Please read AddThis Privacy for more information. We know that this duct also requires a volume flow rate of 0.79m3/s so we can use the velocity and volume flow rate to find the missing data. If we look at the air travelling straight though first, we find the velocity ratio first using the formula velocity out divided by velocity in. Discover more than 50 free on-demand webinars on different topics, from ventilation or data center design and wind load analysis to aerospace, F1, and sports aerodynamics here: Read more about the benefits of using cloud-based engineering simulation and the SimScale Community here: Find thousands of ready-to-use simulation templates created by SimScale’s users which you can copy and modify for your own analysis: Velocity reduction method: (Residential or small commercial installations), Equal friction method: (Medium to large sized commercial installations), Static regain: Very large installations (concert halls, airports and industrial). Methods Used for Sizing. Avoid over-sizing the main system by multiplying the accumulated volume with a factor less than one (this is probably the hard part - and for larger systems sophisticated computer-assisted indoor climate calculations are often required). So we just drop those numbers in and we get the density of air being 1.2 kg/m3. In this example the air out is 3.3m/s and the air in is 4m/s which gives us 0.83, Then we perform another calculation to find the area ratio, this uses the formula diameter out squared divided by diameter in squared. Use maximum velocity limit when selecting size of main ducts. Use 1) to summarize and accumulate total air volume flow - qtotal - in the system. A much more detailed breakdown is available on the EngineeringToolbox.com for those interested in the math behind the process. The sharp turns cause a large amount of recirculation regions within the ducts, preventing the air from moving smoothly. We'll assume you're ok with this, but you can opt-out if you wish. Now if we look at the comparison for the two designs we have a standard design on the left and a more efficient design on the right which has been optimised using simscale. Then we draw another line on the other diagonal grid to find our duct diameter which in this case is about 0.27m and we’ll add that to the table too. Use a pressure drop table or similar to determine static pressure drop in main duct. From the rough drawing we measure out the length of each duct section and enter this into the chart.
The equal friction method for sizing air ducts is often preferred because it is quite easy to use. For example, if we had two ducts, with equal dimensions, volume flow rate and velocity, the only difference is the material.
This template is based on the figure above. Then we find the area ratio using the formula diameter out squared divided by diameter in squared.
You can see on the chart I’ve filled that in. These simulations were produced using a revolutionary cloud based CFD and FEA engineering platform, by SimScale, who have kindly sponsored this article. This category only includes cookies that ensures basic functionalities and security features of the website.
There is a high amount of backflow here which again increases the static pressure and reduces the amount of air delivery. However, I have a question, in calculating the overall pressure drop per duct length. With SimScale, however, all can be done from a web browser. On the chart we start by drawing a line from 0.65 pa/m all the way up and then draw a line across from our required volume flow rate, in this case for section C we need 0.21m3/s. With these considerations in place we can come back to the duct design. It’s usually the longest run but could also be the run with the most fittings. Sizing Ducts. Do that for all the ducts and branches on the table. Ductwork sizing, calculation and design for efficiency. Then to find the pressure drop we draw a vertical line down from this intersection. The equal friction method is straightforward and easy to use and gives an automatic reduction of air flow velocities through the system. The method can be summarized to. Use the air volumes calculated in 1) for the calculation. There are three methods used for most modern duct sizing. Incorrect measurements result in improper delivery of that air and a system that doesn’t quite get the job done. Now repeat that calculation for the other tees and fittings until the table to complete. We can start to fill some of the data in, we can first include the volume flow rates for each of the branches, this is easy as its just the volume flow rate for the room which it serves. The layout of the chart does vary a little depending on the manufacturer but in this example the vertical lines are for pressure drop per meter of duct. We can calculate how much pressure drop each damper needs to provide simply by subtracting the loss of the run from the index run. To save time we’ll just use an online calculator to find that, link here (watch the video to learn how to perform a bilinear interpolation). Use the actual heat, cooling or air quality requirements for the rooms and calculate required air volume flow - q. Out of these cookies, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. The first one being the shape of the ductwork. Then we locate where the velocity line is of 5m/s and we draw a line across until we hit that. Cookies are only used in the browser to improve user experience. © 2008–2020 Carney Plumbing Heating & Cooling. Round duct is by far the most energy efficient type and that’s what we’ll use in our worked example later on. Thank you very much fo this info guys. Now we calculate the dynamic loss for the straight path through the tee, using the formula co multiplied by rho multiplied by v squared divided by 2. Then we scroll up again and align our intersection with the upward diagonal lines to see this requires a duct with a diameter of 0.45m so we add that into the table also. Use the static pressure drop from 4) as a constant to determine the ducts sizes throughout the system. The next fitting we’ll look at is the tee which connects the main duct to the branches, we’ll use the example of the tee with the ID letter H between G and J in the system. Thanks for sharing this. We take our values from our table and use 3.5m/s divided by 4m/s to get 0.875 for the velocity ratio and we use 0.26m squared divided by 0.33m squared to get 0.62 for the area ratio. For that we use the formula Co multiplied by rho multiplied by v squared divided by 2 where co is our coefficient, rho is the density of the air and v is the velocity. The first thing we need to do is calculate the heating and cooling loads for each room.
Friction in a system means the fan needs to work harder and this results in higher operating costs. Equivalent diameter is the diameter of a circular duct with similar pressure loss as an equivalent rectangular duct. A i = 144 q i / v i (1b) where. If we drop our values in we get the answer of 0.934 pascals so add that to the table. They also offer free webinars, courses and tutorials to help you set up and run your own simulations. I used this method with the same pressure in all ducts from the main to all the branches with the quantity of air needed, I found out that some duct and branches fall below 2 m/sec. You can just do that in excel very quickly (copy paste this =1.2^-1) to get the answer of 0.83m3/kg. Also can you do another illustration on the return duct size and location. We’ll specify 21*c and assume atmospheric pressure of 101.325 kPa. In the guides we find two tables the one you use depends on the direction of flow, we’re using the straight direction so we locate that one and then look up each ratio to find our loss coefficient. We can look this up in our air properties tables but I like to just use an online calculator http://bit.ly/2tyT8yp as its quicker. Thank you for helping me learn more about the methods used for ductwork designs. Only emails and answers are saved in our archive. Once you have these, just tally them together to find which is the biggest Load as we need to size the system to be able to operate at the peak demand. Excellent post. A rectangular duct with an equal cross sectional area has a perimeter of 3.87m Now we need to convert the cooling loads into volume flow rates but to do that we first need to convert this to mass flow rate so we use the formula: Where mdot means mass flow rate (kg/s), the Q being the cooling load of the room (kW), cp is the specific heat capacity of the air (kJ/kg.K) and Δt being the temperature difference between the designed air temperature and the design return temperature.