DESIGN CONSIDERATIONS FOR MORE RELIABLE AND DURABLE REFRIGERATION PLANT AT MINE SITE
Refrigeration plant including chillers is used extensively for cooling a variety of mining machinery. From crushers, scrubbers and mills through to safety showers and cooling for potable water, the optimal design of your refrigerant plant can make a big difference to running costs and can help you to maximise return on investment and improve reliability.
Some important factors to consider to ensure longer life and greater reliability of any refrigeration plant are summarised below
Plant cabinet and frame
Refrigeration plant cabinets are normally fabricated from baked enamel powder coated sheet metal with a welded or bolted steel sub frame. The thickness of sheet metal panels can vary between vendors, and some vendors use galvanised panel or offer stainless steel alternatives. Sub frame components can be fabricated from bent sheet metal or can be cut from more solid thicker steel sections like I beams or C sections. Welding on sub frames can be of various qualities. Fabricated sub frames could be sand blasted (to remove mill scale) and primed, or could be hot dipped galvanised. Fasteners for sheet metal could range from galvanised screws to stainless steel. Component mounting bolts can black steel, galvanised, or stainless steel with fasteners secured using nylock nuts, spring washers, nutserts, or a combination of these. Containerised options are available which use standard sea worthy shipping containers modified to accommodate the refrigeration plant. Compromising on the quality of some of these components can make the upfront cost cheaper, however maintenance cost and part replacement need to be factored into the total cost of ownership
Engineers can specify a thicker grade of sheet metal for panels for increased durability. 1.6mm is a common size and mill certificates can be requested to give traceability regarding batch quality. Galvanised steel panels could be specified prior to powder coating. For extremely harsh environments stainless steel could be specified along with the grade. Typically 304 and 316 stainless steel are used with 316 stainless (2% molybdenum) having superior corrosion resistance. The use of sub frame materials will depend on the weight and vibration expected from the components to be mounted. Sheet metal mountings should be avoided for compressors. A weld map should be produced by any vendor prior to fabrication and a hold point put in place for weld inspections. A sample of welds can be non destructive tested by a certified organisation such as Bureau Veritas. The fastening method can be specified depending on the application.
The refrigerant gas selected for a project should reflect the expected operating conditions for the plant. If high ambient conditions are expected for long periods then R134a is the best option for guaranteed continuous performance in hot weather.
The refrigerant gas selected will perform differently depending on the ambient conditions. R407C is suitable for medium temperature applications (evaporating temperature up to 15 degrees C) and is the most efficient refrigerant gas at most ambient conditions within Australia. Refrigeration efficiency is given by the coefficient of performance (COP) which is a ratio of kilowatts of refrigeration generated to kilowatts of electricity used. R407C loses efficiency dramatically at higher ambient temperatures. R407C is a replacement gas for the phased out refrigerant R22. The composition refrigerants HFC-32 (23%), HFC-125 (25%), HFC-134a (52%) have been selected to match R22 as closely as possible. R407C is a zoetrope with glide of 9.7 degrees C which means that for a constant pressure there is a difference in the evaporating and condensing temperatures of the component gasses. If a leak occurs at a point in the system where both liquid and vapour are present then the system may not leak the component gasses equally, changing the ratio. As such R407C cannot be topped up and in the event of a leak a new gas charge must be used and the old R407C reclaimed and destroyed.
R410A is a newer refrigerant and operates at much higher pressure than R407C. R410A gives a lower (worse) COP than R407C at most conditions in Australia and like all refrigerants becomes less efficient as the ambient temperature increases. R410A operates at much higher pressure than R407C and as such the two gasses are not interchangeable. Copper tubing wall thicknesses must be increased for R410A and because of the higher operating pressure a lower gas charge is required and a smaller compressor, evaporator and condenser can be selected to achieve the same refrigeration capacity, which means a lower capital cost. R410A is composed of HFC-125 (50%) and HFC-32 (50%) and is a near azeotrope meaning that it has a very small glide of 0.2 degrees C. A R410A leak in a mixed state vessel (condenser or evaporator) can still leak the component gasses unequally however the change in composition does not give as dramatic a performance change compared to R407C. Manufacturer DuPont recommends that any R410A leaks, after system repair, can have the gas charged topped up rather than replaced.
