Pumps: Pulley Drive, VFD, and Direct Drive; Simplified

Some answers to frequently asked questions about high pressure misting pumps. We now have the wonderful Variable frequency drive (VFD) pumps also known as inverter drive pumps or variable speed mist pumps. We have a few in stock to choose from and can custom build most any size.

We always carry Pulley Drive Mist Pumps and Direct Drive High Pressure Mist Pumps in stock. They both have their pros and cons:

Advanced Systems Direct Drive Mist Pumps
Most Direct drive misting pumps are usually the least expensive and most common although they can and in most cases be the loudest, This is due to the pump and motor spinning at 1750 rpm and they are usually maxed out at 1000 psi. Although the .5 and 1 gpm mist pumps are not too loud at all. The pump would shut off and not burn up. We have these mist pumps both with and without enclosures. We do suggest a low-­‐pressure safety option to prevent damage to the pump if the water was ever turned off accidentally.

Advanced Systems Pulley Drive Mist Pumps
Pulley Drive Mist Pumps are typically more expensive than direct drive because of the extra cost of the pulleys, belts, and all the extra hardware that goes with it. All of the belt drive mist pumps are built with full enclosures that house all the electrical components and eliminates the need for a belt guard. One of the advantages of choosing a belt drive mist pump system is the noise factor; they are quiet and last much longer than the Direct Drive Pumps. The fact is that we size the pumps gallon per minute output much larger and then we slow the pumps down to about 750 rpm. This makes a large pump run much slower and produce less noise. These are good pumps for residential use.

Advanced Misting Systems Variable Frequency Drive Misting Pumps (VDF)

VDF’s are the most expensive mist pump option and these pumps have their own unique characteristics. They consume approximately 25% less electricity than non-­‐VDF systems due to the inclusion of the VFD inverter and the way it utilizes electricity. This is one of the first of three electricity related efficiencies associated with a VFD mist pump but it is the primary reason VFD units are desired for many commercial and industrial applications.

Because of the inclusion of a pressure transducer in the Advanced Misting System VFD Misting Pump, the system also provides the unique capability of only turning the motor at the required hertz based on the flow demand at any given time. This unique feature provides a variable operating speed for the motor. In most cases, the motor AND the pump are both spinning at an rpm level that is less than a standard Direct Drive or Pulley Drive unit. The result is not only a substantially quieter operating mist pump, but also a mist pump that requires less electricity to operate. This is the second of three electricity related efficiencies associated with a VFD misting pump.

As an example, if a 2 gpm system utilized a Direct Drive pump, both the pump and motor would turn at 1750 rpm. A Pulley Drive unit would require the motor to turn at 1750 rpm but the pump would only turn at about 920 rpm (using a bare pump that provides 3.8 gpm at 1750 rpm). A VFD unit would allow the motor AND the pump to both turn at 920 rpm (using a bare pump rated at 3.8 gpm at 1750 rpm). The result of this is the Direct Drive would be the loudest; the pulley drive would be quieter than the Direct Drive; the VFD would be significantly quieter than the Pulley Drive. This scenario and explanation does not take into considerations other on site conditions that may affect the noise levels but it is a good standard to measure when considering the noise levels associated with a Direct Drive, a Pulley Drive, or a VFD Mist Pump System.

As system flow requirements increase, the VFD will progressively increase the speed of the motor until the pump is providing the required flow to achieve the required 1000 psi pressure. As this motor speed increases, it will increase the noise level and it is possible that if the motor and pump are required to spin at full hertz and rpm (60 and 1750 respectively) due to the maximum flow requirement for the system, the noise level for the VFD in that scenario could be equivalent to the noise level of a Direct Drive or Pulley Drive unit of comparable size. However, because the VFD unit affects the speed of the motor being used, it is always possible to resize the VFD unit so that it never spins above a desired rpm.

Other than the power consumption savings on commercial or industrial systems and not accounting for any noise level considerations, a VFD is most typically used when the system includes multiple zones but flow for the system at any given time is less than 50% (and may as low as 5%) of the pumps capacity. In these cases, the VFD has the ability to manipulate the power supply to the motor and thereby affect the motor rpm such that the pumps resulting flow output exactly matches the systems requirements at that point in time.

That means there is no bypass in the system (since there is no excess water being pressurized and bypassed, there is no heat build up in the water supply for the pump so damage to the internal components of the pump is eliminated) which means the system operates more efficiently because the pump is not over working itself by producing excess pressurized water. This is the third of three electricity related efficiencies associated with a VFD pump.

So, those are three options available for Mist Pump Design and each has its own characteristics that make it appropriate for a given project. The most appropriate option should be determined based on the needs and desires of the end user.