Large Format Coolers

The SA-2 cooler interior design can be extended over large areas. The figure at right shows conventional SA-2As adjacent to a pair of large format coolers. These coolers are built on a custom basis, with internal cooling designs, internal manifolding, external manifold ports, and mounting holes designed to meet the customer's particular requirements.

In the particular case shown in the photo at the right, the large format coolers have inlet and outlet holes on the bottom face (the large holes visible in the rightmost cooler). Threaded mounting holes arelocated along the long sides of the cooler, and two alignment pin holes are located on the meridian through the inlet and outlets. The component(s) is then mounted on the opposing face.

Description.

Internal Design. As with all MC² impingement coolers, the internal cooling design is largely independent of the exterior configuration: three internal designs are offered with the "conventional" large format configuration. These designs, designated Types "A", "B", and "C", employ 25µ, 50µ and 125µ passages, respectively. The Type A versions of the coolers have approximately 1000 internal micro-impingement cooling circuits per cm^2 spread uniformly over their surfaces. These passages are located roughly 250 µm below the cooler surface.

Alternatively, tailored internal cooling designs may be supplied to either: 1) provide cooling at designated locations; 2) provide varying cooling performance over the cooler face; 3) lower pressure drops; and/or 4) reduce costs.

External Design. As mentioned above, the external design is generally customized for each application. Nominal values for the tolerances, surface finish, and plating are provided in the drawings supplied for the SA-2 coolers, although these specifications may be tailored as required in most cases.

Construction and Chemical Compatability. The coolers are fabricated from a dispersion-strengthened copper alloy which has about 90% of the thermal conductivity of pure copper, and much higher strength at elevated temperatures. The chemical resistance is the same as that of copper, so the material compatibility and corrosion resistance tables for copper may be used.

Performance. The graph below show the thermal performance of a large format cooler with an SA-2A internal design. The data was taken from evaluation units with a 1 cm x 2 cm cooled area. The thermal resistance in the left-hand graph is defined as: (T_s - T_in)/q, where T_s is the surface temperature, T_in is the inlet water temperature, and q is the heat flux. Due to the unique nature of large format coolers, no flow performance data is supplied.

Usage.

Component Mounting. Typical installations involve soldering or brazing a component or component carrier to the surface of the cooler. The coolers can be processed at temperatures up to 1500 °F, and thus are compatible with conventional soldering and brazing processes. Due to the grain structure of the copper alloy used in the coolers, it is advisable to plate the interface with copper or nickel prior to brazing with silver-based brazes to prevent migration of the silver into the cooler (depleting the silver at the braze joint). If excessive mechanical loads are applied to the cooled surface, it is possible to deform the surface, crushing the internal channels. This may be avoided by either: 1) limiting loading on the surface to less than 1000 psi, and/or 2) requesting a custom design with a more robust surface.

Installation in Flow Systems. Saddleback's micro-impingement cooler packages require special mating manifolds. These manifolds are designed to allow coolers to be closely arrayed without interference from the manifolds or the coolant lines. The coolers are bolted to the manifolds, with o-rings sealing the inlet and outlet ports. Manifolds and manifold arrays can be custom-designed for the users' applications, or may be fabricated by the users themselves.

A common question during installation is: Does it matter which way the coolant flows through the cooler? The answer is yes, it definitely does matter - flowing the wrong way will yield higher pressure drops and higher thermal resistance values. Even with the flow reversed, however, the coolers are still likely to outperform any competing products.

Filtering and Cleaning. Upstream filtering should be provided for the coolers as follows: Type A < 10 µm, Type B < 20 µm, and Type C < 50 µm. Flow lines between the filter and the cooler should be cleaned prior to use of the cooler. Sealing aids such as teflon tape should be avoided as much as possible between the filter and the cooler. The coolers have integral internal filters which are slightly larger than the designated upstream filtering values. Periodic backflushing may clear these filters and improve flow performance. Standard coolers can survive pressurization to 250 psi with no ill effect. This may be compared to standard operating pressures, which are expected to be less than 20 psi.

Long Term Performance. To date only limited long-term performance data is available on MC²'s micro-impingement coolers. Continuous testing for 1200 hours has resulted in only a modest increase in the flow rate of the coolers. The coolers should be resistant to clogging, given proper filtering and periodic backflushing. Copper and its alloys are also known to be inherently resistant to biofouling. If in an application corrosion is determined to be a problem, a commercial additive such as OptiShield (Opti Temp, Inc., Traverse City, MI) may be introduced into the cooling water.