# Auto Equipment Sizing

FluidFlow includes a powerful auto-size feature which allows engineers to automatically size a range of elements such as pipes, centrifugal pumps, fans, compressors, PD pumps, orifice plates, nozzles, pressure and flow control valves. Pressure relief valves and bursting disks can also be auto-sized to API & ISO standards for liquids, gases, steam and two-phase flow systems. This example involves designing a cooling water distribution system to a bank of heat exchangers where we shall use orifice plates to balance the flow distribution. We shall also use the auto sizing functions to develop the system design and size the pipes, pump and orifice plates. Problem Statement: It is desired to provide a balanced cooling water flow to four shell and tube heat exchangers HE1, HE2, HE3 and HE4. The size of the heat exchangers has already been determined from the process requirement and is summarized in table 1. Table 1 The cooling water is to flow through the heat exchangers and the design system inlet temperature will be 15°C. The design temperature rise of the cooling water across each heat exchanger is 30°C. The elevation of all elements in this model is zero. The design duty pressure rise for this system is also known, 2.5 bar. Building the model in the FluidFlow flowsheet: 1. Place two known pressure boundary nodes on the flow sheet. 2. Using the steel pipe element, connect the known pressure boundary nodes together as per Figure 1 below. 3. Add the branch pipe connections, ensuring the branch at each tee junction is assigned correctly. 4. Insert the heat exchangers into each branch line. 5. Insert thin orifice plates into the branch pipe connection serving each heat exchanger. The basic model connectivity should appear as set out in Figure 1. Figure 1 If you wish to check that your data entry is correct, you can load up the example "[FluidFlow3 folder]\Examples\Auto Sizing Example Rev 1.ff3" file. Each element placed on the flowsheet is provided with default data which the engineer can easily edit based on known design parameters. In this case, we will define the pipe lengths as set out in Table 2. Note, we will accept the default data provided by FluidFlow for the boundary nodes. Table 2 Select all the heat exchangers at once by holding the SHIFT key and left mouse-clicking on each. All heat exchangers should now be highlighted on the flowsheet. From the Input tab of the Data Palette, set the Heat Loss Model to Fixed Transfer Rate, the heat transfer direction to Into the Network and the Heat Transfer Unit to kW. We have now set all common parameters for the heat exchangers in one step. The next step is to define the heat load for each heat exchanger (see Table 1). The design pump pressure rise is 2.5 bar. We can therefore set the centrifugal pump to Automatically Size from the Input Tab on the Data Palette. In doing so, we have two sizing options available: Size for Flow and Size For Pressure Rise. Select Size For Pressure Rise and define the Design Pressure Change as 2.5 bar. Using the data available for the heat exchangers, we can determine the design mass flow rate from the heat balance (Q = m x c x ΔT). The heat transferred to the cooling water will therefore be: Heat Transferred (W) = mass flow (kg/s) x specific heat capacity (J/kg) x temperature rise (°C) The specific heat of water at 30°C is approx 4154 J/kg, so from Table 1 we see that the mass flow needed to HE1 will be 370000 / (4154 x 30) = 2.969 kg/s. Summarizing in Table 3. Table 3 Since we can auto-size our components, we can define the design flow rate at each orifice plate by setting the Automatically Size option on the Input tab of the Data Palette to On. We now have two sizing models to choose from, Size for Flow and Size for Pressure Loss. As we know the flow rate, select Size for Flow. The next step is to enter the mass flow rates noted in Table 3 for each orifice plate. The Sizing Model for pipes is Economic Velocity by default. This means that FluidFlow will determine the Exact Economic Size pipe diameter based on the calculated Economic Velocity. This velocity is a function of the fluid physical properties, the pipe materials, various capital and installation costs and the operating hours/year. This value is calculated from the Generaux equation and the calculation uses the values and constants stored in the pipe sizing database. Economic velocity is changing (generally decreasing) with time, particularly as energy costs have increased rapidly in recent times. Flite Software keeps these values up to date, which is one of the many reasons you should keep your software current. Economic velocity is meant to be a guide for pipe sizing; it is NOT a strict criteria for sizing pipes. For example you would not use this value to size pipes where two phase flow is present, or where plant operation is intermittent, or where materials can degrade at high velocities, or for Non-Newtonian flows. We wish to develop an efficient system design and as such, we are going to retain the Economic Velocity sizing model. We are now in a position to calculate the model. The solved system should appear as set out in Figure 2. Figure 2 The flow distribution has been shown and if we view the results for any of the four heat exchangers, we can see that the inlet temperature is 15°C and the outlet temperature is 45°C (based on our design 30°C temperature rise). Note, we also have three warning messages indicating high velocities in the pipelines highlighted in RED. If you wish to check that your data entry is correct, you can load up the example "[FluidFlow3 folder]\Examples\Auto Sizing Example Rev 2.ff3" file. A quick check on the results for each of the pipes with a high velocity warning indicates velocities in the range of 4.5 m/s which is considered high. We therefore need to review the pipe diameter. The diameter of each of these pipes is the default value of 2 inch which is 52.5mm. FluidFlow has determined an economic pipe size of approx. 101mm for each of these three pipes. We therefore need to select the next closest standard size match. Lets try a 4 inch schedule 40 pipe. You can multi-select the three pipes by holding the SHIFT key and left mouse-clicking on each pipe. From the Input tab on the Data Palette, access the pipes database and change the pipe to 4 inch schedule 40 pipe. Press Calculate to refresh the results for the system. If we view the results for the orifice plates, we can see that FluidFlow has automatically determined the K value and the size of orifice required to provide the desired mass flow rate. The pump duty has also been established for us as 9.95 kg/s @ 2.5 bar (26 m fluid). The warnings have also been corrected and as such, it can be considered that the system is now a relatively efficient design. Based on the data entry, FluidFlow has therefore automatically sized the pipework, pump and orifice plates for us, thus simplifying the design process and reducing design time considerably. If you wish to check that your data entry is correct, you can load up the example "[FluidFlow3 folder]\Examples\Auto Sizing Example Rev 3.ff3" file. The final design should appear as set out in Figure 3. Figure 3 Design Note: When using the auto size function, it's important not to over constrain the model as this could lead to model convergence difficulties. For instance, in this example, we could have set the sizing model for the pump to Size for Flow. As we had effectively already defined the total flow in the system by defining the flow rate at each orifice plate, we would have duplicated the flow in the system. It is best to avoid duplicating design parameters when developing your model. Video: You can view a video of this example on our website. Visit http://www.fluidflowinfo.com/webinars.php and select "FluidFlow3 (v3.31) - New Features".