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Understanding Temperature, Heat & Pressure in HVAC Systems

Learn about temperature measurement, heat transfer methods, laws of thermodynamics, different types of heat, specific heat, pressure gauges, and more. Increase your knowledge of HVAC fundamentals!

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Understanding Temperature, Heat & Pressure in HVAC Systems

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  1. HVAC 100 UNIT 1

  2. 1.2 TEMPERATURE • TEMPERATURE IS THE LEVEL OF HEAT IN A SUBSTANCE • HEAT IS DESCRIBED AS MOLECULES IN MOTION • HENCE, THE STARTING POINT OF MOLECULAR MOTION IS THE STARTING POINT OF TEMPERATURE- KNOWN AS ABSOLUTE ZERO (-460*) • THERMOMETERS ARE USED FOR MEASURING TEMPERATURE

  3. 1.2 TEMPERATURE • WATER FREEZES AT 32* & BOILS AT 212* AT STANDARD CONDITIONS • STANDARD CONDITIONS ARE: • 70* AMBIENT AIR • 14.696 PSIA • PURE WATER

  4. 1.2 TEMPERATURE • THERMOMETERS: • HOLLOW GLASS TUBES FILLED WITH MERCURY OR ALCOHOL • USE FARENHEIT AS THE BRITISH SCALE • USE CELCIUS AS METRIC SCALE

  5. Temperature measurement Celsius Fahrenheit Water boils 212°F 100°C °F = (9/5 x °C) + 32 °F = (1.8 x °C) + 32 °F = (1.8 x 100) + 32 °F = (180) + 32 °F = 212 0°C 32°F Water freezes Convert °C to °F: °C = .55 x (°F – 32) °C = .55 x (32 -32) °C = 0

  6. 1.3 INTRODUCTION TO HEAT • LAWS OF THERMODYNAMICS CAN BE APPLIED TO HEAT: • HEAT CANNOT BE CREATED OR DESTROYED • HEAT CAN BE ACCOUNTED FOR WHEN TRANSFERRED • QUANTITY OF HEAT IS MEASURED IN BTU’S (BRITISH THERMAL UNITS) • 1 BTU IS REQUIRED TO RAISE TEMP OF 1# OF WATER 1* AT STANDARD CONDITIONS • RATE OF TRANSFER IS CALCULATED BY FACTORING IN TIME

  7. 1.3 INTRODUCTION TO HEAT • WHEN A TEMPERATURE DIFFERENCE OCCURS, HEAT WILL ALWAYS FLOW FROM HOT TO “COLD” • THE GREATER THE TEMPERATURE DIFFERENCE, THE FASTER THE HEAT FLOWS • FURNACES & AC’S ARE RATED IN BTUH • THE METRIC COUNTERPART TO THE BTU IS THE JOULE

  8. THREE TYPES OF HEAT TRANSFER • CONDUCTION: • FROM MOLECULE TO MOLECULE • CONVECTION: • FROM ONE PLACE TO ANOTHER PLACE • RADIATION: • THROUGH SPACE TO THE NEAREST SOLID OBJECT

  9. 1.4 CONDUCTION • CONDUCTION: • FROM MOLECULE TO MOLECULE • EXAMPLE: COPPER ROD & BURNER (FIG. 1-8) • HEAT TRANSFERS AT DIFFERENT RATES DUE TO THE MAKE UP OF THE SUBSTANCES (FIG. 1-9)

  10. 1.5 CONVECTION • CONVECTION: • FROM ONE PLACE TO ANOTHER PLACE • EXAMPLE: AUTOMOBILE HEATER • TWO TYPES OF CONVECTION: • NATURAL- WITHOUT A FAN • FORCED- WITH A FAN

  11. 1.6 RADIATION • RADIATION: • THROUGH SPACE TO THE NEAREST SOLID OBJECT • EXAMPLE: SUN HEATING THE EARTH • HALF THE DISTANCE, QUADRUPLE THE INTENSITY

