# Designing a Cooling Water System. Part 2

This is the concluding part to the design of a cooling water system, started in Part 1. In this part we will select a specific manufacturer's pump to perform at the operating duty calculated from the centrifugal pump element and consider the effect on the system if the heat load on exchanger 2 (HE2) is increased by 33%. Click on the centrifugal pump element on the flowsheet and the results tab on the data palette. If you are using the default "System International" environment you should see a result similar to: OR That is, the design flow of 12.99 kg/s of water at 25° C needs a pressure rise of 176601 Pa in order to operate at the design flows. We may wish to stop here and ask our preferred pump manufacturer to suggest a pump to supply this duty. To do this we will need to convert the pump duty flow to a suitable volumetric unit and the calculated pressure rise needed to head units. You can use FluidFlow to do this for you automatically, however you may find it instructional to do the conversion now to m3/h and m Fluid. The density of water, obtained from the results table is 997 kg/m3 and so the calculated volumetric flow will be 12.99 x 3600/997 = 46.9 m3/h. The head that the pump is required to produce will be 176601/(997 x 9.80665) = 18.1 m Fluid. This information, together with the operating fluid and temperature conditions is enough for the pump manufacturer to make a selection and under normal circumstances this is all that is required. It is better to allow the manufacturer to make the selection for the following reasons: pump selection is often more than a simple hydraulic selection. Pump configuration, sealing and shaft load considerations, materials of construction, etc., are best handled by the manufacturer. Even though the manufacturer is in a better position to make the selection it is still worthwhile and you can often get a more flexible design by making some additional hydraulic considerations. For the purpose of illustration we will use the inbuilt pump selector to make the selection at this duty point. From the menu select 'Tools | Equipment Performance Viewers | Pump Performance' to create the dialog shown below. To use this tool change the Design Flow to be 46.9 m3/h, and as you click on each pump in the list the performance data (Head, Efficiency, Best Efficiency and NPSH required) are shown for the currently highlighted pump. This means that you can move through each pump in the database and view how it will perform in this system. The pump shown above would be a suitable selection from NPSH and efficiency viewpoint, but is unsuitable because the head developed is greater than that required by our duty (18.1 m fluid). Viewing the pump database provides a number of pumps that will fulfill the hydraulic and system needs. You may wish to consider some of the following models: Girdelstone 32ns. DNP85-165. FA 253-4402Z. NM3196 2x3-10. The best choice is to select a pump operating near the best efficiency or a pump with the highest efficiency at the duty point. To complete the example we will select the Gould pump NM 3196 2x3-10 MTX. The operating performance of this pump based on a design flow rate of 46.9 m3/h is as shown below. To assign this pump to the model, click on the centrifugal pump node (6) and set "Automatically Size" to "Off". Change the pump model from the default pump, by clicking on the button in the Pump Model row in the Input Editor and select the Gould NM 3196 2x3-10 MTX. Notice that the pressure developed by this pump is higher than that needed and that the orifice size on each branch has changed slightly in order to maintain the design flow rates specified. As engineers we like to over design. Right !!!. Before we congratulate ourselves on the design, we may wish to check that the selected pump is operating near to the Best efficiency point (BEP) and that the duty point is not near the pump run-out condition. If we are operating in the latter region, it would be wasteful of energy, have a larger NPSH requirement, increase pump wear and we would most likely need to oversize the motor. This particular pump is operating in a reasonably good position. However, there may be an opportunity to reduce the pump power requirement and thus, improve the system operating efficiency. It is important however to give due consideration to the system design pressure requirements, in particular, the design pressure losses advised by the cooling tower manufacturer. Let's consider the current set of results for the pump. If we wished to study energy saving measures, we could select a smaller pump, or better still we can reduce the impeller diameter from 254 mm to around 224 mm. This will allow us some additional capacity if needed in the future. Lets apply this change to the impeller diameter from the Input tab. of the Data Palette. When we recalculate the model, we can see that the affinity laws have been applied and the power requirement has reduced from 4.03 to 2.97 kW. We will also note however that the duty pressure rise has also reduced from 188296.5 to 133284.2 Pa. You may therefore wish to review this operating condition with the cooling tower manufacturer. Details of the results of this simulation are shown below. Note, by reducing the pump impeller diameter, the pressure drop across the system reduces and the orifice plate diameters increase to maintain the desired flow rate which is reflected in the updated flowsheet as shown below. WARNING: You should note that reducing the impeller diameter or changing the speed will produce a flexible design, but in this example, you can see from the adjusted pump chart that we are still operating away from the best efficiency point and so this may not be the best solution. Finally we need to consider increasing the heat load to HE2 by 33%. Change the heat transferred to be 226,100 Watt, then recalculate. The temperature at the exit of the heat exchanger rises from 34.9°C to 38.2°C. The temperature to the cooling tower increases from 34.9 to 36 °C. This increase in temperature is considered to be acceptable and we do not need to rebalance the system.