Fan coil units
HVAC fan failure monitoring
Geoff Edwards, technical product engineer of Axair Fans examines a method of detecting fan failure within fan coil units in heating and ventilation
T
he undetected failure of fan coil units is a long standing problem in heating and ventilation. Fan coil units like the types found in H&V
applications tend to be unmonitored and system failures can often go unnoticed to the point where a refit or refurbishment is required or where air simply ceases to flow from the ductwork. The oldest format for monitoring the FCU is to use
differential pressure switches. Within the fan coil unit, the installer would place a differential pressure switch on either side of the fan inlet and outlet which would monitor the pressure difference between the two. Tubing from each sensor is connected to a mechanical switch and when a change in pressure is detected, the switch will open or close to raise an alarm. The use of pressure differential switches has its limitations. They are easily clogged by air born debris, requiring regular maintenance to ensure reliable operation. The switch itself is mechanical and prone to corrosion which will lead to ceasing and non-operation, and it can be time consuming to calibrate the switches. A more elegant solution is to monitor the current
draw of the fans within a fan coil unit. A fan failure, depending on its nature, will often result in a drawn current spike or, of course, zero current. This requires the installation of additional components in the control circuit.
A part solution has been offered for a number of
years utilising the tacho pulse output generated by the hall affect sensors found within many AC or EC motors. However, this is a limited solution. At failure, the fan when used in a multi-fan system will start to
free wheel backwards, due to the pressure differential on the downstream side of the plenum forcing air upstream past the fan. A tacho pulse will therefore still be generated and fan failure is not picked up (this limitation is applicable to multi-fan systems). One fan manufacturer offers a solution to this problem that will help to keep fan systems running at maximum efficiency, for longer. With the on-board alarm now available in Ecofit EC motors, it is possible for a building management system (BMS) to constantly monitor the air handling systems that are on the network. Allowing for the identification of the faulty air handling unit very quickly, no inspection of multiple assets is required to locate the fault. If required, individual fans can be monitored depending on the requirements of the BMS specification. This has significant advantages when looking to
resolve faulty FCUs. Down time of the FCU is minimised through rapid identification of faulty assets. Furthermore, the labour required to resolve the issues is significantly reduced because faults are quickly identified, productivity of the work space is kept high because disruption to the work environment is reduced, and because air quality is so vital to productivity, the FCU is operational again very quickly. The European fan manufacturer Ecofit, in
partnership with Axair, has developed a double inlet, forward curved centrifugal fan (pictured above) that features a highly efficient EC motor, surpassing ERP 2013 and 2015, but also has a built-in failure monitoring system to detect failures of individual
fans or fans within a system. And that’s along with all the other well known benefits of using EC motors over traditional AC motors. The failures are monitored via a normally closed
contact within the motor which will open when the fan speed drops below 200RPM but only when the fan is receiving a run command, so as the motors RPM falls below 200, the monitoring circuit is opened. This switch is controlled internally on the motor’s PCB by an opto-coupler. This is used as it isolates the alarm circuitry from electromagnetic radiation that could cause false readings. The on- board alarm system is such that multiple fans can be wired in series and powered either using the fan’s own 0-10v output or an external input from the BMS system, 48V 5mA max. The flexibility of EC allows for the customisation of the RMP “trigger” point and whether the system operates normally closed or normally open. This circuity within the fan’s printed circuit board will generate an alarm signal which the building management system will detect and respond to accordingly. For both the end user and the fan coil manufacturer, on board alarms hold many advantages. For the end user, downtimes for maintenance are reduced, expensive building assets are running at optimum efficiency for longer and productivity of the work space is maintained. For the fan coil manufacturer, no additional components are required to monitor tacho outputs before integration with the building management system, meaning reduced costs, and quicker construction times and less complex assembly procedures.
22 July 2017
www.heatingandventilating.net
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