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Pressure Enthalpy without Tears

Pressure Enthalpy without Tears. Presented by Eugene Silberstein Suffolk County Community College. HVAC EXCELLENCE EDUCATORS CONFERENCE Imperial Palace, Las Vegas, Nevada March 8-10, 2009. If we change the way we look at things, the things we look at change. LINES OF CONSTANT ENTHALPY.

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Pressure Enthalpy without Tears

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  1. Pressure Enthalpy without Tears Presented by Eugene Silberstein Suffolk County Community College HVAC EXCELLENCE EDUCATORS CONFERENCE Imperial Palace, Las Vegas, Nevada March 8-10, 2009

  2. If we change the way we look at things, the things we look at change

  3. LINES OF CONSTANT ENTHALPY LINES OF CONSTANT PRESSURE Pressure (psia) PRESSURE DROPS PRESSURE RISES HEAT CONTENT DECREASES HEAT CONTENT INCREASES Heat Content Btu/lb

  4. Pressure (psia) SATURATION CURVE Heat Content Btu/lb Btu/lb

  5. THE SATURATION CURVE • Under the curve, the refrigerant follows the pressure-temperature relationship • The left side of the saturation curve represents 100% liquid • The right side of the saturation curve represents 100% vapor • For non-blended refrigerants, one pressure corresponds to one temperature

  6. Pressure (psia) LINES OF CONSTANT TEMPERATURE Heat Content Btu/lb

  7. Pressure (psia) LINES OF CONSTANT VOLUME (ft3/lb) Heat Content Btu/lb

  8. Pressure (psia) LINES OF CONSTANT ENTROPY Heat Content Btu/lb

  9. Pressure (psia) LINES OF CONSTANT QUALITY Heat Content Btu/lb

  10. PUT IT ALL TOGETHER… Pressure (psia) Heat Content Btu/lb

  11. Pressure-Enthalpy (p-h) Diagram for R-12 (Simplified) Pressure (psia) 160°F 140°F 221 120°F 172 100°F 132 80°F 99 60°F 72 40°F 52 20°F 36 0°F 24 12 2025 31 35 8 8 8 8 8 9 9 9 9 9 1 1 1 1 1 0 2 4 6 8 0 2 4 6 8 0 0 0 0 0 Enthalpy in btu/lb (Heat Content) 0 2 4 6 8

  12. Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified) Pressure (psia) 160°F 140°F 352 120°F 275 100°F 211 80°F 159 60°F 117 40°F 84 20°F 58 0°F 39 15 24 31 40 46 110 112 119 123 Enthalpy in btu/lb (Heat Content)

  13. Liquid Vapor High Pressure High Temperature High Pressure High Temperature Low Pressure Low Temperature Low Pressure Low Temperature Liquid Vapor CONDENSER METERING DEVICE COMPRESSOR EVAPORATOR

  14. Subcooled Liquid Saturated Refrigerant Superheated Vapor CONDENSER METERING DEVICE COMPRESSOR EVAPORATOR

  15. Pressure Subcooled Region Superheated Region Saturated Region Heat Content

  16. Pressure Heat Content

  17. Pressure (psia) Heat Content Btu/lb

  18. Height Above Saturation Saturation VAPOR LIQUID

  19. Saturation VAPOR LIQUID Distance Below Saturation

  20. Pressure (psia) Heat Content Btu/lb

  21. PUT IT ALL TOGETHER… Pressure (psia) A E B C D Heat Content Btu/lb

  22. PUT IT ALL TOGETHER… Pressure (psia) A A E E B B C C D D Heat Content Btu/lb E to A: CONDENSER (Including discharge and liquid line) A to B: METERING DEVICE B to C: EVAPORATOR C to D: SUCTION LINE D to E: COMPRESSOR

  23. A E D NET REFRIGERATION EFFECT The portion of the system that provides the desired cooling or conditioning of the space or products being treated. B C

  24. NET REFRIGERATION EFFECT • The larger the NRE, the greater the heat transfer rate per pound of refrigerant circulated • NRE is in the units of btu/lb • Cooling effect can be increased by increasing the NRE or by increasing the mass flow rate • The cooling effect can be decreased by decreasing the NRE or by decreasing the rate of refrigerant circulation through the system

  25. NRE Example • Heat Content at point B = 35 btu/lb • Heat Content at point C = 85 btu/lb • NRE = C – B = 85 btu/lb – 35 btu/lb NRE = 50 btu/lb • Each pound of refrigerant can therefore hold 50 btu of heat energy • How many btu does it take to make 1 ton?

