Expansion Valve Control Using Subcooling for Stable Evaporator Capacity
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Solution Overview
Problem
Vapour compression systems face instability and reduced evaporator capacity due to the need to prevent liquid refrigerant from reaching the compressor, which limits the utilization of refrigeration capacity and introduces operational risks.
Innovation Solution
A method that controls the refrigerant supply to the evaporator by measuring and accounting for both superheat and subcooling values, utilizing an internal heat exchanger to evaporate any liquid refrigerant and maintain optimal superheat, thereby ensuring stable operation and maximizing evaporator capacity without risking compressor damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If liquid refrigerant is allowed to pass through the evaporator to maximize evaporating capacity, then the refrigeration efficiency is improved, but the risk of liquid refrigerant reaching the compressor increases causing potential damage
Solution Approach 1:
The suction line heat exchanger acts as an intermediary device between the evaporator and compressor. It uses heat exchange with the condenser to evaporate any liquid refrigerant that passes through the evaporator, preventing liquid from reaching the compressor while allowing the evaporator to operate with liquid refrigerant throughout for maximum capacity
Solution Approach 2:
The system changes the temperature parameter of the refrigerant by using the suction line heat exchanger to heat the refrigerant leaving the evaporator. This temperature increase causes liquid refrigerant to evaporate before reaching the compressor, resolving the contradiction between allowing liquid in the evaporator and protecting the compressor
2Reliability
If superheat is maintained positive to prevent liquid refrigerant from reaching the compressor, then compressor safety is improved, but the evaporating capacity of the evaporator is not fully utilized
Solution Approach 1:
The suction line heat exchanger serves as a mediator that allows the evaporator to maintain zero or negative superheat for maximum capacity while still ensuring positive superheat at the compressor inlet through additional heating in the heat exchanger
Solution Approach 2:
The superheating process is segmented into two stages: first in the evaporator where liquid refrigerant is present throughout for maximum capacity, and second in the suction line heat exchanger where additional heating ensures compressor protection, separating the conflicting requirements of the two components
3Reliability
If an internal heat exchanger is added to evaporate liquid refrigerant, then compressor safety is improved, but the system complexity increases
Solution Approach 1:
The suction line heat exchanger performs multiple functions: it heats the refrigerant to evaporate liquid, cools the condenser outlet refrigerant to increase subcooling, and prevents liquid from reaching the compressor. This multi-functionality reduces the need for separate components, offsetting the added complexity with functional consolidation
Solution Approach 2:
The heat exchanger merges the functions of superheating and subcooling into a single component located in the suction line. By combining these functions and utilizing the temperature difference between condenser outlet and evaporator outlet refrigerants, the system achieves compressor protection without requiring multiple separate devices
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 method stabilizes the operation of vapour compression systems, optimizes refrigerant supply, and maximizes evaporator capacity by precisely controlling the expansion device based on superheat and subcooling, reducing the risk of liquid refrigerant reaching the compressor and enhancing refrigeration efficiency.
Implementation Method 1
refrigerant leaving the evaporator is heated in the internal heat exchanger
Implementation Method 2
in the case that a small amount of liquid refrigerant leaves the evaporator, it is evaporated in the internal heat exchanger
Implementation Method 3
heat exchanging fluids flow in parallel in order to stabilise the system
Implementation Method 4
an expansion device and an evaporator arranged along a refrigerant path. Refrigerant flowing in the refrigerant path is alternatingly compressed and expanded
Data Source
Figure 1~2
Figure 3a~3b
AI summary
A method for operating a vapour compression system (1) is disclosed. The vapour compression system (1) comprises a compressor (2), a condenser (3), an expansion device (4), e.g. in the form of an expansions valve, and an evaporator (5) arranged along a refrigerant path. The method comprises the steps of: obtaining a superheat value being representative for the superheat of refrigerant entering the compressor (2); obtaining a subcooling value being representative for the subcooling of refrigerant entering the expansion device (4); and operating the expansion device (4) on the basis of the obtained superheat value and on the basis of the obtained subcooling value. It is an advantage that the subcooling value is taken into account when operating the expansion device (4), because variations in the subcooling value has significant influence on the refrigerating capacity of the evaporator (5) at a given opening degree of the expansion device (4). A more stable operation of the vapour compression system (1) is therefore obtained when taking the subcooling value into consideration. The vapour compression system (1) may advantageously further comprise an internal heat exchanger (6), e.g. in the form of a suction line heat exchanger.