Design Point, Baseline Test, Performance Monitoring, Periodic Test, Maintenance. What is the correlation ?
Performance Monitoring for Industrial Ventilation System is much crucial than any compliance monitoring
What is System Design Point ?
Fan Curve and System Curve are crucial tools in designing Industrial Ventilation System
System Design Point is the intersecting line of the system curve and the fan curve based on the design basis which is the system flow rate and the resistance the fan has to overcome to enable the displacement of air either in positive or negative pressure. It is very important to select system providers who can accurately define both the system flow rate and the system static pressure within 10% contingency as per AMCA (Air Movement and Control Association) standards. Failing to derive an accurate system flow rate will cause the actual performance to be off the objective to meet the criterion such as inability to remove airborne hazard concentration below Action Level of the Permissible Limit defined by the Industrial Hygiene Statutory Regulation, fall short to meet ACGIH Industrial Manual derived capture velocity ranges or the ASHRAE 62.1 & 62.2 stipulated air changes or other acceptable range of standards. Similarly, failing to define accurate system static pressure will create imbalance for the fan rotation to churn the adequate flow rate as it has to allocate its power to overcome the system static pressure losses. The scenario can be at both extremes which is to be over designed or under designed.
What is Baseline Performance Report ?
A baseline performance report is the maiden record of the performance after it is commissioned based on its success in meeting the design point within 10% allowance while meeting there ventilation system objective. The report will record all the peak numbers which should be used to set the criterion of acceptable range of performance and kick start the periodic performance monitoring. Logging of the performance will establish the trend of operation in comparison to the baseline / peak point. The peak numbers will also serve as reference point as to gauge the severity of system degradation.
The importance of performance logging not only for chimney performance but applicable for any set of parameters.
What is Performance Monitoring ?
Performance monitoring is the simplified quick gauge of the system performance index to ensure it is operating within the baseline effective ranges which is also within its original system design point. A performance monitoring is crucial to be conducted preferably daily before the shift works comments. A day to day logging will provide the trend to indicate if the system is performing outside the acceptable range. This statement may not sound important when we merely look at it as repetitive action in logging data. However, if we understand that the system is actually performing while being in balance to overcome resistance across the system while needing to generate adequate flow rate, then we will start focusing on the resistance build up. However prior to that, the first area of focus should always be the fan speed.
Why monitor the fan speed ?
Fan speed determine the amount of power delivered to generate the desired flow rate after overcoming the system static pressure losses.
A fan’s functionality is just to rotate at the rated speed. However the configuration of the impellers and its blade are designed in such a way to deliver the desired flow rate. The fan capacities are made available in various range of flow rate and system static pressure. However any design point selected is based on the SPECIFIC FAN SPEED. This means if the fan speed changes, the fan will not be able to deliver the demand of the system point. It will be underpowered. As per the illustration above, the curve of a dropped fan speed is far below the design system point. The rotating fan will not deliver the desired air flow. The fan speed must be immediately rectified. The causes of fan speed failures are :-
coated heavy impeller
misalign shaft
bearing failure
motor coil failure
As the fan speed is the first point of checks in the event of dwindling performance, it is a good practise to have the fan speed test logged regularly. Apart from that, if there is a fan speed drop, never conduct any thorough periods testing until the fan speed is rectified. Fan speed is highly recommended to be taken first during stack / chimney emission monitoring or periodic lev test.
What is Performance Monitoring ?
Once the baseline documents are established and the performance monitoring protocol are set, the person in charge taking the records must be trained to know what are they recording and how to trigger improvement action in the event they detect out of specs. results. Every inspector must have a mantra that the system must operate within the system design point / baseline acceptable range of criterion. They also must know that the moment there is an off specs, the will be risk in the control integrity or the objective of the system. The other simple fundamental the user needs to be taught is that resistance build up is the culprit.
Actual performance skewed away from the design point due to resistance build up increased the system static pressure beyond the allowance range.
System resistance build up can take place due to these reasons :-
restriction in hood access
damper unapproved balancing
poor engineering change of management - plug and play addition of hoods without proper engineering calculation
excessive settlement, coating, scaling or condensation along the duct
clogging of air pre-filter, adsorption filters, packed towers, demister pad, fabric filters
poor throttling of liquid spray flow rate outside its allowable range causing increasing resistance for gas to pass through
As there are many factors possibly contributing to the build up, a diligent effort to conduct the performance monitoring will easily detect the drop in the flow rate. As it can be seen in the illustration above, increase of flow rate above the design point, will make the fan to compensate from the flow rate it suppose to deliver. This will cause drop in failure and generate a chain reaction crippling the entire systems performance by undergoing these scenarions :-
failure to generate adequate capture velocity causing failure in local capture efficiency increasing the risk of airborne hazard exposures.
will not generate adequate air changes causing failure of airborne hazard dilutions increasing the risk of airborne hazard exposures.
increase of duct settlement due to drop in flow rate causing further constriction of air flow passage.
failure of air cleaning device due to low filtration velocity
down wash of chimney efflux velocity increasing ground level concentration causing environmental impact.
excessive amp draw back causing motor overload and complete idle of system.
Periodic Test
Periodic test are thorough test which is usually performed during any upset or annually to check in detail every component to ensure it does not contribute to unwanted increase in resistance. The ultimate objective is to tweak the system to design point and cross check any areas showing sign to cause process upset. Periodic test must be based on criterion set in Baseline test and should not establish their own criterion as the reliability of the test results may be affected. It is very common for lots of tester even the competent ones to set their own criterion which goes beyond the capability of the fan showing imminent failure as the criterion set is not suitable and relatable.
When to conduct maintenance ?
Drop of performance below the lower limit must trigger corrective maintenance to retain performance back to peak level
Maintenance is an act to retain the system performance within its baseline condition. As it can be seen above, action must be taken to correct fan speed or reduce the excessive resistance build up to tune back the system to its original condition. A comprehensive understanding system design point and what causes its off set can enable the service provider to easy reset the system to its original condition. Maintenance can be made predictive based on certain cycle or preventive based on performance logging. A combination of both approach is crucial for a continual high efficiency performance eliminating unwanted faulty or sudden failure.
