The following variables must be analyzed and specific improvements implemented to achieve a completely optimized system:
- Operating cost considerations, including exhaust rate and recirculation, exhaust air temperature, radiant heat losses, web exit temperature and maintenance costs.
- Stable web handling.
- Heat and mass transfer.
- Dryer length and capital cost.
- Direct, semidirect or indirect heat source.
In addition, process requirements such as coating type, substrate type and temperature limitations must be considered. To begin a comprehensive optimization process, first examine how each variable can be improved independently.
Reducing Exhaust RatesDryer exhaust rates represent a major factor in the operating cost of dryers. Reducing exhaust rates cuts fuel costs by:
- Lowering the amount of makeup air to be heated in the dryer.
- Increasing solvent concentrations, which reduces fuel consumption when an oxidizer is part of the process.
- Smaller fans, motors and heat sources can be purchased, resulting in reduced energy usage.
Reduced exhaust rates also lessen the tendency for web flutter, thereby ensuring end product quality. At the absolute minimum, in direct-fired systems, the exhaust rate must be high enough to maintain a slight negative pressure in the dryer and exhaust combustion products. For large exhaust rates, electrical consumption can be greatly reduced by using variable speed motors. Exhaust rates can be minimized as long as product quality and safety goals are not compromised.
Recirculation within the dryer will lower the exhaust rate and increase solvent or moisture concentration. Care must be taken in recirculation design. If the solvent is organic, the increase in solvent concentration may retard drying or cause condensation in the dryer or the ductwork. Dry-ness and solvent condensation issues created by increased recirculation normally can be mitigated by modest increases in air temperature well and good air mixing. For temperature-sensitive processes, a modest increase in dryer length may be required in lieu of higher temperatures.
Typically, safety is not an issue with recirculation because the process air in the dryer may increase from 3% of the lower flammability limit (LFL) up to 15%. Concentrations of 25% LFL are considered safe without monitoring equipment and 50% LFL is considered safe with monitoring instrumentation. Nevertheless, the exhaust rate at any given time must be based on the potential worst-case upset scenario under specific process conditions. LFL monitoring, combined with exhaust rate control, enables processes to run at minimum exhaust without compromising safety. However, optimum LFL concentrations are process specific and may be below the maximum achievable level.
When using water-based or 100% solids coatings, recirculation will not be limited by LFL but may be limited by dryer negative pressure, removal of combustion products, or formation of powder from traces of inorganic material that pass through the burner flame. Powder, or snow, can be minimized by indirect heat or by semi-indirect heat.
Zoning and Temperature ControlAs recirculation increases, proper zoning and temperature control become more important. Temperature stratification must be minimal, especially at the nozzle. Poor mixing of return air can cause cold spots where solvents may condense and possibly drip on the web. In general, the more zones there are, the more control the operator will have over the process -- especially if each zone is controlled independently.
The first zone should have minimal recirculation so that solvent concentrations in the recirculated air can be kept at a minimum. When the cold web enters the dryer, there is a tendency for the solvent or particulate in the recirculated air to condense on the web. More solvent in the air will cause more condensation on the web. This can have detrimental effects on coating quality. Makeup air to the first zone can come from the ambient air or possibly from one of the final zones where solvent concentrations normally are low. Exhaust from the first zone normally can be cascaded into the second zone.
Beginning with the second zone, the remaining zones are process dependent. Typically, the highest solvent evaporation rates are in the second and third zones. These may be exhausted separately or cascaded, but the final system exhaust most likely should be from these zones, where solvent concentrations are the highest. Careful design will ensure that the final zones will have low solvent concentrations.
Radiant Heat LossesHeat losses from radiation can be significant, especially if they increase the cooling load of an air-conditioning system. Insulation is a one-time capital cost but radiation losses are forever. Supply fan motors also radiate a great deal of heat. Through-metal structural support beams in the dryer walls can increase losses through the dryer if their design allows uninterrupted conduction of heat from the drying chamber to the outside wall.
Designs minimizing radiation losses must be balanced against increased capital expenditure for more insulation and engineering-intensive design modifications. Estimated radiation losses, including added air-conditioning loads, can generate a yearly cost associated with this variable. Savings resulting from lower radiation losses can be compared against the costs required to generate the savings.
High web exit temperatures also can add to the air-conditioning heat load and may require additional cooling on a chill roll or other cooling device. In general, shorter dryers require higher peak web temperatures and thus higher web exit temperatures. This impact should be evaluated for each given process.
Maintenance CostsContinuously welded internal linings will help prevent external corrosion. Equipment must be accessible with ample ways for entry and inspection. Vendors, experienced operators and maintenance personnel can help develop an appropriate preventive maintenance program. Complex instrumentation such as LFL monitors may require off-site repair. Thus, production must be able to continue in the absence of such instrumentation even though the process may not be optimized at that time.
