30 Questions: Specifying a Dryer, Part 1
Specifying equipment is potentially the most important step in any project. The specification is the primary means of communicating the process requirements to a supplier or designer, so it is imperative that it be complete, correct and thorough. There can be no assumptions - an error in the details can potentially flaw an otherwise perfect project.
My first few columns will detail 30 questions that should be asked - and answered - when preparing the specification of a thermal drying plant. While some of the information covered may appear obvious and mundane, it will be included for completeness.
1. What is the contact name, title, company, department, address, telephone and fax numbers, and e-mail address?This is, plainly, quite an obvious requirement. However, surprisingly, it frequently is incomplete or overlooked. Contact or departmental information should be covered for both technical and commercial aspects.
2. What is the plant location?Describing the plant location provides essential information such as atmospheric pressure, altitude, relative humidity, ambient temperatures and other important aspects that may affect the equipment design. For example, altitude will provide the ambient air's density at the site as well as atmospheric pressure. Density is fundamental in the calculations: It relates volume to mass (weight), and in performing a heat and mass balance for the system, the air requirement will be calculated as a mass. The required volume of air will be a function of the local air density.
If the plant is at a high elevation and a sea level density is used in the calculation, the air volume and associated air-moving equipment probably will be undersized significantly. Rectifying this design error would increase velocities through the system and could potentially require replacing the air-moving equipment - a costly mistake.
Atmospheric pressure also has an influence on the wet solid's vapor pressure. As the atmospheric pressure falls, less energy is required to achieve evaporation. Consequently, as the altitude increases and atmospheric pressure falls, the boiling point of the liquid to be evaporated will fall.
Relative humidity defines the quantity of water vapor in the surrounding air. If relative humidity is high, the amount of water the process air can hold before reaching its dewpoint is reduced. With high relative humidity and without proper design, the system may not operate efficiently or meet process requirements.
Knowing the ambient temperature is important when calculating how much energy is needed to heat up the carrier (usually air) to the operating condition. Obviously, the larger the difference between the ambient and operating temperatures (referred to as gT), the more energy required to achieve the increase.
Ambient humidity and temperature are seasonal conditions, but be sure that the "worst case" scenario is used during design. The supplier may want to use a statistically average figure to reduce the required investment for the system and propose a more "realistic" option. These options should be seriously considered.
Whether the equipment is installed indoors or outside also will affect the design. For example, dealing with the effects of direct sunlight and ultraviolet (UV) degradation as well as other elemental considerations could increase costs. Indoor facilities have fewer operating-condition fluctuations. This buffers the operation and produces a more consistent product.
3. What is the product to be dried?A clear statement about the product to be dried is necessary. Cite all essential aspects of the product - for example, mineral concentrates containing copper, nickel, chrome, sulfur and trace amounts of precious metals. This defines the project and also will categorize the application for future reference.
4. What is the production rate?The rate of production (product per hour), together with the operating schedule (hours of production per day), has significant value to a designer. Plant availability - the proportion of time the plant is required to be in operation during the operating schedule - must be detailed, normally as a percentage. For example, a 90% plant availability would indicate that the plant must be in operation 328 days/yr for a 24 hr, 7 days/wk.
Also, the rate of production should be defined on either a dry or a wet basis; typically, wet basis is the industry standard. Capacity problems can occur if a unit is sized for a throughput assumed to be wet basis when, in fact, it was for dried product. For instance, suppose a capacity of 10,000 lb/hr of product with a 40% (by weight) initial moisture and 0% final moisture content is specified. Does this indicate the production is for 10,000 pounds of dry product or 6,000 pounds of dry product? Be specific!