Specialist article in DIE KÄLTE+KLIMATECHNIK
R290 / R744 cascade
Kratschmayer successfully uses SKADEC chillers for supermarket refrigeration with exclusively natural refrigerants for the refrigeration supply of a supermarket that opened in August 2018.
Due to the high refrigeration requirements of supermarkets and the resulting energy consumption, the concept developed focuses in particular on efficient and future-proof refrigeration supply. Due to its criteria for refrigerant selection and taking into account the future availability of refrigerants in accordance with the F-Gas Regulation, only natural refrigerants are used. By combining an R290 (propane) brine set in conjunction with an R744 (CO2) compound, as well as a propane heat pump for air conditioning, only natural refrigerants are used. Since the current phase-down stage for placing HFC refrigerants on the market came into force and the associated enormous price increases for conventional refrigerants, natural refrigerants are more in demand than ever. In the supermarket sector, it is state of the art to use a CO2 (R744) refrigeration cascade in the deep-freeze (freezer) sector. In the normal refrigeration (NK) and air conditioning sector, conventional refrigerants are often still used.
KRATSCHMAYER has succeeded in implementing an innovative concept in a newly built supermarket. As a natural alternative to the R134a or R450A systems with direct evaporation that are otherwise widely used in the refrigeration sector, a specially developed R290 cold brine unit from SKADEC was chosen. This not only provides the -5°C cold brine for the low temperature range, but also condenses the CO2 compound used in the low temperature range using an intermediate circuit with heat exchanger (cascade). In addition to the advantage of not having to install an external condenser, the condensation is almost independent of the outside temperature and is therefore much more stable and easier to control. The cold brine unit with a nominal cooling capacity of 78 kW at the design point was installed as a compact unit for outdoor installation on the roof of the supermarket. In addition to saving space in the technical center, this design also allows the danger zone to be completely removed from the building. With dimensions of 3.86 x 2.20 x 2.43 m (L x W x H), the chiller also requires only a small amount of space on the roof. Maximum operational reliability is ensured by splitting the installed capacity into two redundant refrigeration circuits. Thanks to the compact design, the refrigerant charge is minimized to 3.5 kg per circuit. Each circuit has a two-compressor system with FC-controlled lead compressor, its own condenser coil with two EC high-efficiency fans and a plate heat exchanger with electronic expansion valve. An internal hydraulic module with two redundant speed-controlled charge pumps further increases machine availability. Each refrigeration circuit is equipped with a 53 kW plate heat exchanger for heat recovery. This can be used for desuperheating as well as for full condensation.
The SKADEC machine software is individually tailored. Temperature sensors in the brine buffer tank and in the flow and return of the machine record the current cooling requirements of the consumers and adjust the cooling capacity provided. The control system works independently of the refrigeration point controllers used as standard in this supermarket chain, so there are no critical interfaces. The PLC-based regulation and control software developed by SKADEC is designed for maximum operational reliability. All relevant process values are continuously monitored so that faults caused by irregularities in operation can be detected at an early stage. In this case, KRATSCHMAYER is automatically informed by e-mail and can intervene before the system breaks down. Should a malfunction nevertheless occur, the machine is put into emergency mode. In this mode, a standstill of the brine set is delayed as long as possible. By analyzing various process values, the system provides the maximum available cooling capacity so that the chilled product temperature can be maintained for as long as possible to prevent product damage.
For manual process analysis and reconstruction of the operating behavior, the SKADEC software has a data logger that records all relevant process values. In combination with the installed remote maintenance router, this enables convenient analysis of the system behavior and fine adjustment of the control parameters, especially during the commissioning phase. The intuitive operating concept via touch display also made it easy for the installers from the specialist company carrying out the work to record process data and adjust settings. The SKADEC heat pump with 214 kW heating and 187 kW cooling capacity, which is used for air conditioning the sales rooms, also impresses with maximum efficiency and availability. The machine is divided into two redundant cooling circuits, similar to the cold brine set. Each circuit contains an inverter-controlled compressor, a separate air heat exchanger with two EC fans and two separate electronic expansion valves for the cooling and heating modes. The water heat exchanger is used independently by both circuits at the same time. This design allows both circuits to be operated independently of each other in heating or cooling mode and offers redundancy.
Particularly with regard to defrosting in heating mode, each circuit can be defrosted at the optimum time in terms of energy consumption. This optimum defrosting time and the type of defrosting (circuit reversal/recirculation) is calculated by the SKADEC heat pump software, taking into account various process data (including pressure conditions in the system, outside temperature, etc.). The additional functions of emergency operation and early fault detection are also implemented in the heat pump. The refrigeration supply by SKADEC heat pump and chiller are innovative components within KRATSCHMAYER’s holistic supermarket concept and contribute to the sustainable operation and high quality level of the new building, as everyone involved agrees.
