Friday, 5 August 2016

ASHRAE - Are You Pro Propane?

Are You Pro Propane?

By Andy Pearson, Ph.D., C.Eng., Fellow ASHRAE

The motto from the walls of the energy efficiency office.

We have thought about various refrigerant choices, and now we turn to the hydrocarbon family, the most common of which, as a refrigerant, is propane. It has often been said that choosing a refrigerant is an exercise in compromise because there are so many competing factors to consider, and they are sometimes contradictory. Hence, there is no perfect refrigerant.

Propane scores very highly in all criteria bar one. It has a suitable pressure-temperature relationship (very similar to R-22); it has a relatively high latent heat but a low index of compression; it is cheap and readily available, and it is compatible with mineral oils - not a surprise as it is a kind of mineral oil itself! Environmentally, it also performs well, having no effect on the ozone layer and a low global warming potential - even lower than the HFOs.

Hydrocarbons are very widely used in the domestic refrigerator market, particularly in Europe where more than 85% of all refrigerators use one or other of the family. This was not always the case, but in the early 1990s when the Berlin Wall had recently fallen and the two halves of Germany were figuring out how to live together again after a 45-year separation, a small, struggling East German refrigerator manufacturer hooked up with Greenpeace to develop a "climate-friendly" refrigerator.

Their timing was excellent; the concept of CFC-free refrigeration was quickly adopted by the German government and translated soon after into a European regulation, and all of the major European manufacturers quickly followed suit. Early systems used a mixture of propane and isobutane to match the performance of R-12, but as the technology matured equipment was developed to use isobutane.

Among the spin-off benefits of this change, it was noted that units are significantly quieter on starting than traditional R-12 and R-134a systems and the weight of refrigerant required is much less. The reduced noise is due to the absorption of refrigerant in the oil when the compressor is not running, enabling a gentle start against lower pressure. The reduced charge is due to the low liquid density - the volume of liquid required is about the same but it weighs less. They are also very efficient, helping manufacturers meet the stringent energy labelling requirements.

The labelling laws introduced in Europe in the 1990s had a scale from G (least efficient) to A (most efficient) but stepwise improvements in the requirements over the years have meant new, more efficient bands have been added; A+, then A++ and then (you've guessed it), A+++. Since mid-2012 all models must be at least A+ efficiency; it's a shame they didn't start further back in the alphabet.

The transition from CFC to HC in the European refrigerator market was quick and relatively simple because all the hard stuff took place at the factory. Production lines had to be retooled and in some cases completely reengineered to cope with the flammability of the refrigerant now being used, but the technology of the refrigerator did not change dramatically, and in the eye of the householder there was no change at all. The fridge is installed and used in exactly the same way as it always was and there has been no discernible change in product safety.

This highlights a key difficulty when introducing hydrocarbons to an existing service organization. If a tech is being introduced to CO2 as a refrigerant, he is naturally wary - the pressures are higher and the gear looks a bit different. However, his regular habits in terms of service practice will stand him in good stead and not cause problems. On the contrary, a hydrocarbon system looks and feels much like a normal HFC system but regular habits, such as sweating off an expansion valve, can cause major problems unless great care is taken to ensure the refrigerant is well out of the way. Propane has been adopted by a major UK supermarket as its preferred solution for its display cases - for them the various benefits of cost, efficiency and reliability outweigh the challenges they have had to overcome to take this road to environment-friendly refrigeration.

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. 

This article was first published in ASHRAE Journal, October 2013. Copyright 2013 ASHRAE. Reprinted here by permission from ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

ASHRAE - Itchy Foes?

Itchy Foes?

By Andy Pearson, Ph.D., C.Eng., Fellow ASHRAE


Is it a bird, is it a plane...?

Last month's column looked at good things and bad things; now we turn our attention to a family of refrigerants that transcends that discussion and causes deeply divided opinions to emerge on many fronts - the fluorinated alkenes, more commonly known as HFOs.

Even the adopted name of this family has generated polarized reactions. Opponents have dismissed it as "a marketing stunt." These chemicals contain hydrogen, fluorine and carbon so they are obviously HFCs, aka f-gases, which are currently the subject of restrictive legislation around the world. Furthermore, the "O" in the name is potentially misleading since they don't contain oxygen.

