FEASIBILITY OF AMMONIA IN U.S. SUPERMARKETS Caleb Nelson, EIT, LEED AP CTA Architects Engineers December, 2009 Abstract: As the supermarket industry in the United States looks ahead to the phase out of environmentally non friendly Halo Carbon Refrigerants, the safety, efficiency, and feasibility of an outdoor ammonia chiller system utilized in a secondary loop or cascade application should not be overlooked. If global warming and ozone depletion are realities that are indeed contributed to and shared across the globe, then the refrigerants and technologies that exist to combat these negative effects on our environment should likewise be embraced by all the contributors. The United States is in a great position to take advantage of the ammonia system technologies that have developed over the years and that have been safely and efficiently implemented in the supermarket industry in other countries.
Introduction In the U.S. supermarket industry, misconceptions of ammonia (R717) and the codes that govern it, coupled with a lack of knowledge pertaining to the systems, serve as major hurdles that will need to be cleared before ammonia can be accepted as a viable alternative to Halo Carbon refrigerants. Thus, it is the purpose of this paper to: one, focus on the refrigerant R717 and expose its safety record, energy efficiency, and sustainability attributes; two, look at the International Code governing the majority of the U.S. and determine what restrictions may apply; three, expose what types of systems have successfully been utilized elsewhere in the world in supermarket applications, and four, provide some basic design considerations regarding the selection and operation of the basic components of these types of ammonia systems. Ammonia (R717) Through the many years that ammonia has been utilized as a refrigerant, engineers have been able to develop R717 systems to a point where they can operate at high levels of efficiency and as safely as any other type of refrigeration system provided they are installed and operated in accordance with the safety codes. Beyond ammonia s safety and efficiency, it is also important to review the environmental impacts that are associated with this natural refrigerant. In a comparison presented in Dr. Daniel Colbourne s paper, Opportunities for the Application of Natural Refrigerants, published in Proklima s, Natural Refrigerants 3, a potential efficiency range (PER) has been reported for ammonia and other natural refrigerants (including C0 2 and propane), for multiple HFCs and HFC drop ins, and for R 22 (an HCFC). In summary, by observing the thermophysical properties of the multiple refrigerants, modeling these refrigerants in similar system designs, and accounting for actual experimental results, a potential efficiency range (relative to R 22) has been assigned to each refrigerant. Since there are many factors that influence a system s overall efficiency, the reported efficiency percentages have a wide enough range to account for the unknowns while still providing accurate insight to the refrigerant s performance. R 22 was assigned a 100% efficiency for which all the other refrigerants were compared to. Ammonia came in first place with a PER of 100 125%. Second and third place are propene (R1270) and propane (R290) reporting ranges of 90 120% and 90 115% respectively. Since supermarkets in the U.S. have been moving away from HCFCs such as R 22 for some time now and have mostly reverted to HFCs such as R404a, it would be better to see a potential efficiency range of ammonia with respect to R404a instead of R 22. Since R404a reported a range of 85 100% relative to R 22, we can update the ammonia potential efficiency range (relative to R404a) to be 100 147%.