Stable Web HandlingStable web handling is nonfluttering, nonwrinkling, uniform nozzle-to-web clearance. It is a function of nozzle design and geometry, web tension and return or spent air management in the dryer enclosure. Nozzles are the devices through which heated air is delivered to the web and coating to remove solvent or water. Return or recirculated air is air that has been discharged from the nozzle and is being returned to the heat source to be reheated to the desired supply air temperature.
Nozzle design and geometry differ by nozzle-to-web clearance, nozzle-to-nozzle spacing, number of supply air discharge slots, and width and orientation of the discharge slots to the web. The greater the clearance between the web and the nozzle, the less potential for touch down if tension upsets occur.
Return air management is a function of internal dryer geometry. Proper dryer return air design will provide a stable web as it moves through the dryer enclosure. Re-turn air is exhausted from the drying system to maintain a negative pressure in the dryer enclosure and to prevent heated air from escaping into the room through the web slots. In addition, return air is exhausted to sustain an internal dryer humidity that does not retard drying and to maintain safe LFL concentrations.
Exhaust rate affects web handling. An excessive exhaust rate near the web can cause web flutter, especially with lightweight webs, resulting in touch down and potential marking of the substrate and coating.
Heat and Mass Transfer
Heat transfer is the energy imparted to the web and coating expressed in units such as BTU/hr-ft2-oF or kJ/hr-m2-oC. In the case of air flotation dryers, mass transfer of the solvent out of the coating is directly proportional to heat transfer. It usually is expressed in units such as pounds per hour per foot squared or gallons per minute per hour per meters squared. Nozzles with higher heat transfer coefficients require lower air temperatures and velocities to remove the solvent from the coating. This can greatly enhance product quality.
Heat and mass transfer is a function of dryer nozzle geometry and shape, nozzle velocity, supply air temperature and the distance between the nozzles and the web. It is necessary to control heat and mass transfer -- not just maximize it. High mass transfer during the early stages of drying can cause defects if the evaporating solvent disrupts the integrity of the coating.
Some nozzle types provide high heat and mass transfer but do so while operating with less clearance between them and the substrate. Depending on the coating, you may want to maximize the clearance between the nozzle and the web to minimize touch down. Nozzle choice involves trade-offs between maximum heat and mass transfer and web handling.
Nozzle velocity is the speed of the air discharging from the nozzle expressed in units of feet per minute or meters per second. A high nozzle velocity results in greater heat and mass transfer. Some products are nozzle-velocity limited, meaning they may be damaged by high velocity air impinging on them or their flotation characteristics may be unstable. Low nozzle velocity might be a consideration with light-gauge films and low wet-strength materials.
Heat and mass transfer also are a function of supply-air temperature and the distance between the nozzles and the web. The higher the supply-air temperature, the greater the heat and mass transfer. Some substrates (polyethylene, for example) and coatings are supply-air-temperature limited. In addition, the closer the nozzles are to the web, the greater the heat and mass transfer.
Dryer Length and Capital CostTypically, dryer length and capital cost go hand in hand. Thus, you need the appropriate length to meet the line speed target while achieving acceptable product quality in the available space and within the project budget.
While reducing length cuts costs, dryers that are too short may require high temperatures that can cause blistering and other coating or substrate defects. Heating the web is one aspect of drying, but residence time is just as important. In general, higher temperatures are not a good substitute for inadequate residence time.
Dryer length projections can be made using computer drying models, existing production information and lab trials. Theoretical dryer lengths derived from computer modeling do not necessarily guarantee a quality product. For some coatings, the optimized dryer length requires lab trials. Fitting the required dryer length in the available space may require creative web paths involving multiple web passes in either a vertical or horizontal orientation.
Direct, Semidirect or Indirect HeatAlthough direct heat is the most efficient approach for heating dryer air, indirect heat may be necessary to prevent formation of inorganic oxides in the recirculated air. For example, traces of silicone will form particulate silicon dioxide snow if the silicone passes through the direct flame of a burner. Such particulate is unacceptable in most coating applications and can cause significant downtime for cleaning. Thus, indirect heat may be mandatory if burner combustion products or inorganic oxides compromise the quality of the product. Of course, if traces of inorganic species from the process are negligible, direct-fired systems are suitable.
MonitoringThere is no substitute for a system that has instrumentation to track key operating parameters. LFL monitors are useful even if they do not directly control the process. Electrical and gas consumption seldom are optimized unless they are measured.
Even when the above processes are optimized and running smoothly, there is another dimension to achieving the perfect drying system design. Optimization of an air flotation dryer system requires that other process demands be considered in conjunction with any reduction in operating costs. Therefore, the complicated relationship between the dryer and the emissions control device (i.e., the oxidizer) also must be taken into consideration when optimizing systems processes.