Oh ok but of course 1060FM is much too fast.
Yes it is! You need to bring it down to about one quarter of that.
So if I use a silencer, which uses a cross section that is 2 X the size of the 6" duct, does that means that the speed will be cut in half to 530FM?
If the cross sectional area is twice as large, then the speed will be half. Yes. This is because you cannot create or destroy energy. A basic rule of science. Energy can only be converted from one form to another form, but it cannot be destroyed, or created. So the TOTAL energy of your moving air remains the same at every single point along the path, and cannot change. That total energy is made up from several components, but you can think of the most important two being the flow rate and the flow velocity. The only relationship between those two is the cross sectional area of the duct. You can see this by looking at the units of each. The flow RATE is measured in units of "cubic feet per minute" or "f^^3 / m". The flow VELOCITY is measured on units of "feet per minute" or "f / m". If you divide one by the other, you end up with units if "f^^2", or "square feet", which is the measurement of units for the cross sectional area. And because the flow RATE is fixed (that's the maximum amount of air being moved by the fan, and pushed into the ducts: that does not change), that means that the other two are related by a simple, linear, and directly proportional relationship. As cross-sectional area goes up, flow VELOCITY goes down by the same amount. If you increase the area by 10%, the velocity goes down by 10%. If you increase the area by 200%, the velocity goes down by 200%.
Or will the baffles further reduce the speed because of the 180 degree turns as well?
No. Once again, the flow RATE is fixed: it does not change. It is fixed by the fan, and the static pressure of your system. The fan can only move a certain volume of air, and that amount is set by the manufacturer, and limited by the static pressure. When the manufacturer design the fan, they designed it to work with a a certain range of possible static pressures, and a certain set of speeds. With some fans, you can change the speed by using a variable speed controller, and with most mini-split system, that is something you can set via the remote control unit: you often have a choice of three speeds: low, medium, or high. The "rating" published by the manufacturer refers to the "high" speed, since that's the maximum performance that the fan can produce. So when the fan is running on "high", then it will produce the flow rate stated in the manual. Normally there is a table or a graph that shows what the rate will be vs. the static pressure. The higher the static pressure, the lower the maximum possible rate. The static pressure of your system is fixed: it does not change. Once you have built it, it won't change any more (unless you have variable dampers in the system, but that's an entirely different subject). So no, the baffles in your silencers do not "slow down" the air. They have already been accounted for when you calculated or measured the total static pressure for your system. The "static pressure" refers to the entire system of your ducts, filters, silencer boxes, dampers, registers, the room itself, etc. The "static pressure" is one single number that takes into account all of the "resistance" offered by every part of the system, from beginning to end. That "total resistance" or "static pressure" is what the fan "sees" as it tries to move air through your system, and it does not change. That single number is what sets your flow rate for every given fan setting, and therefore also sets the air flow speed at any given point along that path, based entirely on the cross sectional area.
- Stuart -
Thanks for the kind words! I've sort of been thinking of writing a book about studio design.... maybe I will, one day!