The way to design a duct system. On this article we’ll be studying the right way to measurement and design a ductwork system for effectivity. We’ll embrace a full labored instance in addition to utilizing CFD simulations to optimise the efficiency and effectivity utilizing SimScale. Scroll to the underside to observe the FREE YouTube video tutorial!
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Strategies of ductwork design
There are a lot of completely different strategies used to design air flow programs, the most typical methods being:
- Velocity discount methodology: (Residential or small industrial installations)
- Equal friction methodology: (Medium to massive sized industrial installations)
- Static regain: Very massive installations (live performance halls, airports and industrial)
We’re going to give attention to the equal friction methodology on this instance because it’s the most typical methodology used for industrial HVAC programs and its pretty easy to observe.
So we’ll bounce straight into designing a system. We’ll use a small engineering workplace for instance and we wish to make a structure drawing of the constructing which we’ll use for the design and calculations. It is a actually easy constructing it has simply 4 places of work a hall and a mechanical room which is the place the fan, filters and air heater or cooler will probably be situated.
The very first thing we have to do is calculate the heating and cooling masses for every room. I received’t cowl how to try this on this article we’ll should cowl that in a separate tutorial because it’s a separate topic space.
After getting these, simply tally them collectively to search out which is the largest Load as we have to measurement the system to have the ability to function on the peak demand. The cooling load is normally the very best, as it’s on this case.
Now we have to convert the cooling masses into quantity movement charges however to try this we first must convert this to mass movement charge so we use the method:
mdot = Q / (cp x Δt)
The place mdot means mass movement charge (kg/s), the Q being the cooling load of the room (kW), cp is the particular warmth capability of the air (kJ/kg.Okay) and Δt being the temperature distinction between the designed air temperature and the design return temperature. Simply to notice that we’ll use a cp of 1.026 kJ/kg.ok as customary and the delta T needs to be lower than 10C so we’ll use 8c.
We all know all of the values for this so we will calculate the mass movement charge (what number of kilograms per second of air must enter the room). If we take a look at the calculation for room 1, we see it requires 0.26 kg/s. So we simply repeat that calculation for the remainder of the room to search out all of the mass movement charges.
Now we will convert these into quantity movement charges. To try this we want the particular quantity or density of the air. We’ll specify 21*c and assume atmospheric strain of 101.325 kPa. We will look this up in our air properties tables however I like to simply use a web-based calculator photoshopservices.web/2tyT8yp as its faster. So we simply drop these numbers in and we get the density of air being 1.2 kg/m3.
You see that density has the items of kg/m3 however we want particular quantity which is m3/kg so to transform that we simply take the inverse which suggests to calculate 1.2 to the ability of -1. You may simply try this in excel in a short time (copy paste this =1.2^-1) to get the reply of 0.83m3/kg.Now that we now have that we will calculate the amount movement charge utilizing the method:
vdot = mdot multiplied by v.
the place vdot equals the amount movement charge, mdot equals the mass movement charge of the room and v equals the particular quantity which we simply photoshopservices.web if we drop these values in for room 1 we get a quantity movement charge of 0.2158m3/s that’s how a lot air must enter the room to satisfy the cooling load. So simply repeat that calculation for all of the rooms.
Now we’re going to sketch out our ductwork route onto the ground plan so we will begin to measurement it.
Earlier than we go any additional we have to think about some issues which can play an enormous function within the general effectivity of the system.
The primary one being the form of the ductwork. Ductwork is available in spherical, rectangular and flat-oval form. Spherical duct is by far essentially the most vitality environment friendly kind and that’s what we’ll use in our labored instance in a while. If we examine spherical duct to rectangular duct we see that:
A spherical duct with a cross sectional space of 0.6m2 has a fringe of two.75mA rectangular duct with an equal cross sectional space has a fringe of three.87mThe rectangular duct subsequently requires extra steel for its building, this provides extra weight and prices to the design. The bigger perimeter additionally means extra air will come into contact wit the fabric and this provides friction to the system. Friction in a system means the fan must work more durable and this leads to greater working prices. All the time use spherical duct the place doable though in lots of circumstances rectangular duct must be used as area is proscribed.
The second factor to contemplate is the fabric getting used for the ducts, and the roughness of this materials as this causes friction. For instance, if we had two ducts, with equal dimensions, quantity movement charge and velocity, the one distinction is the fabric. One is constructed from customary galvanised metal the opposite from fibreglass, the strain drop over a 10m distance for this instance, is round 11 Pa for the galvanised metal and 16 Pa for the Fibreglass.
The third factor we now have to contemplate is the dynamic losses brought on by the fittings. We wish to use the smoothest fittings doable for vitality effectivity. For instance use lengthy radius bends quite than proper angles because the sudden change in route wastes an enormous quantity of vitality.
