Vincent,

Superficially this appears a simple question, but there are various subtle hazards in the duty. First, it is presumed that the application is a screw conveyor, to handle a feed that is controlled to the inlet by prior equipment, and not a screw feeder with a flood feed, which has a different mode of operation for material transfer. Secondly, the feed rate is not given, therefore the nominal loading factor cannot be determined.

Turning to the equipment mechanics, a centre bearing is a definite no-no at this inclination. A 20 foot span therefore requires a large centre tube to avoid excessive deflection and have stiffness to avoid casing contact. The screw also must be well straightened after welding to eliminate ‘waggle’. The size of auger tube required will only leave a short height for the flights on a 10” diameter screw, so the potential conveying capacity, relatively low at 20 rpm, is further inhibited. Further, an inclination of 35 degrees is a nasty angle to move a fine powder by screw conveyor because flight tip leakage and over-tube fall-back place a high reflux demand on the conveyor loading. In fact, a fine powder in a loose condition will be likely to fill the screw cross section. If this occurs there will be problems of clearing the contents, and then of re-starting the conveyor after shut-down in a full condition. The state of the powder is therefore a key issue, particularly if it is warm because it is then more prone to dilatation, and starting and stopping conditions relative to the discharge requirements must also be addressed.

Conditions may dictate a short pitch screw, at the start of the screw or over the whole length, and perhaps the use of dual or multi-start flights. As the application is sensitive to a number of features I would advise the user to seek expert advice. This is considered desirable, (a) to conduct a thorough design audit to determine the rate and state range of the product and ensure the machine will handle the required rate of transfer, and (b) to establish the working and extreme conditions to be accommodated in terms of drive power with an appropriate safety factor.

Alternatively, if the application and responsibility are not critical, fit a 30 horse powder drive unit and hope (or pray), whichever is your choice.

Lyn Bates ■

1. your material.

Calcium powder known as limestone dust is a very light - sluggish material, midely abrasive tends to aerate and become uncontrolable fluid.

2. your screw conveyor.

For a horizontal U through conveyor (not feeder) of 10" screw you need a housing of 11" MAX. Supposing you have no intermediate bearing and a regular screw, you need a 6" centeral pipe. With a 30% loading your capacity will be 91 cubic - feet per hour (3 metric tone per hour).

since you have a 35 deg. inclenation you will have a 20% efficiancy wich means 0.6 metric tone per hour. because the large clearence and the material tendcy to aerate and flow back your real capacity will be 0 (ZERO) metric tone per hour.

You need no drive motor for this capacity.

3. improve your screw conveyor.

You may use the existing U through as a base for your improved conveyer and change \ replace the screw only. It's preferable to take an all new half pitch circular through screw conveyer. Supposing the existing through is a 12" sch 10 the inner diameter will be 314.7mm. A new screw with a diameter 12" (304mm) with a 6" pipe and half pitch screw will give a capacity of 5 metric tone per hour at 75 rpm for 30% load and 35 deg. inclination.

Use a 5.5 HP (4 Kwatt) motor.

You have to control the feeding to the screw conveyor.

For further info. you may contact me at : shraga@ludan.co.il ■