors
Typical Use on Ships	Older provision/refrigeration systems	Modern provision and A/C systems

Recommendation for 50,000 DWT Oil Tanker:

Preferred Refrigerant: R-134a

Reasons:

Zero ODP – Fully compliant with MARPOL Annex VI and IMO guidelines.
Moderate pressures – Safer and easier to maintain on board.
Good chemical stability and non-flammability – Suitable for shipboard crew environment.
Readily available and approved for marine provision and air-conditioning plants.

R-22, though thermodynamically efficient, is being phased out due to its ozone depletion potential (HCFC type).

✓ Verified Model Answer (Text Solution)

Desirable Properties of a Refrigerant

Comparison of R-22 and R-134a for Provision Plant on a 50,000 DWT Oil Tanker

Recommendation for 50,000 DWT Oil Tanker:

Reasons: