The economic life of the cooling unit depends on the selection of materials that are appropriate for the operating conditions. The cooling coil within the unit in particular must be manufactured in conformity with industrial specifications.
Properties of the Cooling Coil Cooling coils must be manufactured in conformance to the SEP (Sound Engineering Practice) defined under 97/23/EC PED (Pressure Equipment Directive).
Tubes In consideration of performance and cost-friendliness, the most appropriate tubing material for cooling coils is copper. The quality of the copper used for the tubes, is among the most important properties determining the life of the dry cooler. In cases where weak material is used, problems occur particularly in inflection points and solder joints. For this reason, the copper tubes used must be manufactured per international standards. Since measurement over the product is not possible, the information regarding the wall thickness of the tube should be obtained from the manufacturer while selecting a dry cooler. Contact between the tubes and the ring plate should be avoided and long working life without leaks should be ensured in pipes.
Fins The fin material commonly used in dry coolers is Aluminum. In most applications, epoxy coated Aluminum is favored for longevity of the dry cooler. The epoxy coating significantly increases the resistance of the fin to the abrasive effect of the environment. Particularly in facilities near the sea and in power plants, the epoxy coated fin application is a necessity. In highly corrosive environments where the epoxy coating may prove inadequate, epoxy and polyurethane coating is recommended.
Fin Geometry In dry cooler design, fin geometry which defines the diameter of the tube and distances between tubes influences capacity and pressure losses. The fin geometry is selected among its own standards by the manufacturer so as to provide the required cooling capacity within the appropriate pressure losses. Geometries with intensive tubing can be said to yield more advantageous capacity/price ratios; however in this case, optimization is required since pressure losses will increase in tandem. Under practical conditions, it must be kept in mind that dry coolers having the same heat transfer surface, yet different geometries will yield different cooling capacities and pressure losses under the same conditions
Casing Dry coolers must have a self-supporting construction and allow placing on the floor where there will be installed without the aid of additional constructs. Casing material for dry coolers is selected according to environmental conditions. In mainstream applications, galvanised steel coated with electrostatic powder paint is used as casing material. In instances where sturdier materials are required, stainless steel casing may be preferred; however this will be quite costly. Each fan compartment should be separated from the other from plates and the reverse rotation effect of standing fans should be prevented. The placement of control covers on top of units is recommended.
Noise Level and Fans Particularly in applications near residential areas, low noise levels for the operation of dry coolers becomes an important criterion. The noise level which results primarily from the fan motor and the design of the fan blades is determined by evaluating manufacturer’s data and checked in terms of conformance to the prescribed specifications. The sound level may be reduced by decreasing motor speed, if need be; in this case the heat transfer area of the heat exchanger should be increased in order to provide the required cooling capacity. Another point which requires consideration in radiator selection is the necessity to assure the adequacy of the design to provide the cooling capacity required in conditions of high ambient temperature. In periods where air temperatures are low, operating all fans at maximum speed to achieve the desired capacity will be superfluous and costly. In systems monitored by cooling water outlet temperature, operating fans at low speed or disabling them will provide an air supply of sufficient flow to the system.
Two Speed Fans The most practical means of supplying air of varying flow is to use a two speed fan. Thanks to these fans that can operate at a secondary speed like 3/4ths of the highest operating rotation, a substantial amount of energy can be saved in periods where the air intake temperature falls far below design temperatures. For example an 870 kW dry cooler with four fans may be operated with lowered fan rotation when ambient temperature drops from 33 ºC to 20 ºC . In this case, 0.75 kW less of power will be consumed per fan, which means an energy consumption of nearly 40 %. This example pertains to 4 fans, systems of a much higher number of fans are being operated in most plants. The power consumed by