I am starting a series of blogs for explaining important HAVC (Heating ventilation and air-conditioning) concepts into simple language. Theoretical knowledge we all can gain from books. But for practical knowledge we have to work hard in our field. I have 12 years and counting experience in HVAC industry. Thus, I will focus more on practical aspect of HVAC. If you find it interesting please like, comment and share the blog to keep me motivated. We all need motivation. Without constant motivation we lose interest and stop doing whatever we started. All type of questions based on the current topic are welcome.
Today, I want to discuss about aspect ratio in ducting. What is it and how is it so important?
Aspect ratio is very simple term. It is the ratio of two sides of duct, i.e. height: width. We consider 1:4 as maximum acceptable aspect ratio as per standards for rectangular duct. It is always better to have lower aspect ratio up to 1:1, if space permits. That means square duct. As aspect ratio increases, friction to air flow increases and thus cost of fan too increases. Similarly, cost of material also increases with increase in aspect ratio. Because thickness (gauge) of GI sheet (or any other duct material) used for duct-work depends on the size of duct. If you increase one side of duct, thickness of sheet will increase after specified size. As we move from square to rectangle, surface area increases. Increase in surface area increases duct heat loss. Furthermore, insulation material is increases because of increase in surface area.
Though, aspect ratio looks like very trivial term, but cost wise it plays very important role. Therefore, always keep aspect ratio in range of 1:1 to 1:2 and maximum up to 1:4 which is acceptable in case when there is no enough space to install the duct. In all such conditions you are supposed to re-calculate the ESP of fan before making decision to change the size. It is interesting to know that how ESP of duct and Aspect ratio are connected. For simplicity, equations of friction loss are derived for round duct. Thus, first we must calculate equivalent diameter for all rectangular ducts. Formulas are available in carrier hand book-chapter 2. Now, as aspect ratio increases, equivalent diameter decreases for two duct with same cross section area at constant air flow. Friction loss of duct is inversely proportional to the equivalent diameter for constant flow. Hence, friction loss increases with increase in aspect ratio for same air flow.
There is one big misconception about re-sizing of duct when we have to avoid any obstruction like structural beam. Most of people increase width and reduce height, keeping cross section of rectangular duct same. This is a wrong practice. Keeping cross section area same does not mean that velocity and pressure loss will remain same. The correct practice must be to first convert rectangular duct by calculating equivalent diameter.
Equivalent Diameter Deq= 1.3 ((a.b)^.625 / (( a+b)^.25) )
Where, a and b are sides of duct.
Now, check from chart, for same equivalent diameter what rectangular duct sizes are available from ASHRAE or Carrier. Use any of those rectangular size which keep equivalent diameter same. It will keep velocity and pressure drop same even after reducing the height and increasing the width.