Is the Equation h = u + (0.185)Pv Accurate for Converting Energy Units?

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In summary, the formula for h is h = u + (0.185)Pv (Btu/lbm), where h and u are measured in Btu/lbm and P and v in psia and ft*3/lbm, respectively. On a temperature and specific entropy graph, h represents the area under the curve at a constant specific pressure, u represents the area under the curve at a constant specific volume, and (0.185)Pv represents the area between the h and u curves. This equation can be converted to metric units as h = u + (1000)Pv, with units of kg/kj for h and u and Mpa and m3/kg for P and v. The equation has
  • #1
wrenchtime
9
0
Should h = u + Pv

be h = u + (0.185)Pv (Btu/lbm)

where units for h and u are (Btu/lbm)

P (psia) and v (ft*3/lbm)

On a Temperature and specific entropy graph

h... represents the area under the curve at a constant specific pressure

u... represents the area under the curve at a constant specific volume

(0.185)Pv... represents the area between the h and u curves

I have tested the equation on several medium and it seems to work?
 
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  • #2
Here’s a hint: 1 foot pound = 0.00129 BTU
 
  • #3
To convert (psia) x (ft3 / lbm) to Btu / lbm

1 Btu / lbm = (1 lbf/in2) x (1 ft3/lbm) x (1 Btu/778 ft-lbf) x (144 in2/ 1 ft2) = 0.185 Btu/lbm

The equation in metric units is

h = u + (1000) P v

With units
h = kg /kj
u = kg /kj
P = Mpa
v = m3/kg
 

FAQ: Is the Equation h = u + (0.185)Pv Accurate for Converting Energy Units?

What does the equation H = u + (0.185)Pv (Btu/lbm) represent?

The equation represents the enthalpy of a substance, which is the total energy of the substance including both its internal energy (u) and the energy associated with its pressure and volume (Pv).

How is enthalpy related to internal energy?

Enthalpy is equal to the internal energy of a substance plus the product of its pressure and volume. In other words, enthalpy takes into account the energy required to change the volume of a substance at a given pressure.

What is the significance of the constant 0.185 in the equation?

The constant 0.185 is known as the specific heat ratio and is a measure of the ratio of specific heats of a substance at constant pressure and constant volume. It is used to calculate the enthalpy of a substance at a given pressure and volume.

What are the units of measurement for H, u, P, and v in the equation?

H and u are both measured in Btu/lbm (British thermal units per pound-mass), while P is measured in pounds per square inch (psi) and v is measured in cubic feet per pound-mass (ft3/lbm).

How is the equation H = u + (0.185)Pv (Btu/lbm) used in scientific research or applications?

The equation is commonly used in thermodynamics and heat transfer calculations to determine the enthalpy of a substance at a given pressure and volume. It is also used in various engineering applications, such as in the design of heat exchangers and power plants.

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