Reading the Load Profile on the System Curve
Learn how to interpret load profiles for typical pump applications.
Understanding the load profile for your application is an essential part of pump selection. However, the load profile is not actually an aspect of the pump curve, but of the system curve. This is due to pumps with a constant duty point will soon become outdated.
In this module, we will look at how the reduction of break tanks and control valves, and the popularisation of variable speed drives, would mean that most of the pump applications would have variable demand.
The water consumption pattern of a system depends on the application. For example, in a residential building there are peak hours of hot and cold-water consumption in the morning when everyone is taking a shower and low demand hours in the night when everyone is sleeping.
Another example is the water consumption pattern in a commercial building’s air-conditioning system which depends on the thermal load inside the building. This is affected by other factors such as the amount of people at a given time; the climate; building insulation; building orientation against solar radiation; building lighting; and, equipment and chiller selection.
If we look at a zoned irrigation system, we see that the water consumption pattern depends on the: crop type; irrigation system type; precipitation levels; ambient temperature; and; evaporation and humidity.
Our final example is an industrial production site, where the water consumption pattern could be totally unpredictable and tied to: production levels; number of shifts per day; and, specific machine water demand.
The load profile for any these examples can be determined by estimating how many hours per day the system will demand a specific water flow. Once we have the load profile, we can go to grundfos.com to select our pump. If this information is not available, the product sizing tool at grundfos.com can provide a non-specific load profile for many applications.
The most important thing to consider when selecting a pump for variable flow is that the peak flow does not occur most of the time, so it is imperative to select pumps which offer the best efficiency with lower frequent flows. This is counterintuitive, as we have the tendency to select a pump with the best efficiency at the peak flow. Which will lead to an application that uses much more energy than needed.
In the following example, we will select a pump for water consumption in a typical 13 story apartment building with 146 units. A consultant monitored consumption in this building and concluded that 95% of the time the water consumption was less than 10% of the design flow. The product sizing tool on Grundfos.com will provide an optimal selection suited for the application.
To get started, click on “pump family”, “Hydro MPC”, and “Hydro MPC-E”. Enter the flow and head and click on the arrow. Scroll down to the “life cycle cost calculation” and expand. Select “Standard profile”. This will enable the product sizing tool to use a non-specific load profile for pressure boosting. Further options are available. For example we can select “proportional pressure” with zero decrease at low flow to simulate “constant discharge pressure” control and set the interest rate to 0.3%. Click “start sizing”. Now select the top result which has the lowest life cycle costs. Each orange circle in the pump curve, corresponds to a duty point in the load profile. The bigger the orange circle, the longer the pump will run at that duty point. The orange line that goes across the orange circles is the control curve.
Pump selection can also be attained by using a customised load profile. This option is for advanced users. To do so, start a new product selection, select “application”, “commercial buildings” and “commercial water pressure boosting”. Click on the arrow. Select “Boost up from break tank” and enter the values for “flow”, “Geodetic height”, “Friction losses”, “Discharge pressure”. In this example we will not use friction losses to match the given load profile. Scroll down. Expand “Edit load profile”, select “User profile”, and “Direct Q/H input”. Notice that when you use this option there is a 25% safety factor for the maximum flow. Enter the values for the load profile. Expand “Configuration” and chose “2 pumps” with no standby pump. Click on “Start sizing”. You can either select the top product which has the lowest lifecycle cost, or the product family you require. For this example, we require a Hydro MPC-E because it offers lower energy consumption. The load profile is important for energy consumption estimations that can be used to compare pump alternatives.
You are now ready to use your new knowledge on load profiles to make better pump selections.