How many percent of contingency shall I add to my Design ? Design Points and Safety Factors in Designing an Industrial Ventilation System (LEV & GV Systems).
The figure above illustrates the SYSTEM DESIGN POINT.
What is The DESIGN POINT of operation or also known as duty point (adopted from pump design nomenclature) are the primary objective of the design exercise. The sequence of design process ranges from calculating flow rate, determining system static pressure then using these data to select a fan. The point of intersection of the system curve and the fan curve determines the point of operation (actual). Determining the Design point is the primary objective of any design exercise. The design point is essential in selecting the fan for the system. The design point will also serve as the peak design value the operation should maintain all along with the implementation of effective preventive maintenance when the systems succumbs to dwindling performance.
The design point also plays a very important role in the design approach sequence which is optimising the need of the airflow into a space without needing to have excessive energy losses. This scenario becomes imminent when the system is designed using ‘guestimation’ approach sizing a big flow rate of air which increases the cooling load in the room unnecessarily high. On top of it the ‘guestimation’ approach will end up having high amperes being drawn into oversized Industrial Ventilation and MVAC systems. The high amps is also equivalent to unnecessary high operation expenditures. This is something usually get easily overlooked as flow rate, capture velocity, air change per hours or even temperature seems to be the only form of design selection criterions adopted while the operation parameters are completely overlooked, mostly due to ignorance at that point of design exercise. The after effect, you may end up running an expensive system.
Design point which derives the sizing of components using reliable empirical values taken from acceptable standard such as ACGIH, ANSI or ASHRAE ensures the actual performance will fall within the design parameters established. Safety factors or contingency values are added to eliminate any under or excessive performance due to changes in physical construction or wrong selection of coefficients. These methodological approach technically should be providing a desirable performance. The figures illustrated below depicts various scenarios when the design point approach are not practises.
The figures illustrates design point or the intersection of the fan curve and the system curve at the point of operation. In the designing sequence, the system static pressure (using the available fan static pressure curve) will be calculated using standard formats such as ACGIH, ANSI and etc. These standards provides parameters such as recommended capture velocity or air changer per hour, entry and friction coefficients, system effects coefficients and etc. The numbers which have been derived based on actual built projects serves are important empirical basis for any new design. Failure in using these approaches accordingly provides impact as illustrated in the figures below.
Figure (A) illustrate a successful design meanwhile figure B,C and D illustrates a poorly design system.
The fan curves intercepts at the design point of the system curve. The flow rate desired will be delivered as long the operation is within the design static pressure losses.
The actual system is undersized due to wrong selection of fan or wrong setting of fan speed. Each fan curve is represented by its speed.
This above figure illustrates a classic example where the system resistance was over estimated. In actual operation, when the system static pressure is much lower, the excess energy of the fan will be converted into additional flow rate. The system will over performed causing possible energy losses and unwanted turbulence. You may have imminent noise issues plus enhanced wear and tear issues.
The figure above indicates classic example of ‘guesstimation’ approach where the design point and the fan selection approach do not correlate. This kind of issues sometimes are also caused by failing to do non standard air correction for processes without higher temperature, or higher static pressure or at much higher altitudes. Fan curves which are derived at standard condition, will not provide the desired design numbers if the corrections in design parameters are not done as per the non standard air physical numbers.
As by know we know how important it is to have a proper design point derived, the safety factor or the contingency we add into the design process plays and equal crucial role. The figure above describes the ACGIH Design Manual 10% contingency recommendation. Any numbers above this margin will create deviation completely off the design point similar to some of the figures illustrated above.
