Cascade Air Conditioning Startup Sequence for Two-Phase CO2 Loops
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Solution Overview
Problem
Refrigerant systems with two-phase CO2 as a secondary heat transfer fluid face increased complexity and operational inefficiency, particularly during startup, due to the need to maintain CO2 in a supercritical fluid state, which can lead to equipment and operational challenges such as pump cavitation.
Innovation Solution
A dual heat transfer fluid circulation loop system is implemented, where a first vapor/compression loop is initialized before starting a second two-phase loop, allowing for coordinated operation and heat transfer between the loops through internal heat exchangers, with specific fluid and pump configurations to prevent cavitation and enhance efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If CO2 is maintained in a supercritical fluid state as a secondary heat transfer fluid, then heat transfer efficiency is improved, but equipment complexity and operational difficulty increase
Solution Approach 1:
The system performs preliminary action by starting the first heat transfer fluid circulation loop before starting the second loop with CO2. The first loop is initialized to establish proper operating conditions, and only after it is running does the system introduce the CO2 loop, preventing operational issues while maintaining heat transfer efficiency.
Solution Approach 2:
The first heat transfer fluid loop acts as an intermediary system that prepares the thermal environment before introducing the CO2 loop. This intermediary approach allows the system to manage the complexity of supercritical CO2 operation by having a buffer system already in place to handle thermal transitions.
2Use of energy by moving object
If CO2 is maintained in a supercritical fluid state, then heat transfer performance is improved, but operational inefficiency and pump cavitation occur during startup
Solution Approach 1:
The system performs preliminary action by starting the first heat transfer fluid circulation loop before starting the second loop with CO2. The first loop is initialized to establish proper operating conditions, and only after it is running does the system introduce the CO2 loop, preventing operational issues while maintaining heat transfer efficiency.
Solution Approach 2:
The system provides beforehand cushioning by having the first heat transfer fluid loop already operational before introducing CO2. This buffer system cushions against thermal shocks and pressure fluctuations that would otherwise cause pump cavitation and operational inefficiency during startup.
3Loss of time
If a two-phase CO2 circulation loop is started simultaneously with the first loop, then system startup time is reduced, but pump cavitation and operational inefficiency occur
Solution Approach 1:
The system performs preliminary action by starting the first heat transfer fluid circulation loop before starting the second loop with CO2. The first loop is initialized to establish proper operating conditions, and only after it is running does the system introduce the CO2 loop, preventing operational issues while maintaining heat transfer efficiency.
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 approach reduces operational inefficiencies and equipment complexity by optimizing startup sequences and fluid circulation, improving the overall efficiency and reliability of the refrigeration system while maintaining the CO2 in a supercritical state.
Implementation Method 1
transfers heat to the first heat transfer fluid circulation loop through the heat exchanger evaporator/condenser
Implementation Method 2
heat exchanger evaporator/condenser
Implementation Method 3
heat transfer fluid is compressed in a compressor from a lower to a higher pressure
Implementation Method 4
heat rejection heat exchanger, commonly referred to as a condenser... where heat is typically transferred from the heat transfer fluid to ambient environment
Implementation Method 5
high-pressure heat transfer fluid flows to an expansion device where it is expanded to a lower pressure and temperature
Implementation Method 6
evaporator, where heat transfer fluid cools a secondary heat transfer fluid to be delivered to the conditioned environment
Data Source
AI summary
A method of operating a heat transfer system includes starting operation of a first heat transfer fluid vapor/compression circulation loop including a fluid pumping mechanism, a heat exchanger for rejecting thermal energy from a first heat transfer fluid, and a heat absorption side of an internal heat exchanger. A first conduit in a closed fluid circulation loop circulates the first heat transfer fluid therethrough. Operation of a second two-phase heat transfer fluid circulation loop is started after starting operation of the first heat transfer fluid circulation loop. The second heat transfer fluid circulation loop transfers heat to the first heat transfer fluid circulation loop through the internal heat exchanger and includes a heat rejection side of the internal heat exchanger, a liquid pump, and a heat exchanger evaporator. A second conduit in a closed fluid circulation loop circulates a second heat transfer fluid therethrough.