Interview with Professor Hrnjak on reducing NH₃ charges

ammonia21.com: Professor Hrnjak, in your paper presented at the Ammonia Refrigeration Technology Conference in Ohrid, you mention that “smaller low charge ammonia systems, chillers in particular, used for refrigeration or air conditioning as part of secondary loop or cascade system provide excellent potential for market penetration of ammonia”. What advantages does ammonia charge reduction offer in your opinion?

Hrnjak: In a nutshell, low charge opens applications for ammonia in densely populated areas such as a supermarket chiller with secondary refrigerants (either single phase, like glycols or similar, or volatile, like CO₂), air conditioning, or in various processes. A good location is on the top of a building because, unlike other refrigerants, ammonia is lighter than air so any leak would escape upwards, with no undesirable effect.

Generally, ammonia is an excellent refrigerant with the exception of being flammable and toxic; even flammability is very low since the LFL is very high and ignition is very difficult. Ammonia is self alarming due to its strong odor which makes it unbearable even at concentrations orders of magnitude below dangerous limits. Excellent performance is the reason why ammonia is dominant in industrial applications where efficiency and cost matter, but where locations are far from populated areas and operations are typically overseen by qualified personnel. Typical ammonia charges are several tons, and that is a potential for problems in the case of catastrophic leaks. Various regulations prescribe ways of handling and reporting their charges and eventual leakage. Low charges, less than 50 kg, are unregulated, to my best knowledge. So, the question becomes what capacity can be obtained with such low charge. Even our early prototypes in 1997 have demonstrated charges as low as 18 g/kW in an air cooled chiller with a 15 kW cooling capacity. I have presented these results at the IIAR conference in Palm Beach in 2001 and at Purdue in 2000. These are very good results compared to any earlier air cooled systems, but water cooled chillers, where it is typically easier to reduce charge, have demonstrated similar charge levels (23 g/kW). Since then, we have been continuously working on the subject.

ammonia21.com: According to your work, ammonia has excellent potential for charge reduction compared to other refrigerants. Did you expect these results?

Hrnjak: I expected low charges, but I must admit that I was surprised how good the results were. In all honesty, I doubt that anyone would choose ammonia over some other refrigerant only because of the charge reduction potential, but it is good to have an additional argument.

ammonia21.com: What are the potential problems one is likely to confront when working towards reducing a system’s ammonia charge? Are there any trade-offs one needs to consider?

Hrnjak: Yes, the system efficiency of a low charge system is much more sensitive to the leak than a system with more refrigerant in the receiver . If there is a refrigerant pump in the system, it can be affected earlier by the lack of liquid in the case of refrigerant leaks.

ammonia21.com: Your latest work has focused on charge reduction in heat exchangers, especially by using a microchannel approach, technology which you assert that it achieves the lowest charges at this point of heat exchanger development. What were the main findings of your work when comparing microchannel heat exchangers with other technologies?

Hrnjak: Microchannel heat exchangers are the best option for charge reduction of all known designs because they have the best refrigerant side surface to volume ratio but some plate type heat exchanger designs and spray evaporators for large capacities are also good options.

ammonia21.com: Through your work you developed the lowest specific charge air-cooled chiller for ammonia reported in the literature so far. However, you mention that the experimental chiller was not really optimized for charge, as the smallest reliable ammonia compressor that it incorporates was still oversized. Do you have any estimates concerning potential additional charge reductions that could be achieved by using a smaller compressor?

Hrnjak: Certainly significant, but it is hard to be exact at this point. My expectation would be a reduction of specific charge by as much as 30-40% compared to current values.

ammonia21.com: Recent advances in manufacturing microchannel heat exchangers allow their use in mass production markets and, therefore, a further reduction in their cost. What are the areas of application for which mass production currently takes place and which could also encourage the application of microchannel heat exchangers in novel areas such as ammonia?

Hrnjak: I believe air cooled chillers will be the strongest beneficiary of increased microchannel heat exchanger manufacturing and lowered cost. At this point microchannel condensers are the main emphasis but there are more microchannel components to come.

ammonia21.com: Besides microchannel heat exchangers, you mention some other approaches that have achieved good results in achieving charge reduction, such as using plate evaporators or condensers with water or other fluids as well as spray evaporators. Could we expect Creative Thermal Solutions or the industry in general to focus future work on these technologies as well?

Hrnjak: Yes, Creative Thermal Solutions is home to a group (over 20 at this point) of excellent, creative, enthusiastic, and experienced engineers, most of whom graduated with advanced degrees from the Air Conditioning and Refrigeration Center at the University of Illinois, and I am very proud most of them were my students. CTS laboratories are as busy as a beehive with several ongoing projects at various stages: prototyping, development, and research. Some of the projects are focused on certain technologies for known customers but there are several projects that are internal R&D and Creative Thermal Solutions will be happy to offer those solutions to the companies interested in bringing new attractive products to the market.

ammonia21.com: You describe the new Mycom hermetic compressor for ammonia based refrigeration and heat pumping as probably the most important recent development. What kind of market opportunities could this development open for ammonia?

Hrnjak: You are right, I believe that the hermetic ammonia compressor is probably the most important missing technology for penetration of small ammonia systems into the market. It is, because the system with hermetic compressors will not require constant presence of qualified personnel, will reduce concerns of ammonia leak and smell, potentially reduce the cost, etc. In short, ammonia systems will look like, feel like, and work like any system with a conventional refrigerant offering well known benefits.

ammonia21.com: Thank you very much!