A chiller is a generic term for a temperature controlled liquid recirculator.  They are typically used to remove heat from an instrument through the use of a fluid as the heat transfer medium.  In many cases, the instrument must be kept at a constant temperature as well.  This is a typical requirement of laser cooling.  There are many types of liquid cooling methods but the vast majority of chillers utilize vapor phase refrigeration (VPR) as the method of fluid cooling.  It is one of the oldest means of mechanical refrigeration and is used extensively in home refrigerators and automobiles.  Very few other refrigeration technologies have proven to be more efficient and as cost effective as VPR.  But, there are some promising new methods that may be commercially available within the decade.

The current direction in chiller technology, dictated mostly by the laser and semiconductor industry, has been to supply a chiller with a high amount of heat removal in a very small package, using a relatively low amount of electricity, all with a high level of reliability.  Many companies have been able to offer VPR systems that meet most of these requirements with the use of plate style evaporators, new refrigerants, and condensers with improved heat transfer materials.  Many laser users are asking for yet smaller and more efficient chillers.  VPR chillers may reach their size limit soon.

A vapor phase refrigeration system consists of a compressor, condenser, and evaporator.  A chemical (refrigerant) with a very low boiling point is used to remove heat from the liquid to be recirculated for cooling (process fluid).  Two common refrigerants used today are R22 and R134a. These refrigerants are in a gaseous state at room temperature and atmospheric pressure.  Here is a simplified version of how this system works.  The refrigerant is recirculated internally and is used to absorb heat from one spot (process fluid) and moving it to another (rejection to the air or another source of water).  The refrigerant enters the compressor as a hot gas and is compressed.  After that it is routed to the condenser where it condenses into a cold liquid.   This cold liquid then enters the evaporator.  This is where the process fluid is cooled to the user defined temperature set point. As the refrigerant removes heat from the process fluid, it changes state back to a hot gas.  This hot gas returns to the compressor and the process is repeated.

In a chiller, the process fluid cooled by the evaporator is recirculated to the laser using a built in pump.  The process fluid absorbs heat from the lasers and returns to the chiller to be cooled to the user defined set point.  A temperature controller on the chiller monitors the process fluid temperature and adjusts the refrigeration system accordingly to maintain this setpoint.

Vapor phase refrigeration is still the most efficient commercially accessible refrigeration method available today.  It is also the most powerful.  No other type of system can provide the level of heat removal over a wider temperature range as VPR.  The major disadvantage of VPR is that it is a mechanical method.  In other words, there are many moving parts.  Some of them include the compressor, fan, fan motor, and refrigerant metering valves.  Moving parts, unfortunately, wear out over time.  A more ideal refrigeration method would be a powerful, energy efficient cooling system with no or minimal moving parts.  

One of the more simple cooling devices that is well suited to small laser cooling is an air to water heat exchanger.  It consists of a condenser similar to a car radiator, a fan, and a pump.  For small heat removal applications such as cooling a laser diode, an air to water chiller can meet the small size and low electricity requirements. Since air to water exchangers are relatively inefficient, a heat removal requirement of greater than 1000 watts will drive the size of the unit to be quite large physically.  This is due to the amount of surface area required of the condenser.  Additionally, an air to water heat exchanger cannot cool the process fluid below the ambient temperature.  Therefore, air to water exchangers can only meet a narrow range of cooling requirements.

Thermoelectric (or Peltier) cooling is a commercially available refrigeration method that can cool air or fluid below ambient without the use of moving parts.  Thermoelectric cooling uses p and n-type semiconductors sandwiched between two ceramic plates.  If a positive DC voltage is applied to the n-type semiconductor, electrons will move to the p-type semiconductor, creating a hot side and a cold side.  Process fluid can be cooled by circulating it through a heat exchanger in contact with the p-type side.   Thermoelectric cooling can be an excellent method of removing heat from small heat loads.  There are virtually no moving parts, the system is very quiet, and can be quite small physically.  This method, however, does have its disadvantages.  The electricity requirements of a thermoelectric chiller can be 2 to 3 times higher than a VPR system and the power supply required is quite complex.  Additionally, the efficiency of a thermoelectric chiller is much lower than a VPR system making it non practical for large heat removal applications such as CO2, YAG, or Argon Ion lasers.   

There are a few experimental refrigeration methods that may prove to be practical for cooling small lasers within the next 5-10 years.  Thermoacoustic refrigeration was developed by the Aerospace industry as a cooling method that would be able to work in any orientation and at zero gravity.  A loudspeaker produces high-amplitude sound waves to compress, displace, and expand a gas that is between two heat exchangers.  One heat exchanger is hot and the other is cold.  The gas moves heat from the cold exchanger to the hot due to the expansion and contraction.  The advantages are no moving parts and no use of potentially ozone depleting gases.  

Magneto caloric refrigeration uses a wheel embedded with segments of magnetic material spinning around a strong magnet.  As the magnetic material passes through the magnetic field, it heats up.  The material cools as it moves out of the magnetic field.  If the wheel is in contact with a heat exchanger, the process fluid can be cooled.  This technology is still in its infancy but the potential for magneto caloric refrigeration to remove large amounts of heat is excellent.  

Overall, the laser industry’s requirement for small cooling capacity chillers is being driven by the growing popularity of low heat load diode lasers.  While vapor phase refrigeration is by far the best, most economical, and efficient refrigeration method, other technologies are also suitable for removing small amounts of heat.  Whether or not lasers will be cooled with magnets or loudspeakers in the future remains to be seen but it is safe to say the chiller industry is constantly striving to produce smaller, more powerful and more efficient chillers to meet the needs of the laser market.

(As seen in Laser Focus World, November 2000)