In the world of process cooling and freezing, there are diverse operations and needs. As such, selecting equipment appropriate to the application is essential if you are to achieve good results.
As a young engineer, one of things that attracted me to this industry was the diversity of the processes we needed to satisfy, and the myriad demands of the applications to which we were applying refrigeration solutions. My mentors would always emphasize that choosing the correct equipment, and implementing an effective means to control it, were paramount. Moreover, the course — that is, the application and its requirements — would play a big part in the decision-making process to identify the appropriate cooling solution.
Choices start with the basics such as which refrigerant to use. This should be aligned with the ever-growing need to switch from synthetic refrigerants to natural solutions like ammonia, carbon dioxide (CO2) and hydrocarbons. Not only are these natural refrigerants better for the environment, they are, in most cases, more efficient in terms of energy usage.
Many advances have been made in recent years to improve the performance of systems using natural refrigerants. Major international suppliers have invested large research and development budgets to concentrate on natural refrigerant solutions. As a result, industrial processors can select from several natural refrigerant-based systems:
- Low charge ammonia systems.
- CO2 trans-critical systems.
- CO2 cascade systems.
While the latter is being used in almost all domestic refrigerators today, the others are used widely in industrial process cooling, among other uses.
Speed control gives modern chillers good part-load characteristics while their compact design and high efficiency heat exchangers provide design alternatives. They also offer the use of low charge ammonia in the installation.
Know Your Course
Each application has its own specific requirements — and plenty of them! There are the obvious requirements like temperature level but also the more complex like part-load, for instance.
It never fails to amaze me how often plant owners or operators really do not know how their plants perform, either daily or seasonally. But, with a better understanding of the application, the experts, by measurement and know-how, can help improve the accuracy of these predictions. It is an essential part of the equipment-selection process in matching the load and choosing the correct number of compressors based on the system part-loads or pull-down requirements.
Batch cooling and freezing also has its challenges. Again, understanding the process requirements and the demands of the operation will result in appropriate equipment selection and control. The use of cascade systems that employ multiple refrigerants like ammonia and CO2 is also a trend that can have benefits for the end user. Also, CO2 can be used in trans-critical mode; as such, depending on climatic conditions, it can be an interesting alternative.
Certain modern compact chillers offer solutions for systems that are suited to applying secondary refrigerants like glycols and brines. Speed control gives these chillers good part-load characteristics while their compact design and high efficiency heat exchangers provide design alternatives. They also offer the use of low charge ammonia in the installation.
Speed control generally has been a welcome addition to control-panel capability during the last decade. As the price of inverters fell and the technology improved, there was much more integration of their use with compressor drives, fans and pumps. This also opened the opportunity to match and mix compressor types for various applications.
For instance, adding together both screw compressors and piston compressors in the same installation can give good part-load benefits and energy savings. It is interesting to note that running a piston compressor at low speed increases the coefficient of performance (COP) while the opposite is true of screw compressors, where increasing the speed provides the benefit. Modern control systems offer these possibilities built into the software, so adding differing compressor types and sizes on the same system is really a function of the initial on-site setup. This approach results in load trimming; that is, running the most efficient combination of compressors for any given load situation.
In well-designed systems, the use of intercoolers and economizers also can be tailored to balance conditions. The degree of subcooling and adjustment of intermediate temperatures can enhance the overall plant performance as well as offer opportunities to satisfy different temperature requirements.
A combination of screw and piston compressors can deliver significant part-load benefits and energy savings. Shown here is a piston compressor.
Well-built cold rooms and stores also benefit from good quality insulation and door-management control. Large energy losses occur if best practices are not in place. The use of airlocks also can be considered. They often help prevent the mistakes that poor door management can create.
High-bay cold stores are another trend evolving to reduce store footprints and to provide energy-efficient designs. Often integrating automated storage-and-retrieval systems (ASRS), these stores can operate without forklifts, personnel or the need for lighting. Another benefit of the high-bay store design is that designers can utilize the cold lake principle with the use of computational fluid dynamics (CFD) modeling.
CFD is a powerful technique widely used for solving engineering challenges related to fluid and thermal behaviors. Using the CFD technique, designers can simulate cold-air circulation and temperature distribution within the cold store and, by design, reduce the fan power compared with traditional stores. A fan-speed reduction of 10 percent can result in a 27 percent reduction in fan-power usage.
As the price of inverters fell and the technology improved, it opened the opportunity to match and mix compressor types for various applications. Shown here is a screw compressor.
Evaporators and Condensers
Evaporator selection is another important aspect. Using the correct fin spacing or tube configuration can positively affect the cooling performance. The choice of condenser also will contribute to system efficiency. With so many options available — like evaporative adiabatic, water- and air-cooled condensers — this can be a complicated choice. Ultimately, the selection often depends on the availability of condensing mediums and space restrictions. The importance of making the final choice should never be underestimated, and careful consideration on the effects of system performance should be explored fully.
The condenser is an interesting subject. Refrigeration systems produce a lot of heat, and it is normally expelled to the environment. A modern trend is to look at how to harness that heat to be reused in other parts of the process. For instance, in food processing, both heating and cooling are often used to manufacture the product. The system designer can take the heat used in the cooling system to reduce the energy he would otherwise take from boilers. For instance, one site in Sweden uses a heat pump to upgrade the heat normally sent to the condenser through a heat pump to provide hot water for a district heating scheme for the surrounding village. This is typical in Scandinavia, where heat pumps are commonly installed and payback on investment is relatively short. But, as mentioned, there are many ways plants can use the heat that would normally be rejected, which is of considerable benefit to both the environment and the end user’s bottom line.
In conclusion, process cooling is a fascinating subject with many variables to consider. Make your choices carefully and consider all of the aspects and requirements of the system. My old boss would say that repeat business is the best business. It means you did it right the first time — you did not try to use a racehorse to plow a field. PC