Heat exchangers transfer [[Heat|thermal energy]] from one [[Fluid|fluid]] or fuel to another without [[Mixtures|mixing]]. Ideally, the heat lost by one fluid will be entirely gained by the other. Heat exchangers come in many forms, differing in [[Efficiency|efficiency]], however, for simplicity we will assume the aforementioned ideal condition. Boilers, condensers, and evaporators are all forms of heat exchangers which may or may not use another fluid to heat/cool the main fluid line.
![[heat_exchanger.png|center|500]]
Equation Summary:
| Conservation Item | Condition | Result |
| -------------------- | -------------------------- | ------------------------------------------------------------ |
| Heat | Transferred between fluids | |
| Work | Zero work | $\dot{W}=0$ |
| Enthalpy | Non-negligible | |
| Kinetic | Typically negligible | $\Delta K=0$ |
| Potential | Typically negligible | $\Delta P=0$ |
| Mass Conservation | Per fluid line | $\dot{m}_i=\dot{m}_e$ |
| Total Energy Balance | | $\dot{m}_1(h_2−h_1)=\dot{Q}_1=−\dot{Q}_2=\dot{m}_3(h_3−h_4)$ |