System and method for capacity control in a multiple compressor liquid chiller system
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Solution Overview
Problem
Existing multiple compressor chiller systems face challenges in efficiently controlling capacity during high ambient air temperature conditions, particularly in reducing compressor cycling and optimizing the operation of multiple compressors to maintain a stable leaving chilled liquid temperature setpoint, while also addressing the drawbacks of variable speed drives such as increased cost and potential failure leading to system-wide shutdown.
Innovation Solution
A method and system that control the capacity of a multiple compressor chiller by adjusting the operating speed of compressors and the number of compressors in operation using a variable speed drive (VSD) with a control panel and algorithm to manage the output voltage and frequency, allowing for optimal compressor management and reducing cycling, especially during high ambient temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single variable speed drive is used to power multiple compressor motors in parallel, then the overall system cost is reduced, but a fault in one motor may disable the variable speed drive and prevent other motors from operating
Solution Approach 1:
The system divides the variable speed drive functionality into separate independent drives for each compressor motor. This segmentation ensures that a failure in one motor or its associated drive does not affect other compressors, as each has its own dedicated drive unit. The segmentation principle resolves the contradiction by sacrificing the cost benefit of a shared drive to achieve the reliability benefit of independent operation.
Solution Approach 2:
The control system acts as an intermediary that coordinates multiple independent variable speed drives. It monitors system conditions and individually controls each compressor motor's speed based on overall system requirements, enabling the motors to work together as a coordinated team while maintaining independence. This intermediary control allows the system to achieve both cost efficiency through intelligent coordination and reliability through independent drive units.
2Device complexity
If compressor capacity is controlled by sequential engaging and disengaging, then the control process is simple, but compressor cycling increases and system efficiency decreases
Solution Approach 1:
The system dynamically adjusts compressor motor speeds continuously rather than simply engaging or disengaging compressors. Each motor's speed is varied in real-time based on system conditions, eliminating the need for frequent on/off cycling. This dynamic speed control reduces energy losses associated with startup/shutdown cycles while maintaining relatively simple control logic through standardized motor control algorithms.
Solution Approach 2:
Instead of changing the operational state (on/off) of compressors, the system changes the operational parameters (speed) of running compressors. By adjusting the speed parameter of variable speed motors, the system can modulate capacity continuously without cycling compressors on and off, thereby reducing energy losses while keeping control complexity manageable through parameter-based regulation.
3Reliability
If multiple separate variable speed drives are used for each compressor, then system reliability is improved, but the overall system cost increases
Solution Approach 1:
Multiple variable speed drives are designed with universal characteristics, allowing them to be identical or standardized units that can interchangeably control any compressor motor. This universality enables the system to use the same drive design across all compressors, reducing overall cost through standardization, economies of scale, and simplified inventory management, while still providing the reliability benefits of having separate drives for each motor.
Data Source
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AI summary
A capacity control algorithm for a multiple compressor liquid chiller system is provided wherein the speed and number of compressors in operation are controlled in order to obtain a leaving liquid temperature setpoint. In response to an increase in the load in the chiller system, the algorithm determines if a compressor should be started and adjusts the operating speed of all operating compressors when an additional compressor is started. In response to a decrease in the load in the chiller system with multiple compressors operating, the algorithm determines if a compressor should be de-energized and adjusts the operating speed of all remaining operating compressors when a compressor is de-energized.