By Raul Angulo
There is a difference between positive pressure ventilation (PPV) and positive pressure attack (PPA). Both involve controlling the interior flow paths to your advantage, and both include the usage of high-powered 15,000-20,000 cubic feet per minute (cfm) blowers. If your fire department uses PPA or PPV or both, you’re probably familiar with a blower running at full throttle bouncing away from the door opening from its vibration. I call this the “dancing fan.”
I want to quickly address two issues to fend off the critics. First, to the purists out there, the correct term for these high-volume, gas or electric, air-moving machines is blower. A fan refers to the old smoke ejectors that were popular in the 1970s and 1980s–it’s all we had for mechanical ventilation back then. But like a 2 ½-gallon, water pressurized, portable fire extinguisher is referred to as “the can” (for brevity), the PPV blower and the smoke ejector are referred to as “the fan.” We all know our department cultural routines and nomenclature so there shouldn’t be any confusion when the terms are used interchangeably.
The important difference to understand is that blowers positioned on the exterior of a structure create a higher interior atmospheric pressure (blowing in), where smoke ejectors are usually positioned on the inside and create a negative interior atmospheric pressure (blowing out). But they’re still just fans.
The second issue is, many might say what I’m about to present is a trick of the trade, not a drill. And I always reply, “yes.” Come on! It’s all in how you present the information–you can make a drill out of anything. To me, a drill is a more comprehensive approach to sharing a vital piece of information attached to other teaching points as opposed to simply offering a piece of advice. So here is the drill scenario.
Positive Pressure Ventilation and the Fans
Start by going over the basics of the fan, including maintenance procedures. See if your firefighters know the make and model as well as the cfm capability. Then go over the placement of the fan on the fireground. The optimum placement for a PPV fan depends on the size of the door opening, the area of the landing (like a front porch), and the size of the fan.
The first objective is to seal the opening of the door with a cone of air. Ideally, a 15,000-20,000 cfm PPV fan should be placed approximately 8-10 feet from the door. However, small porches and stairs can make the ideal place impossible, so do the best you can. The PPV fan can be tilted back approximately 20-30 degrees to aim the center of the air cone towards the center of the door opening. Some ventilation is better than no ventilation. But pose the question to your crew: “How can we still create an effective positive pressure ventilation profile given the design of the structure?” See what they come up with. Multiple PPV fans can be set up in series or next to each other to increase the volume of air directed at the door. Here’s another hint–if you think that the fan needs to be set up in front of a door, your options may be limited. If you think the fan can be set up in an opening, you might have a few more creative options.
The second objective is to ensure that there is an adequate-sized exhaust port (doors or windows) for the smoke and hot fire gases. An adequate size means approximately one to three times the size of the ventilation opening. PPV fans can increase the volume of air movement 20 times more than thermal air convection currents generated by the fire. The goal is to effectively horizontally ventilate in a relatively short time. Interior smoke conditions should immediately begin to improve, otherwise entry and exhaust ports may have to be adjusted.
A Tactical Safety Note: Let’s say the PPV fan is set up at the front door on the A side of the building and the exhaust port is a window on the C side of the building–it is always a good idea to have a charged hose line on the exterior C side. Often the hot smoke venting from the exhaust port finds the right air mixture at the window and flames can come roaring out like a blowtorch. It’s quite impressive. However, the siding and soffits on the C side are being exposed to radiant heat and possible direct flame contact. A firefighter needs to flow water above and to each side of the exhaust window and soffits to keep the fire from spreading or entering the attic space. The firefighter is protecting the exposure by keeping surface areas wet. The firefighter should not direct the hose stream back into the window. Allow the fire to vent.
Back to the fans–if your fire department isn’t a fan of PPV fans, stand by. The dangers related to vertical ventilation roof operations combined with lightweight truss construction and synthetic fuels are causing many department to switch to PPV for sheer risk-benefit management. Regardless, the fans get a lot of use and abuse. Even with wheels and handles, the fans aren’t light. Depending on the height of the storage compartment on the apparatus, the fans are constantly being forcefully pushed, pulled, bumped, yanked, bounced, and dropped with countless occasions to remove the fans from the rig and then put them back on. This doesn’t include all the abuse they take on a rugged fireground. After a while, a tread on the wheels starts to wear out and the rubber boots that are on the front leg posts begin to chip, crack, disintegrate, and then fall apart. After a while, the wheel locks either lose their strength or the wheels are too smooth for the brake clamp to grip. Then, when the fan is running at full throttle, you get…the dancing PPV fan!
The dancing PPV fan happens when the engine is running full throttle and the fan is on concrete or some other smooth surface. The vibration from the engine and the backward thrust from the blades place strong centrifugal forces in motion. If the rubber boots and the wheel lock are not strong enough to hold the fan in place with friction and traction at the contact points, the fans will start to vibrate and bounce in a clockwise position. If this motion goes un-noticed, eventually the fan will turn away from the target opening, which completely defeats its purpose. The seal will be lost, changing the ventilation profile and immediately change the flow path. This can have disastrous results, especially in a PPA scenario. In PPA, you want the wind at your back. This is always the safest way to attack a fire. The PPV fan creates this wind-at-your-back flow path; so if the fan bounces out of position, the flow path can reverse itself, sending the hot smoke and fire back towards the entry attack team.
Since the fan is usually is placed at the entry opening that firefighters are also going to use, it can also get bumped out of position by an advancing hoseline, getting bumped by tools and other equipment, or by firefighters tripping over the fire hoses entering the door space. Unfortunately, hose, tools, firefighters, and the fans are all competing for the same entry opening.
Raul Angulo and company demonstrate the “dancing fan” effect.
Raul Angulo and company demonstrate a solution to the problem of the “dancing fan.”
The Solution to Dancing Fans
One solution, and the best solution, is to keep one firefighter posted at the front door with the primary responsibility of manning the fan. However, fire departments with limited staffing may not be able to spare one when other critical tasks still need to be performed.
I asked my guys on Ladder 6 to come up with a solution. After a few experiments that were successful but impractical, here’s what we came up with:
If you steal two four-inch hose gaskets from engine company’s workbench and place one underneath each wheel of the PPV fan, it acts like a mini-wheel chock. There is enough thickness in a four-inch gasket that it acts like a curb or a wedge and keeps the wheel in place. For more powerful fans like the 20,000 cfm fans, we had to double-up on the four-inch gaskets. We stacked two gaskets together and secured them with electrical tape. That definitely provided enough of an edge that locked the wheels in place and, voila, we solved the problem of dancing fans. Another member suggested we tie the gaskets to the engine frame so they would always be there…another good idea.
Try this Ladder 6 common sense solution next time you get dancing fans. You’ll see…it really works!
Positive Pressure Attack for Ventilation & Firefighting, By Kriss Garcia, Reinhard Kauffmann, and Ray Schelble (Fire Engineering, 2006)
RAUL A. ANGULO is Captain Emeritus of Ladder Co. 6 and retired from the Seattle (WA) Fire Department with over 35 years of dedicated service. He is an international author and instructor and serves on the advisory board for Fire Apparatus and Emergency Equipment magazine.