Proponents point out calmly and often that "O" stands for "olefin" (pronounced oalyfin), a term currently used by the International Union of Pure and Applied Chemistry to describe any molecule that contains a carbon-to-carbon "double bond." Molecules with these types of bonds are called "unsaturated" because, like a half-damp sponge, it is possible for them to absorb more elements by adding them to either end of one of the double bonds, turning it into a single bond.

However, the adoption of the new family name has historical precedent. All synthetic refrigerants were simply known as "halocarbons" until it became necessary in the 1980s to discriminate between those containing only chlorine, fluorine and carbon, those also containing hydrogen, and those without any chlorine. Unfortunately, the new name, HFO, is not perfect. What about unsaturated fluorocarbon compounds that don't contain hydrogen? What about those, like ethylene and propylene that only contain hydrogen and carbon - are they hydroolefins or just olefins? And furthermore, to hyphenate or not to hyphenate?

This unusual family of molecules is not new. They have been familiar to chemists for decades (under the name fluoroalkenes). The addition of the double bond into the mix makes the compounds generally more reactive than their fluoroalkane (that is, saturated) equivalents. This means that they do not last as long in the atmosphere which, in last month's terms is "a good thing," but that they are flammable, clearly "a bad thing." Just how flammable is a subject of great and detailed debate. The most commonly proposed substances occupy a grey area between the completely inert and the highly combustible.

In fairness, it must be said that traditional refrigerants containing hydrogen, such as R-22 and R-134a also occupy this grey area. They happen to lie on one side of the flammability fence and the HFOs lie on the other side. The exact location of the borderline is a question of opinion and much debate. It is not a clear-cut scientific fact but depends on factors such as pressure and temperature and even leads to such philosophical questions as "what is a flame?" and "what is a spark?" A lot of additional work, including fresh insights and new understanding, is required to enable us to determine how to deal with the reactivity of HFOs in the myriad applications of the refrigeration and heat pump world.

The world of refrigerants became more complicated when we decided that in addition to having gases that were nonflammable and non-toxic we also wanted them to have no effect on the ozone layer and not cause global warming. At the same time systems are to be "cost-effective" (usually a synonym for "as cheap as possible") but also must use minimal energy. Now we are contemplating the addition of further constraints - for example "no persistent effect on the local environment" or "no toxic products of combustion." It is clear none of the proposed working fluids for refrigerating and heat pump systems satisfy all of these criteria, and the prospect of finding something that satisfies them all is nonexistent.

The answer to this riddle, therefore, lies in the realms of the economists, legislation-writers and accountants, not the chemists and engineers. We need a method of assessing relative merits of different options, taking all relevant factors into account. As cheap as possible is a good measure, but the key question is "what is possible?" or, to be more exact, "what is permissible?"

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. 

This article was first published in ASHRAE Journal, September 2013. Copyright 2013 ASHRAE. Reprinted here by permission from ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

ASHRAE - What's Up With CO₂?

What's Up With CO₂?

By Andy Pearson, Ph.D., C.Eng., Fellow ASHRAE

King Harold couldn't see why there was such a fuss about CO₂.

There has probably been more written about possible applications for CO₂ as a refrigerant in the last 20 years than about any other refrigerant at any time in the history of mechanical refrigeration. The classic text "1066 And All That," a spoof history of England, characterizes all history as either "a good thing" or "a bad thing," or occasionally "a very good thing" or "a very bad" thing. Here is the same technique used to analyze the prospects for CO₂ refrigeration.

CO₂ contributes more than any other gas to global warming—this is clearly a bad thing. However, without global warming the earth would be uninhabitable, with an average surface temperature of –2°F (19°C). This is a very bad thing, so global warming is a good thing provided, as Ralph Waldo Emerson said, we have “moderation in all things, especially moderation.” So CO₂ as a substitute for high-GWP HFC refrigerant can help reduce the additional global warming that pushes the thermometer beyond the tolerable band to which we have been accustomed for at least the last thousand years.

