In most pump applications, the demand varies over the day, the week and year. Centrifugal pumps are often used to move liquid from one location to another in a variety of applications from heating, ventilation and air-conditioning (HVAC), such as circulating chilled water between chillers and air handling units for cooling, to municipal water supply, transferring clean water from reservoirs or wells into distribution systems. These applications typically operate inefficiently at fixed speeds. Therefore, pairing a centrifugal pump with a variable frequency drive (VFD) allows precise speed control, energy efficiency improvements, reduced mechanical stress and extended equipment life.
There are two main reasons for adding a VFD to a pump: process control and energy savings.
- Process control is about getting what is needed at any point in time. The user might need a certain pressure independent of the flow, or a specific flow through a filter independent of the actual filter status. Using a VFD and the right sensors makes it possible for the user to control the process in the way they want.
- Energy savings come from reducing the speed of centrifugal pumps. Just a small decrease in speed will decrease the power consumption considerably. It is known from the affinity laws that a 10% reduction in speed will lead to a 27% reduction in power, and with pumps typically running a high number of hours, even a small speed reduction can lead to high energy savings.
The typical energy savings in heating, ventilation, air conditioning and refrigeration (HVACR) pumping applications are in the range of 15%-45%, whereas water and wastewater applications typically experience 15%-30% savings. The difference can partly be explained by the static lift requirements in water and wastewater.
For example, a fixed speed pump will always run at the same speed, and the actual load point of the pump will be where the system curve intersects with the pump performance curve (flow vs. head). This leads to changes in head, depending on the actual flow. In most cases, this variation of head is unwanted. However, using a VFD to control the pump leads to a more constant performance of the whole system. The VFD allows the pump to work only at the speed required to meet the current demand, rather than at full speed all the time. As a result, energy savings are achieved, since the speed is now variable, so all possible load points can be reached within the operating range of the pump.
The operating range of a pump is given with a minimum and maximum flow, but also with the minimum and maximum speed for the specific pump. The maximum speed is, in most cases, the nominal speed for the motor (i.e., the motor nameplate data) and the minimum speed is typically in the range of 30%-50% of the nominal speed.
When the pump speed is decreased, the head also decreases. Mechanical stresses on components reduce, leading to a longer service lifetime. Additionally, the VFD solution has the benefit of a soft start, slowly ramping up the pump speed, which reduces mechanical stress on the pump and on other pump system components.
Adding a PLC
The VFD must have some information about the process to control the speed of the pump. This should come from a sensor or an application controller like a programmable logic controller (PLC). When the sensor is connected to the VFD, known as closed-loop control, the sensor closes the loop by sending information on a single parameter back to the drive. This information is compared to the setpoint. The setpoint is the target for control, and the speed will change if there is a deviation between the feedback value and the setpoint.
A PLC would be used in applications where multiple factors are utilized for control. A set speed reference would be sent to the VFD, increasing or decreasing the speed accordingly. A modern VFD can interface to a wide range of sensors such as pressure, flow, temperature, level, humidity, etc. The most common signals from sensors are still analogue 4-20 milliamps (mA) and 0-10 volts direct current (VDC).
Pump Performance Limits in Actual Applications
Static head is one of the elements that sets the limits for the pump performance when using a VFD. The pump minimum speed, in this case, is the speed where there is flow. The pump selection can be a limiting factor, depending on the actual demand of flow and head vs. the full-speed performance of the pump. Additionally, as is known from wastewater applications, a factor such as the minimum flow speed demand in pipes can have an influence on achievable speed changes in an application.
At extremely low-flow velocities, solids in wastewater can settle in the pipes, leading to clogging, reduced system efficiency and increased maintenance. While the VFD reduces the pump speed to save energy during low demand periods, if the speed (and therefore flow) drops below the minimum required velocity, it can compromise the self-cleaning ability of the piping system.
Therefore, there is a limit to how much the speed can be reduced using a VFD in wastewater applications. Operators must set a minimum allowable speed (or flow rate) in the VFD control logic to ensure solids remain suspended in the flow and the system stays clean.
Additional Benefits for Pump System Optimization
VFDs dedicated to running pumps can also include additional features to support the optimization of pump systems. There are features for running multiple pumps as duty-standby, where one pump is running and one pump is in standstill. The system control will still be running even through a pump shift. This type of pump operation allows for both redundancy and the benefit of a VFD. The other option is duty-assist, or lead-lag, where pumps are running in parallel to support each other at high demand. At low demand, only one pump will be running to satisfy demand, and one or more pumps will run to meet elevated demand. There will be one VFD per pump in optimal systems, but older systems may be designed with one VFD and a series of direct online/across the line pumps. These older systems are typically not as optimal as modern types with one VFD per pump.
Most modern VFDs designed for pump operation will also have a feature that allows the VFD to stop the pump at low flow. It is not recommended to let a pump run for extended periods if it is maintaining constant pressure. The stop function works by utilizing a pressurized tank (known as a hydrophore), where the pump boosts water into it at a slightly elevated pressure before stopping the pump. The pump will then restart when the pressure drops below the required setpoint. The hydrophore will supply water for a period long enough to restart the pump and build the flow and pressure back up to the desired level.
Additionally, a VFD will include a set of monitoring functions, where energy consumption, the number of starts and the running hours are collected. Analyzing this data is highly valuable, as it can be utilized to better optimize pump performance and changes made can be monitored closely.
Adding a VFD to a centrifugal pump allows precise speed control and energy savings, as well as ensuring better performance across varying demand conditions. Unlike fixed-speed pumps, VFD-equipped systems adapt flow and pressure, improving process control while reducing energy use and mechanical stress. This leads to greater efficiency, longer equipment life and more reliable system operation. With added features like closed-loop control, sensor integration and duty cycling, VFDs are crucial for optimizing HVAC and water applications.
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