The fact that the popular head loss coefficient concept, may become negative in branched junctions, is a symptom that something is not correctly managed. The paper makes a review of recent works which have sought new models based on physical concepts, as a way to avoid speaking about “negative losses”. Herwing and Schmandt [1], showed that the origin of the negative sign was a diffusive shear work exchange between the two streams of a branched junction. Traditionally, the head losses at the branched junctions are neglected, but definitely it cannot be done in HVAC air-duct networks.

Firstly, the paper illustrates how, by ignoring this “negative loss” contradiction, traditional duct network analysis may encounter unexpected numerical difficulties. Secondly, it shows that the Minimum Energy Dissipation Principle (MinEDP) can be successfully applied to analyze the steady-state of any flow network (not necessarily HVAC ductworks), with or without shear work at junctions. Moreover, the new method does not need to know the latter, beforehand, although the nature of the solution is very different in either case.

Finally, the paper includes a practical example of an HVAC ductwork to illustrate the outcomes. The new method works smoothly and quickly and does not need any ad hoc modification to cope with an eventual “negative” head loss.

流行的水头损失系数概念可能在分支连接处变为负值，这是某些未正确管理的症状。该论文回顾了最近的工作，这些工作寻求基于物理概念的新模型，以避免谈论“负损失”。Herwing 和 Schmandt [1] 表明负号的起源是分支连接的两个流之间的扩散剪切功交换。传统上，分支连接处的水头损失被忽略，但在 HVAC 风管网络中绝对不能这样做。

首先，本文说明了在忽略这种“负损失”矛盾的情况下，传统的风管网络分析可能会遇到意想不到的数值困难。其次，它表明最小能量耗散原理 (MinEDP) 可以成功地应用于分析任何流动网络（不一定是 HVAC 管道系统）的稳态，无论是否在交界处进行剪切功。此外，新方法不需要事先知道后者，尽管两种情况下解决方案的性质非常不同。

最后，本文包含一个 HVAC 管道系统的实际示例来说明结果。新方法运行平稳、快速，不需要任何临时修改来应对最终的“负”水头损失。