Flow-Weighted Average (Mixed Input Streams)

Determines the mixed condition (temperature, conductivity, dissolved oxygen, or any property) when two streams combine, by weighting each input property by its flow rate. This is the core mass-balance calculation for blended streams such as boiler feedwater (condensate return plus makeup).

$$\text{Result}=\frac{Q_1\times C_1+Q_2\times C_2}{Q_1+Q_2}$$

Variables

Symbol	Meaning	Unit
$Q_1,Q_2$	Flow rate of stream 1 and stream 2	gpm
$C_1,C_2$	The property being calculated for each stream (temperature, conductivity, dissolved oxygen, etc.)	varies (°F, µS, mg/L)

Worked example

Boiler feedwater at 75% condensate return. Temperature: (75 x 170 + 25 x 70) / 100 = 145°F. Conductivity: (75 x 50 + 25 x 500) / 100 = 163 µS. Dissolved oxygen: (75 x 0.2 + 25 x 8.5) / 100 = about 2.3 mg/L.

---

Part of The Blueprint of Water