Expansion Valve Feed-Forward Control to Prevent Compressor Flooding

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Solution Overview

Problem

Existing control methods for expansion valves in refrigeration systems are inadequate in handling fast load changes, often leading to low suction pressure faults or compressor flooding, and fail to account for load reductions, which can cause compressor flooding.

Innovation Solution

A method combining feedback control and anticipatory feed forward control is used to adjust the expansion valve position based on the rate of change of operating parameters, such as compressor speed, temperature, and liquid level, to anticipate and respond to load changes, preventing compressor flooding and ensuring efficient operation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If the expansion valve is pre-opened by a fixed number of steps to accommodate fast loading, then the valve response speed is improved, but compressor flooding occurs if the number of steps is too large

Engineering Contradiction:

Improvevalve response speedVSAvoidcompressor flooding

Core Design Contradiction:

SpeedVSObject-generated harmful factors

Solution Approach 1:

The system performs preliminary action by pre-opening the expansion valve before fast loading occurs. The feed-forward controller calculates the required pre-opening amount based on the rate of change of operating parameters, allowing the valve to be positioned in advance to accommodate upcoming load changes without causing compressor flooding.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system uses feedback control to continuously monitor the actual valve position and operating parameters, comparing them with the desired values. The feedback controller adjusts the valve position in real-time based on the error signal, ensuring that the valve responds appropriately to load changes while preventing compressor flooding through closed-loop control.

Inventive Principle:

Principle #23Feedback

2Speed

If the expansion valve is pre-opened by a fixed number of steps to accommodate fast loading, then the valve response speed is improved, but low suction pressure fault occurs if the number of steps is too small

Engineering Contradiction:

Improvevalve response speedVSAvoidsuction pressure stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system transitions from static fixed-step pre-opening to dynamic adaptive pre-opening. The feed-forward controller continuously adjusts the pre-opening amount based on the real-time rate of change of operating parameters, enabling the valve to respond optimally to varying load conditions while maintaining suction pressure stability.

Inventive Principle:

Principle #15Dynamics

3Device complexity

If existing control methods do not employ provisions for preclosing the valve in case of load reduction, then the control system is simple, but the chiller is exposed to potential compressor flooding

Engineering Contradiction:

Improvecontrol system complexityVSAvoidcompressor flooding

Core Design Contradiction:

Device complexityVSObject-generated harmful factors

Solution Approach 1:

The system performs preliminary action by pre-closing the expansion valve before load reduction occurs. The feed-forward controller detects the rate of change of operating parameters and calculates the appropriate pre-closing amount, positioning the valve in advance to prevent refrigerant accumulation that would lead to compressor flooding during load reductions.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system applies preliminary anti-action by pre-closing the expansion valve to counteract the potential harmful effect of compressor flooding. The feed-forward controller anticipates load reductions and adjusts the valve position in advance to prevent refrigerant accumulation, thereby neutralizing the potential harm before it occurs.

Inventive Principle:

Principle #9Preliminary anti-action

Data Source

PatentEP3268682B1Expansion valve control

Publication Date: 2022.08.24 CARRIER CORP

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AI summary

A method for controlling a refrigeration system having a compressor, heat rejecting heat exchanger, expansion valve and heat absorbing heat exchanger circulating a refrigerant in series flow, the heat absorbing heat exchanger in thermal communication with working fluid, the method includes obtaining an expansion valve position set point; using a feedback control loop to generate a controlled expansion valve position; obtaining a rate of change of an operating parameter of the system; using the rate of change of the operating parameter to generate an adjustment; modifying the controlled expansion valve position using the adjustment; and controlling the expansion valve using the modified controlled expansion valve position.