Analysis Solves Heat Exchange Problem

Krakow, Poland-based Dynalabs designs energy efficient heat exchangers and noise and vibration reduction devices. When its customer ATS Stahlschmidt & Maiworm Aluminium Wheels Foundry needed to reduce production costs, Dynalabs developed a heat exchanger for continuous recovery of waste heat for one of the foundry’s 1.6 MW furnaces. By using NEi Fusion – NEi Software’s 3D CAD modeller with NEi Nastran FEA – with Femap software, Dynalabs met the client’s expectations in less than 30 days and under budget.

The illustrations show translation results calculated using NEi Nastran static simulation. The load is from
results of previous steady state thermal simulation in NEi Nastran.

There were two significant issues to tackle: space restrictions and reduced energy costs. The diameter of the heat exchanger needed to be less than 1.2 m. However, the area of heat exchange needed to be large enough to let exhaust gases and a large volume of air flow through it. The heat exchanger operates 24 hr/day, seven days/week, and the temperature of the air leaving it is 400° C, so thermal load and stresses are quite large. Usually, this kind of “tube and ribs” heat exchanger has to be almost 15 m long to emit gas at 400° C. The final design ended up being only 4.5 m long.

The stress results were calculated using NEi Nastran static simulation. Load from results of a previous steady state thermal simulation in
NEi Nastran.

The amount of heat exchanged by successive stages of the unit were calculated, as were the temperature of the exhaust gases and air, heat transfer coefficients, and temperature of the walls separating the media.

This load was mapped from a previous thermal flow calculation. The load is in the form of a fluid temperature and convection coefficient distribution stored in a text file (x,y,z, alpha, T) and mapped using the Data Surface Editor function in Femap.

Using the results of the preliminary heat flow calculations, two simulations were performed: An NEi Nastran heat transfer simulation to determine the temperature gradient across the heat exchanger, and a structural simulation to determine the stresses and strains caused by thermal elongation.

Here is a heat exchanger finite element mesh and quad mesh of plate elements consisting of approximately 40,000 elements.

With the installation of the heat exchanger for continuous recovery of waste heat for one of the foundry’s furnaces, Aluminium Wheel Foundry benefits from the following factors: The design saves approximately 38% of gas consumed by the furnace; it reduces CO2 emissions by 800 tons/yr; and it increases the amount of melted aluminum by 200 kg/hr.

The temperature distribution was calculated using an NEi Nastran steady-state thermal simulation.