Authors: D.O. del Castillo, A. Pieters, B. Loots,
M. Kontu
9th International Mine Ventilation Congress, New Delhi, India, November 10, 2009
Abstract
Impala Platinum is the second largest producer of Platinum in the world. The mine property consists of many shafts and has some 69 000 kW of installed ammonia refrigeration capacity distributed across the property. The oldest machine was commissioned in 1985 and the most recent one was commissioned in 2002.
The older machines are required to be overhauled due to ageing. In 2006 the decision was taken to replace all the plate-and-frame evaporators installed in the 1980’s because they were starting to fail and posed the risk of leaking ammonia. The units, using titanium plates, were becoming too costly to replace due to the huge price increases in titanium, and considerable lead times, during the years 2006 and 2007.
Impala Platinum also decided to modify the cooling distribution system of those shafts so that chilled water would no longer be sent underground, requiring only a closed-loop system supplying chilled water to the surface bulk air coolers. This modification meant that the mine water in the circuit could be replaced with potable water, thus obviating the need for titanium plates.
Once the decision was made to replace the old plate-and-frame evaporators in the ammonia refrigeration machines, a selection criterion for the type of heat exchangers to be considered was developed. The new units had to have, amongst others, the following main characteristics:
- Comparable heat transfer efficiency to the older units
- Robust construction
- Easy to maintain
- Reduce the risk of ammonia leaks
- Allow the existing chilled water pumps to be re-used, which meant having units with equal or lower pressure drop than the existing plate-and-frame heat exchangers
- Offer minimum modifications to the original cooling system, including piping and controls
- Develop a “universal” design suited for the thermal duty of all the ammonia evaporators installed on the property
The refrigeration plant at No.1 Shaft was selected for the first upgrade. This plant consists of two ammonia refrigeration machines installed in parallel, each with a nominal cooling capacity of 8 300 kW. The evaporator of the older machine was chosen for the first upgrade.
The initial plan was to install a single unit, but due to physical constraints it was decided to split the unit in two (parallel), which in turn gives the refrigeration plant extra versatility.
