Application of Ammonia in U.S. Supermarkets by Caleb Nelson, PE, LEED AP, CTA Architects Engineers Introduction In the U.S. supermarket industry, the fear of ammonia (R717) and the misconceptions of 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 traditional halocarbon refrigerants. The goals of this article are to: 1) expose ammonia s potential for safe application; 2) look into the codes governing the majority of the U.S. and determine what restrictions may apply; 3) introduce what types of systems have successfully been utilized elsewhere in the world in supermarket applications; and 4) provide some basic design considerations regarding the selection and operation of the basic components of these types of ammonia systems. Safety 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. Conceptions of ammonia as an unsafe refrigerant are, to some degree, justifiable due to the fact that it is toxic and classed as slightly flammable. It is the belief of the author, however, that due to the type and application of the system that will be described here, any risk due to ammonia s toxicity and flammability can be nearly eliminated. The majority of ammonia s use as a refrigerant today is in large, industrial systems, which may contain as much as 200,000 lbs. of ammonia or more. In most industrial systems, ammonia plants are located within the building and ammonia is carried in distribution piping to areas where employees work. The combination of this type of application and large ammonia charges creates a Ammonia in U.S. Supermarkets continued on page 6 4 Condenser August 2011 A Publication of the International Institute of Ammonia Refrigeration
Ammonia in U.S. Supermarkets continued from page 4 scenario where the potential for accidents and unsafe practice can sometimes be substantial. In recent years, industrial systems have utilized secondary fluids to minimize potential accidents involving employees within the building by restricting the ammonia charge to the machinery rooms; however, there still exists the possibility of releasing these large charges to the atmosphere. Since the subject of this paper is the application of ammonia in supermarkets, it is extremely important to differentiate between the industrial application of ammonia and the commercial, supermarket application of ammonia. Less than 200 or 300 pounds of ammonia can typically be expected for a supermarket application. Also, due to codes that will be discussed later, an ammonia system cannot be installed within a commercial space. Therefore, an outdoor, rooftop, ammonia chiller is the most feasible option. A packaged type of system with the refrigerant charge residing outdoors alleviates the potential for accidents involving employees and customers within the building. These two key differences suggest that a supermarket ammonia system would be inherently safer than most industrial systems; however, it would be foolish to rely on these qualities alone to assure a safe utilization of ammonia in supermarkets. Both the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) have safety programs that are required for systems with more than 10,000 lbs of ammonia. Although these regulations are not required for supermarket-size systems, there are still many system design considerations that should be made to ensure safe applications of ammonia for supermarkets. Just as with any refrigeration system, a risk assessment should be performed to determine the appropriate safeguards and measures to reduce the risk to an acceptable level. Performing an atmospheric dispersion model of an ammonia release is something that is common for industrial systems, and can be done for small commercial systems as well. It would be handy if a single analysis could be performed to cover all worst case conditions; unfortunately, weather patterns, humidity levels, and the off-site receptors change with each new location. Therefore, the off-site consequences for a worst case ammonia release is something that should be explored for each location until experience exposes the safety of small rooftop systems. Depending on the site location, we can predict the behavior of an ammonia release based upon a scenario such as a pressure release valve (PRV) lifting. When released, the pressure in the system drops and allows the PRV to close which allows for an intermittent release. The natural rise and dissipation of ammonia gas (due to ammonia gas being lighter than air) teamed with an intermittent release and a small charge generates a scenario where a release through a PRV will most likely go unnoticed in most locations. Other scenarios do exist that may interfere with the rise and dissipation of ammonia such as down drafts from surrounding buildings or extreme humidity levels. As previously mentioned, the worst case needs to be considered separately for each location. Using an outdoor ammonia chiller enclosed in removable housing (instead of applying an outdoor chiller house ) eliminates the possibility for technicians to ever be trapped in a toxic environment due to a leak while still providing protection for the chiller from the outdoor elements. Regardless, leak detection should be used in coordination with an exhaust system to prevent ammonia concentrations at the chiller from ever reaching flammable levels. Since ammonia gas rises, an exhaust fan at the top of the