Electronic Expansion Valve Startup Control for Stable Refrigeration
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Solution Overview
Problem
Conventional refrigerating cycle heat source apparatuses face challenges in stabilizing operation quickly after startup due to rapid pressure and temperature changes, leading to potential liquid back and oil film shortage in screw compressors, especially with pseudo azeotropic refrigerants where temperature gradients are absent, making precise control of electronic expansion valves difficult.
Innovation Solution
A heat source apparatus with temperature and pressure detectors that control the electronic expansion valve by initially adjusting the opening degree based on discharge side refrigerant superheat, then transitioning to suction side superheat control, ensuring a stable operation without rapid suction pressure decrease, using a combination of temperature and pressure detection for precise valve control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the electronic expansion valve is controlled based only on suction side superheat detection, then the control system is simple, but the control precision is insufficient leading to liquid back and compressor damage
Solution Approach 1:
The patent uses discharge side superheat as an intermediary parameter to indirectly infer suction side superheat conditions. Since discharge side superheat has a larger value range (around 25K) compared to suction side superheat (3-5K), it serves as a more reliable mediator for control decisions, avoiding the precision limitations of direct suction side measurement while still achieving effective suction side control through the relationship between discharge and suction parameters.
2Loss of time
If the electronic expansion valve opening degree is rapidly decreased after startup, then the stabilization time is shortened, but the suction pressure decreases rapidly causing water or brine freezing in the evaporator
Solution Approach 1:
The patent applies preliminary action by first opening the electronic expansion valve to a predetermined opening degree at startup, then gradually decreasing it while monitoring discharge side superheat. This preliminary opening prevents rapid pressure drop and evaporator freezing, while the subsequent controlled closing based on discharge superheat achieves stabilization. The system prepares the valve in a safe state before initiating the closing process.
Solution Approach 2:
The patent implements dynamic control by adjusting the electronic expansion valve opening degree based on real-time discharge side superheat measurements. The control speed is dynamically adapted: initially fast opening to prevent freezing, then gradual closing as discharge superheat increases, optimizing the balance between stabilization time and preventing evaporator freezing throughout the startup process.
3Adaptability or versatility
If pseudo azeotropic refrigerants are used, then the system operates with common refrigerants, but temperature gradient is absent in gas-liquid saturation region making suction dryness undetectable
Solution Approach 1:
The patent inverts the detection approach by not directly measuring suction side parameters (which are undetectable with pseudo azeotropic refrigerants), but instead measuring discharge side parameters and using the known relationship between discharge and suction superheat to infer suction conditions. This inversion allows the system to work with pseudo azeotropic refrigerants despite the absence of temperature gradient in the saturation region.
Data Source
AI summary
After a chiller unit is started, an opening command of 10 pulses is output to an electronic expansion valve, and after a predetermined period of time (at the time of starting with unload) elapses, an opening command of 100 to 150 pulses is output to open the electronic expansion valve up to a predetermined opening degree. Thereafter, a discharge side refrigerant super-heat TdSH is monitored and a closing command of 1 pulse/sec is output to the electronic expansion valve to drive the same in a closing direction in a short time until TdSH becomes 20 K. After TdSH has reached 20 K, a closing command of 1 pulse/3 sec is output to the electronic expansion valve, until TdSH becomes 25 K, to drive the electronic expansion valve 3 at a smaller speed in the closing direction than that speed in the closing direction until TdSH becomes 20 K. After TdSH reaches 25 K, it is judged that an operation has shifted to a stable operation, and the opening degree of the electronic expansion valve is controlled while a suction side refrigerant super-heat TsSH is monitored.