Refrigeration System

These are required to operate the process plants at low temperatures, production of organic chemicals, crystallisation and separation of crystals from concentrated solutions, condensation of volatile solvents, etc. 

When the compressor is driven by a variable-speed turbine, the temperature controller adjusts the setpoint of the turbine speed controller, which manipulates the flowrate of high-pressure steam to the turbine. 

The temperature of the chilled process stream is controlled by adjusting the setpoint of the evaporator level controller (the heat transfer area is varied to change the heat transfer rate). 

The water stream used to absorb the ammonia vapor is ratioed to the ammonia flowrate. 

The level in the base of the column is controlled by manipulating the feed flowrate to the column. 

The temperature profile is controlled by manipulating heat input to the reboiler. 

The compressed refrigerant vapor is discharged from the compressor. This vapor is superheated, meaning that it is above its dew-point temperature. 

The vapor is next cooled and condensed in the refrigerant condenser. Typically, 75 to 80 percent of the condenser heat duty is the latent heat of condensation of the refrigerant. The remaining heat duty is sensible-heat removal. The sensible heat 

The refrigerant liquid partially flashes to a vapor as it flows through the letdown valve. The flashing represents the conversion of the sensible heat of the refrigerant to latent heat of vaporization. In Fig. 31.1, the refrigerant is chilled from 

100 to 40°E Approximately 25 percent of the liquid flashes to a vapor to provide this autorefrigeration. 

The compressor, motor, condenser, receiver, and letdown valve are all components of your home central air ondition-ing unit. They are installed as a package, surrounded by the condenser, outside your house. The evaporator is located in your attic. To continue our description of the process flow  

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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. 

Most refrigeration systems are essentially the same as the heat pump cycle shown in Fig. 6.37. Heat is absorbed at low temperature, servicing the process, and rejected at higher temperature either directly to ambient (cooling water or air cooling) or to heat recovery in the process. Heat transfer takes place essentially over latent heat profiles. Such cycles can be much more complex if more than one refrigeration level is involved. 

As with heat pumping, the grand composite curve is used to assess how much heat from the process needs to be extracted into the refrigeration system and where, if appropriate, the process can... 

The cost of shaftwork required to run a refrigeration system can be estimated approximately as a multiple of the shaftwork required for an ideal system. The performance of an ideal system is given by... 

A schematic of the two-level refrigeration system is shown in Fig. 6.406. It should be noted that the single exchangers represented in Fig. 6.406 might in practice be several exchangers. 

Figure 6.40 A two-level refrigeration system for Example 6.6 with heat rejection to cooling water.

Chilled water lines (refrigeration system or well)... 

Gas-Cycle Systems. In principle, any permanent gas can be used for the closed gas-cycle refrigeration system however, the prevailing gas that is used is air. In the gas-cycle system operating on the Brayton cycle, all of the heat-transfer operations involve only sensible heat of the gas. Efficiencies are low because of the large volume of gas that must be handled for a relatively small refrigera tion effect. The advantage of air is that it is safe and inexpensive. 

Cooling. A compression refrigeration system, driven by an electric motor, suppHes cooling for either direct expansion or ice bank systems (Fig. 12). In the former, the milk is cooled by the evaporator (cooling cods) on the bulk tank liner opposite the milk side of the liner. The compressor must have the capacity to cool the milk as rapidly as it enters the tank. 

Heat Pumps. Because of added capital and complexity, heat pumps are rarely economical, although they were formerly commonly used in ethylene/ethane and propylene/propane spHtters. Generally, the former spHtters are integrated into the refrigeration system the latter are driven by low level waste heat, cascading to cooling water. 

Heat Pumps. A heat pump is a refrigeration system that raises heat to a useful level. The most common appHcation is the vapor recompression system for evaporation (qv) (Fig. 14). Its appHcation hinges primarily on low cost power relative to the alternative heating media. If electricity price per unit energy is less than 1.5 times the cost of the heating medium, it merits a close look. This tends to occur when electricity is generated from a cheaper fuel (coal) or when hydroelectric power is available. 

Use in distillation systems are rare. The reason is the recognition that almost the same benefits can be achieved by integrating the rehoiling—condensing via either steam system (above ambient) or refrigeration system (below ambient). 

Fig. 1. Components of a refrigeration system A, condenser outlet B, evaporator inlet C, evaporator outlet D, compressor discharge.

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. 

The world fleet of tmck and tmck/trailer refrigeration systems is estimated at 125,000 units, of which 30% are trailer units, 40% independent tmck units, and 30% units driven off the tmck engine. The container population is estimated at 300,000 units. Tmck and tmck/trailer units have traditionally used CFC-12 or R-502, but in the 1990s some have switched to HCFC-22 or HFCs such as R-404a and R-507. Container units have typically used R-12, but some are suppHed with HFC refrigerants such as R-134a. 

The processes using physical absorption require a solvent circulation proportional to the quantity of process gas, inversely proportional to the pressure, and nearly independent of the carbon dioxide concentration. Therefore, high pressures could favor the use of these processes. The Recitsol process requires a refrigeration system and more equipment than the other processes. This process is primarily used in coal gasification for simultaneous removal of H2S, COS, and CO2. 

The dephlegmator process recovers a substantially higher purity C2+ hydrocarbon product with 50—75% lower methane content than the conventional partial condensation process. The C2+ product from the cryogenic separation process can be compressed and further separated in a de-ethanizer column to provide a high purity C3+ (LPG) product and a mixed ethylene—ethane product with 10—15% methane. Additional refrigeration for the deethanization process can be provided by a package Freon, propane or propylene refrigeration system. 

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