Education & Training
Not all PPV fans are designed the same, perform the same, or even have the same airflow type. So understanding how your fan works and what its Performance Potential is will help you accomplish your goals- to Pressurize, to Ventilate, or both.
So here’s what you to know right off the bat: The very best PPV Performance you can have is from a huge fan with huge power because it offers a big airstream, big airstream momentum, and big entrainment to pressurize and ventilate. But, battery-operated PPV fans are built smaller for taking up less space on apparatus and for easy maneuverability. A small fan gives a small airstream which means the airstream needs to do a little magic for optimal performance.
We’ll be breaking up this article into the 3 primary systems in PPV Performance: System 1: The PPV Fan itself, System 2: The Airstream it generates, and System 3: The Structure. We’ll look at the variables of each system to understand how they influence the function and performance of the fan.
System #1: The Fan Itself
The two most important components of fan performance are its Airstream Size and Power. The way a PPV fan works is that it uses power to accelerate air into an airstream. Like we said before, the very best PPV Performance you can have is from a huge fan with huge power. Based on the Airstream Size and the amount of Power applied, a fan generates a certain level of Momentum in the airstream. So Airstream Size + Power = Airstream Momentum.
The amount of Momentum a fan can induce into the airstream defines the Performance Potential of that airstream. So the bigger the airstream and the more power you can put into it gives you higher Momentum and better performance.
For example, a small fan with a small airstream and small power will provide low performance because it is limited in the amount of Momentum it can induce into the airstream. A large fan with a large airstream and big power will provide high performance because it can induce high Momentum into the airstream.
Now don’t get us wrong, most PPV fans out there on the market today are either large but with small power, small fans with small power, or even small with big power, but all are limited in performance! PPV Fans are firefighting tools, and BlowHard believes they should be designed and optimized for that purpose. But for this blog, we’re focusing fan Performance Potential within ANY airstream.
So how do we measure a fan’s airstream potential, its ability to do the work? Well, a direct measurement of the Momentum in the airstream is called Thrust. Thrust, not CFM, is the most accurate method of measurement to determine the performance potential of any fan! You’ve heard the saying “every action has an equal and opposite reaction.” Well, Thrust gives a precise measurement of the airstream momentum because IT IS the equal and opposite reaction to the Momentum the fan has induced into the airstream. Thrust measures Momentum and it’s Thrust that measures your fan’s performance!
So to recap System #1 The Fan itself: it’s is the first component in PPV performance and it creates Momentum in the airstream in response to the fan’s Airstream Size and its Power. There is a Thrust reaction against the fan in direct response to the Momentum induced into the airstream.
System #2: The Airstream
Now historically CFM or volumetric airflow measured in cubic feet per minute, has been used to gauge a fan’s performance. CFM is a key component of performance, but it isn’t everything! You’ve got to factor in the velocity or speed to truly define a fan’s performance. You see, Airstream Velocity is actually Airstream Pressure. This combination of CFM and Pressure together are the building blocks of Momentum and Thrust. These are the true performance metrics of an airstream. So let’s dig in…
We talked about how battery-operated PPV fans should be small in size to reduce storage requirements on apparatus as well as provide easy portability for reaching more places quickly. However, because of their small size, the airstream is also small and is in a compressed, high-pressure state. But that small, compressed state is not an effective, usable form when high-flow is needed for ventilation. In order to get it into a usable high-flow form, the airstream has to be able to convert into a High-Flow / High CFM state (the magic we talked about earlier). It’s got a job to do and if it can’t do it, it won’t perform optimally because the state of the airstream will define how airflow works within the structure. Let’s break this all down…
So what is the job that the airstream has to do? It’s got to increase in CFM by Entraining air! Which means that as it travels, it pulls air into it from the surrounding environment and as a result, gets larger. So, the airstream actually changes state as it travels. The 1st state is right where it exits the fan and the 2nd one is anywhere downstream after it’s grown.
Now remember in System #1, the fan itself, it created Momentum from its Airstream Size and Power. For System #2, after it has left the fan, it’s the combination of CFM and Pressure that makes up the Airstream Momentum. So you can’t gauge a fan’s performance simply on a CFM number because you can have a fan that has high CFM and very little Pressure-it can’t do the work. You’ve got to factor in both CFM and Pressure to know its Momentum– the fan’s true performance, and we can directly measure Momentum by its opposite reaction which is Thrust.
Momentum and Thrust are the heart and soul of PPV Performance! Fans which generate high Momentum airstreams will be capable of Entraining more air to generate higher CFM, and more importantly, will maintain higher pressure when expanded!
Let’s look at those two Airstream Momentum States. State #1 is right when it exits the fan. At this point, it hasn’t entrained or pulled any air in yet. It is tightly condensed, tightly compressed, and has High Pressure. So, Airstream Momentum State #1 = High Pressure, Low CFM.
Airstream Momentium Sate #2 is anywhere down the airstream. Now it’s traveling, Entraining air and growing bigger. This state = Lower Pressure, High CFM. Remember we said that the airflow has to convert to a High-Flow state to get to high-CFM? We’ll it’s the effective and efficient Entrainment that will get it to transition.
