Fun with Hydrocarbons
I was going to call this post „Playing With Fire“ but didn’t for obvious reasons. Anyways, in my last post we saw how CO2 (R744) can be used in both a traditional sub-critical as well as in trans-critical cycles which operate almost backwards to each other. Also, when I typed in trans-critical without the hyphen, Spell Wreck asked me if I meant to type ‚transatlantic‘ instead. Um no, but it did remind me of a little bit of history to touch on before we move onto R290. I apologize for the digression but I think you will find it interesting.
Back in the day, several decades before CFC’s made the scene, CO2 was the big player on the block. Refrigeration was still in it’s infancy and the cold chain that we rely on today just didn’t exist. If we go back 20 years earlier still, refrigeration occurred between December and March when the ice on the lakes was thick enough to harvest and then store for the rest of the year. I kid you not. So even though we may wonder how life existed without a bar fridge in the basement, the advent of early, large scale refrigeration was nothing short of dramatic.
Think about it. Your body does not make vitamin C. Combine this with the fact that the other critical vitamins that we require are only found in fruits and vegetables available for a short period in the year, and it is easy to see why malnutrition was prevalent in the northern hemisphere during winter. When your grandfather tells you he received an orange for Christmas, you can understand why he would have been excited.
Refrigerated ships using R744 allowed for the transportation of citrus fruits and other essential foodstuffs from across the globe to be delivered fresh to areas that would not otherwise see them for months. From a technical standpoint this was amazing, as the on board engineers did not have a long history of operating characteristics to go on and it seemed they were essentially going by the seat of their pants in many ways. A good example of this was when they entered warmer tropical waters and the sea water temperature jumped. They relied on this water to cool the condensers of the refrigeration system as the system was operating in a traditional sub-critical cycle, which makes sense when you consider the temperature of the northern Atlantic. However, when the water temperature rose further south they found that their capacity started to drop rapidly. Apparently somebody „discovered“ that if they partially closed the valve leaving the condenser their capacity suddenly went up, which was a lot better than the alternative, which was their ship went down. Now to the engineers, this made no sense what so ever so it was duly noted in the ships log and can be seen to this day as an example of a big discovery occurring that nobody had the foggiest idea of what it was. Today we know of course that the system was tipping over from a sub-critical into a trans-critical operating mode. The condenser was now operating as a gas cooler so when the valve was closed, it dropped the pressure of the vapor causing it to condense and supply liquid to the evaporator. Apparently the on board engineers did not know this, but it worked and that was all that mattered 8 days from shore.
Getting back to R290, we can put the whole idea of trans-critical cycles away for a bit as it operates just like every other refrigerant we are familiar with, only better. In fact, from a performance standpoint, it has got every CFC, HCFC and HFC beaten hands down. To prove the point, let’s take a look at the pressure enthalpy chart for several traditional refrigerants and compare them with R290. Now I can’t actually post the images here, but if you pull up the diagrams, you can follow along.
Let’s focus on how much heat the different refrigerants will absorb in the evaporator (Net Refrigeration Effect or NRE) at the following conditions:
- HBP 45F° Evaporator, 110 F° condensing, 100F° Liquid, 10F° super heat
- MBP 20F° Evaporator, 110F° condensing, 100F° liquid, 10F° super heat
- LBP -10F° Evaporator, 110F° condensing, 100F° liquid, 10F° super heat
By plotting the conditions, you can see that the NRE for R-290 at HBP conditions is 207-94 = 113 btu/lb, at MBP conditions it is 201-94 = 107 btu/lb and finally, at LBP conditions is 196-94 = 102 btu/lb.
With R22 for HBP we get 110-41 = 69 btu/lb, 107-41 = 66 btu/lb at MBP and for LBP we have 105-41 = 64 btu/lb.
Moving on to R404A, HBP = 98- 46 = 52 btu/lb, MBP = 94- 46 = 48 btu/lb and 91- 46 = 45 btu/lb at LBP.
