The figure below shows a simplified representation of the HVAC system of an auto
ID: 2073616 • Letter: T
Question
The figure below shows a simplified representation of the HVAC system of an automobile, when the air conditioner is used during a hot summer day. T he cabin blower and the inlet damper mix fresh air from the ambient to recirculated air from the cabin. The mixture is sent first to the A/C evaporator coils to extract moisture and then to the cabin heater to regulate the temperature to the desired setting Assume the system operates at 100 kPa steady-state conditions. The blower sets the volumetric air flow rate to the cabin to a constant value of 10 m3/min 1) Using the attached psychrometric chart, draw an accurate representation of the complete process, indicating the thermodynamic states at the inlet and outlet of each section of the HVAC system 2) Calculate the cooling load and the heating load of the two heat exchangers (kW) The A/C evaporator is connected to a simple vapor compression refrigeration cycle, where the condenser operates at least 10 above ambient conditions, and the evaporator operates 5°C below the temperature of the moist air that exits the coils. Assuming the refrigerant is R-134a and the compressor has 75% isentropic efficiency, provide the following: 3a) Sketch the corresponding l-s diagram of a simple vapor compression cycle and number your states, 3b) Calculate the heat rejection at the condenser (kW) 3c) Calculate the compressor power consumption (kW), 3d) Calculate the COP of the system T35 C Amblent alr Outside air intake damper = 80% Li eGol ecat To cabin air 2 A/C Evaporator Cabin Heater ,-30% T, = 30°C =6096 Cabln blower Reclrculated alr from cablrn Use the attached psychrometric chart to obtain numerical values as well as to visualize the process for Questions 1 and 2.Explanation / Answer
1) Given that,
Pressure , P = 100 Kpa
Mass flow rate , mI = 10 m3 /min
cooling load of the heat exchangers , Qcool = m Cp ( T2 - T1 )
= 10 x 956 ( 30 - 20 )
= 95.6 KW
heating load of the heat exchanger , Qheat = m Cp ( T2 - T1 )
= 10 X 956 X ( 58 - 24 )
= 325 KW
2) b) heat rejection , Q = 1000 X 4.187 X 10 X ( 35 - 25 ) / 60
= 6.97 KW
c ) Compressor Power consumption = ?
0.6 = o/p / i/p
Input = 2.13 KW
Compressor power consumption = 2.13 X 0.6 = 1.278 KW
d) COP of the system = T2 - T1 / T3 - T1
= 35 -25 / 30 -25
= 10 / 5
= 2
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