The physical laws that govern centrifugal fans and pumps dictate that the volume of air or water decreases directly with fan or pump speed. Power consumption, on the other hand, decreases with the cube of speed. So a small reduction in speed produces a significant reduction in power consumption.
Since a variable speed HVAC system usually operates at less than full volume, adjusting fan and pump speeds as the method of adjusting volume requires considerably less energy than methods where motors run continuously at full speed.
Comparison of Variable Air Volume (VAV) Control Options:
Outlet Dampers
These are metal blades positioned in the air stream on the outlet side of the fan. The fan operates at constant speed. AIr volume is reduced by closing the damper to restrict airflow.
|
Inlet Guide Vanes
These are mounted in the fan inlet. Volume is reduced by altering the vane position to pre-rotate the air going into the fan causing reduced flow capacity. Fan speed remains constant.
|
Variable Speed Drives
These reduce air volume by lowering fan speed.
|
Now let's compare the savings:
Take the example of an HVAC system operating 3,120 hours per year (5 days a week, 12 hours a day, 52 weeks per year), and two 30 Hp motors operating supply fans in two air handling units. The HVAC system runs at peak load 25% of the year, 80% load for 50% of the year, and 70% load for the remaining 25% of the year.
At peak load for the year, the outlet damper would consume approximately 41,200 kWh of power, the inlet vanes 40,500 and the VSD 39,000.
At 80% load for the year, the outlet damper would consume approximately 77,200 kWh of power, the inlet vanes 61,000 and the VSD 42,300.
At 70% load for the year, the outlet dampers would consume approximately 36,800 kWh of power, the inlet vanes 27,800 and the VSD 16,300.
The annual power savings using a VSD in place of outlet dampers would be 57,600 kWh, or $518.00 based on $0.09 per kWh and 31,700 kWh or $2,850.00 in place of inlet vanes.
Contact Calvert-Jones today to discuss a Variable Speed Drive for your HVAC unit today.