Chapter 7 - Fan Selection

The air exchange capacity of a mechanical ventilation system is provided by fans. Fans discharge a volume of air per minute from the building and, in concert with inlets and a static pressure difference, cause fresh air to enter the building to replace the exhausted air.

An exhaust fan creates a slight vacuum within the structure compared to outside static pressure. The static pressure difference required to ventilate a building is very small – on the order of 0.05-inch water (pressure is often measured as the depth of water in a column). This can be visualized as the amount of suction needed to draw water 5/100 of an inch up a straw. This may not seem like a lot of suction, but it is enough to create sufficient airflow to properly ventilate a building. Static pressure should be maintained within a reasonably constant range. Creating a static pressure difference requires relatively tight building construction, however, and not all poultry buildings meet this criterion. Mechanical ventilation buildings need a static pressure gauge (manometer) so the operator can verify that desired static pressure (0.05 to 0.08-inch water) is being maintained.

Fans for the poultry house ventilation are belt- or direct-drive propeller fans and are designed for providing large volumes of air against low airflow resistance. Poultry house fans require totally enclosed motors for protection from dust and gas damage. In a conventional system, fans are often banked, or installed side by side, in sets of tow to four fans approximately every 50 to 100 feet along one or both sidewalls of long poultry buildings. Some producers locate summer fans on or near one end wall for tunnel ventilation applications.

The resistance to airflow that must be overcome by fans is affected by ventilation inlets and fan shutters and guards. Additional pieces of equipment, such as wind protection devices, evaporative pads, or light traps, further restrict airflow. Fan airflow capacity is influenced in turn by static pressure, which is most effective when kept at 0.05 to 0.08 inches water gauge across the poultry house inlets. This is monitored as part of the ventilation system control, but it only represents one component of the static pressure difference against which the fan must operate. Total resistance along the airflow path from outside to the building interior and back outside can be as high as 0.20 inches in a water gauge if the fan is moving air through evaporative pads or exhausting air into strong winds. Obstructions within twenty fan diameters distance downstream of the fan should be minimized. For example, a 36-inch fan should have no obstructions within 60 feet of its exhaust side. Light trap hoods violate this rule, but they are often necessary for light-controlled pullet and layer houses.

Fans are used in mechanical ventilating systems to supply the energy needed to exchange the desired amount of air in a poultry house each minute. In a negative pressure system, fans are installed to exhaust stale or used air from the building and bring in fresh clean air. It is very important to use only rated fans.

Fan ratings are typically given in cubic feet of air per minute (CFM), or in SI units – cubic meters of air per hour, at specific static pressure levels. Fan ratings are given in table form, similar to Table 7.1. Look for certification by an organization like the Air Movement and Control Association (AMCA - http://www.amca.org/) when purchasing fans. With rated fans, there is some assurance that the CFM rates given in the table are valid. Individual fan ratings depend on motor horsepower (HP) and fan speed (RPM), shape, and shroud design around the blades. Fans with the same diameter can have very different CFM values. Because it is very difficult to accurately determine a fan’s CFM capacity when it is already in place in an existing facility, it is very important to use rated fans when selecting or replacing a fan so that the air exchange or ventilation rate is known.

Table 7.1 - Typical rating table for exhaust fans (Delivery rate in cubic feet per minute (CFM) at listed static pressures)

Agricultural ventilation fans may be chosen for CFM delivery at 1/10^th(0.10) or 1/8^th (0.125) inch static pressure. Mechanically ventilating systems, including negative, positive, and neutral pressure, operate at static pressures slightly below these levels. However, when the fans are selected at static pressures slightly above operating conditions, a small safety factor is provided, to ensure that sufficient air exchange is provided through the building when the wind is blowing into the fan exhaust. When higher resistance conditions are expected, as when drawing air through evaporative cooling pads or light traps, choose fans capable of delivering airflow at the higher resistance. The maximum airflow of a fan at any speed occurs at free air, or zero static pressure. Wind blowing against the fan increases the static pressure the fan experiences. A fan’s ventilation efficiency ratio, in CFM per watt, represents its performance versus operating cost. Electrical energy efficiency is most critical in large-capacity fans that operate primarily during warm weather. Energy efficiency criteria for the continuous or cold weather operating fan(s) is less important because they move a small percentage of the total airflow in the ventilation system.

Airflow capacity and efficiency of a fan are improved by good blade design, small clearance between blade tip and fan housing, smooth panel design, and the presence of inflow and/or discharge cones. Fans should also be selected with maintenance requirements, noise levels, dealer service, and cost in mind.

