Refrigerant vaporization

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. 

Refrigeration Systems Refrigerant vapor pressure at temperature 10°-15° F. above condensing water. 

High pressure steam lines Low pressure steam lines Vacuum steam lines Compressed air lines Refrigerant vapor lines High pressure Low pressure Refrigerant liquid Brine lines Ventilating ducts Register grilles... 

A refrigeration system requires that 52,400 Ib/hr of propylene refrigerant vapor from the compressors be desuperheated and then condensed. 

Figure 10-151. Exchanger rating for refrigerant vaporizer-water chiller.

Refrigerating Effect. This is the heat absorbed in the evaporator per lb of refrigerant. It is determined by the difference in enthalpy of a lb of refrigerant vapor leaving the evaporator and that of a lb of liquid just upstream (ahead) of the expansion valve at the evaporator. From Figure 11-48A,... 

Figure A-6 Viscosity of (a) gases and (b) refrigerant vapors. (From Crane Technical Paper 410, Crane Co., Chicago, 1991.)...

Table 3. Pressurized Combustor Modelled by a Bed Fluidized with Refrigerant Vapor 134a at Ambient Conditions...

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

The partially vaporized refrigerant flows into the evaporator. In Fig. 22.1, the evaporator shown is similar to a kettle-type reboiler (see Chap. 5). The process fluid flows through the tube side of the kettle evaporator. The refrigerant liquid level is maintained by the letdown valve. The refrigerant vapor flows from the top of the kettle, to the compressor suction. 

The motor-driven compressor boosts the pressure of the refrigerant vapors from the evaporator pressure, up to the condenser pressure. 

Now let s assume that there is a refrigerant receiver between the condenser and letdown valve. This traps any uncondensed vapors. The accumulation of these vapors raises the pressure in the receiver. This puts backpressure on the condenser. The higher condenser pressure promotes more complete condensation of the refrigerant vapors. 

Lowering the compressor discharge pressure can be achieved by changing to cooler water to the refrigerant condenser. But how does this affect the horsepower load or electrical power demand on the compressor s driver Well, it depends. It depends on how much the refrigerant vapor flow increases, as the compressor discharge pressure is reduced. 

The tubes in the condenser required for subcooling steal heat-transfer surface area required for condensation. In effect, the condenser shrinks. This makes it more difficult to liquefy the refrigerant vapor. The vapor is then forced to condense at a higher temperature and pressure. Of course, this raises the compressor discharge pressure. And, as we have seen in the pressure section, this increase in compressor discharge pressure invariably reduces the compressor s capacity and may also increase the horsepower needed to drive the compressor. 

Figure 3.11. Vaporizers (reboilers), (a) Vaporizer with flow-rate of HTM controlled by temperature of the PF vapor. HTM may be liquid or vapor to start, (b) Thermosiphon reboiler. A constant rate of heat input is assured by flow control of the HTM which may be either liquid or vapor to start, (c) Cascade control of vaporizer. The flow control on the HTM supply responds rapidly to changes in the heat supply system. The more sluggish TC on the PF vapor resets the FC if need be to maintain temperature, (d) Vaporization of refrigerant and cooling of process fluid. Flow rate of the PF is the primary control. The flow rate of refrigerant vapor is controlled by the level in the drum to ensure constant condensation when the incoming PF is in vapor form.

Figure 3.12. Fractionator for separating ethylene and ethane with a refrigerated condenser. FC on feed, reflux, and steam supply. LC on bottom product and refrigerant vapor. Pressure control PC on overhead vapor product.

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