  26. How Many btu = 1 Ton? • 12,000 btu/hour = 1 Ton = 200 btu/min • From the previous example, how many lb/min do we have to move through the system to get 1 ton? • 200 btu/min/ton ÷ 50 btu/lb = 4 lb/min • We must circulate 4 pounds of refrigerant through the system every minute to obtain one ton of refrigeration • Mass Flow Rate Per Ton

  27. NRE and MFR/ton • The NRE determines the number of btu that a pound of refrigerant can hold • The larger the NRE the more btu can be held by the pound of refrigerant • As the NRE increases, the MFR/ton decreases • As the NRE decreases, the MFR/ton increases • NRE = Heat content at C – Heat content at B • MFR/ton = 200 ÷ NRE • Cool, huh?

  28. A E B D THE SUCTION LINE The line that connects the outlet of the evaporator to the inlet of the compressor. This line is field installed on split-type air conditioning systems. C

  29. SUCTION LINE • The suction line should be as short as possible • The amount of heat introduced to the system through the suction line should be minimized • Damaged suction line insulation increases the amount of heat added to the system and decreases the system’s operating efficiency • Never remove suction line insulation without replacing • Seal the point where insulation sections meet

  30. A E E B D D C

  31. A E B C D HEAT OF COMPRESSION The quantity, in btu/lb that represents the amount of heat that is added to the refrigerant during the compression process.

  32. HEAT OF COMPRESSION (HOC) • The HOC indicates the amount of heat added to a pound of refrigerant during compression • As the pressure of the refrigerant increases, the heat content of the refrigerant increases as well • Heat gets concentrated in the compressor • As HOC increases, efficiency decreases • As HOC decreases, efficiency increases • HOC = Heat content at E – Heat content at D

  33. A E B C D TOTAL HEAT OF REJECTION The quantity, in btu/lb that represents the amount of heat that is removed from the system. THOR includes the discharge line, condenser and liquid line.

  34. TOTAL HEAT OF REJECTION (THOR) • THOR indicates the total amount of heat rejected from a system • Refrigerant (hot gas) desuperheats when it leaves the compressor (sensible heat transfer) • Once the refrigerant has cooled down to the condensing temperature, a change of state begins to occur (latent heat transfer) • After condensing, refrigerant subcools • THOR = Heat content at E – Heat content at A • THOR = NRE + HOC

  35. SUBCOOLING & FLASH GAS • Subcooling is a good thing, right? • Flash gas is a good thing, right? • Are flash gas and subcooling related? • How can we tell? • Stay tuned...

  36. A E B C D HIGH SUBCOOLING.... (Only a slight Exaggeration) What happened to the amount of flash gas?

  37. A E B C D LARGE AMOUNT OF FLASH GAS.... (Only a slight Exaggeration) What happened to the subcooling?

  38. SUBCOOLING & FLASH GAS • Subcooling and flash gas are inversely related to each other • As the amount of subcooling increases, the percentage of flash gas decreases • As the percentage of flash gas increases, the amount of subcooling decreases

  39. A E High-side pressure Low-side pressure B C D COMPRESSION RATIO Determined by dividing the high side pressure (psia) by the low side pressure (psia)

  40. COMPRESSION RATIO • Represents the ratio of the high side pressure to the low side pressure • Directly related to the amount of work done by the compressor to accomplish the compression process • The larger the compression ratio, the larger the HOC and the lower the system MFR • The larger the HOC, the lower the efficiency • Absolute pressures must be used

  41. ABSOLUTE PRESSURE • Absolute pressure = Gauge pressure + 14.7 • Round off to 15, for ease of calculation • Example 1 • High side pressure (psig) = 225 psig • High side pressure (psia) = 225 + 15 = 240 psia • Low side pressure (psig) = 65 psig • Low side pressure (psia) = 65 + 15 = 80 psia • Compression ratio = 240 psia ÷ 80 psia = 3:1

  42. Low Side Pressure in a Vacuum? • First, convert the low side vacuum pressure in inches of mercury to psia • Use the following formula  (30” Hg – vacuum reading) ÷ 2 • Example • High side pressure = 245 psig • High side pressure (psia) = 245 + 15 = 260 psia • Low side pressure = 4”Hg • Low side (psia) = (30”hg – 4”Hg) ÷ 2 = 13 psia • Compression ratio = 260 ÷ 13 = 20:1

  43. Meet Tammy…

  44. 90th Floor 2 Lawyers + 1 Tammy = Wasted Time 2nd Floor

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