Back migration in vacuum systems can be caused by several factors:

1. Leaks: One of the main causes of back migration is leaks in the vacuum system. Even small leaks can allow gas molecules from the external environment to enter the vacuum chamber, leading to an increase in pressure and reduced vacuum effectiveness. Leaks can occur at various points in the system, including seals, gaskets, valves, and connections.
2. Temperature changes: Changes in temperature can also cause back migration. When the temperature inside the vacuum chamber increases, the gas molecules inside gain energy and can escape back into the external environment. Similarly, when the temperature decreases, gas molecules from the external environment can enter the chamber.
3. Outgassing: Outgassing refers to the release of gases from materials used in the vacuum system. Some materials, especially those with high vapor pressure, can release gas molecules over time. These gas molecules can migrate back into the chamber, increasing the pressure.
4. Residual gases: Even after thorough evacuation, there are often residual gases present in the vacuum system. These gases can be difficult to remove completely and can contribute to back migration.

To prevent back migration, it is important to design vacuum systems with proper seals, gaskets, and valves to minimize leaks. Regular maintenance and monitoring of the system are also crucial to identify and address any potential sources of back migration. Techniques such as bake-out and degassing can be used to remove trapped gases from the system.

Source: https://www.lesker.com/newweb/vacuum_technology/vacuum_technology_handbook/vacuum_system_design.asp

To choose materials for a corrosion-resistant heat exchanger, you need to consider several factors such as the operating conditions, the corrosive environment, and the compatibility of the materials with the fluids being processed. Here is a step-by-step guide on how to choose materials for a corrosion-resistant heat exchanger:

1. Identify the operating conditions: Determine the temperature, pressure, and flow rate of the fluids that will be processed in the heat exchanger. These factors will help you select materials that can withstand the specific operating conditions.
2. Assess the corrosive environment: Identify the corrosive elements present in the fluid being processed. This can include chemicals, acids, alkalis, or other corrosive substances. Consider the concentration and temperature of these corrosive elements, as they can significantly impact the material selection.
3. Understand material compatibility: Research the compatibility of different materials with the fluids being processed. Some materials may be resistant to certain corrosive elements but may corrode when exposed to others. Consider the chemical composition, temperature limits, and corrosion resistance properties of the materials under consideration.
4. Consult corrosion resistance charts: Corrosion resistance charts provide valuable information about the compatibility of different materials with specific corrosive environments. These charts rate materials based on their resistance to corrosion in different conditions. Use these charts as a reference to narrow down your material options.
5. Consider the cost and availability: Evaluate the cost and availability of the materials you are considering. Some materials may be highly corrosion-resistant but may be expensive or difficult to source. Balancing the cost and availability with the required corrosion resistance is important in the material selection process.
6. Seek expert advice: If you are unsure about the material selection, consult with corrosion engineers or experts in heat exchanger design. They can provide valuable insights and recommendations based on their experience and knowledge.

It is important to note that the selection of materials for a corrosion-resistant heat exchanger is a complex process that requires careful consideration of various factors. Consulting with experts and conducting thorough research is crucial to ensure the optimal material selection for your specific application.

Sources:

“Materials Selection for Corrosion Control” by Roger Francis, John P. Broomfield

“Corrosion Resistance Tables: Metals, Nonmetals, Coatings, Mortars, Plastics, Elastomers, and Linings and Fabrics” by Philip A. Schweitzer

Liquid nitrogen cold traps prevent products arising during the equipment operation from passing from the chamber into the pump where they could either contaminate the pump or cause the breakdown of the pump fluid. In either case, severe loss of efficiency and poor vacuum levels result. In the example shown the diffusion pump was only capable of reaching 4 x 10-3 Torr rather than the typical 1 x 10-5 Torr range achieved with a clean pump. In addition, these types of cold traps prevent backstreaming of the pump.

SOURCE: Cold Traps / VAC AERO International Inc.