Extended Format SA-2 Cooler

The SA-2 cooler design can easily be extended in length as shown in the figure at right. The roughly 2 cm length of the standard SA-2 has been extended to 7 cm in the SA-7A cooler and to 14 cm in the SA-14A cooler shown at right. The depth (distance between the heated surface and bottom of the cooler) may increase as the length of the cooler is extended to allow more room for internal manifolding. The amount of the depth increase will depend upon the intended operating flow rate and requirement for flow uniformity along the bar.

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 SA-2 extended configurations. These designs, designated Types "A", "B", and "C", employ 25µ, 50µ and 125µ passages, respectively. The Type A versions of the coolers have approximately 2000 internal micro-impingement cooling circuits spread uniformly over their surfaces. These passages are located roughly 250 µm below the cooler surface.

External Design. The external configuration of the coolers can be inferred from the exterior design of the SA-2. Nominal values for the tolerances, surface finish, and plating are provided in the drawing, 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 with 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 graphs below show the thermal and flow performance of the nominal SA-2A design compared with the two extended formats shown above (SA-7A and SA-14A). 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.

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, 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. MC2'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, although use of UV filters is recommended for long-running applications. 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.