What’s important to note is that an airstream with high Momentum (CFM and Pressure) will maintain that increased Pressure when it gets to the expanded state, which now means it has increased potential to DO THE WORK. A second point to note is that different fan designs will produce airstreams with higher or lower rates of entrainment. Some fans will have limited air entrainment like a straight jet because they haven’t expanded by the time they get to the door. Some utilize modes of entrainment, like the cone which have over expanded, resulting in reduced pressure in the airstream. Be sure to watch our short video on the BlowHard High-Flow expansion Jetstream.
So to recap System #2 The Airstream: the 2 primary factors of airstream performance are Momentum, and Entrainment. Momentum is the driving force within the airstream and Entrainment allows it to transition into different CFM & Pressure states.
System #3: The Structure
Every structure and situation are different. Doors are different sizes, buildings are different sizes, and so the pressure and CFM requirements are also going to be different. In order to generate the best outcome and maximize fan performance relative to the structure, it is important to understand those situations and to have both the proper fan and proper fan placement. Are you needing to pressurize, ventilate, or both?
A large part of performance is a fan’s ability to seal the air into the structure with its airstream. This is accomplished by expanding the airstream to match the size of the door as much as possible to limit the amount of air that can escape.
When using a fan to pressurize or ventilate a structure, you have to take into account both the structure inlet and outlet openings and choose the correct airstream state that we discussed.
- Momentum State #1: Condensed (High Pressure, low CFM) is used with the fan closer to the door when high pressure is needed. To increase pressure in the structure, outlet openings would need to be limited. In this case you would also need an air curtain to contain the pressure in the structure. An example of a high-pressure scenario is when needing to overpressure a region of a building to prevent smoke from entering and to provide an escape route for victims.
- Momentum state #2 Expanded (High Flow/CFM, Low Pressure) is most commonly used, and works best with the fan being set back from the door to entrain air and generate high volumetric flow/high CFM. To increase flow in the structure, outlet openings need to be increased. Examples of scenarios requiring High Flow/CFM are clearing smoke for visibility and providing fresh air, and to cool down a structure.
- Some circumstances will require both Pressurizing and High Flow. These cases would require a large fan with high power for extreme performance. A scenario requiring both High Pressure and High Flow/CFM are clearing smoke for visibility in larger and taller structures with restricted exit points, and for PPA attack.
To recap all of this information: fans with large airstreams and high power will produce high Momentum in the airstream. Airstreams can utilize either their High-Pressure state or leverage the Entrainment process to expand into a High-Volumetric Flow/CFM state. Your PPV ventilation performance will be highly influenced by your fan size, its power, its entrainment ability, how you set up your fan, and the configuration of the structure.
That was a lot of information but still just skimmed the surface of PPV fan performance. To see this information in video form with visuals, check out our YouTube channel: www.youtube.com/@BlowHardFans. Please reach out if you have any questions or want more information or clarification. info@blowhardfans.com
As always, take care and stay safe!
The three most important components of PPV Fan Performance is Power, Airstream Size, and Entrainment. So let’s talk about the Entrainment piece and what it is exactly. Well, you know that when a fan blows air, it creates its own an airstream. The entrainment process happens when as that airstream travels, it picks up more air grows bigger, and that volume of air, or CFM, increases. This entrainment process is super important because in order to provide high-volumetric flow (CFM) for proper ventilation, the airstream has to convert from its small, high-velocity, high pressure sate, into a high volumetric, high CFM state. So the airstream has a job to do and if it can’t do it, it cannot and will not perform optimally.
There are three different types of Entrainment. Some types are more rapid than others and some are more efficient and effective than others, giving a fan better performance. Now these types of entrainment are characteristics of specific types of airstreams: the cone, the jet, or the BlowHard High-Flow Jet that we discuss in another video. It is important to note that a fan’s given airstream type and it’s entrainment abilities are set by the fan’s design and there’s nothing you can to do in the field to change it or make it better other than moving the fan further back from the door and that is not efficient or in some cases even possible.
So the first type of air entrainment is called Venturi –This entrainment process is similar to that of air going over the wing of an airplane, where the high velocity air in the airstream creates low pressure within it. So that low pressure in the airstream actually pulls in air from the surrounding environment; literally sucking it into air into the airstream, making it grow bigger. Venturi Entrainment is super-efficient because it does not directly induce turbulence!
The second type of entrainment is Viscous Drag and it’s just like it sounds. It’s initiated from the friction between the outer surface of the Jetstream and the environment. The Viscous Drag entrainment process happens when the speed of the air outside of the airstream pulls, or drags, the surrounding, stationary air along with it. Now because viscus drag pulls surrounding air along with the airstream, that now entrained air will have a directional flow with the airstream. A fan which has an airstream that can aggressively utilize both the Venturi and viscous entrainment in combination will have rapid entrainment, making it both effective and efficient!
The last type of Entrainment is Turbulent Mixing. This is where turbulence in the airstream mixes with the environment and is the classic “cone” effect historically used with PPVs. The chaos of the turbulence in the flow causes aggressive mixing with the environmental air, causing it to get caught up in the airstream. This NON-directional, chaotic flow is inefficient because it has a limited amount of air traveling in the direction of the airstream. This results in a lower percentage of air that’s able to drive into the structure.
