Refrigerant cycle control system and control method for reliquefaction system for ship
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Solution Overview
Problem
Existing reliquefaction systems for boil-off gas in LNG storage tanks on ships struggle to effectively adjust the amount of cold heat in response to changes in load while maintaining process stability, leading to inefficiencies and potential destabilization.
Innovation Solution
A refrigerant cycle control system that includes a control unit to manage refrigerant flow using PID control and on/off control based on suction and discharge pressure differentials, adjusting refrigerant charging and discharging to stabilize the reliquefaction process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If PID control is used to continuously adjust refrigerant flow in response to load changes, then the amount of cold heat can be adjusted to match demand, but the system becomes unstable due to frequent refrigerant charging and discharging
Solution Approach 1:
The control system dynamically switches between PID control mode and on/off control mode based on operating conditions. When load changes require adjustment, PID control is used to modify refrigerant flow. When the system approaches steady state, the control switches to on/off mode to maintain stability, preventing excessive refrigerant charging/discharging cycles.
Solution Approach 2:
The control strategy changes parameters (control mode) based on system state. The system monitors refrigerant flow rates and temperature differences, and when certain thresholds are met, it transitions from continuous PID adjustment to discrete on/off control, thereby stabilizing the reliquefaction process while still adapting to load changes.
2Productivity
If refrigerant flow is increased to meet higher load demands, then more boil-off gas can be reliquefied, but the system becomes inefficient due to excessive refrigerant charging and discharging
Solution Approach 1:
Instead of continuous refrigerant charging and discharging, the system uses periodic on/off control actions. When the refrigerant flow reaches optimal levels for the current load, the system maintains that state through on/off control rather than continuously adjusting, thereby reducing energy waste from frequent cycling while still meeting productivity demands.
3Ease of operation
If separate refrigerant is used for reliquefaction, then the refrigeration cycle can be independently controlled, but the system complexity increases
Solution Approach 1:
The control system merges the control of refrigerant flow and temperature regulation into a unified control strategy. By using a single control unit that implements both PID and on/off control modes, the system achieves independent refrigeration control without requiring separate complex control systems for each function, thereby reducing overall system complexity while maintaining operational flexibility.
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
Enhances reliquefaction efficiency by stabilizing the process and optimizing cold heat utilization, preventing destabilization due to frequent refrigerant fluctuations.
Implementation Method 1
a refrigerant compressor disposed on the refrigerant circulation line to compress the refrigerant discharged from the heat exchanger after cooling the boil-off gas
Implementation Method 2
boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line
Implementation Method 3
a control unit configured to receive the suction pressure set point and adjust the amount of cold heat in the refrigerant circulation line by charging the refrigerant into the refrigerant circulation line or discharging the refrigerant from the refrigerant circulation line
Implementation Method 4
a suction pressure detector configured to detect a suction pressure of the refrigerant introduced into the refrigerant compressor on the refrigerant circulation line and to transmit a current suction pressure value (PVs)
Data Source
Figure 1(a)~1(b)
Figure 2(a)~2(b)
Figure 3~5
AI summary
Disclosed herein are a system and method for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line. The system according to the present invention includes: a refrigerant compressor disposed on the refrigerant circulation line to compress the refrigerant discharged from the heat exchanger after cooling the boil-off gas; a suction pressure detector configured to detect a suction pressure of the refrigerant introduced into the refrigerant compressor on the refrigerant circulation line and transmit a current suction pressure value (PVs); a load controller configured to transmit a suction pressure set point (SPs) for load adjustment of the reliquefaction system; and a control unit configured to receive the suction pressure set point and adjust the amount of cold heat in the refrigerant circulation line by charging the refrigerant into the refrigerant circulation line or discharging the refrigerant from the refrigerant circulation line, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on/off control to stop charging and discharging of the refrigerant.