So if we were to look at it from a strictly evaporator NRE perspective, R290 is more than twice as efficient as R404A. This is demonstrated by the mass circulation rate required for a given capacity calculated below.
Using the MBP example, the circulation rates in lbs./min per ton of refrigeration for the 3 refrigerants are as follows:
- R290 =12000 btu/hr /60 = 200 btu/min /107 btu/lb = 1.87 lbs./min
- R22 = 12000 btu/hr/60 = 200 btu/min / 66 btu/lb = 3.03 lbs./min
- R404A =12000 btu/hr/60 = 200 btu/min/ 48 btu/lb = 4.17 lbs./min
Of course there is more to it as real world systems require hardware that can compress refrigerants and pump them around the system. Being a hydrocarbon which has a low density works against R290 as it requires a greater volume to be moved through the compressor which reduces how much capacity a particular compressor can have.
Looking at it from this perspective, we have the following:
The vapor specific density for each refrigerant is:
- R290 = 1.89 ft3/lb
- R22 = .935 ft3/lb
- R404A = .649 ft3/lb
What this means is that, even though only half the mass of R290 is required to absorb the same amount of heat as R404A, the R290 is only a third as dense. In other words, it takes up 3 times the volume for a given mass. When we combine the 2 properties (By multiplying the mass flow by the vapor specific density) we can calculate the swept volume of the compressor, which is a fancy way of stating the displacement required to have equal capacity.
Compressor displacement is:
- R290 = 3.53 ft3/min
- R22 = 2.83ft3/min
- R404A = 2.70ft3/min
What this means is that to have the same capacity at this condition, the R290 compressor requires a displacement that is ~ 35% larger. We see a similar result if we compare R134a with R404A. R134a is more efficient at absorbing heat, but it requires a much greater displacement compressor to achieve similar capacities but in this case, it is due to the lower pressure that R134A operates under. However, R290 has more up its sleeve which makes it even more favorable as a refrigerant.
If we look at the R134a and R404A example above, the biggest thing R134a has against it for use in low temperature applications is it’s low vapor pressure. At -20F/-29C, its pressure and resulting vapor density is so low, that the capacity for a given compressor falls off the chart. The low mass of returning refrigerant also makes cooling the compressor more of a challenge. That is why we do not see R134a used in these applications even though it is more efficient than R404A.
R290 on the other hand has the best of both worlds in this regard as it has both excellent efficiency and the appropriate P-T characteristics to make it suitable for everything from low temperature freezers right up to AC applications. In fact if you compare it to R22, the PT curve is very similar. What set R290 apart from R22 is the resulting discharge temperature that it produces? While R22 at -10F/-23C results in discharge temperatures that turns oil into something resembling carbon black , R290 has a much lower discharge temperature and does not require the extra efforts of cooling the oil, compressor heads and the rubber mounting grommets while you’re at it.
If you want to get an idea of how efficient R290 is, just compare the power usage of a compressor using R290 and the same compressor using R404A. Below is that information for a compressor at the conditions one might expect in this application.
Test Conditions: C/L/S, 130F°/115F°/95F° Evaporator = 20F/-6C,
R290: 2753 btu/hr, 399 watt power consumption, EER = 6.9
R404A: 2789 btu/hr, 529 watts power consumption, EER = 5.27
Keeping the other conditions the same and dropping to a -10F/-23C evaporator temperature we get:
R290: 1340 btu/hr, 290 watts power consumption, EER =4.62
R404A: 1316 btu/hr, 368 watts power consumption, EER = 3.58
Both of the compressors were of the same family design so the R290 was not a high efficiency model compared to the R404A model. The efficiency gains were due to the properties of R290. You can do the same thing and look up the same glass door merchandisers from a major manufacturer and compare one with R134a and the same size unit with R290 and you will get similar results. With these units already optimized to meet CEC requirements etc., it would be very hard to squeeze a 130 watt saving out of the unit and still keep R134a as the refrigerant. I think it would be almost impossible without adding significantly to the upfront cost.
