What is an HFO?

Hydrofluoroolefins, HFOs, are a relatively new class of low global warming potential refrigerants. They are actually composed of the same chemicals found in an HFC: hydrogen, fluorine, and carbon. If you define an HFC as a chemical containing those three elements, then HFOs are actually HFCs. The difference is in how they are constructed.

Ethane

HFC 134a

 Both HFCs and HFOs start out as a hydrocarbon, containing a chain of carbon atoms surrounded by hydrogen atoms. To make a traditional HFC you replace some of the hydrogen atoms with fluorine atoms. Standard hydrocarbon molecules and traditional HFC molecules are composed exclusively of single atomic bonds. You can think of an atomic bond as a type of Velcro strip holding the atoms together. Carbon has four atomic Velcro strips while hydrogen and fluorine just have one. Single bonds just attach one strip between atoms. Using only single bonds a carbon atom will connect to four other atoms because it has four bonds. Molecules constructed this way are called saturated. They have the maximum number of atoms joined together.

Propane

HFO 1234yf

What makes HFOs different is that they use a double bond between two carbon atoms. These two caron atoms are connected together with two Velcro strips instead of just one. Since carbon atoms only have four connections, using two to connect to each other means that each carbon can only connect to two other atoms besides each other. This reduces the total number of atoms that can be connected together. This type of molecular construction is called unsaturated.

Why does this make a difference? Unsaturated molecules are far less chemically stable and tend to break down easier. Since HFOs are less chemically stable, they do not survive long in the atmosphere – and so they do far less harm than the more stable saturated HFCs. The difference is dramatic. HFC 134a has a GWP of 1430 while HFO-1234yf has a GWP of 4.

However, HFOs have a design challenge to overcome: they are mildly flammable. The very instability that reduces their GWP increases their flammability. At present, building codes in the US generally do not recognize a difference between highly flammable refrigerants and mildly flammable ones. Most building codes do not allow the use of significant amounts of flammable refrigerant inside the building. ASHRAE is working on revising their Safety Standard for Refrigeration Systems,  Standard 15. It is projected to be ready by January 2018. For more information on the work being done on flammable refrigerants check out this article in Contracting Business
http://contractingbusiness.com/refrigeration/codes-preparing-technology-refrigerant-changes

Personal Gas Safety

Time for furnace tune-ups is here. Soon you will be taking the first no-heat calls of this heating season. No doubt you will be inspecting equipment for safety concerns that affect the customer. Don’t forget to pay attention to your own personal safety. Gas leaks, leaky vent systems, incorrect combustion, and lack of combustion air can all create immediate hazards to the service tech working on the system. There are pro-active steps you can take to stay safe while working on gas fired equipment.

Gas Leaks

If there are existing gas leaks it is possible that a combustible atmosphere exists in the space. Ideally, you want to check the air in the space for combustible gas before entering. You can use an electronic combustible gas detector for this. Many techs now use these to check for gas leaks, so you might already have the tool. Just turn it on and let it sample the air in the room as you enter. There are also personal detectors which you can wear to monitor the conditions wherever you are. If you do detect combustible gas in the room, do NOT turn on or off any electrical switches and leave the room. Shut off the gas outside and ventilate the room before continuing. You will have to turn the gas back on to find the leak, but you can ventilate the room and start with a safe atmosphere while you are doing it.

Carbon Monoxide

Negative room pressure, incorrect or leaky gas venting, and lack of combustion air can all contribute to a build-up of carbon monoxide in the room. You can’t smell or see carbon monoxide, so it is especially dangerous. Again, you should test the atmosphere in the room. There are electronic detectors made to check for carbon monoxide in the air, and some are made to wear to monitor the air continuously. If the monitor indicates a build-up of CO in the room, you should shut off the gas outside the room and ventilate the room. Once the room is cleared of CO you can start to look for the cause. Before turning the gas back on make sure to check for obvious things such as blocked combustion air intakes or compromised gas vents. After restarting the furnace you should check the flame color and test the flue for proper operation. Also be sure to check the flue gasses for CO.

Gas Ignition Problems

When observing a gas furnace light, you should never place your face directly in front of the furnace. If there is a delayed ignition or flame roll-out you can be inside the fire-ball. I have made that mistake. It was over before I knew what happened. For a second all I could see was yellow because my head was in the fireball that came out of the furnace. Luckily it just singed my eyebrows a bit and gave my face a slight burn similar to a sunburn. Afterwards I worked out what had happened: a leaky gas valve had allowed a build-up of gas prior to turning the furnace on. Had my face been off to the side I would not have been in the flames when they came out.

Don’t leave your personal safety to chance. 

Take steps to detect and avoid possible personal safety hazards. It is an inconvenience to the customer to have to call 911.

Sorting Out Refrigerant Flammability

Most techs know that ASHRAE Standard 34 originally established three categories of refrigerant flammability: 1,2, and 3. They ranged from 1 – nonflammable, to 3, highly flammable. Category 2 was listed as mildly flammable, or somewhat flammable. Then they added a new category – 2L – for an even lower category of still flammable refrigerant. I admit that I always found that a bit confusing. In my mind, it either burns or it doesn’t.  In order to get a better handle on this I have done some studying.

Two characteristics are used to differentiate category 1 and category 2 refrigerants: the lower flammability limit and the heat of combustion. The lower flammability limit is the lowest percentage concentration of gas in a gas-air mixture that will ignite. Concentrations lower than the lower flammability limit will not burn. Even highly combustible gasses such as gasoline have a lower flammability limit. Refrigerants with a lower flammability limit of 3.5% or less are considered class 3, highly flammable. For comparison the lower flammability limit of gasoline is 1.4%  and propane‘s is 2.1%. Another way for a refrigerant to be considered class 3 is for its heat of combustion to equal or exceed 19 million joules per kilogram. In general terms, it does not take very much class 3 refrigerant to burn and when it does it is very hot.

Class 2  refrigerants have a lower flammability limit greater than 3.5%. It requires more than 3.5% concentration in order to ignite. Class 2 refrigerants do not burn as hot as class 3 refrigerants: their heat of combustion is lower. The lower heat of combustion is important because that is what sets other things on fire. It is possible for a class 2 refrigerant to burn without burning up everything around it.

So where does the 2L come in? Flammability class 2L is really a subclass of 2. Refrigerants with a 2L designation have a burning velocity of 10 centimeters per second or slower. The burning velocity is how fast the flame travels. A burning velocity of 10 cm/s means that the flame will travel about 4 inches in a second. In contrast, the class 2 refrigerant HFC-152a  has a burning velocity of 23 cm/sec.  – a little more than twice as fast.  Propane, a class 3 refrigerant, has a flame velocity of 39 centimeters per second – 4 times as fast.

Why is this important? The flame velocity and heat of combustion are what determine whether or not an explosion can occur. Rapid burning and high heat of combustion expand the air and combustion gasses so rapidly that great pressure is created, blowing things apart. Class 2L refrigerant cannot burn fast enough or hot enough to blow anything up. In many cases, a burning class 2L refrigerant will not even catch other combustible things around it on fire.

To summarize:
Class 3 Refrigerants have a lower flammability ratio of 3.5% or lower and/or a heat of combustion equal to or greater than 19Mj/kg. They burn fast and hot.

Class 2 Refrigerants have a lower flammability ratio exceeding 3.5%. It takes more of them to burn and they do not burn as hot as class 3 refrigerants.

Sub-Class 2L Refrigerants in addition to a lower flammability ratio exceeding 3.5% also have a flame velocity of 10 cm/sec or less. They burn slowly and without releasing as much heat.

Class 1 Refrigerants do not burn.