The movement of air for heat transfer and ventilation is an essential function of a postharvest cooling and storage facility. Thus, the proper selection and use of air moving equipment is necessary for a well-designed system. Fans of various types are the prime movers of air in postharvest facilities. A fan is simply a type of air pump. There are three types of air pumps depending on the required pressure range. A compressor is a high pressure device that can produce air pressure differentials above 50 lb per sq in. (psi) at relatively low flow rates of a few cubic ft per minute (cfm). A blower is an intermediate pressure device suited for air pressure differentials from about 5 to 50 psi with a moderate flow rate. A fan is a relatively low pressure device capable of pressures from 0.0 psi (free air) to about 5 psi and capable of the largest flow rates.

Fans are used when there is a need to move large volumes of air at a relatively low differential pressure such as situations that are common in postharvest cooling facilities. There are two basic types, the propeller fan and the centrifugal fan, as shown in Figure 6-1.

Both types of fans can operate in the 0.0 to 0.5 psi pressure range. However, deciding which type is more suitable for a specific application depends on individual fan characteristics. These include the relationship of pressure to flow, level of noise produced, limitations on mounting space, level of dust and contaminants in the air stream, cost, and energy efficiency. Most fans used in postharvest cooling applications are propeller fans.

With low fan air pressures, it is customary to specify the pressure differential in terms of inches of water. Low pressures are conveniently measured with a simple device called a manometer (Figure 6-2), which is a “U” shaped tube partially filled with water. If there is a difference in air pressure at the openings as indicated by P1 < P2, then there will be a difference in the level of water. The static pressure exerted by a depth of one in. of water is 0.036 psi. For example, if the differential height of the water level in the two sides of the manometer is 1.5 in., then the static pressure is 1.5 in. of water [(1.5) (0.036) = 0.054 psi]. It is common for fan specifications to display the static pressure in terms of “in–wg” (in-water gage) or alternately “WP” (water pressure).

Figure 6-1. Propeller fan, left and centrifugal fan, right.

Source: M. Boyette.

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Figure 6-1. Propeller fan, left and centrifugal fan, right.

Source: M. Boyette.

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Figure 6-2. Manometer.

Source: NC State University.

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Figure 6-2. Manometer.

Source: NC State University.

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There are two basic relationships that characterize a particular fan and fan system. One relates to the fan alone at a constant number of revolutions per minute (RPM) (supply side) and the other to the system into which the fan is operated (demand side). Alternatively, the fan supplies the energy (driver) and the system provides the energy sink (resistance). Until the relatively recent introduction of variable frequency drives (VFD’s), fan impeller speed was fixed to the motor RPM, the belt, and the pulley arrangement that drives the impeller. When selecting a fan for a certain application, the most important characteristic of the fan is the relationship between the pressure rise or pressure differential, (∆P), expressed as in. of water and the volume flow rate (cubic ft per minute or CFM) at a certain fan impeller speed. In general, as the pressure differential increases, the flow rate decreases.

Figure 6-3 is a graph of static pressure versus volume flow rate for a propeller fan at a constant shaft speed (RPM), (the red line). Point A is the operating area where the pressure is highest but the flow rate is essentially zero as if the outlet were completely blocked. Point B is the area where the pressure is high but the flow is minimal. A fan should normally not be operated in this area because of the very low power efficiency. Point C represents the stall region where air flow and pressure are unstable. In this range, air flow past the fan blades is not smooth and the pressure and volume can oscillate widely. There is a dramatic increase in fan noise and vibration and the air flow can actually reverse. At this point, fan efficiency is also very low and the vibration can cause fan metal fatigue and failure.

Fan manufacturers design fans that will operate efficiently over a wide range. Most fans do not operate in the large unstable range shown in the graph, although some do. For this reason, it is wise to review the fan curve before purchasing. In selecting a fan for a particular application, it is best to select a fan that will provide the necessary pressure and flow in its best operating range well past the stall region and with reasonable efficiency.

