Methods of control for transport refrigeration units
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Solution Overview
Problem
Scroll type compressors in refrigeration systems experience stress and potential failure due to high density restart conditions resulting from repeated on-off cycles, which can lead to adverse effects and reduced reliability.
Innovation Solution
The implementation of an electronic valve assembly, such as an electronic expansion valve or suction modulation valve, controlled by an electronic controller to perform a controlled pump-down operation during compressor shutdown, reducing refrigerant density and minimizing stress on the compressor during restarts by maintaining low density conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If repeated on-off cycles are used to maintain desired temperatures when excess compressor capacity exceeds load demand, then temperature control is achieved, but high density restart conditions cause stress and potential failure of the scroll type compressor
Solution Approach 1:
The system performs a pump-down operation before compressor shutdown to remove refrigerant from the evaporator and discharge line, creating favorable low-density restart conditions in advance. This preliminary action prevents the harmful high density conditions that would otherwise occur during restart after on-off cycles.
Solution Approach 2:
The system changes the refrigerant density parameter in the discharge line and evaporator by performing a pump-down operation. The electronic expansion valve is closed to isolate the evaporator, and the compressor runs at high speed to pump refrigerant into the condenser, thereby reducing refrigerant density in the evaporator and discharge line to favorable levels before shutdown.
2Productivity
If scroll type compressor is used to provide advantages in refrigeration system, then system performance is improved, but repeated on-off cycles generate adverse loads and effects on the compressor
Solution Approach 1:
Before each compressor shutdown, the system performs a pump-down operation to prepare favorable restart conditions. This preliminary action ensures that when the compressor restarts after on-off cycles, it does so with low refrigerant density in the evaporator and discharge line, minimizing adverse loads and protecting the scroll type compressor from damage.
Solution Approach 2:
The system converts the potential harm of repeated on-off cycles into a benefit by using the pump-down operation to create favorable restart conditions. The high-speed compressor operation during pump-down, which could be considered excessive, actually serves to reduce refrigerant density to levels that protect the compressor during subsequent restarts.
3Object-affected harmful factors
If pump-down operation is performed prior to compressor stop to reduce start noise, then noise is reduced, but the operation adds complexity to the control system
Solution Approach 1:
The pump-down operation serves multiple functions: it reduces compressor start noise, creates favorable restart conditions to protect the compressor from high density conditions, and prepares the system for efficient operation after shutdown. By combining these functions into a single operation controlled by the existing electronic expansion valve and controller, the system achieves multiple benefits without proportionally increasing complexity.
Solution Approach 2:
The system uses feedback control to determine when to perform the pump-down operation and when to shutdown the compressor. The controller monitors system conditions and makes decisions based on temperature, pressure, and operational patterns, thereby managing the complexity through intelligent control rather than additional mechanical components.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the reliability and lifespan of scroll type compressors by reducing the impact of on/off cycles, minimizing unwanted scroll set motion, and maintaining favorable restart conditions, thereby reducing the risk of compressor damage and failure.
Implementation Method 1
perform a controlled pump-down operation during compressor shutdown, reducing refrigerant density
Implementation Method 2
maintaining favorable restart conditions... maintaining low density conditions
Data Source
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AI summary
Systems and methods of operating a refrigeration system including initiating a compressor shutdown operation (302), recording shutdown conditions (304), calculating one or more restart characteristics based on the recorded shutdown conditions (306), comparing the calculated restart characteristics with one or more compressor restart safety limits (308), when the calculated restart characteristics do not satisfy the restart safety limits, performing a temperature modulation pump down operation (310), and when the calculated restart characteristics satisfy the restart safety limits, completing the compressor shutdown operation (312).