W1D1

Fundamentals of Thermodynamics and Heat Calculations

BTU Calculation

• To calculate the BTUs for a given airflow, use the formula:

  • $ext{BTUs per hour} = CFM imes 0.68 imes (T<em>{in} - T</em>{out})$

  • Example:

  • For 1,600 CFM:

    • $1,600 ext{ CFM} imes 0.68 imes (78 - 60) = 19,584 ext{ BTUs per hour}$

Tools for Measurement

• Simple measuring tools:

  • Thermometer: Does not measure a change in state.

  • Psychrometer: Not required for simplicity.

  • Buckets or Measuring Cups: Can capture data over time to equate to BTUs:

  • Capture moisture over a period (e.g., 10 minutes) and weigh it.

  • Use the conversion: 7,000 grains per pound to calculate equivalent BTUs.

Heat Change Concepts

Sensible Heat vs. Latent Heat

• Sensible Heat Change:

  • Refers to a temperature change without a change in moisture (or grains).

  • Formula used:

  • $ext{BTUs} = CFM imes 4.5 imes ext{delta} h$

• Latent Heat:

  • Change in state without a temperature change.

  • Always measured in grains, whereas the total heat formula uses delta h (enthalpy).

Total Heat Calculation

• Total Heat Formula:

  • $ext{BTUs per hour} = CFM imes 4.5 imes ext{delta} h$

  • Example with calculations:

  • Previous enthalpy values were:

    • $H<em>{1} = 30.4 ext{ and } H</em>{2} = 22.4$

    • $ext{delta} h = 30.4 - 22.4 = 8$

    • Then,

    • $ext{Total Heat} = 1,600 imes 4.5 imes 8 = 57,600 ext{ BTUs per hour}$

    • Total heat calculation confirmed among participants.

Sensible Heat Ratio Calculations

• Understand how to manipulate various heat ratios using the formulas for specific or latent heat calculations.

  • Note: Delta h is always expressed in BTUs per pound, while latent heat uses grains.

Specific Heat Calculations

• To find specific heat, use the formula:

  • $ext{Specific Heat} = rac{BTUs}{ ext{Weight} imes ext{Change in Temperature}}$

• Example Problem:

  • Add 1,150 BTUs to a substance increasing the temperature from 35°F to 129°F:

  • For 50 pounds of material, specific heat calculated as:

  • $ext{Specific Heat} = rac{1150}{50 imes (129 - 35)} = 0.245$

Example Problems from Textbook

Heating Coils, Fluids, and Gases

1. Heating Coil Calculation:

   • Aluminum heating coil of 300 pounds heated up to steam temperature (using specific heat: 0.23 BTUs per pound per degree).

   • For heating means:

     • $BTUs = 300 ext{ lbs} imes 0.23 imes (T_{steam} - 55)$

2. Air Temperature Change in Heating:

   • Heating coil raises the air temperature from 40°F to 110°F with 400 lbs of air:

     • $BTUs = 400 ext{ lbs} imes 0.24 imes (110 - 40)$

     • Resulting in 6,720 BTUs, then for one hour multiply by 60 minutes.

Cooling Processes

1. Brine Cooling Calculation:

   • A calcium brine, 20% solution cooled from 45°F to 17°F with refrigerating plant:

     • $ext{BTUs} = rac{BTUs_{total}}{ ext{Specific Heat} imes ext{Temperature Change}}$

   • Using tables for finding specific heat.

2. Refrigerant Property Reference:

   • Sodium chloride observed cooling aspects with considerations of increased/ decreased temperature based on flow rates.

Vacuum and Pressure Calculations

• To convert gauge pressure to absolute pressure:

  • Add atmospheric pressure (14.696 psi) to gauge value.

  • Furthermore, calculations for temperatures at various psi values will be executed with reference to steam tables.

Conclusion

• Review refraining upon dimensions of latent and sensible heat and their concurrent applications in real-world HVAC scenarios.

  • Continuing education, as noted from end-of-chapter problems, guided through realistic practice in calculating heat transfers, pressures, and efficiency in thermodynamic applications.