- Ammonia is the most dominant refrigerant within industrial refrigeration over the years. It’s a well-proven and effective refrigerant with more than 100 years of experience.
- But even if it has been well-proven, new applications and increased requirements for product and methods are still challenged.
For several years, the global refrigeration industry has been going through several changes. In particular, the global environmental impact of some refrigerants — such as the ozone depletion, global warming and the greenhouse effect — has changed the industry considerable. The environmental impact is mainly focused on the synthetic refrigerants such as CFC, HCFC, HFC and HFO.
Natural refrigerants, such as ammonia, carbon dioxide (CO2) and hydrocarbons are not affected by environmental restrictions; however, toxicity and flammability are issues that restrict the use of these refrigerants.
Compared to many other industries, the industrial refrigeration industry, is often claimed to be “old fashioned”. Compared to some industries, the development within industrial refrigeration moves slowly, however, taking a look back, many new applications, product and methods has been implemented over the last years.
Ammonia is the most dominant refrigerant within industrial refrigeration over the years. It’s a well-proven and effective refrigerant with more than 100 years of experience. But even if it has been well-proven, new applications and increased requirements for product and methods are still challenged.
New refrigerants within industrial refrigeration are not very common, but a couple of “old” refrigerants have been re-introduced: CO2 and hydrocarbons.
The number of industrial refrigeration system with CO2 has increased considerable over the last 10 years.
Ammonia is generally a well-proven and effective refrigerant, but unfortunately it also has a couple of negative features.
Due to ammonia’s classification as toxic, there are a number of restrictions which are important to consider when designing ammonia systems.
1. Generally, it’s favorable to limit the ammonia charge in the system, in order to minimize the safety precautions.
2. Due to the low density of ammonia, the efficiency of ammonia systems becomes less attractive at low temperature, compared to e.g. ammonia/CO2 cascade system.
An ammonia/CO2 cascade system is a unique way to combine the benefits of two natural refrigerants. Typically the ammonia charge can be reduced to approximately one-tenth of a traditional ammonia system, which minimizes the safety requirements.
An ammonia/CO2 cascade system has an attractive efficiency at low temperature, compared to traditional Ammonia systems.
An ammonia/brine system is another way to reduce the Ammonia charge, but compared to the ammonia/CO2 cascade system, the brine system has a less attractive efficiency (see Table 1).
The increased focus on energy cost has brought more attention on re-use of energy, in particular within industrial processes, this has increased the focus on Ammonia heat pumps, operating at high pressure.
Applications with Ammonia for high temperature has been limited, due to the lack of suitable components, but recently components with higher pressure and higher temperature ratings has been made available on the market.
In the past ammonia was exclusively used on large scale installations, but with new environmental requirements on traditional large commercial systems, several of these are today using ammonia, such as large food retail installations that are installing ammonia/CO2 systems.
Small ammonia DX systems are not widely used yet, mainly due to lack of the proper components. This is still one of the challenges for the industry.
New Technologies, New Performance Targets
Moving forward, new industrial refrigeration technologies will be implemented, and new targets will have to be met.
Safety and environmental requirements are definitely a must for industrial refrigeration systems. Manufacturers simply need to follow the requirements and ensure compliance.
Safety requirements can be divided into two groups: Safety requirements for products, and safety requirements for the operation of refrigeration systems.
For components and system manufacturers, leak requirements are an essential issue in being in compliance with the environmental requirements. Increased focus on tightness is a global trend. This means that components and systems need to be tight, and therefore, all possible leak potentials need to be eliminated/minimized. Flanged, screwed and similar joints have a higher leak potential, than permanent joints, and therefore it is obvious to eliminate these, where possible.
• The global trend for safety and environmental requirements is supported by the international standards for refrigeration systems. ISO 5149 and ISO 14903 are two new international standards for refrigeration systems, which will be released shortly. Both standards are based on the existing European standards (EN 378 and EN 16084)
Design pressure of Components
The need for components suitable for higher pressure is definitely increasing.
The refrigeration industry is a relatively small industry, and the different manufacturers need to have a strategy, for which market/application they want to serve.
Looking into the actual need for valves with PS 65 bar for ammonia heat pumps, the potential is very limited, however if this potential is combined with the need for high pressure CO2 valves, then it could be attractive to design valves for this pressure rating.
Components for 90 bar are attractive in CO2 systems, due to the fact that this would ensure that the system pressure during stand-still will not increase above the maximum working pressure, and thus a backup cooling system would not be needed. However, it’s not likely that the larger refrigeration valves and components will be available for 90 bar in the near future. Increasing the pressure above 65 bar will require a completely new platform, which will lead to dramatic investments for component and equipment manufacturers, and the additional cost will most likely not be financially attractive.
Higher Requirements for Components
There is and will continue to be a lower tolerance for system leaks. One step toward this goal is to eliminate flanged valves, and all parts and components that have a leak potential.
Valves must be suitable for several types of refrigerants and oils and be able to operate in an increased operating range.
Only qualified low temperature steel/stainless steel is acceptable valve material for housings and bonnets. Refrigerant neutral sealing elements are preferred.
Cost Effective Solutions & System Efficiency
There is a higher demand for high performance valves with low pressure drop, and a higher demand for valves with enhanced control performance. This will include an increased use of electronic components, control strategies to bring “tighter” temperature control, and flexible control strategies to ensure optimum operating conditions at full and part load.
There has also been an increased focus on system efficiency with electronic controls and motorized valves. Wen properly designed and coupled to electronic controls, motorized valves can provide optimum and precise response from control signals to control flow and/or pressure — and indirectly, temperature — by the following:
• minimizing or eliminating pressure surges which frequently lead to compressor loading
• minimizing or eliminating transient conditions motorized valves can be configured and controlled to match load requirements
• using motorized valves, which provide their own source of power and don’t rely on system pressure or pressure differential to function
• on suction lines, traditional piloted control valves require a minimum 0.2 bar of pressure differential to fully open.
N.P. Vestergaard is a research scientist for Danfoss A/S Industrial Refrigeration.
For the complete article, as presented during the 2013 IIR Ammonia Refrigeration Conference, visit bit.ly/industrialreftrends. Used by permission.