At FDIC this year I will be presenting a classroom session titled Ventilation Principles and Practices on Thursday 4/19 from 1330 to 1515 in Rooms 134/135. Below is a preview of some of the information I will be covering. Choosing a class to attend from the schedule is very difficult due to the quality of programming and sheer number of sessions. If you are unable to attend FDIC this year or you choose another session I would still like to make my information available to you as a resource. Fire Engineering goes to great lengths to ensure our profession is furthered through the sharing of information and that is not limited to the pages of the magazine or the week in Indianapolis. Take a look at the write up, consider seeing more on Thursday April 19th at FDIC and contact me through the community if I can help you or your department further.
When it comes to the topic of ventilation the fire service tends to get drawn into the how’s of the tactic. Ways to cut holes, tying off haligans, and CFM ratings on fans. If we spend more time looking at what we are actually using to vent fire and smoke our understanding of the tactic is expanded and our decision making process is simplified. Consider the fact that inside an enclosed structure the fire creates heat which builds pressure while consuming oxygen and property.
Of all the different techniques of ventilation there are only 3 methods. The ventilation tactic of cutting a h*** over a room and contents fire is using heat to ventilate the structure. Taking a window to vent horizontally is using pressure differentials. When we cannot take advantage of fire behavior to ventilate we need to employ a mechanical means (fan) to overpower natural fire behavior (heat and pressure) in order to vent.
When we start to consider our ventilation options we should take into account our odds of success. The “hierarchy of ventilation” reinforces the thought process of using the most reliable method with the least variables first and using the method rooted in overpowering Mother Nature with the most variables as a last resort.
Heat is a fire’s greatest strength and most predictable attribute, it wants to go up. Using the thermal column to ventilate fires has allowed us to live and work around fires in enclosed spaces for thousands of years. Because of the simplicity and reliability of the chimney effect, using heat to ventilate should be the first option considered.
Here we have an excellent example of vertically ventilating a room and contents fire to utilize the chimney effect. In the first picture it is clear the fire is confined to an interior room and not in the attic. Smoke is present but it is easily influenced by the breeze and is “lazily” pushing out of the roof. Using a pike pole the firefighter punches in the “lid” of the room providing all the stored up heat and energy a direct path up and out.
Out of that ventilation opening a significant amount of fire/heat is being released. With that there is very little if any smoke. Inside that room conditions have without a doubt improved tenfold. The particulate and increased toxic gasses which are more prevalent in incomplete combustion are being burned and fresh air is being entrained in to that room at an adequate rate. One last note is due to the fact that the ventilation has such a direct route up and out this tactic requires very little direct coordination with interior crews.
Using the innate need for heat to rise as a ventilation method is by far the most reliable. When we open up an enclosed fire over the top we stand to mitigate a long list of problems with one tactic. Vertical ventilation and using the desire for heat to rise also takes our ventilation opening out of our plane of operations. Remember we as firefighters operate horizontally in structures so venting horizontally puts us at greater risk than venting vertically. It is understood there is certainly more to consider in the operation, for one getting over the fire is not always an option, however the focus of this post is association of fire behavior with ventilation practices.
We already covered the fact that heat wants to rise and how we use that to our benefit. When we release the heat from an enclosed space out a horizontal ventilation opening we are taking advantage of the higher interior pressure the fire has built up through the heating of the air and breakdown of solids to gas. Exterior atmospheric pressure is 14.7 which will be lower than the interior pressure of an enclosed structure which has had it's atmosphere heated. Additionally when solids are broken down into gasses through combustion the volume is expanded furthering the increase of interior pressure. We know from filling our SCBAs on cascade systems that gas pressures seek to equalize with higher pressures traveling to low developing a pressure gradient and "Flow Path".
The high pressure environment of an enclosed fire area wants to seek out lower pressures almost as bad as the heat wants to rise. This is why when we open the door to a house with a fire in it we need to expect that conditions will change. Unless a wind is present that change will occur in our direction as the exterior will be the consistent atmospheric pressure. Many people claim that the reason conditions change and we see the smoke and fire show at the front door just after opening is because “it needs oxygen”. Yes and no. The pressure gradient brought that nasty smoke and heat from the fire area to the front door, not a thirst for oxygen. When we open a door to a fire room, fresh air is entrained along the floor and built up heat and smoke (pressure) moves out at the “lid” seeking areas of lower pressure. When those byproducts of incomplete combustion move into areas that have more oxygen available it may or may not still be hot enough to combust. This flash or ignition is secondary to the driving force (pressure) that is moving the smoke. For greater detail on flow paths and pressure differentials consult the UL study.
