MAGUIRE LPD-30, Установка, эксплуатация и обслуживание

Страница: 93 / 127

Maguire Products, Inc.
Edition: November 5, 2010
93
LPD Dryer
®
6.5 - Theory of Operation / Performance
THEORY OF VACUUM DRYING
Water boils at 212 F (100 C) degrees. However, this is only true at sea level, which is to say at
standard atmospheric pressure, which is 14.7 pounds/sq in (1 bar), also expressed as 29.92
inches (760mm) of Mercury (Hg).
At lower pressures the boiling point of water is reduced.
Standard atmospheric pressure can support a column of Mercury 29.92 inches (760mm) high. If
we pull a perfect vacuum above a column of Mercury, the mercury will rise in that column 29.92
inches and, for that reason, the number we can expect to read on the vacuum gauge, at full
vacuum, is 29.92 inches. Lesser vacuums read lower numbers. No vacuum reads zero.
When water is subjected to a vacuum level of 25 inches (635mm) of mercury, it will boil at 133F
(56C) degrees. When plastic pellets are heated to 160F (71C) degrees, or greater, and subjected
to a vacuum of
25 inches (635mm), the water vapor within wants very much to boil. This increased molecular
activity within the pellet and the greatly reduced pressure surrounding the pellet drives the
moisture from the pellet in a remarkably short time. This then is the reason for the remarkable
short drying time of a vacuum dryer.
PERFORMANCE
The true measure of a dryer's performance is determined by the moisture content of the resin
after the dryer has done its job. Resin moisture content, however, is not easily measured, so
dryer manufactures use other criteria to assure performance.
Conventional "desiccant" dryers use DEW POINT as a measure of performance. This is a
measure of the dryness of the air passing over the resin, but not the dryness of the resin itself.
For example, for a particular resin, experience may tell us that 180f (82c) degree air dried to
minus 40 dew point, and passed over the material for 4 hours, is sufficient to reduce the moisture
content of that resin to the required level of dryness.
Since our LPD dryer does NOT use dry air, we have no "dew point" to measure.
In our case, for the same resin, experience tells us that a vacuum of 25 inches (635mm) applied
for 20 minutes to material that has been heated to 180f (82c) degrees, is sufficient to reduce the
moisture content of that same resin to the correct level of dryness.
Therefore, just as desiccant driers assure dry material by measuring temperature and DEW
POINT over time, we assure dry material by measuring temperature and VACUUM over time.
When we assure that a certain temperature has been reached and a certain vacuum level
achieved for a correct length of time, we can then be assured the material is dry.
You may visually assess performance by monitoring temperature and vacuum levels yourself. Of
course, the final test is in the quality of the product you manufacture. We welcome your
comments and observations.