Direct Drive, Pulley Drive, and Variable Frequency Drive Pumps
There are three basic design options when it comes to misting pumps: direct drive (DD) pumps, pulley drive (PD) pumps, and variable frequency drive (VFD) pumps, also known as an inverter drive pumps. Each option has its own set of pros and cons, which we will detail in this blog post.
Pros and Cons of Direct Drive, Pulley Drive, and Frequency Drive Pumps
A direct drive misting pump will usually be the least expensive (except in some cases with sizes above 3gpm), but will also be the loudest because the motor and pump will both be spinning at full rpm (1750 for standard 60 Hz 4 pole motors and 1450 for standard 50Hz 4 pole motors).
A pulley drive is typically more expensive than a direct drive because it includes two pulleys and a belt, and requires either an enclosure or a belt guard. A pulley drive misting pump will also include an oversized bare pump (the standard 1 gpm pump utilizes a 3.8 gpm bare pump) to slow down the rpm of the pump and still achieve the necessary flow.
Most pulley drive misting pumps include additional components compared to direct drive units, which contributes to its higher cost basis. These components add function and feature to the pulley drive pump design. We use a full enclosure (except in tabletop designs, which already include a separate control box) because it provides a place for us to include various custom electrical components without having to provide a separate control box, and eliminates the need for the belt guard.
Even though the motor in a pulley drive unit will turn at full rpm, it will always be quieter than a comparably sized direct drive pump, as the pump is turning at a slower rpm than the motor and creates less vibration and less mechanical noise. The pulley drive misting pump also produces less noise because it is typically fully enclosed where the direct drive pump typically is not.
A variable frequency drive mist pump is the most expensive option, but it does have some unique and valuable characteristics. First, it consumes between 20% and 25% less electricity to operate a VFD system than a non-VFD system, due to the inclusion of the VFD inverter and the way it utilizes electricity. This is only 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 pressure transducers in VFD misting pumps, the system has 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 (usually much less) than a standard direct drive or pulley drive unit. The result is not only a substantially quieter operating mist pump, but 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 used 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 is that 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 do not take into consideration 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, pulley drive, or 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, so does the noise level. If the motor and pump spin at full hertz and rpm (60 and 1750 respectively) due to the maximum flow requirement, the noise level could be equal to that of a direct drive or pulley drive unit. However, because the VFD unit affects the speed of the motor, it is always possible to resize the VFD unit to never spin 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 typically used for multiple zones with less than 50% (and as low as 5%) flow capacity. In these cases, the VFD can manipulate the power supply to the motor, thereby affecting the motor rpm and resulting in a flow output that exactly matches system requirements.
That means there is no bypass in the system. Since there is no excess water being pressurized and bypassed, there is no heat buildup in the pump’s water supply, so damage is eliminated. This allows the system to operate more efficiently because the pump is not overworking itself by producing excess pressurized water. This is the third of three electricity-related efficiencies associated with a VFD mist pump.
Now that you know the three options available for mist pump design, remember that 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. To learn more, or to get started with your own misting system, contact Fogco today.