Comparing compressor performance and plant performance
An operator is asked to compare two shifts of refrigeration performance. One spreadsheet divides cooling duty by compressor input only, while another includes the secondary pump and condenser fans. The pressure-ratio column uses gauge pressures directly on one shift and absolute values on the other. A trainee also uses a Celsius temperature in an ideal Carnot formula. Meanwhile, the load and cooling-water conditions differ substantially between the shifts. The resulting numbers appear to show a dramatic efficiency improvement, but the calculation boundaries and reference conditions have not been reconciled. The operator must correct the definitions, units, and comparisons before reporting a performance conclusion or proposing changes to operating setpoints.
1. Which calculation issues should be corrected first?
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Define the same performance boundary and include the corresponding energy inputs for both shifts. Convert pressure ratios to absolute-pressure references and use absolute temperatures where the formula requires them. Keep power rates and interval energy quantities distinct, with consistent units throughout.
2. What limits the conclusion even after arithmetic is corrected?
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Different loads and heat-rejection conditions can change legitimate performance, so compare like conditions or use an appropriate normalized analysis. Instrument uncertainty and operating state also matter. A better calculated COP does not authorize violating equipment limits or changing protective setpoints without the required review.