R134A is azeotropic – a single component refrigerant that operates at lower pressure than R407C. Because of its lower operating pressure R134A can operate at much higher ambient air conditions and is recommended by compressor manufacturers for use at ambient air temperatures up to 60 degrees C. R134A is used extensively in automotive air conditioning, in domestic refrigerators and in large screw compressor and centrifugal compressor chillers. R134A is less efficient than R407C at most conditions in Australia however it is the most efficient of the three refrigerants at air temperature above 37 degrees C. Because of the lower operating pressure the main refrigerant components: compressor, evaporator, condenser, fans, must all be larger and as such the capital cost for R134A plant is generally higher.
The refrigerant compressor should be selected for the expected operating environment and duty. For longevity a compressor should be within its standard envelope of operation for all expected operating conditions. Additional factors to consider include multiple compressors within a single plant for partial redundancy and the availability of back up service for new compressors as scroll compressors come to the end of their life or for bearing changes on screw compressors.
Commonly used refrigerant compressors types include scroll, reciprocating, screw and centrifugal. Each has a different mechanism for compressing refrigerant gas and the type used will depend on the plant size and the operating conditions. Scroll compressors from manufacturers such as Copeland and Danfoss are widely used in plant up to up to approximately 200kW cooling capacity. These are hermetically sealed and if damaged require replacement in full. Multiple compressors are often used to reach the required refrigeration capacity as the largest scroll compressor on the market will deliver approximately 90kW refrigeration.
Reciprocating compressors are used primarily at low evaporating temperatures for refrigeration applications.
Screw compressors are used for both low temperature and medium temperature applications for refrigeration, air conditioning, or process cooling. Screw compressors are semi hermetic meaning that bearing replacement can be undertaken at the manufacturers recommended interval. The recommended interval between bearing changes is 30,000 to 50,000 hours depending on the manufacturer. Screw compressor inspection and rebuilding is available in Australia either directly with a number of vendors or through one of several after market compressor rebuilders.
Centrifugal compressors are used primarily in large air conditioning chiller applications.
The evaporators and condensers should be selected taking into account the operating temperature, the need for maintenance, and any expected corrosive elements at the site.
Evaporators could be direct expansion type (fan coil units) or plate type or shell and tube type for chilled water applications.
In direct expansion fan coil units refrigerant flows through a series of copper / aluminium / stainless steel tubes which are mechanically expanded into equally spaced metal fins. Common fin materials include aluminium, copper and stainless steel. Fin spacing can be varied where dust or ice are anticipated. Fin coating can be specified or the fins can be left untreated. The disadvantage of untreated fins is an increased risk of corrosion.
Plate evaporators in chillers are used predominantly in small plant. They consist of multiple stainless steel pattern pressed plates brazed together at the edges with copper. Alternate circuits of refrigerant and water pass next to each other affecting the heat exchange. The benefit of the plate evaporator is the small size relative to the possible kW of heat transferred. This is also a disadvantage given that a momentary interruption of water flow can lead to freezing of water within the evaporator, rupturing of the plates and damaging the compressor by water ingress.
Shell and tube evaporators typically contain a solid steel shell and a network of copper tubes within the shell. The refrigerant can be within either the tubes with water in the shell side or vice versa. Shell and tube evaporators have a wider tolerance for water flow variations and can be customised for materials such as stainless steel or cupro-nickel for sea water applications. Where the water is in the tube side the end section can be easily removed for period maintenance.
Condensers can be air cooled via finned coils as per evaporators above or water cooled by plate or shell and tube heat exchangers.
Electrical equipment within refrigerant plant includes contactors, circuit breakers, safety controls, a central controller or PLC, and a controller interface.
Component selection can be specified to match site requirements for named brands in order to reduce the essential spare parts cost and duplication. Brands specified should be widely available in Australia through multiple resellers. Specified items could also include the IP rating on the electrical box and motors, the electrical box material, gland types, cable type and cable number types.
The electrical standard could be specified as AS 3000. In Australia the wiring standard for manufactured machinery that most vendors comply to is AS 60204.1 Safety of Machinery – Electrical.
Equipment manufactured to AS 3000 (Electrical Installations) will allow consistency of electrical standard site wide. Electrical tradespeople will be instantly familiar with the layout and function of electrical boards allowing for faster maintenance.
These are just some of the considerations for increased reliability of refrigeration plant at remote sites and may not be applicable to every installation. A detailed discussion with a number of refrigeration plant or chiller manufacturers will yield further considerations to optimise plant for higher quality, greater reliability and longer life.