  12. 1.7 & 1.8 TWO TYPES OF HEAT • SENSIBLE: • HAS TO DO WITH A CHANGE IN TEMPERATURE • CAN BE SENSED • LATENT: • FROM LATIN WORD MEANING “HIDDEN ” • HAS TO DO WITH A CHANGE OF STATE (PHASE)

  13. 1.9 SPECIFIC HEAT • DIFFERENT SUBSTANCES RESPOND DIFFERENTLY TO HEAT • EXAMPLE: ICE & STEAM REQUIRE ½ THE BTU’S TO RAISE THE SAME AMOUNT IN TEMPERATURE AS WATER • THIS DIFFERENCE IS KNOWN AS SPECIFIC HEAT • DEFINED AS THE AMOUNT OF HEAT REQUIRED TO RAISE 1# OF A SUBSTANCE 1*F

  14. 1.9 SPECIFIC HEAT (fig.1.15)

  15. 1.10 SIZING HEATING EQUIPMENT • SPECIFIC HEAT IS USED IN CALCULATING HEAT LOADS • QUANTITY OF HEAT (Q) = WEIGHT X SPECIFIC HEAT X TEMPERATURE DIFFERENCE IN DEGREES

  16. 1.11 PRESSURE • PRESSURE = FORCE / UNIT OF AREA • USUALLY STATED AS PSI • PRESSURE ASSERTED AGAINST A BODY IS GREATER UNDERWATER & LESS IN AN AIRPLANE

  17. 1.12 ATMOSPHERIC PRESSURE • ATMOSPHERE HAS WEIGHT LIKE THE OCEAN ONLY LESS • AT SEA LEVEL, AVERAGES 14.696 PSIA • MEASURED BY A BAROMETER IN INCHES OF MERCURY (Hg) • AT SEA LEVEL, AVERAGES 29.92” Hg • CHANGES ABOUT 1” Hg PER 1000’

  18. Mercury (Hg) Barometer (fig 1.20)

  19. 1.0” 25.0” 29.9” Elevation and boiling temperatures At 30,000’ elevation water boils at 100 F At 5000’ elevation water boils at 203 F At sea level water boils at 212 F

  20. 1.13 PRESSURE GAUGES • THE BOURDON TUBE MEASURES PRESSURES IN A CLOSED SYSTEM LIKE A REFRIGERATION SYSTEM • THE COMPOUND GAUGE IS USED TO MEASURE PRESSURES BOTH ABOVE & BELOW ATMOSPHERIC PRESSURE • THE HIGH PRESSURE GAUGE IS USED TO MEASURE DISCHARGE PRESSURES • THESE GAUGES READ IN PSIG

  21. Fig 1.24

  22. Low Side & High Side Gauges • “Low Side” (suction) gauge Bluefor cold Reads ECT Compound gauge • Both positive pressure and vacuum • “High Side” (discharge) gauge Redfor hotReads CCT

  23. 60 250 300 50 70 20 160 40 30 10 80 200 350 120 130 30 110 40 140 60 0 20 30 70 100 50 40 50 90 10 120 130 160 150 170 180 140 90 150 140 70 90 110 120 80 130 100 80 10 0 400 150 80 25 100 20 200 190 70 10 90 160 60 90 20 100 20 70 60 0 R-134a 50 95 50 10 65 30 50 10 30 40 40 R-134a 10 50 20 R-22 40 R-22 100 55 450 110 50 R-404a 30 50 10 R-404a psi 0 in Hg vac 120 RETARD 10 psi 50 20 500 350 30 0 Pressure Gauges High Side Low Side

  24. Low Side Gauge • Read suction pressure • Read suction temperature- ECT • Read vacuum

  25. Reading the Low Side Gauge 69 psig 40º (R22)

  26. High Side Gauge • Read condensing pressure- CCT • Read condensing temperature

  27. 278 psig 125º (R22) Reading the High Side Gauge

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