Parallel Compressor Oil Level Balancing Using Feedback Control
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Solution Overview
Problem
Existing thermodynamic systems with compressors of varying capacities, especially those with a variable-speed compressor and a fixed-capacity compressor, face oil level imbalances due to pressure differences, leading to potential damage from oil depletion in one compressor and accumulation in another, regardless of operating conditions or compressor configurations.
Innovation Solution
A compression device with a variable-capacity first compressor and a fixed-capacity second compressor, equipped with oil level detection means and control means that stop the second compressor when the oil level in the first compressor falls below a predetermined value, allowing oil to be transferred back and maintaining a minimum oil level, ensuring balanced oil levels across both compressors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a variable-speed compressor and a fixed-capacity compressor are used in parallel, then the system can operate under varying cooling demands, but oil level imbalance occurs causing depletion in one compressor and accumulation in another
Solution Approach 1:
The patent employs feedback control by continuously monitoring the oil level in the variable-speed compressor's crankcase and using this information to control the operation of the fixed-capacity compressor. When oil level drops below a threshold, the fixed-capacity compressor is shut off, preventing further oil depletion. This closed-loop feedback mechanism resolves the contradiction by adapting system operation based on real-time oil level conditions.
Solution Approach 2:
The system uses the variable-speed compressor's operation to naturally service the oil distribution issue. By controlling the variable-speed compressor's speed and operation cycles based on oil level feedback, the system self-regulates oil circulation and distribution between the two compressors without requiring external intervention or complex additional components.
2Reliability
If a throttling member is positioned in the suction conduit to maintain pressure balance, then oil transfer between compressors is promoted, but the solution is not satisfactory for compressors with very different capacities or variable speed operation
Solution Approach 1:
The patent replaces the static throttling member approach with a dynamic control strategy. Instead of using a fixed pressure-balancing component, the system dynamically adjusts the operation of the fixed-capacity compressor based on real-time oil level conditions in the variable-speed compressor. This dynamic approach adapts to varying operating conditions and compressor configurations, resolving the limitation of static throttling solutions.
Solution Approach 2:
The system changes the operational parameters of the fixed-capacity compressor (on/off state) based on oil level parameters in the variable-speed compressor. This parameter-based control approach allows the system to adapt to different compressor capacities and operating conditions, making the oil level balancing solution universally applicable rather than configuration-specific.
3Productivity
If the second compressor operates at high capacity, then cooling demand is met, but oil accumulates in the second compressor's oil sump causing imbalance
Solution Approach 1:
The feedback control mechanism monitors oil level in the variable-speed compressor and responds by shutting off the fixed-capacity compressor when oil depletion is detected. This feedback loop automatically adjusts system productivity (cooling capacity) to maintain oil level balance, preventing the accumulation issue that occurs when the fixed-capacity compressor operates continuously at high capacity.
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 balances oil levels in both compressors regardless of operating conditions or compressor types, reduces the qualification process duration, and protects the more expensive compressor from pressure variations, while allowing the use of any tubing configuration.
Implementation Method 1
a pressure unbalance between the oil sumps of both compressors is established causing a transfer of the majority of the oil from the evaporator towards the second compressor
Data Source
AI summary
A compression device includes a first compressor and a second compressor mounted in parallel, each compressor including a leakproof enclosure including a low pressure portion containing a motor and an oil sump, an oil level equalization conduit putting into communication the oil sumps of the first and second compressors, and control means adapted for controlling the starting and the stopping of the first and second compressors. The first compressor includes first detection means coupled with the control means and adapted for detecting an oil level in the oil sump of the first compressor. The control means are adapted for controlling the stopping of the second compressor when the oil level detected by the first detection means falls below a first predetermined value.