Electric fans are a great...
Electric fans are a great option for many cars and may be mounted in front of the radiator as a pusher or behind as a puller (pulling is preferred). Note the blades on this example are straight.
Engine-driven fans are usually criticized for consuming a large amount of horsepower-of course if you think about it from another perspective, can a 1hp electric fan move as much air as an engine-driven fan that takes 10 hp or more to spin? When an engine is under a heavy load and the A/C is on, an engine-driven fan is hard to beat, as they can move lots of air through the radiator. Of course when the cooling demands aren't as great, horsepower is being lost unnecessarily. So the trick is to determine the vehicle's needs and what fan works best the majority of the time.
Rigid fans: simple and effective, they move lots of air but they do consume power and can be noisy.
Flex fan: a sophisticated version of a rigid fan. The pitch of the blades on a flex fan will pull lots of air at low speeds then flatten out at higher speeds for less drag.
Curved blade electric fans...
Curved blade electric fans have become popular thanks to their lower noise level. In terms of efficiency they're about the same as straight blades.
Rigid fans are capable of...
Rigid fans are capable of moving a lot of air, particularly through thick radiator cores and A/C condensers. The downside is they can be loud at speed and it takes horsepower to spin them.
Clutch fans: there are a variety of these, but the major drawback for all of them is the space they require.
Flex fans are made from a...
Flex fans are made from a variety of materials, including aluminum and stainless steel. Keep in mind, some serpentine drive systems reverse the direction of the water pump so a reverse rotation fan is required.
these are designed to reduce the load in the engine. The theory is simple enough, a fluid coupling provides some drive, or if you prefer some slip, all the time.
Viscous/Temperature-controlled (not thermostatic): these are unique in that they slip less as temperature raises increasing fan speed.
Viscous (thermostatic): turns on and off at a predetermined temperature. The most sophisticated design, the fan "idles" and doesn't run at engine speed until needed. While this sounds like the ideal scenario, the difficulty with using these is that temperature calibration is critical. According to Scott Leon, a former GM proving ground technician, production OEM fans are normally calibrated to turn on when the coolant temperature reaches 220 degrees and every car or truck gets its own calibration. The average guy doesn't have the option of going into the parts store and asking for a 178-degree clutch for a 16-inch fan, so successfully adapting an OEM design would be pure luck. Too low a cut-in it would run all the time, too high and it will never turn on.
Shrouds are a great addition...
Shrouds are a great addition to make any fan more effective. The blades should be approximately 50 percent inside the opening. The homemade example is on the Varanas family tub.
Electric fans are often a good choice for street rods. They are particularly effective at low vehicle speeds-and as they aren't powered directly by the engine, horsepower isn't lost to their operation. However, big electric or multiple electric fans can draw lots of current and that current has to come from somewhere. At some point the alternator has to provide the energy, which does require horsepower to produce. Nonetheless, as electric fans don't operate continually, the total amount of energy needed to operate them is less than a mechanical fan. Another advantage to an electric fan is packaging. In many cases, they will fit where other fans will. For the greatest effectiveness, electric fans should have a shroud, and they work better pulling the air through the core than pushing it.
While many electric fans look similar, the shape of the blades may vary; some fans have straight blades, others have curves. The primary difference between the two is the noise they make, as S-blades are generally quieter.
When shopping for an electric fan beware of cfm ratings as they are not all determined in the same way. How much air a fan will move, its cfm rating, depends on the static pressure it is exposed to-in other words is there free air in front of it or is the fan trying to pull air through some sort of restriction, like the radiator core? Another way fans are rated is by the electrical power they consume. Generally, a higher amp motor will have the potential to turn a more aggressively pitched fan blade, which will move more air.
Fan clutches are used with...
Fan clutches are used with rigid fans to reduce parasitic loss by "slipping" at low engine temperatures or high engine speeds. Note the unevenly spaced blades to lower sound levels.
Regardless of the type of fan used, for the radiator to be effective air must pass thorough the core rather than go around it. That means if there is a path behind the grille (assuming the car has one) that allows air to pass around the outside the radiator it should be blocked. Sheetmetal baffles that direct air through the core rather than letting it go around the sides over the top tank are extremely beneficial.
Of course another method of directing air is with the use of a fan shroud (see "Which Way the Wind Blows" on page 172 in this issue) as they are extremely effective-in fact in some instances with an electric fan, or fans, shrouds can be too effective. The problem can be at speed with the fan off-the restriction offered by the stationary blades in the shroud's opening restricts airflow. The solution to that problem that can be found on some shrouds (and added to others) are vent holes covered with flexible flaps. When the fan(s) are on, the flaps are pulled closed and air is drawn through the radiator core-when the fan is off the flaps it will blow open, eliminating any restriction and allowing more air to flow through the core.