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Evaporator air-side fouling: effect on performance of room air conditioners and impact on indoor air quality.

Article Excerpt
INTRODUCTION

Heat exchangers are the main components of room air-conditioning systems. Thus, even fractional performance degradation due to fouling has the potential to cause further energy consumption. Furthermore, heat exchangers are directly in contact with the indoor airstream and have extended surfaces. They are typically a fin-and-tube configuration in order to increase the heat transfer rate between the refrigerant and the indoor air. However, a certain material that deposits on the condensate over the surface of these heat exchangers will react with other indoor airborne contaminants and produce mold compounds. These contaminants include airborne particulate matter and dusts. If some of those deposited materials are biological in nature, they will multiply and spread into the indoor air. Fouling of indoor air-side fin-and-tube heat exchangers, particularly air-conditioner evaporators of room units, leads to a reduction in airflow and to discomfort for occupants a few years after installation, particularly in dusty regions. The available literature concerning air-side heat exchanger fouling of room or unitary air conditioners or related subjects are summarized as follows. Krafthefer and Bonne (1987) investigated the cost-effectiveness of using high-efficiency air cleaners instead of more commonly used dust stop filters in heat pumps. Rossi and Braun (1996) evaluated the maximum cost savings associated with using optimal service scheduling for the cleaning of heat exchangers in packaged air-conditioning equipment. Bultman et al. (1993) simulated the effect of partially blocked condensers of vapor compression units and reported that the coefficient of performance (COP) was predicted to decrease by 7.6% when the airflow across the condenser is reduced by 40% for a constant-speed fan. Siegel (2002) and Siegel and Nazaroff (2003) conducted an experimental study to verify their model of predicting particle deposition on HVAC heat exchangers. Yang et al. (2004) investigated experimentally the impacts of air-side fouling on the performance of coil and filter combinations, which include filter pressure drop, coil air-side pressure drop, and coil air-side effective heat transfer coefficient. Breuker and Braun (1998) conducted an experiment on condenser fouling of a 3 ton rooftop unit by simulating the fouling in the test by blocking the condenser coil with strips of paper; the evaporator fouling was simulated by reducing the airflow. Pak et al. (2005) investigated experimentally the impacts of air-side fouling and cleaning on the performance of various condenser coils used in unitary air-conditioning systems.

Despite the potential importance and impact of air-side fouling of room air-conditioner unit evaporator coils on indoor air quality (IAQ) as well as the overload energy consumption during the cooling period, there has been relatively little research concerning this topic. Also, it was found from the available literature that experiments on fouling of HVAC heat exchangers and filters are done by using pure dust particulates. In practice, most of these dust particulates are usually trapped by the unit filter normally located in the air inlet side before reaching the indoor evaporator coil surfaces. In addition, most of the previous work focuses on the effect of the evaporator coil fouling on the air-side pressure drop. In the work of Siegel (2002), a picture taken using an optical microscope for real fouling material on the evaporator face was presented. The image showed a mixture of animal and human hair, textile fibers, and other spherical supermicron particles as some of the constituents of fouling material. Therefore, this study aims to investigate experimentally, by using real fouling materials collected from dirty evaporator coils, the impact of evaporator coil fouling on IAQ. Also, the fouling materials' effect on the measured performance of the room air-conditioner unit for the cases of a partially fouled and completely fouled evaporator coil (compared with a clean coil) are also investigated. This will be done to clarify the degradation of the unit's COP at the standard air face velocity of 1.53 m/s as well...

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