Capitalizing on pressure differentials to ventilate structures is not just for fires contained in the occupancy. Attic fires are very difficult for many reasons recognition of the high pressure environment may save your life. One of the greatest challenges of attic fires is fire location. Due to the lack of windows and the fact that it is an uncompartmentalized space, the odds of cutting a h*** over the seat of the fire is like finding a needle in a hay stack. So when an attic space is vented the main goal is to relieve the pressure and prevent the catastrophic event rather than seeking to create the thermal column.
Think about what we are taught to do when we identify a back draft situation. The textbook answer is to vertically ventilate before initiating any interior operations. An attic fire is a back draft / smoke explosion in the making. There are countless reports of firefighters killed or injured in smoke explosions every year. The most common report is that the firefighter entered the structure and upon pulling ceiling a smoke explosion occurred. Why? Because the pulled ceiling became the ventilation opening for all the heat and pressure built up in that confined space. By being more disciplined on these fires we can prevent these cases by choosing to ventilate known attic fires to the exterior first, directing everything up and out.
Why does mechanical ventilation end up last on the decision tree? If we are using mechanical ventilation we have made a decision that we need a machine to overpower natural fire behavior (the heat and pressure). In this framing it should make us all a little more cautious.
Of course, there are situations where we must rely on mechanical ventilation. So we need training, education, and plans to put the tactic to use. We never employ the tactic to compensate; we employ the tactic because the situation calls for it.
When it comes to mechanical ventilation the tactic can be divided up in to 3 areas.
We found through testing in our district (5-10 story occupancies) that our new electric fans with a CFM rating of 14,000 at 100% power can pressurize most stairwells in the area of 1 minute. While this finding is impressive we must keep in mind that loosing that pressurization will result in an instantaneous return to prior conditions. This is no different to loosing battery on our thermal imaging camera, technology is only effective when it is working. Consider that when you watch this video from NIST and observe how quickly the fan provides enough of a pressure differential to prevent smoke movement into the stairwell from the fire floor.
Zone pressurization is very similar to and may include stairwell pressurization. The way it works is air coming into the zone is continued while return dampers are shut increasing the pressure in that area. This creates a pressure barrier to prevent smoke travel into these areas. The specifics and capabilities of this tactic depend on the building and the system. Typically a zone is an entire floor, stairwell or elevator shaft. Some of the more modern systems at larger malls can actually pressurize separate zones on the same floor. In most departments this type of operation is only performed with the resource of a building engineer.
PPV can be used to support a positive pressure attack which has specific criteria and requires precision coordination. PPV can also be used for smoke removal post control which is by enlarge the common place. Click on the slide below to review some of the considerations the Chiefs Garcia and Kauffmann presented in Fire Chief Magazine following PPV related firefighter injuries.
Nationally there is a move away from “fan and a line to the front door”. With the greater understanding that years of experience has brought us PPV is now primarily a post control ventilation method. The key reasons for this change are as follows.
Our last method mechanical ventilation to discuss is hydraulic ventilation. Tried, true, and simple. While hydraulic ventilation won’t clear out a big box structure the CFM that it will move is more than adequate for post control room clearing or salvage venting.
This article is a presentation of a thought process and organizational tool from the FDIC classroom session I will be presenting again this year. When presented with a structure fire the size-up process begins, this is just one component. When we can use fire’s greatest strength and most predicable attribute against itself with vertical ventilation and the release of heat we should. In some organizations even if a top floor room and contents fire presents itself perfectly, staffing or policy may prevent the option of vertical ventilation. To the counter, regardless of staffing and conditions the application of mechanical ventilation requires going the extra mile on coordination and planning. The Hierarchy of ventilation may be a new frame however, the science, practice and information presented here can be found in any number of sources between our two coasts and beyond. Through the most recent UL study and one which will begin soon on vertical ventilation, we have been provided with means for more informed decision making. Unfortunately the study only information and it is incumbent on you to make the time to review it and create these processes for yourself.