Compression couplings win district heating test

Edited by the Engineeringtalk editorial team Apr 6, 2006

Rubber-in-compression couplings at a district heating utility in Sweden have proved their worth over rubber-in-shear couplings that were installed at the same time in 1986.

Rubber-in-compression couplings, installed by Renold Hi-Tec Couplings at a district heating utility in Sweden, have proved their worth over rubber-in-shear couplings that were installed at the same time in 1986.

During the 20 years of operation, Renold's rubber-in-compression couplings have only required maintenance twice to replace the rubber elements, a simple job that costs less than 10% of a replacement coupling.

Conversely, when the rubber elements in the rubber-in-shear couplings needed replacing it took two days and, significantly, the cost was higher than fitting new rubber-in-compression couplings from Renold.

As a consequence, the company eventually decided to replace the rubber-in-shear couplings with rubber-in-compression.

The deciding factor was the speed with which the new rubber-in-compression couplings could be installed.

It took just half a day to fit the couplings but would have taken two days to replace the rubber elements in the rubber-in-shear couplings.

The couplings operate between a series of heat pumps and ABB synchronous motors transmitting 9400kW at 1500rev/min.

In 1986, when the couplings were first specified, engineers working for the heating utility were unsure whether rubber-in-compression or rubber-in-shear couplings would be best suited to the application.

As a trial, both technologies were specified, but now, 20 years later, it can be seen that the rubber-in-compression couplings have provided the most reliable and cost effective for the plant.

Rubber-in-compression couplings are made up of two round, metal sections fitting one inside the other with what looks like the paddles of a paddle steamer projecting inwards from the outer section and outwards from the inner.

Rubber blocks are placed in the spaces between the paddles and, as the motor turns the outer section, it drives the inner section through the rubber blocks.

As this happens, the rubber is compressed, hence the term "rubber-in-compression".

Rubber-in-shear couplings are made up of two metal discs with a rubber block fused between.

The motor turns one disc, which in turn drives the other through the rubber element.

District heating facilities are fairly common in Sweden and are environmentally friendly.

Ambient heat is taken from the earth, air or water and transferred to local homes and businesses, usually in the form of hot water for heating radiators and providing a domestic hot water supply.

The system works a bit like a fridge in reverse.

Fridges remove unwanted heat from the food compartment and dispense with it as a waste product outside.

District heating utilities remove heat from the environment and use it as the desired end product, which is then used to heat homes and supply hot water.

In cold countries such as Sweden the local community is entirely dependant on the district heating supplier and the reliability and cost effectiveness of plant and equipment is essential.