The blue dashed line on the graph represents the pressure-flow relationship of the system or the resistance to air flow of the system. These system curves often begin nearly linear with respect to pressure and flow, although at some point, these curves increase dramatically in response to turbulence and other factors. Where the fan curve and these system curves intersect represents the operating point of the fan-system combination. When this point is located in the best operating region for the fan and near the peak of the system curve, efficiency is at its peak. The green dashed line represents the horsepower curve as a function of pressure and volume. Again, for efficiency, the operating point of the fan/system should be located near the peak horsepower.

When selecting a fan for a particular system, it is important to remember that a fan operating at any given speed can have an infinite number of operating conditions along the fan curve line. After the fan is connected to the system with its own unique system curve, the fan will operate at the one (operating) point that balances the ∆P and flow of the driver (fan) with the resistance of the system.

This is the point where the fan curve and the system resistance curve cross. A fan works by compressing a certain volume of air to a certain pressure per unit of time. As such, the power produced by the fan is simply the product of the increase in pressure (∆P) and the flow per unit time (CFM).

Figure 6-3. Propeller fan static pressure curve.

Source: M. Boyette.

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Figure 6-3. Propeller fan static pressure curve.

Source: M. Boyette.

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Scientists and engineers have long recognized that there are certain relationships in electrical, thermal, mechanical, and fluid systems that generally fit the form of a driver moving a fluid through a resistance. These relationships can be very useful in visualizing and understanding complex systems.

For example, the basic equation for conductive heat transfer takes the general form of:

q = ∆T/R

where:

q = the flow of heat

∆T = the temperature differential

R = the thermal resistance

This equation is analogous to Ohm’s Law (V=IR or I=V/R) as applied to electrical circuits. Relationships analogous to Ohm’s Law may also be applied to the operation of fans under certain conditions. If the power (P) in an electrical circuit is simply the product of the current (I) and the voltage (V), as in P=IV, the analogy of Ohm’s Law may also be applied to fans. This relationship of flow, pressure, and resistance follows the same general form where V is analogous to pressure (∆P), I is analogous to flow (CFM), and R is analogous to system resistance. Thus, power produced by a fan is the product of pressure and flow. Fan Power = (∆P) (flow). (See Figure 6-4).

There is one important difference when comparing Ohm’s Law for electrical circuits to the fan power equation. In an electrical circuit, the resistance does not vary with voltage. The electrical resistance remains constant even when the voltage is increased or decreased. This is not the case when a fan forces air through a resistance. Air, unlike electrons, is a compressible fluid that work can be applied to in varying amounts that are proportional to the increase in pressure. When the air pressure increases, the air through the resistance becomes more turbulent and the resistance increases. Consequently, the resistance to airflow of a system increases as the flow increase, as shown in Figure 6-5.

This is the reason why the efficiency of a fan changes dramatically along its fan curve. It is also the reason a fan should always be selected to match the resistance load as close to its peak efficiency as possible. A fan with a poor match between flow and resistance can have gross energy efficiencies in the single digits. Figure 6-5 illustrates how losses can accumulate in a fan as electrical power is converted to air power. Engineers tasked with the process of specifying fans for postharvest cooling applications should be aware that oversized fans can be costly in equipment cost in the short run and operating costs in the long run. Even small fans can be very inefficient if they are mismatched to their loads.

Figure 6-4. The balance of the fan performance and system resistance curves.

Source: M. Boyette.

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Figure 6-4. The balance of the fan performance and system resistance curves.

Source: M. Boyette.

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Figure 6-5. Sources and typical percentages of fan efficiency losses.

Source: NC State University.

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Figure 6-5. Sources and typical percentages of fan efficiency losses.

Source: NC State University.

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Consider a 36-in., 2 horsepower propeller exhaust fan operating at the constant speed of 861 rpm. The fan curves are shown in Figure 6-6. The fan specifications provide a flow of 18,434 cfm at a ∆P of 0.125 in. of water. At these conditions, the fan curve indicates that it requires 2.3 BHP from the electric motor.

The equation for fan power is:

Fan power = (∆P) (flow),

Converting units

= (0.125 in-wg) (18,434 cfm) = (0.6503 lb./ft2) ( 307.2 ft3/sec.)

= (199.8 ft-lb/sec.) / (550 ft-lb/sec./hp) = 0.36 Hp.

