Welch Vacuum 1376N Chemstar - Manual (Page 26)

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C 3. Particulate Trap With A Filter Element
In some corrosive gas applications, particulates are generated. A particulate trap with a fi lter element provides positive
protection for CHEMSTAR pumps when dusty or loose particulates generated in the application can be ingested by the
pump.
Welch offers a particulate trap, cat. No. 544002 for Models 1400N-01, 1402N-01 and 1376N-01. Cat No. 544003 is the
particulate trap to use with Model 1374N-01. The particulte trap incorporates a cleanable, radial fi n fi lter element. This
design gives maximum fi lter area in a compact vertical fi n design. The fi lter element can be fi eld cleaned with a vacuum
cleaner or replaced when the element is damaged or uncleanable.
Filter elements are mainly polyester providing 10 micron solid retention at 98% effi ciency. Filter housings is stainless steel
and can be quickly serviced without breaking inlet/outlet lines that are in a horizontal in-line confi guration.
Like any porous fi ber, a small amount of water is absorbed by the fi lter element during manufacture. When the element is
used in a trap, you should expect it will take a few minutes for this trace water to be drawn out. The replacement element
cat. No. is 544982 for trap cat. No. 544002. The replacement element cat. No. is 544983 for cat. No. 544003.
D. Insuring an Adequate Exhaust System
In order to protect yourself and your co-workers install an exhaust system which will allow safe remove of corrosive or
hazardous gases. The exact details of the design should be developed after reviewing standard designs as given in
sources such as:
“Industrial Ventilation, A Manual of Recommended Practice, “American Conference of Governmental Industrial Hygienists,
Cincinnati, OH. Publication updated biannually.
The American Vacuum Society’s recommended practices for exhaust systems may be found in the following article:
Part 3 Operating a Vacuum System Containing Corrosive Gases
A. Removal of Condensable Corrosive Vapors Using the Gas Ballast
Vacuum systems, which contain undesirable vapors, will cause diffi culties. The two major ones are the degrading of the
pump oil which is acting as a lubricant and from the standpoint of attaining desirable ultimate pressure.
A vapor is defi ned as the gaseous form of any substance which is usually a liquid or a solid at standard conditions. Water,
acetic acid, and formic acid vapors are examples. When such vapors exist in a vacuum system, the vapors or mixtures
of gas and vapor are subject to condensation within the pump. The precipitated liquid may ultimately dissolve, react or
become emulsifi ed within the oil. Not only will this lead to premature failure of the pump, but also, the ultimate pressure
achievable by the pump will be degraded as a result.
Condensation takes place particularly in the compression stroke of the second stage in the CHEMSTAR pump. The
compression stroke is that portion of the cycle during which the gas drawn from the intake port is compressed to the
pressure necessary to expel it pass the exhaust valve. Condensation takes place when the ration between the initial
pressure and end pressure of the compression is high. By adding air through the gas ballast valve to the mixture of vapor
and gas being compressed, the pressure required for delivery past the exhaust valve is reached with a considerably
smaller reduction of the volume of the mixture. Depending upon the amount of air added, condensation of the vapor is
either entirely avoided or substantially reduced.
With the gas ballast open, the sound of the exhaust is similar to that of a pump operating against a large leak. Because
of the increased pressure introduced into the compression stroke, the pump must work a little harder to function, thus
resulting in an increased operating temperature of approximately 8°C over a prolonged period of time. Tests have shown
that continuous and prolonged operation for several weeks under these conditions is not injurious to the pump.
The ultimate pressure achievable by the vacuum pump increases by a factor of thirty when the gas ballast is open as
measured with a McLeod gauge.
B. Diluting Corrosive Gases
The lower the % of the gas fl ow into the CHEMSTAR pump which is corrosive gases the better. Hence, if you have
extra pumping capacity in your application and corrosive gases making up more than a few percent the gas load, Welch
recommends diluting the gas with dry nitrogen or argon (See specifi c recommendations for various gases in Section 4). If
you are planning a new system, Welch recommends you use a pump with a higher capacity than initially calculated if you
plan to dilute the corrosive gases.
The dry nitrogen or argon can be bleed into the intake of the CHEMSTAR pump through a needle valve attached to
vacuum piping joint to the vacuum chamber. Be sure not to add too much dry nitrogen or argon such that the total
pressure of the system rises above 10 Torr.