FEATURE
INCORPORATING COLD CHAIN
GEA Refrigeration Technology
A refrigeration and heat pump plant installed at Wipasz - Poland ’ s leading poultry producer .
refrigerant . The vapour refrigerant is drawn out of the intermediate cooler and flows to the high-stage compressor , before flowing into the condenser and repeating the entire cycle .
• Cascade systems : In this configuration there are two set of compressors , one in a high temperature circuit and another in a low temperature circuit . A heat exchanger between the two circuits , called the cascade condenser , acts as an evaporator for the high temperature circuit and a condenser for the low temperature circuit . The two refrigerants can be the same or different for each circuit One common practice is to use R717 for the high temperature side and R744 for the low temperature side . This means that less ammonia is used , and the system is more efficient compared to a two-stage ammonia-only system .
These systems primarily utilise large reciprocating compressors – although screws are used as well – for the CO₂ . These compressors , however , are only capable of subcritical CO₂ compression . As a result , the compressors may look a bit odd to someone familiar with ammonia systems . The compressors themselves are relatively small , and the motors are relatively large – the exact opposite of an ammonia system .
• Transcritical CO₂ System : Transcritical CO₂ systems are continuing to grow in capability and size . They offer a particularly great solution for cold-storage freezers . Transcritical CO₂ systems utilise racks of small compressors ; however , as compressor technology allows them to become larger , the racks will in turn get smaller . Additionally , these systems commonly use hybrid heat-rejection systems called adiabatic gas coolers that utilise less water , send less water to the sewer and often require no chemical treatment of the water . Along with such systems , it is reasonable to expect to see improvements on the controls side .
• Low Charge Ammonia System : Optimisation trends of current technologies for low charge ammonia refrigeration systems are moving toward reduced ammonia charges for every application . The key is the application of correctly engineered and sized equipment .
Overall , stick-built low charge ammonia refrigeration systems will operate like a typical ammonia system in use today with key exceptions : All components will be smaller , and a large high-pressure receiver typically will not be used . More precise engineering will mean the ammonia charge is no more than necessary to operate the system . Expect these systems to continually be optimised , utilising less refrigerant while becoming more efficient and getting smaller at the same time .
Packaged low charge ammonia refrigeration systems offer the benefit of faster installation and are excellent for cold storage in high ambient temperatures and coolers . They also eliminate the need for an engine room ; instead , they use localised systems .
As the name indicates , the charge for these systems typically is lower , and the systems are even smaller than stick-built ammonia systems . Some employ special feed and control techniques to keep the charge to an absolute minimum . In practice , these systems will often appear as a sort of miniaturised engine room located on a facility roof or on a concrete pad outside .
A properly designed low-charge optimised system uses less than 2-7 kg of ammonia ( from 0 06 kg / kW to 1 3 kg / kW⁹ ), and therefore fewer vessels , fewer pipes , smaller pipe diameters and no pumps . Nevertheless , it still needs an equipment room . A packaged ammonia system eliminates the huge quantities of ammonia inventory and piping by moving to smaller self-contained systems that are usually placed on the roof / ground outside , avoiding any dangers due to leaks These self-contained systems have an ammonia charge of about 0 6 kg / kW and usually combine the compressor , evaporator valve system and control systems into one easily installed and movable packaged system . CLA
22 www . coldlinkafrica . co . za COLD LINK AFRICA • October 2021