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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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**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**LINES OF CONSTANT PRESSURE Pressure (psia) PRESSURE DROPS PRESSURE RISES HEAT CONTENT DECREASES HEAT CONTENT INCREASES Heat Content Btu/lb**Pressure**(psia) SATURATION CURVE Heat Content Btu/lb Btu/lb**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**Pressure**(psia) LINES OF CONSTANT TEMPERATURE Heat Content Btu/lb**Pressure**(psia) LINES OF CONSTANT VOLUME (ft3/lb) Heat Content Btu/lb**Pressure**(psia) LINES OF CONSTANT ENTROPY Heat Content Btu/lb**Pressure**(psia) LINES OF CONSTANT QUALITY Heat Content Btu/lb**PUT IT ALL TOGETHER…**Pressure (psia) Heat Content Btu/lb**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**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)**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**Subcooled Liquid**Saturated Refrigerant Superheated Vapor CONDENSER METERING DEVICE COMPRESSOR EVAPORATOR**Pressure**Subcooled Region Superheated Region Saturated Region Heat Content**Pressure**Heat Content**Pressure**(psia) Heat Content Btu/lb**Height Above Saturation**Saturation VAPOR LIQUID**Saturation**VAPOR LIQUID Distance Below Saturation**Pressure**(psia) Heat Content Btu/lb**PUT IT ALL TOGETHER…**Pressure (psia) A E B C D Heat Content Btu/lb**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**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**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**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?**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**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?**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**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**A**E E B D D C**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.**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**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.**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**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...**A**E B C D HIGH SUBCOOLING.... (Only a slight Exaggeration) What happened to the amount of flash gas?**A**E B C D LARGE AMOUNT OF FLASH GAS.... (Only a slight Exaggeration) What happened to the subcooling?**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**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)**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**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**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**90th Floor**2 Lawyers + 1 Tammy = Wasted Time 2nd Floor

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