unit or in a chimney exhausting upward would further promote the rise and dissipation of ammonia into the air. Beyond these design considerations, a preventative maintenance program is necessary for all ammonia systems to assure that all equipment and safety systems are functioning properly. Periodic testing of components such as the leak detection system, alarms, evacuation system, and safety cut out switches should be performed and pressure relief valves should be replaced every five years. Inspection checklists should also be developed to aid in the swift discovery of any physical damage or corrosion to the system. Finally, training and education on the systems and the safe work practices for ammonia is critical to ensure that technicians can safely and effectively carry out these tasks. Codes Through researching the international codes (IBC 2006, IMC 2006 chapter 11 & IFC 2006 section 606) and others that apply (ANSI/IIAR 2-2008 & ANSI/ASHRAE Standards 15-2007 & 34-2007), 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 commercially. 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 a phase out of HFCs at which time, the pressure for authorities to be more open-minded toward the use of natural and efficient refrigerants will be much greater. Before discussing further the code restrictions, it is important to acknowledge the EPA s Significant New Alternatives Policy (SNAP) program. This program has been set up under the Clean Air Act and is designed to identify and regulate approved alternatives to replace ozone-depleting chemicals. It is illegal to utilize a refrigerant that has not been approved by the SNAP program. Before the first system could be installed in the US in recent years, had to traverse a year-and-a-half-long application process to become approved for use. The main focus of this process is to ensure that the chemical is safe for people and the environment. It is 6 Condenser August 2011 A Publication of the International Institute of Ammonia Refrigeration
comforting to know that ammonia has in fact been deemed safe, and has already been approved by the EPA s SNAP program for use in secondary applications in supermarkets. Until recently, ASHRAE 34 classified R717 as a B2 refrigerant, which means it was designated as toxic and flammable. As the result of a recent addendum, R717 is now classed as a B2L refrigerant along with other mildly flammable refrigerants that have a proven burning velocity less than or equal to ten cm/s. Ammonia s toxicity class remains unchanged but it is now recognized to be less flammable than a B2 refrigerant. This classification shift is still very new and has yet to materialize into any tangible changes in the codes pertaining to ammonia s use. Therefore we must still treat ammonia like a B2 refrigerant. This B2 classification 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 represents the only choice in order to comply an example being, an ammonia chiller located on a rooftop. This type of system could both chill a secondary fluid, such as a propylene glycol-water mixture or, and pump it into the store to refrigerate 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. 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. It should be noted that the preferred method of release by ASHRAE and the International Institute of Ammonia Refrigeration (IIAR) is a direct release to the atmosphere and that only in special applications may it be determined necessary to utilize a burning, diffusing, or diluting system. 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 the outdoors. 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. 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. 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 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). Systems One specific example of an ammonia, supermarket system that has been operating since mid November of 2009 can be seen in a Pick n Pay store in Strand, South Africa. This store uses one rack that contains three ammonia compressors and three compressors. Each ammonia compressor is an open-drive, reciprocating compressor equipped with cylinder unloading. The ammonia evaporator is a plate and shell type which is connected to a low pressure surge vessel. One side of the evaporator condenses the low-temp system and the other side cools the glycol for the secondary, medium-temp system. This entire system is located in a rooftop, machinery room equipped with leak detection and an automatic exhaust system. Adjacent to the machinery room is the ammonia evaporative condenser which uses a high pressure float system to expand the liquid and regulate the flow directly to the evaporator. Because of the float system there was no liquid receiver needed which allowed for a small ammonia charge of approximately 286 lbs. to be achieved. Another system that is a good candidate for a supermarket application is the Low Pressure Receiver (LPR) system. The evaporator in this type of system remains fully wetted and actually realizes a slight overfeed driven by pressure differences within the system. Figure 1 is a basic schematic of a low charge LPR system cascaded with a system: Figure 1 (Low Pressure Receiver System) Ammonia in U.S. Supermarkets continued on page 20 Condenser August 2011 A Publication of the International Institute of Ammonia Refrigeration 7
