Question 1

Primary functions of a refrigerant compressor  In a vapour-compression refrigeration system, what does the compressor accomplish?

Accepted Answer

Introduction / Context:The compressor is the work-input device of a vapour-compression cycle. Recognizing all of its effects clarifies why condenser and expansion device conditions are what they are.Given Data / Assumptions:Compression of superheated or saturated vapour from evaporator pressure to condenser pressure.Real compressors have isentropic efficiency Concept / Approach:By compressing the vapour, the compressor increases its pressure to the condensing level, simultaneously increasing temperature (due to compression work) and providing the driving force for circulation around the closed loop.Step-by-Step Solution:Pressure: suction (low) → discharge (high), enabling condensation at ambient conditions.Temperature: rises due to work input; crucial for superheat management and oil return.Circulation: the compressor draws vapour from the evaporator and pushes it through condenser and expansion device.Verification / Alternative check:Energy balance: W_in to the compressor equals the enthalpy rise across it (neglecting heat loss), explaining temperature increase.Why Other Options Are Wrong:Any single-function choice is incomplete; the compressor performs all three.“Only maintains mass balance” ignores the required pressure rise and work.Common Pitfalls:Overlooking that discharge temperature depends on isentropic efficiency and suction superheat.Final Answer:All of the above

Question 2

Why aircraft use air-cycle refrigeration  Aircraft commonly employ air (reversed Brayton) refrigeration cycles primarily because:

Accepted Answer

Introduction / Context:Air-cycle (reversed Brayton) systems are favored in aviation. Understanding the engineering trade-offs explains why COP is not the sole selection criterion.Given Data / Assumptions:Aircraft already carry air compressors/turbomachinery (bleed air/ram air).Weight, reliability, and maintenance are critical constraints.Concept / Approach:Although air-cycle COP can be lower than that of vapour-compression, the system offers major advantages: light weight, no refrigerant handling, tolerance to leaks, and effective use of available ram air and high-altitude low ambient temperatures.Step-by-Step Solution:Assess competing drivers: mass and simplicity dominate in aircraft.Air cycle uses air as the working fluid; no evaporator with phase change refrigerant is required, reducing complexity and weight.Altitude provides cold sink air to enhance cooling performance.Verification / Alternative check:Aircraft environmental control systems (ECS) widely use bootstrap/air-cycle packs that prioritize weight and reliability over peak COP.Why Other Options Are Wrong:Highest heat-transfer coefficients: not the primary justification.“Only” low ambient temperature: it helps but is not the sole reason.Higher COP than vapour cycles: generally untrue.No compressor: air cycle still uses a compressor or utilizes bleed/ram compression.Common Pitfalls:Equating selection solely with COP while ignoring weight and system integration.Final Answer:They provide low weight per tonne of refrigeration and simple, robust hardware.

Question 3

Naming pressures in a refrigeration compressor  What is the standard term for the pressure measured at the inlet of a refrigerant compressor (i.e., the evaporator pressure at the suction line)?

Accepted Answer

Introduction / Context:Correct terminology helps in reading P–h charts, service gauges, and troubleshooting logs. The compressor has a low-pressure side (suction) and a high-pressure side (discharge).Given Data / Assumptions:Standard vapour-compression layout with evaporator, compressor, condenser, and expansion device.Inlet line to compressor originates from evaporator outlet/suction accumulator.Concept / Approach:The pressure at the compressor inlet is called suction pressure. It corresponds to the evaporating pressure and is read on the low-side gauge in service practice. Some texts also refer to evaporator pressure as back pressure; however, “suction pressure” is the unambiguous term at the compressor inlet.Step-by-Step Solution:Identify component: compressor inlet.Associate pressure name: suction pressure (low side).Note: discharge pressure is the high-side pressure at compressor outlet; critical pressure refers to fluid property, not line pressure.Verification / Alternative check:Service manifolds label LP gauge for suction (evaporator) pressure and HP gauge for discharge (condensing) pressure.Why Other Options Are Wrong:Discharge pressure: outlet, not inlet.Critical pressure: property at critical point, unrelated to component location.Back pressure: sometimes synonymous with evaporator pressure, but the standard inlet term is suction pressure.Common Pitfalls:Using “back pressure” interchangeably can cause confusion in multi-stage or cascade systems.Final Answer:Suction pressure

Question 4

Power requirement per tonne of refrigeration (PTR)  For a vapour-compression plant, express the power per tonne of refrigeration (in kW per TR) in terms of the coefficient of performance (COP). Take 1 TR = 3.5 kW of cooling.

Accepted Answer

Introduction / Context:Linking COP to input power per unit cooling capacity is essential for comparing chiller and refrigeration plant efficiencies across sizes.Given Data / Assumptions:Definition: COP = Q_L / W_in.One tonne of refrigeration (1 TR) = 3.5 kW of cooling capacity.Concept / Approach:For 1 TR, Q_L = 3.5 kW. Rearranging the COP definition gives W_in = Q_L / COP. Therefore, power per TR is 3.5 / COP (kW per TR).Step-by-Step Solution:Start with COP = Q_L / W_in.Rearrange: W_in = Q_L / COP.For 1 TR, W_in = 3.5 / COP kW.Verification / Alternative check:Numerical example: COP = 3 → PTR = 3.5/3 ≈ 1.167 kW/TR. This matches typical plant data.Why Other Options Are Wrong:COP/3.5 or 3.5COP invert or misapply the relationship.1/(3.5COP) has wrong dimensions and magnitude.Common Pitfalls:Confusing COP with EER or SEER; forgetting that PTR decreases as COP increases.Final Answer:3.5 / COP

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