Oh, and did I mention that R290 is a hydrocarbon? Well, guess what refrigerant oil is? That’s right, just a slightly thicker hydrocarbon. And the 2 of them love each other. In fact, to date I have not come across an oil that would not work with R290, at least in theory anyways. Of course this can have its draw backs as well. Since oil is basically a hydrocarbon with a lower vapor pressure than R290, the latter will dissolve quite handily into the oil so keeping the refrigerant out of the oil is a necessity. However it is also a necessity for any other refrigerant for that matter and since R290 can only be utilized in package units for the time being (More on that later.) and will have low charges, this is not as big an issue as it might seem. It is used in millions of domestic refrigerators and freezers around the world and compressor replacement has not been an issue as far as I can find.
Concerning moisture, the molecular sieve driers that are used on HFC systems can be used on HC systems as well. It is critical that these systems stay dry as it is relatively easy for ice to form in cap tubes and in TXV’s so once again, follow best practices by using a micron gauge and everyone will be happy. By the way, you will hear the term „best practices“ quite a bit from here on.
So what will R290 be used in? For starters, it cannot be used as a retrofit refrigerant. You cannot swap out the R22 in your medium temperature unit and vacation on the energy savings. According to the people involved in the SNAP program, only new build (OEM) package units can utilize hydrocarbon refrigerants. No split systems with remote condensers are allowed over here on this side of the pond. They are used over in Europe and the results have been fantastic concerning energy use but we will have to wait for them over here if at all.
There are some other applications that are allowed that will prove to be interesting and given the opportunity for energy savings, I am almost positive that they will be. PTAC (Packaged Terminal Air Conditioner) units are allowed, as are their heat pump brethren. Think about this for a minute. Everybody loves to keep their hotel room nice and cool and some of these hotels can have hundreds of these units, most of which run most of the time. Show a hotel owner savings of 20 or 30% in energy use and this will grab his interest, especially since the government has given the OK to use them which cover his/her behind. We are talking serious money savings here and I can’t fathom that anyone would pass up on this given how competitive the hotel industry is.
You may have heard all of the talk about very small refrigerant charges such as 75 or 150 grams which is true for domestic and commercial refrigerators and freezers and think, how can I operate a 12 000 btu/hr PTAC unit with 150 grams of refrigerant regardless of how efficient it is? The truth is you can’t, but it is also true that you don’t have too and this is due to how the maximum allowable refrigerant charges are calculated.
The folks at the EPA and the other involved organizations figured out early on that it is much easier to control a program from the design side than it was form the service side. OEM’s design and test their products so the results can be catalogued and tracked so it is much easier to ensure compliance than when the equipment is in the field. What this means for refrigerant charge relates to how and where the equipment will be used.
Everything basically hinges on the chances of a combustible mixture forming and all efforts are on minimizing that chance as much as possible. How the equipment is designed is important so that any leakage from the unit will either vent outside, or if there is the possibility that it can be trapped within say a fridge, the electrical components that are a possible ignition source, are either outside of the unit or they utilize explosive environment rated enclosures to surround the contacts etc.
Maximum refrigerant charge is calculated based on the expected room/space size the unit will be operating in. For a domestic fridge, the 75 gram charge, if leaked out, will be far too small to form a combustible mixture given the room volume that these fridges are placed within. The evidence supporting this approach is very strong, as just about all documented accidents where there was a fire or explosion occurred when the hydrocarbon was trapped within the fridge itself, not when it leaked outside of the unit. This makes sense since it requires a small volume to achieve a combustible mixture with a small amount of fuel. Believe it or not, the safest place for the hydrocarbon to leak into is the outside and hence why design takes such precedence to minimize ignition within the unit.
