Abstract
This paper presents a new air-heating system concept for energy-efficient dwellings. It is a system designed to heat a low-energy building by coupling a heat-recovery ventilation system with a three-fluid heat exchanger located on the chimney of a wood-pellet stove. The proposed work focuses on the heat transfer that occurs between flue gases, the ventilation air and the combustion air within a triple concentric tube heat exchanger with no insulation at its outer surface. The main objective is to predict outlet temperature for the specific geometry of the heat exchanger studied here. Thus, the governing differential equations are derived for a counter-co-current flow arrangement of the three fluids. Then analytical solutions for the steady-state temperature distribution are obtained as well as the amount of heat transferred to the outside. An expression for the effectiveness of the heat exchanger is also proposed. Based on these results, calculations are performed on a case study to predict the fluid temperature distribution along the heat exchanger. Finally, a parametric study is carried out on this case study to assess the influence of the relevant parameters on the effectiveness of the heat exchanger. In addition, computation of heat losses to the outside justifies whether insulation is needed.
Highlights
Low energy buildings are characterized by a suitable orientation, a high thermal insulation and a very low air leakage [1]
First this study presents various types of heat recovery used in buildings namely fixed-plate, heat pipe, rotary wheel and run-around units
The system operates under steady-state conditions; Phase change does not take place; Bulk mean temperatures only depend on axial (x) direction; The thermo-physical properties of the fluid streams are constant and uniform; Thermal conduction is not assumed in fluids or walls parallel to the fluid flow direction; Heat transfer coefficients are independent of time and axial position; Temperature of the outside remains constant
Summary
CERIC Laboratory (Centre d’Essais et de Recherche des Industries de la Cheminée), Poujoulat. Received: 9 November 2012; in revised form: 24 December 2012 / Accepted: 5 January 2013 /
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