Abstract
High indoor humidity is a leading cause of degradation of the interior of buildings [1], [2] and promotes an unhealthy environment for the occupants [1], [3], [4]. Domestic refrigerative dehumidifiers are employed by 25% of New Zealand (NZ) homes to mitigate problems associated with high humidity [5].
Studies on the average temperatures in NZ homes have revealed a mismatch between the conditions at which imported dehumidifiers are performance tested and the temperatures at which they are operated. The imported systems are tested by the Association of Home Appliance Manufacturers (AHAM) in the range 18.3°C – 32.2°C [6]. NZ homes on the other hand have a mean evening temperature range of 10 – 23.8°C [7] with the average annual room temperature for some being as low as 13.4°C [8]. This results in dehumidifiers operating outside of their optimum performance envelope. Furthermore, refrigerative dehumidifiers are known to operate ineffectively at 10°C [6], which is far below the lower limit of the AHAM test range.
Evaporator gearing is a widely recognized method for improving dehumidifier performance [9]. To assess the impact of air-side gearing on domestic scale dehumidifiers, an existing empirical heat pump model, developed for industrial scale systems, was adapted to small scale systems. This required the integration of a domestic scale compressor sub-model which was generated as part of this work through calorimetric testing of a rotary compressor. The model predicted large gains in the moisture extraction rate with evaporator gearing. In conjunction with the model, a prototype domestic scale geared dehumidifier was designed, constructed and rigorously performance tested over the temperature range typical of the NZ home. Measured performance data and numerical simulation results are presented for the direct retrofitting of the ungeared dehumidifier with an air-side economiser. The results show that evaporator frosting plays a significant role in the under performance of geared systems operating at low-temperatures. Experimental results for the performance of the geared system, setup for tilted operation to improve economiser drainage, are also presented.
The results also show that gearing a domestic dehumidifier increases the ambient dry-bulb temperature (for a fixed relative humidity) at which the refrigerant evaporator enters the frosting region. Numerical work was carried out to establish opportunities for lifting the evaporating temperature in order to push the frosting limit to lower ambient temperatures. The thermal effectiveness of the economiser and the size of the refrigerant evaporator coil were identified as having a strong influence on the geared system evaporating temperature and hence the ambient temperature at which frosting onset occurs. Subsequently, experimental work was carried out to test these methods by increasing the number of refrigerant evaporator rows and deactivating economiser hot-side ducts. The results show that the geared system can be operated down to an ambient air temperature of 15.4°C without evaporator frosting.
Air-side economiser condensation was found to contribute significantly to the overall geared system dehumidification capacity at low-temperatures – which is not the case for warm-temperature operation. Details of the economiser condensation have not previously been reported in great detail experimentally nor has it been given considerable attention from a numerical modelling standpoint. A lack of agreement between experimental and numerical economiser condensation data, at some ambient conditions, was also shown. In response to this, a comprehensive, two-dimensional, air-to-air plate heat exchanger economiser model was developed which solves the steady-state mass, energy and momentum balance equations within the economiser ducts. This model has been integrated with an existing dehumidifier model and a preliminary assessment of the integrated dehumidifier and economiser model was made. Development of this model allows for a quantitative approach when engineering a domestic scale geared refrigerative dehumidifier equipped with an air-to-air plate heat exchanger economiser. Numerical results from this model are presented in this thesis.