Extractor hoods: maintenance tips and fault detection

Extractor hoods or exhaust fans are rotating machines used for the volumetric displacement of gaseous fluids. They are therefore useful for renewing the ambient air and as part of a system for controlling atmospheric pollutants or toxic agents.

The most common models are wind, centrifugal and axial. To know which is the best option, you need to take into account factors such as temperature, types of gases and particles present in the environment.

Wind turbines are the simplest models, consisting of fins, bearings, rims and rivets. They are installed on roofs and serve to exchange air masses by means of the difference in weight between them.  On average, the flow rate varies from 1,500 m³/h to 7,000 m³/h, which is why they are generally used in homes and businesses and do not require maintenance. For industrial applications, the recommended models are axial or centrifugal.

Types of hoods

Axial hoods

These units are light and resistant and are used for ventilation, removing heat and humidity from the environment. These models are suitable for places such as warehouses, factories, and environments that do not have high levels of pollutants (dust, toxic gases or grease).

In this way, axial extractor fans ventilate the room, controlling humidity and heat. They are so named because of the direction of the air flow generated by the movement of the blades around the axis, providing a large volume but with low pressure.

Centrifugal hoods

Centrifuges are suitable for high-pressure areas and can be placed directly on the machines. This application therefore allows for the rapid dissipation of gases, vapors and flammable or corrosive materials produced in woodworking, chemical, textile and metalworking industries, etc.  

These assets are able to generate high pressure by moving the rotor at high speed, which creates the pressure to move the air radially and alter its outlet by 90°. These characteristics therefore make them suitable for situations where there is a significant loss of pressure in the system (above 40 mmCa).

Maintenance on extractor hoods

One of the main ways of detecting initial faults in extractor hoods is through vibration analysis. It is ideal to monitor the condition in order to prevent damage to the equipment and thus extend its useful life.

Obtaining continuous vibration data allows you to establish a history and gain a better understanding of the hood. To do this, sensors need to be installed on the fan’s bearings to enable condition analysis. In the case of the Dynamox Solution, it is recommended to install HF+ sensors: 2 on the motor, at least 3 on the reducer and 1 on each bearing. Sensitive inspection can also be used to obtain a more complete set of data.

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In addition to sensing, sensitive inspection can be used to obtain a more complete data set. This is because experience and practice are important allies in assessing operating conditions and thus noticing a possible fault in advance. 

With regard to measurements, manufacturers recommend that they be at least radial and axial. Once it has been decided which axes will be observed, it is important to maintain the standard for all analyses. The criteria published in the ISO 2372 and VDI 2056 standards can be used to verify and conclude the vibration levels of a machine.

Careful monitoring and analysis of data is recommended when vibration levels reach the “fair” level. Now, when the levels are excessive or reach the “bad” range, the equipment should be taken out of operation.

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Frequency of common rotor faults

There are some relatively common faults that can also occur in extractor hoods. These include unbalance, misalignment, wear, lack of or excess lubrication, looseness and overload.

The following table helps to identify the most common characteristics and causes of vibration in relation to the frequency at which they occur. However, it is important to stress that this information is a baseline and may not be applicable to all operating conditions. It is therefore essential to know the specifics of the asset and, if necessary, put these values into perspective:

Extractor hoods: possible causes of vibration in these assets
Source: Manual Sicflux

Sometimes, the subtle increase in vibration in the rotor, bearings or belts can be related to the presence of impurities in the components. In this case, cleaning will solve the problem and normalize vibration levels. If the problem is one of wear, then the item needs to be replaced before it becomes unbalanced.

Another common situation is the loosening of bolts, which can be extremely dangerous, given that if the problem occurs in the rotor blades, detachment at high speed can cause serious accidents. This is why checking the tightness of nuts and bolts is so important.

Detecting a fault in an extractor hood avoids a cost of $6,000

Mining company Nexa has Dynamox sensors installed in its industrial assets at the Vazante unit in Minas Gerais. During online monitoring, an increase in vibration levels was noticed in the mine’s extractor hood, with frequencies typical of bearing looseness. On opening the bearing, the team confirmed the existence of the looseness:

Unbalance in extractor hoods

With the Dynamox Solution, major damage was avoided, such as the breakdown of bearings, shafts, bearings and the unavailability of equipment. It is estimated that the savings generated by identifying the fault in advance and taking preventive action exceed $ 6,000.

Read the full story: With Dynamox, Nexa avoids a corrective maintenance cost of $6,000