Investigation of Air-side Fouling of Split System Outdoor Heat Exchangers: Characterization, Performance Effects, and Frost Formation Interactions

Fouling is generally believed to have a negative impact on heat exchanger performance, and has been studied extensively. Split systems, such as those that serve typical residential air-conditioners, have an air-to-refrigerant coil located outdoors, where it is subject to many airborne fouling agents, such as grass clippings, leaves, and dust. This research project took novel approach to testing the effects of air-side fouling. Instead of applying standardized fouling agents in a laboratory test, we collected fouled outdoor coils from field-deployed air-conditioners, and tested them in a laboratory. This allowed tight control and measurement, but with real fouling. The effects on aerodynamic performance and heat transfer were assessed under two conditions: (1) while controlling airflow rate across the coil to be constant; (2) allowing the airflow rate to vary. The latter case mimics the performance of a typical split system that has a single-speed fan, in which case the fouling can reduce bulk airflow, as well as affecting the boundary layer on the heat transfer surfaces. Based upon the findings of the effects on field-fouled coils, as well as laboratory analysis of the fouling material, we developed a new fouling agent that simultaneously impacts airflow resistance and heat transfer in a way that matches the field-fouled coils. An additional question about fouling on an outdoor coil is whether it increases frost formation for heat pumps in heating mode. A set of experiments was conducted with the synthetic fouling agent to explore the interactions between fouling and frost. This paper will: show the impacts of fouling on performance of the field-fouled coils; describe the synthetic fouling agent; show results from testing new coils that were fouled with the agent; and show the results of a set of experiments on the interactions between fouling and frost formation. One surprising finding that will be discussed and explained is that air-side fouling often improves the heat transfer performance of a coil, even while bulk airflow is reduced. Although other experimenters have observed this phenomenon, it has been controversial, and led to the author’s article being widely distributed and discussed online