Pressure losses in ventilation, how to minimize them?

For the correct operation of mechanical ventilation systems and their better optimization, one of the phenomena we must consider most is pressure losses. This will determine the consumption of the fans and the strength required by the installation.

Pressure losses are a phenomenon that arises from the air resistance in the ventilation system, which is greater the larger the air flow. As air passes from the fan to its final destination, greater or lesser resistance occurs, caused by friction as it passes through the air ducts and other equipment, such as filters, silencers, the heat unit, or others.

The loss of force of the air caused by these frictions is what we call pressure losses. Whether they are greater or lesser will largely depend on a correct installation and design, avoiding very long routes and changes in duct section that cause the air to lose strength. From this planning, greater or lesser ventilation efficiency will be achieved, and if not taken into account, there would be greater energy consumption by the fans.

How to calculate pressure losses and optimize the system?

• The total distance between the fan and its final destination. The longer the duct, the more force will be required.
• The changes in section or elbows. When the air changes direction, pressure losses will occur.
• The diameter of the ducts. The larger the diameter, the more air transported.
• The speed of the air and its density. It is essential that the fan's force is exactly what the system needs after calculating the pressure losses.
• Roughness of the inner tubing coating.

To calculate pressure losses, nomograms are commonly used, which are analytical geometry calculation methods and are very helpful to calculate these losses in ventilation systems. However, as they arise from experience and their use, the calculation of these nomograms is valid only in ducts with the most common materials and the most used roughness.

The calculation of the estimated pressure losses in a ventilation system will allow us to push the air at the necessary speed without having to increase the power of the fan, in addition to avoiding insufficient ventilation. Regarding the speed that the air should acquire, the following values are taken into account:

• Ventilation in ducts at low speed: between 6 and 12 m/s.
• Ventilation in ducts at high speed: >12 m/s.

The calculation of the speed in ventilation ducts can be resolved using the following formula:

• V= L /(3600*F) (m/s)

In this formula, "L" represents the capacity of the air in m³/hour. And "F" represents the section changes of the ducts in m^2.

The pressure losses can be reduced by a duct network with fewer section changes, as the air travels at much higher speeds. A longer straight length is preferable to a system with many crossings and elbows. Similarly, if there are many ventilation areas connected with grilles, the fan can be located in a central area, as this avoids requiring greater energy to reach the most distant end.

To calculate the pressure losses in air supply or extraction systems, we can use the constant pressure loss method. It is performed by calculating the ducts so that they assume the same pressure loss per unit length throughout the entire route.

The most common in this method is to find a critical velocity based on the maximum noise that can be assumed. How do we determine this velocity? Taking the main duct as an example, which follows the supply since it is collected from the outside or from filters. Thanks to this velocity that serves as a starting point, and knowing the air flow that needs to be supplied, the unit pressure loss that must be constant throughout the network of ducts is calculated.

The pressure losses are a phenomenon in ventilation that we cannot overlook. Moreover, they are the most important aspects to consider in an installation if we want it to be truly efficient the mechanical ventilation system. If unnecessary air paths and very complex layouts are avoided, we can have a much more reasonable consumption of the fans and the noise will be manageable.