This means that the net power required to raise the pressure of 18,434 cfm of air from zero to 0.125 in-wg requires only 0.36 hp. The amount of horsepower that must be supplied to the fan by the electric motor is 2.3 hp.

The fan efficiency = 0.36 ÷ 2.3 = 15.8%. Over 84% of the power from the electric motor is wasted on friction and aerodynamic inefficiencies, both of which generate heat. This is an exceptionally low efficiency at this operating point. When the fan inefficiency is added to the motor and controls inefficiencies, the gross efficiency of the fan system approaches single digits.

Fan energy efficiency can vary over a wide range depending on operating conditions, as well as fan design and application. When specifying a fan for a specific application, it is important to select the correct combination of pressure and flow and also to increase energy efficiency by applying the correct fan. A misapplied fan with a relatively low energy efficiency can cost far more during its lifetime in the operating costs of electricity than any savings in equipment costs.

Instead of a 2 hp, 36 in. fan, consider a 2 hp, 48 in. fan that uses the same electric motor with a BHP of 2.3. However, this fan has a flow of 24,803 cfm at 0.125 in-wg while operating at 576 rpm. The efficiency of this fan is 21.2%. This means that with a larger fan diameter operating at a slower speed, there is about 35% more air flow with the same electrical power. When more air flow is required at a certain pressure, it is always better to select a larger diameter fan that is turning at a lower rpm than to use a larger electric motor and a faster rpm.

Figure 6-6. Typical curves for 36 in. 2 horsepower propeller fan.

Source: M. Boyette.

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Figure 6-6. Typical curves for 36 in. 2 horsepower propeller fan.

Source: M. Boyette.

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Fan affinity laws govern the operating characteristics of turbo pumps such as fans, blowers, and certain liquid pumps. In their simplest form, the laws apply to a fan with fixed geometry (the size does not change) and with varying shaft speed (the only independent variable).

If we allow for:

Q = the flow

RPM = the shaft speed

P = the pressure differential (∆P)

hp = the power requirement

Then:

Law 1. Flow (Q) is directly proportional to shaft speed:
Q2 ÷ Q1 = RPM2 ÷ RPM1

Example: If we double the shaft speed of a certain fan, we can expect the flow from that fan to double. This relationship generally holds true over a reasonable range (2 to 5 times). However, if we increase the shaft speed 100 times, we cannot expect the flow to increase 100 times.

Law 2. The pressure differential (∆P) is proportional to the square of shaft speed:
∆P2 ÷ ∆P1 = (RPM2 ÷ RPM1)²

Example: If we double the shaft speed, we can expect the pressure to increase by a factor of 4, over a reasonable range.

Law 3. The power required to drive the fan is proportional to the cube of shaft speed:
HP2 ÷ HP1 = (RPM2 ÷ RPM1)³

Example: If we double the shaft speed, the power required to drive the fan will increase by a factor of 8, over a reasonable range.

There are several important and practical implications of the fan laws in postharvest cooling. (See Figure 6-7). First, in a forced air cooling system, the limiting factor that controls the rate of cooling is often the conduction of heat from the interior to the surface of the produce. After the airflow rate past the produce surface is sufficient to remove the heat at the rate it reaches the surface, any additional air flow is wasted. When appropriate, reducing air flow by slowing the shaft speed with a variable frequency drive (VFD) can result in significant savings in electrical power. For example, reducing the shaft speed by 25% can reduce the power consumption by about 58%. Further, as shown in Table 6-1, when comparing fans of different size that produce the same pressure and flow, the larger and slower fan will always be more energy efficient.

* Brake horsepower (BHP) is the measurement of a motor’s maximum sustainable output without consideration of any friction losses from belts or bearings. The BHP is the real measure of a motor’s highest possible output that will not shorten its service life.

In summary, energy efficiency in fan systems has recently come under closer scrutiny due to high electrical energy costs, improved fan and air flow computer modeling software, and more reliable data collection techniques. In response, fan manufacturers have developed more efficient fan blade design, more efficient electrical motors, and industry-wide standardizations.

Figure 6-7. The fan affinity laws.

Source: M. Boyette.

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Figure 6-7. The fan affinity laws.

Source: M. Boyette.

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