Variable Frequency Drives or Flow Control Valves For Your Project

Municipalities and businesses are continuously searching for ways to decrease electrical demand, reduce operational costs, and reduce maintenance requirements with associated downtime.  In a system with constant changes to flow, whether that medium is air or water, it can be adjusted with a simple flow control valve.  Is that the best option?  There are other choices and decisions that can influence the outcome, such as installing a variable frequency drive instead of a control valve.  Depending on what you need, either can work, but which is the better choice? 

Following the Traditional Approach

The traditional approach to controlling the flow of centrifugal pumps and fans has been done with the installation of mechanical control devices, such as flow control valves and louvers.  This was the standard taught for many years in every textbook, but this approach has drawbacks concerning power consumption.  The centrifugal pumps run at a constant speed regardless of the amount of flow required when connected to an electrical motor.  If the operation or process requires a flow reduction, the only means to reduce the flow is via control valves or louvers.  The pump energy consumption will not change, and there is chance of wasting energy by overproducing flow capability that isn’t utilized.  Wasting energy leads to higher electrical costs, and that can be passed on to overall facility operations and per-product cost.

Beyond higher electrical costs, the standard approach to flow control can increase maintenance and repair investment over time.  Reducing flow output while keeping a continuous operating pressure adds unnecessary wear on the centrifugal pump.  The pump can cause leaks elsewhere in the system, increase vibration, and overheat because it is trying to push additional flow volume that isn’t needed by the system.  The better solution is to use a centrifugal pump that can alter its output based on the needs of the system at the source, rather than overproducing flow that isn’t required.

Changing the Status Quo with Variable Frequency Drives

If your operating system stays at a constant flow and pressure, a simple centrifugal pump and electrical motor may be the best option to use.  What happens if that flow needs to be adjustable based on operating conditions, and you can’t afford to waste energy and maintenance time?  Enter the Variable Frequency Drive (VFD).  Replacing a constant speed electrical motor with a variable frequency drive can reduce energy demand from 10% to 60% depending on the application.  The associated cost of replacing the standard motor with a VFD can be paid back typically within 24 months, if not sooner depending on the process load and demand reduction.

Beyond electrical savings, a VFD can offer additional benefits:

• Reduced maintenance costs – Running a pump at a continuous flow volume doesn’t add additional stress on the pump, but continually reducing the flow with a valve can push pressure back to the pump.  That pressure can cause vibrations, leaking, and mechanical failure at the pump itself, or elsewhere in the system.  Using a VFD to adjust the pump flow volume based on the requirement reduces the stress on the pump.  Less maintenance equates to less time and money spent on equipment repairs and replacements.
• Reduced process variability – Flow Control Valves respond well to changes when they are initially added to the system, or at start-up of the process.  Over time, they can become “sticky” and can respond poorly to input changes and process change requirements.  Their lack of response, or inability to adjust correctly as needed, can cause the process flow to become slightly unstable.  If your product quality and output is dependent on that precise flow control, the valve may lead to poor or reduced product quality over time.
• Improved plant overall power factor – Facilities often use capacitor banks to decrease the initial electrical demand at start-up.  The capacitors also require maintenance, which typically is a separate item from standard equipment maintenance performed by trained personnel.  Capacitors also require a large investment and can have a long return on investment.  Replacing motors or installing VFDs initially can alleviate the need for the capacitor bank and its associated costs.

Are VFDs the Perfect Solution?

Variable Frequency Drives are a great solution to changing operational flows, but there are limitations.  VFDs may be poor performers if used at very low speeds, such as less than 20% of their maximum flow rate.  They should be used between 20% to 100% of their capable flow rate, as anything less than 20% may struggle to overcome the static system head pressure to create flow.  If that situation can occur, a Flow Control Valve should be added to the system to increase the system restriction and increase the system pressure.

It may seem like a clear choice to make the Variable Frequency Drive the standard answer to every flow problem.  They are a great choice, but as mentioned they aren’t the perfect solution to every situation.  Deciding which type of equipment should be used can be complex process, and Automation Electric and Controls can offer support to your business to determine the better choice for your application.  AEC is a licensed ETL 508A panel building shop, and our work is defined by the quality products we produce.  We offer service for every industrial panel after installation, and can support your business with enclosures, breakers, electrical motors, variable frequency drives, wiring, relays, contactors, and much more.  We are locally owned and have operated in the Pacific Northwest since 2003.  When you’re ready for a new custom designed industrial panel, a custom mobile trailer and container, or simply need support of one currently in use, give us a call at (866) 452-6947 or email us at sales@automationelectric.com.