It is important to keep in mind that different refrigerants modeled in similar systems can sometimes produce misleading results. Since latent heats and critical temperatures of refrigerants vary, their efficiencies can either be capitalized or dulled by the type of system they are modeled in. For example, ammonia can offer greater benefits than other refrigerants when applied with heat reclaim; conversely, ammonia finds less benefit than other refrigerants when sub cooled. The lesson, then, when comparing the economics, would be to consider the efficiencies and the costs for the types of systems that each refrigerant would most likely use. The truth of the matter is that there are many different factors that will decide the economic feasibility for installing an ammonia system in a supermarket. If we re talking about ammonia s efficiency however, the fact that ammonia continues to be the refrigerant of choice for industrial applications around the world, speaks loudly for its efficiency. On the other hand, many misconceptions are held regarding ammonia s safety because it has been labeled as flammable and toxic. Since ammonia is a one of a kind refrigerant, it is necessary to take a closer look at the accuracy of these labels. Through science and a long history of experience, the behaviors of ammonia in our atmosphere are well documented in such sources as provided by the International Institute of Ammonia Refrigeration (IIAR) and by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). For example, the IIAR has stated that experience has shown that ammonia is difficult to ignite and, under normal conditions, is a very stable compound. 1 Also, ammonia concentration must be about 1000 times above that considered hazardous to humans before flammability becomes a concern. 1 These facts serve well to assure that an ammonia ignition is very unlikely. Moreover, if ammonia was to burn, the question becomes: how would it burn, and what would result? Anders Lindborg has explored this very question in his paper, Ammonia and its Reputation as Refrigerant, published in Proklima s, Natural Refrigerants 3. Lindborg begins by explaining that ammonia cannot explode, but rather, can only flash burn. Due to the large amount of energy required to ignite ammonia, not even a spark from a 440 volt, 3 phase system has enough energy to do the job. Furthermore, ammonia must be confined to catch fire so it can be considered non flammable in outdoor applications. It should be noted that the products of combustion of ammonia nitrogen and water are completely harmless even to [the] global environment. This is in marked contrast to the fluorocarbons [HFCs and HCFCs] which may form hydrofluoric acid, hydrochloric acid, carbonyl chloride (phosgene), carbon monoxide, etc. when burnt, which are highly corrosive and extremely toxic, even in small quantities. 3 Ammonia s toxicity must also receive a more accurate analysis of its implications. The characteristically strong odor of ammonia has been the root of many unwarranted fears regarding its toxicity. Ironically, ammonia s odor is a major contributor to its safety.
Ammonia s sharp and unpleasant odor actually acts as a built in, highly efficient, and free ofcharge leak detection system. Not only does the odor facilitate the discovery of leaks, but it demands a swifter repair in order to eliminate the odor. It should also be noted that unlike halocarbon refrigerants, ammonia gas is less dense than air. This is an important characteristic which allows ammonia to disperse as it rises instead of causing it to concentrate as it pools. In Lindborg s paper, the table entitled Physiological effect of ammonia on man, shows that the threshold value for discovering ammonia is 5 ppm. At this concentration, a person would be aware of a leak and could safely leave the area. Jumping up to 300 ppm, there is still no danger; however, the strong odor will cause most to leave the area. It isn t until the concentration reaches somewhere between 2,000 5,000 ppm before an exposure of 30 minutes or less can result in death. 3 To give a realistic idea of the level of safety these numbers represent in the industry, Lindborg states the following, Lethal accidents in the USA (last 11 years), UK (1975 2007), Sweden (since 1940), Denmark, Norway and Finland (since 1945) and Germany (last 20 years) are documented. This data gives an Annual Death Rate (ADR) of <2 per 1,000,000,000 population per year. As a benchmark, the ADR of lightning strike in the USA is 32 per billion per year. 3 Although it is unclear how this statistics would change as a result of ammonia becoming much more widely used, it is very likely that there would be no change due to ammonia s usage in small systems. Discussing ammonia s environmental impacts may be the easiest subject to tackle thus far simply because it is a natural refrigerant. ASHRAE points out that of the 100 million metric tons [120 million metric tons currently] of ammonia produced commercially throughout the world each year (14 16 million metric tons in the United States) over 80% is used for agricultural purposes. This leaves 20% to be used in many other industrial applications. So if up to 150 pounds of