Firefighting with Refrigeration

IIAR has had much success in the past 15 years in the model code arena, updating codes to more appropriately regulate ammonia refrigeration. I’m sometimes asked how we were able to sway the opinions of so many fire safety regulators to support approval of IIAR proposals, and the answer, in a word, is “education.”

There’s an unwritten rule in the code development and code enforcement business that is sometimes applied to ammonia refrigeration and other complex topics: over-regulate what you don’t understand. With this in mind, it makes sense that, to reduce the burden of unjustified regulations, IIAR needed to educate inspectors and emergency responders to better understand how ammonia refrigeration systems work. Code requirements for firefighter emergency control boxes serve as a good example of how this approach was effectively used to motivate a regulatory change.

On the surface, it might seem to be a daunting challenge to explain – to a firefighter, a fire inspector, or for that matter, anyone who has no education on the subject of mechanical refrigeration – how a refrigeration system works. However, firefighters and fire inspectors, who often come from firefighting ranks, actually know a lot more about the concept of refrigeration than they might realize. Firefighters use refrigeration every time they use water to extinguish a fire. In fact, we could coin a new firefighter job title that conveys a more superhero-like status, “The Evaporator.” Let me explain . . .

Fundamentally, the role of a firefighter in extinguishing a room or compartment fire is all about absorbing the heat that is generated. As contents burn, heat is released into the compartment, and the compartment temperature will increase exponentially, often exceeding 1,000 degrees Fahrenheit in just a few minutes. Exposure to these conditions leads to progressive ignition of nearby combustibles, causing the fire to grow.

To stop this progression, firefighters use water deployed from fire hoses. A common initial attack technique taught in recruit school is discharging water through a fog nozzle, which breaks the discharge into small droplets that are “stirred” into the compartment as the nozzle is moved about by a firefighter.

The primary objective of this technique is absorbing heat, which in turn lowers the air temperature in the compartment. Firefighters are taught in recruit school that small droplets produced by a nozzle set to a fog pattern have a high surface-to-mass ratio, which enhances the efficiency of heat transfer over what can be achieved by setting the nozzle to a solid stream setting. Firefighters also learn that the process of absorbing heat by converting water to steam takes advantage of the latent heat of vaporization, which is the amount of heat required to change a liquid to a vapor at the liquid’s boiling point. In the case of water, a pound of liquid (roughly 16 fl uid ounces) captures 1 BTU for each 1 degree Fahrenheit increase in temperature. However, when the temperature reaches the boiling point of 212 degrees, 970 BTUs are required (the latent heat of vaporization) for each pound of water to accomplish the phase change from liquid to steam while the temperature remains at 212 degrees. It’s clear from these values that the process of changing water to steam is far more efficient in capturing heat than simply heating water in the liquid phase. I’m obliged to point out that some fire departments choose to use solid streams rather than fog patterns for initial fire attack, espousing the benefits of reduced risk of steam burns to firefighters and more effective extinguishment by initially putting water directly onto whatever is burning. The merits of one approach versus the other continue to be a topic of spirited debate in firefighting circles.

By now, if you understand how ammonia refrigeration works, you should be seeing some parallels between ammonia refrigeration and firefighting. In the case of firefighting, water can be used as a refrigerant because compartment fires are hot! Capturing the latent heat of vaporization by converting water to steam at 212 degrees provides a satisfactory level of refrigeration to reduce compartment temperatures below the auto-ignition temperature for ordinary combustibles, which helps to stop a fire from spreading.

In contrast, water doesn’t typically make a good industrial refrigerant because the heat transfer properties are poor at temperatures below the boiling point (latent heat of vaporization can’t be utilized for heat transfer if you’re below the temperature at which the liquid-to-vapor phase change occurs). Ammonia, on the other hand, facilitates much lower temperatures because ammonia boils at -28 degrees Fahrenheit. Accordingly, the latent heat of vaporization for ammonia, which is less than water, at 589 BTUs per pound, but is still orders of magnitude better than most other refrigerants – can be utilized all the way down to -28 degrees, at atmospheric pressure. In a refrigeration system, the evaporation of liquid ammonia to vapor ammonia typically takes place in an evaporator.

The parallels between ammonia refrigeration and firefighting actually run even deeper. For example:

  • the water carried on a fire truck can be compared to liquid ammonia in an accumulator;
  • the pump on a fire truck can be compared to the pump on a liquid ammonia recirculation system;
  • the fire hose can be compared to liquid refrigerant supply piping;
  • a fog nozzle can be compared to expansion valve, and;
  • the firefighter using a fog nozzle essentially plays the role of an evaporator.

Theoretically, if a burning compartment were tented and drained in such a way that evaporated water could be condensed, captured and transported back to the intake on a fire truck, along with runoff, the process of firefighters putting out a fire would equate to a closed refrigeration system.

With all of this in mind, you now know that we in the ammonia refrigeration industry do, in fact, have a lot in common with firefighters. We’re all in the refrigeration business, but we use refrigeration for different purposes. Accordingly, when the crew from your local fi re station shows up at your facility to do a pre-plan, or a local fi re inspector shows up to do an inspection, you’ll now be able to “feel the love” of a common cause. Escort them to an evaporator, point, and explain to them that they’re a lot like that big box hanging off the ceiling. I bet that will be a great conversation starter!