Rectifier heat sink and cooling system (water-cooled thyristor and diode heat sinks with air-over-water cabinet cooling)

01

Water-cooled thyristor heat sinks: flow per device and series versus parallel piping

Advanced Energy's water-cooled thyristor stack, built for outputs above 2000 A and above 1500 kW per bridged rectifier at up to 690 V AC, cools each thyristor between two liquid-cooled heat sinks of copper alloy, connected with polyurethane hoses and stainless-steel fittings. Coolant flow is limited to 10 L/min (2.6 gpm) or less per thyristor. [1]

The same user guide ranks three ways of piping the heat sinks. All heat sinks in series, each inlet fed from the previous outlet, is the simplest and the lowest-performing, because the water warms as it passes each one. Piping only the two heat sinks of each thyristor in series improves it, and feeding every heat sink in parallel from a supply manifold to a return manifold gives the best performance. AE states that the stack's maximum current depends on the thyristor chosen, the plumbing configuration, water temperature and flow rate. [1]

Editorial calculation: water at 10 L/min is about 0.167 kg/s; with a specific heat of about 4.18 kJ/kg·K (assumed) and a 10 K rise from inlet to outlet, it carries 0.167 × 4.18 × 10 ≈ 7 kW. Three AE stacks make the six-thyristor B6C bridge the guide shows for DC output, so at the 10 L/min limit per thyristor the bridge would take up to 6 × 10 = 60 L/min. [1]

Two of the stack's heat sinks carry a normally-closed temperature switch that opens at 85 °C and can be wired to alarm or shut down. The hose fittings suit hose of 9.5 mm inside diameter with a minimum bend radius of 70 mm, and the hose is rated to a maximum operating pressure of 16 bar. [1] GD Rectifiers, which supplies water-cooled heat sinks for isolated-base thyristor, diode and IGBT modules, pressure tests them to 7 bar (100 psi) as standard. [5]

Ask for the flow and pressure drop per heat sink at your inlet temperature, the piping arrangement the current rating assumes, and where the over-temperature switches sit.

02

Cooling-water purity, live heat sinks and the closed secondary loop

Whether the water touches live metal decides how clean it must be. In Trystar's Series 30 plating and anodizing supply (6 to 100 V DC, 500 to 10,000 A), the diodes are on the low-voltage side of the main transformer and sit on all-copper water-cooled heat sinks with no isolation between the cooling water, the heat sink and the diode. The SCRs on the line side are electrically isolated from their copper heat sink and its water. Trystar states that all voltages above 100 V are isolated from the cooling water. [2]

AE asks for much purer coolant in its thyristor stack: salt content of 1 mg/L or less, deionized water, with up to 40% glycol. [1]

Where plant water cannot meet such limits, KraftPowercon's CoolKraft places a water-to-water heat exchanger between the raw water and the rectifier; the maker says the secondary circuit reduces concerns about water-quality supply. It is standardized for 1 to 30 kW installations, limits coolant to the power electronics to 35 °C with a minimum 10 °C difference between raw water and coolant, holds a 5 L expansion tank, is rated PN5 (5 bar design, 7.5 bar test) with 6 bar maximum on the cooling circuit, and can sit up to 70 m of pipe from the rectifier. The unit measures 870 × 500 × 610 mm, weighs 67 kg and is rated IP34 for +5 to +50 °C ambient. [3]

Send the supplier a recent analysis of the plant water and state whether the rectifier will run on a closed deionized loop.

03

Transformer and cabinet: fan, air-to-water coil, condensation and leak shutdown

In Trystar's water-cooled version the transformer is not water-cooled directly: a direct-driven high-pressure axial fan circulates internal air through the main transformer and an air-to-water heat exchanger of copper water tubes and aluminium fins. [2]

Water flow is switched on and off by a solenoid valve to prevent condensation and save water; the cabinet and device temperatures are controlled by two solenoid valves on the inlet line operated by sensors inside the rectifier. The valve is interlocked with a leak detector, so water detected in the cabinet shuts the unit down and closes the valve. Over-temperature sensors on the diode and SCR heat sinks protect the devices if water flow becomes insufficient. [2]

For comparison, the air-cooled version of the same supply draws ambient air through the components with a fan and runs at full rating up to 40 °C ambient, with derating above that. Its devices are clamped to extruded heat sinks with less than 10 °C difference between device and heat sink, and the heat sink limits temperature rise to 40 °C at the specified airflow. [2] KraftPowercon describes water cooling as the more reliable choice when temperatures reach 40° or more or the air is dusty or contaminated, since the closed system avoids filter clogging and does not return heat to the room. [4]

04

Data for quoting a rectifier cooling system

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Rectifier: [DC current; DC voltage; SCR or diode; process — plating, anodizing, electrolysis]
Losses to remove: [kW, if known]
Plant water: [inlet temperature range; pressure; flow available; water analysis]
Loop: [direct plant water; closed deionized loop; water-to-water exchanger]
Heat sinks: [copper or aluminium; isolated or live; piping series or parallel]
Cabinet: [air-to-water coil; fan; enclosure rating]
Protection: [over-temperature switches; flow monitor; leak detector; solenoid valve]
Site: [ambient temperature; dust or corrosive fumes]

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