ammonia can be directly injected as a fertilizer into 1 acre of soil over a year s time, and elsewhere it can be injected into a buildings stack emissions to neutralize other harmful gases, one might argue that ammonia most definitely must be a safe and environmentally friendly compound. Another source of ammonia is the human body; it can release as much as 17g per 24 hours. (Lindborg) Beyond this, ammonia is biodegradable and is the only refrigerant with a less than zero Global Warming Potential (GWP) and a zero Ozone Depletion Potential (ODP), thus the main reason for its growing popularity as a replacement for HCFCs and HFCs. 2 Finding suitable alternative refrigerants for HCFCs is becoming more important for supermarkets as time moves forward. In the U.S., as of January 1 st, 2010, R 22 (HCFC) has been banned from production and import for new systems. Five years from now, it will be illegal to sell or use R 22 except to service existing systems. By 2020, all remaining R 22 systems in the U.S. will rely on a limited amount of stockpiled and recovered refrigerant. 2020 can seem like a
comfortable deadline until the vast number of systems that are still in operation with R 22 is considered. Although the phase out of R 22 is certain, it is uncertain the amount of stockpiled R 22 that will be available after 2020 and the price tag that will be associated with it. It should also be noted that this step in the phase out of R 22 systems occurred ten years ago in Europe. As HCFCs such as R 22 are phased out, HFCs will prove to be a suitable replacement or drop in where it is desired to reuse existing systems. While HFCs are not currently regulated and prove to be an alternative to R 22, it is possible that, in time, HFCs, similarly to HCFCs, will be phased out due to their characteristically high Global Warming Potentials. In April of 2009, the Environmental Protection Agency (EPA) proposed a rule to classify the mix of six different greenhouse gases as an air pollutant based on scientific evidence [that] compellingly supports a positive endangerment finding for both public health and welfare. 13 Included in these six different gases is hydrofluorocarbons (HFCs). On December 6 th, the EPA officially signed this finding which enabled the regulation of the gases under the Clean Air Act. This provides more evidence pointing toward the likely phase out of HFCs similar to HCFCs. It should also be noted that the R 22 ban that took effect on January 1 st, 2010, occurred ten years ago in Europe. This fact leaves reason to believe that the phase out of HFCs is likely to follow closer behind the phase out of HCFCs in the U.S. For this reason, forward thinking supermarkets will look beyond HFCs to natural refrigerants where new systems are required. International Codes Through researching the International codes and others that apply, no major deterring restrictions have been found for the use of an ammonia system in a supermarket. This does not disregard that certain counties and states within the U.S. have adopted their own local requirements for an ammonia system regardless of the system size. New Jersey, Chicago, and Los Angeles are examples. Possibly the best approach would be to avoid these areas (to begin with) when considering locations to implement ammonia. Doing so could allow the commercial ammonia market to grow and become familiar prior to the day that the U.S. is possibly faced with the phase out of HFCs at which time, the pressure to be more open minded toward the use of natural and efficient refrigerants will be much greater. Ammonia classifies as a B2 refrigerant, which means it is designated as toxic and flammable. This B2 rating is the foundation for which all codes and restrictions are applied to ammonia; the most influential of which, restricts ammonia from being used in any occupied space. Therefore, an indirect, secondary system becomes the natural choice in order to comply an example being, an ammonia chiller located on a rooftop or behind the store. This type of system could both chill a secondary fluid, such as a propylene glycol water mixture or
C0 2, and pump it into the store to cool the product. In such a system, the ammonia is fully limited to the outdoors; and as an added bonus, the ammonia charge is dramatically reduced. Both the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) have their own safety requirements for ammonia systems. The EPA s Risk Management Program (RPM) and OSHA s Process Safety Management (PSM) program are both designed to increase the safety of ammonia usage and are required to be implemented when the 10,000 pound ammonia threshold is exceeded. While these programs do promote a safer utilization of ammonia in large quantities, they are not required for smaller systems. In a supermarket, we should not expect to have any more than a couple hundred pounds of ammonia, so these programs will not apply. Depending on the state or county, one may need to incorporate a system to dilute, diffuse, or burn ammonia in the event of a discharge. However, none of these discharging methods are necessary if the fire code official determines upon review of a technical opinion report submitted by a professional engineer that a fire, health, or environmental hazard would not result from discharging ammonia directly to the atmosphere. 