One of the most frequent causes of operational problems with both freezer and normal chilling temperature refrigeration systems is ice build up on air coolers and evaporators. Ice formation on the evaporator's fins for example is detrimental to the heat transfer and results in a temperature increase in the cold room. To minimise the energy consumption as far as possible, an effective and efficient defrosting system is required. This will help to keep the whole refrigerating system running efficiently in the long term. eurammon member Michael Freiherr, Chief Technical Officer at Güntner GmbH & Co. KG, talks about various influencing factors on the energy efficiency of defrosting systems and looks at the special requirements when using natural refrigerants.
How can the energy efficiency of a defrosting system be calculated?
Michael Freiherr: The energy efficiency of a defrosting system is the sum of the latent and sensible heat necessary to convert the ice build up on the evaporators to water just above 0°C, divided by the energy actually consumed by the system during defrosting. Well rated systems reach a defrosting efficiency of approx. 0.5 in real operation, however the efficiency of many systems is far lower.
What aspects influence the energy efficiency of a defrosting system in practice?
Michael Freiherr: The energy demand of the defrosting system depends primarily on the defrosting method, for example, air defrost, water, electric or hot gas. The efficiency also depends on equipment layout and fine tuning during commissioning by the engineer on site. Important aspects here include good positioning of the defrost sensor as well as a correct calculation of defrosting times and intervals. Defrost-on-demand is particularly efficient, as this is only activated when a sensor fitted on the evaporator or cooler detects the corresponding demand.
Another possibility of minimising defrost energy consumption is to use the waste heat already present in the system rather than consuming additional defrosting energy. Possibilities include hot glycol/brine/Thermobank, hot gas defrosting and Bäckström defrosting method in particular.
Energy can also be saved by keeping as much of the heat inside the evaporator/cooler during the defrost process. If less heat escapes into the cold room, this reduces the refrigerating capacity necessary to maintain the room set-point temperature after a defrost. Smart solutions include the use of a hood on the backside of the cooler and socks or dampers on the air outlet side impeding hot air heat circulation, or an insulated cooler preventing it entirely.
Which special aspects apply to defrosting methods in NH3 systems?
Michael Freiherr: Basically, every established defrosting method can be used in systems with natural refrigerants, taking account of all safety aspects that apply to normal chilling operation. Hot-gas defrosting is widely used for NH3 pump systems on a very broad global scale. One of the reasons is that in NH3 systems, it is particularly easy for the refrigerant condensate generated in the evaporator during defrosting to flow back into the wet return pipe or to the separator. Ammonia also has a relatively high evaporation enthalpy compared to other refrigerants which makes it possible to achieve shorter defrost times than with other refrigerants.
One characteristic of NH3 systems are the relatively high discharge temperatures. If this is not taken into consideration during the system design, defrosting could cause unwanted vapour formation and result in ice formation in the cold room. Another advantage of hot gas defrosting , regardless of the specific refrigerant used is the very uniform heat supply from the inside. This removes frost and ice very quickly with shorter defrosting cycles and as such enhanced efficiency.
Which special aspects apply to defrosting methods in CO2 refrigerating systems?
Michael Freiherr: If CO2 is used for hot gas defrosting, it would require the air cooler and the hot gas piping to be rated to the same pressures as the CO2 gas cooler / condenser. However, CO2 air coolers are usually designed for far lower pressure levels. In terms of efficiency, hot brine defrosting would therefore appear to be a better alternative when compared to electric defrosting in CO2 cascade systems.
How can energy efficiency be evaluated in general for defrosting systems with natural refrigerants?
Michael Freiherr: It is not possible to generalise here. The efficiency of the defrosting system depends on many different individual parameters so that it is relatively difficult to make comparisons. However, when deciding which refrigerant to use, it is far more important to note that systems with natural refrigerants fundamentally offer operators a high degree of future viability. Ammonia and CO2 are not affected by current and future restrictions imposed by the F-Gases Regulation or other environmental requirements and are therefore suitable for long-term planning.