Vapor Compression Cooling – Active Thermal Management

Technology – Vapor Compression Refrigeration


Aspen’s Technology Background

Aspen Systems specializes in the design and manufacture of compact, vapor compression refrigeration systems to meet the requirements of many challenging applications in a wide variety of industries.  It all began in the early 1990s when we developed our own miniature, variable speed, rotary compressor for a portable application for cooling dismounted soldiers wearing heavy protective gear while operating in high ambient temperature battlefields.  By the late 1990s we were producing small volumes of systems to cool fighter jet pilots.  In the early 2000s we built a modern, state-of-the-art, compressor manufacturing facility to produce our miniature compressors in relatively high volume (>100,000 units annually) and low cost while ensuring high reliability through a repeatable automated process.  Soon after that, we developed and began manufacturing electronic cooling systems for the U.S. military’s Warfighter Information Network.   A couple short years later, we were manufacturing unique compact systems for medical aesthetics, DNA sequencers, and semiconductor manufacturing.  We now supply thousands of cooling systems annually to many industry leading equipment manufacturers for applications in medical, life science, laboratory, pharmaceutical, military, electronics, electric vehicle, lasers, and several other industries.

Our miniature compressor, operating at variable speeds and utilizing a high efficiency, high energy density brushless DC motor gives us a unique advantage when it comes to compact, lightweight, cooling systems.  We can design complete high capacity cooling systems smaller than a shoe box that can easily be incorporated inside the tight confines of your device chassis.  We specialize in air cooling and circulating systems, chilled liquid systems, and direct expansion systems where the refrigerant is circulated directly through a cold plate evaporator in contact with the heat load.  Our advanced compressor and system components, combined with our innovative packaging approaches can result in the most compact, highest performance solution at the minimum cost, for your specific needs.  If your application needs a unique, high efficiency, compact cooling system, Aspen has the technology and decades of experience necessary to develop and manufacture a custom refrigeration system to meet your requirements and provide you with a distinct market advantage.

Why Use Vapor Compression?

Vapor compression cooling uses refrigerant phase change to efficiently cool heat sources to temperatures below the surrounding ambient air.  This is the same technology used in your home refrigerator to make the inside of your refrigerator colder than the room it sits in. It was developed more than a century ago and has been continuously optimized with the latest technological advances.  Vapor compression uses a compressor to pump refrigerant from an evaporator where it absorbs heat (the inside of your refrigerator) to a condenser where it rejects heat (to the air in the room outside the refrigerator). The refrigerant changes phase from a liquid to a vapor in the evaporator and from a vapor to a liquid in the condenser.  This phase change and the pumping by the compressor allow this process to take place very efficiently.  Typical vapor compression systems produce more cooling (measured in Watts or Btu/hr) than the amount of power (Watts of electricity) put into the system.  With the use of an appropriate refrigerant, compressor, and heat exchangers, a vapor compression refrigeration system provides a very efficient and highly reliable means of cooling in a wide range of applications from your well known home refrigerator to medical devices, life science research tools, laboratory equipment, high tech lasers, military communications, and electric vehicle charge cables.  Aspen uses only approved, environmentally compatible refrigerants that are in compliance with all international regulations for the reduction of Ozone Depleting and Global Warming substances.  We are continually monitoring the evolution of the international regulations and the development of new refrigerants.  We are currently testing alternative refrigerants that are even more environmentally beneficial than those in use throughout the world.

The Basics of Vapor Compression Refrigeration
The major components of a vapor compression system are the compressor, condenser, expansion valve, and evaporator.  The compressor is the heart of a refrigerant system: it uses a small amount of energy to generate the necessary refrigerant flow and subsequent heat transfer as desired.  Aspen developed a miniature rotary compressor which efficiently generates a significant cooling capacity within a small volume. The compressor takes in low temperature, low pressure refrigerant vapor and compresses it to a high pressure and temperature. The refrigerant undergoes an isothermal phase change (gas to liquid) in the condenser while rejecting heat to the ambient environment. The warm, high pressure liquid refrigerant out of the condenser is throttled to a low pressure and temperature (typically below ambient) through the expansion valve.  Refrigerant enters the evaporator primarily as a liquid and again undergoes an isothermal phase change (this time from liquid to gas) as the evaporator absorbs heat from the heat source (the hot device that needs cooling). The refrigerant returns to the compressor as a low temperature and pressure vapor to complete the cycle as shown below. See How Vapor Compression Refrigeration Works for a short video explanation.

Vapor Compression Refrigeration Diagram

Other refrigeration technologies, such as thermoelectrics (Peltier cooling), offer alternatives to vapor compression, but vapor compression offers distinct advantages in capacity, efficiency, flexibility, and cost. And with Aspen’s miniature compressor technology, the size and weight of vapor compression systems are smaller than that of thermoelectrics. This article compares the performance of vapor compression and thermoelectric systems in more detail:  Vapor Compression vs Thermoelectric Cooling

Vapor compression cooling systems can be put into the three major categories described below.

  • Air Circulating Systems

Air Conditioning Systems use recirculated air in a sealed environment as the cooling mechanism. Aspen’s refrigeration systems remove heat and humidity from the air which is circulated through the system to cool the heat source. Air conditioning systems are ideal for cooling bulk heat loads or multiple distributed loads (e.g. electronic components) where minimal modification to the system is desired.

Benefits: Least invasive solution, low cost, can remove humidity as well as heat.

  • Liquid Circulating Systems

A Liquid Chiller System offers high heat transfer rates because the secondary fluid has a high thermal conductivity and specific heat. Liquid chiller systems can be quite compact because a pump circulates a coolant which offers high heat flux. Using a secondary coolant enables the size of heat exchangers to shrink, while thermal performance increases. A Liquid chiller is ideal for cooling high heat flux components and in applications where a solution must be rugged and reliable.

Benefits: high heat transfer rate, compact, adaptable/configurable

  • Direct Expansion (Direct Refrigerant) Systems

Refrigerant circulating systems known as Direct Expansion or Direct Refrigerant Systems typically offer the highest heat transfer rates. A direct expansion system does not use a secondary coolant, instead the refrigerant absorbs heat directly at the heat source through a contact heat exchanger (cold plate), minimizing the size and component count of the system.  Direct expansion systems are custom designed to integrate directly into a system and provide very high heat fluxes at the heat source.  See Advantages of Direct Refrigeration Cooling for a short video.

Benefits: much smaller, lighter and compact system with higher reliability and longer MTBF due to elimination of the liquid pump and the secondary liquid loop, higher system efficiency due to the highest overall heat transfer rate, easier to maintain narrow temperature variation.