Typical industrial refrigeration systems — approximately 90 percent of installations today — utilize ammonia as the primary refrigerant. The refrigerant charge of these systems, however, can range from 5,000 lb on the smallest systems to as much as 200,000 lb in large food processing plants. Using high volumes of any kind of chemical requires an added layer of responsibility, including structured safety protocols, risk mitigation and a proactive approach to maintenance. Thus, the demand to quantify, measure and mitigate the risks associated with these industrial facilities has accelerated the adoption of new technology, changes to best design practices and a formal structured approach to plant operation.
In recent years, the cold chain has relied primarily on the use of natural refrigerants to meet food safety requirements, which, fortunately, has positioned the industry such that it is less likely to be burdened by the uncertainty and challenges of hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) refrigerant regulations and availability in the future. Most of the industrial refrigeration plants in operation today have been designed for the use of ammonia, which provides outstanding thermodynamic efficiency with lower mass-flow rates than synthetic refrigerants. With its inherent efficiency, natural properties and low price point, ammonia is expected to remain the dominant refrigerant in this market segment.
In addition to its many benefits, however, an ammonia-based refrigeration system presents risks, including the chemical itself. The industry has been challenged to meet a higher level of regulation to mitigate the inherent risks of this natural refrigerant choice. One way to limit risk is to implement methods to minimize the refrigerant charge.
As processing and cold storage plants have undergone significant growth in capacity in the last few decades, so have refrigeration systems. The need for larger vessels and pump-circulated ammonia has become the standard, and it aligns quite well with the expansion requirements of the industry. Today, investments in the cold chain are favoring technologies that enable the growth, sustainability and safety of new and existing plants. This emerging trend has been seen across small, medium and large facilities, regardless of the nature of the product being refrigerated. The emergence of more stringent regulations, food safety requirements and constraints in space has led to the investigation and implementation of alternatives to reduce ammonia charge overall. Some of the most widely known technologies include:
- Ammonia electronic direct expansion (DX) systems. This provides electronic injection control for centralized plants.
- Ammonia/carbon dioxide (NH3/CO2) refrigeration systems.
- Compact packaged and self-reliant refrigeration systems with direct expansion operation.
Each provides advantages and deserves a place in refrigeration plant design, construction and operation.
Ammonia Electronic Direct Expansion Systems
Direct expansion — or DX, as they are often called — systems have been known in the industry for a long time due to the use and availability of thermostatic expansion valves. In the past, however, facility owners and contractors have questioned the performance and reliability of ammonia thermostatic expansion valves. Unstable liquid injection and flooding typically lead to poor food quality.
While electronic expansion valves have overcome many of these barriers — rendering clear and sound solutions to the market — progress also must be credited to the advancement of dedicated algorithms used by stand-alone controllers. These controllers facilitate the task of PLC-based control as well as changes in evaporator and heat exchanger designs.
Electronic ammonia direct expansion systems use a simple, well-known and reliable principle of controlling the superheat of the refrigerant at the outlet of the evaporator. Because electronic superheat control adjusts to capacity and changes in system pressure conditions, it results in a highly efficient room temperature or process control. The main benefit, however, can be measured as the dramatic reduction of the ammonia charge both at the evaporator and the engine room sides. In most applications, an electronic direct expansion system easily can be added to a plant expansion where vessels would be at maximum capacity and the need for larger investments in equipment and real estate could lead to cost concerns.
Electronic control of superheat in ammonia systems has successfully been applied to both medium and low temperature evaporators, making it an attractive alternative to pump-circulation systems.
Superheat control via electronic controllers and sensors, coupled with improvements in heat exchanger design, has become a proven option for reducing refrigerant charge. In ammonia systems, superheat has been reduced to a stable, middle-single-digit figure compared to mechanical thermostatic valves that require anywhere between 10 and 15 degrees. Pressure transmitters and temperature sensors connected to an advanced stand-alone superheat controller provide accuracy and flexibility in small or large plants, releasing central PLCs from tedious, time-consuming iterations that tend to slow down the communication networks. When it comes to the choice of liquid-injection valves, it is important to select a technology that works at both full and partial load.
Both pulse-width-modulating valves and motorized valves have been carefully crafted with advanced materials to provide a reliable, safe performance in evaporator flow control. They can enable the leap to successful use of direct expansion systems on an industrial scale, allowing facilities to reduce the overall ammonia charge.
A traditional method of reducing refrigerant charge is to utilize a secondary cooling medium. In the past, pumped glycol was used to exchange heat between the ammonia system and the refrigerated space. While the system offered a simple alternative, offsetting negative consequences such as reduced efficiency and multiple design compromises had to be considered. Electrical energy typically is the second largest operating cost in most industrial facilities, so glycol solutions often were not a cost-effective option.
A number of developments have occurred that have improved the options for using a secondary cooling medium to reduce the ammonia charge. They include simple pumped carbon dioxide systems (often known as volatile carbon dioxide brine systems) and cascade systems where carbon dioxide compressors are used. Regardless of the choice of system, the introduction of carbon dioxide to all of the refrigerated areas significantly reduces the ammonia charge in the plant and brings some additional benefits during the installation of pipelines.
In principle, ammonia is contained in the engine room, where screw compressors and heat exchangers — typically plate-and-shell units — are used to condense the returning carbon dioxide vapor from the refrigerated areas. The technology has made a leap from process industries — where very low temperatures were required for deep-freezing — to cold storage and process areas where sensitive products and large numbers of people could be exposed to an ammonia release. In both cases, electronic direct expansion for carbon dioxide can be used, making it ideal to further reduce overall refrigerant charge.
Regardless of the design specifications, using carbon dioxide as a secondary refrigerant reduces the ammonia charge by up to 90 percent. In terms of energy efficiency, carbon dioxide can provide the benefits of a glycol system while reducing the power consumption an additional 20 percent compared to pumped glycol.
Packaged Refrigeration Systems
The next step in the evolution of industrial design and construction has been the development of self-contained, somewhat portable and easy-to-install ammonia ultra-low charge, or ammonia/carbon dioxide, packaged systems.
Packaged systems offer further reduction in ammonia charges thanks to efficiencies gained in evaporator design, defrost strategies and liquid-injection control. Ammonia/carbon dioxide packaged systems are available from a number of OEMs and vary depending upon the size and the needs of the plant. These packaged refrigeration solutions can be used for new construction, expansion or retrofit applications. These systems allow for similar energy usage while eliminating the need for centralized pumped recirculation vessels and piping across the facility, which are the primary drivers of large ammonia charges.
Ammonia ultra-low-charge systems are emerging as a potential solution for cold storage or distribution centers. These packages offer a reduced refrigerant charge and the ability to work with a single refrigerant while utilizing a refrigerant that is well known by most stakeholders in the industry. Package manufacturers assert that the units can be installed in a fraction of the time of centralized systems.
There is no single answer to mitigating risk and reducing charge in industrial refrigeration installations. Recently, a general consensus that refrigerant charge reduction needs to be accelerated and prioritized in facilities has led to multiple viable solutions for the industry. Regardless of the circumstances of the facility, these solutions can be implemented today to provide the risk mitigation required without penalties to energy efficiency and while maintaining or improving food safety and quality.