Principle of liquid nitrogen cold traps

LN₂ Cold Traps (CCT Series) - Introduction

Long life liquid nitrogen reservoir.
Good pumping speeds, high reliability.
Available in vertical or horizontal configurations.
Manual or pneumatic isolation valve option.
Alternative pump and system flanged versions available.

The diffusion pump was one of the earliest pumps to be used in the UHV industry; its attractive feature being its ability to pump both active and inert gases without a memory effect. With the addition of a CCT trap it becomes a true UHV pump. On going chemical developments that have constantly improved the range and quality of the pump oils, which has ensured that these pumps have kept retained their importance in the market. The diffusion pump is popular with customers requiring reliability, high pumping speeds, low maintenance and cheap running costs.

There are two historical arguments against using a diffusion pump: namely the damage resistance of the pump when accidentally exposed to air whilst running, and oil contamination of the vacuum system.

The performance and design of modern diffusion pump oils minimises the problems encountered by accidental exposure of the pump to air.

With the combination of VG Scienta CCT traps and good UHV operating practise, we have been able to reduce oil contamination to a minimum. The combination of liquid nitrogen cooling and a specially designed chevron array allows a large surface area to be kept at nearly -196° C for a long time.

This long term improvement of performance has permitted the diffusion pump to continue to be operated in the UHV industry, especially where high pumping throughput at a low price exist is required.

Operating Principle (CCT Series)

The CCT trap consists of 2 basic parts: an outer body and an inner liquid nitrogen reservoir. The inner reservoir is demountable from the CCT outer body allowing it to be replaced in the event of a leak or damage occurring. The reservoir is filled with liquid nitrogen through a series of ports. The vertical versions have 3 (one for filling, one for venting, and one for liquid nitrogen level detecting devices), whereas the horizontal versions have only 2 ports (filling and venting).

Mounted on the base of the reservoir is a series of nickel chevrons which form the major trapping surface area of the pump. As they are welded at the bottom of the liquid nitrogen reservoir, their performance is unaffected by the level of the liquid nitrogen in the trap (until it is empty). Since the bulk of the oil condensate builds up on the chevrons, it means that a vertical CCT trap can be left operational for typically 24 hours at 10-10 mbar (and approx. 4 hours for a horizontal version) before the liquid nitrogen charge runs out.

Any oil condensate evaporating from the reservoir as the level falls is recondensed lower down on the reservoir, since it is unlikely to escape from the narrow annular gap between the reservoir and the outer trap body. Experiments show a constant system pressure is held until the trap reservoir is empty, at which point the pressure rises rapidly over the following 30 minutes or so.

Above the diffusion pump mounting port (or pump port for short) is mounted a water cooled baffle. This baffle serves 2 purposes. In the first instance it acts as a primary baffle, cutting out gross back-streaming of oil (the hot oil drops condense on the surface and drip back down into the diffusion pump under gravity) and as a ‘creep’ baffle. Secondly, it protects the Indium wire seal between the diffusion pump and the CCT body. This seal is most likely to melt during bakeout if it is not cooled (Indium melts at 157°C). If the water cooling is not connected (or flow switch protected), severe damage to the system / pump may result. It is essential to incorporate a water flow switch into the cooling circuit to indicate cooling water flow.

Horizontal and Vertical Mounting

There are 2 distinct styles of CCT trap, horizontal and vertical. The operating principle, is exactly the same for both styles of trap.

The horizontal units are normally mounted underneath the system bench, whereas the vertical ones normally have the base flange (and connected Diffusion pump) underneath the bench and the rest of the body (and system flange) in the baked system area. This gives several advantages and disadvantages to each style of CCT.

The horizontal versions free up much more bench top area, but are more expensive (due to the more complicated nature of the design). They can also have a slightly reduced pumping speed compared to the vertical design.
They are used when overall height is a problem. The lifetime of the liquid nitrogen in a horizontal CCT is also about 1/5 for that in a vertical version.

The vertical versions use up more of the bench top area; however, this also means that they can be easily included in the system bakeout area (essential for UHV use). Vertical traps can also give slightly better overall performance as the gas flow path is less tortuous compared to the horizontal versions. The liquid nitrogen has a longer lifetime than the horizontal version.

Please note that the CCT bakeout zone extends down the water cooling pipes and baffle at the base of the CCT, any parts below this are NOT bakeable.