CO2 Refrigeration Pressure Control With Flash Tank Unloading
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Solution Overview
Problem
Controlling refrigerant charge in transcritical refrigerant vapor compression systems using carbon dioxide is complex due to the absence of distinct liquid or vapor phases, making it difficult to maintain system equilibrium and efficiency.
Innovation Solution
A carbon dioxide refrigerant vapor compression system with a flash tank receiver and a compressor unload circuit, along with a controller that modulates the expansion device and unload circuit flow control valve based on sensed operating parameters to maintain discharge pressure below a preselected upper limit, ensuring operation within a subcritical cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a transcritical cycle is used with carbon dioxide refrigerant, then the system can operate in environments with ambient temperatures above the critical point, but controlling refrigerant charge becomes complex due to the absence of distinct liquid or vapor phases
Solution Approach 1:
The patent introduces a receiver as an intermediary component in the refrigerant circuit. The receiver stores liquid refrigerant and works with the expansion device to control refrigerant charge, providing a mechanical means to manage the refrigerant despite the absence of distinct phase boundaries in transcritical operation.
Solution Approach 2:
The patent implements a control system with sensors that monitor operating conditions and provide feedback to automatically adjust the expansion device and receiver operation. This closed-loop feedback mechanism dynamically controls refrigerant charge based on real-time system conditions, simplifying the complexity of transcritical charge management.
2Temperature
If the compressor discharge pressure is allowed to exceed the critical point pressure, then the system can reject heat in high ambient temperatures, but the refrigerant heat rejection heat exchanger operates less efficiently
Solution Approach 1:
The patent employs dynamic control of the expansion device and receiver operation to adjust refrigerant flow and compressor discharge pressure in real-time. This dynamic adjustment allows the system to optimize the balance between heat rejection capability and efficiency based on varying ambient conditions.
Solution Approach 2:
The control system monitors and adjusts key operating parameters including compressor discharge pressure, refrigerant flow rate, and receiver liquid level. By dynamically changing these parameters, the system maintains optimal heat rejection efficiency while adapting to high ambient temperature conditions.
3Adaptability or versatility
If the liquid level in the receiver tank fluctuates to establish equilibrium, then the system adapts to changing operating conditions, but this makes it difficult to maintain stable refrigerant charge
Solution Approach 1:
The control system continuously monitors the receiver liquid level and operating conditions, using feedback signals to automatically adjust the expansion device and refrigerant flow. This feedback control stabilizes the refrigerant charge by counteracting fluctuations that would otherwise occur as the system adapts to changing conditions.
Solution Approach 2:
The receiver and expansion device work together as a self-regulating system where the expansion device automatically meters refrigerant flow based on receiver liquid level and operating conditions. This self-service mechanism maintains stable refrigerant charge distribution without requiring constant external intervention.
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 solution effectively regulates refrigerant charge and maintains the system in a subcritical cycle, ensuring efficient operation and stability by controlling discharge pressure and temperature, thereby optimizing system performance and preventing excessive refrigerant accumulation or depletion.
Implementation Method 1
Liquid refrigerant from the condenser enters the receiver tank and settles to the bottom of the tank. As this liquid will be at saturated temperature, refrigerant vapor will fill the space in the tank not filled by liquid refrigerant.
Implementation Method 2
a refrigerant compression device, a refrigerant heat rejection heat exchanger for passing refrigerant received from the compression device at a high pressure
Implementation Method 3
a refrigerant heat rejection heat exchanger for passing refrigerant received from the compression device at a high pressure in heat exchange relationship with a cooling medium
Implementation Method 4
an expansion device disposed in the refrigerant circuit downstream of the refrigerant cooling heat exchanger and upstream of the refrigerant heating heat exchanger
Implementation Method 5
a refrigerant heating heat exchanger for passing refrigerant at a low pressure refrigerant in heat exchange relationship with a heating medium
Data Source
AI summary
A carbon dioxide refrigerant vapor compression system and method of operating that system are provided. The refrigerant vapor compression system includes a compression device, a flash tank receiver disposed in the refrigerant circuit intermediate a refrigerant heat rejection heat exchanger and a refrigerant heat absorption heat exchanger, and a compressor unload circuit including a refrigerant line establishing refrigerant flow communication between an intermediate pressure stage of the compression device and the refrigerant circuit at a location downstream of the refrigerant heat absorption heat exchanger and upstream of a suction inlet to the compression device, and a unload circuit flow control device disposed in said unload circuit refrigerant line. In response to at least one system operating parameter sensed by at least one sensor, the controller selectively positions the unload flow control device to maintain the refrigerant vapor compression system operating below a preselected high pressure limit.