This is one of the property of a pure fluid or a mixture which is never available to you when you need it most specially when you need it urgently. That time you are never able to recall the source where did you see it last time.

So don't worry now. There are few easy methods available which can give you quick & accurate estimate of this be it for pure fluid or for a liquid mixture.

**The first method I am discussing is Riedel's Correlation.**

This method is limited to calculating latent heat at normal boiling point only. However, this is a property which can be used for the derivation of other properties also. We will see it in future & coming posts.

Hvb = 1.093 R Tc [ Trb x {Ln(Pc)-1}/{0.930 - Trb}]

Where

Hvb = Latent heat at normal boiling point. (Remember this is an important definition)

Trb = Tb / Tc Reduced Boiling Point Ratio in Kelvin

Pc = Critical Pressure atm

Hvb Unit is Lit-Atm/Gm-mole

R = Gas Constant in Lit-Atm/Gm-mole/K

So change R value in different unit & you will get Hvb in desired unit accordingly, because the value in parantheses is unitless.

Also note that 1 Lit-Atm/Gm-mole is equal to 24.12 Cal/gm-mole

**The second method I am discussing is Pitzer's Correlation.**

This method is applicable for a wide range from normal boiling point to critical point.

The equation given is as below.

(Hv / R Tc) = 7.08 ( 1 - Tr)^0.354 + 10.95 * omega ( 1 - Tr)^0.456

Hv = Latent heat at t °C.

Tr = (t+273.15)/Tc

omega is accentricity factor - a std property

**The third method I am discussing is Watson's Correlation.**

This method is most useful for the fluid where you know the latent heat at a given temperature & want to calculate it at another temperature, or using some simulation where accurate estimate is required so instead of using basic equations you can use this escalation equation.

Hv2 = Hv1 [ (1 - Tr2) / (1 - Tr1)]^0.378

Hv1 = latent heat at T1

Hv2 = latent heat at T2

Units are same as described in first method.

__List of other property estimation methods on this Blog.__- Calculate Diffusion Coefficient in Gases
- Calculate Diffusion Coefficient in Liquids
- Heat Capacities of dissolved solids & organic solutions - Quickest methods
- Heat capacities with dissolved solids
- Kinematic viscosity of air Vs Temperature
- Thermal Conductivity of air Vs Temperature
- How to calculate viscosity of liquid mixtures
- Surface Tension
- Viscosities of Pure Gases at Low Pressure - 1
- Viscosities of Pure Gases at Low Pressure - 2
- Viscosities of Pure Gases at High Pressure
- Viscosity of Gaseous Mixture
- Steam Properties - I
- Vapor Pressure Vs Temperature