In today’s construction industry the need to design and build energy efficient buildings has never been higher. With commercial buildings being one of the largest consumers of energy in the United States, architects and engineers alike are always looking for better ways to make buildings more energy efficient. One way to make the mechanical systems within buildings more energy efficient is to use variable frequency drives (VFD) to control motor speed. This technology can be applied to both water side mechanical systems (pumps, chillers, cooling tower fans, etc.) and air side mechanical systems (supply/return fan motors in air handlers, exhaust fan motors, etc.). For the purpose of this article we will be examining the application of a VFD as it relates to pumping in chilled water and heating water systems. A majority of the concepts discussed below are directly related to the air side equipment mentioned above.
The use of VFD’s in hydronic systems is done for many reasons. The following is a list of those reasons: increased flow and pressure control, more precise heating and cooling control within a building, soft start capabilities, and prolonged life-cycle costs. Below is a brief description of the benefits related to VFD integration into hydronic mechanical systems.
1. Increased Flow and Pressure Control
On traditional constant flow pumping systems there are many mechanical ways to control system flow and pressure such as with the use of throttling valves or hydraulic couplings to mention a few. The most common flow control method within mechanical hydronic systems is with the use of manual throttling valves and control valves. If smaller levels of flow are desired the valve closes a corresponding amount to allow less flow, and the opposite is done if larger amounts of flow are required. In a lot of cases the valves are two positions, either open or closed. Due to the fact that the pump remains at a constant flow even though the valve has changed its flow characteristics, or even closed completely, these scenarios cause energy to be wasted, non-constant system pressures, and unnecessary wear on the pump, pump motor and piping system. With the use of a VFD change in flow is accomplished by changing the speed of the pump motor to align with the flow requirements of the valve. The VFD will allow the pump to operate at its most efficient operating point no mater the position of the valve in the system. Allowing the pump motor to slow down or speed back up as the valve is opened or closed, allows system pressure to remain relatively constant over the entire operating range of the valve.
2. Precise Control of Heating and Chilled Water Systems
Based on the descriptions above and the VFD allowing the pump motor to speed up or down based on valve control allows for very precise control of both heating and chilled water systems. The pump will only circulate the required flow called for from the system, rather than over flowing based on heating/chilled water valve positions. This allows for more control related to the actual amount of heat and/or cooling provided to building spaces and increases human comfort levels within the building.
3. Soft Start and Prolonged Life Cycle Costs
Soft start capabilities are related to when the pump is first activated if the system has been shut down. The VFD allows the pump motor to progressively increase the speed of the motor rather than just starting it at full speed. This reduces stress on the pump motor and other working parts within the pump casing. In return due to the lower stress on the pump in general, the equipment will last longer and have an overall longer life cycle. Maintenance costs related to pump motors and internal pump parts can also be diminished based on lower stress levels on the pump.
Above are a few reasons to apply VFD technology into HVAC systems. Not only do they save considerable amounts of energy, but they allow the equipment to have a longer service life and operate more efficiently. The above information is related to pumps, however as mentioned before this technology can be applied to supply air fans, return air fans, and exhaust fans in many different types of applications.