Food Process Engineering & Operations

Refrigeration systems operate on the Second Law of Thermodynamics (specifically the Clausius statement), which dictates that heat cannot spontaneously flow from a colder reservoir to a hotter reservoir without the input of external mechanical work provided by a compressor or heat engine.

A throttling process (fluid expansion through an expansion valve, orifice, or porous plug with no work transfer and adiabatic conditions, $Q = 0$, $W = 0$) is an isenthalpic process, meaning the specific enthalpy remains constant ($h_1 = h_2$) between the inlet and exit states.

Equivalence of one ton of refrigeration (TR):

• One ton of refrigeration (1 TR) is defined as the rate of heat removal required to freeze 1 short ton ($2000\text{ lb}$) of water at $0^\circ\text{C}$ into ice at $0^\circ\text{C}$ in 24 hours. In SI units, $1\text{ TR} = 3.517\text{ kW} = 211\text{ kJ/min} = 50.4\text{ kcal/min}$, practically taken as $3.5\text{ kW}$.
• $2.5\text{ kW}$ is below the thermal equivalent of 1 TR.
• $1.0\text{ kW}$ equals $0.284\text{ TR}$.
• $4.5\text{ kW}$ equals approximately $1.28\text{ TR}$.