Vapor-Compression Refrigeration Systems

Present day refrigeration is viewed as a necessity to keep our popsicles cold and our perishables fresh. But have you ever wondered what people did to keep their food from spoiling hundreds or even thousands of years ago? Or just what goes into a refrigerator system today? In this blog, we’ll take a look at how refrigeration works; the history behind it; and examine the cycle, working fluids, and components.

Schematic of Refridgeration System in AxSTREAM NET
Figure 1. Modern-day Refrigeration System in AxSTREAM NET
Introduction

Refrigeration is based on the two basic principles of evaporation and condensation. When liquid evaporates it absorbs heat and when liquid condenses, it releases heat. Once you have these principles in mind, understanding how a refrigerator works becomes much more digestible (pun intended). A modern-day refrigerator consists of components such as a condenser, compressor, evaporator and expansion valve, as well as a working fluid (refrigerant). The refrigerant is a liquid which as enters the expansion valve the rapid drop in pressure makes it expand, cool, and turn into a gas. As the refrigerant flows in the evaporator, it absorbs and removes heat from the surrounding. The compressor then compresses (as the name suggests) the fluid, raising its temperature and pressure. From here, the refrigerant flows through the condenser, releasing the heat into the air and cooling the gas back down to a liquid. Finally, the refrigerant enters the expansion valve and the cycle repeats. But what did we do before this technology was available to us?

History

The first glimpse of cooling technology originated in China around 1000 BCE. During this time, people would harvest ice from rivers and lakes to cool, and conserve food. Later down, Hebrews, Greeks, and Romans all adapted a similar idea, digging out pits to capture snow and insulating it with things such as grass or tree branches. By 400 BCE, Persian engineers had begun using Yakhchāls which were dome shaped structures constructed with thick heat-resistant materials to insulate the walls and preserve food and ice year-round.

Yakhchal still in use Today
Figure 2. Yakhchāl still in use today. Source

As time progressed, our basic needs changed, and new ways to manipulate temperature began to be explored. In the mid 1700s through early 1800s, people preserved their food by storing food and ice in iceboxes. The mid 1800s marked the development of the world’s first workable vapor compression refrigeration system which is still the most common refrigeration system used today due to its low cost, high efficiency and capacity to be driven directly via mechanical or electric energy.

Vapor-Compression Refrigeration

Vapor-compression cycles are used in most household refrigerators today as well as in many large commercial and industrial refrigeration systems. Figure 3 shows some common configurations. In its simplest form, a vapor compression system consists of a condenser, compressor, evaporator and expansion valve as mentioned earlier, although these components may be used in different configurations depending on:

  • – Operating conditions and system function
  • – Type of working fluid(s)
  • – System size
  • – Efficiency requirements
  • – Economic requirements

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Figure 2. Vapor-Compression Refrigeration Cycle Configurations
Figure 3. Vapor-Compression Refrigeration Cycle Configurations in AxCYCLE

Once you have a cycle configuration for your needs, it is important to establish a working fluid (refrigerant) for the system which comes with its own set of requirements and considerations. For example, the refrigerant should have a low boiling point (to ensure minimal energy is used for phase changes) and low freezing point (to prevent freezing of the fluid in the system). Additionally, it should operate in a reasonable pressure range, be economically viable, and the working fluid cannot be thermally temperamental (as you can imagine, exploding refrigerators would not bode well for investors or consumers) and must be within international standards for Ozone Depletion Potential (ODP) and Global Warming Potential (GWP) limits . Figure 4 shows commonly used refrigerants.

Figure 4 - Commonly Used Refrigerants
Figure 4. Commonly Used Refrigerants

As you can see, there are different pros and cons for each class of refrigerant/working fluid.  At present however, it is very difficult to find a working fluid which satisfies all environmental, economical, and safety requirements. To combat this, companies are exploring the use of Hydrofluoroolefin (HFO) refrigerants due to their environmental friendliness, cost-effectiveness, and great energy efficiencies. Figure 5 presents this emerging class of refrigerant.

Figure 5. Hydrofluoroolefin Refrigerant Properties
Figure 5. Hydrofluoroolefin Refrigerant Properties

While on paper this working fluid checks all the boxes, in reality a new refrigeration system must be designed in order to meet the requirements of HFOs such as operating pressure and capacity, flammability risks, increased stresses, and adjustments of system components and compressor.

Compressors are an integral part of any refrigeration system, and some types of compressors handle specific refrigerants better than others, the ways to use them varies for some (turbomachines vs. positive displacement devices), etc. Figure 6 shows the main types of compressors. Types of compressors found in refrigeration cycles include:

  • – Reciprocating
  • – Helical-Rotary
  • – Centrifugal
  • – Scroll
  • – Rotary­

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Compressor Types
Figure 6. Common Types of Compressors
Modern Design Methods

Back in the 1800-1900s, the calculations and specifications required to design a vapor compression refrigeration system were done by hand with a pen and paper. Due to advancements in technology, engineers are now able to complete tasks which once took years in a few short weeks.

An example of one such time saving technology is our very own, fully integrated software platform, AxSTREAM®. Utilizing this software, companies can design, analyze and improve refrigeration systems for various requirements and applications. On the system level, engineers can simulate the entire refrigeration cycle to determine the overall performance, how the cycle operates at off-design modes, and define boundary conditions for the system components. On the component level, companies can perform detailed design of the centrifugal compressor including performance map generation, FEA, CFD and rotor dynamics. Users can also model and analyze the heat exchanger in AxSTREAM NET™. Figure 7 shows a centrifugal compressor designed using AxSTREAM®.

Compressor Blade Design in AxSTREAM
Figure 7. Component designed using AxSTREAM®

Are you looking to develop your HVAC or refrigeration system? Send us an email at Sales@Softinway.com to arrange a demo!