# Enthalpy change in a flow process

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• ethonodon
ethonodon
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...

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?

## What is enthalpy change in a flow process?

Enthalpy change in a flow process refers to the change in enthalpy (a measure of total energy) of a fluid as it flows through a system. It accounts for the energy added or removed from the fluid due to work, heat transfer, and mass flow. This concept is crucial in thermodynamics, especially in the analysis of turbines, compressors, and heat exchangers.

## How is enthalpy change in a flow process calculated?

The enthalpy change in a flow process is calculated using the first law of thermodynamics for open systems, often expressed as ΔH = H_out - H_in, where H_out is the enthalpy of the fluid leaving the system and H_in is the enthalpy of the fluid entering the system. The specific enthalpy values are typically obtained from thermodynamic tables or calculated using equations of state.

## What factors affect the enthalpy change in a flow process?

Several factors affect the enthalpy change in a flow process, including the temperature and pressure of the fluid, the type of fluid (e.g., gas or liquid), the specific heat capacity of the fluid, and any phase changes that occur during the process. Additionally, heat transfer to or from the surroundings and work done by or on the fluid can significantly influence the enthalpy change.

## Why is enthalpy change important in engineering applications?

Enthalpy change is crucial in engineering applications because it helps in the design and analysis of energy systems such as power plants, refrigeration systems, and HVAC systems. Understanding enthalpy change allows engineers to optimize processes for energy efficiency, predict system performance, and ensure safe operation by managing thermal and mechanical stresses.

## How does enthalpy change relate to the conservation of energy in a flow process?

Enthalpy change is directly related to the conservation of energy principle in a flow process. According to the first law of thermodynamics, the total energy entering a control volume must equal the total energy leaving it, plus any changes in internal energy within the control volume. Enthalpy change accounts for the flow work and internal energy changes, ensuring that energy is conserved throughout the process.

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