Cascade Systems – Defrost Methods
A cascade refrigeration system can have energy consumption advantages over a more conventional two-stage industrial refrigeration system, especially where very low temperatures (in the order of -30°C and below) are required for a particular process.
In a cascade plant, a conventional industrial refrigeration system is employed for the high-side (chiller and “interstage”) duty with the required compressors, evaporative condensers, evaporators and pipework. A high-side evaporator heat exchanger on this plant is employed to act as a condenser to a separate, “low-side” (freezer) cascade refrigeration system, which also includes its own compressors, evaporators and pipework. By utilising an alternate refrigerant for the low-side (such as carbon dioxide) with a suitable high-side refrigerant (ammonia with larger industrial compressor systems or R134a on smaller plants) energy usage of the compressors can be reduced.
Where carbon dioxide (CO2) is used as a low-side, cascade refrigerant, care must be taken in the application of defrost method to provide a holistically suitable system for the specific duty required. Although carbon dioxide provides an advantageous coefficient of performance (COP, a measure of compressor efficiency) in a cascade arrangement, it does so whilst operating at pressures well in excess of those experienced in a conventional two-stage ammonia plant.
This means that traditional hot gas defrosting (where “hot”, high-pressure discharge gas is piped from the compressor/s to the various evaporators, and introduced into the coils to provide a heat source for melting ice growth) involves considerable investment and complexity of controls, as it is necessary to maintain the hot gas loop at sufficiently low pressure to be suitable for the maximum allowable design pressure of the currently available components, whilst still being at sufficiently high pressure (i.e. warm enough) to defrost ice from the heat exchanger surfaces. This nominally means the requirement of an added hot gas circuit with hot gas generator (i.e. compressor) with smooth modulation and fine controls to suit, and to date the whole of life costs and maintenance complications have pushed most designs to other methods of defrost.
One alternate solution is to use electric defrost elements as the heat source for evaporator defrosts, but care should be taken to ensure the liquid refrigerant contained within the internal volume of the evaporator is “pumped out” (i.e. transferred to another part of the system) prior to defrost, otherwise the heat gained by the volatile CO2 liquid can cause rapid vapour generation, leading to excess pressure and possible evaporator tube failure. It is therefore prudent to install suitable controls to confirm that large evaporators have no residual CO2 liquid (and are therefore safe to add heat) and have these controls integrated into the industrial refrigeration system’s safe operation protocol.
Water defrost can also be employed in lieu of electric elements (with a similar pump out requirement), but safeguards also need to be in place to ensure ice, frost and snow growth in water supply and drain pipework doesn’t cause water overflow from defrost drain trays and subsequent ice issues on the floor of the room.
Finally, carefully regulated and modulated warm ambient air defrosts are also possible, but as with all defrost methods noted here, the design criteria are dictated by the required pressure of defrost for the system at hand.
Gordon Brothers has designed and installed the largest carbon dioxide/ammonia cascade system in Australia (1,740 kW operating down to -52°C in the Northern Territory) incorporating both water and electric defrost systems, and this has proven to be efficient and reliable.
For more information on cascade plants and the best refrigerant and defrosting method for your industrial refrigeration system, please contact your Gordon Brothers Pty Ltd representative.