Cooling Fans

[RPMagoo]

Have been told that a 4 blade fan cools better than a fan with more blades. -- Is this true ?

 

[PurpleBeeper]

No way unless the fan with more blades spins a lot slower

 

[Black_Sheep]

Have been told that a 4 blade fan cools better than a fan with more blades. -- Is this true ?

That has not been my experience. There’s too many variables such as pitch, diameter, shroud or no shroud, etc… to make a blanket statement that 4 blade fans cool better.

 

[RJRENTON]

Have been told that a 4 blade fan cools better than a fan with more blades. -- Is this true ?

Please acquaint yourself with the basic fan laws...

Fan Law 1: CFM is directly proportional to RPM.​

Formula: CFM2 = CFM1 X (RPM2 ÷ RPM1) or RPM2 = RPM1 X (CFM2 ÷ CFM1)

What it means: As you increase fan RPM, CFM increases at a 1:1 ratio. So if you need to increase CFM by 10%, your RPM has to increase by 10%. Since it is a 1:1 ratio, we can interchange RPM for CFM in Fan Laws 2 and 3. We use Fan Law 1 all the time in the field. If we need to change the airflow, we change fan speed by changing a speed tap, VFD output, pulley diameter, or other means.

Apply it in the field: If your blower is moving 1000 CFM at 1100 RPM, and you need to decrease airflow by 10% to 900 CFM, Fan Law 1 says your RPM must decrease by 10% also. Let’s put that in the formula:

RPM2 = RPM1 X (CFM2 ÷ CFM1)

RPM2 = 1100 X ( 900 ÷ 1000)

RPM2 = 990 This is your new RPM.

We also need to understand that for us to make predictions using this fan law and fan laws 2 and 3, everything else about the air and the system needs to stay the same, including air temperature and density. System friction must also stay constant, so these fan laws cannot be used with automatic dampers that self-adjust to maintain flow.

Fan Law 2: Total Static Pressure changes with the square of CFM (or RPM).​

Formula: SP2 = SP1 X (CFM2 ÷ CFM1)² or SP2 = SP1 X (RPM2 ÷ RPM1)²

What it means: A 10% increase in CFM will result in a 21% increase in static pressure. Think about that. A small increase in airflow creates a significant increase in duct pressure. This increased pressure will be evenly distributed across components like coils and filters. So, this fan law can be applied to total static pressure or a static pressure drop across a single component in the system. That matters because some components have static pressure limitations that affect their performance. Air filters work best when they have a low-pressure drop across them. This usually means the air velocity is low enough to allow for “dwell time” through the filter material, catching more particulates. Condensate traps that are already close to their limit may have to be made deeper so that they don’t get overwhelmed. Air proving switches must be adjusted so that they do their job at the new CFM and static pressure.

Apply it in the field: At 1000 CFM, you read a 0.9″w.c. pressure drop across a media filter. You need to increase your airflow to 1200 CFM. What will be the new pressure drop?

SP2 = SP1 X (CFM2 ÷ CFM1)²

SP2 = 0.9 X (1200 ÷ 1000)²

SP2 = 1.3″ w.c.

This new pressure drop will probably be too high, according to most filter manufacturer specs that recommend less than 1″. It will perform like a dirty filter, even when brand new. The filter surface area now has to be increased. Using Fan Law 2 to predict static pressure will prevent you from creating unintended consequences by increasing airflow on a system that is already close to its limit.

Fan Law 3: Horsepower changes with the cube of CFM (or RPM)​

Formula: HP2 = HP1 X (CFM2 ÷ CFM1)³

What it means: A 10% increase in airflow results in a 33% increase in horsepower required to do that work. If your motor is already close to its rated HP, a small airflow increase can overload it. Let’s demonstrate that.

Fan Curve Charts
Manufacturers test their equipment under a variety of conditions and plot fan performance on a “Fan Curve Chart.” This is useful for predicting how the performance changes as other variables change.

Fan curve charts look different from manufacturer to manufacturer. Most look like a graph shown below. The curve represents a constant RPM for a specific model. Plot a horizontal line starting at the static pressure axis until it intersects with the curve. Then plot another line straight down to the CFM axis. This is the CFM at those conditions.

Some manufacturers add a brake horsepower (BHP) curve to this chart to show how much power is required to do the work we are asking the fan to do at a given RPM and SP. This intersection is called the operating point. When a BHP curve is added, we can determine the horsepower required by plotting a vertical line up from our operating point to intersect with the BHP curve.

Using the 3 Fan Laws with a Fan Curve Chart​

The manufacturer will always provide a “system line” representing the path the fan has to stay on as conditions around it change. Any point plotted on the chart must be along the system line. Once an operating point can be plotted on a fan curve chart at a known RPM, we can now use the 3 fan laws to predict what will happen if RPM or SP changes. CFM and horsepower will change with RPM and SP changes.
The curve for a propeller type fan is slightly different but the fan laws apply. The selection of the fan is determined by many factors including cost and operating conditions.........FYI
BOB RENTON

 

[davek]

Have been told that a 4 blade fan cools better than a fan with more blades. -- Is this true ?

Use the factory setup and you cant go wrong. JMO

 

[LowBikeMike]

Unfortunately all that fancy jargon and math didn't provide an answer.

 

[RJRENTON]

Unfortunately all that fancy jargon and math didn't provide an answer.

Sure it did...perhaps you just don't understand....search for propeller fans, select your diameter, pitch angle, operating RPM, to see static pressure and CFM....the fan laws apply....there is no yes/no answer...you have to determine conditions and requirements or are you waiting for someone to make the determination for you?.....
BOB RENTON

 

[RemCharger]

Unfortunately all that fancy jargon and math didn't provide an answer.

It's good for those looking to get into hvac manufacturing anyway.

 

[WileE]

A fan must move a certain amount of air ( CFM ) for it's intended purpose. A fan's design incorporates the blade size, pitch angle, as well as number of blades. The size will be the surface area that pushes/pulls the air molecules. It will be engineered to match what it's used for; enough for its purpose. Not too small, not too big. Let's exaggerate... imagine using a computer fan for a hemi, or a shop exhaust fan for a lawn-mower engine. Both behind a radiator that's intended to remove heat from the liquid passing through it. Mismatch. So, the design of the fan will fulfill it's purpose. All else equal, should a 5-blade fan pull more air than a 4-blade ? Should a 9-blade fan pull even more air than that 4-blade ? You then have to consider other factors, one of which is turbulence. More blades spinning at higher rpm can encounter turbulence from the other blades; which will result in lower efficiency. So, it's not as simple as 4 vs 5.

 

[Sinitro]

If U do decide to add electric fans...
Be sure to double check your electrical system capacity, to provide enough current capability.
As most fans draw significant current.

Just my $0.02...