Heat exchangers are devices that transfer heat from one medium to another. The purpose of the heat transfer typically is to lower temperatures in a device or within a specified area. The two media can be the same — for example, air-to-air — or different — air-to-water.
With their simple scheme, heat exchangers are a mature technology. They have been used for many years and are available in an array of designs. Modern heat exchanger designs leverage the laws of thermodynamics to provide cooling or heating in systems that range from simple to quite complex. This article will provide a brief look at how heat exchangers are used in three different process cooling applications. Heat Exchangers for Cooling Lasers
Industrial laser technology requires precise, automated processing for high speed and high accuracy marking, cutting, perforating and welding. High power industrial lasers generate large amounts of heat. If not controlled, this can result in an unwanted increase in wavelength. Proper cooling will benefit the system by maintaining precise laser wavelengths and more efficient laser output. This helps ensure a better-quality laser beam and reduce thermal stress on the laser system.
Maintaining lower operating temperatures also can increase the lifetime of a laser system’s components. An integrated cooling system helps ensure that the mean time before failure (MTBF) is extended, and downtime is reduced. These measures can help save on operation and maintenance costs.
Examples of higher power lasers include carbon dioxide (CO2) lasers, excimer lasers, ion lasers, solid-state lasers and dye lasers. Each of these laser types uses liquid cooling to remove excess heat. The cooling systems incorporate recirculating chillers and cold plates as well. In addition, the systems commonly include some form of heat exchanger.
For example, the wavelength of light emitted from gallium arsenide (GaAs) diode laser bars shifts at a rate of approximately 0.3 nm/°C due to temperature-related changes in bandgap energy and refractive index. To keep a high overall optical conversion efficiency of light from GaAs diode bars in some solid-state lasers, it is critical for the wavelength of light energy from each emitter to be within a narrow wavelength band, or within 1 to 2°C of each other. Cooling can help to keep the beam aligned in front of the emitter (±5 microns).
Integral heat exchangers for lasers commonly include liquid cooling loops. Heat exchanger often are found within cooling systems such as chillers, liquid cooling systems and ambient cooling systems. Some laser manufacturers prefer to purchase a heat exchanger separately and integrate it themselves, connecting it to their own pump and reservoir.
Medical Electronics Cooling with Heat Exchangers
Medical devices require effective thermal solutions to control the heat from electronics enclosed in confined spaces. Common uses include:
- Maintaining a safe external device temperature for patient-contacting surfaces.
- Cryogenic cooling for surgery applications.
- Cooling for the electronics in medical equipment.
Incorporating heat exchangers into the design of medical devices and equipment can boost the instruments’ accuracy, longevity and efficiency.
Heat exchangers typically are used for sealed medical enclosures or devices. If the enclosure does not need to be sealed, a fan circulating ambient air through it often will cool the device better and less expensively than a heat exchanger. Most medical devices, however, are sealed to maintain cleanliness and minimize contamination.
For many medical electronic devices, air-to-air heat exchangers — which transfer heat from the internal air and transport it to the ambient — typically provide sufficient cooling performance. Double-sided extrusion-type heat exchangers, which include fins on both sides, transfer the heat from the inside air to internal fins. The heat travels from the inside half of the extrusion by conduction. (For aluminum, the thermal conductivity is 180 to 200 W/m-°K.) Outside air then circulates over the outer fins to remove the heat.
When it comes to incorporating heat exchangers into medical device designs, consider appropriate materials for the application. For example, heat pipes normally are copper, which can be toxic to the human body or can corrode in certain environments. In most medical applications, customers ask designers to use gold or nickel plating on heat pipes to preserve conductivity and improve patient safety as well as to resist environments and cleaners that might be corrosive to copper.
Heat Exchangers for Food and Beverage Processing
Heat exchangers are a good solution for many applications because they remove waste heat from cabinets and panels without letting contaminants enter the enclosure. Like with medical applications, this is useful for cooling electronics used in food industries.
Regular washdown of equipment is required in many food processing plants as means of keeping the areas clean and free from debris or harmful bacteria. Many of a plant’s sensitive instruments must be enclosed in order to protect them from the high pressure cleansing spray. While enclosing electronics or a piece of equipment will keep it dry and protected, it also will allow for the buildup of heat.
A common material choice for heat exchangers used in producing food and beverages is stainless steel. Stainless steel heat exchangers also are common to pharmaceutical and other healthcare industries because they can be steam cleaned, providing a hygienic, antibacterial surface.
Features and options that typically can be specified when ordering stainless steel heat exchangers include:
- NEMA 4X enclosures for protection against corrosion and extreme environments.
- Type 304 or 316 stainless steel housings.
- Surface mount on outside of enclosure.
- VAC or VDC fan configurations.
- A device that is UL listed or recognized.
Another heat exchanger solution for food processing is a closed-loop cabinet cooler. These units draw heated air from a cabinet, thermodynamically cool it and then return the cooled air to the cabinet. Providing energy-efficient cooling for electrical and electronic enclosures, cabinet coolers often do not have any moving parts except the fans. This can make them a low maintenance solution.
To function, the cabinet cooler heat exchangers employ a powerful fan to draw heated air from the cabinet and force it across a heat-absorbing core. As the heat is absorbed from the airstream, the heat is transferred to the ambient side of the core. A fan on the ambient side then moves cool ambient air across the core. The cool air absorbs the heat and is dissipated as exhaust from the heat exchanger. The newly cooled air is then recirculated back into the cabinet, which lowers the interior cabinet temperature. At no time does the cabinet air contact the outside or ambient air.
Another heat exchanger design often used in food processing, medical electronics and other industries is a plate heat exchanger. An alternative to tube-based heat exchangers, plate heat exchangers can range from a few inches to several feet in size. They can accommodate many cooling needs in large food and beverage processing facilities.
A plate heat exchanger uses metal plates to transfer heat between two fluids. This offers advantages over conventional shell-and-tube heat exchanger designs in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat and greatly increases the speed of the temperature change.