Thermal Engineering, Refrigeration & Power

Throttling and expansion devices in refrigeration systems include: capillary tubes, thermostatic expansion valves (TEV), automatic expansion valves (constant pressure valves), high-side float valves, low-side float valves, and electronic expansion valves (EEV). There is no such device as a 'Centrifugal expansion valve' in refrigeration technology.

Option analysis:

• Low side float valve: Standard expansion device used in flooded evaporators.
• Centrifugal expansion valve: Correct, non-existent device in refrigeration systems.
• Capillary tube: Universal fixed-orifice expansion device in domestic refrigerators.
• Thermostatic expansion valve: Standard superheat-controlled expansion valve in commercial refrigeration.

One ton of refrigeration (TR) is defined as the rate of heat extraction needed to freeze $1\text{ short ton}$ ($2000\text{ lb}$ or $907.18\text{ kg}$) of pure water at $0^\circ\text{C}$ into ice at $0^\circ\text{C}$ in $24\text{ hours}$:

$$1\text{ TR} = \frac{2000\text{ lb} \times 144\text{ BTU/lb}}{24\text{ hours}} = 12000\text{ BTU/h} = 200\text{ BTU/min}$$

Converting to SI metric units:

$$1\text{ TR} = 210.98\text{ kJ/min} \approx 211\text{ kJ/min} = \frac{210.98}{60}\text{ kJ/s} = 3.5167\text{ kW}$$

Option analysis:

• $1\text{ TR} = 311\text{ kJ/min} = 5.5167\text{ kW}$: Incorrect values.
• $1\text{ TR} = 111\text{ kJ/min} = 4.5167\text{ kW}$: Incorrect values.
• $1\text{ TR} = 411\text{ kJ/min} = 2.5167\text{ kW}$: Incorrect values.
• $1\text{ TR} = 211\text{ kJ/min} = 3.5167\text{ kW}$: Correct standard thermodynamic equivalents.

A constant-volume gas thermometer (using an ideal gas like helium or hydrogen) serves as the primary standard thermodynamic thermometer used to establish the absolute thermodynamic temperature (Kelvin scale) based directly on Charles's and Gay-Lussac's ideal gas laws ($T \propto P$ at constant volume).

Option analysis:

• Constant-volume gas thermometer: Correct, fundamental primary thermodynamic instrument for Kelvin temperature determination.
• Thermocouple: Secondary electrical thermoelectric temperature sensor.
• Resistance thermometer: Secondary resistance temperature detector (RTD).
• Thermistor: Secondary semiconductor resistance device.

An isentropic process is a reversible adiabatic process. Because it is adiabatic, no heat is transferred ($dQ = 0$); because it is internally reversible, entropy generation is zero, meaning entropy remains constant ($dS = dQ_{\text{rev}}/T = 0$, hence $S = \text{constant}$).

Option analysis:

• Polytropic process: General thermodynamic process following $PV^n = C$ with heat transfer.
• Isothermal process: Constant temperature process ($T = \text{constant}$).
• Adiabatic process: No heat transferred, but entropy can increase if irreversible.
• Isentropic process: Correct, uniquely specifies both zero heat transfer and constant entropy.