Jump to main content.

Module 4: Liquid Characteristics - Enthalpy - Practice Problems

Features
Instructions:
Work these problems on a sheet of paper and check your answers against those provided below.

Helpful Calculators:
The following calculator may be useful in solving these problems. You can access it either from the "Calculators" link in the Features box or from the link below.

Gas Flow Rate Converter (Actual Standard Conditions)

Break Line



Q icon
#1
A cylindrical vessel, 10 ft high and 17 ft in diameter, is half-filled with water at 82°F. How much energy in the form of heat (Btus) is required to heat the water to 300°F? See Figure 3.
See Table 2 [Values for the Enthalpy of Vaporization Figure 3of Water (Approximate)]. See Table 3 (Enthalpies of Gases, Btu/lbm).
A icon
Answer: ~80,740,000 Btu

Solution:

Step 1. Calculate the mass of water in the vessel (in pounds mass).

Practice Equation 1(1)

Reminder: Consult "Numerical Constants" under the "Quick Reference" button for information on the density of water.

Step 2. Calculate the change in enthalpy error-file:TidyOut.log of 1 lbm of water from 82°F to 300°F.

Practice Equation 1(2)

  1. Calculate the change in sensible energy as the water rises in temperature from 82°F to 212°F.

    Practice Equation 1(2)(1)

  2. From Table 2, the enthalpy of vaporization of water is 970.3 Btu/lbm.

  3. Calculate the change in sensible energy as the water rises in temperature from 212°F to 300°F.

    Practice Equation 1(2)(3)
Step 3. Calculate the total change in enthalpy per pound of water.

Practice Equation 1(3)

Step 4. Calculate the total change in enthalpy of the entire vessel of water.

Practice Equation 1(4)

Break Line

Q icon #2
Determine how long the vessel of water introduced in Problem 1 can adequately cool a 5,000 ACFM gas stream entering an evaporative cooler. The gas enters at 1,000°F and -12 in. W.C. of pressure and exits at 300°F. The gas is similar to air. Its molecular weight is 29 lbm/lb mole and its volume is 385 SCF/lb mole. See Table 2 [Values for the Enthalpy of Vaporization of Water (Approximate)]. See Table 3 (Enthalpies of Gases, Btu/lbm).

Figure 4


A icon
Answer: 57.76 hours

Solution:

Step 1. Calculate the inlet gas flow rate (lbm/min).

  1. Convert the gas flow rate from ACFM to SCFM.

    Practice Equation 2(1)(1)

  2. Calculate the inlet gas flow rate (lbm/min).

    Practice Equation 2(1)(2)

Step 2. Calculate the change in enthalpy of the air as it cools from 1000°F to 300°F on a Btu/min basis. See Table 3. (Enthalpies of Gases, Btu/lbm).

Practice Equation 2(2)

Step 3. Calculate the change in enthalpy per pound of water as it is heated from 82°F to boiling, as it boils, and as the water vapor is heated to 300°F. Calculated in Practice Problem 1.

Practice Equation 2(3)

Step 4. Calculate total quantity of water required in cubic feet per hour.

Practice Equation 2(4)

Step 5. Calculate how long the vessel of water can adequately cool the gas stream.

From Problem 1, the amount of water available is 1135 ft3.

Practice Equation 2(5)

Therefore there is enough water available to cool the gas stream for 57.76 hours.

Break Line

Return To Lesson
Enthalpy
Next Lesson
pH
 

APTI Virtual Classroom

Local Navigation

Jump to main content.