Tarmac Topblock adds heat recovery system

Cheltenham, UK -- Tarmac Topblock has achieved a significant reduction in its annual energy costs through the installation of a new system to recover heat from waste steam at its Alfreton factory, which manufactures products for the construction industry.

At the Tarmac Topblock plant, aerated concrete blocks are mixed and moulded, before curing for several hours in a bank of eight autoclaves fed by steam at 12 bar g. The steam is vented rapidly at the end of each cycle, drawing out any excess moisture from the blocks.

Instead of sending the vented steam to atmosphere as was previously done, the new heat recovery system - Spirax Sarco Engineered System - feeds the steam through a heat exchanger to pre-heat the site’s boiler feed water.

As the supply of waste steam and the demand for hot water from the boiler do not always match, the system includes a storage tank that acts as a buffer between the heat exchanger and the boiler feed tank. The ideal temperature of the boiler feed water is 94°C, which reduces the amount of energy the boiler uses to raise steam.

“Heating the boiler feed water minimises the cost of making every 1,000 kg of steam,” explains Tarmac Topblock’s engineering manager Robin Stapleton. However, the temperature can’t be raised beyond 94°C without risking cavitation in the boiler feed pump.

Cold water circulates repeatedly through the Spirax Sarco heat exchanger and storage tank, getting progressively hotter. The water can be drawn off at any time to meet demand from the boiler, but if it reaches 94°C, excess waste steam is vented to prevent overheating. If the pre-heated water reaches the boiler feed tank at a lower temperature, Tarmac Topblock’s system brings it up to 94°C by direct steam injection.