Liquid temperature control has been a critical component in laboratories for many years.  Before the advent of chillers, scientists typically used tap water, dry ice, and liquid nitrogen for temperature control and cooling of analytical instrumentation and condensers.  But, with the rising costs of tap water, regional restrictions on tap water usage, and the need for tighter stability with today’s more sophisticated equipment, chillers have become a necessity.  Additionally, tap water may contain harmful particulates which can, over time, build up in the instrument’s cooling lines.  This can cause equipment failure, leading to and expensive repair.

A chiller is a refrigerated recirculating liquid cooling system consisting of a compressor, condenser, evaporator, pump, and temperature controller, all in one package.   The chiller recirculates temperature controlled fluid to the application to remove heat.  The heat is picked up by the recirculating fluid and is returned to the chiller.  The chiller cools the fluid to the setpoint specified by the user and is then recirculated back to the application.

A chiller can regulate temperature in a variety of ways: compressor cycling, heater cycling, or hot-gas bypass.  Compressor cycling entails turning the compressor on and off continually to maintain setpoint.  This can be acceptable for an application that does not require tight temperature control, such as chilled water for building air conditioning.  For laboratory applications, temperature stability is more crucial.  The continual cycling of the compressor will cause premature wear and failure.  A compressor will actually last longer if it is run constantly.  Heater cycling and hot-gas bypass are two methods that allow this.  In the heater cycling method, a heater fights against the constant refrigeration to maintain the setpoint.  While this provides excellent stability, it will result in higher power consumption, adding unnecessarily to the cost of ownership.

Hot-gas bypass, on the other hand, offers highly stable fluid temperatures as well as lower power consumption.  Very cold liquid refrigerant cools the recirculating fluid via a coil in the fluid reservoir or through a heat exchanger.  As the refrigerant removes heat from the recirculating fluid, it changes state to a hot gas.  If the fluid requires more cooling, the hot gas is sent to the compressor and condenser to return to a cold liquid state.  If the fluid needs to be heated, the hot gas is routed directly back to the coil or heat exchanger by a solenoid.  By running the compressor continually, the chiller will be more reliable and have a longer life.

Once heat is removed from the application, the chiller must remove its own waste heat.  This can be done by rejecting the heat into the air (air-cooled condenser) or to a secondary water source (water cooled condenser).  Which version to use is dependent on the area where the chiller will be placed.  If the room is large and the chiller is small, heat rejection into the ambient will not be a problem.  If the room is small and the chiller is relatively large, a water cooled condenser might be an alternative.  A water cooled condenser requires a secondary source of water to be circulated to the chiller.  This is typically tap water, building chilled water, or tower water.  

A typical laboratory will have many applications that require liquid temperature control.  Some only need cold water that can vary in temperature as much as a few degrees.  These include cooling of condensers, evaporators, and diffusion pumps.  Other applications need a higher degree of temperature stability such as lasers, spectrophotometers, inductively coupled mass spectrometers (ICP/MS), atomic absorption graphite furnaces (AAGF), electrophoresis, gas chromatograph mass spectrometers (GCMS), and electron microscopes.

To size a chiller for the application, one must know the heat removal requirement, temperature, fluid flow, and fluid pressure.  These requirements, in many cases, can be obtained from the instrument manufacturer.  If this is not known, a heat load calculation can be performed (See table 1).  With the application running and being cooled properly,  measure the temperature of the incoming and exiting water.  Also, measure the flow rate of the water entering the application.  If a flow meter is not available, use a 5 gallon bucket and a stop watch.  Enter these three numbers into the formula to determine the amount of heat the cooling water has removed from the application.  This is how much heat the chiller needs to remove.

Condensers and evaporators typically necessitate the recirculation of a cold fluid.  Tap water can be used as long as it is cold enough to condense or evaporate the material from solution.  A common working temperature is 4 C, which is colder than what is commonly available from tap water.  Fluid temperature stability is not usually critical but fluid pressure is if the condenser or evaporator is made of glass.  Glass can withstand fluid pressures up to 25 psi.  Therefore, when choosing a chiller, it is wise to make sure the pump used is a centrifugal type pump (low pressure).  If a centrifugal pump is not available and a positive displacement or turbine (high pressure) pump is used, an internal or external pressure reducer will allow the user to reduce the fluid pressure so as not to damage the glassware.

Diffusion pumps, on the other hand, require a high pressure pump due to very small diameter cooling lines.  They can be as small as 1/8” inside diameter.  But, similar to condensers and evaporators, temperature stability is not very important.  An electron microscope is an example of an analytical instrument that uses diffusion pumps.

Conversely, there are many laboratory cooling requirements where fluid temperature stability is critical.  For lasers, a consistent temperature from day to day will ensure a coherent beam as well as repeatable results.  Commonly cooled components are the laser tube or the laser diode, depending on the type of laser.  

There are a number of analytical instruments used in laboratories for analysis and identification of compounds in a sample.  The majority of them require water cooling at precise temperatures.  ICP/MS is an analysis method where the sample is introduced into an Argon plasma heated to 8000 K by an RF source.  Each element in the sample then produces a characteristic emission spectrum.  A chiller is used to cool the torch that heats the sample.   GC/MS also identifies unknown compounds in a sample.  The diffusion pump requires cooling, if present.  An AAGF is used to quantitatively define up to 60 elements that are found in a sample.  The process uses atomation (by a flame or electric heat source) to create a vapor which is interposed into the light path.  The chiller cools the torch or the graphite furnace.  

Overall, there are a multitude of applications for water cooling in the laboratory.  A refrigerated, recirculating chiller is an ideal replacement for tap, tower, or building chilled water.  The recirculating fluid from a chiller is stable in temperature, flow, and pressure.  In addition, a closed loop recirculating system will ensure the fluid is clean and void of particulate matter, which is commonly present in tap water.  These harmful deposits can damage equipment causing equipment failure and loss of experimental data.

Table 1: Heat Load Calculation

Flow Rate (gallons/hour) x Cooling Fluid Weight (pounds/gallon) x Specific Heat of Cooling Fluid x  DT ° F (Temperature Out – Temperature In) = Heat Load (BTU/hr)

Notes:

Weight of water/gallon = 8.35 pounds/gallon 

Specific heat of water = 1 

12,000 BTU/hr = 1 ton

1 watt = 3.41 BTU/hr

(As seen in Process Cooling, June 2000)