Concluding my discussion, this column is the last in a series to detail the questions to be asked - and how that information should be used - when preparing a drying specification

25. What is the electrical power?

Specify the electrical power, including the voltage, number of phases, and cycles, and the area classification. Also, detail whether the motor control center will be provided by the supplier or purchaser, and clearly state the battery limits of the supply.

26. How is the equipment controlled?

Whether you have a distributed control system (DCS), programmable logic controller (PLC) or solid-state discrete controllers, specify the control system type, make and model. Once again, the specification must define who is providing the hardware and software. In some cases, a functional specification with logic diagrams can be supplied for inclusion into the overall plant operating program. Programming can be performed by the owner's preferred contractor or the supplier.

Do not take on the task of developing a control system unless you have extensive experience. OEM and other vendors have tricks of the trade know-how that may take months to learn. Additionally, operational guaranties may not be offered if the system control is not provided by the supplier. At best, this creates a gray area should the operation not meet your expectations.

27. What are the materials of construction?

Specifying materials of construction is a multi-faceted consideration. Material requirements may be dictated by corrosion, contamination, aesthetic or abrasion concerns, and the materials' treatment and finish also must be taken into account. For products that are not corrosive, the requirements most frequently will relate to the contact parts. Noncontact parts will be galvanized, enamel- or epoxy-coated carbon steel.

For example, in food and pharmaceutical applications, contact part requirements may demand a highly polished stainless steel finish. Should a caustic or other chemical agent need to be used during wash down, the remainder of the fabricated components also may need to be stainless steel. Specifying whether the design is to meet sanitary, clean or general standards will assist in material and surface treatment selection.

As a rule, Grade 304 and 316 (and the L derivatives) are used in corrosive environments while other series 300 stainless steels such as Grade 309, 310 and 321 are used in high temperature applications. For some acidic environments, treated carbon steel may provide better protection than stainless steels. For extremely aggressive environments such as fluorides, using refractories may offer better corrosion resistance.

There are many effective ways to reduce wear caused by abrasive products. For example, using high chrome and/or nickel-content steels, titanium or ceramics can minimize wear. But, there will be high wear areas in the dryer. Specifying replaceable sections with wear-resistant liners in these areas will allow you to replace only the at-risk sections instead of the entire dryer.

Should you know the exact materials needed, specify them for both contact and noncontact components. If you do not have this information, specify what mechanism (wear or contamination, for example) would necessitate a special requirement. Also, consider whether other components such as seals, gaskets, grease, wear strips and other accessories have special requirements.

Testing Information and History

The next group of questions will look at testing information and history.

28.Are samples available?

If possible, furnish a sample of wet and desired dry products with the specification so the supplier can use them for testing purposes. A larger sample may be solicited at a later stage for pilot plant trial runs. Provide the sample with a material safety and data sheet (MSDS) and cite any specific handling precautions.

29. What are the present installations?

If the product already is being dried, state the dryer type being used and give the following information: drying air temperature, drying time, air velocity through/over material, thickness of layer and principle problems currently experienced.

30. Are there any additional remarks?

Lastly, mention any expected difficulties in drying this product.

This series of columns has defined the detailed information required by a designer to engineer the optimal dryer for your application. Understanding why this information is required and how it is used will certainly empower the end user to prepare better specifications, be more enlightened in the bid analysis, and make better choices at purchase.