1 / 28

HVAC317 - Refrigeration

HVAC317 - Refrigeration. Refrigeration Theory. Terms. Heat: A form of energy . Refrigeration: The process of removing heat from a space. British Thermal Unit (Btu): The conventional unit of measurement of heat. Terms.

dolan
Télécharger la présentation

HVAC317 - Refrigeration

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. HVAC317 - Refrigeration Refrigeration Theory

  2. Terms • Heat: A form of energy. • Refrigeration: The process of removing heat from a space. • British Thermal Unit (Btu): The conventional unit of measurement of heat.

  3. Terms • A Btu is defined as the amount of heat required to raise or lower the temperature of one pound of water (or any substance) one degree Fahrenheit. • Substances other than water require different amounts of Btu.

  4. Terms • Thermodynamics: The laws of thermodynamics state that heat will always travel from warmer to cooler areas. • Pressure: Standard atmospheric pressure is 14.7 psia (pounds per square inch absolute pressure).

  5. Terms • Refrigeration gauges are calibrated to read zero psig (pounds per square inch gauge pressure) at standard atmospheric pressure. • Vacuum: Pressures below atmospheric pressure.

  6. Terms • Compound Gauge: A gauge that can read above and below atmospheric pressure (this is always the low side gauge on your manifolds). • Above atmospheric pressure these gauges will read psig. • Below atmospheric pressure these gauges will read inches of mercury (Hg).

  7. Terms • Sensible Heat: A change in temperature measured with an ordinary thermometer. • Latent Heat: Heat that causes a change in state. • Change in State: Conversion of a substance from one form to another. This is NOT a temperature change, it is a molecular structure change. • For example: • Water to Ice, or Ice to Water • Water to Steam, or Steam to Water

  8. Terms • Specific Heat: The amount of Btu it takes to raise the temperature of one pound (1 lb) of any substance one degree (1) • Ambient Temperature: The temperature of air that is around an object or device.

  9. Terms • Enthalpy: The total amount of heat that is around a substance. • Ton: A refrigeration rating that is calculated when Btu are known. • 1 ton = 12,000 Btu per hour • Compressor tonnage and horsepower are proportionally equal. • 1 ton = 1 hp = 12,000 Btu/h

  10. Terms • Temperature Pressure Chart (TP Chart): A cross reference between the temperatures, pressures and the boiling points of refrigerant.

  11. Terms • Methods of heat transfer • Convection: Heat transfer by fluid or air movement. (Air Conditioning / Forced air systems) • Conduction: Particle to Particle transfer of heat. (Radiant Heat) • Radiation: Transfer of heat by rays. (sunlight)

  12. Classification of Systems • High Temperature • Start at about 45°F • End at about 60°F • Used in flower shops and candy storage.

  13. Classification of Systems • Medium Temperature • Start at about 30°F • End at about 45°F • Used in products such as dairy goods, beer, wine. • Many products will not freeze at 32°F.

  14. Classification of Systems • Low Temperature Systems • Start at 32°F, and decrease from there • Used in anything frozen such as ice, ice cream, frozen foods, meat storage. • Must have a defrost cycle.

  15. Temp / Pressure Relationship • The temperature at which a substance changes state. It is dependent on pressure. • Lowering the pressure lowers the saturation or boiling point. • Raising the pressure raises the saturation or boiling point.

  16. Refrigeration Calculations • To find the total Btu needed to cool a substance from one temperature to another (without a change in state): • Find the net weight of the substance. • Find the degrees of temperature change (Delta T or T). • Find the specific heat of the substance. • Btu/hr = weight * T * Spec. Heat

  17. Example 1 • Cooling 500 lbs of Cranberries from 70F to 35F T = 70 - 35 = 35 Btu/hr = weight * T * Spec. Heat Btu/hr = 500 * 35 * .91 Btu/hr = 15,925

  18. Example 2 A grocery store takes in 400 lbsof cucumbers and needs to cool them from the 93°F truck to the long term storage temperature of 45°F. Calculate the Btu. T = .93 - 45 = 48 Specific Heat of Cucumbers = .93

  19. Example 3 Need to chill 50 gallons of water from 75°F degrees to 35°F. • Specific heat water = 1.0 • T = 75 - 35 = 40 • 1 gallon water = 8.345 lbs • Weight = 8.345 * 50 = 417.25

  20. Example 3, cont’d: So, it takes 16,690 Btu to cool 50 gallons of water from 75°F to 35°F. The only substance with a specific heat of 1.0 is water.

  21. Refrigeration Calculations • To find the total Btu needed to cool a substance from one temperature to another with a change of state: • Find the net weight of the substance. • Find the degree of temperature change (delta T or T). • Find the specific heat of the substance. • Find the latent heat of the substance.

  22. Refrigeration Calculations • Find the temperature at which the substance changes state (32°F for water). • Calculate the number of Btu prior to the change in state. • Calculate the numer of Btu after the change in state. • Calculate the number of Btu for the state change. • Add them all together for total Btu needed.

  23. Example 1 • A warehouse gets 100 pounds of celery. It is delivered at 70°F. It needs to be cooled to -10°F for long term storage. How many Btu are needed?

  24. Example 1, cont’d • Find the latent heat changes: • Celery freezes at 29.7° F 70 Sensible 29.7 29.7 Latent -10

  25. Example 1, cont’d • Two areas of sensible heat change: • 70 - 29.7 degrees = 40.3°F • 29.7 - -10 degrees = 39.7°F • Latent Heat = 135 Btu

  26. Example 1, cont’d Conclusion: It takes 19,234.1 Btu to cool celery from 70°F to -10°F.

  27. Example 1, cont’d • Note that some substances have a different sensible heat above and below their freezing points.

More Related