REFRIGERANTS Intelligent defrosting
Michael Freiherr, managing director (engineering) at Güntner, explains how to influence the energy efficiency of defrosting systems and which factors generally play an important role in this regard.
W
hen it comes to evaluating the efficiency of air coolers, defrosting plays a key role. Ice on the fins has a negative effect on the overall heat transfer rate and can cause a significant temperature rise in the cold room. This is why defrosting at regular intervals is indispensable, so that as little energy as possible is required, thereby maintaining the efficiency of the entire cooling system in the long run.
Q: Is it possible to calculate the energy efficiency of defrosting systems?
A: The energy efficiency of a defrosting system is the ratio of the sum of fusion enthalpy and sensible heat necessary to heat up ice to just above 0°C and the energy actually consumed by the system during the defrost phase. In real operation, well rated systems reach a defrosting efficiency of about 0.5, although the efficiency of many systems is far lower.
Q: Can the energy efficiency of defrosting systems be evaluated globally?
A: There is no single answer to this question as the efficiency of a defrosting system depends on many different individual parameters that cannot be compared with one another. These parameters, in turn, mainly depend on the actual application, the design of the plant as a whole and the installation on site.
Q: Which factors determine the energy efficiency of a defrosting system in practice?
Q: All in all, the defrost method determines the energy consumption of the defrosting system. This means it all depends on how defrosting is carried out – does it take place via recirculated air, water, hot gas or electrically? The fitter on the spot plays a major role as his or her fine tuning during commissioning largely determines the defrost behaviour. The positioning of the defrost sensor and the defrosting times and intervals require special attention. As a general rule, defrosting in line with demand is always more efficient than it is at fixed intervals. With defrost on demand, a sensor attached to the cooler detects the need for defrosting and activates the defrost phase accordingly. Defrost methods using the heat or waste heat that is present in the system reduce the energy consumption compared with methods consuming additional defrost energy. Warm brine/Thermobank and hot gas defrost systems have become established in this respect, with Bäckström
18 June 2019 defrosting as a specific variant.
If the cooler is also thermally insulated during the defrost phase, less heat energy needed for defrosting enters the cold room. This way, ‘twice as much’ energy is saved. On the one hand, less energy is needed for defrosting and, on the other, less energy is required to maintain the setpoint temperature in the cold room. Thermal separation is achieved, for instance, by a defrost sock or an inlet hood both impeding heat circulation into the cold room, or by an insulated unit cooler preventing it entirely.
Q: Are there any differences between plants operated with natural refrigerants and those operated with synthetic refrigerants?
A: In principle, systems using natural refrigerants need no special design. However, all safety aspects that also apply for
normal cooling must be considered. Hot gas defrost for NH3 pump systems have been established worldwide. One of the reasons for their widespread use is that the refrigerant condensate that forms in the cooler during the defrost process can be fed back to the pump return line or to the separator very easily. Ammonia also has a relatively high evaporation enthalpy compared to other refrigerants, allowing for comparatively short defrost times. Regardless of the refrigerant, systems with hot gas defrost have very specific requirements regarding design as the cooler is defrosted by feeding superheated refrigerant gas. This is also why the low pressure area of the plants must be designed for the operating pressure of the high pressure side. Due to the high discharge temperatures, it is also important to make sure when designing the plant that no vapour forms, as this would lead to an increasing ice formation in the cold room.
Another point in favour of hot gas defrost is that regardless of the specific refrigerant, the heat supply from the inside is very uniform. Hot gas defrost removes frost and ice very quickly from the fins, resulting in shorter defrosting cycles and in increased efficiencies. For CO2
plants, the focus is more on the pressure stage of the heat exchangers as these are usually not designed for high operating pressures. However, if CO2
hot gas is used for defrosting, the
air cooler and the hot gas tubing must have the same pressures as the CO2
gas cooler or condenser. This is why,
from my point of view, warm brine defrosting is a good alternative to electric defrost if the defrost efficiency is to be increased.
www.acr-news.com
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