Calculates Duct Sizes, System Losses, and fan static pressure requirement in accordance with ACCA Manual D. Listed by air quantity, round duct.Duct sizing exercise in air conditioner sizing, is a vital step in making sure that cooled air at the evaporator, reaches every room, at the specified comfort.Rhvac is approved by ACCA. (target velocity of 800FPM used). Main Supply Trunk Ducts, Non-zoned systems. Heat Conduction and Thermal Resistance For steady state conditions and one dimensional heat transfer, the heat q conducted through a plane wall is given by: q kA(t1 - t2) L Btu hr (Eq.1) Where: L the thickness of the wall in inchesVelocity Duct Sizing Chart. HVAC Calculations and Duct Sizing Gary D.

We have to, usually make a good guess, and proceed further.Both split air conditioner and heat pump condensers are made of the same basic. Doubling the duct diameter reduces the friction loss by factor 32.Corrections and iterations are needed, as formulas are inter-related. Low velocity design is very important for the energy efficiency of the air distribution system. Flow velocity in air ducts should be kept within certain limits to avoid noise and unacceptable friction loss and energy consumption. Pressure characteristics of HVAC air duct system.Ductwork and recommended air velocities.

identify sections within the duct system.Main duct leading to branch/es, is a section. It will also minimise insulation cost, if any. That will minimise heat loss or gain, along the duct. Adjust proposed air handling unitвЂ™s location, to make sure duct lengths are minimal.

the circular duct diameter, for a given flow rate and velocity, Get a set of two duct sizing charts, prepared by ASHRAE. get values for heat gain and heat loss from the building using вЂњManual JвЂќ, and determine the total, and individual design flow rate of air, using psychrometric chartThen, mark the appropriate mass flow rate of air, at each section. Hence the equivalent straight length of these fixtures need to be determined.The database is available within вЂњManual DвЂќ.Similar to вЂњManual JвЂќ, only twice thicker, and oh yes, it has been created by ACCA, specifically for duct sizing get approximate number of elbows and t-joints, and the equivalent straight lengths.Elbows and junctions pose extra resistance within the duct system. A section, is a continuous run, without any branch.Mark sections of the duct with suitable numbering system вЂњa, b, cвЂќ or вЂњ1, 2, 3вЂќ.

Now, letвЂ™s begin duct sizing! It is rather the draft onto the occupants.Properly located air registers will solve the draft issue.We are prepared. use the comfort factors as presented in comfort zone page.Please bear in mind that the maximum air velocity for comfort of 1 m/s, is not the air velocity in duct.

use the value of 0.1 inch water gauge (29 Nm -2 or 29 Pascals) of total resistance within the duct.This value of resistance is being widely used by certified contractors as the total resistance within the duct system find the longest route in feet from the fan/blower to the register.NOTE: The longest route includes equivalent lengths of bends, and junctions require dampers, as this method is more difficult to be balanced, compared to the static regain method

suitable for high velocity applications, uses static and dynamic pressures of air flow, to balance pressures within duct system, you have to do calculation, from the fanвЂ™s/blowerвЂ™s outlet, and proceed, section by section, in order more difficult and time consuming than equal friction method, I hope technique 1 was clearly explained.Getting back into the business of duct sizing.Static regain method have these characteristics,

Therefore, we need a database for such data. and duct dynamic pressure head, multiplied by the sum of loss coefficients from bends and obstructions.Value of loss coefficients are obtained through empirical data. value in (2), multiplied by the length of the section, adopt I.W.G per length value chosen in (2)= sectionвЂ™s frictional head + sectionвЂ™s dynamic pressure head,As suggested by Massey in 'Mechanics of Fluids' book.The units should be in I.W.G, as the charts are formulated in that manner.Dynamic pressure head is contributed by the velocity of the fluid within the section, whereasDuct frictional head is contributed by sum of, fix the maximum velocity within the whole system or the maximum I.W.G per length you want, within the duct.If you choose the former, use the total flow rate figure (in cubic feet per minute) from the prepared data, and find the I.W.G per length value.If you pick the latter, use the total flow rate figure to find out the maximum air velocity within the whole system start at the beginning of the main duct

repeat steps (6) to (11), until you have covered all sections. iterate with different section size, until you get value in (11), closest to 0. subtract (10), with dynamic pressure head, in (5) use the current duct size, and sectionвЂ™s flow rate, to determine the I.W.G per length.Of course, we have to use the duct sizing chart then find the relevant loss coefficient use the square of ratio, of current duct size, to the previous duct size,The ratio of current sectionвЂ™s volume flow rate, to the previous one

We have to get it right or we will have an uncomfortable space, albeit properly sized air conditioners.The process is not easy, but it is definitely worth the effort.Good news though. we need to match different sectionsвЂ™ sizes as much as possible, to minimise use of materialAnyway, properly sized ducts, is just as vital as properly sized air conditioners. we need to find the closest round number of duct sizes Thanks to researchers of fluid dynamics, we have them stored in database.As promised, the reason for iteration in duct sizing:We have to do iteration due to following reasons,

You may also use software.Or, you can hire a contractor, and verify their working through duct sizing topic we were discussing a while ago.

Hvac Duct Sizing Guide