Microchannel coils are becoming quite popular as an alternative to bulky fin/tube heat exchangers. For the desired capacity, a microchannel coils are smaller in size compared to a fin/tube heat exchangers, resulting in less refrigerant charge in the system.
The most frequent issue with which users are faced when considering microchannel coils for some applications, like heat pumps, is the issue of a shorter frosting time compared to fin/tube coils, and the further reduction in time, with each cycle due to residual water retained at the end of each defrost cycle.
Empirical testing has shown that frost cycle time for microchannel heat exchangers is about 25% less than a fin/tube coils. Moreover, the capacity degradation of fin/tube and microchannel coils at the end of the frosting period is not the same. In case of microchannel heat exchangers, the following methods of defrost control can be adopted:
The most common method of defrosting is by electric heater. Although simple to install and control, electric defrost may be expensive in terms of energy consumption. The electric heater is energized for the duration of each defrost cycle and this may lead to significant power consumption of the HVAC system.
Off-cycle defrost simply involves turning off the refrigeration system while letting the fans run continuously. Since the air must be warmer than 0˚C, off-cycle defrost is only useful on coolers for the storage of beverages and produce. The main advantage of off-cycle defrost is the simplicity and economy of installation.
Hot gas defrost
This is the most complex defrost system, based on the quality and effectiveness of the hot gas defrost, it is better than the other defrost methods. In hot gas defrost, the liquid refrigerant flowing through the evaporator is interrupted, and instead, a supply of gas directly from the discharge of the compressor is used. The hot discharged gas applies heat to the frost where it forms on the microchannel coil. The hot gas travels throughout the entire circuit, and will therefore defrost areas of the evaporator that may not be as effectively reached by electric resistance heaters. In some applications, especially for cold storage rooms, the flow of the hot gas is in the reverse direction compared to that of the cooling mode (reverse cycle hot gas defrost).
Continuous defrost involves sequential closing and opening of independent microchannel coil segments for off-cycle defrosting. When one coil segment needs to be defrosted, the control system shuts it off, while remaining segments allow the evaporator to function normally. Closed segment will be opened as soon as frost is removed from it by running fans, while the next segment will be closed for defrosting.
Despite the fact that microchannel heat exchangers have a shorter frost cycle in comparison to fin/tube coil designs, the common defrosting methods for microchannel coils are still the same as they were for fin/tube heat exchangers.