Heat Exchangers

Heat exchangers are devices where two moving fluid streams exchange heat without mixing.

The conservation of mass principle for a heat exchanger in steady operation requires that the sum of the inbound mass flow rates equal the sum of the outbound mass flow rates. This principle can also be expressed as follows: Under steady operation, the mass flow rate of each fluid stream flowing through a heat exchanger remains constant.

Typically:

• Adiabatic outside ($\dot{Q}\approx 0$)
• No work ($\dot{W}=0$)
• $\Delta \text{PE}\cong 0$ (effectively zero for practical purposes)
• $\Delta \text{KE}\approx 0$ (approximately equal to zero)

When the entire heat exchanger is selected as the control volume, $\dot{Q}$ becomes zero, since the boundary for this case lies just beneath the insulation and little or no heat crosses the boundary. If, however, only one of the fluids is selected as the control volume, then heat will cross this boundary as it flows from one fluid to the other, and $\dot{Q}$ will not be zero, it will be the rate of heat transfer between the two fluids.