physics

Lee–Kesler method

Lee–Kesler method is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Lee–Kesler method rather than just read about it. In short: The Lee–Kesler method allows the estimation of the saturated vapor pressure at a given temperature for all components for which the critical pressure Pc, the critical temperature Tc, and the acentric factor ω are known. Equations ln ⁡ P r = f ( 0 ) + ω ⋅ f ( 1 ) {\displaystyle \ln P_{\rm {r}}=f^{(0)}+\omega \cdot f^{(1)}} f ( 0 ) = 5.92714 − 6.09648 T r − 1.28862 ⋅ ln ⁡ T r + 0.169347 ⋅ T r 6 {\displaystyle f^{(0)}=…

Key takeaways

Lee–Kesler method belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Lee–Kesler method to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lee–Kesler method from memory before moving on to harder problems.

Reference excerpt

The Lee–Kesler method

allows the estimation of the saturated vapor pressure at a given temperature for all components for which the critical pressure Pc, the critical temperature Tc, and the acentric factor ω are known.

Equations

ln ⁡ P r = f ( 0 ) + ω ⋅ f ( 1 ) {\displaystyle \ln P_{\rm {r}}=f^{(0)}+\omega \cdot f^{(1)}}

f ( 0 ) = 5.92714 − 6.09648 T r − 1.28862 ⋅ ln ⁡ T r + 0.169347 ⋅ T r 6 {\displaystyle f^{(0)}=5.92714-{\frac {6.09648}{T_{\rm {r}}}}-1.28862\cdot \ln T_{\rm {r}}+0.169347\cdot T_{\rm {r}}^{6}}

f ( 1 ) = 15.2518 − 15.6875 T r − 13.4721 ⋅ ln ⁡ T r + 0.43577 ⋅ T r 6 {\displaystyle f^{(1)}=15.2518-{\frac {15.6875}{T_{\rm {r}}}}-13.4721\cdot \ln T_{\rm {r}}+0.43577\cdot T_{\rm {r}}^{6}}

with

P r = P P c {\displaystyle P_{\rm {r}}={\frac {P}{P_{\rm {c}}}}} (reduced pressure) and T r = T T c {\displaystyle T_{\rm {r}}={\frac {T}{T_{\rm {c}}}}} (reduced temperature).

Typical errors The prediction error can be up to 10% for polar components and small pressures and the calculated pressure is typically too low. For pressures above 1 bar, that means, above the normal boiling point, the typical errors are below 2%.

Example calculation For benzene with

Tc = 562.12 K Pc = 4898 kPa Tboiling = 353.15 K ω = 0.2120 the following calculation for T = Tb results:

Tr = 353.15 / 562.12 = 0.628247 f(0) = −3.167428 f(1) = −3.429560 Pr = exp( f(0) + ω f(1) ) = 0.020354 P = Pr · Pc = 99.69 kPa The correct result would be P = 101.325 kPa, the normal (atmospheric) pressure. The deviation is −1.63 kPa or −1.61 %. It is important to use the same absolute units for T and Tc as well as for P and Pc. The unit system used (K or R for T) is irrelevant because of the usage of the reduced values Tr and Pr.

See also Vapour pressure of water Antoine equation Tetens equation Arden Buck equation Goff–Gratch equation

References

Worked examples

Example 1 — a first encounter with Lee–Kesler method

Start with the simplest possible case. Write down what Lee–Kesler method claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lee–Kesler method before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lee–Kesler method ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lee–Kesler method

In research: Lee–Kesler method appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lee–Kesler method in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lee–Kesler method is common in secondary-school and first-year university syllabi. It links to neighbouring topics Thermodynamic models, so understanding it makes those chapters shorter.
In everyday life: Look for Lee–Kesler method outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Lee–Kesler method in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lee–Kesler method means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Lee–Kesler method out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lee–Kesler method in simple terms?

The Lee–Kesler method allows the estimation of the saturated vapor pressure at a given temperature for all components for which the critical pressure Pc, the critical temperature Tc, and the acentric factor ω are known. Equations ln ⁡ P r = f ( 0 ) + ω ⋅ f ( 1 ) {\displaystyle \ln P_{\rm {r}}=f^{(0…

Why does Lee–Kesler method matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lee–Kesler method?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lee–Kesler method.