Heater and Fan Discussion

How long does it take to dry?

Well, everybody knows the time required for drying something is represented:

Yeah, right.... 

Even if you understand that equation, all those symbols are unknowns until you find out thru experiments what values to use.  You can't simply figure it out with simple math and physics. It is complicated.

The rate of air drying of any item is dependent on several factors:

  • Relative humidity of the air
  • Air flow rate
  • The characteristics of the material to be dried and how hard it is for water to move thru it.

Let's dig into these 3 factors: 


Roll of Relative Humidity

A damp item such as a piece of leather or cloth that is placed in a room, will eventually reach a stable level of moisture for that particular material and we would call it “dry".  The only way it dries is thru evaporation from the surface of the material.  To get it fully dry, not only must the surface water evaporate but the water molecules inside the material must move (usually slowly) thru the material to the surface so they can evaporate.  Because water always moves toward the dryer part of the material, the faster it evaporates from the surface, the more pull there is for the molecules inside the material to move to the surface to evaporate.  Once those molecules are at the surface of the material, that’s where wind and heat come into play.


Evaporation is essentially the water molecules moving into air. The drier the air , the faster the molecules can evaporate.  Think about sweating in the dessert at 90 deg F and sweating in the rain forest on a 90 deg day.  In the dry desert air you dry so fast the sweat never accumulates on your skin unlike the muggy day in the tropics. 

But we cant all operate our dryers in the desert.  So we have to make the air we have, seem dryer so that it will allow for faster evaporation and that is where relative humidity plays a big role. 

As the temperature of air increases, the more water it can hold.  We want to do the opposite of what happens when air cools over night and we get dew or fog.  So we heat the air up and quite simply it can absorb more water.  At 120 deg F, (50 C) air can hold about 6 times the water than it can at 68 deg F (20C).  So if we simply heat the air up by this 52 degrees, we have changed the relative humidity drastically and the water will evaporate into this dry air much faster. (think desert conditions)  In a boot dryer application, we could increase the temperature further and continue to reduce the relative humidity, but temperature is typically limited to avoid damage to the leather or plastic.  The ultimate would be to cool the air first, essentially to get the dew to fall out, and then heat it back up.  That is energy expensive, but that would get the fastest drying!


How Much Air is Needed?

Air flow is complicated. 

If the material to be dried is soaking wet, the first drying happens quite fast as this "free" water evaporates.  Once that water on the surface has evaporated, things slow down, sometimes drastically before things are really dry inside.  To get the inside dry, the water has to move thru the material to get to the surface and that can be a slow process.  So air flow at the beginning of drying has a great effect to move away that easily evaporated water, but later on, air flow is still needed but high flow is not as critical. 

The exact relationship of airflow to drying rate is not easily modeled with pure physics and most engineering applications rely on empirical data obtained thru experimentation.  This is true for drying wood, grain or foods as well.

Most ski boot dryers dry quite adequately in an 8 hour session, many without heat.  This amount of time is sufficient for most footwear that is to be dried for the next day.  A few applications benefit from faster drying like hockey gloves between periods or turnout gear for fire and rescue where rapid turn-around is desired.  A focus on high airflow is not always relevant to the casual user.  As long as the relative humidity of the air is lowered thru heating and there is enough turbulence and air exchange, the boots will dry perfectly fine.  Even the inexpensive plastic floor models with tiny fans will dry a wet boot overnight.  (They just look like hell)

Some skepticism is due on claims of high air flow.  Some dryer fabricators claim to have very high air flows.  These ratings may come from the manufacturer and are based on maximum flow with no resistance at the outlet of the fan.  But the long thin tubes to the boots create a lot of back pressure that must be overcome by the fan.  Normal blade fans have a big flow, but cant create much pressure.  Centrifugal blowers can create the pressure needed to push the air thru the tubes.  If the back pressure goes up, the flow goes down.  A true air flow from each peg could be measured and totalized but it will be significantly lower than the “rated” flow.  But do you care what the number is?  The only thing that matters is that enough pressure is created to blow thru the tubes and get warm turbulent air flow into the boot toe. 

Some exaggerated flow claims can be demonstrated mathematically as incorrect.  The temperature increase of the air can be easily calculated based on the air flow rate (cubic ft / min ) and electrical power consumed (Watts):           

Temperature increase in degs F = Watts x 2.5 / cfm 

So if there is a claim of 200 cfm, the dryer would need 2000 Watts to get 70 deg F air up to 90 deg F.  If it is to get to 110 deg F, it will need 3200 Watts which is a good bit more than a standard 120V outlet can provide.

So check those Watts vs air flow for credibility.  If amps are listed instead of Watts, multiply the amps x the volts, 120 or 240 to get Watts.

We at Puelche™ will be doing some experiments in the near future and will present our findings here.

Some discussion of heaters:

There are two main ways to heat air with electrical current.  We are all familiar with the hot wires inside a toaster.  Blowing air across such a hot wire is a very effective heater but the airflow must be maintained or the filament can get too hot and burn itself out.  To prevent this, an airflow switch is included in the circuitry and if the fan fails or the air flow is blocked, the heater will be turned off.  The power usage in this element is typically fairly constant and the air flow will change the temperature slightly.

Hot Coil Heater  

The other style of heater is a PTC heater that does not get as hot but needs more fins and surface area to heat the air as it passes by.  This style of heater has inherent characteristics that limit the element’s temperature and if more air blows by, sucking away more heat, it will increase power usage to get itself back up to temperature.  Many PTC heaters will not burn paper when it is placed on the element and are selected as an additional safety feature.

PTC Heater 

PTC heaters are used on all Puelche™ Boot Dryers because of their safety and simplicity.  The feature design of the Puelche™ Boot Dryers is that unused pegs can be closed, which shuts off the air flow to them.  The PTC style of heater helps to accommodate for this change in airflow.  An additional thermostat switch is located inside the dryer to limit the air temperature to 120 F and some cycling of temperature may be noticed especially with low air flows.  But the boots will not get hot and they will dry!


The Puelche™ Dryer Concept

 Face Plates and Body Design Discussion