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# Enthalpy value calculation

## Enthalpy value calculation

(OP)
how can i calculate enthalpy value tank inside. If i know vapor pressure of liquid tank inside. e.d:

tank volume : 18m3
Vapor pressure at 20ºC =9 bar
Pressure tank= vapor pressure

### RE: Enthalpy value calculation

You don't tell us what's the fluid. So I'm guessing, if it is R22, a refrigerant, the enthalpy of the liquid is 53.55 kcal/kg, and that of the vapor, 98.39 kcal/kg.
R22's saturation v.p. at 20oC is 9.1 bar abs. This is no estimate, data were taken from published tables.

### RE: Enthalpy value calculation

(OP)
Is a furane type , isn´t it.
and the NPSHa = 0
This mean That :
absolute pressure = vapor pressure.
What is the pressure at the pump inlet ?

Pressure inlet= absolute pressure-Vapor pressure+ static head- pipe losses .
If NPsha =0
Pressure inlet= vapor pressure???
Could you help me ?

### RE: Enthalpy value calculation

The equation you presented is not the pressure of the system at the suction of the pump. You can have 100 atm pressure and still the NPSH=0.

Since the NPSH expressed in m or ft of liquid, measured at the pump suction, is

NPSH=(P-Pv)/sp.gr. + u2/2g

a value of zero would mean that

P=Pv-(u2xsp.gr.)/2g

When the second term is small in comparison with Pv, as it generally is, one may say that when NPSH=0, P=Pv.

where

P is the absolute suction pressure
Pv is the vapor pressure of the liquid at the pumped temperature
u is the velocity of the liquid at the section where measurements were taken
g is the acceleration of gravity.

Hasta pronto y buena suerte.

### RE: Enthalpy value calculation

(OP)
how can I know Initial conditions tank inside ?
Raoult- Dalton laws?

### RE: Enthalpy value calculation

Look for the thermophysical properties of the fluid at the prevailing conditions in the tank in specialized handbooks. The nist webbook on line can be of help. Good luck.

### RE: Enthalpy value calculation

(OP)
I´ve got a differents values at Webbook nist .
But I don´t know what dose mean some of this.

Isothermic conditions:
The value of volume of differents phase , is lts of vapor or liquid per each mol of Propane at this pressure and temperature.
sorry
Thank you

### RE: Enthalpy value calculation

Gorkus,

Please work with a local chemical or pump engineer to calculate what you need.  You seem confused on the concepts of enthalpy, NPSH, and composition.  At this rate I'm worried you will improperly apply these formulas and unknowingly proceed with the wrong value in purchasing equipment.  Good luck.

### RE: Enthalpy value calculation

The NIST webbook allows you to select the units you prefer among: mol/L, mol/m3, g/mL, kg/m3, lb-mole/ft3, lbm/ft3, and so is also with specific volumes (the reverse units). The table also says at the end of each row whether the properties belong to a liquid or a vapour. While switching from saturated liquid to saturated vapour on two consecutive lines at different pressures you get the properties of both liquid and vapor. By combining the proportional individual enthalpies you get the enthalpy of the mix.

### RE: Enthalpy value calculation

The fraction wt% vaporized or flashed can be approximated using the following equation:

wt% flashed = 100 x (Hl,p1-Hl,p2)/Hv,p2

where

Hl,p1 is the enthalpy of the liquid at pressure P1
Hl,p2 is the enthalpy of the liquid at pressure p2
Hv,p2 is the latent heat of vaporization at p2.

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