Efficiency, compressor refrigerators

Commercial refrigeration - Alternatives to CFCs for new commercial refrigeration equipment include HCFCs (including HCFC mixtures) and HFCs and HFC mixtures. Retrofit of existing equipment is possible by using both HCFCs and HFCs, in conjunction with reduced charges and more efficient compressors. Hydrocarbons are, to a limited extent, applied in hermetically sealed systems. 

Much attention has been given in recent years to the power consumed in the refrigeration process and the development of more efficient compressors. A few points to consider are ... 

For the general purpose of minimizing air flow, transport velocity, wear and power, the fluidized dense-phase mode of flow is preferred for long-distance applications. Efficient blow tank feeders, rotary-screw compressors, refrigerated dryers and stepped-diameter pipelines also are recommended. For products that are not suited to fluidized dense-phase, the possible modes of flow include dilute-phase (suspension flow) or bypass conveying (Wypych, 1995a). 

Build the three-stage-compressor refrigeration cycle as shown in Fig. 6.42a. Assume the compressors are adiabatic with 85% efficiency, and the heater and cooler are isobaric. 

Refrigerators and freezers account for about 20% of the total electricity consumption of household appliances. For this reason the appliance industry is under pressure to improve the energy efficiency of their products to cope with the need to reduce carbon dioxide emissions, as recently mandated by the Kyoto Conference. This objective has to be achieved without penalising product performance. Several options to decrease the energy consumption are under evaluation, ranging from high efficiency compressors to the adoption of intelligent electronic devices [57, 58, 59]. 

Use the most efficient compressor and the details of the same should be looked into. Cooling water requirements for the unit temperature and flow of cooling water required in m /hour. when the refrigeration unit is running with a water cooled condenser for the working fluid (e.g., ammonia/freon)... 

If the refrigerator in Problem 5.33 had been utilized as a liquefier, determine the liquid yield and the figure of merit for the system. Compare these results to those obtained for an ideal system (100% effective heat exchanger and 100% efficient compressor) operating under the same pressure and temperature conditions. 

A simple cooling cycle serves to illustrate the concepts. Figure 1 shows a temperature—entropy plot for an actual refrigeration cycle. Gas at state 1 enters the compressor and its pressure and temperature are increased to state 2. There is a decrease in efficiency represented by the increase in entropy from state 1 to state 2 caused by friction, heat transfer, and other losses in the compressor. From state 2 to states 3 and 4 the gas is cooled and condensed by contact with a heat sink. Losses occur here because the refrigerant temperature must always be above the heat sink temperature for heat transfer to take... 

For the actual systems, compressor work will be higher than for ideal for the isentropic efficiency and other losses. In the case of hermetic or accessible compressors where an elec trical motor is cooled by the refrigerant, condenser capacity should be ... 

In summary, starting with 105°F gas at atmospheric pressure, the theoretical work necessary to liquify one pound of methane is 510.8 Btu or 352 hp/MMcfd. The simplified liquefaction process, as illustrated, uses a turboexpander/compressor and a small propane refrigeration unit. The 41.25% efficiency breaks down as follows one-fourth contributed by the turboexpander/compressor at 35.8% efficiency one-sixteenth contributed by the mechanical propane refrigeration unit at 43% efficiency, at a moderate temperature where its efficiency is high and a large fraction—eleven-sixteenths—contributed at 58.2% efficiency by compression and Joule-Thomson condensation energy. 

Ward reported power consumption for a plant using the mixed-refrigerant cycle. Compressor power was 9,460 kw for an LNG production rate of 28,550 m /hr. This converts to 3.37 kw-hr/lb-mole of liquid. The minimum work for a feed-gas pressure of 37 atm, an assumed feed-gas temperature of 80°F, and an assumed heat-sink temperature of 80°F = 1.09 kw-hr/lb-mole of liquid. This gives an efficiency of 32.4%. 

Flooded compressors use the asymmetric profile rotor extensively because the rotor s efficiency is most apparent in this size range. Flooded compressor size has, over the more recent times, been increased. The upper range is in the 7000 cfm range. While most applications are in air and refrigeration, certain modifications can make it applicable for process ga.s service. One of the considerations is the liquid used for the flooding. 

To meet the 1993 Energy Standards, the industry undertook, at considerable cost, the optimization of the various refrigeration system components. The most significant improvement was the increase in compressor efficiency, from an EER of about 4 to about 5.5. Other system improvements included more efficient fan motors, more effective heat transfer by the evaporator and the condenser, and less defrost energy. In the early 1980s, both the Whirlpool Corporation and White Consolidate Industries introduced electronic defrost controls. Heretofore, an electric timer initiated the defrost cycle, typically every t A elve hours, whether the evaporator needed it or not. With the electronic control the defrost inteiwal is more a function of frost accumulation than of time, and thus referred to as a variable defrost control or as adaptive defrost. It saves energy by being activated only when needed. 

Rotary Compressor. Inherently a rotary compressor is more efficient than the current reciprocating compressor, (room air-conditioners have been using rotary compressors for decades.) Several manufacturers in the U.S. and Japan have produced refrigerators with rotary compressors, but experienced long-term quality problems. 

An actual vapor compression refrigeration cycle operates at steady state with R-134a as the working fluid. The evaporator pressure is 120 kPa and the condenser pressure is 600 kPa. The mass flow rate of refrigerant is 0.1 kg/sec. The efficiency of the compressor is 85%. Determine (a) the compressor power,... 

R-134a enters the compressor of a steady-flow vapor compression refrigeration cycle as superheated vapor at 0.14 MPa and — 10°C at a rate of 0.04 kg/sec, and it leaves at 0.7 MPa and 50°C. The refrigerant is cooled in the condenser to 24°C and saturated liquid. Determine (a) the compressor power required, (b) the rate of heat absorbed from the refrigerated space, (c) the compressor efficiency, and (d) the COP. 

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