Presented by George R. Cozzarin, Xaloy, Inc.
The paper industry found the heat transfer roll, a steam drum commonly called the
Yankee cylinder, to be very effective when used to drive the moisture from one-side coated paper and crepe tissue. The addition of a forced-convection hood created a conduction, convection and radiation system that provided the process with the necessary drying control as line speeds increased. Advances in heat transfer roll design and technology, combined with the evolution of the products being produced and ever increasing production rates, have continued to make the roll effective when precise control is required. Beyond drying and moisture removal, heat transfer rolls have been used to temper and cure substrates more efficiently than other methods, with temperature control and uniformity being key to their performance.
Today, there are three commonly used methods of drying: infrared (IR), impingement (hot air or flotation dryer) and contact (drum or heat transfer roll). Dryers are used to drive off the water or solvents that become laden in a continuously moving web from operations such as coating or printing. A specific amount of energy is required to remove a defined amount of liquid, be it to completely dry the web or to retain a desired amount of moisture as it exits the drying system.
In the ideal drying system, the web is heated as quickly as possible shortly after the
coater so that a substantial amount of liquid contained in the coating can evaporate. This will help reduce the amount of liquid being absorbed by the base web while elevating the webs temperature to continue to facilitate the evaporation process. Evaporation of additional liquid occurs by the continuation of heat input to the web while evacuating and replacing the moisture-laden air surrounding the web. As the removal of liquid from the web continues to occur, so does the increase in temperature of the web, which in turn, increases the rate of evaporation. Soon thereafter, the web will reach a point of relatively constant temperature and the rate of evaporation will begin to slowly decrease. As the liquid level continues to fall and the rate of evaporation continues to decrease, the sheet temperature will begin to rise.(1) Once the desired level of drying is achieved, the web is typically cooled for handling purposes or to lock in a desired amount of residual moisture.
As previously stated, a specific amount of energy is required to remove a defined amount of liquid from a continuously moving web. Along with the energy required for the initial heat up of the drying system itself, there is the process energy which includes the energy to heat the web, the latent heat of evaporation of the liquid from the web and the energy given away to ambient losses. With regards to contact drying utilizing heat transfer rolls, the initial heat up energy required and the process energy required are both examined to assure that the temperature control unit (TCU) that supplies the heating medium to the rolls is properly sized.
Heat transfer rolls have advanced in design since the advent of single-shell steam drums, which are still manufactured and perform adequately in certain applications. Today’s high performance heat transfer rolls consist of a double shell-spiral design that provides efficient operation and precise temperature control. They can be designed to operate using different types of heating mediums (water, oil, steam, glycol solutions), manufactured from a variety of materials (carbon steel, stainless steel, aluminum) and finished with numerous types of coatings (chrome, nickel, Teflon, ceramic). Within the high performance roll, the heating medium is channeled and directed across the face of the roll by a spiral insert between the inner and outer shells. The design of the flow channel is based on the volume of the heating medium supplied to the roll, which is dictated by the energy requirement of the drying system. The channel is sized for turbulent flow, and then optimized for the maximum coefficient of heat transfer that can be achieved with the medium being used. The design assures that 100% of the operating surface area of the roll face is in contact with the heating medium. This maximizes the ability to precisely control the operating temperature of the roll.
As the term contact drying implies, the substrate being dried is in direct contact with the surface of the heat transfer roll. As opposed to most non-contact dryers, this system allows for a high rate of heat transfer between the dryer (heat transfer roll) and the product. The accuracy in modeling the system by computer analysis allows the designer to determine the requirements to completely dry a web, or retain some percentage within the product by removing a specific amount of moisture. Once the overall heat transfer coefficient between the product and the heating medium is calculated, the roll designer can determine the length of contact, or dwell time, required to achieve the desired results and properly define the complete system. This includes the number and size of the rolls along with defining the operating requirements of the TCU. Precise control of cross
product temperature uniformity is achieved with the proper design of the flow channel within the rolls and the volume of medium being supplied to the rolls. High performance rolls provide the ability to achieve a temperature uniformity across the width of the product within +/-1°F. Controlling and maintaining product temperature uniformity with the high performance heat transfer roll can minimize or eliminate such problems as inadequate tension control, edge curl, poor web tracking or uneven temperature distribution throughout the web.
Heat transfer rolls are also being used for tempering, annealing and curing products within a variety of applications. The same principles apply in that the roll is used to heat the product to the desired temperature, then a series of rolls maintain that temperature for the period of time required to achieve the desired properties of the product. The key to their performance in these various processes is attributed to the ability to precisely control the temperature of the high performance roll and maintaining the consistency of temperature distribution throughout the product.
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