CO₂ systems have to run at very high pressure, typically eight to 10 times higher than an average ammonia system, for them to be cooled by the surrounding air. Equipment needs to be designed to contain this pressure safely; this is clearly a good but expensive thing. Many people seem to be put off exploring the possibility of using CO₂ for this reason, so it has a bad consequence. However, those who persevere and spend the little extra it takes to accommodate higher operating pressures have learned that there are lots of reasons to like CO₂—more of this good thing later. One hundred years ago CO₂ systems were commonplace, often capable of operating at pressures up to 1,500 psig (over 100 bar gauge), but they were relatively expensive compared to lower pressure refrigerants, so they fell out of favor.

CO₂ systems cooled by the surrounding air operate on their high pressure side above the maximum pressure at which the gas can be liquefied by cooling. This is a confusing thing and has led to a lot of latter-day mythology about these so-called “transcritical” systems. In reality, they are not so different from what we are used to; their major challenge is that the system capacity is greatly reduced on a hot day, which is not just a bad thing, but bad timing, too.

However, this high pressure makes CO₂ gas very dense, which delivers the opportunity to do amazing things with system design. Pushing the discharge pressure above the condensing zone results in a temperature drop as the gas cools, but on the low pressure side of the system the temperature remains constant as the liquid evaporates. This is a "best of both worlds" deal: glide on the heating side and no glide on the chiller side—allowing CO₂ to be used in high temperature heat pumps very effectively.

High operating pressure delivers the good things about CO₂—the small compressor size, the low pressure loss in suction pipes and the good heat transfer in heat exchangers, so it ought to be welcomed. The high density allows crazy things to be done in the evaporator design department, even for very low temperature systems such as cooling to -50°F (-45°C) without suffering large pressure drop.

I have never yet been able to design a CO₂ evaporator that suffered from the circuits being too long, despite trying several times. The circuits are almost impossible to overload, especially in chill applications. As we push toward making system efficiency better, this radical feature of CO₂ will be crucial to the success of new system designs. Unfortunately, system designers and particularly evaporator designers are following old rules for old refrigerants and are not capitalizing on this advantage. This is a bad thing, but it is not irredeemable.

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. ■

This article was first published in ASHRAE Journal, August 2013. Copyright 2013 ASHRAE. Reprinted here by permission from ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

Thursday, 4 August 2016

ASHRAE - I'm On You

I'm On You

By Andy Pearson, Ph.D., C.Eng., Fellow ASHRAE

Ammonia: you love it or you hate it.

One of the most puzzling things to an outsider about the world of refrigeration is the polarizing effect that ammonia has on people. It is like that quintessentially English spread, Marmite - you either love it or hate it. Ammonia is, in truth, nasty stuff with a strong unpleasant smell that reminds us automatically of toilets and putrefaction, like hanging around the wrong end of a dead cow.

For me, the downside of ammonia was captured perfectly by the hair colour ad that showed a gorgeous girl with waves and waves of amazingly beautiful auburn hair under the slogan “look—no ammonia!” The hidden subtext of course is “the fridge guy doesn’t get the girl—again!”

However, on the other side of the debating chamber are the dyed-in-the-wool ammonia guys who have been using this stuff as their refrigerant of choice for more than 150 years. In fact, it is the only substance from that era to have been in constant use since its introduction as a refrigerant all those years ago, so there must be some points in its favor.

Here is an insight into some of the weird things about ammonia that have endeared it to so many people.

It is an exceptionally easy chemical to work with, which is why many emergency responders choose to train on ammonia rather than other, more “difficult” chemicals such as chlorine or sulfur dioxide. The strong smell gives a very clear signal of its presence long before it reaches harmful concentrations, even to people who have no sense of smell. The high latent heat makes it easy to contain and manage if there is a large liquid spill. Although it can be burned, it is usually very difficult to ignite and burns with a low velocity flame that generally does much less damage than other flammable substances. For industrial systems it is relatively easy to design and install ammonia systems with exceptionally high standards of safety because of these odd characteristics.

For a variety of reasons ammonia works exceptionally well in evaporators and condensers. On a like-for-like basis its performance as a heat transfer fluid is more than four times better than R-134a, which is the best of the currently used fluorocarbons. Ammonia also responds well to abuse; for example, it is amazingly tolerant of water in a refrigerating system and will carry on cooling even when the water content in the recirculated system is as high as 25%. It also copes with high condensing pressures rather better than most fluorocarbon fluids, so it is more tolerant of a certain lack of condenser maintenance.

