Ammonia Water Absorption Cycles

Ammonia-water (NH3-H2O) based absorption machines have been used for most of this century, notably in residential refrigerators and air conditioners. Presently, the technology is used in custom-designed systems for low-temperature industrial applications and in small-capacity, direct-fired air-cooled units. Typical units are 3, 4, or 5 tons (10, 14, or 17 kW^) capacity, with up to five units assembled on a single skid. Figure 38-23 shows a 50 ton (166 kWr) system featuring two 25 ton (88 kWQ factory-assembled modules. Larger capacity, higher efficiency models are becoming more common and are the subject of extensive R&D efforts.

In the ammonia-water cycle, water acts as the absorbent and the ammonia-water solution acts as the refrigerant. Both the refrigerant and the absorbent boil in the generator, and the regeneration of strong absorbent is a fractional distillation process. Figure 38-24 shows a schematic diagram of an ammonia-water absorption chiller. In this diagram, heat energy is designated Q, with the arrows showing the direction of flow. A refrigerant heat exchanger (RHX), also called a precooler, is commonly placed upstream of the evaporator to transfer heat to the absorber.

Currently, double-effect LiBr absorption machines achieve higher cooling cycle COPs than do ammonia-water units. There are, however, several advantages offered by ammonia-water cycle systems. A limitation of LiBr machines is their inability to achieve evaporator temperatures below about 40°F (4.4°C). In contrast, the ammonia-water cycle allows extremely low evaporator temperatures to be achieved, and for very low applications, is competitive with vapor compression technology on a cost and efficiency basis. Such systems have been effectively applied for various process refrigeration applications, including ice storage.

The ammonia-based refrigerant also allows the cycle to be operated at condenser pressures of up to 300 psia (20.1 bar) and at evaporator pressures of about 70 psia (4.8 bar), allowing vessels to be under 6 in. (15 cm) diameter. The solution pumps in small-capacity units must be positive-displacement types. Also, due to the higher condenser pressures, ammonia-water systems can be more effectively air-cooled than LiBr-based systems, avoiding the capital cost and complexity associated with evaporative cooling.

GAX Cycles

One of the most promising areas of absorption technology development involves the generator-absorber heat exchange (GAX) cycle. While this cycle has gone undeveloped since its invention at the beginning of this century, its simplicity and potential for adaptation to more efficient variations has led to recent intensive development activity. In

Fig. 38-23 Two 25-Ton Factory-Assembled, Direct-Fired, Air-cooled

Ammonia Absorption Chiller Systems on Single Skids.

Source: Robur Corporation and The American Gas Cooling Center

Fig. 38-23 Two 25-Ton Factory-Assembled, Direct-Fired, Air-cooled

Ammonia Absorption Chiller Systems on Single Skids.

Source: Robur Corporation and The American Gas Cooling Center the GAX cycle, generation (desorption) and absorption each occur over a large temperature glide (a wide concentration range), allowing for a temperature overlap between the hot end of the absorber and the cold end of the generator. In this overlap zone, heat can be transferred from the absorber to the generator, directly or through a circulating heat transfer fluid. As this temperature overlap is increased, the potential cycle COP is increased. In addition, heat can be recovered from the absorber section at temperatures of up to 180°F (82°C), making the cycle well-suited for heat pump application. This is one of the primary focuses of this developing technology.

The basic GAX-cycle system, shown in Figure 38-25, has two pressure levels and a single sorbent circulation path. There is no temperature overlap until the generator temperature increases to a point at which the temperature difference between the generator and the condenser is about 2.6 times the lift (temperature difference between the condenser and the evaporator). At full load, high COP can only be achieved when the input energy is significantly hotter than required for a conventional single-effect absorption machine. Under part load, at reduced lift, temperature overlap increases, improving COP.

In one design variation, the branched GAX cycle, solution flow through the temperature overlap portion of

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