Professor Pega Hrnjak sees a robust future for low-charge packaged ammonia systems in industrial refrigeration – and potential for ammonia secondary chillers in a range of applications.
Last month at the IIAR Conference and Exhibition in Orlando, Florida, Accelerate America sat down with Professor Predrag (Pega) Hrnjak to discuss the direction of ammonia refrigeration.
Born in Belgrade, Yugoslavia, Pega is one of the leading researchers, teachers and experts in the refrigeration industry. He started his activities in the US during the 1980s, and in 1993 joined the faculty of the University of Illinois, Urbana, Ill., where he is a professor in the Department of Mechanical Science and Engineering.
He is also director of the University of Illinois’s Air Conditioning and Refrigeration Center, an NSF-founded, industry-university cooperative research center, and is president of Creative Thermal Solutions (CTS), his private research-and-development company, also based in Urbana.
In the following interview, he offers new insights into the application of low-charge ammonia cooling systems in industrial and commercial scenarios.
ammonia21.com: Where is the growth in low-charge ammonia systems taking place?
Pega: I believe the growth at this point is coming from owners of larger ammonia systems interested in charge reduction. I wish it would come from smaller plants using HFCs.
ammonia21.com: What is your opinion of low-charge packaged units?
Pega: I like packaged units whenever possible because of the shorter length of the lines and the reduction of the charge. And, you can place them locally near the cooling loads. The only disadvantage is that when you have multiple units, maintenance technicians need to go from unit to unit on the roof, instead of doing all activities in a machine room.
I think you will see more and more premade, packaged systems. That is the direction to go. There is almost always better craftsmanship in the factory as well as better control and lower costs with faster delivery time, in principle. This is how every industry has developed: by increasing the quality and lowering the cost.
So what we are seeing now is a reduction of custom-made industrial refrigeration systems and an increase in premade, factory-built packaged systems that one can almost buy off the shelf.
ammonia21.com: Are packaged units as efficient as central systems?
Pega: It’s very tough to perfectly compare them, but I’d say the efficiencies are similar. If manufactured well, packaged systems should have even higher efficiency.
ammonia21.com: Would you need multiple packaged units to deliver large industrial capacities?
Pega: Yes, but also in a single machine room of large industrial plants there are multiple compressors and condensers.
ammonia21.com:Are CO2-only transcritical systems becoming more suitable for industrial applications?
Pega: I think we will see more and more transcritical systems. Ammonia/CO2 is more energy-efficient than CO2-only. But, having two refrigerants and two types of oils in a cascade system increases the complexity and is a burden.
When we make CO2-only systems operate more efficiently it is a simpler solution. And packaged CO2 systems will be equally good and better than custom made systems in machine rooms.
ammonia21.com: What is the opportunity for ammonia in chillers?
Pega: In chillers, ammonia cools some secondary liquid – it could be CO2 or glycol or some other brine. One could use an ammonia/glycol chiller in industrial cooling or in comfort cooling for a large hotel lobby or airport. The advantage of CO2 is that it can be used in large and small capacities, in industrial refrigeration or supermarkets, and in HVAC applications.
Most chillers don’t currently use ammonia as the primary refrigerant; they use R22, R410A, or R123 and R11 in old centrifugal chillers. But ammonia, being indisputably efficient, can easily replace especially the first two refrigerants. They all use secondary liquids, so there is a market that is prepared for using chillers.
A relatively new refrigerant, R1233zd, which is characterized as non-toxic and nonflammable with a GWP close to zero, will be very strong competition for ammonia in chillers, especially centrifugal. New oil-free centrifugal compressors using magnetic bearings will make it even more competitive.
In the chiller market for air conditioning, there are typically smaller units that use reciprocating, scroll or screw compressors. That is a potential market for ammonia chillers. Hermetic or semi-hermetic ammonia compressors would be a great boost in that direction.
ammonia21.com: So you think ammonia will be used in comfort cooling?
Pega: I believe so and I’m working on that approach for comfort cooling. It’s possible and I believe it is very reasonable. At Creative Thermal Solutions we have developed a chiller system using microchannel condensers that has an ammonia charge of 2.3oz/ton (18g/kW).
But we need to promote regulations that would lift restrictions on a charge as low as say 3 kg in a unit that can provide a minumum capacity of 100 kW.. Then ammonia would be really competitive in the HVAC market with R22 systems.
When using hydrocarbons, with less than 150 grams (some say even more) one can put a unit anywhere in a building. We don’t have that freedom right now with ammonia, even though ammonia presents fewer concerns than R290 from a flammability viewpoint. We just have the ASHRAE-15 standard, which allows ammonia equipment to be used outside the machine room if the charge is less than 6.6 pounds.
ammonia21.com: With a very low charge of ammonia, the health risk posed by a leak would be low. But in a supermarket, a leak could still produce an odor. Is that enough to be a problem?
Pega: I would say, keep the ammonia outside, not in the areas with high concentration of people and just use a secondary fluid inside. Ammonia can be dangerous as a liquid in contact with skin or eyes, but not as a vapor. The vapor has an odor only, which can be create a panic, but that is a different type of problem. In any case, most of the chillers are outside, on the roof because it is easy to remove heat by air.
I think ammonia could play a significant role in HVAC applications because there are three realistic natural refrigerant options: ammonia, propane and CO2. Propane chillers, used outside, could give you almost the same performance as ammonia.
But propane is heavier than air while ammonia is lighter; thus in the case of any leak ammonia gas will rise up, far from populated zones. Heavier gas can be trapped close to the floor and reach potentially dangerous concentrations.