KEVOS® MAKEOS · Engineering
Wet Steam Dryness by Throttling Calorimeter (Dryness Fraction, Calorimeter Validity)
From a throttling calorimeter reading, what is the dryness fraction of the wet steam in the line, is the reading valid (steam superheated after throttling), and is the steam as dry as the process needs?
Dryness fraction of wet steam in a line from a throttling calorimeter reading: the throttled sample keeps its enthalpy, so the enthalpy found from the calorimeter’s pressure and temperature gives the line steam’s dryness, with a check that the sample is superheated enough for the method to be valid.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Line steam (saturation values at line pressure) | ||
| Line pressure (absolute) | kPa | Absolute pressure of the steam line at the sampling point (gauge reading plus atmospheric pressure). |
| Saturated liquid enthalpy at line pressure | kJ/kg | Saturated liquid enthalpy at the line pressure, from the saturated steam table. |
| Enthalpy of vaporisation at line pressure | kJ/kg | Enthalpy of vaporisation at the line pressure, from the saturated steam table. |
| Calorimeter reading | ||
| Calorimeter pressure (absolute) | kPa | Absolute pressure inside the calorimeter after throttling, usually close to atmospheric. |
| Steam temperature in the calorimeter | °C | Steam temperature measured inside the calorimeter. |
| Saturation temperature at calorimeter pressure | °C | Saturation temperature at the calorimeter pressure, from the steam table. |
| Superheated steam enthalpy at calorimeter pressure and temperature | kJ/kg | Enthalpy of superheated steam at the calorimeter pressure and temperature, from the superheated table (interpolated) or software. |
| Limits | ||
| Least superheat for a valid reading | K | Least superheat in the calorimeter you accept as proof the sample is fully evaporated; your test procedure. |
| Minimum dryness fraction required | Lowest dryness fraction the process or equipment needs, from its specification. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Line steam (saturation values at line pressure)
What this means: Line pressure (absolute)
- What it is
- The absolute pressure of the steam in the line at the sampling point, at which the saturation values below are read.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 110 kPa to 22000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only checked against the calorimeter pressure; the saturation values you enter carry its effect.
- Get it right
- It must be above the calorimeter pressure; add atmospheric pressure to a gauge reading.
What this means: Saturated liquid enthalpy at line pressure
- What it is
- The enthalpy of saturated liquid water at the line pressure, the starting point for the dryness calculation.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 0 kJ/kg to 2100 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 762.51 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher value lowers the calculated dryness.
- Get it right
- Read it at the line pressure, not at the calorimeter pressure.
What this means: Enthalpy of vaporisation at line pressure
- What it is
- The enthalpy of vaporisation at the line pressure, which divides the enthalpy above saturated liquid to give the dryness fraction.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 1 kJ/kg to 2600 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2014.6 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger value lowers the calculated dryness for the same reading.
- Get it right
- It falls as pressure rises; use the value at the line pressure.
Calorimeter reading
What this means: Calorimeter pressure (absolute)
- What it is
- The absolute pressure inside the calorimeter after the sample has been throttled, usually close to atmospheric.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 1 kPa to 1000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only checked against the line pressure; it sets which saturation temperature and enthalpy you look up.
- Get it right
- Read it on the calorimeter gauge or take atmospheric pressure if it vents freely.
What this means: Steam temperature in the calorimeter
- What it is
- The temperature of the throttled steam inside the calorimeter, which with its pressure fixes the steam’s enthalpy when it is superheated.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 400 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher temperature raises the superheat and so the confidence that the reading is valid.
- Get it right
- Use a calibrated thermometer in a well in the steam flow, and wait for a steady reading.
What this means: Saturation temperature at calorimeter pressure
- What it is
- The saturation (boiling) temperature of water at the calorimeter pressure, which the calorimeter temperature must exceed for the method to work.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 200 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 99.61 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher value reduces the superheat and can make the reading invalid.
- Get it right
- About 100 °C at atmospheric pressure; read it from the steam table at the actual calorimeter pressure.
What this means: Superheated steam enthalpy at calorimeter press…
- What it is
- The enthalpy of the superheated steam in the calorimeter at its pressure and temperature, which equals the enthalpy of the wet steam in the line because throttling keeps enthalpy constant.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 2400 kJ/kg to 3300 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2776.6 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher value raises the calculated dryness.
- Get it right
- Interpolate in the superheated table at the calorimeter pressure, or use steam software.
