5 Reasons to Choose an Impingement Dryer for Process Heating
For those of you who are new to process heating, or even if you’re just developing a new application, here’s your chance to learn the fundamentals about the various types of process heating ovens and dryers. Over the next several weeks, we’ll be explaining the advantages and limitations of each and the common applications they’re used for. This week’s focus: impingement dryers.
What is an impingement dryer?
Impingement drying involves blowing medium to high-velocity hot air onto the material to be dried. In most cases, the material is a sheet or continuous web, which is transported through the dryer using rollers or a conveyor.
The heated air can be generated using gas burners, electricity, steam; or even recovered from other processes, such as oxidizers. Using a controlled pressure blower, hot air is forced through nozzles or slots which are precisely positioned in the dryer according to the heating requirements of the material.
Impingement dryer nozzle array
The high velocity of the air allows for very high heat transfer rates, which results in more moisture removal and faster, more efficient drying and curing. Processing times and dryer length can be greatly reduced as compared to lower velocity or non-impingement convection air dryers.
Factors that affect the rate of heat transfer to the web include:
- Impingement Air Velocity
- Air temperature
- Humidity of the supply air
- Distance between the nozzles and the material being dried
The ability to position nozzles throughout the length and width of the dryer means that temperature can be consistently controlled, even at the edges of the web. Multiple heating zones in the machine direction, can be created using groups of nozzles that are independently controlled for temperature and velocity.
Common applications for impingement drying
Impingement dryers are typically used for drying coating on webs such as paper, films, foils, nonwoven and fabric. Impingement is a very efficient means of drying coatings on webs. In most cases, coated and non-permeable webs are the best applications for impingement drying. Permeable webs, where the coating may “bleed” through the material or the applications where the web is saturated with coating can also be run with impingement dryers in a vertical pass configuration. Typical end products that benefit from a higher efficiency impingement drying system include adhesive tapes, protective films, coated fabrics, architectural fabrics etc.
5 reasons to choose an impingement dryer:
- Accurate and uniform temperature control. Since these dryers are designed to provide uniform temperatures throughout the dryer, impingement can be used for drying various coatings on many different substrates.
- Heat transfer is independent of web handling. Impingement dryers can be designed to use various methods of transport through the dryer. Rolls (either idler or driven) are commonly used for webs. Conveyors or vertical tower designs are also available depending on the material and the requirements of the process. The adjustable control of the drying is independent of the transport method.
- Ease of creating multiple heat transfer zones. In an impingement dryer, groups of independently-controlled nozzles can be arranged to create different heating zones within the dryer. As the material moves through each zone, the temperature and velocity of heated air is adjusted to the requirements of the process. As an example; depending on the thickness of the coating, the first zone may be lower on velocity so as not to disturb the coating. As the coating dries, the velocity and temperature can be increased to improve the rate of heat transfer and reduction of the dryer length.
- Nozzle configuration can be easily modified. This comes in handy when you’re using the dryer for multiple products or processes that may have different requirements.
- Ability to create recipes. Just as you have the ability to adjust the nozzle configurations, you can also adjust the velocity and temperature of each zone to create recipes for different products.
Impingement dryer limitations:
- Higher capital costs. An impingement dryer requires a separate material handling mechanism, such as rolls or a conveyor, which adds cost, especially when driven-roll support is needed. Also, rollers and bearings require regular maintenance, and these expenses need to be taken into account.
- Web tension issues. Especially when a longer dryer is needed, the required tension to move the web may be a problem for some materials. Also, longer dryers may not be able to use idler rolls but may require more expensive driven-roll support.
- Risk of marking or damage to web. If you’re drying a material that could be damaged by contact with the conveyor or support rolls, you may want to consider a flotation dryer where the web never touches a support mechanism.
- Possible heat transfer spikes. For processes requiring slower line speeds, impingement drying can sometimes cause heat transfer spikes on nozzle centers
Important design considerations for an impingement dryer
If you determine that your product is a good candidate for impingement drying, remember that the dryer needs to be designed with your exact process requirements in mind. When you work with an experienced dryer manufacturer, these design options can be precisely configured according to your needs:
- Material handling mechanism
- Temperature and air velocity
- Nozzle gap and opening
- Nozzle spacing within the dryer and distance to the web
- Heating zones
- Access for cleaning and maintenance