Study of a triple concentric-tube heat exchanger integrated into a ventilation and heating system of dwellings

1. Introduction

Low energy buildings are commonly characterized by a suitable orientation, an important thermal insulation and a high level of air tightness. As a consequence, traditional heating systems are no longer required nor even suited to these constructions. Moreover, the additional heating demand can be provided by an efficient ventilation system only.

In this context, a new concept of air heating system has been developed to heat a whole low energy building by coupling a heat recovery ventilation system and a triple concentric-tube heat exchanger placed on the chimney of a room sealed wood pellet stove.

2. Methods

The heat exchanger comprises of three concentric-tubes. The flue gases are evacuated through the inner tube. The ventilation air flow to be heated is carried by the intermediate tube. The combustion air which ensures the correct operation of the wood pellet stove is supplied by the outer tube. Thus, flue gases and ventilation air are in a counter-flow arrangement, while ventilation air and combustion air are in a parallel-flow arrangement.

A mathematical model is developed for steady-state conditions and governing equations are expressed in dimensionless form. Three third order ordinary differential equations are obtained and solutions (outlet temperatures and efficiency) are expressed in the form of exponential functions. The inlet temperatures of the three fluids and the external temperature provide the required boundary conditions to solve the set of equations analytically. An expression for the amount of heat transferred across the non-adiabatic outside surface is also presented.

For validation, experimental measurements are conducted in the CERIC laboratory to evaluate mass flow rates and temperatures of the three fluids at the entrances and exits of a prototype built by the manufacturer Poujoulat.

3. Results

A comparison of outlet temperatures and efficiency obtained using the mathematical model with the experimental data validates the analytical solutions. A parametric study is also conducted to estimate the influence of mass flow rate variations and determine the optimum design for the heat exchanger.

4. Conclusions

Analytical solutions for the steady-state temperature of three heat exchanging fluids along the length of a triple concentric-tube heat exchanger have been obtained with non-adiabatic condition at the outer surface. The heat transferred to the outside is also estimated. The heat balance analysis and the comparisons with experimental measurements allow model validation. The equations derived in this study can be used for both performance and design calculations.

12.05.2010 / 09.00-10.30