Taking care of air

We take a look at maintenance of air handling units and how to maximise performance with vibration monitoring

Air control has been used in the preparation of medicines for centuries, ever since witch doctors began using the method to treat their herbs and spices.  Even today, the practice remains popular in some corners of the globe; in 2011, the National Geographic Channel television series The Witch Doctor Will See You Now showed viewers dried animal medicinal products on sale at a Kowloon shop including deer antlers sliced into thin strips and then ground into a powder, supposedly to promote bone health.  In the west, air handling for medicinal purposes is a very different business, requiring the skill to manage machinery that can maintain air conditions within precise limits during the manufacture of pharmaceutical supplies. 

Air handling units are key to maintaining the quality of output on a pharmaceutical production line.  Consistent indoor air quality must be maintained, with no sudden changes to air flow rate, temperature and humidity that could potentially compromise production quality.  To ensure that these levels are carefully managed, machinery must be well specified, maintained and run, since a mechanical failure can make a degradation in air standards almost inevitable.

 So how can operators protect themselves against such eventualities?  To learn how best to optimise your systems, it pays consider the general maintenance issues surrounding air handling units and then look at the relevant vibration monitoring techniques that can ensure consistent air quality.  In addition, we need to understand what vibration sensors are and how they function in order to install and use them to their fullest potential. 

 Accelerometers and air handling units

A typical air handling unit consists of a large metal enclosure with removable panels, behind which we find air filters, coils that circulate hot water or steam for heating and chilled water for cooling, and a fan and motor assembly, plus associated bearings, shafts, pulleys and belts.  By evaluating the state of the rotating elements of the air handling unit, engineers can exercise an efficient preventative maintenance programme and a highly effective way to carry out condition monitoring is to use vibration sensors. 

Vibration sensors, which measure a quantity of acceleration and are therefore a type of accelerometer, typically contain a piezoelectric crystal element bonded to a mass.  When the accelerometer is subject to an accelerative force, the mass compresses the crystal, causing it to produce an electrical signal that is proportional to the level of force applied.  The signal is then amplified and conditioned using inbuilt electronics that create an output signal, which is suitable for use by higher level data acquisition or control systems.  Output data from accelerometers mounted in key locations can either be read periodically using sophisticated hand-held data collectors, for immediate analysis or subsequent downloading to a PC, or routed via switch boxes to a centralised or higher level system for continuous monitoring.

Accelerometers can be divided into two categories, AC and 4-20mA.  Each of these accelerometers is capable of delivering a different level of data aqcuisition, a key fact to consider when specifying a vibration monitoring programme for your air handling units.  4-20mA devices are typically used with a PLC (programmable logic controller) to measure lower value assets, such as fans and pumps, while AC accelerometers can be used with a data collector for monitoring the condition of higher value assets.  4-20mA devices are generally considered to be the ‘cost-effective’ option and, like AC accelerometers, are highly efficient in detecting imbalance, bearing condition and misalignment but there are occasions when AC accelerometers are specified for their ability to identify further issues, such as cavitation, looseness, gear defects and belt problems.  To see how these different sensors might be applied, let’s return to our examination of a typical air handling unit.

Planning your programme

A series of options are available when planning a vibration monitoring programme for air handling units.  Fans, for example, may require as many as eight vibration sensors, depending upon their specification.  A supply fan may need a vibration sensor installed on both the drive-end and non-drive-end bearing if it is a direct-drive type.  However, when monitoring a non-direct drive fan it is advisable to add two accelerometers on the journal bearings of the gear shaft.

By mounting 4-20mA sensors on to the bearings and shafts of the air handling unit, velocity levels can be fed back to a PLC, allowing vibration trends to be effectively monitored.  If engineers also require more-in-depth vibration analysis via a data collector, other options are available.  For example, Hansford Sensors offers a dual output sensor (HS-421 Series) that provides not only a 4-20mA output but also an AC output to deliver more detailed information on balance and alignment.  A further option is provided by fixed AC sensors, which can be hard-wired to switch boxes located in accessible positions outside the air handling unit; this enables vibration data to be collected routinely and safely but the data gathered is limited in its potential, since measurements relate only to the time at which they were logged.

Engineers should be aware of the fact that there are also sensors available for use in installations where there is limited space for installation or access.  For example, side entry sensors can be used where access is difficult, such as in proximity to belt guards, while smaller air handling units can be effectively and conveniently monitored using compact, small footprint sensors.

When specifying equipment, choose a supplier with a broad range of options and expertise in a variety of sectors.  For example, a number of configurations and finishes are available for switch boxes but stainless steel units are often compulsory in pharmaceutical applications so you need to select a balance between cost-effectiveness, legislative requirements and reliability.

Installation issues

There is no point in specifying the ideal equipment for your application and then failing to install it correctly so, again, use a well established supplier who will offer valuable advice.  For example, engineers should mount accelerometers directly onto the machine surface on a flat, smooth, unpainted surface, free from grease and oil, that is larger than the base of the accelerometer itself.  Another useful tip is to ensure that the accelerometer is positioned as close as possible to the source of vibration.  If conditions (and product warranties) allow, the preferred mounting technique is to drill and tap a thread directly onto the machine so that an accelerometer with an integral ¼-28UNF, M6 or M8 mounting thread can be screwed in to place.  This ‘drill and tap’ method can also be used to fix a mounting stud, to which an accelerometer can then be attached, and specialised installation kits are available for performing this task.  It is important to take care and use the right tools for the job, as tightening the sensor outside the appropriate torque levels can damage equipment or reduce its effectiveness.  For example, over tightening can damage the sensor by stripping the thread, while under tightening will lead to inaccurate readings due to poor contact with the vibrating surface.

If drilling and tapping is not an option, the next best thing is to attach mounting studs using adhesive.  The issue to be aware of here is that you may need to consider the temperature present within your application and choose an adhesive that is capable of coping with that temperature.  For applications in temperatures up to 100°C, a metal-filled epoxy adhesive would be the appropriate substance to use.

Having installed accelerometers as carefully as possible, ensure that the accelerometer cable is clamped to the body of the accelerometer itself with a cable tie.   This will not only prevent strain but also prevent the false readings that can be generated when loose cables make excessive movement.

Although there are several important specification and installation issues to be considered, the result of implementing a vibration monitoring programme will be well worth the effort, extending the operating life of equipment beyond recommended maintenance intervals and guarding against the expenses generated when increases in vibration lead to machine failure, downtime and unwelcome reductions in production volume or quality. 

Author biography
Chris Hansford is a qualified electro-mechanical engineer with over 30 years’ experience in the vibration monitoring industry.  In 1986, he was involved in the formation of a sensor manufacturing company and, as Managing Director for 20 years, successfully grew the business and gained a wealth of commercial experience within the UK market.  In 2006, Chris moved on to set-up Hansford Sensors Ltd, a manufacturer of accelerometers and ancillary equipment that has already become a global market leader.


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Date: 21/02/2012