Drying – Basic Hot Air Dryer Design

We have discussed earlier the importance of drying in a cleaning process.  Despite the several hardware options available, hot air dryers remain the “go to” in industrial cleaning processes today.  Today’s blog will discuss the basic design concept of hot air dryers and how they function.

The main components of hot air dryers are a drying chamber where the work is placed for drying, a heat source and a fan to circulate heated air.

Hot Air Dryer Schematic

Basic Hot Air Dryer

These basic components can be arranged in a myriad of ways depending on the application and the space available.

Drying Chamber –

Typically, the drying chamber is at least partially enclosed.  A lid prevents escessive leakage of hot air to the environment.  The chamber is constructed of materials that will resist the shedding of particles that would otherwise contaminate the parts being dried.  Unpolished stainless steel is ideal.  In cases where thermal leakage to the environment is a concern (in a clean room for example) the drying chamber and associated ductwork can be insulated.  Most drying chamber lids do not provide a tight seal although it would be easy to argue the importance of sealing the chamber to prevent contamination of parts during drying as we will discuss later.

Blower –

The blower, as the chamber, should be reltively immune to damage due to humidity and chosen to prevent particle shedding during operation.  The blower is the largest potential source of particle contamination in the typical dryer because of its moving parts.  The use of electrically driven “squirrel cage” blowers is common although there are many other options available including simple fans.  The blower must be able to withstand high temperatures as the return air from the drying chamber will be hot.  The flow capacity of the blower should be based on the application.  In some cases, air velocity is key while in others, temperature is the primary important factor in drying effectiveness.  Search “drying” in the search box below for additional background on this issue.

Air Heater –

The next major component of the basic dryer is a source of heat.  As is the case with the blower, the air heater should be relatively immune to humidity and should not generate particles.  The air heater has the second largest potential for particle generation in the drying system.  Although electric heaters dominate in dryers, other sources of heat including steam and gas are used occasionally (especially in larger systems).  The heater should have sufficient capacity to raise the temperature in the dryer in a relatively short period of time but should not be so powerful that excessive cycling is required for temperature control in normal operation.  Excess capacity may result in temperature spikes due to the “thermal inertia” of the heater.  Finally, the heater should be chosen to provide as little resistance to the flow of air as possible.

Air Exchange –

As discussed in previous blogs, moisture in the form of humidity is created as water evaporates from the surfaces of parts being dried.  In a totally closed system, the moisture in the air will eventually reach a saturation point (100% relative humidity) resulting in a decrease in drying efficiency despite adequate temperature and air flow.  In order to prevent this, some means of exchanging humid air for less humid air must be provided.  In the illustration above, a bleed-off is provided after the blower to exhaust humid air.  A damper is provided to control the amount of air exhausted.  Makeup air, then, is ideally acquired through a filtered inlet in the “suction” line going to the blower.  In the case of a drying chamber that is not totally sealed, some makeup air may also be acquired as it directly enters the chamber.  In this case, of course, the air is unfiltered and may introduce particles into the dryer.  This may or may not be important depending on the application.

The above has described the basics of a hot air dryer.  As you may assume there is a lot more to talk about when it comes to hot air dryers and a lot of both valid and unfounded theories about the best way to configure them.  I’ll cover these and more in future blogs.

–  FJF  –

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