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Pressure Enthalpy without Tears

. If we change the way we look at things, the things we look at change. . . . . . . . . . . . . . Pressure. Heat Content. . . (psia). Btu/lb. LINES OF CONSTANT PRESSURE. LINES OF CONSTANT ENTHALPY. . HEAT CONTENT INCREASES. . HEAT CONTENT DECREASES. . PRESSURE RISES. . PRESSURE DROPS. . . Pressure. Heat Content.

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Pressure Enthalpy without Tears

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    6. 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

    25. 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

    26. 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?

    27. 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

    28. 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?

    31. 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 systems operating efficiency Never remove suction line insulation without replacing Seal the point where insulation sections meet

    34. 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

    36. 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

    37. 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...

    40. 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

    42. 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

    43. 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

    44. 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 = 4Hg Low side (psia) = (30hg 4Hg) 2 = 13 psia Compression ratio = 260 13 = 20:1

    45. Meet Tammy

    55. Tammys 8-Hour Day 9am 10 am Work on 2nd Floor 10am 11am Walk up 11am 12 noon Work on 90th Floor 12 noon 1pm Walk down 1 pm 2pm Lunch 2pm 3 pm Work on 2nd Floor 3 pm 4 pm Walk up 4pm 5 pm Work on 90th Floor

    56. Hmmmmmmmmmmmm What if the law firm moves its 90th floor office to the 3rd floor? How will this affect Tammys productivity? Will she do more work? Less? What the heck does this have to do with air conditioning? How many licks does it take to get to the chocolaty center of a Tootsie Pop?

    57. If Tammys office moves from the 90th floor to the 3rd floor, we get something like this.

    58. Tammys 8-Hour Day 9:00 am 10:00 am Work on 2nd Floor 10:00 am 10:05 am Walk up to 3rd Floor 10:05 am 11:05 noon Work on 3rd Floor 11:05 am 11:10 am Walk down to 2nd Floor 11:10 am 12:10 pm Work on 2nd Floor 12:10 pm 1:10 pm Lunch 1:10 pm 1:15 pm Walk up to 3rd Floor 1:15 pm 2:15 pm Work on 3rd Floor 2:15 pm 2:20 pm Walk down to 2nd Floor 2:20 pm 3:20 pm Work on 2nd Floor 3:20 pm 3:25 pm Walk up to 3rd Floor 3:25 pm 4:25 pm Work on 3rd Floor 4:25 pm 4:30 pm Walk down to 2nd Floor 4:30 pm 5:00 pm Work on 2nd Floor

    59. Office Comparison 2nd Floor ? 90th Floor 4 hours of work 3 hours of walking up and down the stairs 1 hour lunch Day ends on the 90th Floor 2nd Floor ? 3rd Floor 6 hours of work 30 minutes of walking up and down the stairs 1 hour lunch Day ends on the 2nd Floor

    60. COMPRESSION RATIO Lower compression ratios ? higher system efficiency Higher compression ratios ? lower system efficiency The closer the head pressure is to the suction pressure, the higher the system efficiency, all other things being equal and operational

    61. Causes of High Compression Ratio (High Side Issues) Dirty or blocked condenser coil Recirculating air through the condenser coil Defective condenser fan motor Defective condenser fan motor blade Defective wiring at the condenser fan motor Defective motor starting components (capacitor) at the condenser fan motor

    62. Causes of High Compression Ratio (Low Side Issues) Dirty or blocked evaporator coil Dirty air filter Defective evaporator fan motor Dirty blower wheel (squirrel cage) Defective wiring at the evaporator fan motor Closed supply registers Blocked return grill Loose duct liner Belt/pulley issues

    63. THEORETICAL HORSEPOWER PER TON Determines how much compressor horsepower is required to obtain 1 ton of cooling The ft-lb is a unit of work The ft-lb/min is a unit of power 33,000 ft-lb/min = 1 Horsepower The conversion factor between work and heat is 778 ft-lb/btu 33,000 ft-lb/min/hp 778 ft-lb/btu = 42.42 btu/min/hp

    64. THEORETICAL HORSEPOWER PER TON THp/ton = (MFR/ton x HOC) 42.42 For example, if we had a system that had an NRE of 50 and a HOC of 10, the THp/ton would be:

    66. THp/ton Example If we had a 20-Hp reciprocating compressor and the THp/ton calculation yielded a result of 2 hp/ton, what would the expected cooling capability of the system be?

    67. What Affects the THp/ton Number? The Net Refrigeration Effect (NRE) The Heat of Compression (HOC)

    69. MASS FLOW RATE OF THE SYSTEM The amount of refrigerant that flows past any given point in the system every minute Not to be confused with MFR/ton MFR/system is the actual refrigerant flow, while MFR/ton is the flow per ton MFR/system can be found by multiplying the MFR/ton by the number of tons of system capacity, or

    70. COOL STUFF As the HOC increases, the MFR/system decreases, and vice versa As the Compression Ratio increases, the HOC increases As head pressure increases, or as suction pressure decreases, the Compression Ratio increases As the MFR/system decreases, the capacity of the evaporator, condenser and compressor all decrease Lets take a closer look

    71. EVAPORATOR CAPACITY A function of the MFR/system and the NRE The MFR/system is in lb/min, the NRE is in btu/lb and the capacity of the evaporator is in btu/hour

