The question asks about the specific term for the amount of heat removed in the evaporator per unit mass flow of refrigerant. This is a fundamental concept in refrigeration and air conditioning.

Refrigerating Effect Defined

The refrigerating effect is the amount of heat absorbed by the refrigerant in the evaporator per unit mass of refrigerant circulating through the system. In a refrigeration cycle, the evaporator is where the refrigerant undergoes a phase change from liquid to vapor, absorbing heat from the refrigerated space or substance. This absorbed heat is what provides the cooling effect.

Mathematically, the refrigerating effect (RE) is typically calculated as the difference in enthalpy between the refrigerant leaving the evaporator (vapor) and the refrigerant entering the evaporator (liquid after throttling). If \(h_1\) is the enthalpy of the refrigerant vapor leaving the evaporator and \(h_4\) is the enthalpy of the refrigerant liquid entering the evaporator, then the refrigerating effect per unit mass is:

\[ \text{Refrigerating Effect (RE)} = h_1 - h_4 \]

This value is crucial because it represents the cooling capacity achieved for every kilogram (or pound) of refrigerant that flows through the system. A higher refrigerating effect means more cooling per unit mass of refrigerant, which can lead to better system efficiency.

Analyzing the Options

Let's look at why the other options are not the correct answer for the heat removed in the evaporator per unit mass flow of refrigerant:

• One tonne of refrigeration: This is a unit of cooling capacity, defined as the rate of heat removal required to freeze one short ton (2000 lbs) of water at \(0^\circ\text{C}\) to ice at \(0^\circ\text{C}\) in 24 hours. It is equivalent to 211 kJ/min or 3.517 kW. It represents a rate of heat removal, not the heat removed per unit mass of refrigerant.
• Volumetric efficiency of compressor: This refers to the actual volume of vapor compressed by the compressor compared to the theoretical volume displaced by the compressor's piston. It's a measure of compressor performance and relates to the capacity of the compressor to move refrigerant, not the heat absorbed in the evaporator.
• The coefficient of performance (COP): COP is a dimensionless ratio that indicates the efficiency of a refrigeration or heat pump system. For a refrigeration system, it is defined as the ratio of the desired cooling effect (heat removed from the refrigerated space) to the work input required to achieve that cooling. It's a performance metric for the entire cycle, not the heat removed per unit mass in the evaporator alone.

Therefore, based on the definitions, "the refrigerating effect" precisely describes the amount of heat removed in the evaporator per unit mass flow of refrigerant.