So, using this logic, the greater the capacity of the PTAC unit, the greater the space it must be utilized within. As well, it must be a certain height above the floor to make sure there is enough space and free volume to allow dilution should a leak occur. As well, leakage outside of the space into the great outdoors is obviously preferable. Again, these can all be achieved during the design and building of the unit. PTAC units are not the only AC application given the green light, so are window units. A 5000 BTU/hr window shaker can have a maximum charge of 130 grams of R290. A 10 000 btu/hr unit can have a maximum charge of 260 grams, almost 10 ounces! If you look at the mounting requirements, the minimum height of the evaporator fro0m the floor is .6 meters and the maximum height is 1 meter. I assume the maximum height is to avoid somebody standing under it and smoking? Not sure.
Getting back to PTAC units, a 12 000 btu/hr unit can have a maximum charge of 160 grams and all models must be no more than .6 meters above the floor. A 24 000 btu/hr model can have a max charge of 290 grams. Wall mounted AC units, being the highest off of the grounds, can have very large charges, up to 1 kilogram which is 2.2 pounds! All these units must be mounted between 1 and 1.8 meters off of the ground so this extra height allows for greater dilution.
Concerning service, the greatest threat stems from first and foremost not knowing what the unit is charged with. We have seen some dramatic (And unfortunate) examples of this with the hydrocarbon based R22 replacements which are mostly propane. Unfortunately these were sold online and through other retailers and quite a bit of this stuff went out before the factory that made it blew up and burned to the ground. Seriously.
Assuming that the refrigerant is benign and skipping on the best practices has led to some very dire situations and people have gotten injured. The specific examples I have heard about stem from using a torch to sweat out a component when there was still a few pounds of pressure left from when the refrigerant was recovered. The resulting fireball, if it didn’t burn you, probably had you going home to change your underwear.
Luckily, it is pretty straightforward to control this by requiring that only new equipment be charged with hydrocarbon refrigerants and that the equipment is duly labelled in as many places as possible to make it obvious. In fact, there is a specific color required for this task and it goes by the dreadful name of Pantone Matching System 185 or PMS 185 for short. Most of us will just call it red.
As far as how to service the equipment, RSES has a great bit on this and in fact, the last time I checked, most of the manufacturers were recommending the RSES program to any technicians who will be working on hydrocarbon equipment. I went through the RSES materials and they are good and pretty much stress what we have all been saying regarding best practices. In fact if you want a good example of why we should all be following best practices, the fact that they eliminate the chance of needing a skin graft should help focus everyone’s attention. Below is a quick synopsis of the major points.
- Follow the manufacturer’s directions regarding servicing as they best know their equipment. Stay up to date on procedures and never assume the service requirements for a HC unit is the same as for a HFC unit.
- Use a digital meter that actually has R290 in its data base so you can correctly ascertain performance.
- Never mix refrigerants and this includes inside hoses.
- Always open the door of the unit if present and make sure you have good ventilation. Providing a moving air stream from a fan is an awesome idea so do it.
- Leave the compressor terminal box cover on until power has been turned off! It acts as a protection device against combustion should the fusite by the terminal pins be compromised.
- Reclaim the refrigerant even though by law it is not required and use a HC rated reclaim unit.
- Once you have recovered the refrigerant, purge with nitrogen as it is inert and will remove any chances that a combustible mixture remains.
- Do not use a torch to remove components (Duh), however, you can use one to install components after purging with nitrogen.
- Always replace components either with the exact replacement or if applicable, one that is approved by the manufacturer.
- Weigh in the charge. Refrigerant charges are small in HC units so an incorrect charge can have a serious performance impact. Besides, would you want an overcharged unit containing R290 in your space? I know I wouldn’t.
Obviously there are more, but the point is that for the most part, these are what should be done anyways with the added emphasis on safety around a potentially flammable mixture. When servicing R290 equipment, it will not be an unknown regarding what is inside of the unit. The expectation is to do it right by ensuring that one is prepared with the correct knowledge and tools and to put them to good use.
To my American friends, have a great Thanksgiving holiday next week.
Training Manager at Danfoss