    72. EVAPORATOR CAPACITY If the NRE or the MFR/system decreases, the evaporator capacity also decreases The 60 is a conversion factor from btu/min to btu/hour, given that there are 60 minutes in an hour Divide the evaporator capacity in btu/hour by 12,000 to obtain the evaporator capacity in tons

    73. CONDENSER CAPACITY A function of the MFR/system and the THOR The MFR/system is in lb/min, the THOR is in btu/lb and the capacity of the condenser is in btu/hour

    74. COMPRESSOR CAPACITY A function of the MFR/system and the Specific volume of the refrigerant at the inlet of the compressor Calculated in cubic feet per minute, ft3/min

    75. COEFFICIENT OF PERFORMANCE (COP) The ratio of the NRE compared to the HOC, assuming a saturated cycle If the cycle is not saturated, add the suction line heat to the HOC If the HOC remains constant, any increases in NRE will increase the COP If the NRE remains constant, any decrease in HOC will increase the COP The COP is a contributing factor to the EER of an air conditioning system COP is a unitless value

    76. COP EXAMPLE #1 Heat content at point B = 35 btu/lb Heat content at point C = 104 btu/lb Heat content at point D = 104 btu/lb Heat content at point E = 127 btu/lb NRE = 104 btu/lb 35 btu/lb = 69 btu/lb HOC = 127 btu/lb 104 btu/lb = 23 btu/lb COP = 69 btu/lb 23 btu/lb = 3 Notice that the 3 has no units

    77. COP EXAMPLE #2 Heat content at point B = 35 btu/lb Heat content at point C = 105 btu/lb Heat content at point D = 110 btu/lb Heat content at point E = 140 btu/lb NRE = 105 btu/lb 35 btu/lb = 70 btu/lb HOC = 140 btu/lb 110 btu/lb = 30 btu/lb SL superheat = 110 btu/lb 105 btu/lb = 5 btu/lb COP = [70 btu/lb] [30 btu/lb + 5 btu/lb] = 2

    78. ENERGY EFFICIENCY RATIO (EER) A ratio of the amount of btus transferred to the amount of power used In the units of btu/watt The conversion between btus and watts is 3.413 One watt of power generates 3.413 btu For example, if a system required 50,000 btu of heat, 14,650 watts of electric heat (14.65 kw) can be used

    79. ENERGY EFFICIENCY RATIO (EER), Contd. The efficiency rating of an air conditioning system is the COP For each btu/lb introduced to the system in the suction line and the compressor, a number of btus equal to the NRE are absorbed into the system via the evaporator To convert the COP to energy usage, we multiply the COP by 3.413

    80. EER EXAMPLE The NRE of a system is 70 btu/lb The HOC of the same system is 20 btu/lb The COP is 70 btu/lb 20 btu/lb = 3.5 The EER = COP x 3.413 EER = 3.5 x 3.413 EER = 11.95

    81. SEASONAL EER (SEER) Takes the entire conditioning system into account Varies depending on the geographic location of the equipment Ranges from 10% t0 30% higher than EER So, if the EER is 10, the SEER will range from 11 to 13

    82. From the P-H Chart, We Can Find Compression Ratio NRE HOC HOW THOR COP MFR/ton THp/ton MFR/system Evaporator Capacity Condenser Capacity Compressor Capacity EER of the System SEER

    83. An R-22 A/C System Condenser saturation temperature 120F Condenser outlet temperature 100F Evaporator saturation temperature 40F Evaporator outlet temperature 50F Compressor inlet temperature 60F Compressor Horsepower: 4 hp

    103. Properly Operating System Heat Content A = 40 btu/lb Heat Content B = 40 btu/lb Heat Content C = 109 btu/lb Heat Content D = 111 btu/lb Heat Content E = 125 btu/lb High side pressure = 267 psig High side pressure = 282 psia Low side pressure = 70 psig Low side pressure = 85 psia Compressor Hp = 2.5 Hp Specific Volume = 0.7 NRE = 69 btu/lb HOW = 14 btu/lb HOC = 16 btu/lb THOR = 85 btu/lb Comp. Ratio = 3.32 MFR/ton = 2.9 lb/min/ton THp/ton = 0.96 Hp/ton COP = 4.3 MFR/system = 7.58 lb/min CAP/evap = 31,381 btuh CAP/cond = 38,658 btuh CAP/comp = 5.3 ft3/min EER = 14.68 SEER = 16.15 19.1

    105. Clogged Cap Tube System Heat Content A = 39 btu/lb Heat Content B = 39 btu/lb Heat Content C = 112 btu/lb Heat Content D = 118 btu/lb Heat Content E = 134 btu/lb High side pressure = 226 psig High side pressure = 241 psia Low side pressure = 59 psig Low side pressure = 74 psia Compressor Hp = 2.5 Hp Specific Volume = 0.9 NRE = 73 btu/lb HOW = 16 btu/lb HOC = 22 btu/lb THOR = 95 btu/lb Comp. Ratio = 3.26 MFR/ton = 2.74 lb/min/ton THp/ton = 1.03 Hp/ton COP = 3.3 MFR/system = 6.63 lb/min CAP/evap = 29,039 btuh CAP/cond = 37,791 btuh CAP/comp = 5.97 ft3/min EER = 11.26 SEER = 12.39 14.64

    108. Contact Information... Eugene Silberstein Suffolk County Community College 1001 Crooked Hill Road Brentwood, NY 11717 (631) 851-6897 E-mail: silbere@sunysuffolk.edu

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