How to select a fan?

[Vicsidhu]

I work in a company (very small) where my mentor is this web site, books and the Internet. I am the only designer (very fresh) – soon to be an engineer (increased responsibility). So I am turning to the form for help.

Some fundamental questions are the least asked and clarified.
My question in short is: How to select a fan / HVAC unit?

Designing a ductwork is not a problem. I go with COOK ductulator and design for 0.1” pressure loss for main and 0.08” for branch ducts. I have difficulty understanding what method to use to calculate system pressure (Static Pressure) for fan selection. I have asked many people – but have received no good answer. Some say just go by “Equivalent lengths” and find the static required for the critical length. While some say – you have to calculate with “Equal Friction method” for each item of critical path. My superior (part-time) engineer just looks at the system and says – it is 1.2” w.c SP for the fan. No matter what the system looks like, he invariably reaches the same number in seconds – which defies all logic of calculations.

I have tried both – “Equivalent length” is too easy to be true, while "Equal Friction Method" is so time consuming (ASHRAE Fundamentals 2001 – page 34.24).

I would like to hear from you, if you have any feedback. How do you do this? Do you use any software programs? I want to learn this and be a confident and good engineer. Thanks for reading this message.

 

[arabei]

Hi Vicsidhu, hang on it's a very long way to go, and it seems that you're the person for it with your great will to learn. anyways, you must look at the issue from the point of how the fan functions to perform it's duty.
There's the thermal load of the system that the fan must deliver the adequate air flow (cfm, L/s) to carry that load according to your design temp. difference, and thatt's the first task on the fan, which is also the first step in the proceddure of fan selection.
Then this fan must be able to deliver the required air flow to the most HYDRULICALLY remote air outlet, and here comes the importance of the static pressure calculations. so what you must do is that you should make your sizing using the equal friction method (for low velocity systems) then you messure the longest duct run from the hydraulics point of view as highlighted before, then you multiply the length that you've calculated in feet with the friction figure you've used (99% it's 0.1" Wg) but you must take into account the losses in the fittngs and the air outlets.
For the fittings you can use the equivalent length method, and for the air outlets you will find the figure in any manufacturer's catalogue, then you add all the figures to get the total static pressure on the fan.
But if this fan is an AHU's fan you must add the internal pressure losses inside the AHU (which is in most cases catered for by the manufacturer's data in the selection part of the catalogue).
For more info. i recommend Trane AC manual, it's really a good one.
Good Luck

 

[atangent]

there are some companies that offer good software if you want to take the easy way out like Cook. Since you already use their ductulator. Just check out their website.