Foreline Traps - Introduction

An Introduction to Foreline Traps.

As part of the fight against oil contamination, it is essential that any vacuum system is properly protected against contamination from rotary pump oil. All the time a rotary pump has a sizeable gas volume passing through it, there is little chance of oil contamination; however, once the limiting pressure of the pump is reached (with little gas volume passing through) there is a possibility of rotary pump oil molecules moving up the backing line against the gas flow direction, creating a potential contamination problem known as back-streaming. To overcome this, the Foreline Trap was developed; its method of operation is described below.

Greater than 99% of back-streaming stopped
Regeneration in-situ (except ZRT3)
Choice of molecular sieve or activated alumina sorbent
Available with wide range of flange connections

Method of Operating the Foreline Trap.

The Foreline Trap is placed upstream of the rotary pump and contains a material with the ability to trap any passing oil molecules. The material is placed in a line of sight position so that any gas molecules moving in either direction still have to pass through it.

There are various forms of material used, but the two most common versions that used are Activated Alumina (AA) and Molecular Sieve (MS). The main property of these materials is that they have a very large surface area relative to their physical size. They have gas molecule sized pores in their outer surfaces that lead to large internal pathways. Any molecules straying down these pores get lost in the internal maze of pathways, and are effectively ‘captured’.

Molecular sieves are supplied in various grades, we supply types 5A and 10A. The number refers to the size of pore on the surface. To get the sieve to trap large oil molecules, we need to use the large pore size (10A). The 5A sieve would be used for trapping smaller gas molecules (such as O₂, N₂ etc..) and is also used in sorption pumps as the pumping medium.
Molecular sieve is a crystalline Potassium Alumino-silicate.

Activated Alumina is used in systems where there is the possibility of having a higher water content than normal as it is better at trapping water than molecular sieve.

As there are a finite number of pores, the sieve eventually becomes exhausted; the time period for this depends heavily on system type and usage. At this point the sieve needs to be regenerated. The trap must be shut off from the system (a valve should really be used (or an air bleed valve)) and the integral heater started. At the same time, the rotary pump air bleed screw should be opened to stop the oil from being saturated by the heated contaminants (mainly water from the sieve).

Once the sieve has baked for a few hours, the heater can be turned off, the trap given at least an hour to cool, and it can then be re-used.

Sieve Lifetime.

It should be mentioned that a molecular sieve does not last forever. In systems where processes such as evaporation are being carried out, the sieve can become quickly saturated with particles such that it cannot not be regenerated.

The colour of the sieve normally indicates the level of contamination. An off-white colour if it is satisfactory, whereas if it is black then replacement of the the sieve is indicated.

Remember that when a new sieve is first installed, it must be baked to regenerate it before it can be used.