The condenser is essentially a heat exchanger. Process 1a to 2a is irreversible as it does is not an isentropic process (due to there being small amounts of heat transfer) hence it cannot be reversed. Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air-conditioning of buildings and automobiles. Examination of the saturation table for R-22 shows that at atmospheric pressure, the saturation temperature is already very cold (about -40°C). Haywood, R.W. The higher temperature in vapor compression cycle occurs at, (e) compressor discharge 3. The Vapor Compression Refrigeration Cycle is nearly 200 years old, but it does not seem ready to leave the scene any time soon. An ideal refrigeration cycle looks much like a reversed Carnot heat engine or a reversed Rankine cycle heat engine. In addition, this is as good a place as any to specify the working fluid. We have several working fluids available for use in refrigeration cycles. However, if T2 is too high (that is, higher than the critical temperature TC for the working fluid), then we will be beyond the top of the saturation dome and we will loose the benefits of the large energy the fluid can reject while it is being cooled. Compressor Inlet (S4) Figure 3: Vapor-Compression Refrigeration Cycle COP versus Thigh in the cooler For an efficient air conditioner, we want this quantity to be large compared to the power needed to run the cycle. (T2 is just the saturation temperature at Phigh). Many issues could lead to this- blocked air filters, dirty air passages etc. Compressor Inlet (S4) Contributed by: M. E. Brokowski The vapor compression refrigeration cycle is the most common method used for removing heat from a lower temperature level to a higher temperature level using a mechanical work. Since this process involves a change of phase from liquid to vapor, this device is often called the evaporator. Phigh is the same as P2, and P2 determines the temperature at state S2, T2. The usual design assumption for an ideal heater in a refrigeration cycle is that it is isobaric (no pressure loss is incurred from forcing the coolant through the coils where heat transfer takes place). We note that the higher Tlow, the better the COP. Since the heating process typically takes place entirely within the saturation region, the isobaric assumption also ensures that the process is isothermal. We want to design a vapor-compression refrigeration cycle to absorb heat from a cool environment and reject it to a warm environment. Phigh is the same as P2, and P2 determines the temperature at state S2, T2. R-22 (CHCLF2) Since the vapor compression cycle is against the Second Law of Thermodynamics, some work is necessary for the transfer to take place. This is due higher work done during compression in actual vapor compression cycle by talking friction in account. Does the ideal vapor-compression refrigeration cycle involve any internal irreversibilities? This allows us to absorb as much energy from the surroundings as possible before leaving the saturation dome, where the temperature of the working fluid starts to rise and the (now non-isothermal) heat transfer becomes less efficient. One cannot afford to ignore condenser safety and performance. Heat transfer from surroundings to refrigerant è Entropy increases (S2>S1). Of course, we would get the same isothermal behavior if we were to start the compression before the fluid was completely saturated. Vapor Compression Refrigeration Cycle.The majority of cooling systems are based on the vapor compression refrigeration cycle. In theory, we can use a turbine to lower the pressure of the working fluid and thereby extract any potential work from the high pressure fluid (and use it to offset the work needed to drive the compressor). Since this process involves a change of phase from liquid to vapor, this device is often called the evaporator. We can choose if T2 to be anywhere between that number and the 96°C TC. An examination of the saturation tables for our refrigerants shows that setting Tlow at, for instance 15° C, still allows for fairly high pressures (4 to 7 atmospheres, typically). This process is maintained in the cycle to make it a more realistic model for the actual vapor-compression refrigeration cycle. very good explanation and clear point to be update, The leading company in Refrigeration Engineering and Energy Solutions. At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. ISBN: 0-19-856255-1 How to choose Tlow 132.35 Replacing the expansion valve by a turbine is not practical since the added benefits cannot justify the added cost and complexity. For reference, TC for our four working fluids are given below. R-22 (CHCLF2) Initial Entry: 12/14/97 Initial Entry: 12/14/97 The final state depends on the quantity of heat absorbed and the same may be wet (B’), dry and saturated (B), or superheated (B”) as shown in Fig. It is for this reason that we choose the inlet to the compressor to be completely saturated vapor, ensuring that the compressor can do its work entirely in the superheat region. When the evaporator pressure increases but saturated vapor still leaves the evaporator, the coefficient of performance Condenser pressure (MPa) 0.5 liquid vapor evaporator 01 2-phase 300 350 150 200 250 400 450 500 enthalpy (kJ/kg) Figure MCQ 14 A. increases B. decreases C. does not change D. is uncertain 37. 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