It is cheap and widely available, which are the benefits of being easy to make and used in a wide number of industrial applications. This is important when systems become so large that the necessary inventory of refrigerant to make them work properly runs to tens of thousands of pounds. If the value of the refrigerant is considered as a fraction of the total cost of the job then the value of ammonia charge for an industrial system might be 1% of the contract value. With modern fluorinated refrigerants, the value could be 10% or more of the contract value. This is partly because they are more expensive anyway, but also because the liquid has a higher density and the heat exchangers and pipes tend to be larger, so more stuff is required to make the system work.

Finally, and perhaps nowadays most importantly, ammonia makes a phenomenally efficient refrigerant. This is true whether it is used for cooling stuff in chillers and freezers or for heating stuff in heat pumps. In a recently completed district heating system that was driven by a set of ammonia heat pumps, the heating energy supplied to the district was 20% higher than would have been achieved by a best-of-class R-134a heat pump with the same electrical input. That means that the district was getting 20% more heat completely free compared to the standard approach to heating. Now that’s something that everybody should like.

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. ■

This article was first published in ASHRAE Journal, June 2013. Copyright 2013 ASHRAE. Reprinted here by permission from ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

ASHRAE - Direct Talking

Direct Talking

By Andy Pearson, Ph.D., C.Eng., Member ASHRAE

Stanley thought it was a direct expansion indirect system, but he wasn't sure.

The skill of fridge guys in confusing others (and often themselves) has been mentioned in these columns before. This month, we tackle a word that is frequently used, but seldom understood, although most people seem to be oblivious to this. The word is "direct."

When we talk about direct and indirect systems, the meaning is usually clear. A direct system uses refrigerant to cool the target stuff: air, product or whatever; whereas an indirect system has one or more intermediate steps of heat transfer, usually using water, brine or glycol to move heat from the stuff to be cooled to the suction side of the compressor. So far so good, but now it gets complicated.

Safety standards such as ASHRAE Standard 15-2010, Safety Standard for Refrigeration Systems, refer to the risk of refrigerant leaking from the system into the place where people are located. Such systems are also sometimes loosely referred to as direct systems, but there is a subtle difference. If the evaporator is in the same place as the people being cooled, then the system is described as direct: a leak from the refrigeration circuit would expose occupants to refrigerant gas. The term “direct” applies in this sense even if the evaporator is cooling a secondary fluid (for example, a water chiller in a production area). It should be understood in the sense of “directly releasable,” a phrase that was tried in the standards a few years ago but never really caught on.

The amount of refrigerant allowed in the system is determined by the hazardous effect that it would have if it leaked into the occupied space. To mitigate this risk a secondary coolant, such as water, is used to provide the cooling effect to the people, and the evaporator is placed outside the area where the people are located.

If refrigerant leaks into the water, it does not affect the people being cooled. However, if the water circuit was sealed, with no means to relieve excessive pressure, then the refrigerant leak could, in theory, pressurize the water circuit and damage it.

To avoid this nomenclature clash, a secondary circuit without any vent is called an “indirect closed system,” and is treated in the same way as a direct system. Likewise, a water circuit that had automatic air purgers installed in the pipes within the area that was occupied also would have to be treated as a direct system.

If the refrigerant leaked into the water circuit, it would be released into the same place as the occupants. To avoid this problem, some method of venting the water circuit is required. This could be an automatic air purger outside the occupied area, or a header tank or relief valve, provided any refrigerant venting from the system could not reach the occupants of the place being cooled.

Such systems are called “indirect open,” “indirect vented” or “indirect closed vented,” depending on how the refrigerant would be released if it leaked into the water.

We also talk about “direct expansion” systems (but funnily enough nobody ever refers to an “indirect expansion” system). Often shortened to “DX,” this actually means a system where the refrigerant goes directly from the expansion valve to the evaporator. If you think all systems are like that, then you need to get out a bit more!

