Turndown of FD fan control?

Hi, Can anyone give me a rough number for turndown likely to be achievable with a boiler FD fan with adjustable suction vanes? Not the boiler turndown,
just how low the air flow is likely to be able to be set in comparison to MCR. If the flow can't be run all the way to zero, what's the cause? Can these single stage axial fans surge?
Fan drive is around 500KW, the system characteristic is essentially just resistance. TIA
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

achievable
turndown,
Apologies, centifugal fan.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.