Parallel Expansion Valve Control for Low-Flow Refrigerant Accuracy
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Solution Overview
Problem
Existing refrigerant systems face challenges in achieving accurate and repeatable refrigerant flow control, particularly at low flow rates, due to non-ideal behavior of real-world expansion valves, leading to inefficiencies and inaccuracies.
Innovation Solution
Implementing a control system that uses multiple real expansion valves in parallel, modeled as a virtual expansion valve, to compensate for individual valve inaccuracies and achieve a combined flow rate equivalent to a single ideal valve, through dual stage control and weighted operation of each valve based on their characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single real expansion valve is used, then the device complexity is low, but the refrigerant flow control accuracy deteriorates at low flow rates
Solution Approach 1:
The system divides a single expansion valve function into multiple parallel expansion valves (first, second, and third expansion valves). Each valve handles a portion of the total refrigerant flow, allowing individual valves to operate in their optimal range even when total flow is low, thereby improving control accuracy without requiring an impractically small single valve.
Solution Approach 2:
Multiple expansion valves are combined in parallel to work together as a unified flow control system. The valves operate simultaneously with their respective control inputs coordinated to achieve the desired total refrigerant flow rate, merging their individual capabilities to overcome the limitations of any single valve at low flow conditions.
2Reliability
If multiple real expansion valves are used in parallel, then the refrigerant flow control accuracy improves, but the device complexity increases
Solution Approach 1:
The control function is segmented across multiple valves, with each valve equipped with its own control input. This segmentation allows the system to maintain reliable flow control by distributing the control burden, so that if one valve performs suboptimally, others can compensate, thereby improving overall reliability.
Solution Approach 2:
The system changes the operational parameters of each expansion valve individually through separate control inputs. By adjusting opening degrees, flow coefficients, or other parameters of each valve independently, the system can optimize the combined performance of all valves to achieve reliable and accurate flow control across varying operating conditions.
3Measurement precision
If individual control inputs are calculated for each expansion valve based on flow characteristics, then the aggregate flow rate accuracy improves, but the control system complexity increases
Solution Approach 1:
The control system incorporates feedback by continuously monitoring the actual refrigerant flow characteristics and comparing them against the desired aggregate flow rate. Based on this feedback, the control inputs for each expansion valve are adjusted to minimize the difference between actual and target flow rates, thereby improving aggregate flow accuracy through closed-loop control.
Solution Approach 2:
The control system dynamically changes operational parameters (such as valve opening degree, flow coefficient, or control pressure) for each expansion valve based on real-time conditions. By calculating individual control inputs that account for each valve's specific flow characteristics and adjusting parameters accordingly, the system achieves accurate aggregate flow control while adapting to varying operating conditions.
Data Source
AI summary
A control input represents directed flow rate of refrigerant through a virtual refrigerant metering device. For a plurality of parallel-connected, real, non-virtual, refrigerant metering devices, control input is determined for each, based on refrigerant flow characteristic of each refrigerant metering device, to produce individual flow rates through the refrigerant metering devices that provide an aggregate flow rate equivalent to the directed flow rate. Control signals are sent to the refrigerant metering devices.