Ammonia in U.S. Supermarkets continued from page 7 As apparent in Figure 1, there is 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. The compressor will maintain the saturated suction pressure in the LPR by pumping dry ammonia gas to the condenser. Liquid draining from the condenser is then sub-cooled through a heat exchanger located at the bottom of the LPR and then is fed to the evaporator. A mild expansion is typically provided by a motorized valve at the evaporator inlet which is controlled by a condenser drain float. The return line from the evaporator is a wet return (liquid/suction mix) which feeds into the LPR where the suction and gas are then separated. The LPR system has been introduced in this article because it can deliver the same efficiency and performance as other ammonia systems while containing a very small ammonia charge. Figure 1 depicts plate-type heat exchangers for both the evaporator and condenser which allows for the lowest charge possible. 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. 1 Since these numbers have been derived from industrial applications, it would be unrealistic to expect the same ratios for smaller systems in supermarkets. Conservatively, if we assume 1.5 pounds per ton of refrigeration and consider a standard, 55,000 square-foot supermarket with a 1.5 million BTU load (125 tons), we re left with 188 pounds of ammonia for the entire store. Beyond the low charges that both of these types of systems are able to achieve, the fundamental means for which high levels of efficiency can be reached in comparison to standard supermarket systems are listed below: 1. Flooded or slightly overfed evaporators: These evaporators are extremely efficient since they maintain a fully wetted evaporator surface. Therefore, all the heat absorbed into the ammonia is effectively used to evaporate it instead of superheat it. 2. Low suction superheats: Compressor efficiency is increased due to the extremely low suction superheats seen leaving the evaporator. 3. Floating head pressure: Just as with any high side of a secondary or cascade system, head pressures can easily be floated as low as the compressor will allow. 4. Open drive compressors: Suction gases are not used to cool the compressor motors and so this additional heat does not end up in the refrigeration system. In addition to low charges and efficiency, there are additional system qualities that are equally crucial in order for ammonia to be successfully implemented in U.S. supermarkets. First of all, tight, factory-built systems will need to be applied in order to reduce the probability of leaks. Secondly, manufacturers will need to look for every opportunity to keep the system costs as low as possible. It should be understood that initially, commercial ammonia system will be more expensive than standard systems; therefore, manufacturers will need to eliminate any unnecessary components and consider less complicated designs without compromising efficiency and/or reliability. Thirdly, it will be desirable for the systems to be maintainable by commercial technicians. Although additional technician training will be necessary for this to be possible, any opportunity to integrate familiarity into the systems should be taken. For example, utilizing reciprocating compressors instead of screw compressors would be the more familiar option since the overwhelming majority of U.S. supermarkets use reciprocating compressors today. System Components Beyond the consideration of reciprocating versus screw compressors, one must also consider the application of single stage versus two stage compression. These questions cannot be answered independently since the compressor technologies and application ranges are so different between the two types of compressors. For example, ammonia evaporating at low temp conditions ( 20ºF) with a condensing temperature of 105ºF would force a traditional, open drive, reciprocating compressor to operate outside of its envelope. This is due to the characteristically high discharge temperatures seen with ammonia. In this case, a two-stage system would be required. Another option for this scenario would be to use a screw compressor since they have a much larger application range and can effectively use oil cooling to keep discharge temperatures within the desired range. For most supermarket applications, however, ammonia will be evaporating at medium temperature conditions (15º 20ºF) on the high side of a cascade or secondary system. In the majority of these applications, a single stage, reciprocating compressor could be used. Measures can also be taken to reduce condensing temperatures, such as using an evaporative condenser or a fluid cooled condenser in order to allow for the use of reciprocating compressors in warmer climates. Although it can be agreed that reciprocating compressors would be the most familiar option for commercial technicians in the U.S., applications with ammonia do not permit the use of traditional, semi-hermetic compressors. This is due to the incompatibility of ammonia and copper (in the motor windings) which means that the added challenge of aligning shafts and dealing with shaft seals will be present. This task will not be new to all contractors, though, thanks to the increased use of secondary systems which require similar attention to alignment and seals on the secondary pumps. In some ways, Ammonia in U.S. Supermarkets continued on page 22 20 Condenser August 2011 A Publication of the International Institute of Ammonia Refrigeration
