DEAR MAHMUD

CEMA INDICATES DESIGN CAPACITY only IF you want design capacity to be 10 or 15 percent higher han rated capacty you can desig the conveyor based on your requirement.

A R SINGH ■

Dear Mr. Mahmud,

I am not in a position to comment about your statement regarding CEMA (which is also not very clear to me).

However, for conveyor design by any method, if you want to see the tension power etc. with reference to design capacity, then the values corresponding to design capacity has to be used in formula.

In fact, conveyor should not be designed for one capacity. The conveyor can be running sometimes empty, sometimes at part load and sometimes at maximum load, which normally happens to be design capacity. The belt tensions at various capacities are designed not in isolation but in totality of various situations.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

As with any equipment what rate do you want?

If you want 1000 te/hr then design on 1000 te/hr, but if you want 1100 te/hr then design on 1100 te/hr the choice is yours. If you don't believe the book formula why use it? Create your own that gives you the results you are happy with.

I see a tendancy for enquiries to state something like "normal rate 500 te/hr, design rate 600 te/hr". Clearly you have to design on 600 te/hr because if you don't your machine may not be physically large enough or have sufficient power. So why waste time on saying "normal rate 500 te/hr"! ■

Dear Designer,

THe purpose of the two ratings: design and nominal or normal or operating capacity is to distinguish between design limits and operating ratings.

Nominal or normal is used to establish equipment life such as idler, pulley, reducer and motor bearings and wear components such as belt covers et al.

Design is used to set motor, belt and structural loads.

There is yet a third, fourth and fifth set of criteria practiced by some:

3. Design for momentary or peak and partial load conditions which include uphill and downhill separate conditions that exceed a fully loaded belt at design

4. Design for motor nameplate should include the take-up to be compatible and meet points 1-3. THis insures the operator can apply tonnage that meets the motor nameplate without structure and pulley failures

5. Design for motor breakdown torque in case the belt is jammed and you do not wish to see the structures and pulleys and belt catastrophically fail. This may or may not occur during the conveyor's life, but it is a safety issue. As such the safety factor is reduced for this once in a life time condition. THis is also and arguement for some protective device to limit "what if" scenarios so that the economic impact is controlled by special means. ■

Nordell,

The points you make are valid. Those "experienced in the art" will have their own past experiences to draw on and their design will take into account any number of conditions that may occur, it's not just a case of plugging numbers into a formula from a book.

In the range of materials handling equipment I have recently been having to explain that just because the "normal capacity" is, say, 80% of the "design capacity" it does not mean that that a smaller machine can be used with a lower power motor. The machine must have a sufficient size to pass the "design" m3/hr and be provided with sufficient power for the "design" te/hr. ■

Designer,

Yes, some become confused when they are new to "why" and "how" designs evolve. The factor for "design capacity" verses " nominal capacity" has its roots in:

Design: denotes a make-up capacity or temporary surges to contain spillage. Makeup is usually applied to 2 days of a 7 day week with 48 hours to catchup on a 8 hour PM down time. Thus, 8/48 = 17% minimum addition to the nominal. Traditionally, then you add the mechanical outage, other non-reliability factors, and plant operating factors, which puts you close to adding 25%. This later factor should not be considered normal. It is a peak which is rarely operated at for more than a two days a week.

Nominal: denotes the expected daily throughput that governs 70% of production time, with the other times divided between empty belt (15-25%) and design capacity operations (10-15% of the day to day which can differ a little from the above).

Operators tend to have differing factors for these parameters when poled. ■