One large fan is usually more energy-efficient and less expensive to purchase and operate than several smaller fans. Larger fans will also save on installation costs because less wiring and carpentry are required. Fewer controls are needed, and larger fans usually give lower total power consumption from improved efficiency. The energy efficiency of winter fans is less important than their reliability at higher static pressures.

It is important to evaluate and select fans that have been tested under conditions similar to those expected in the poultry facility. Manufacturers offer fan performance data for bare fans with no additional equipment in place. This is not typical of an installed agricultural fan. Most manufacturers also offer fan performance data with various equipment options in place. Ask for this more appropriate data. For example, if an installed fan will have shutters and guards, evaluate data obtained when the fan was rated with shutters and guards in place. If that information isn’t provided, add the static pressure resistance associated with these accessories to your estimate of the total static pressure against which the fan will operate.

Other factors influencing fan performance and/or operating cost include electrical cost, blade revolutions per minute (rpm), motor size and design, fan/motor matching, maintenance, and bearing design and lubrication.

Fan performance can vary widely among different models and manufacturers. At 0.10-inch static pressure, a 36-inch fan may deliver as little as 6,200 CFM for the worst performer and as much as 13,000 CFM for the best. The best fans have relatively flat performance curves across the range of operating static pressures (0.08 to 2.0 inches of water). By selecting the best-performing fan over the worst-performing, one can double the airflow capacity of a ventilation system. These data underscore the necessity of checking rated fan data, rather than relying on a ‘"rule of thumb" that indicates that a 36-inch fan provides 10,000 CFM. A rule of thumb is acceptable for a first estimate, but specific rated fan data should be used when selecting fans for the system.

Select energy-efficient fans which have a high CFM per watt ratio at the ventilation system’s operating static pressure. Efficient fans have high output in CFM with lower input costs in kilowatt-hours (Kwh) of electricity. Again, the fan efficiency should represent the conditions in which the fan will be operated. The best fans are almost twice as energy efficient as the worst fans.

The annual electricity cost of a fan is calculated as A = (8.76 x N x T x P x C x K)/VER where:

• A = annual energy cost, $/yr
• N = number of bird batches per year
• T = time each bird batch spends in the house, in days
• P = current electricity price, $ per kilowatt hours
• C = installed fan capacity, CFM
• K = fan utilization fact, fraction
• VER = ventilation efficiency ratio, CFM per watt

The fan utilization factor indicates the proportion of time a fan is operating. For example, one or two fans in a poultry house may run continuously all winter and throughout warm weather for a fan utilization factor of nearly 100% of the time or K=1. Other fans are staged to come only under the hottest conditions and may only be used 25% of the year, for a K=0.25.

Variable-speed fans have the advantage of continuous variation between their minimum and maximum ventilation rates. Smooth airflow changes reduce temperature swings that can occur with staged, on-off fan control. When properly sized and controlled, variable-speed fans can reduce building energy costs. Variable-speed fans are direct-drive, and motor voltage varies the revolutions per minute (RPM) of the fan blade, thus modifying the airflow rate. When operated at low speeds, however, variable-speed fans have the disadvantage of losing their ability to resist wind-induced back-pressure on the fan. Variable-speed fan performance is reported as a set of characteristic curves reflecting static pressure versus airflow.

At low speeds of 20-50% capacity, most variable-speed fans will not deliver adequate, reliable airflow under typical agricultural ventilation conditions. For example, a 36-inch fan will not provide any airflow when static pressure exceeds 0.15 inches at 120 V (50% of fan capacity). This will often result in stalling, blade reversal, and motor overloading. Variable-speed fans are also inherently prone to wind interference when operating at low voltages because they generate negligible pressures when compared to wind pressures. A wind blowing into a fan can easily provide an amount of static pressure against which a low-speed fan cannot provide airflow. When this happens, the fan may still have blades turning and appear to be working, but the air is actually entering the building rather than exhausting.

Variable-speed fans equipped with electronic motor-speed controllers automatically adjust motor voltage and thus fan speed continuously for smooth changes in airflow rate. Decades ago, field experience and research reported inefficiencies, motor overheating, speed instability, insufficient torque, mechanical vibration, and acoustical noise associated with variable-speed fans. However, recent design improvements, such as speed and airflow feedback devices, have minimized or eliminated many of these problems. Variable speed fans are not common in poultry houses but they may be an attractive option.

Consider the following when using variable-speed fans:

1. Limit the lowest speed setting to 50% of the fan supply voltage unless the fan system is equipped with speed or airflow feedback. For example, if the fan speed setting corresponds to 100 volts with a 220-volt supply voltage, the fan motor may overheat.
2. Protect fans from wind by locating them away from prevailing winds and/or installing wind protection devices.
3. As voltage (and airflow) is reduced below 100% capacity, the fan efficiency in CFM per watt is also reduced.