Limits
What this means: Least superheat for a valid reading
- What it is
- The least amount by which the calorimeter temperature must exceed its saturation temperature before you accept the steam is fully evaporated and the reading valid.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0 K to 50 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the line between a valid and an invalid reading.
- Get it right
- A few kelvin guards against thermometer error near saturation.
What this means: Minimum dryness fraction required
- What it is
- The lowest dryness fraction the process or equipment needs, as a fraction: 0.97 means at most 3 % of the steam mass is water.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.97 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line of the dryness check.
- Get it right
- Sterilisers and turbines often specify a dryness; take it from their standard or maker.
Method and formulas
- Throttling through a valve or orifice with no heat or work transfer keeps the enthalpy constant: h₁ = h₂.
- With the calorimeter steam superheated, h₂ follows from its pressure and temperature; the line steam’s dryness is x = (h₂ − h_f1) ÷ h_fg1 at the line pressure.
- The reading is valid only if the calorimeter temperature is above the saturation temperature at its pressure by at least the superheat you set.
- Example: steam at 1 MPa (h_f 762.51, h_fg 2,014.6 kJ/kg) throttled to 100 kPa reads 150 °C (h 2,776.6 kJ/kg, 50.39 K of superheat): dryness 0.9997, above 0.97.
Limitations
- Steam wetter than about 0.95 at moderate pressures does not superheat on throttling to atmospheric pressure, so the throttling calorimeter cannot measure it; a separating calorimeter is used ahead of it.
- Heat lost from the sampling line and calorimeter makes steam look wetter than it is; insulate both and sample from a proper probe.
- One sample at one moment; carryover and wetness vary with boiler load.
- Example values are steam-table values chosen as an illustration. Replace them with your own readings and table values.
Typical uses
- Check the dryness of steam supplied to a process, steriliser or turbine
- Investigate wet steam from boiler carryover, poor trapping or missing separators
- Teach constant-enthalpy throttling with a practical measurement
Who it is for
- Steam plant and boiler engineers
- Maintenance technicians
- Process engineers in food, pharmaceutical and hospital plants
What you get
- Dryness fraction and moisture content of the line steam
- Superheat in the calorimeter and whether the reading is valid
- Dryness against the requirement
Interpreting the result
- A dryness of 0.97 means 3 % of the mass is water droplets, which carry no latent heat and erode valves and turbine blades.
- If the reading is not valid, the steam is wetter than the method can measure: that is itself a finding.
Before relying on the result
- Repeat the test at the boiler’s normal and peak loads, and check the thermometer and gauge calibration.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 3 (section 3-5: saturated liquid–vapour mixtures and quality) and chapter 5 (section 5-4: throttling valves). Method reference; no tables, figures or examples are reproduced.
- IAPWS-IF97 for steam properties.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · KEVOS® original book
Reading the steam table
In Process Steam by the Numbers
What it covers: Understand saturated steam through its table: absolute pressure and boiling point, sensible and latent heat, specific volume, why steam is distributed high and used low, and wet steam.
Learning pathway
Measure how wet your steam is
Learn what dryness fraction means and how a throttling calorimeter measures it, then work out the dryness from your own reading.
1 Learn
Read Reading the steam table in Process Steam by the Numbers.
Understand saturated steam through its table: absolute pressure and boiling point, sensible and latent heat, specific volume, why steam is distributed high and used low, and wet steam.
2 Apply
Open Wet Steam Dryness by Throttling Calorimeter (Dryness Fraction, Calorimeter Validity) and enter your own figures.
From a throttling calorimeter reading, what is the dryness fraction of the wet steam in the line, is the reading valid (steam superheated after throttling), and is the steam as dry as the process needs?
3 Verify
Read the tool’s feasibility and verification checks under the result, and test your answer against its stated limitations:
- Steam wetter than about 0.95 at moderate pressures does not superheat on throttling to atmospheric pressure, so the throttling calorimeter cannot measure it; a separating calorimeter is used ahead of it.
- Heat lost from the sampling line and calorimeter makes steam look wetter than it is; insulate both and sample from a proper probe.
- One sample at one moment; carryover and wetness vary with boiler load.
- Example values are steam-table values chosen as an illustration. Replace them with your own readings and table values.
4 Improve
Download the decision report or copy the link with your inputs to keep a record, then change one assumption at a time and compare.
Next reading: Pressure reduction and distribution in Process Steam by the Numbers.
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