Advancements in vapor compression systems include the miniaturization of compressors, advanced heat exchangers, and expansion valves commensurate with lower-capacity systems.  These modern very compact high-efficiency vapor compression systems have been replacing thermoelectric systems in multiple markets due to their small size, superior efficiency, tight temperature tolerance, and low noise performance advantages.  With the advent and introduction of the Aspen Mini Compressor, vapor compression cooling is now accessible in laboratory equipment, medical devices, laser systems, military electronics cooling, and electric vehicle applications.  The small size of these modern systems makes the well-known advantages of vapor compression cooling available for those customers who wish to integrate advanced cooling directly into their products. The photograph below shows the 10X size reduction attained by rotary compressors over previously available reciprocating designs.  The rotary compressor uses a variable-speed brushless DC motor that enables the integrator to control temperature directly with a simple PID control signal.  

Concomitant with the release of the compact compressor, Aspen Systems has integrated efficient advanced microchannel condensers into all of our systems.  By incorporating heat exchangers that include microchannels connected by high-density fins, such as shown in this photograph, the delta T to ambient air is minimized, which lowers the power draw.  This also allows the use of quiet low-speed fans while keeping the condenser size at a minimum.  All important features for engineers who wish to integrate cooling into their devices.  See the photograph of one of Aspen’s liquid chiller modules.  Note the fin density on this unit.  The low heat transfer coefficient between a surface and air (h) is a driving factor in this design.  The heat transfer coefficient h is defined as: h=Wm-2/∆T

With a typically very low h, there are three ways to drive heat transfer into the air.  

  1. Use a very high Delta T.  The downside of this is excessive compressor power.
  2. Use a very high surface area.  This can be accomplished with an effective compact heat exchanger design. 
  3. Use a very high airflow.  The downside of this is a very powerful fan that is also quite noisy.

With an efficiently designed heat exchanger with close proximity between the heat source and the ultimate heat sink, the issues of a high-power noisy fan and high compressor power are mitigated.  In the system shown below, a microchannel heat exchanger with microchannel tubing and a high-density fin network are utilized to shorten the fin length and increase the surface area to air while minimizing the pressure and airflow required to achieve the necessary heat transfer.   

These features differentiate vapor compression significantly from the old-fashioned thermoelectric systems they are replacing.  A typical hot-side heat exchanger used in thermoelectric systems is shown in this figure.  Note that with a TEC device, the heat is dissipated on the hot side of the semiconductor material.  There is no efficient method of removing the heat directly to the air like there is with a refrigerant system.  In a typical TEC hot side heat exchanger an extruded aluminum heat sink fin block is attached to the hot side.  The heat is conducted through thick fins and a fan is blown across the structure.  This arrangement requires high-capacity fans and also produces a high delta T across the fin surfaces, forcing the TEC device to draw more power to drive the heat transfer into the air. 

To keep the fan size lower, and the power down these heat exchangers can become quite large for even relatively minor heat loads.  Side-by-side comparison tests we have run between equivalent vapor compression and TEC air cooling systems show that the vapor compression system is 65% smaller in size and 80% lighter. 

Next time, we will investigate the noise generation of a modern vapor compression system. 

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