11 These technical opinion reports have been successful in the industrial world since it can be proven that ammonia is difficult to ignite, it has positive effects on the atmosphere, and the concentrations needed to endanger a human would not be present in low charge systems. Per the International Code, a supermarket is classified as a mixed occupancy since the sales floor classifies as a commercial occupancy and the receiving area and utility rooms classify as Industrial Occupancies. In fact, if an ammonia system was limited to the industrial portion of the building, it would fall under the same restrictions as any other Industrial ammonia application. However, there are additional freedoms realized by limiting the ammonia to an outdoor unit. For example, the International Mechanical Code (IMC) allows us to classify our system as Low Probability if the system components are isolated from the building. Or per ASHRAE 15, an outdoor unit is considered Low Probability if there is no way the refrigerant can enter an occupied space. 8 With an outdoor, Low Probability system, the ammonia restriction found on table 1103.1 in the IMC (2006) of 0.022 pounds per 1,000 cubic feet does not apply. Section 1104.2 supports this by stating that the only way to exceed the refrigerant amounts shown in table 1103.1 is for the system to be located outdoors or in a machinery room. Furthermore, section 1104.3.3 actually excludes ammonia from the 1,100 pound (total for all occupancies) restriction that other B2 refrigerants must adhere to. 10 Now that it has been established that an outdoor ammonia system should be utilized in a supermarket application, it may be helpful to know what the restrictions are for storing additional ammonia on site. This question leads us to the International Building Code (IBC). 12
Since ammonia is not flammable in liquid state, but gaseous, we must reference the liquefied flammable gas row in Table 307.1(1) in the IBC (2006). This table restricts the storage of a liquefied, flammable gas to a quantity of 30 gallons located within a 1 hour, fire rated, control area. (The IBC defines a control area as essentially any room within the building that contains a hazardous material.) Table 414.2.2 (2006 IBC) shows how the quantity of control rooms and their fire ratings can change, and how the allowable quantities of the hazardous material adjust, based on what floor level the control rooms are located on. The IBC and the International Fire Code (IFC) have similar restrictions for toxic materials; however, the flammable restrictions just mentioned are more stringent. In summary, ammonia can be stored inside the building by simply fire rating the room and installing the appropriate signage per the IFC and IBC. Finally, it should also be noted that all ammonia systems, regardless of the application, should adhere to all the requirements and specifications provided by the International Institute of Ammonia Refrigeration (IIAR) and by ASHRAE 15. ASHRAE 15 provides similar requirements to those found in the International Mechanical Code, and ANSI/IIAR 2 2008 covers everything the equipment manufacturers will need to observe (emergency pressure control systems, system component specs, acceptable grades of steel for flanges and fittings, etc). These specifications will still apply for ammonia in a supermarket application, so there is really nothing new to apply for a supermarket system that manufacturers aren t already designing around for their industrial, ammonia systems. Types of Systems 1. Liquid Overfeed System Liquid Overfeed systems are typically used for Low temp ammonia systems with multiple evaporators. They utilize a Low pressure receiver in conjunction with a standard high pressure receiver and a liquid pump to circulate (overfeed) liquid to the evaporators in order to maintain a fully wetted evaporator surface (Figure 1). Due to the higher compression ratios seen with ammonia for low temperature evaporators, evenly distributed refrigerant becomes an issue because of the high amount of liquid boiling within the evaporator. Thus, Liquid Overfeed systems have been industrially utilized over direct expansion ammonia systems when evaporator temperatures less than 0 o F are necessary. (The issue of even refrigerant distribution in evaporators can also be solved by using a gravity fed, flooded evaporator.) It is also recommended to utilize a pumped overfeed system when the evaporators are located significantly far away from the Low Pressure Receiver, or again, when there are multiple evaporators used. 5 In a grocery store application, where only 1 or 2 gravity fed, flooded
evaporators in a packaged system would be utilized it is not necessary to apply a Liquid Overfeed System. 2. Low Pressure Receiver System Figure 1 (Liquid Overfeed System) Low Pressure Receiver (LPR) systems utilize a Low Pressure Receiver but eliminate the need to pump the liquid to the evaporator. Figure 2 is a basic schematic of a low charge LPR system: Figure 2 (Low Pressure Receiver System) As apparent in Figure 2, there is also no High Pressure Receiver needed; however, the LPR must be sized large enough to contain the majority of the ammonia charge to allow for maintenance. Liquid draining from the condenser is sub cooled through a heat exchanger located at the bottom of the LPR and then is gravity fed to the evaporator. Just like the Liquid Overfeed System, a wetted evaporator is maintained and the wet return (liquid/suction mixture) is
