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In some countries liquid nitrogen is used on dairy trucks instead of mechanical refrigerators. A 3 hour delivery trip requires .200 m^3 of liquid nitrogen which has a density of 808 kg/m^3, assuming it is already at its boiling temperature of -195.8C, calculate the heat transfer necessary to evaporate this amount of liquid nitrogen and raise its temperature to 4.0C (Nitrogen: Lv=201 kJ/ kg, cgas = 1040 J/kgC

Respuesta :

Answer:

Amount of necessary heat transfer, q = [tex]66.1\times 10^{6} J[/tex]  

Given:

Volume of liquid Nitrogen, V = 0.200 [tex]m^{3}[/tex]

Density of liquid Nitrogen,  [tex]\rho = 808 kg/m^{3}[/tex]

Temperature, [tex]T = - 195.8^{\circ}C[/tex]

Temperature Rise, T' =  [tex]4.0^{\circ}C[/tex]

Latent heat of Vaporization, [tex]L_{v} = 201 kJ/kg[/tex]

Specific heat capacity of gas, [tex]C_{gas} = 1040 J/kg^{\circ}C[/tex]

Solution:

Now, calculation of heat transfer required to evaporate liquid nitrogen and raise its temperature is given by:

[tex]q = mC_{gas}\Delta T + mL_{v}[/tex]                        (1)

Now, mass, m can be given by:

[tex]m = \rho V = 808\times 0.200 = 161.6 kg[/tex]

Now, from eqn (1):

[tex]q = m(C_{gas}\Delta T + L_{v})[/tex]  

[tex]q = 161.6(1040(T' - T) + 201\times 10^{3})[/tex]  

[tex]q = 161.6(1040(4 - (- 195.8)) + 201\times 10^{3})[/tex]  

q = [tex]66.1\times 10^{6} J[/tex]