The alternative is that the refrigerant goes from the expansion valve to a receiver and the liquid from there is circulated to the evaporators by pumps or gravity. This means that the gas created during the expansion process does not go through the evaporator but passes straight to the compressor suction. Such systems are usually called “flooded” because the outlet from the evaporator is a mix of gas and liquid in contrast to the typical DX system where the outlet is superheated. However, the terms “DX” and “flooded” are not opposites; it is not uncommon to have a direct expansion system that uses flooded evaporators. For this reason, the opposite of a flooded system is sometimes called a “dry expansion” system, also abbreviated to “DX.” So next time you hear a system described as “direct,” pause for thought.

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. ■

This article was first published in ASHRAE Journal, May 2013. Copyright 2013 ASHRAE. Reprinted here by permissionfrom ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

Wednesday, 3 August 2016

ASHRAE - Sit Back, Enjoy the Glide

Sit Back, Enjoy the Glide

By Andy Pearson. Ph.D., C.Eng., Member ASHRAE

*This joke is courtesy of Flanders and Swann, music hall comedians of the 1950s, of whom I learned all the important bits of thermodynamics.

Last time, we thought a bit about the effect water has on ammonia when they are mixed, and it was noted that ammonia refrigeration systems have been known to carry on working even when as much as 25% of the liquid in the system is water. The effect of the water is to move the boiling point of the ammonia, so that in an evaporator the temperature shifts as the liquid turns to gas.

This effect is known as “glide.” It is tolerable in an evaporator where the outlet is intended to be a mixture of liquid and gas, for example, in a pumped circulation system. But, it can be disastrous if the flow control is dependent on superheat produced at the end of the evaporator.

The thermostatic expansion valve—from the Greek words “thermos,” meaning “hot” (if you don’t drop it*) and “statos,” meaning “standing,” so “thermostatic” = “keeps it hot”—or its modern electronic equivalent measures the inlet pressure and the outlet temperature of the evaporator and adjusts the refrigerant flow rate to make sure there is only gas in the evaporator outlet. This is good news for the compressor, which doesn’t suffer from unsquashable liquid in the place where gas squashing takes place, and also good news for the evaporator manufacturer who gets to sell a larger evaporator to provide the necessary heat exchange surface to do this superheating.

It is not such good news for system efficiency, particularly if the only way to get a large enough temperature difference to do the superheating is by lowering the evaporating temperature, but that’s another story (see “The Temperature Lift” in March 2012). There are two drawbacks to this approach. First, it assumes that the pressure is the same all the way through the evaporator. Second, it assumes that the temperature is the same all the way through the evaporator.

In fact, there needs to be a little bit of pressure drop to make the refrigerant move in the right direction, but this is not usually enough to cause a problem. The boiling temperature of the refrigerant usually follows the pressure (see “Temperatures, Pressures, and Refrigerants” in April 2012), and so it drops a little bit as the pressure reduces. A traditional mechanically operated thermostatic valve can usually control steadily if there is at least 10°F (5.5 K) superheat. A good electronic valve can manage to keep steady at about half this level.

If the refrigerant is actually a cocktail of two or more chemicals then sometimes the boiling temperature does not follow the pressure, but slides from one condition to another as the fluid boils. This is called “glide” and can make your head hurt. Glide happens because the components of the mixture evaporate at different rates, so the proportions of the cocktail change. The pressure drops slightly on the way through the evaporator but the boiling temperature rises a lot. If a standard mechanical valve is set to maintain 10°F (5.5 K) difference between the boiling point at the coil inlet and the actual temperature at the coil outlet, this might not be enough to allow the valve to do its stable control thing. The result is unsquashable liquid in the compressor.

For example, a coil with liquid entering at –15°F (–26°C) that has a pressure drop equivalent to 1°F (0.5 K) and is set to control the superheat to 10°F (5.5 K) will have an outlet temperature of –5°F (–21°C) and the superheat will actually be 11°F (6 K) on normal refrigerant. If there is also an 8°F (4.4 K) glide in the refrigerant because it is a cocktail, then the actual superheat at the coil outlet will only be 3°F (1.6 K) and a mechanical expansion valve will not be able to control accurately. If the pressure reading is taken at the coil outlet and a “true” superheat value is being measured, as is the case with some electronic valves, then it will work OK, but with mechanical valves this is not possible due to the way the valve cleverly balances internal pressures to vary the amount of opening.