fed back to the LPR where the liquid is separated from the suction. The Compressor(s) can then safely pull the dry suction from the top of the LPR. The LPR system has greatly grown in popularity around the world and has been effectively used in many different types of applications. Its popularity and usage can be attributed to the fact that it can deliver the same efficiency and performance as other ammonia systems while containing a very small ammonia charge. Typically charges of 0.8 pounds per ton of refrigeration have been accepted for these systems, when historically, systems with shell and tube evaporators and high pressure receivers have needed as much as 12 pounds per ton of refrigeration. 6 Since these numbers have been derived from industrial applications, it would be unrealistic to expect the same ratios for a smaller system in a supermarket. To be safe, if we assume 1.5 pounds per ton of refrigeration and consider a standard 55,000 square foot supermarket with a one million BTU load (83.3 tons) we re left with 125 pounds of ammonia for the entire store. Three of the biggest differences and greatest advantages of an ammonia LPR system compared to a typical halo carbon, parallel rack, DX system (most common in the United States) are as follows: 1. Evaporator efficiency is increased since the inner surfaces are fully wetted. A fully wetted evaporator surface means that all the heat absorbed into the ammonia is effectively used to evaporate it instead of superheat it. Also, since the superheat can be minimized, the compressors can also gain efficiency. 2. The system is given the ability to float head pressure at low ambient conditions since the flooded evaporator doesn t rely on a high liquid pressure to allow for proper operation of a thermal expansion valve. 3. Ammonia s high vaporization heat and sustainability (previously discussed). Compressors 1. Two Stage vs. Single Stage Savings seen by applying a two stage Ammonia compression system aren t usually significantly realized at evaporator temperatures warmer than 20 o F. Also, the first costs of a two stage system are more swiftly paid back in larger, industrial size systems. Sometimes, the first costs of a two stage system can be minimized when two different suction temperatures are required on one unit, since two separate suction temperatures are naturally gained by using a two stage system. These items, coupled with operation and maintenance considerations, should be accounted for when deciding what type of compression is best for a specific ammonia system; however, as a general rule, smaller systems (the size you would expect to see
in a supermarket) operating with evaporator temperatures above 20 o F will typically be singlestage. 2. Reciprocating vs. Screw Both Reciprocating and Screw compressors can be, and are, used in ammonia applications. Some of the main criteria found for deciding which type of compressor to use is listed below: Reciprocating compressors have been found to be significantly more efficient than Screw Compressors at part load conditions. 7 In smaller applications with evaporator temperatures above 50 o F, reciprocating compressors are more efficient. In a case with reverse parameters, the Screw compressors are more efficient. 7 In a retrofit scenario where more capacity is needed, a screw compressor generally can double the capacity of a reciprocating compressor without needing additional space for installation. 7 Up front costs are generally lower when using screw compressors in very large applications, since fewer screw compressors would be needed in comparison to reciprocating compressors. In smaller applications, 1 or 2 reciprocating compressors are cheaper than 1 screw compressor. 7 There are fewer parts to replace on a screw compressor, but a failure usually means replacing the entire compressor. A reciprocating compressor has more parts that can fail, but they usually can cheaply be replaced in the field. 7 3. Additional Considerations Thus far, when the design parameters of a typical supermarket are considered, the above information seems to point in favor to reciprocating compressors as the more appropriate option, but there are further considerations that need to be taken. Due to ammonia s high index of compression, evaporator temperatures below 10 o F will cause the discharge temperature for a single stage reciprocating compressor to be too high. In this case, two stage reciprocating compressors would be required. Another option would be to use a screw compressor with oil cooling to reduce the discharge temperature. Beyond this, it is important to think about the fact that the overwhelming majority of existing supermarket refrigeration systems in the U.S. utilizes reciprocating compressors. Therefore, we can be certain that reciprocating compressors will be the most familiar option for servicing contractors. However, since hermetic reciprocating ammonia compressors aren t available due to the incompatibility of ammonia and copper (in the motor windings), the added challenge of aligning shafts and dealing with shaft seals will be present. This task will not be new to all
