Click to enlarge Portable liquid chillers, rack-mounted liquid chillers, water chillers and heat exchangers are completely self-contained devices, able to remove heat from laser heat-dissipating components.

Many types of recirculating cooling systems or chillers are used to circulate the fluid. For most laser applications, however, a closed-circuit refrigerated cooling unit is used to maintain proper coolant temperature. The size of the refrigeration system depends upon the heat load produced by the laser when it is operated for long periods of time at its maximum average power. Proper chiller sizing and selection are critical to obtaining optimum laser performance and long-term reliability.

While many refrigerated recirculating water chiller designs are offered, not all water chillers provide the same performance. Many may use control methods that do not provide for widely varying loads or ensure good temperature stability.

To understand the differences in water chiller technology, it is helpful to first understand why coolant temperature control is important and how, in general terms, it can affect laser operation.

Click to enlarge Chillers with an automatic zero-to-full-load capacity control eliminate compressor cycling.

Temperature Variation and Laser Performance

Coolant temperature is a critical factor in the proper operation of solid-state lasers. Higher temperatures can result in lower laser efficiency, so generally it is desirable to operate the system at the lowest temperature that is practical. If the temperature is reduced to or below the dewpoint, however, condensation may begin to form on some laser components that are exposed to the atmosphere. If the laser-rod ends are cooled to the dewpoint, for example, condensation can form there. If the system is started, the laser beam can burn off the condensation layer - and damage the surfaces in the process. Therefore, solid-state laser systems should never be operated with coolant temperatures low enough to result in condensation. Because the dewpoint depends on ambient temperature and relative humidity, each application must be analyzed to determine a safe operating temperature based on the conditions to which the laser is exposed.

Moisture condensation also is a problem for industrial carbon dioxide lasers, which often have separate, or secondary, cooling circuits designed specifically to cool the optics. Usually, these circuits are maintained at higher temperatures than the primary circuit; that is, they are designed to operate above the ambient dewpoint.Coolant temperature stability also is an important factor in chillers designed for lasers. Laser-diode devices must be controlled at a specific temperature even in applications with the potential for wide ambient temperature ranges. Failure to maintain a stable temperature can result in volatility of the laser diode’s output wavelength. The output wavelength of a diode typically shifts with operating temperature at a rate of approximately 0.33 nm/°C. In these lasers, the absorption bandwidth of the gain medium normally is about 2 to 4 nm, so temperature regulation of the pump diodes to ensure a stable power output from the laser head is essential. Temperature changes in the laser head can result in laser output power drift, frequency drift in single-frequency operation and drift in the output beam direction (beam pointing).

Selecting a Refrigerated Chiller for Lasers

Click to enlarge Some process water chillers have refrigeration circuitry that provides zero-to-full-load capacity control without cycling the compressor. Chillers also can provide temperature stability to ±0.1°C.

When selecting a water chiller for a laser application, several factors should be considered. The most important is cooling capacity.

Typically, laser manufacturers will provide cooling water specifications that indicate a certain minimum cooling capacity required to properly cool the laser under full power conditions. It is important to note, however, that lasers do not always operate at the maximum output power. Frequently, lasers are sold to an integrator or end-use operator that may be using the lasers for several applications. The laser may often be operated at much less than full time (low-duty cycle). As a result, the average output power will be much less than the maximum design output. If the duty cycle falls below 50 percent of the chiller’s cooling capacity, it can lead to control problems. To eliminate control problems, the chiller must provide capacity control over the full cooling range of the chiller.

Traditional refrigerated recirculating chiller systems have a fixed design cooling capacity. When the chiller is not operating at its full capacity, it is necessary to offload the excess capacity to ensure proper operation. If the excess capacity is not reduced, it can prevent system stability and possibly cause damage. To reduce capacity within the refrigeration system, a hot gas bypass usually is incorporated within the refrigeration circuit.Although there are design variations for hot gas bypass systems, the goal of reducing system capacity is the same. The hot gas bypass diverts some of the system’s mass flow of refrigerant before it can be used effectively in the condensing and evaporation processes within the system. The hot gas system is used when the actual process load falls below the design cooling capacity of the chiller.

This method for capacity control generally works out satisfactorily until the cooling load falls below approximately 50 percent of the design capacity of the chiller. At this point, the hot gas bypass quantity is so large that it can affect the returning gas temperatures for the refrigeration compressor. The refrigeration system’s compressor can become heated to levels exceeding the compressor manufacturer’s recommendations. Elevated return-gas temperatures cause compressor overheating and can lead to premature compressor failure and shortened life. Consequently, some manufacturers will cycle the compressor on and off when the loads fall below this 50 percent threshold.

Frequent compressor cycling, however, can cause increased wear and tear on the system, temperature instability and premature component failures. Excessive compressor cycling (or short cycling) can cause lubrication problems within the refrigeration system. Because a refrigeration compressor is bathed in oil, it will naturally circulate oil throughout the system. Constant cycling can prevent proper oil circulation and cause the compressor to progressively pump oil from the compressor’s oil reservoir so that the entire oil charge is lost from the crankcase, starving the components of proper lubrication.

A second source of compressor damage can result from liquid refrigerant entering the compressor due to lack of cooling load or improper system design. Methods such as control timers can be incorporated to prevent the system from cycling too frequently. Usually, however, the result is poor system temperature control.

Click to enlarge Liquid cooling systems can be used to remove heat from lasers and avoid overheating of critical laser components.

Advanced Refrigeration Control Technology

As a result of advances in process chiller controls, portable water chillers that provide zero-to-full-load capacity are available. Chillers like these offer several advantages, including the ability to operate multiple lasers or processes on one water chiller. The zero-to-full-load capacity control allows one or more lasers to be operated simultaneously or separately, as needed, without sacrificing temperature control or stability. Some of these chillers can also be equipped with heaters to help bring the process fluid to a temperature above the dewpoint, minimizing the possibility of condensation. PH

Links

• Opti Temp Inc.