Cycles refrigeration

A refrigeration system is a heat pump in which heat is absorbed below ambient temperature. Thus the appropriate placement principle for heat pumps applies in exactly the same way as for refrigeration cycles. The appropriate placement for refrigeration cycles is that they also should be across the pinch. As with heat pumps, refrigeration cycles also can be appropriately placed across utility pinches. It is common for refrigeration cycles to be placed across a utility pinch caused by maximizing cooling water duty. 

Tc = temperature at which heat is taken into the refrigeration cycle (K)... 

It must be emphasized that Eq. (6.7) is only an approximate method for calculating the performance of refrigeration cycles. If greater accuracy is required, the refrigeration cycle must be followed using thermodynamic properties of the refrigerant being used. °... 

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... 

Absorption Systems. Absorption refrigeration cycles employ a secondary fluid, the absorbent, to absorb the primary fluid, refrigerant vapor, which has been vaporized in the evaporator. The two materials that serve as the refrigerant—absorbent pair must meet a number of requirements however, only two have found extensive commercial use ammonia—water and water—Hthium bromide. 

Heat Pumps. Heat pumps involve the appHcation of external power to pump heat from a lower temperature to a higher temperature. Heat pumps are frequently used for space heating and are simply refrigeration cycles operated in reverse. The heat rejected in the condenser becomes the primary objective of operation. Consequently, refrigerants used for mechanical compression refrigeration have similar appHcation in heat pumps. 

A simplified flow diagram of a modern H2SO4 alkylation unit is shown in Eigure 1. Excess isobutane is suppHed as recycle to the reactor section to suppress polymerization and other undesirable side reactions. The isobutane is suppHed both by fractionation and by the return of flashed reactor effluent from the refrigeration cycle. 

Fig. 2. Basic refrigeration cycle. A or A/ condenser outlet B or B/ evaporator inlet C or C/ evaporator outlet S/ compressor inlet D, compressor discharge and D/ condenser inlet. To convert kPa to atm, divide by 101.3. To convert kj/kg to Btu/lb, multiply by 0.4302.

Flooded refrigeration systems are a version of the closed-cycle design that may reduce operating problems in some appHcations. In flooded systems, the refrigerant is circulated to heat exchangers or evaporators by a pump. Figure 11 shows the flooded cycle, which can employ any of the simple or compound closed-refrigeration cycles. 

Open-Cycle Operation. In many chemical processes, the product to be cooled can itself be used as the refrigerating Hquid. An example of this is in gathering plants for natural gas. Gas from the wells is cooled, usually after compression and after some of the heavier components are removed as hquid. This Hquid maybe expanded in a refrigeration cycle to further cool the compressed gas, which causes more of the heavier components to condense. Excess Hquid not used for refrigeration is drawn off as product. A typical open-cycle is shown in Figure 12. 

Vapor-Compression Cycles The most widely used refrigeration principle is vapor compression. Isothermal processes are realized through isobaric evaporation and condensation in the tubes. Standard vapor compression refrigeration cycle (counterclockwise Ranldne cycle) is marked in Fig. ll-72<7) by I, 2, 3, 4. 

The optimum intermediate pressure for the two-stage refrigeration cycles is determined as the geometric mean between evaporation pressure (pi) and condensing pressure (p/, Fig. 11-79) ... 

The mixed refrigerant cycle is a variation of the cascade cycle and involves the circulation of a single multicoiTmonent refrigerant stream. The simplification of the compression and heat exchange services in such a cycle can offer potential for reduced capital expenditure over the conventional cascade cycle. 

FIG. 11-116 Propane precooled mixed-refrigerant cycle cooling curve for natural gas. 

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%. 

Expansion Turbine, 296-300 applications, 300 condensation, 299 operating limits, 297 power recovery, 297 refrigeration cycles, 298 reliability, 480 types. 206 Experience... 

Critoph, R.E., Evaluation of alternative refrigerant - adsorbent pairs for refrigeration cycles. Applied Thermal Engineering, 1996,16(11), 891 900. 

Turner, L., Improvement of activated charcoal-ammonia adsorption heat pumping/refrigeration cycles. Investigation of porosity and heat/mass transfer characteristics. Ph.D. Thesis, University of Warwick, UK, 1992. 

The chiller is usually a kettle type exchanger. Freon (which is cooled in a refrigeration cycle to -20 F) is able to cool the gas to approximately - 15°F. Propane, which can be cooled to -40°F, is sometimes used if lower gas temperatures and greater recovery efficiences are desired. 

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