Enthalpy is a thermodynamic concept that can be hard to get a grip on. Perhaps, the reason is that it appears in several contexts so that understanding of it becomes confusing.
In this post the usage of enthalpy in the context of steam is presented. This is an interesting case since steam services are present in almost every chemical process industry and different daily applications, such as the heating of buildings, for example.
On the other hand, the interpretation of enthalpy, among the professionals of the steam, is as thermal energy (as simple as that). Why? Because, engineers and technician need something easy to use and measurable rather than something that makes their work harder.
Some parameters used in steam tables
Data in steam table are usually order either by pressure or by temperature. Also there are different two types of steam: saturated and superheated. For simplicity, we will only refer to saturated steam tables.
For reference, consult the ASME Steam tables. Compact edition book.
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| Fig. 01 Sample steam tables by temperature and pressure. These tables were taken from the ASME Steam Tables. Compact edition book. |
The steam tables present data for volume, enthalpy and pressure. In this post, we will focus on volume and enthalpy only.
Volume of liquid and vapor
These parameters, appearing in the columns 3 and 4 in tables of Fig. 01, partly indicate how much liquid water is present in the steam. Unless the steam is completley dry the steam will be a two phase fluid made of: vapor and liquid (in the form of small droplets). As you can see from the data in tables of Fig. 01, the saturated steam is not dry at all.
This volume is presented as [volume of liquid or vapor]/[unit mass of steam]. In other words, and for the present case, this is the $ft^3$ of liquid or vapor per $lb_m$ of steam. Also, the summation $V_L+V_V$ is also called the specific volume.
The enthalpies
In Fig. 01 only two enthalpies ares shown: $h_L$ and $h_V$. However, a third enthalpy parameter usually appear in this kind of tables: $h_{evap}$. Theses parameters can be referred as,
- $h_L$ the enthalpy of the liquid phase,
- $h_V$ the enthalpy of the vapor phase and
- $h_{evap}$ the enthalpy of evaporation,
all in the same units: $Btu/lb_m$ or $kJ/kg$ (for SI units). In fact,
For illustration purposes, let us consider the first row, for steam at $32\,F$, in Table 1 of Fig. 01. Thus, $h_L$ is interpreted as the energy required to increase the temperature of $1\,lb_m$ of water (or $1\,kg$ of water, in SI) from its freezing point to its boiling temperature $32\,F$ for the present case. Notice, that $h_L$ will only make sense if the water remains liquid.
For the same row in Table 1 in Fig. 01, $h_{evap}$ is interpreted as the energy required for the change of phase of $1\,lb_m$ (or $1\,kg$ in SI) of liquid water at its boling temperature into vapor. Notice, that either liquid or vapor are at $32\,F$, the boiling temperature. This amount of energy referred by the steam professionals as the one that can be used for heating purposes since the steam can very quickly transfer it.
Finally, $h_V$ is interpreted as the thermal energy carried by the vapor. This is the energy of the liquid at its boiling temperature plus an added energy so that the fluid can be in vapor phase.
One parameter I have no mentiones is the pressure. All above comments are for constant pressure. If you change the pressure, the values of temperature, volume and enthalpies will also change but the interpretation will be the same.
Other stuff of interest
- Understanding the law of Boyle
- Measuring atmospheric pressure
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
- Composition variables for mixtures
Ildebrando.
