Vibration sensors monitor AHUs
March 1st 2011

Hansford Sensors has supplied vibration sensors to a range of companies in Ireland to monitor air handling units (AHUs).Their use helps to prevent machinery failures in critical applications and reduces the exposure of the engineer to safety hazards by enabling data to be collected from inaccessible points

AHUs are employed across a variety of industries to circulate air as part of a heating, ventilating and airconditioning system. They are required to operate continuously providing consistent environmental parameters such as temperature, humidity and level of flow.

There are numerous applications whereby it is critical that safe, consistent indoor air quality is maintained 24/7 with any significant changes having the potential to cause huge repercussions concerning health and safety issues and inferior, damaged or loss of production.

Take for example the consequences on a pharmaceutical production line where to ensure a quality product it is imperative to maintain air flow rate, temperature and humidity throughout a total production process, which could last in excess of 4 weeks. If there is a mechanical failure the cost of loss production could be exceedingly high, with other implications such as customer dissatisfaction and penalties resulting from late orders. In laboratories too, it is vital to monitor air quality, often within extremely tight tolerances, to prevent the destruction of months of experiments and stored data, which can be wiped out if certain environmental conditions are not maintained.

It therefore makes economic sense to monitor the rotating elements of the AHUs to enable vibration engineers to pick up early signs of any degradation so maintenance can be planned and carried out with minimal expense and disruption to service.

Hansford Sensors supplies a variety of vibration sensors to this industry in Ireland, UK and further afield with various approaches having been applied. Typically AHUs include a supply fan and extractor fan and can either be direct drive or non-direct drive. On direct drive fans a vibration sensor would be installed on both the drive end (DE) and non drive end (NDE) bearing. For non-direct drive fans, in addition to monitoring the DE and NDE bearings you would install two sensors on the journal bearings of the gear shaft. Therefore, depending on the configuration of the fans employed a typical installation would need either six or eight vibration sensors A simple cost-effective on-line option for monitoring AHUs requires 4-20mA sensors (HS-420 Series) mounted on to the bearings and shafts of the AHU with the velocity levels being fed back to a PLC or similar system where overall vibration trends can be monitored. This is an established technique for identifying in particular out of balance and misalignment. An example is the M12 Connector option used in conjunction with separate M12 cable assemblies. The main advantages of the M12 connector are its compact size; associated cable assemblies are said to have a tighter bend radius, and it is an established connector used across many industries. Another sensor option to consider is a dual output unit (HS-421 Series) which provides not only a 4-20mA output, but an AC output along which allows the engineer to take more-in-depth vibration analysis via a data collector, including detecting bearing defects, mechanical looseness, along with more detailed information on out-of-balance and possible alignment issues.

Another method that can be applied is the use of fixed AC sensors (HS-100 Series) hard-wired back to switch boxes located in accessible, safe positions outside the AHUs.

This set up enables vibration data to be collected safely from the same positions on a regular basis, although it should be noted that the readings are only applicable to the moment in time in which they were taken.

Analysis of this data over time enables the maintenance engineer to obtain an effective picture of plant operating conditions and schedule any necessary maintenance procedures accordingly. For installations where there is limited access, for example, near the belt guards, a side entry sensor (HS-100S Series) can be used. A further choice suitable for the smaller AHUs is to install the small footprint sensor, the HS- 130 Series. Switch boxes are available in a number of configurations and finishes ranging from the cost-effective HS-BE Series rated to IP55 and suitable for clean, indoor environments, to the painted mild steel (rated to the IP66), to the GRP for outdoor locations to the stainless steel boxes compulsory in many pharmaceutical installations.

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