Direct Drive, Pulley Drive, and Variable Frequency Drive Pumps

Whenever Fogco begins the process of designing a misting system for a customer, we have three basic pump design options. These options include a direct drive (dd) pump; a pulley drive (pd) pump; or a variable frequency drive (vfd) pump also known as an inverter drive pump. Each option has its own set of pros and cons.

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 our standard 60 Hz 4 pole motors and 1450 for our standard 50Hz 4 pole motors).

A pulley drive is typically more expensive than a direct drive because it includes 2 pulleys, a belt and requires either an enclosure or a belt guard. Also, a pulley drive misting pump will include an oversized bare pump (our standard 1 gpm pump utilizes a 3.8 gpm bare pump) so that we can slow down the rpm of the pump and still achieve the necessary flow.

Most Fogco pulley drive misting pumps also include additional components compared to a direct drive unit (which contributes to its higher cost basis) to add function and feature to the pulley drive pump design. We use a full enclosure (except in our table top 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 it 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 because 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 inclusion of a pressure transducer in the Fogco 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 (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 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, bucasue 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, there 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.