Considerations in Selecting Fans for Retrofits and New Houses

11/30/2018

When retrofitting high-rise layer houses to aviary systems and erecting new cage-free housing for pullets and hens, ventilation plays a major role in optimizing performance and minimizing cost.  Sustainability is an important consideration for producers, especially now that customers are imposing more rigorous standards on the conservation of resources including, water, electrical power and avoiding waste.

 

SKOV has introduced a range of direct driven variable speed BlueFan, offering more precise control of ventilation and improved energy efficiency. These fans exceed the capabilities of conventional belt-driven fixed-speed fans that are the mainstay of the U.S. egg industry.  SKOV is a multinational company based in Denmark with over 60,000 poultry houses in operation, spanning 65 nations on five continents.

 

The advantages of a variable speed fan are based on physical laws determining performance.  The ability of a fan to move air, measured in cubic feet per minute, increases proportionally with fan speed when using the same diameter blade.  Sometimes producers change the diameter of drive pulleys to increase airflow, but unfortunately this approach bears a heavy cost.  Power consumption increases exponentially with fan speed.  At a blade speed of 400 rpm, power consumption for a typical 48” fan may be 500 watts.   At 500 rpm, representing a 25 percent increase in speed and air displacement, the power requirement will double to 1,000 watts which will be reflected in the cost of operation over a specific period.  The opposite is also relevant in that slowing the fan speed to reduce airflow results in disproportionately lower power consumption. It can be shown that at full capacity, a variable speed fan operating at 550 rpm uses 1,400 watts of power.  If fan speed is reduced by 20 percent to 450 rpm, power is reduced to 780 watts.  A 38 percent reduction in fan speed, representing a practical limit, to 340 rpm effects a further reduction in power use to 350 watts, 65 percent lower than at maximum capacity.

Fan efficiency can be measured by the airflow ratio which influences power consumption.  Airflow ratio is defined as:

 

                          The air displaced at 0.2” wg ÷ airflow @ 0.05” wg

 

More efficient fans will have higher airflow ratios at 0.2” wg..  SKOV variable speed fans displacing 21,000 cfm operate with an airflow ratio of 0.72.  Models rated at 35,000 cfm displacement attain an airflow ratio of 0.88.  The energy consumption of fans is measured t in cfm per watt. A SKOV fan operating at 35,000 cfm has a power consumption of 17.8 cfm per watt compared to a SKOV fan with 21,000 cfm displacement yielding 25 cfm per watt.

 

Conventional single-speed fans installed in a house are staged to operate according to stepwise changes in temperature as detected by sensors.  As house temperature rises, more banks of fans are activated.  The efficiency of conventional belt-driven fans was compared to variable speed fans in a structured trial conducted by the College of Agriculture and Environmental Sciences at The University of Georgia*.  The study was designed and conducted by Michael Czarick and Dr. Brian Fairchild.  The study was completed on a farm with two identical 40ft x 500ft conventional slat and litter broiler breeder houses.  One house was equipped with eight 48-inch slant-wall fans and two 36-inch side-wall exhaust fans. In the other house, six 55-inch direct-drive variable-speed SKOV BlueFans were installedThe variable range for the SKOV fans was between 60 and 100 percent of air displacement.  Each of the SKOV fans were set to displace 16,000 cfm at the low rate measured in rpm equivalent to each of the 48-inch belt-driven fans.  Once all SKOV fans were on at low speed, increased air demand increased the speed of the SKOV fans to match the capacity of the single-speed fans.  Set temperatures were the same in both houses and air speed was 480 ft. per minute over the litter area and 300 ft. per minute over the slats.  Through a laying cycle, it was demonstrated that variable speed fans used 65 percent less power during the period from October through April with savings of approximately 45% during summer.  The cost of operating conventional 48-inch fans at a rate of $0.105 per Kw-Hr was $1,880.  The 55-inch variable-speed SKOV fans incurred a cost of $874 representing a 54 percent reduction.  At $0.20 per Kw-Hr, it is estimated that the 48-inch fans would have an operating cost of $3,758 compared to the variable speed fans at $1,675.

 

The cost saving is however associated with a higher initial cost.  Variable speed fans require electronic controllers to help control fan speed that are different and slightly more expensive than the simple belt-driven motor of a single speed fan.  However, SKOV optimizes efficiency and practicality by adding certain features like optional air-tight motorized shutters, safety guards, a bell-mouth opening, cone for air outlet and a direct drive motor.  Studies conducted under farm conditions in several countries showed payback periods of two to three years for the additional cost of a complete installation, only considering the cost of the power consumed.

 

SKOV fans are constructed of thermo-plastic and stainless steel to resist corrosion allowing for high-pressure spraying for cleaning. The direct-drive motors do not require maintenance of drive belts, and the motor and speed controller are separate units which facilitates replacement should this be necessary. The air-tight motorized shutters help to maintain negative pressure within the house and ensure the shutters always open fully to get maximum fan performance.

 

The bottom line is that when selecting fans, it is advantageous to consider the initial investment as well as the total operating cost of an installation over a 15-year period. Savings in power and maintenance can accumulate to show a positive return on investment from variable-speed fans versus single-speed fans.

 

* Czarick, M. and Fairchild, B. (2018) Variable Speed Tunnel Fan Study. Poultry Housing Tips. Vol.30: (7)






















































































































































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