Homemade Air Respirator

Hello.

I am building a homemade air respirator from scratch and my research has led me to some confusion about which fan to select based on what is out there.

Having taken apart some air purifiers and respirators to understand how they work I have come across different styles of fan within them and don't understand the choices they have made. I will give an example.

Some air purifiers use this type of fan: [ATTACH]951[/ATTACH] where the fan is enclosed in a case that it mostly fills in terms of it's height (where height is the vertical height of the edges of the fan).

Another type of respirator uses a similar design with a blower (that is their own design) - I have tried to draw here:
[URL="http://bit.ly/1avh3m0"]
[ATTACH=CONFIG]952[/ATTACH]


[/URL]
as there are no schematics for it online. As you can see I have highlighted the large gaps above and below the impeller section within the blower case.

If anyone can explain why they have done this and what the advantages are as opposed to just putting a bigger impeller inside the blower I would appreciate it.

(I assumed the the bigger fan i.e. with taller blades would process more CFM and pressure for the blower?)

The fan used was likely selected because of space, flow, electrical properties and other design centric requirements.

OEM fans will come with performance specifications..

Thanks Kelly but I was looking for a more detailed answer as to the exact reasons why. I'd like to understand why there are variations in design and what their reasons are. I can't find any information on fan design theory.

I can select a fan and it will work but ideally I would like the most efficient fan available within an efficient design accounting for battery use, noise level and CFM and pressure throughflow.

If you're wanting to design a fan - well, this is a different question..

In engineering design and specification using OEM components, we would search for available components/technologies using the parameters listed in my previous post. When we identify those best-fit components we will then make our selection.

You're going to have a challenging time identifying an unbiased and complete analysis of "[COLOR=#333333]most efficient fan available" as most manufacturers and suppliers will be biased to the products they are selling.[/COLOR]