Hello all I am new to this group and found some interesting discussions iin this so i thought i could get soome help from you.
My question may be sort of silly but i am coming out.
Can any one explain what are system curves for an electrical fan and what are the fan curves. what is the difference between those. If we are given an electrical with motor specification how can we draw system curves and fan curves to defiine a operating point for the fan. what data should i need to collect for that.
I appreciate if any one could help me in understanding this in detail or help me with some websites where i could get better info. Thanks in advance Regards smitha
Fan loading curves, depends upon type of fan. speed vs power used, or speed vs airflow, depends upon the type of fan blades too. ( at least 6 types I can think of) noise vs speed. Look in a Granger' cataloge in the back. for starters
hi Guys thanks to all. I would go through the details. I have the el.motor spec as 2Kw, 200rpm variable speed DC motor. I was given a question in the class that " if you are asked to draw sytem curves and fan curves and find the operating point of the fan with the given motor spec. assume a cooling scenario in the car and think of a radiator fan to be designed" so this is my actual queest.
Aprreciate for the help and any more suggestions I welcome.
For flow through a system, the flow can be either laminar or turbulent. Laminar flow in forced flow systems is very rare, it is almost always turbulent, so let's go with that.
Turbulent flow of most fluids (water, oil, low-pressure air) follow a 2nd order relationship. Flow is proportional to the square root of the differential pressure. Or, in other words, the pressure drop through a given set of pipes, valves, dampers, fittings, etc... is proportional to the flow rate squared.
So for a given 'system', one can plot a series of measurements of flow rate through the system, and required driving pressure to achieve that flow rate. When plot the points graphically, you will see they form 1/2 of a parabola, concave upwards, with the vertex at the origin. Change a valve, or pipe and the curve will change to a new 'system curve'.
Centrifugal pumps have a curve of flow versus developed head that can be approximated as a parabola that is rotated 90 degrees and concave to the left. (as flow through a pump is increased, the developed head decreases). Fans come in all sorts of varieties of shapes and designs. Each fan has a 'head curve' (sometimes referred to as a 'performance curve') that shows the static pressure developed between inlet and outlet for a given test flow rate. Van-axial fan curves often look similar to pump curves over a range of operation. But fans often have 'stall' conditions so that the curves have abrupt shifts or 'walls' in their shape.
If one has a 'system curve', and a 'head curve' for a fan, you can figure out the operating point by simply overlapping the two curves (assuming the axis have the same scales). Where the downward sloping fan curve intersects the upward sloping system curve is where the system will reach equilibrium when the fan is running. From there, knowing the flow and static head of the fan at that operating point, one can determine the motor requirements.
If the system curve doesn't intersect the fan performance curve (check units and scale), or it does so in a 'stall' region, then the fan is not appropriate for that system (e.g. not enough static pressure, or too much flow)
Generally, a system curve can be computed/estimated from the duct/piping design with a fair amount of accuracy. Fan curves often come in 'families' with adjustable pitch fan blades. So once a system curve is known, and the desired operating conditions (flow) decided, one can plot the system curve with a few points on the fan vendor's curves. Find the fan curve that intersects the system curve at the desired flow and that tells you which fan and/or which pitch setting to go for.
If the pitch setting is all the way to one extreme, pick the next larger/smaller fan and adjust the blade pitch accordingly. That way you'll have a fan with some 'adjustment' left in the blade pitch. This can be important for future expansion, or if the system's *actual* performance differs somewhat from the *design* performance (something all prudent engineers plan for).