Thermodynamics: Enthalpy of Steam

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Questions and Answers

What is enthalpy defined as?

The sum of the internal energy and the product of pressure and volume.

What is the mathematical representation of enthalpy?

h = u + pv

What is the condition for dp=0 in the first law of thermodynamics?

Constant pressure process

What is the significance of enthalpy in a constant pressure steam generation process?

The amount of heat supplied to the water is equal to the change in enthalpy. Signup and view all the answers

What is the enthalpy of dry saturated steam denoted as?

hg Signup and view all the answers

What is the relationship between hg, hf, and hfg?

hg = hf + hfg Signup and view all the answers

Why does the enthalpy of wet steam depend on the dryness fraction?

Because the wet steam contains water molecules entrained in it, absorbing only a fraction of the latent heat of evaporation. Signup and view all the answers

What is the enthalpy of wet steam denoted as?

h Signup and view all the answers

What is the temperature at which water begins to boil at a stated pressure?

Saturation temperature (Ts) Signup and view all the answers

What is the amount of heat required to raise the temperature of 1kg of water from 0°C to the saturation temperature?

Sensible heat (hf) Signup and view all the answers

What happens to the temperature of the water when further heat is added after reaching the saturation temperature?

It remains at the saturation temperature (Ts) Signup and view all the answers

What is the process represented by the horizontal line BC on the graph?

Constant pressure and constant temperature heat addition process Signup and view all the answers

What is the amount of heat required to evaporate 1kg of water at the saturation temperature?

Latent heat of vaporisation (hfg) Signup and view all the answers

What is the temperature of the steam above the saturation temperature at a given pressure?

Superheated temperature Signup and view all the answers

What is the point on the graph where the heat addition process occurs at constant pressure and constant temperature?

Point C on the graph Signup and view all the answers

What is the heating process of water from 0°C to Ts°C at constant pressure represented by on the graph?

The inclined line AB Signup and view all the answers

What is the enthalpy of steam at 10 bar absolute pressure and what is the temperature of saturated steam at this pressure?

h_g = 2776.2 kJ/kg and T_sat = 179.9°C Signup and view all the answers

What is the formula to calculate the enthalpy of superheated steam per kg?

h_2 = h_g + c_p_s (T_sup - T_sat) Signup and view all the answers

What is the specific volume of superheated steam at 207°C?

v_2 = 0.20592 m³/kg Signup and view all the answers

What is the internal energy of superheated steam at 207°C?

u_2 = 2627.2 kJ/kg Signup and view all the answers

What is the enthalpy of 5 kg of wet steam at 0.8 bar absolute pressure and 90% dry?

H = 12191.95 kJ Signup and view all the answers

What is the enthalpy of 5 kg of superheated steam at 20 bar absolute pressure and 300°C?

H_sup = 14971.5 kJ Signup and view all the answers

Study Notes

Enthalpy

Enthalpy (h) is the sum of internal energy (u) and the product of pressure (p) and volume (v): h = u + pv
Enthalpy is a measure of the total energy of a system

Constant Pressure Process

For a constant pressure process, dp = 0, and the amount of heat supplied (dQ) is equal to the change in enthalpy (dh)
dQ = dh, i.e., the heat supplied is equal to the change in enthalpy

Enthalpy of Steam

Enthalpy of dry saturated steam (hg) is the total amount of heat supplied to convert 1kg of water into 1kg of dry saturated steam at its saturation temperature
hg = hf + hfg, where hf is the sensible heat and hfg is the latent heat of evaporation
Enthalpy of wet steam (h) is the total amount of heat supplied to convert 1kg of water into 1kg of wet steam at a specified dryness fraction
h = hf + xhfg, where x is the dryness fraction

Superheated Steam

Enthalpy of superheated steam (hsup) is the total amount of heat supplied to convert 1kg of water into 1kg of superheated steam
hsup = hg + cP(Tsup – Tsat), where cP is the specific heat of superheated steam

Specific Volume of Steam

Specific volume of superheated steam (v2) is given by v2 = vg.Tsup / Tsat, where vg is the specific volume of dry saturated steam

Internal Energy

Internal energy (u) is given by u = h - pv, where h is the enthalpy and pv is the product of pressure and volume

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Description

This quiz covers the concept of enthalpy, its definition, and its relation to internal energy and pressure-volume product. It also explores the first law of thermodynamics.