So to sum it up, The type of entrainment a fan is designed with highly impacts it’s performance because if the airstream is not able to do its job and entrain air fast enough, the airstream will not transition from that high pressure state into a high flow or high CFM state which imperative for effective ventilation.
Be sure to check out the follow-up video or write-up on the Rate of Entrainment: Cone, Jet and BlowHard High-Flow Expansion Jet Airstreams, because they all have these different types of Entrainment we talked about. Be sure to subscribe to our YouTube channel to stay up to date! Thanks for reading, and always, take care and stay safe!
PPV Airstream Types-Pros and Cons
In a previous article/video we discussed the 3 different ways airstream entrain air: Venturi, Viscus Drag and Turbulent Mixing. Here we will explain how those entrainment types correlate with the 3 different types of airstreams that a fan creates: the Cone, the Jet, and the BlowHard High-Flow Expansion Jet along with their pros and cons.
Let’s start with the historical Cone airstream. This is an airstream which uses Turbulent Mixing Entrainment.
This type of entrainment occurs when the chaos of the turbulence in the flow causes aggressive mixing with the environmental air, causing it to get caught up in the airstream. Cone airstreams actually have the fastest rate of entrainment of all the airstreams. This means they can expand quickly to fill large openings and seal the structure within a short distance. So that’s a plus for the cone and why it was thought at the time to be efficient.
But looking at the physics of it, here are the cons: The Turbulent Mixing is induced through “tripping” the airstream right when it exits the fan and is often achieved with a wire grill. This tripping effect induces micro turbulence throughout the airstream. This requires a significant amount of power which is lost by having to push air through the grill, reducing the airstream’s Momentum or Pressure. So, a cone may achieve high CFM by having rapid entrainment but wastes so much power to do so, that by the time the airstream gets to the structure it has little pressure within that volume of air. Because of the turbulent, chaotic, non-directional flow of the cone airstream, it has a limited amount of air traveling in the direction of the airstream which reduces the percentage of air that can drive into a structure for ventilation. Because the cone keeps getting wider and wider, there is a very small window of placement of the fan, and a firefighter must be very accurate in order to achieve optimal performance. Now, Cone airstreams can still be very useful but are best used with gasoline engines where high power can offset the inefficiencies.
After the Cone came the Straight Jet and has been heavily adopted into battery operated fans. This is a super compressed, high-velocity, high pressure airstream that maintains its shape over a long distance. The high speed of the Jet airstream is great at generating high pressure in structures through small openings. This was thought to be the new, great thing in PPV, but the main drawback to the Straight Jet is that this airstream has a very low rate of entrainment and limited expansion. This means the airstream has difficulty transitioning from its compact, high-velocity state into a usable, high CFM/high-volumetric flow state in order to fully seal a doorway. So, for pressurizing you would need to use an air curtain or will experience significant blow back out of the structure. Because it holds that narrow shape and doesn’t expand, then like in many AMCA tests, fans that have a Straight Jet require extreme setback distances in an attempt to entrainment air and gain CFM, but the effort is still limited. For high flow air movement, Straight Jets are not effective and long setback make them unrealistic in the field. With these long setback distances, the Straight Jet turns into a Semi-Cone inducing turbulent mixing, making the airstream gusty and chaotic. Ultimately, because sufficient entrainment is not achieved, the full Momentum potential of the airstream cannot be utilized. Another big challenge with the long setback of the Straight Jet is that if there is any kind of cross wind blowing, it easily pushes the airstream off course making it difficult to maintain the accurate targeted doorway.
Last is the Expansion Jet: Created by BlowHard, High-Flow Jet is an Expansion Jetstream engineered to utilize the most efficient modes of entrainment of both Viscus Drag and Venturi, which gives it an aggressive rate of entrainment. This one-two punch keeps the power and directional flow of a jet but quickly expands the airstream to a high CFM state without over entraining or creating turbulence. This gives the airstream the ability to both seal the door yet also keeps the Momentum and Pressure within it providing high performance. The efficiency of the entrainment also allows for an optimal placement of the fan for real-world situations and is less likely to be impacted by cross wind effects.
To recap: Cones rapidly entrain to quickly seal the door but the wasted power through the grills and the non-directional turbulence results in low efficiency and performance. Straight Jets are great for high-pressure situations if you have an air curtain. However, effective ventilation is difficult because they lack the ability to entrain air and transition to high flow unless set far back far from the door, ultimately reducing performance. Last is the BlowHard Expansion Jet that that starts out small like a straight jet but then expands quickly through aggressive entrainment to seal the door within an optimal working distance. It extracts Momentum so that by the time it makes it through the door, it has still maintained that Momentum and ability to do the work required to ventilate or pressurize.
So, there you have it! If you know what you’ve got and what you’re working with, you’ll know how to best use your tool! To keep up on cutting-edge knowledge and technology, make sure to subscribe to our YouTube channel and as always, take care and stay safe!
