When we are first introduced to equations, the equation format we are taught is

**closed equation format**, that is y = f(x).

For example,

| y = mx + c | | equation of a straight line |

| Q = UALmtd | | design equation of heat exchanger |

To write an equation in

**open equation format**, the closed format equation is rearranged with all the terms on one side. The equation is then evaluated to find the residual. For example;

| **Closed Equation Format** | | **Open Equation Format** |

| y = mx + c | | R = y - mx - c |

| Q = UALmtd | | R = Q - UALmtd |

So why would we want to work with open equations?

- For linear equations, any term in the equation, (not just the left-hand term), can be found without the need rearrange the equation or solve it via iteration
- If the model is over-specified the use of open equations provides a method to reconcile the data
- Inter-related equations can be linked by simple connection equations, rather than by eliminating variables and equations