Carole Musgrave
Over the last 20 years, the technical evolution of lasers has resulted in more optical power from a smaller package. This has created cooling challenges for laser design engineers. Output wavelength and power are sensitive to temperature variations so careful temperature control is often necessary. Even in applications that do not require precise temperature control, most lasers still need to be cooled.
For low-power lasers needing temperature control, air cooling is sufficient to remove waste heat. A steady flow of ambient-cooled water to a cold plate, or directly to the laser head, will usually suffice for a medium-power laser (see "Tap water vs. a closed loop system"). But for high-power lasers, a recirculating chiller is the most efficient cooling method. The chiller delivers coolant at a consistent temperature, flow rate, and quality. Much like an air conditioner, recirculating chillers use active refrigeration for applications requiring below-ambient temperatures or precise temperature control (see figure). Standard chillers are capable of cooling capacities ranging from 600 W to more than several kilowatts and are offered with many optional features.
Laser manufacturers may supply cooling equipment with their instrument, recommend a chiller manufacturer, or provide the cooling specifications and let the user select a chiller. The price of installing a recirculating chiller represents a very small fraction of the total cost of purchasing and installing a laser. The chiller must be correctly sized, however, as well as adequately equipped, reliable, and delivered on time. A chiller that does not meet these criteria could spell disaster for the laser performance. The first step is to define the basic chiller requirements such as capacity, coolant, and flow rate; the second, to determine which options are needed; and the third, to look at other considerations such as warranty and ISO 9001 certification. The following information contains guidelines to help in selecting the right chiller to meet the needs of particular applications.
Specifying the basic chiller
Determining the cooling requirements for the laser is the first step in selecting the best chiller. The laser vendor should supply this information if it is not included in the equipment's operating manual. Several issues should be considered, including the cooling capacity the laser requires. Cooling capacity defines the chiller size. The amount of waste heat that must be removed from a laser or other equipment is called the heat load. Usually expressed in units of watts or BTU/hr, the heat load is the minimum capacity to look for in a recirculating chiller to cool a laser.
Each chiller has a characteristic performance curve. Nominal chiller capacity is usually given at 20°C coolant output temperature and 20°C ambient air temperature. A chiller rated at 425 W with a coolant output temperature of 10°C, for example, might operate at 600 W with an output temperature of 20°C. Ambient temperatures significantly above 20°C will also reduce the chiller's cooling capacity.
Another consideration is the type of coolant required by the laser. Most standard-model chillers can handle ordinary distilled or tap water as a coolant. Additives include algaecide to prevent algae growth, and/or ethylene glycol to prevent corrosion and lower the freezing point of the fluid. Some lasers require deionized (DI) water, which can be very aggressive toward many materials. If this is the case, the chiller must have only DI-compatible materials in contact with the coolant, and it should be equipped with a deionization cartridge to maintain the required resistivity level.
The proper pump for a chiller depends on required coolant flow rate and pressure for cooling the laser. Laser vendors usually specify flow rates in liters (or gallons) per minute, and delivery pressure in bars or pounds per square inch, such as 1 L/min at 1.5 bars. Most manufacturers of chillers offer a variety of pump options. A positive displacement pump produces the same flow rate regardless of the system pressure drop. A centrifugal pump is pressure-dependent and will produce a higher flow rate at lower pressures. And a turbine pump is similar to a centrifugal pump but is ideal for applications with a large pressure drop. Some pumps have longer operating lives than others. Chiller vendors can assist in the selection of the right pump for the application. If the pressure or flow rate of available pumps is not low enough, the chiller vendor should provide an optional flow bypass loop. Plumbed in parallel with the laser, the bypass loop allows the unneeded flow to bypass the laser.
Some lasers specify precise coolant temperature and temperature stability. The chiller vendor will provide the temperature stability of the coolant as it leaves the chiller. For smaller chillers, the standard temperature control is ±0.1°C, while in larger chillers it is typically ±0.5°C.
Optional features
Many safety, control, and convenience features should be considered to ensure the best performance of the laser. Most chillers offer optional protection features to safeguard lasers and guarantee that they receive a consistent, reliable, coolant flow. Since the cost of such features is very small compared to the initial cost of the laser and losses due to downtime, it makes sense to incorporate them in the purchase of a chiller.
Communications features, for example, allow the chiller to be connected to a computer. Interfaces such as RS-232 enable remote start-up and power-down of the chiller from a computer. With this kind of interface, it is also possible to remotely monitor and change parameters such as the temperature setpoint, actual coolant temperature, coolant pressure, and fault conditions. Furthermore, it is possible to track conditions, such as declining coolant pressure, that forewarn of potential problems in the chiller's operation.
High/low temperature alarms and shutoff help prevent damage from coolant that is too hot or too cold. A visual display, audible alarm, or a computer (if RS-232 interface is used) signals the fault. Low-flow alarms and shutoff protect both chiller and laser from frozen coolant. These features are critical because if the coolant freezes, it will expand and may cause the evaporator (the component in which the coolant is chilled) to crack, possibly resulting in extensive repairs. This and other causes, such as a kinked hose, can cause low flow that can harm the laser and impair its function. Low coolant-level alarms and shutoff protect the laser from coolant loss due to evaporation or leaks.
Coolant filters protect the laser and the pump from harmful particles. Air filters prevent dust and dirt from accumulating on the condenser (which can cause a loss of cooling capacity). Hot-gas bypass prevents unnecessary wear on the compressor. Unlike home refrigerators, in which the compressor is cycled on and off, the flow of hot refrigerant is diverted to bypass the condenser and flow directly to the evaporator.
Water-cooled condensers enable removal of heat from the room housing the chiller. If "facilities water" is available, a water-cooled chiller can be used to reject heat from its condenser into the facility water stream instead of into the ambient air. Accurate temperature setpoints can be obtained by looking for a chiller that offers fractional degree setpoints, such as 0.1°C increments. Heaters can be used to maintain above-ambient temperatures and to quickly raise the temperature.
Defining other considerations
Numerous secondary considerations also play a role in selecting a chiller, such as ease of use. Many operators will probably use the chiller over the course of its lifetime. Several specific questions apply: Is the chiller control intuitive and easy to program? Does it display understandable symbols for the various chiller functions? Is the display digital? Does it show both temperature and pressure? How about an easy-to-read coolant level indicator?
The noise level of the chiller unit may be a concern. People situated near the chiller will appreciate a quiet machine. If the chiller will be relocated often, select a chiller with casters for easier mobility. Locking casters will keep the chiller stable when it is in operation.
The reliability of the vendor is an important concern, as it is in all consumer purchases. Most chillers come with a one-year warranty and technical support, but some vendors may offer a two-year warranty, and round-the-clock technical support may be available. If installation will include other heat transfer components, like cold plates and heat exchangers, find out whether the vendor will supply them as well. Look for companies with ISO 9000 registration, and chillers with industry certifications such as UL, CE, and CSA.
The key to getting the best chiller value is to ask questions. Talk to the laser vendor about the laser's cooling requirements and question the chiller vendor on product features and options. Most suppliers have engineers who can help in the selection of the right chiller for your application. A chiller should fulfill your cooling requirements for many years, so think ahead and purchase with future needs in mind. Although the chiller represents a small percentage of the total cost of a laser system, it makes sense to carefully select a dependable chiller to support smooth service for years to come.
CAROLE MUSGRAVE is marketing communications coordinator at Lytron, 55 Dragon Court, Woburn, MA 01801; e-mail: [email protected].