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HVAC System Overview and Refrigerant Design 141 table 9.6 Performance of some typical refrigerants at 50 mph and 40% r.H. Parameter description r134a r152a r290 Outside air temp (°F) 100 100 100 Relative humidity (%) 40 40 40 Humidity ratio (lbm dry air) 0.017 0.017 0.017 Evap. air flow rate (CFM) 250 250 250 Evap. air mass flow rate (lbm/min) 17.3 17.3 17.3 Evap. disc. air temp (°F) 50 50 50 Condenser air flow rate (CFM) 2000 2000 2000 Condenser air mass flow rate (lbm/min) 138 138 138 Cond. air out temp. (°F) 118 117 117 Refrigerant charge (lbm) 2.125 1.375 0.918 Ref. mass flow rate (lbm/min) 6.93 4.02 3.71 Comp. suct. press (psia) = P 1 43.5 42.78 71.64 Comp. suct. temp. (°F) = T 1 38.5 40 40 Comp. disc. press. (psia) = P 2 250 224 312 Comp. disc. temp. (°F) = T 2 195 225 184 Comp. isentropic efficiency 60% 60% 60% Comp. horsepower 4.35 3.88 4.17 Cond. out press. (psia) = P 3 238 219 297 Cond. out temp. (°F) = T 3 123 125 122 Cond. cooling capacity (Btu/min) 569 547 562 Ref. evap. in press. (psia) = P 4 53.4 47.34 82.96 Ref. evap. in temp (°F) = T 4 44 42 43 Ref. evap. out press. (psia) = P 5 48.34 45.03 77.2 Ref. evap. out temp. (°F) = T 5 38.5 40 40 Evaporator effectiveness/overall eff. 85% 85% 85% Evap. latent load (Btu/min.) 175 175 175 Evap. sensible load (Btu/min) 207 207 207 Evap. cooling capacity (Btu/min) 382 382 382 Comp. power (BTU/min) 184 165 177 Comp. power loss (Btu/min) 5.53 4.93 5.3 A/C blower power (Btu/min) 16 16 16 System power 205 185 198 Cycle COP 2.072 2.322 2.162 System COP 1.863 2.065 1.929 Source: Goodbane, M. SAE Paper 1999-01-084, 1999. With permission.
142 The Role of the Chemist in Automotive Design table 9.7 comparison of co 2 refrigerant and r-134a a refrigerant co 2 (r-744) r-134a Saturation pressure (MPa) 3.69/4.502/5.086 0.35/0.414/0.488 Latent heat (kJ/kg) 214.6/196.8/176.2 194.8/190.9/186.7 Surface tension (mN/m) 3.53/2.67/1.88 11.0/10.3/9.6 Liquid density (kg/m 3 ) 899.6/861.5/821.3 1277.1/1260.2/1242.8 Vapor density (kg/m 3 ) 114.8/135.3/161.0 17.1/20.2/23.7 Liquid viscosity (uPa.s) 95.9/86.7/77.2 270.3/254.3/239.7 Vapor viscosity (uPa.s) 15.4/16.1/17.0 11.2/11.4/11.7 Liquid specific heat (kJ/kg K) 2.73/3.01/3.44 1.35/1.37/1.38 Vapor specific heat (kJ/kg K) 2.21/2.62/3.3 0.91/0.93/0.96 Source: Goodbane, M. SAE Paper 1999-01-084, 1999. With permission. a At 5°C/10°C/15°C. T c of CO 2 is in the ambient region, the system will operate in the transcritical mode. Because of this, the design of a system will have to incorporate a cooler instead of a condenser (see later discussion) [17]. If we examine simulated data from Table 9.8 [17], we can see that the COPs for the two refrigerants are 3.09 and 3.52 for CO 2 and R134a, respectively. Another measure of efficiency is the energy efficiency ratio, which gives the amount of cooling per unit input of electricity [17]. This efficiency is greater for 134a (value of 12) versus CO 2 (value of 9.87). 9.13 tradItIonal and co 2 reFrIgerant system desIgn Figure 9.5 shows an HVAC loop for a traditional HVAC system as well as a CO 2 system. If we follow the refrigerant path from the evaporator and move toward the compressor, refrigerant will exit the evaporator as a saturated vapor. At stage 1, refrigerant enters the compressor (see Figure 9.6, temperature/ entropy diagram). The refrigerant gas is compressed into a superheated vapor by the compressor and then moves on to the condenser. At stage 2, in the condenser, superheated vapor is cooled and condensed into a liquid. In this stage, heat exits the system. At stage 3, moving out of the condenser, the refrigerant is now a saturated liquid and is routed through the thermal expansion valve (TXV). At the thermal expansion valve (stage 4), adiabatic “flash evaporation” of part of the liquid occurs. Autorefrigeration lowers the temperature of the liquid and vapor refrigerant mixture so that it is colder than the temperature of the enclosed space to be refrigerated. When the refrigerant returns to the evaporator (stage 5), warm air evaporates the liquid part of the cold refrigerant mixture and the cycle begins again.
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THE ROLE OF THE CHEMIST IN AUTOMOTI
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CRC Press Taylor & Francis Group 60
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Contents Preface...................
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Contents ix 6.6 Thermal Properties
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Contents xi 11.4 Glass Microspheres
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The Author Herman Phlegm was born i
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2 The Role of the Chemist in Automo
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3 3.1 IntroductIon Component Materi
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Component Materials in Automobiles
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6 6.1 IntroductIon Engineering Poly
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Engineering and Structural Polymers
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194 Index Power train, 95 Power tra
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