Dear Neeta,

You have posted this thread under “Pneumatic Conveying”.

However, you are asking for information about a screw conveyor, also working as a cooler.

Calculating the screw conveyor alone is not so difficult but calculating the heat transfer from the conveyed material through the screw tube to the cooling water in the jacket is something that should be done by a manufacturer with knowledge.

They should know the field parameters, based on their field experience.

A pneumatic conveying system, working with cooled compressed air and a cooling jacket could also be an option.

Take care of condensation of water, when cooling seeds.

Success

Teus ■

The residence time through a heat transfer screw cooler depends on the equipment specification so it may be useful to set out a few design parameters. The writer does not identify the type of seed, so a basic value needed is the specific heat of the seeds, conductivity and seed dimensions.

The total heat burden can then be related to the heat transfer characteristics of the mode of equipment selected and temperature of the water intake and discharge, to determine the heat transfer surface contact area and the water capacity needed. Iterative comparison of the required cooling area with the handling capacity relative to screw size will establish the necessary size and speed of the equipment. Attention will then be given to whether the feed is controlled by prior equipment or is self-regulated by the machine.

The final temperature required would seem a non-starter on a horizontal screw conveyor as the thermal efficiency is low due to employing only a section of the casing periphery, and even that is insulated by a residue layer in the flight tip clearance that suffers poor inter-particle heat transfer by static contact. A steep, jacketed screw elevator will generate full use of the casing circumference, maximum thermal homogenisation of the contents and generate intense surface wiping of the cooled surface. A courageous design would incorporate a spiral chute around the cooling jacket for the returning grains. In summary, I would expect even a specialist supplier to conduct some trials, unless he had specific experience with the product, and the costs of a custom-built unit to compare unfavourable with a simple, standard pneumatic system that also offers easier testing for cooling in transit.

Screws are great for reducing high temperature products to a degree that leaves a decent differential with the cooling medium, but the combination of high thermal loads and low temperature differences to the cooling media seriously drives up the scale of equipment required. On the other hand, heating or cooling is relatively inexpensive to add to equipment that is necessary for material transport. ■

Your formula is correct you are just executing it wrong. The length and pitch need to be in the same unit of measure.

length in ft / (pitch in ft x rpm) = minutes

20' / (.5'x16RPM) = M

20' / 8 FPM = M

Therefore retention time is 20/8 = 2.5 minutes assuming that the inlet and outlet are 20' apart. ■

As Gary noted, the transit rate of 8 ft/min gives a residence time in mins. of - conveyor length,(ft)/8.

However, to assess the amount of heat transmitted is much more involved. Classical heat transfer equations apply, but the factors used are sensitive to fine design details and may require verification by trials. This is generally a specialist suppliers task. ■

You could cool the temperature of the material in a few ways, yes a water jacket around the outside but also down the middle of the screw conveying shaft depending on how big the shaft of the screw conveyor is.

Regards

Engineering Solutions

www.engineering-solutions.co.za ■

Perhaps this thread has gone past it's sell by date, but anyway. There were earlier threads mentioning an item which might be useful. The manufacturer was offering a screw conveyor which was fully jacketed: casing shaft and flights.

Not much has been heard of it since.

Sealing cooling water while moving a rotor, or anything else, is a daunting proposition. Few vessel manufacturers would consider such a heat exchanger whatever the price. Seal manufacturers would tell you they can seal it all OK and then rub their hands in anticipation of the spares market.

Three tonnes an hour doesn't warrant an expensive throwaway gadget. We don't know your particle sizes or silo dimensions so I can only guess that it should be possible to jacket the riser; spread the material over a water cooled insert and cool the falling curtain along the silo walls. Like any reactor designer would: but instead of trays and rising gas you use skin cooling. It is much easier to cool on shell and tube than it is to have steam pockets spinning around metres from anywhere where they can't easily vent. Everything is against a jacketed screw except the designer's stubbornness.

This is another process engineering issue and although not strictly a bulk handling issue the forum members are happy to oblige when a process seems to be getting into hot water. ■

For the specific heat capacity of a substance is the high temperature required to raise the temperature of 1 kg of the substance by 1k or by 1degree Celsius. ■