Cryogenic Refrigerator Variable-Speed Expander for Fast Cool Down
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Cryogenic refrigerators face inefficiencies during the cooling process, particularly at startup, where the compressor's heaviest load occurs with only partial output flow, and existing systems struggle to maintain optimal power input and refrigeration rate as the system cools down due to changing gas densities and pressures.
Innovation Solution
The solution involves operating the expander at maximum speed near room temperature and gradually slowing it down as the load cools, while maintaining constant supply pressure by transferring gas from a storage tank, and using a variable speed drive with an adjustable orifice to optimize piston speed, ensuring maximum compressor power usage and near-constant pressure differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the compressor is sized to provide the flow rate needed when the unit is cold, then the compressor can meet the refrigeration demand at operating temperature, but the compressor operates with excessive capacity and partial flow utilization during cool down, reducing efficiency
Solution Approach 1:
The expander speed is made variable rather than constant. During cool down, the expander operates at higher speeds to maximize refrigeration effect when gas density is low. As the system approaches operating temperature and gas density increases, the expander speed is reduced to match the compressor flow capacity, ensuring full utilization of compressor power without excessive capacity
Solution Approach 2:
The system changes the operating parameters (speed) of the expander based on the thermal state of the system. At different temperatures and gas densities, the expander operates at different speeds to optimize the match between compressor output and expander input, transforming the fixed-parameter system into a variable-parameter system that adapts to cooling stage requirements
2Device complexity
If the expander operates at constant speed, then the system design is simplified, but the refrigeration efficiency drops during cool down when gas density is low
Solution Approach 1:
The expander transitions from constant speed operation to variable speed operation, where the speed is dynamically adjusted based on the cooling stage and gas density conditions, optimizing refrigeration efficiency without significantly increasing system complexity
Solution Approach 2:
The system incorporates feedback control where the expander speed is adjusted in response to the thermal state and gas density conditions in the system, ensuring optimal matching of compressor and expander performance throughout the cooling process
3Quantity of substance
If gas is added to compensate for increased gas density during cool down, then the mass flow rate can be maintained, but the system complexity increases with gas storage and control mechanisms
Solution Approach 1:
The system uses the natural physical property of gas density change with temperature to automatically regulate mass flow rate. As the system cools and gas density increases, the higher density gas naturally provides sufficient mass flow to the expander at reduced speeds, eliminating the need for external gas addition mechanisms
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 maximizes refrigeration rate by utilizing full compressor power during cooldown, maintaining high refrigeration efficiency and reducing input power consumption, especially at lower temperatures, by maintaining constant pressure and flow rates, thus overcoming the inefficiencies associated with changing gas densities and pressures.
Implementation Method 1
an expander that expands the gas adiabatically to a low pressure, exhausts the expanded gas (which is colder)
Implementation Method 2
a compressor that supplies gas at a high pressure to a counterflow heat exchanger
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A refrigeration system for minimizing the cool down time of a mass to cryogenic temperatures including a compressor, an expander, a gas storage tank, interconnecting gas lines, and a control system. The compressor output is maintained near its maximum capability by maintaining near constant high and low pressures during cool down, gas being added or removed from the storage tank to maintain a near constant high pressure, and the speed of said expander being adjusted to maintain a near constant low pressure, no gas by-passing between high and low pressures.