The use of roped tubes for the enhancement of the secondary heat exchanger of a domestic condensing boiler
thesisposted on 25.10.2010, 11:28 by William J. Fidler
This investigation was carried out in order to determine whether roped (or spirally indented) tubes could be used to improve the amount of heat transferred by the secondary heat exchanger in a domestic natural gas fired condensing boiler. Experiments were carried out using purpose built test apparatus and three types of heat exchanger tubes (plain, roped and finned tubes respectively) that were used to make up four distinct heat exchangers. Two of the experimental heat exchangers were comprised solely of plain and roped tubes while two of the heat exchangers were hybrids, made up of a combination of finned and plain tubes and finned and roped tubes respectively. The results of these experiments showed that the heat exchanger made up solely of roped or spirally indented tubes transferred less heat than an equivalent plain tube heat exchanger. When the roped (or spirally indented) tubes were combined with finned tubes to form a hybrid heat exchanger, it was determined that more heat was transferred than by a similar hybrid heat exchanger that consisted of plain and finned tubes. The experiments also showed that increasing the velocity of the natural gas combustion products as they flowed through the secondary heat exchanger resulted in an increase in the amount of heat transferred by the four heat exchangers. Additionally, information is presented about the effect that air, excess to that required for stoichiometric combustion, had on heat transfer and the condensing regime present on the plain, roped and finned tubes in a secondary heat exchanger in a condensing boiler. In addition to the experimental work, theoretical modelling was also performed using a modified CFD model to calculate the performance of the four heat exchangers. At high fuel gas velocity the theoretical results produced by the modified CFD model for both the plain and roped tube heat exchangers were found to be good, while the low flue gas velocity results were found to be less accurate. The theoretical results for the two hybrid heat exchangers were found to considerably underpredict the experimental results in all cases.
- Mechanical, Electrical and Manufacturing Engineering