contractors, however, thanks to the increase utilization of secondary systems which require similar attention to alignment and seals on the secondary pumps. Evaporators Flooded Shell and Tube style evaporators have traditionally been used in industrial ammonia refrigeration when cooling a secondary fluid; however, out of the necessity for smaller packaged systems and lower ammonia charges, plate and frame and/or plate and shell technology has been effectively incorporated and utilized as both evaporators and condensers. Depending on the size of the system, one may choose a plate and frame evaporator over a plate and shell evaporator. One may also choose a welded or semi welded plate heat exchanger depending on if it may need to be expanded in the future or not. Some of the main criteria found for deciding which type of evaporator to use is listed below: A plate and frame evaporator would typically be used for 30 Ton systems and larger. 6 These evaporators are the largest of the plate heat exchangers but they can most easily accept additional plates to gain future capacity since they are bolted together and not welded. A plate and shell heat exchanger would be most suited as an ammonia evaporator that serves to condense Carbon Dioxide in a Cascade type system where low temps and high pressures need to be accounted for. Typically, smaller plate heat exchangers would not need a frame. These smaller units are usually welded or fused and would be appropriate for smaller tonnages where the capability for future capacity expansion isn t critical. All ammonia heat exchangers need to be Carbon or Stainless Steel. Smaller units can be made with copper to enhance heat transfer but only if they re electro tinned. 6 Condensers Condensing ammonia can be achieved by the same methods and technologies as standard Halo Carbon refrigerants. The questions to ask when deciding what type of ammonia condenser to select would also be the same questions you would ask when designing a Halo Carbon system. The condensing pressures are similar with both refrigerants and many ammonia systems use standard evaporative or air cooled condensers that are equipped with stainless steel tubes instead of copper. Typically, the size of the ammonia charge is the biggest concern and so some systems use adiabatic, air cooled condensers to reduce the condenser size by lowering the condensing temperature. The most effective and most common way to reduce the ammonia charge on the high side is to use a fluid cooled condenser in conjunction with a closed loop fluid cooler; in which case, a plate heat exchanger can be very effectively utilized as an ammonia condenser.
Conclusion Supermarkets that look to apply an ammonia system should be confident in ammonia s safety, efficiency, and sustainability. Moreover, utilizing ammonia won t mean they need to reinvent the wheel. Ammonia systems have been used around the world for many years in various types of industries and applications more specifically in supermarkets. It is the hope that by shedding light on an applicable ammonia system and its design considerations for a supermarket, and by exploring the International Codes and others that apply, this paper will help raise the comfort level of ammonia usage in the U.S. supermarket industry.
References 1. Equipment, Design, and Installation of Closed Circuit Ammonia Mechanical Refrigeration System, IIAR 2 2008, International Institute of Ammonia Refrigeration, Washington, D.C. 2. Ammonia as a Refrigerant, January 2002, ASHRAE, Atlanta, Ga. 3. Ammonia and its Reputation as Refrigerant, Anders Lindborg, and Opportunities for the Application of Natural Refrigerants, DR. Daniel Colbourne, from: Natural Refrigerants, Sustainable Ozone and Climate Friendly Alternatives to HCFCs, July 2008, Proklima, Eschborn, Germany. 4. ASHRAE Handbook 2006, Refrigeration, ASHRAE, Atlanta, Ga. 5. Liquid Overfeed Systems, n.d., Ramesh Paranjpey 6. IIAR Technical Paper #5, Low Charge Ammonia Plants: Why Bother? 2003, Andy Pearson, Star Refrigeration, Glasgow, Scotland, UK. 7. Compressors: screw or reciprocating? Retrieved December 8 th, 2009, from http://www.sabroe.com/download/screw or reciprocating.html 8. ANSI/ASHRAE Standard 15 2007, Safety Standard for Refrigeration Systems, ASHRAE, Atlanta, Ga. 9. ANSI/ASHRAE Standard 34 2007, Designation and Safety Classification of Refrigerants, ASHRAE, Atlanta, Ga. 10. International Mechanical Code 2006, Chapter 11, International Code Council, INC. 11. International Fire Code 2006, Chapter 6 & Chapter 34, International Code Council, INC. 12. International Building Code 2006, Chapter 3 & Chapter 4, International Code Council, INC. 13. Proposed Endangerment and Cause or Contribute Findings for Greenhouse Gases under Section 202a of the Clean Air Act, Lisa P. Jackson, Environmental Protection Agency, 40 CFR Chapter 1, Prepublication Copy, 4/17/09.