Problems with the Static Regain method

Abstract

Many attempts have been made to devise a simple yet accurate algorithm for duct design. The Static Regain Method, which is one attempt, has gained wide popularity due to its simplicity and its seemingly sound theoretical basis. However, a paper by authors Tsal and Behls challenged many long held ideas concerning this method. In this paper some of their arguments are critically reviewed. The Static Regain method equates the static pressure increase across a junction with the pressure drop in the succeeding section. It uses the Bernoulli equation to compute the static pressure regain across a junction. However, Tsal and Behls argued that the Bernoulli equation may not be used for this purpose since it is only valid for streamline flow. Based on the energy equation, Tsal and Behls derived an expression for the static pressure regain across a junction. In this paper it is proposed that the derivation was over-simplified since the energy equation was applied between two junction nodes. Instead it should have been applied to an appropriate control volume. One of the popular applications of the Static Regain method is to create equal static pressures at successive junctions along a main duct. It is claimed that this will result in equal branch flows for identical branches. To disprove this claim Tsal and Behls considered the change in total pressure in a system and showed that irrespective of the static pressures equal flows cannot be attained since total pressure balancing is violated. Once again it is suggested in this paper that their proof is based on an over-simplified view of the problem. It is shown that under certain circumstances it is indeed possible to attain equal flows in identical taps although equal static pressures do not necessarily have anything to do with it. Based on the balancing requirement, a method is proposed purely to demonstrate that when equal static pressures are created at take-off points; it does not follow that equal flows in identical branches will be the result. A realistic duct design problem was solved by using both this method and the Static Regain method. In general only the proposed method succeeded in designing a system that ensures exact total pressure balancing at the design airflows. A duct section is considered here to be a length of duct which has no change of cross sectional area, shape, flow or friction coefficient.