Cryogenic Refrigerator Pressure Control for Faster Cooldown
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Solution Overview
Problem
Cryogenic refrigerators face inefficiencies during the cooling process due to variable gas density and pressure differences, leading to suboptimal power usage and refrigeration rates, especially during the initial stages of cooling when the heaviest load occurs.
Innovation Solution
The solution involves operating the expander at maximum speed near room temperature and gradually reducing speed as the load cools, while maintaining constant supply pressure through a gas storage system, using a variable speed drive for the expander, and optimizing the orifice control in pneumatically driven systems to maintain near-constant pressure differences and maximize compressor power usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
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 inefficiently during cool down when gas density is low and pressure difference is high
Solution Approach 1:
The patent applies a variable speed drive to the compressor, allowing it to operate at optimal speeds across different operating conditions. During cool down, the compressor runs at higher speeds to compensate for low gas density, while at operating temperature it adjusts to maintain efficient operation. This dynamic speed adjustment resolves the contradiction between meeting refrigeration demand and maintaining compressor efficiency throughout the cooling process.
Solution Approach 2:
The patent changes the operating parameters of the compressor by introducing variable speed control and adjustable pressure relief valve settings. These parameter changes allow the system to optimize compressor performance across the entire temperature range, preventing the efficiency loss that occurs when a fixed-speed compressor operates under varying gas density conditions.
2Device complexity
If the expander operates at constant speed, then the system design is simple, but the expander cannot utilize full compressor power during cool down when gas density is low
Solution Approach 1:
The patent introduces variable speed control to the expander, allowing it to operate at maximum speed during cool down to utilize full compressor power, and then reduce speed as the system reaches operating temperature. This dynamic speed adjustment significantly increases the refrigeration rate during the critical cool down phase while maintaining acceptable system complexity through the use of variable speed drives.
3Device complexity
If a fixed orifice is used in pneumatically driven expanders, then the design is simple, but the speed range is limited and efficiency falls off from the best orifice setting
Solution Approach 1:
The patent replaces the fixed orifice with an adjustable orifice that can be tuned to optimize expander performance across different speed ranges. This allows the system to maintain high efficiency whether operating at high speeds during cool down or lower speeds at operating temperature, while the adjustability provides the versatility needed for different operating conditions without excessive complexity.
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 allows for maximizing refrigeration rates by utilizing full compressor power during cooldown, maintaining efficient operation across a wide temperature range, and reducing power input by 25% at 40 K while maintaining high refrigeration efficiency.
Implementation Method 1
an expander that expands the gas adiabatically to a low pressure
Implementation Method 2
a compressor that supplies gas at a high pressure to a counterflow heat exchanger
Implementation Method 3
A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a high pressure
Data Source
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.