Refrigeration systems evaporator temperature

FIG. 11-98 Performance of complete refrigeration system (1), when there is reduction in heat load (2), and when for the same ambient (or inlet in evaporator) evaporation temperature is maintained constant hy reducing capacity of compressor/condenser part (3). 

Absorption Refrigeration Systems Two main absorption systems are used in industrial application lithium bromide-water and ammonia-water. Lithium bromide-water systems are hmited to evaporation temperatures above freezing because water is used as the refrigerant, while the refrigerant in an ammonia-water system is ammonia and consequently it can be applied for the lower-temperature requirements. 

Unhke other refrigeration systems, the chiUed-water flow rate is of no particular importance in steam-jet system design, because there is, due to direct heat exchange, no influence of evaporator tube velocities and related temperature differences on heat-transfer rates. Widely varying return chiUed-water temperatures have Uttle effect on steam-jet equipment. 

The family of short curves in Fig. 29-45 shows the power efficiency of conventional refrigeration systems. The curves for the latter are taken from the Engineering Data Book, Gas Processors Suppliers Association, Tulsa, Oklahoma. The data refer to the evaporator temperature as the point at which refrigeration is removed. If the refrigeration is used to cool a stream over a temperature interval, the efficiency is obviously somewhat less. The short curves in Fig. 29-45 are for several refrigeration-temperature intervals. A comparison of these curves with the expander curve shows that the refrigeration power requirement by expansion compares favorably with mechanical refrigeration below 360° R (—100° F). The expander efficiency is favored by lower temperature at which heat is to be removed. 

Evaporator temperature, "F Figure 19. Single-stage ethylene refrigeration system. 

Example. With the example cascaded system at an evaporator temperature of -100°F, the horsepower requirement is 6.2hp/ton refrigeration. A ton of refrigeration is equal to 12,000 BTU/hr. 

Example. For an evaporator temperature of -10°F, a steam rate (300°F saturated temperature in the generators) of 33.61b/hr/ton refrigeration is required. Also, 5.4gpm cooling water/ton refrigeration, assuming a 7.5°F rise through the condenser, are required in this system. 

Refrigerant evaporating temperature Consider the 25MM Btu/hr —40°F independently as a simple two-stage system. Thus, condensing at +120°F... 

Standard Ton Conditions. These are taken by industry to represent the refrigeration tonnage of a system when operating with an 86°F condenser temperature and a 5°F evaporator temperature. This is a comparative reference condition and does not need interpolation for effective evaluation of other tonnage requirements and conditions. 

While refrigeration technologies can provide a near-constant GT inlet air temperature, and thus a nearly constant GT power output, refrigeration TIC systems are significantly more expensive than evaporative systems. However, the incremental net power output from the refrigeration system, when measured on a unit cost basis ( per kWe),... 

The system shown in Fig. 22.1 has no provision for controlling the evaporator temperature this is controlled solely by the compressor suction pressure. The lower the compressor suction pressure, the lower the evaporator temperature. This is exactly how our home air conditioner works. If the evaporator temperature is too cold, what can we do Well, if this were a steam turbine compressor, gas engine drive, or any other type of variable-speed driver, we could reduce the compressor s speed. This would reduce the flow of refrigerant, and raise both the evaporator and compressor suction pressure. 

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