Fan Replacement could offer over 30% in savings
This article is part 2 of our guide on fan power. Read part 1 of the guide here
Type of Fan
There are three types of fan generally used in HVAC systems, these being:-
- Centrifugal with Forward Curved Impeller
- Centrifugal with Backward Curved Impeller
- Axial Flow
- Centrifugal fans are typically installed in air handling units, whereas axial flow fans are usually duct mounted.
- Forward curved centrifugal fans are about 60% efficient, whereas backward curved fans are 80% efficient but higher (more than twice) in cost.
- Many air handling systems will have forward curved fans, due to lower capital cost, except where the design calls for the higher static pressures which the backward curved version can achieve.
- The efficiency of axial flow fans falls between the centrifugal forward and backward figures (around 70%), and the cost is similar to the centrifugal forward curve. These are normally installed for high volume applications, but with lower static pressure requirements.
The Theory
The output power of the fan is the volume of air delivered multiplied by the pressure generated by the fan. The power output, and hence energy consumption is the power required to drive the fan – including the losses through belts, bearings, motors and inverters.
Fan efficiency therefore = output power / power input
For example, the out power of a forward curved fan delivering 5m2/ sec against 500 Pa is 2.5kW which, assuming an impeller efficiency of 60%, equates to a shaft power required of 4.2/(0.9×0.85) = 5.5 kW
Therefore, an overall fan efficiency of 2.5/5.5 = 45%
From this, we can see that there is scope to reduce input power by considering the type of fan and the way it is driven.
Economics
Using the example above, a fan with an output power of 2.5 kW running constantly would consume 48,180 kWhrs of electricity per annum and would emit 25 tons of CO2.
If it were changed to a backward curved fan and driven directly by an 85% efficient motor instead of being belt driven, then the power consumption would drop by 15,700 kWhrs – i.e. a saving of 32% in electricity and CO2 emissions.
An added benefit would be the reduction in heat emitted from the fan motor into the air stream, which would reduce the cooling load and therefore reduce the power input to the refrigeration system.
Action Points
- Check the original design – is it appropriate to change the type of fan or the method with which it is driven?
- Consider variable speed drive (VSD) fan motors.