Munters controls climate

30 January 2009

Dehumidified shrouds - fitted between 1978 and 1980 around the four main cable anchorage chambers on the M4 Severn Bridge - are still providing a perfect climate to control the main 20” diameter cables.

These, each comprising 8,360 parallel wires one fifth of an inch thick, would have been threatened by industrial contaminates such as sulphur dioxide and moist salt laden air.

This technique for enveloping the vulnerable steel cables in a dehumidified shroud was developed by the environmental specialists Munters who first supplied two of their robust M120 dehumidifiers to handle this task in 1978.

However the shroud made it impossible to carry out regular close inspections of the upper lengths of the main cable wires and the Department of Transport were anxious to have the facility to complete regular visual safeguard checks.

Munters then tendered to provide an alternative scheme to dehumidify the entire unshrouded anchorage chamber. Trials showed it was possible to control relative humidity to a maximum of 25%, around the clock, using three of their powerful MD600L dehumidifiers. The anchor chamber has remained stable and corrosion free despite the vagaries of wind and weather. Pending a decision from the SRC, this system will be adopted as the optimum method, rendering the existing shrouds obsolete.

The Munters condenser type dehumidifiers selected for this project incorporate a desiccant drying wheel which is partitioned into two zones. In the larger ‘working’ zone air is continuously drawn through the drying wheel where moisture is removed. Dry air is then blown into each anchorage chamber at the rate of 600m3 per hour as provided by each of the nine machines supplied. Units are controlled and conditions are monitored by a central control panel situated in the middle chamber for easy access.

Dry air is continuously recycled through the chamber area using Munters MD600L dehumidifiers. The extracted moisture is condensed and pumped from the area. The installed dehumidifiers are capable of extracting over 55 kilograms of moisture from the air every day thus meaning that this important installation is well protected.

The Severn Bridge, which carries an average of around 60,000 vehicles every day, originally cost £10M to build.

Many of the World’s best known bridges utilise Munters dehumidification expertise. These include the Forth, Faro, Bosphoros and Bear Mountain bridge in New York also the Hennipen Avenue Bridge in Minnesota.

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