Flash Tank Receiver Charge Control in Transcritical Refrigeration
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Solution Overview
Problem
Refrigerant vapor compression systems, especially those operating in transcritical cycles, face complexity in controlling refrigerant charge due to the absence of distinct liquid or vapor phases, leading to undesirable responses to system charge requirements.
Innovation Solution
A refrigerant vapor compression system incorporating a flash tank receiver and a controller with sensors and a secondary expansion device to monitor and control the liquid refrigerant level, adjusting the flow of refrigerant based on sensed operating characteristics to maintain a consistent charge consistent with desired operating characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a flash tank receiver is used in transcritical refrigerant systems, then refrigerant charge control is improved, but device complexity increases
Solution Approach 1:
The system divides the refrigerant flow path into distinct segments: a first expansion device controlling flow into the flash tank receiver, and a second expansion device controlling flow from the flash tank receiver. This segmentation allows independent control of refrigerant charge at different stages, improving charge management in transcritical systems where phase boundaries are indistinct.
Solution Approach 2:
The flash tank receiver acts as an intermediary component between the two expansion devices. It provides a buffer volume that decouples the control functions, allowing the first expansion device to regulate supercritical refrigerant flow while the second expansion device regulates subcritical refrigerant flow, thereby simplifying the overall control strategy despite adding a component.
2Ease of operation
If multiple expansion devices are used to control refrigerant flow, then refrigerant charge regulation is improved, but device complexity increases
Solution Approach 1:
The control function is segmented into two independent expansion devices operating at different locations in the refrigerant circuit. The first expansion device controls supercritical refrigerant flow before the flash tank receiver, while the second expansion device controls subcritical refrigerant flow after the flash tank receiver. This segmentation enables precise charge regulation without requiring a single complex control mechanism.
Solution Approach 2:
Each expansion device independently regulates refrigerant flow based on local conditions in its respective zone. The first expansion device self-regulates supercritical flow based on upstream conditions, while the second expansion device self-regulates subcritical flow based on downstream evaporator conditions, reducing the need for complex centralized control.
3Stability of the object's composition
If a flash tank receiver is added to the system, then refrigerant charge consistency is improved, but system complexity increases
Solution Approach 1:
The flash tank receiver serves as a mediating component that separates supercritical and subcritical refrigerant zones. By providing a physical boundary and buffer volume, it stabilizes the refrigerant charge composition, ensuring consistent thermodynamic states at the inlet and outlet of the expansion devices despite variations in operating conditions.
Solution Approach 2:
The system exploits parameter changes in refrigerant density and phase state across the flash tank receiver. Supercritical refrigerant enters the flash tank at high density, undergoes pressure reduction and partial vaporization, and exits as subcritical refrigerant at lower density. This parameter transformation stabilizes charge consistency by creating distinct thermodynamic zones.
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
The system effectively regulates refrigerant charge, ensuring optimal operating conditions by maintaining a desired liquid level in the flash tank receiver, thereby improving efficiency and stability in both subcritical and transcritical cycles.
Implementation Method 1
a secondary expansion device disposed in the refrigerant circuit downstream of the refrigerant cooling heat exchanger and upstream of the flash tank receiver; the secondary expansion device operative to expand the high pressure refrigerant flowing therethrough to a liquid/vapor refrigerant mix at a lower pressure
Implementation Method 2
A flash tank receiver is disposed in the refrigerant circuit downstream of the refrigerant cooling heat exchanger and upstream of the expansion device. 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 3
a liquid level sensing device disposed in operative association with the flash tank receiver for sensing the level of liquid refrigerant within the flash tank receiver
Data Source
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AI summary
A refrigerant vapor compression system includes a flash tank receiver disposed in the refrigerant circuit intermediate the refrigerant cooling heat exchanger and the refrigerant heating heat exchanger. The flash tank receiver, which receives a liquid/vapor refrigerant mix, also functions as a receiver. A refrigerant charge control apparatus includes a liquid level sensing device disposed in operative association with the flash tank receiver for sensing the level of liquid refrigerant within the flash tank receiver, at least one sensor for sensing a system operating parameter, and a controller operative to determine a desired liquid refrigerant level within the flash tank receiver and to selectively adjust a secondary expansion device to increase or decrease the flow of refrigerant passing into the flash tank receiver to provide a circulating refrigerant charge consistent with maintaining a desired system operating parameter.