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Cold trap is a gas trapping device which are widely used in vacuum coating machines

Cold trap is a gas trapping device which are widely used in vacuum coating machines


Cold trap is a gas trapping device which are widely used in vacuum coating machines. It use the low temperature wall to achieve gas trapping effect. In addition to the influence of its design structure, the trapping effect of cold trap often depends on the temperature of the refrigerant used. In general, the lower the refrigerant temperature, the better the performance of the cold trap.


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一.Common refrigerants for cold traps and their refrigerating capacity are as follows:


1The cooling temperature of water at 10⁵Pa is 0ºC;


2The cooling temperature of ice-containing brine at 10⁵Pa is -18ºC;


3The cooling temperature of iced calcium chloride at 10⁵Pa is -48ºC;


4The cooling temperature of the acetone mixture at 10⁵Pa is -78ºC;


5The cooling temperature of liquid oxygen is -183ºC at 10⁵Pa;


6、The cooling temperature of liquid nitrogen is -196ºC at 10⁵Pa;


7The refrigerating temperature of liquid hydrogen at 10⁵Pa is -258ºC.


In addition, solid carbon dioxide is also a commonly used refrigerant in cold traps.


二、Glass Cold Trap


Glass cold traps are divided into internal cold traps and external cold traps according to the way of the refrigerant contained. The advantage of the internally cooled glass cold trap is  low heat consumption and large conductance. The disadvantage is difficult to manufacture and the complex structure ; the advantage of the externally cooled glass cold trap is simple structure and easy to manufacture, There is no high requirement for glass blowing technology, but required to an open dewar bottle to  hold the refrigerant.


三、Metal Cold Trap

The metal cold trap with complex structure as there has many different types.

 In order to ensure the good oil retention effect and large flow conductance , we need to pay attention to the follow design point:

(1)The metal cold trap has a reasonable structure, not only needs to consider the convenience of welding and leak detection, but also consider the problem of heat loss, so there  no need too high heat loss;


(2)The temperature of the condensing surface in the metal cold trap should be kept constant, so that it will not be affected by the liquid nitrogen surface change, and avoid the situations of  liquid nitrogen level drops, the oil steam adsorb on the condensing surface will release due to the temperature of the condensing surface increases:


3Metal cold traps are used in ultra-high vacuum equipment,At least one of the passages in which the oil climbs must be in a low temperature state to prevent the oil from climbing;


4The condensing surface should be aligned with the incident direction of the reflux vapor molecules, and should have at least one optical shielding to obtain the best trapping performance.


(5)For cold traps with larger conductance, enlarged the middle part of the cold trap to obtain the most favorable geometry for gas conductance.


 Vacuum state of vacuum coating machine is maintained by vacuum pump to extract the gas in the chamber. However, some of the gas in the cavity may be corrosive. If these gases enter the vacuum pump, it will cause corrosion and affect the precision and durability of the pumping system. Therefore, it is usually necessary to set up a cold trap between the vacuum pump and the vacuum chamber to trap these gases through low-temperature condensation.


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