# Example Designs

You should familiarize yourself with the many examples found in the Quality Assurance Folder. For gases the main section is: "[FluidFlow3 folder]\QA Compressible Flow". There is also a simple example in the Quick Start Guide, which is a worthwhile exercise. For completeness a further two example designs are considered here. Example 1: Air Flow with Pipe Sizing and Fan Selection. The layout of a stainless steel, Schedule 5S, pipe system is already decided and is shown in Figure 1 below. It is required to select the appropriate pipe sizes and fan to deliver approximately 1750 kg/hr of air to each outlet. Figure 1: Air Distribution System - Layout The topics covered in this example design are: ▪Efficient sizing of pipes. ▪The sizing and selection of a fan. Step 1: In this first step we build the model and make a first stab at appropriate pipe sizes by setting them all 6" stainless steel throughout (except for the suction line which we make 12"). We simulate the fan location by two {Known or Assigned Flow} elements, with the desired flowrate set OUT of the suction pipework and INTO the delivery pipework. The difference in the stagnation pressure between these two elements will eventually determine the required fan pressure. Solve the model and view the flow conditions in each pipe. Clearly we have not achieved anything like the approximate flows required at each outlet. For instance the flow to the first outlet is nearly over 2,000 kg/hr instead of 1,750 kg/hr. A variety of flow control measures could be adopted here such as: ▪Flow control valves in each line. ▪Orifice plates in each line. However, since the outlet flow requirements are only approximate it might be more economical to use pipe sizing for flow balancing. FluidFlow provides guidance on pipe sizing via the Results Inspector. The image below shows the Inspector for the 40 M long pipe. Note that FluidFlow displays the selected pipe size (the internal diameter) and the Exact Economic Size. It can be seen that there is a considerable difference in this case so a change of pipe diameter might be suitable. Figure 2: Air Distribution System – Economic Pipe Sizing. Step 2: Change the 40m long pipe to an 8" line and see how this brings us closer to the economic size, but does not assist with flow balancing. However some further pipe size changes improves the flow balance. Change the pipes sizes to those shown in the image below and also display the flow rates and stagnation pressures at the fan location. Figure 3: Air Distribution System – Pipe Sizing. Step 3: The flow rates now look reasonable and we also have a performance specification for the fan. So replace the two {Known or Assigned Flow} elements with a fan element, model as shown. A suitable fan is the Air Control Industries Radial Blower RB200. Figure 4: Air Distribution System – Fan Selection Part 1. Note, when we solve this system based on the operating speed of 2850 RPM and impeller diameter of 348mm, we achieve a reasonable flow distribution however, the flow rate to outlet boundary node 4 (1,739.8 kg/h) is slightly below the design value of 1,750 kg/h. We can therefore consider increasing the impeller diameter to achieve the desired flow rate. Try an impeller diameter of 350mm. Generally, you would expect a temperature rise across a fan as it compresses the flowing gas. Using the Input Inspector Heat Loss Model field you can include the temperature rise in one of three ways: 1.Fixed temperature change. 2.Fixed heat transfer rate. 3.Automatically calculated by FluidFlow using an adiabatic compression. Step 4: In Step 3 we used the difference in stagnation pressures to determine the required duty point of the fan. An alternative approach would be to activate the "Automatically Size" option for fans and set the sizing model to "Size for Flow" (Refer to: Components | Fans and Compressors | Data Entry). With this option we just specify the desired flow rate and the resulting head is calculated. This then offers the possibility of testing different flow balancing techniques such as flow controllers, orifice plates, economic pipe sizing, etc., whilst maintaining a constant input flow rate, and then judging selections against the calculated fan head. Figure 5: Air Distribution System – Fan Selection Part 2. Example 2: Choked Flow. The image below represents the existing gas supply to two power stations, PS1 and PS2, from a natural gas supply main. A new power station is proposed (Newgen) which will place additional demands on the supply system. Figure 1: Natural Gas Supply System. A minimum of 29.0 barg is required at each take off. Using the layout of pipes shown and the data below, calculate the minimum pressure required in the gas supply main to achieve this. A sonic nozzle is present to restrict flow should the pipe fracture downstream of the transfer station. Simulate this with an inline nozzle, size 100mm. For the transfer station itself, assume a constant pressure drop of 0.15 bar. Data: Main pipeline – 14" Schedule 10. Off-take pipes – 6" Schedule 40. Gas mixture – 90% methane, 7% ethane, 3% butane. Demands: PS 1 & 2 = 10 kg/s, Newgen = 15 kg/s. Hints: ▪Review the General Resistances\User Defined Generic component equation and see how you use this to simulate a constant pressure drop. Question? If the pipe fails completely downstream of the Transfer Station, what is the restricted flow through the sonic nozzle? ▪Copy and paste the model to another flowsheet. ▪Delete all pipes downstream of the elbows. ▪Change the elbow icon to an outlet. ▪Calculate. ▪Compare the escaping flow rate with the sonic nozzle set ON and then with the sonic nozzle set IGNORE PRESSURE LOSS.