In the evaporator of an air conditioning system, dry air at $dot{m}_a$ = 0.75 kg/s, $T_{air,in}$ = 40°C and $P_{air,in}$ = 1.04 bar passes over finned tubes through which refrigerant R134a flows. The air exits the evaporator at $T_{air,out}$ = 25°C and $P_{air,out}$ = 1.01 bar. The refrigerant enters the tubes with a quality of $x_{in}$ = 0.2 and temperature $T_R$ = 12°C and exits as saturated vapor at the same temperature. The jacket of the evaporator is well-insulated. a. Determine the volumetric flow rate of the air. b. Determine the mass flow rate of the refrigerant. c. Determine the rate of energy transfer from the air to the refrigerant. d. Before entering the heat exchanger, the refrigerant existed as saturated liquid, which was then throttled through an expansion valve to the evaporator pressure. Determine the pressure and temperature of the saturated liquid refrigerant upstream of the valve.
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To determine the volumetric flow rate of the air, we can use the equation: V̇ = ṁ / ρ where V̇ is the volumetric flow rate, ṁ is the mass flow rate, and ρ is the density of the air. Given that ṁ = 0.75 kg/s, we need to find the density of the air. We can use Show more…
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6. (20 Pts) Figure 4.A-6 illustrates a vapor separator that is part of a typical refrigeration system. Saturated liquid R134a leaves the condenser at state 1 with mass flow rate ṁcond = 0.05 kg/s and pressure Pcond = 890 kPa. This refrigerant enters the vapor separator (a large, insulated tank) through a valve (shown in fig.). At the exit of the valve (state 2), the vapor is in two-phase condition. The pressure in the tank is Ptank = 500 kPa. There is no pressure drop due to flow through the tank (i.e., the pressure at states 2, 3, and 5 are all the same). The liquid portion of this flow falls to the bottom of the tank due to gravity while the vapor portion remains at the top. Saturated liquid is drawn from the bottom of the tank (at state 3) and expands through a valve to state 4 where it enters the evaporator at Pevap = 120 kPa. The saturated vapor is drawn off of the top of the tank at state 5 and fed to a compressor. Assume that the vapor separator is adiabatic, rigid, and operating at steady state. a.) Determine the quality of the refrigerant at state 2. b.) Determine the mass flow rate of refrigerant that goes to the evaporator (ṁevap) and the mass flow rate of refrigerant that goes to the compressor (ṁcomp). c.) Generate a temperature-specific volume (T-v) diagram. Overlay the state points indicated in Figure 4.A-6 onto this diagram. Number the states.
Adi S.
Consider a two-stage cascade refrigeration system operating between the pressure limits of $1.2 \mathrm{MPa}$ and $200 \mathrm{kPa}$ with refrigerant-134a as the working fluid. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. The mass flow rate of the refrigerant through the lowpressure compressor is $0.15 \mathrm{kg} / \mathrm{s}$. Assuming the refrigerant leaves the evaporator as a saturated vapor and the isentropic efficiency is 80 percent for both compressors, determine $(a)$ the mass flow rate of the refrigerant through the high-pressure compressor, $(b)$ the rate of heat removal from the refrigerated space, and $(c)$ the COP of this refrigerator. Also, determine $(d)$ the rate of heat removal and the COP if this refrigerator operated on a single-stage cycle between the same pressure limits with the same compressor efficiency and the same flow rate as in part ( $a$ ).
Consider a two-stage cascade refrigeration cycle with a flash chamber as shown in the figure with refrigerant- 134 a as the working fluid. The evaporator temperature is $-10^{\circ} \mathrm{C}$ and the condenser pressure is $1600 \mathrm{kPa}$. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. The mass flow rate of the refrigerant through the low-pressure compressor is $0.11 \mathrm{~kg} / \mathrm{s} .$ Assuming the refrigerant leaves the evaporator as a saturated vapor and the isentropic efficiency is 86 percent for both compressors, determine $(a)$ the mass flow rate of the refrigerant through the high-pressure compressor, $(b)$ the rate of refrigeration supplied by the system, and $(c)$ the COP of this refrigerator. Also, determine $(d)$ the rate of refrigeration and the COP if this refrigerator operated on a single-stage vaporcompression cycle between the same evaporating temperature and condenser pressure with the same compressor efficiency and the same flow rate as calculated in part $a$.
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