UNSTEADY CONDUCTION AND FREE CONVECTION FOR THE DESIGN OF A STEAM PIPING

Carlos Alberto de Melo¹

¹ Universidade Federal de Uberlândia

Resumo

As steam flows through the distribution piping additional steam condensation is continuously taking place. This condensate has to be removed immediately to do not cause severe waterhamering conditions in the pipeline damaging all equipments installed. An unsteady state conduction model was developed to calculate the mass flow rate of condensate formed, due to heat dissipation to surroundings and due to heat stored in pipe wall and in thermal insulation. This load of condensate is used for the design of recovery condensate system, which contains the steam trap. The transient outer heat transfer coefficient between the insulation outer surface and surrounds was evaluated, iteratively at each instant, by free convection. Unsteady conduction equation, with boundaries and initial conditions was solved using the control volume difference finite formulation, presented by Spalding and by Patankar. The full implicit method was used to formulate the time term. The discretization equations satisfy the convergence Scarborough criteria. The load of condensate was analyzed in function of steel and insulation Stefan numbers. Considering the coupling between the pressure loss of the condensate piping system (system curve) and the pressure loss obtained from the characteristic curve of the steam trap, a governing global equation appears. The mass flow rate of condensate, that can flows in the recovery condensate system, is obtained by the solution of the global equation. This solution, that is called the effective condensate, must be greater than or equal to the load of condensate estimated in the unsteady conduction model. In the iterative process, the Lagrange or the spine cubical interpolating function is used to obtain the intermediate points of the characteristic curve and the secant method is used to calculate the root of the global equation.

Palavras-chave: steam piping; unsteady conduction; free convection and condensate recovery system