We will examine the efficiency of various ductwork designs, shortly and simply utilizing CFD or computational fluid dynamics. These simulations had been produced utilizing a revolutionary cloud primarily based CFD and FEA engineering platform, by SimScale, who’ve kindly sponsored this photoshopservices.web can entry this software program freed from cost by clicking right here, they usually provide various completely different account varieties relying in your simulation wants.
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Now if we take a look at the comparability for the 2 designs we now have an ordinary design on the left and a extra environment friendly design on the suitable which has been optimised utilizing simscale. Each designs use an air velocity of 5m/s, the colors characterize the speed with blue which means low velocity and crimson representing the excessive velocity areas.
We will see from the speed color scale and the streamlines that within the design on the left, the inlet air immediately strikes the sharp turns which might be current within the system which causes a rise within the static strain. The sharp turns trigger a considerable amount of recirculation areas inside the ducts, stopping the air from shifting easily.
The tee part on the far finish of the principle duct causes the air to abruptly divide and alter route. There’s a excessive quantity of backflow right here which once more will increase the static strain and reduces the quantity of air supply
The excessive velocity in the principle duct which is brought on by the sharp turns and sudden bends, reduces the movement into the three branches on the left.
If we now give attention to the optimised design on the suitable, we see the fittings used observe a a lot smoother profile with no sudden obstructions, recirculation or backflow which considerably improves the air movement charge inside the system. On the far finish of the principle duct the air is split into two branches via a delicate, curved tee part. This permits the air to easily change route and thus there is no such thing as a sudden enhance in static strain and the air movement charge to the rooms has dramatically elevated.
The three branches inside the principle duct now obtain equal air movement making a big enchancment to the design. It’s because a further department now feeds the three smaller branches permitting a few of the air to easily break free from the principle movement and feed into these smaller branches.
With these concerns in place we will come again to the duct design.
Now we have to label each part of ductwork in addition to the fittings with a letter. Discover we’re solely designing a quite simple system right here so I’ve solely included ducts and primary fittings, I’ve not included issues akin to grilles, inlets, versatile connections, hearth dampers and so on.
Now we wish to make a desk with the rows labelled as per the instance. Every duct and becoming wants its personal row, if the air stream splits akin to with a Tee part, then we have to embrace a line for every route, we’ll see that later within the article.
Simply add within the letters to separate rows then declare what kind of becoming or duct that corresponds to.
We will begin to fill a few of the information in, we will first embrace the amount movement charges for every of the branches, that is straightforward as its simply the amount movement charge for the room which it serves. You may see on the chart I’ve stuffed that in.
Then we will begin to measurement the principle ducts. To do that be sure to begin on the important duct which is furthest away. Then we simply add up the amount movement charges for all of the branches downstream of this. For the principle duct G we simply sum branches L and I. For D that’s simply the sum of L I and F and for duct A its then the sum of L, I, F and C. so simply enter these into the desk.
From the tough drawing we measure out the size of every duct part and enter this into the chart.
Duct sizing – The way to measurement ductwork
To measurement the ducts you’re going to wish a duct sizing chart. You may receive these from ductwork producers or from business our bodies akin to CIBSE and ASHRAE. In case you don’t have one, you’ll find them within the following hyperlinks. Hyperlink 1 and Hyperlink 2
These charts maintain plenty of info. We will use them to search out the strain drop per meter, the air velocity, the amount movement charge and in addition the dimensions of the ductwork. The structure of the chart does fluctuate a bit relying on the producer however on this instance the vertical strains are for strain drop per meter of duct. The horizontal strains are for quantity movement charge. The downward diagonal strains are for velocity, the upward diagonal strains are for duct diameter.
We begin sizing from the primary important duct which is part A. To restrict the noise on this part we’ll specify that it may possibly solely have a most velocity of 5m/s. We all know that this duct additionally requires a quantity movement charge of 0.79m3/s so we will use the speed and quantity movement charge to search out the lacking information.
We take the chart and scroll up from the underside left till we hit the amount movement charge of 0.79m3/s. Then we find the place the speed line is of 5m/s and we draw a line throughout till we hit that. Then to search out the strain drop we draw a vertical line down from this intersection. On this occasion we see it comes out at 0.65 pa per meter. So add this determine into the chart. As we’re utilizing the equal strain drop methodology we will use this strain drop for all of the duct lengths so fill these in too. Then we scroll up once more and align our intersection with the upward diagonal strains to see this requires a duct with a diameter of 0.45m so we add that into the desk additionally.
We all know the amount movement charge and strain drop so we will now calculate the values for part C after which the remaining ducts.
For the rest of the ducts we use the identical methodology.
On the chart we begin by drawing a line from 0.65 pa/m all the way in which up after which draw a line throughout from our required quantity movement charge, on this case for part C we want 0.21m3/s. At this intersection we draw a line to search out the speed and we will see that it falls inside the strains of three and 4m/s so we have to estimate the worth, on this case it appears to be about 3.6m/s so we add that to the chart. Then we draw one other line on the opposite diagonal grid to search out our duct diameter which on this case is about 0.27m and we’ll add that to the desk too.
