AHU’s (Air Handling Units) are widely employed across a variety of industries to circulate air as part of a heating, ventilating and air-conditioning system (HVAC). There are numerous applications where it is critical that safe, consistent indoor air quality is maintained around the clock with any significant changes having the potential to cause huge repercussions concerning health and safety issues and inferior, damaged or loss of production. On a Pharmaceutical production line it is imperative to ensure a quality product is produced so there is a AHU monitoring requirement to maintain air flow rate, temperature and humidity throughout a total production process. 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.
Continuous and consistent performance
AHU’s are required to operate continuously providing consistent environmental parameters such as temperature, humidity and level of flow control the air flow and come in a variety of shapes and sizes. A typical unit consists of a large metal enclosure containing a fan or blower, coils that circulate steam or hot water for heating and chilled water for cooling and air filters. The complete fan and motor assembly, comprising bearings, shaft, pulley and belts are contained within the AHU enclosure, which will have removable panels to enable engineers to gain access to carry out any necessary maintenance procedures.
It therefore makes economic sense to monitor the rotating elements of the AHU’s to enable vibration engineers to pick up early signs of any degradation in the equipment so maintenance can be planned and carried out with minimal expense and disruption to service.
Hansford Sensors has supplied a wide variety of vibration sensors to this industry and various approaches have been applied. Typically AHU’s include a supply fan and an 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 AHU monitoring the DE and NDE bearings Hansford Sensors recommends installing two industrial accelerometers 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.
Simple and cost-effective solutions
A simple cost-effective online vibration for AHU monitoring 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 (programmable logic controller) or similar system where overall vibration trends can be monitored. This is a well established technique for identifying out of balance and misalignment. A popular choice of sensor would be with the M12 connector option used in conjunction with separate M12 cable assemblies. The main advantages of the M12 connector is it is a smaller size, associated cable assemblies have a tighter bend radius and it is a well accepted and proven connector used across many industries. Another sensor option to consider is a dual output sensor (HS-421 Series) which provides not only a 4-20mA output, but also an AC output and allows the engineer to take more-in-depth vibration analysis via a Data Collector. This method will detect bearing defects, mechanical looseness and provide 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 AHU’s. 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 valuable 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 which is suitable for smaller AHUs is a compact small footprint sensor (HS-130 Series).
The Hansford Sensors range of switch boxes is 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 or the stainless steel boxes compulsory in many food processing and pharmaceutical applications.
There are many different methods available for installing the vibration sensors. The accelerometer needs to be mounted directly to the machine surface on a flat, smooth, unpainted surface larger than the base of the accelerometer, free from grease and oil, as close as possible to the source of vibration and at right angles to the axis of rotation.
Correct measurement of the levels of vibration can be achieved in several ways. The preferred mounting technique, where conditions allow, is to drill and tap a thread directly on the machine so an accelerometer with an integral ¼-28UNF, M6 or M8 mounting thread can screwed in to place. The same drill and tap method can be applied when using a separate mounting stud and then connecting the accelerometer to the stud. Care needs to be taken to tighten sensor to specified torque levels as over tightening can damage the sensor by stripping the thread and under tightening will lead to inaccurate readings due to poor contact with the vibrating surface.
Where it is not possible to drill and tap, for example where warranty issues do not permit this, adhesive mounting is advised. A metal filled epoxy adhesive provides suitable bonding for applications in temperature up to 100°C. Hansford Sensors can provide Metal Bonding Stud Adhesive (HS-AA003), which provides sufficient adhesive to mount up to 25 studs.
The accelerometer cable should be clamped to the body of the accelerometer with a cable tie to prevent strain and excessive movement of the cable, which if left loose could result in false readings.
Hansford Sensors Ltd has a wealth of experience in the design and manufacturing of industrial vibration sensors for various applications and can offer technical advice on specific applications. For further advice please consult our technical team on +44 (0) 845 680 1957 or email firstname.lastname@example.org.