In principle it is possible to use “glide” to your advantage, provided the heat exchanger can be designed to give maximum benefit from the inlet temperature to the stuff being cooled. This would allow the compressor to operate at a higher inlet pressure, but this is tricky in practice, so it’s not normally done, even with “wide glide” refrigerants.

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. ■

This article was first published in ASHRAE Journal, April 2013. Copyright 2013 ASHRAE. Reprinted here by permissionfrom ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.

ASHRAE - Water On The Inside

Water on the Inside

By Andy Pearson, Ph.D., C.Eng., Fellow ASHRAE

It's still always better to fix the leak than to have to deal with the consequences.

It is important to remember that when air leaks into a refrigeration system it brings other stuff with it. Like an onion, the other stuff is mostly water. In face, every 9 lbs (4 kg) of air that get into a system bring 1 oz (30 mL) of water with them. This doesn't sound like a lot, but when you purge the air out of the system it leaves the water behind so over weeks and months it can build up. A system that leaks 1 lb (0.5 kg) of air every day will have two and a half pints of water (1.2 liter) in it after a year. The effect of that water depends very much on the type of refrigerant used. I've detailed three very different situations; hopefully, at least one of them is of interest to you.

Fluorocarbons do not dissolve in water to any significant extent, so the liquid water passes from the condenser mixed in with the liquid refrigerant. When it gets to the expansion valve, if the temperature drops below the freezing point, the water solidifies and blocks the valve. Game over. The main symptom of this situation is low suction pressure because you can’t get enough refrigerant through the blocked valve to keep the system going. However, it can be quite a puzzle because if the system is off for a long time before it is investigated then the ice may have melted by the time the “blocked” valve is opened up for examination.

Ammonia and water are very fond of each other, and solutions of “aquaammonia” are used in many applications from window-cleaning to print-making. They are also, of course, used in absorption refrigerators. This means that water cannot freeze in the expansion valve and so it passes, in solution with the liquid ammonia, to the low pressure side of the system.

If the expansion valve outlet connects directly to the evaporator, which connects directly to the compressor then the water will pass through the system and back to the compressor. All sorts of problems, from dilution of the oil to rusting of the compressor internals might occur, but if the water quantity is small the plant will probably continue to operate normally.

If the system has a receiver vessel on the low pressure side, such as a surge drum or accumulator, then the water will collect in the vessel and will be pumped around the evaporator. There are reports of ammonia systems with up to 25% water content in the surge drum still functioning, albeit inefficiently. More on that later.

The water builds up in the low pressure liquid because it is much more soluble in liquid ammonia than in gas. The evaporated ammonia gas carries almost no water back to the compressor. The only way it can get out of the system is to draw off a sample of ammonia/water liquid and distill it to boil off the ammonia and leave the water behind. This process can be easily automated if necessary, but as I wrote last month, it is always better to fix the leak than to have to deal with the consequences.

Carbon dioxide is also soluble in water, but not to the same extent as ammonia as any lover of soda pop or champagne can tell you. There are two additional considerations when carbon dioxide systems are contaminated by water. Once the solution is saturated, which requires more than 600 parts of carbon dioxide per million of water at 32°F (0°C) (but only 120 part per million at –40°F [–40°C]), any additional carbon dioxide forms carbonic acid. If oxygen is also present (and remember the water probably snuck in with an air leak) then heavy corrosion can occur.

With very high levels of water, clathrates may form. These are lattices of bonded carbon dioxide molecules that form cages, which trap water molecules. Carbon dioxide clathrates can form solids up to 50°F (10°C), which is another way of blocking expansion valves and filters. They also vanish if the system is idle for too long and so can be difficult to diagnose.

In all cases, with any refrigerant (except R-718), water spells trouble. If you can’t keep it out of the system then you have to get it out, regularly.

Andy Pearson, Ph.D., C.Eng. is group managing director at Star Refrigeration in Glasgow, U.K. ■

This article was first published in ASHRAE Journal, March 2013. Copyright 2013 ASHRAE. Reprinted here by permissionfrom ASHRAE at www.star-ref.co.uk. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about ASHRAE, visit www.ashrae.org.