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What is Static Pressure (and Why You Should Care)

Fan performance is generally measured in terms of two basic parameters. The first is the volume of air the fan delivers per minute or per hour, and the second is the fan static pressure. Fan static pressure can be described in a number of ways, but in simplest terms it is the resistance pressure the fan has to blow against in order to move air in the desired direction. If the fan is blowing against a high resistance pressure, it requires more horsepower and delivers less air.

Understanding what causes the fan static pressure to rise or fall is probably less intuitive than understanding its effect on airflow. Pressure on a molecular level is a measure of the combined kinetic energy (energy resulting from molecular motion) of the molecules in a given volume of gas. If the kinetic energy is increased, the molecules begin to move faster and the pressure will rise. If the energy decreases, the molecules begin to slow down and the pressure will fall. The term static pressure as opposed to dynamic or velocity pressure refers to the pressure a gas would exhibit if it were at rest and represents only the randomized motion of the individual gas molecules. Velocity pressure on the other hand results from the motion of the gas and is the wind force the fan generates by moving the air.

Given that static pressure is the measure of the random kinetic energy in a gas, what can cause the pressure to rise or fall is an increase or decrease in that molecular energy (i.e. molecular speed). When air is blown through lumber it transfers energy to the wood resulting from friction between the air and the surface of the wood. In this process the surface of the wood will heat up slightly and the air molecules will simultaneously slow down. The energy transferred to the wood is thus lost by the air and a fan is required to restore the “air energy” back to its original level. If a large amount of energy is lost, the fan has to work harder to restore it. So high fan static pressure means that the air has lost a lot of energy in its circuit through the lumber, and low fan static pressure means that a relatively small amount of energy has been lost.

The amount of energy lost has a lot to do with how fast the air is blowing through the lumber. Frictional losses are proportional to the speed of the air squared so a small increase in air speed through the lumber stack will result in a larger decrease in the residual energy when the air reaches the other side. For example, the losses in a hardwood kiln where the sticker speeds are e.g. 300 fpm are less than for a softwood drying operation in which the speeds are upward of 1000 fpm. As a result, the fan in a softwood lumber kiln will typically face higher static pressures and will have to work harder (and thus require higher fan horsepower) than for a hardwood kiln.

When determining the right fan for a particular wood-drying operation it is important to understand that the kiln static pressure is not a constant, but rather is dependent upon the performance of the fan chosen. If a kiln is running a small fan generating 45,000 cfm at an estimated pressure of 0.5 inches H2O and it is decided to replace it with a larger fan that is rated at 60,000 cfm at 0.5 inches of H2O, the new installation will not achieve 60,000 cfm. The actual air flow will be less than 60,000 cfm due to the rise in the static pressure. In other words, by trying to force more air through the lumber, the energy losses have increased and thus so has the static pressure. Part of the fan’s increased energy has gone into generating more air flow (as was desired) but part of it has also gone into replacing the additional “static” energy lost by the air (as expressed in static pressure). If this hidden energy lost isn’t accounted for, the result can be disappointing.

This is just one of many of the complications in selecting the right fan for a drying operation. The only rule that can be counted on is that if one thing is changed, then everything else will also change. This is true for a change in motor horsepower, a change in blade pitch setting, a change in the fan speed, a change in sticker spacing, a change in the baffling, etc. All of these changes will affect the fan static pressure and that effect has to be accounted for in the final drying performance estimate.