Bell–Coleman (air) refrigerator comparison: For the same temperature range, a dense (closed) air system requires __________ power per tonne of refrigeration compared to an open air system.
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Athe same
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Blower
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Chigher
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Dindeterminate
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Emuch higher at low pressure ratios only
Answer
Correct Answer: lower
Explanation
Introduction / Context:Air-cycle refrigeration can operate as open or dense (closed) systems. For a fixed evaporator and condenser temperature range, practical implementations show the dense system achieving better performance (higher COP), which directly translates to lower shaft power per tonne of refrigeration (TR).
Given Data / Assumptions:
- Identical temperature limits.
- Comparable component efficiencies.
- Closed system maintains elevated mean pressure and better recuperation.
Concept / Approach:Power per TR is inversely related to COP: Power/TR = 211 kJ/min per TR / COP in consistent units. If COP increases in the dense system due to reduced irreversibilities, improved heat exchange, and better control, then required power per TR decreases.
Step-by-Step Reasoning:
1) Dense system → higher density → smaller specific volume → reduced compressor work per unit cooling.2) Enhanced recuperation → larger refrigeration effect for a given work.3) Therefore COP_dense > COP_open → Power/TR_dense < Power/TR_open.Verification / Alternative check:Cycle analyses on T–s diagrams confirm lower specific work for the dense system, all else equal.
Why Other Options Are Wrong:
- Same/indeterminate: contradicts established comparative practice.
- Higher: would imply COP is lower, which is not the typical case for dense systems.
- “Much higher at low pressure ratios only” is not a general truth.
Common Pitfalls:Assuming ideal cycles yield identical COP; practical differences in heat-exchanger effectiveness and mixing losses matter.
Final Answer:lower