Diamond Systems Tritan Anleitung zur Fehlerbehebung PDF herunterladen (Seite 28)

Drying time. Pellets to be dried need to be in the hopper at the

conditions shown on the data sheets for each speciﬁc polymer.

If the dryer is turned on from a cold start, it must warm up to

the proper temperature and the dew point of the air must be

reduced to –30°C (–20°F) or below before drying time can

be counted.

Choosing the hopper size is critical; only when the hopper size

is adequate for the rate of processing will the proper residence

time in the hopper be possible. For example, if a 454-g (1-lb)

part is being run on a 1-minute cycle, then 27.2 kg (60 lb) of

dry material will be needed each hour. If 6 hours is required for

drying, then at least 164 kg (360 lb) of material must be in the

hopper continuously (27.2 kg/h × 6 h). The hopper should be

built so that plastic pellets in all parts of the hopper will move

uniformly downward as material is removed from the bottom.

Funneling pellets down the center of the hopper while pellets

near the outside move more slowly will result in inadequate

drying.

In routine operation, drying time is maintained by keeping the

hopper full. If the hopper level is allowed to run low, residence

time of the plastic in the hopper will be too short and the

material will not be adequately dried. For this reason, and to

compensate for less-than-perfect plug ﬂow through the dryer,

the hopper should be larger than the exact size calculated.

Dryness of air. Dry air comes from the desiccant beds in the

closed air circulation loop of the dryer/hopper system. Desiccant

beds must be heated and regenerated before they can dry

incoming process air. After regeneration, it is beneﬁcial to cool

down the regenerated bed with closed loop (previously dried)

air as opposed to ambient air.

Returning process air from the top of the pellet hopper is

ﬁltered before it is blown through the desiccant bed and on to

the heater and hopper. Dryers used for polyesters should be

equipped with aftercoolers to cool the returning process air.

Return air temperature should be below 65°C (150°F) to

increase the desiccant’s afﬁnity for moisture, thus improving

efﬁciency.

The desiccant in the beds is typically a very ﬁne clay-like

material in pea-sized pellets. It slowly loses its usefulness and

must be replaced periodically—usually about once a year. Use

of plastic with a high dust content (such as regrind) or materials

containing certain additives will reduce the life of the desiccant

by coating the pellets or saturating them with a nonvolatile

material. Good ﬁlters can help extend the life of the bed and

the heater elements.

Air dryness can be checked by dew point meters, either portable

or installed in line in the dryer. Built-in dew point meters and

alarms are the wise choice for polyesters. These meters give a

direct reading of the dew point of the air tested. When the dryer

has rotating beds, the meter must run long enough for all beds

to be checked. Each bed can normally be on line for 20 to 40

minutes or longer; a new bed should rotate into position before

the dew point rises above –30°C ( –20°F). (Also see the discussion

on “Moisture measurement” on page 29).

Note: Once pellets are dried, they must not be exposed to

moist air in conveying or at the machine hopper. Otherwise, the

pellets may reabsorb enough moisture to cause splay or lower

physical properties.

Airﬂow. The usual airﬂow rate requirement for drying is 0.06

cubic meter of hot dry air per minute for each kilogram of material

processed per hour (0.06 m

/min per kg/h) or 1 cubic foot of

hot dry air per minute for each pound of material processed per

hour (1 cfm per lb/h). For example, if 109 kg (240 lb) of material

is used per hour, airﬂow should be at least 6.7 m

/min (240 cfm).

Minimum airﬂow to ensure good air distribution is usually about

2.8 m

/min (100 cfm) for smaller dryers.

Airﬂow can be checked by in-line airﬂow meters, by portable

meters, or much less accurately by disconnecting a hose going

into the hopper and feeling the airﬂow—basically a yes/no

on airﬂow.

If there are dust ﬁlters in the circulation loop, these should be

cleaned or replaced periodically to avoid reduction in the airﬂow rate.

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 43 44

Kommentare zu diesen Handbüchern

Keine Kommentare

Diamond Systems Tritan Anleitung zur Fehlerbehebung Seite 28

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Elektrische Gehäuse Diamond Systems Tritan