To understand the term "tonnes of refrigeration," let's delve into its meaning and application.

The term "tonne of refrigeration" is a unit of power used primarily in the refrigeration and air conditioning industries. Here's a detailed breakdown of this concept:

Definition:

• One tonne of refrigeration (1 TR) is defined as the amount of heat removed to freeze 1 short ton (2000 pounds) of water at 0°C to ice at 0°C in 24 hours.
• The equivalent of this in power is approximately \(3.517 \text{ kilowatts}\) or \(12,000 \text{ BTU/h (British Thermal Units per hour)}\).

The Concept:

• Cooling or refrigeration systems are rated in tonnes of refrigeration to express their heat extraction capacity.
• In everyday terms, when we say a system is "one tonne," it means it can extract or dispel heat equivalent to what would melt 1 short ton of 0°C ice in 24 hours.

Conclusion with Options Analysis:

• Option 1: "the weight of the machine is one tonne" - This is incorrect as the term doesn't refer to the machine's weight.
• Option 2: "the weight of the refrigerant is one tonne" - Incorrect because it doesn't relate to the refrigerant's weight.
• Option 3: "the rate of the heat extraction is one tonne" - Correct, as it refers to the system's ability to extract heat, equivalent to freezing one tonne of water into ice in 24 hours.
• Option 4: "None of these" - Since Option 3 is correct, this is not applicable.

Therefore, the correct answer is: "the rate of the heat extraction is one tonne."