Using the fan curves to power the capture efficiency of airborne contaminants while saving energy.
Ventilation is a mechanism based on the movement of air, engineered to provide comfort, support industrial processes, remove heat and remove suspended airborne contaminants. Ventilation is a form of engineering control element that has the highest efficiency in control mechanics. As the control integrity is good, the emphasis to optimise (an approach to operate at high efficiency within a feasible rate of power consumption) the design is always overlooked causing a system to run at high expenditure due to an overpowered motor. On top of it, the exhaust ventilation will be designed unnecessarily high in exhaust flow rate causing heavy loss of the conditioned air increasing another cost of operation. The question is, can all these be avoided? Can the word optimum design be put to reflect its meaning and objectives?
The common issue in any design approach is how the numbers are derived. Every design may have its standard design equation however the numbers usually do not accurately count the actual risk factor. The rule of thumb ‘guesstimation’ method is usually assumed to work using a big range of limits and contingency. In fact, this design approach has established a false theory stating the higher the flow rate the better it is for control integrity. This may be entirely true if it is viewed based on the individual project objectives, which are to remove or dilute airborne stressors or heat stress. However, ventilation mechanics is never a standalone condition. A set of overall reviews especially in terms of suitability of the aspiration (exhaust) point of the design hood, the logic behind choosing the excessive flow rate, the overlooking of wasted conditioned air, the continuous running of the fan’s motor incurring higher amps drawback and etc.
A fan curve or performance table of a fan illustrates the efficiency of the fan in delivering the intended flow rate either in exhaust or supply (make-up) air form, depending on how the fan is used. The flow rate and static pressure (resistance) parameters are used in selection ranging from various fan speeds and power. By the time we reach this stage to select the suitable fan, the flow rate of the system and resistance in the form of system static pressure must have been calculated with added contingency. Those numbers are called the DUTY POINT. The derivation of the duty point and how the actual performance meets the duty point upon start-up, determine the prowess of the designer or system provider. Once the duty point which is the heart of the system performance is established, the selection of the fan to drive the system is to be done. At this point, factors such as fan noise level and vibration must also be considered apart from using duty points for fan selection. For instance, if the fan can be replaced in a utility room away from workers day to day location, a high speed 2 pole motor can be chosen as it covers a higher range of resistance and flow rate. However, if the fan is to be placed internally a 4-pole motor that runs at a much lower speed is best to be fitted into the system. 2 pole motor which usually runs about 3000 rpm will produce higher noise compared to a 1500 rpm range, 2 pole motor.
There is a common misconception by a lot of designers and system providers when it comes to plotting the DUTY POINT on the fan curve as a part of the selection process. Most of the designers do not specifically have the duty point intersecting the fan curve. This will place the duty point below the curve. They assume that this action is a form of adding more contingency to the system flow rate. However, this choice of design approach does not deliver an efficient system but on the contrary, messes up the entire design and eventually the actual performance. The duty point is derived from parameters such as capture velocity and air change per hour. When the duty point is placed below the curve, the static pressure available becomes more compared to the actual resistance incurred from the hood to the chimney. This scenario will increase the overall flow rate much higher than the duty point derivation. The higher flow rate means higher capture velocity and air changes per hour. Excessive capture velocity will cause turbulence in front of the hood. Higher air changes per hour mean extra cooling load which also means a waste of energy. This approach will also increase unnecessary operating costs as the fan is sized bigger than the actual design needs.
