Wet bulb and air flow

[HencoV]

@ the forum experts...

What is the effect of more or less air flow over the wet bulb (WB) on the WB reading, and the consequent effect on drying controls. Put differently - will higher or lower air speed effect the evaporation rate and temperature of the WB? Will lower air flow result in a higher WB reading, same for higher air flow result in a lower reading?

[MichaelM]

The wet-bulb measurement is independent of airflow above a certain point.  A figure of 500 ft/min is often cited as the minimum.

Water evaporating from the wet-bulb cools it.  As it cools, heat transfer occurs from the air to the bulb.  The rate of evaporation slows as the bulb cools.  Simultaneously, the rate of heat transfer increases as the bulb cools (more heat moves from the surrounding air to the bulb).  At a certain temperature these two factors balance and the temperature becomes steady.  At the wet-bulb temperature, the energy reaching the bulb by convection just equals the energy carried away by the evaporating water molecules.

The two factors (heat transfer and evaporation) are both controlled by the insulating effect of an air layer at the surface of the bulb.  When airflow increases or decreases, the layer is affected and both factors are similarly affected.  For this reason, the wet-bulb temperature is not affected by airflow (above a certain point).  Heat transfer by the moving air is called convection.

High airflow is desirable to minimize the impact of things (radiation and conduction) that affect amount of heat or energy reaching the bulb.  For example, the metal in an rtd conducts (conduction as opposed to convection) some energy from the rtd mount to the tip.  Also, heating coils in view of the bulb can radiate (radiation instead of convection) energy to the bulb.  Hence, dry- and wet-bulbs may have heat shield in some locations.

Below a certain air velocity, the mode of convection changes from turbulent to laminar.  This depends on the bulb diameter and other factors.  This is why most books say 500 ft/min and up is ok. 

[HencoV]

Ok, so we need 500ft/min (2.5m/s) or more over the wet bulb probe to get a constant accurate reading.

From your description of what happens to the air around the bulb at lower (too low) air speed, can one deduce that at lower air flow over the WB:
- the less the evaporation...the higher the WB reading and that radiation and conduction comes into play more (possibly pushing it up further) than at ideal air flow?

This will mean that the condition shows a smaller WB depression than what it actually is, while the drying condition is much harsher than what the control system is showing or trying to control at. This will result in more active venting or vents staying open for long periods for open/close vents or proportionate vents being open more (bigger %) than what is required. Both cases will result in more energy used, possibly more downgrade due to harsher conditions and longer drying times where energy supply is an issue.

Consider that when vents are open, it actually becomes the air's path of least resistance, possibly reducing the amount of air flow through the stack....further reducing energy transfer to the timber and prolonging drying times while drawing more energy from the boiler to maintain DB Setpoint

[MichaelM]

Yes - low airflow causes an artificially high Tw.  This makes the vents open more so the measured Tw comes to setpoint while the real TW (if measured with an accurate wet-bulb) would be lower.

I've always figured this part of the reason two identical kilns dry differently.  The sensor placement is a little different and they read differently so that the actual conditions in the kilns differ.  Ever hear of a schedule working well for one operator, but not working at another mill?  This could be the reason.

Wet-bulb sensors are placed away from the wall to help with airflow.  In some situations we have (in measurements at OSU) placed out wet-bulb and dry-bulbs inside of a blower to have good airflow.  This was not in a kiln application.