14MW Underground Refrigeration Plant
BBE Projects has secured another mine cooling project for the complete turnkey design and build of two refrigeration and air-cooling installations in Mali. Located on surface at the top of two new dedicated downcast ventilation holes, the coolers will each provide more than 14 MWr of air-conditioning for the deeper levels of the underground workings.

Each installation will comprise two 7 MWr dual-compressor York YD R134a refrigeration machines producing chilled water for a horizontal spray chamber. A particular feature of these installations is that the air will be drawn through the ventilation holes by underground fans, so that the surface air coolers will be under a slight negative pressure from the induced ventilation. This has allowed the selection of light-weight building materials for the shell of the air-coolers which offers savings in terms of cost and also speed of erection. Similarly, the condenser cooling towers for heat rejection will be constructed from light-weight FRP components with similar benefits in cost and time.

The shell of the bulk air cooler will be assembled from 1200 mm wide steel-clad insulated panels fitting together in a tongue-and-groove manner, coupled with a light aluminium structure and attached directly alongside the main plant room. The use of this material and the resulting compact layout also contribute to a high thermal efficiency as the chilled water from the refrigeration machine is sprayed directly into the intake air stream, with no losses from interconnecting pipework. Indeed the entire plant occupies a footprint of only fifty metres square.

Being at a low altitude in an equatorial region, the mine experiences a challenging climate of both humid and hot, through the various seasons, particularly when hot and humid at the same time, which is more than half of the year.

With the all the power for the mine coming from on-site generators, special attention has been paid to overall system efficiency and low power consumption of the cooling system. This affects many design parameters such as the number of tubes in the heat exchangers, water flowrates in the two circuits and the size (and therefore thermal approach) of the condenser cooling towers.

The power station has also required careful consideration of electrical start-up demands and the selection of the dual compressor machines has halved the maximum instantaneous start-up current. Nevertheless, starting the plant will require a handshake protocol with the power station to acknowledge and prepare for the build-up of an additional 3 MW electrical load onto the system.

Being a remote location, the cooling system has been designed to be simple to operate with a minimum of control elements. In fact there is only one temperature-controlled valve on the water circuits in the entire plant and load control of the compressors is achieved with conventional inlet guide vanes.

Being vital to mining operations, the specifications place particular emphasis on overall system availability which has been designed with generous modularity and is well-equipped with stand-by equipment and components. All water circuits have permanently installed stand-by pumps.

This project in Mali comes in the wake of two recently completed refrigeration installations for BBE Projects. The first being the second phase of the air-cooling programme at Acacia Mining’s Bulyanhulu Gold Mine in Tanzania, comprising two 3.5 MWr York ammonia screw compressor refrigeration machines producing chilled water for a surface bulk air cooling tower straddling a 1 000 m deep dedicated ventilation hole for the west section of the mine. This installation uses the same Hamon cooling towers planned for the project in Mali.

The second installation being an underground plant comprising three Trane 1.5 MWr three-stage centrifugal refrigeration machines providing chilled water through a closed-circuit network of cooling cars at a depth of almost 3 000 m at Sibanye’s Driefontein Gold Mine. Heat rejection is via a new horizontal spray chamber into a new return airway connection adjacent to the plant.

BBE Projects is also currently busy with the civil construction and mechanical and piping installation of the second phase of the underground refrigeration and air-cooling system for Lift 2 at Palabora Copper. The refrigeration machines are located between two sets of underground spray chambers, the one set producing cold air for two dedicated sub-vertical ventilation holes into the Lift 2 zone and the other set rejecting heat into the return air circuit.

At the Malian mine, civil construction activities have already begun at the first of the two sites and the first plant will be operational by mid 2016. The construction and commissioning of the second plant runs in parallel to the first plant with a stagger of about two months.

The supply of components comes from around the world and the logistics amount to some seventy-five containers and flat racks from the USA, Brazil, India, Malaysia and Europe with the majority from South Africa. The routing is to the port of Dakar followed by 800 km of overland transport to the mine. In line with most mining company’s localisation programmes, BBE Projects will engage local contractors for the on-site construction, erection and installation works.

BBE Projects delivers tailor-made ventilation, refrigeration and air cooling installations in a single turn-key package. Being totally independent, they are able to select the optimum components to satisfy each client’s needs and preferences, with particular emphasis on overall efficiency and simplicity of operation. The process parameters are uniquely designed to suit local climatic conditions and operational circumstances. BBE Projects has an enviable record of successful projects completed across the African continent (South Africa, Ghana, Tanzania and Zambia) over the past 12 years. BBE Projects is a member of the BBE Group that has been providing a comprehensive world-wide consulting service specialising in mine ventilation, refrigeration, air-cooling and energy for all mines in all climates for over 25 years.