Talk:Discussion Forum/Losses In Ventilation
How you look at things changes what you see about them. This is never more true than in cooling air flow (or fluid flow). In flow devices, as in all processes, there is an input to the drive motor which drives a fan which provided the flow. The flow is characterized by both a flow rate (volumetric in [m^3/s]) and a flow pressure rise in [Pa]. The product of those is the flow power in [W]. The efficiency of the motor/fan set is this flow power divided by the inlet electrical power in [W]. This gives a number less than 1 and some have the habit of changing this to a percentage. The author prefers not to do so but it may sooth some practitioners.
The efficiency of any motor-fan set is often plotted, using a different vertical scale (ordinate), on the flow rate as abscissa (horizontal)versus pressure rise along the ordinate (vertical) plot. The efficiency of the motor/fan set has two zeros, one for zero flow rate and one for zero pressure rise. The curve peaks in between, often about (not always to be sure) about 2/3 of the way towards the maximum flow rate on the abscissa. Very few motor/fan set operate close to that peak efficiency point, although things have improved in the last few decades. Control of motor inlet voltage is often used to bring input power up or down to move the operating point closer to the peak efficiency point, but proper initial sizing of all components offers the greatest lowering of the power input (because of reduced waste).
What is often not plotted is the amount of losses/waste in the process of delivering the correct (or optimal) cooling air flow for devices - especially when cooling coils or heat exchangers are involved. Many who size fans and then motors think that they have done well enough when the set efficiency peaks at 80 odd percent, not realizing that at the actual operating point it may be closer to 50%. That is a whole lotta waste and much of it can be dramatically reduced. When I consulted with some, they did a much better job when looking at the actual losses all along the process and they tried harder to get the losses down with that change of perspective. Overheating of motor insulation and degradation of lubricant added to the reduced system costs. Almost without exception the motors were smaller and the fans looked bigger. Traditional reviewers of the system suggested total failure to perform the task. When all worked out well, they just mumbled that it would all come tumbling down because of something that was forgotten. No one offered congratulation on a job very well done. Years later it was noted that normal motor replacement was not needed. The need to occasionally clean all flow surfaces seemed more important, however.