Repeat that final course of for all of the remaining ducts and branches till the desk is full.
Now discover the whole duct losses for every duct and department, that’s very straightforward to do merely multiply the duct size by the strain drop per meter, in our instance we discovered it to be 0.65pa/m. Do this for all of the ducts and branches on the desk.
Sizing ductwork fittings
The primary becoming we’ll take a look at is the 90* bend between ducts J and L
For this we glance up our loss coefficient for the bend from the producer or the business physique, you’ll find that by clicking this hyperlink.
On this instance we will see the coefficient comes out at 0.11
We then must calculate the dynamic loss brought on by the bend altering the route of movement. For that we use the method Co multiplied by rho multiplied by v squared divided by 2 the place co is our coefficient, rho is the density of the air and v is the speed.
We already know all these values so if we drop the figures in we get a solution of 0.718 pascals. So simply add that to the desk. (Watch the video on the backside of the web page to see the right way to calculate that).
The following becoming we’ll take a look at is the tee which connects the principle duct to the branches, we’ll use the instance of the tee with the ID letter H between G and J within the system. Now for this we have to think about that the air is shifting in two instructions, straight via and in addition turning off into the department so we have to carry out a calculation for each instructions.
If we take a look at the air travelling straight although first, we discover the speed ratio first utilizing the method velocity out divided by velocity in. On this instance the air out is 3.3m/s and the air in is 4m/s which supplies us 0.83
Then we carry out one other calculation to search out the world ratio, this makes use of the method diameter out squared divided by diameter in squared. On this instance the diameter out is 0.24m and the diameter in is 0.33m so if we sq. them after which divide we get 0.53
Now we glance up the becoming we’re utilizing from the producer or the business physique, once more hyperlink right here for that.
Within the guides we discover two tables the one you employ is determined by the route of movement, we’re utilizing the straight route so we find that one after which search for every ratio to search out our loss coefficient. Right here you possibly can see each of the values we calculated fall between vales listed within the desk so we have to carry out a bilinear interpolation. To save lots of time we’ll simply use a web-based calculator to search out that, hyperlink right here (watch the video to discover ways to carry out a bilinear interpolation).
We fill out our values and we discover the reply of 0.143
Now we calculate the dynamic loss for the straight path via the tee, utilizing the method co multiplied by rho multiplied by v squared divided by 2. If we drop our values in we get the reply of 0.934 pascals so add that to the desk.
Then we will calculate the dynamic loss for the air which turns into the bend. For this we use the identical formulation as earlier than. Velocity out didived by velocity in to search out our velocity ratio. Then we discover the world ratio utilizing the method diameter out squared divided by diameter in squared. We take our values from our desk and use 3.5m/s divided by 4m/s to get 0.875 for the speed ratio and we use 0.26m squared divided by 0.33m squared to get 0.62 for the world ratio.
Then we use the bend desk for the tee part, once more its between the values listed within the desk so we now have to search out the numbers utilizing bilinear interpolation. We drop the values in to get the reply of 0.3645 pascales. So simply add that to the desk too.
Now repeat that calculation for the opposite tees and fittings till the desk to finish.
Discovering the index run – duct sizing
Subsequent we have to discover the index run which is the run with the biggest strain drop. It’s normally the longest run however may be the run with essentially the most fittings.
We discover it simply by including up all of the strain losses from the begin to the exit of every department .
For instance to get from A to C we lose 5.04paA (1.3pa) + B (1.79pa) + C (1.95pa)
For A to F we lose 8.8paA (1.3pa) + B (1.7pa) + D (1.3pa) + E (2.55pa) + F (1.95)
For A to I we lose 10.56A (1.3pa) + B (1.7pa) + D (1.3pa) + E (1.34pa) + G (2.6pa) + H (0.36pa) + I (1.95pa)
For A to L we lose 12.5paA(1.3pa) + B (1.7pa) + D (1.3pa) + E (1.34pa) + G (2.6pa) + H (0.93pa) + J (0.65pa) + Okay (0.72pa) + L (1.95pa)
Subsequently the fan we use should overcome the run with the very best loss, that being A – L with 12.5pa, that is the index run.
Ductwork dampers – system balancing
To steadiness the system we have to add dampers to every of the branches to make sure equal strain drop via all to realize the design movement charges to every room.
We will calculate how a lot strain drop every damper wants to supply just by subtracting the lack of the run from the index run.
A to C is 12.5pa – 5.04pa = 7.46pa
A to F is 12.5pa – 8.8pa = 3.7pa
A to I is 12.5pa – 10.56pa = 1.94pa
And that’s our ducting system. We’ll do one other tutorial protecting further methods to enhance effectivity in ductwork system.