HVAC fan selection

"how does the selection of fan takes place for air-handling units, does the selection depend on static pressure required for the ductwork?"

Yes, it is based on static pressure of the duct run.

"how to calculate the fan static pressure"

The static pressure should be based on .08"SP per 100ft of duct, or +/-.7Pa per meter of duct as an industry standard. Measure the duct length, divide by 100ft or meters, multiply by .08" or .7Pa. Maybe someone in the UK can give the accurate Pa number. It is in that range .5-.7Pa.

"does the external fan static pressure refers to the return air pressure?"

Sort of. Return air duct static is directly related to a single fan, draw through type air handler without a seperate return air fan. External static has other factors that come into play before the air reaches the supply fan. For example, if there are filters, there will be a pressure drop across the filters, adding to the overall external static pressure drop.

Air handler manufacturer's catalogs supply fan curve data. Your selection will be based on cfm (L/s) and static pressure. If you calculated a pressure drop of .25"SP, you would select a fan capable of .325"SP for example at a given cfm.

Well put, my friend! 'Nuff said!

Thank your information.

for air handling units used in air-conditioning application, calculating the static pressure is a little bit tricky and would present some acoustic problems if not done properly. for air conditioning ducts, i use the equal friction method for branches and velocity method for main ducts. (there are other methods of duct sizing, though) Total pressure is normally calculated, which is the total pressure losses of the supply and the return pathways, unless there is a return air fan. so again, it depends also on system configuration. considering only static pressure is sometimes misleading as always, air in air conditioning is dynamic thus the dynamic pressure is also considered. so be careful with your calculations. refer to ASHRAE Fundamentals Volume for more precise methods.