Ammonia in U.S. Supermarkets continued from page 20 an open drive compressor represents a more efficient design since the heat generated from the motor doesn t contribute to higher discharge temperatures (which would further limit the reciprocating application range and ultimately represent larger, heat-rejection requirements). Despite this advantage, open drive reciprocating compressors available today in the U.S. will still require water-cooled heads. It should be noted that semi-hermetic, reciprocating, ammonia compressors have recently been developed to withstand higher discharge temperatures. Because of this, they can operate in a larger range of evaporating and condensing temperatures which would allow for the use of air-cooled condensers wherever they are being used today. Since these compressors are semi-hermetic, the small leakage seen with open drive compressors is eliminated. These compressors also utilize an Interior Permanent Magnet (IPM) motor which has been proven to be more efficient than standard induction motors despite the required aluminum windings in the stator. Although they have yet to gain UL approval, these compressors may play a significant part in the future of commercial, NH 3 systems for the reasons discussed here. High discharge temperatures do not pose as great of a threat with the use of screw compressors. Screws have the ability to utilize oil cooling as a means to cool the discharge gas due to the fact that the oil is injected directly into the compression space. Cooling the oil for a reciprocating compressor wouldn t be useful since the oil is mainly contained in the sump where it wouldn t have the same opportunity to influence the discharge gas. Although oil cooling, used as a means to lower discharge temperatures, makes screw compressors more applicable to all climates, it is associated with added costs and system components that may not warrant the use of screws on every system especially if the goal is to reduce system costs and components. The efficiency of screw compressors must also be questioned when considering their usage in small commercial systems since small rotor diameters in small machines adversely affect the compressor s efficiency. Part load operation can also negatively affect screw efficiencies. Screw compressors should only be unloaded down to a 50 percent rotor speed due to the efficiency loss realized at speeds lower than this. Furthermore, screw compressors realize a significant reduction in efficiency as they are unloaded via the use of slider valves. Therefore, radical swings in ambient temperatures can greatly reduce the efficiency of these machines by forcing them to run a significant percentage of time at part-load conditions. These are only some of the issues that must be considered when deciding between reciprocating and screw compressors. For valid reasons, both types of compressors are widely used in ammonia applications today. It is perceivable that as supermarket ammonia systems become more widely used, the advantages and disadvantages of both compressor types in commercial applications will become more apparent after they have had an opportunity to operate within the parameters of system costs, operating costs, familiarity, and maintainability. Flooded shell and tube evaporators have traditionally been used in industrial ammonia refrigeration systems 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 successfully used as both evaporators and condensers. A plate and shell heat exchanger would be most suitable 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. Where ammonia is used to only chill a secondary fluid, smaller welded or fused plate heat exchangers would be appropriate. Additionally, any heat exchanger that is used with ammonia will typically be carbon or stainless steel. Smaller units can be made with copper to enhance heat transfer but only if they re electro-tinned. 1 Condensing ammonia can be achieved using the same methods and technologies used for standard halo-carbon refrigerants. The condensing pressures are similar with both types of 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 increasing the design TD. 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 Supermarket owners that look to apply ammonia should be confident in the fact that a properly implemented system can be extremely safe and efficient. Beyond this, there are no deterring code restrictions preventing its use in the majority of the U.S. Designers should also be re-assured by the fact that utilizing ammonia commercially doesn t require the re-invention of the wheel. Ammonia systems have been used around the world for many years in various types of industries and applications and more recently in supermarkets. Although initial system cost and technician training are hurdles that are indeed real, it is comforting to know that they are only temporary and that they are no different from the hurdles that and other new technologies are facing. References 1. IIAR Technical Paper #5, Low-Charge Ammonia Plants: Why Bother? 2003, Andy Pearson, Star Refrigeration, Glasgow, Scotland, UK. (p. 159) 22 Condenser August 2011 A Publication of the International Institute of Ammonia Refrigeration