Flash Tank Sizing for CO2 Refrigerant Storage and Separation
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Solution Overview
Problem
Refrigerant vapor compression systems operating in transcritical cycles with carbon dioxide lack an adequate buffer volume for refrigerant storage, as the refrigerant heat rejection heat exchanger operates as a gas cooler, preventing the use of traditional receivers for liquid refrigerant storage.
Innovation Solution
Incorporating a flash tank between the refrigerant heat rejection and heat absorption heat exchangers, with an economizer circuit and sizing the internal volume of the flash tank to range between 10% to 30% of the total system volume, providing a buffer for refrigerant storage and ensuring unimpeded separation of refrigerant vapor and liquid.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a refrigerant vapor compression system operates in a transcritical cycle with carbon dioxide, then the refrigerant heat rejection heat exchanger functions as a gas cooler operating above the critical point, but the system lacks an adequate buffer volume for refrigerant storage because traditional receivers cannot store vapor-state refrigerant
Solution Approach 1:
The flash tank is designed to perform multiple functions simultaneously: it serves as a buffer volume for refrigerant storage during low load demands, as a separation chamber for vapor-liquid separation, and as part of the economizer circuit for enhancing refrigeration capacity. This multi-functionality eliminates the need for separate receivers or accumulators in transcritical CO2 systems.
Solution Approach 2:
The system utilizes the phase change parameters of carbon dioxide refrigerant by operating the flash tank at conditions where refrigerant can exist in both liquid and vapor states. The flash tank operates at pressures and temperatures that allow efficient vapor-liquid separation, with the liquid portion being expanded through the economizer circuit while vapor is discharged to the compressor.
2Quantity of substance
If the flash tank internal volume is increased to provide adequate buffer storage, then refrigerant storage capacity improves, but the system volume and device complexity increase
Solution Approach 1:
The flash tank is sized to provide partial buffer storage capacity rather than complete refrigerant storage, as the system operates most efficiently with the flash tank containing a mixture of liquid and vapor refrigerant. The tank volume is optimized to handle transient load variations without requiring excessive storage capacity, balancing buffer effectiveness with system compactness.
3Reliability
If a receiver is used to store liquid refrigerant in a subcritical cycle, then adequate buffer volume is provided, but this solution cannot be applied in transcritical cycles where refrigerant leaves the gas cooler in vapor state
Solution Approach 1:
The flash tank acts as an intermediary device between the gas cooler and the evaporator in transcritical systems. It receives high-pressure refrigerant from the gas cooler, allows partial condensation and vapor-liquid separation, then directs both liquid and vapor phases to appropriate system components. This intermediary function enables buffer storage without requiring the refrigerant to be in pure liquid state as in subcritical receivers.
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 configuration allows for efficient refrigerant storage and separation, eliminating the need for additional accumulators and optimizing system performance across varying load demands and operating conditions.
Implementation Method 1
ensuring unimpeded separation of refrigerant vapor and liquid
Implementation Method 2
refrigerant vapor injection line connecting a chamber of the flash tank in refrigerant vapor flow communication with an intermediate pressure stage of the compression device
Data Source
Figure 1
Figure 2
AI summary
A refrigerant vapor compression system (10) includes a plurality of components, including a flash tank (70), connected in a refrigerant flow circuit by a plurality of refrigerant lines (2, 4, 6, 8). The system internal volume equals to the sum of the internal volumes of the plurality of components and the internal volume of the plurality of refrigerant lines. The internal volume of the flash tank ranges from at least 10% to about 30% of the total system internal volume.