I Enthalpy change in a flow process

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In steady-state flow processes through pipes, the general energy balance equates heat evolved to the change in enthalpy. The discussion questions the validity of substituting the change in enthalpy with heat capacity times the change in temperature, especially when pressure drops. It is noted that this substitution is typically valid only for closed systems at constant pressure or when enthalpy is solely a function of temperature. The approximation may hold if the specific heat is relatively constant, but its applicability depends on whether the fluid is a liquid or gas. Overall, the validity of the dH = c*dT substitution in pipe flow remains uncertain due to the pressure drop.
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In a flow process, why do we substitute dH = c*dT if pressure is dropping during the process
I am looking at simple steady-state flow processes, say for flow through a pipe. The general energy balance for pipe flow reduces to simply heat evolved = change in enthalpy between the two states (or heat rate = mass flow rate * change in specific enthalpy). However, for a flow process, why is it justified to replace change in enthalpy with heat capacity * change in temperature. I understand that for closed systems, one can only do so for processes taking place at constant pressure (or for cases in which the enthalpy is a function only of temperature.) In the case of pipe flow, pressure obviously drops - do we simply approximate the enthalpy of the fluid to only depend on temperature? Otherwise, the dH = c*dT substitution does not seem valid to me...
 
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ethonodon said:
TL;DR Summary: In a flow process, why do we substitute dH = c*dT if pressure is dropping during the process

I am looking at simple steady-state flow processes, say for flow through a pipe. The general energy balance for pipe flow reduces to simply heat evolved = change in enthalpy between the two states (or heat rate = mass flow rate * change in specific enthalpy). However, for a flow process, why is it justified to replace change in enthalpy with heat capacity * change in temperature. I understand that for closed systems, one can only do so for processes taking place at constant pressure (or for cases in which the enthalpy is a function only of temperature.) In the case of pipe flow, pressure obviously drops - do we simply approximate the enthalpy of the fluid to only depend on temperature? Otherwise, the dH = c*dT substitution does not seem valid to me...

In general, dH = c*dT is not valid. If the specific heat c is constant enough, it may be an acceptable approximation, depending on the nature of the calculation. Or it may not be.
 
Are you talking about a liquid or a gas?
 
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