Absorption Chiller Technologies

Equipment Classifications

Currently, the two absorption cycles in widespread use are the lithium bromide (LiBr) cycle and ammonia-water (NH3H2O) cycle. In the LiBr cycle, water acts as the refrigerant and LiBr is the absorbent. In the ammonia-water cycle, an

Fig. 38-2 Illustration of Single-Effect Absorption Cooling Cycle. Source: The American Gas Cooling Center ammonia-water solution acts as the refrigerant with water as the absorbent. Most of the large capacity units on the market today use the LiBr cycle. The ammonia-water cycle is most commonly used in small-capacity direct fuel-fired single-effect units or larger capacity custom-designed systems for low-temperature industrial process applications.

Two variations are used with LiBr cycle absorption chillers: single-effect or single-stage cycles (Figure 38-2) and double-effect or two-stage cycles (Figure 38-3). In two-stage designs, which are more thermally efficient, heat recovered from the first stage condensing process is used to boil additional refrigerant at a lower temperature in the second stage.

Absorption machines can also be classified by energy source:

• Direct-fired units, which have an integral burner assembly to combust fuels such as natural gas, propane, and light distillate oils.

• Indirect-fired units, which use steam or thermal fluid from an outside source to power the cycle.

• Exhaust gas or heat recovery units, which are custom-designed machines that can be directly connected to a heat source such as an engine.

Common characteristics of all currently available absorption machines are:

• Simplicity of design with few moving parts and operation at relatively low temperatures and pressures.

• Low electrical requirement.

• High rate of heat rejection, typically from about 21,000 Btu/ton-h (1.75 kWhh/kWhr) to 30,000 Btu/ton-h (2.50 kWhh/kWhr). This necessitates more cooling tower capacity with higher pump and fan energy use, relative to conventional electric-driven vapor compression systems.

• Large physical size and weight. The operating weight of a 100 ton (352 kW^) unit, for example, is typically about 10,000 to 12,000 lbm (4,500 to 5,400 kg) with dimensions of 13 ft (4.0 m) long by 4.5 ft (1.4 m) wide by 7.5 ft (2.3 m) high. The operating weight of a 1,000 ton unit is typically about 65,000 to 75,000 lbm (29,000 to 34,000 kg), with dimensions of 25 ft (7.6 m) long by 8 ft (2.4 m) wide by 12 ft (3.7 m) high.

• Use of low-global warming and ozone-safe refrigerants.

Single-Effect LiBr-H2O Cycle Absorption Systems

Low-pressure steam or hot water can be used as an energy source in single-effect systems. Typical temperature requirements range from 200 to 270°F (93 to 132°C).

Exchanger

Exchanger

Chilled Water

Cooling Water

Fig. 38-3 Illustration of Double-Effect Absorption Cooling Cycle. Source: The American Gas Cooling Center

Chilled Water

Cooling Water

Fig. 38-3 Illustration of Double-Effect Absorption Cooling Cycle. Source: The American Gas Cooling Center

Heat Exchanger

Heat Exchanger

Steam-powered systems are generally designed for operation at pressures between 9 and 15 psig (1.6 to 2.0 bar). When hot water or steam temperatures are below design specifications, chiller capacity is reduced.

Although single-effect absorption technology is relatively thermally inefficient by today's standards, it is useful when steam costs are low or when recovered heat can be used. Under ARI conditions, full-load steam rates typically range from 18.3 to 19.0 lbm/ton-h (2.4 to 2.5 kg/kWhr), corresponding to COPs between 0.65 and 0.69. At 80 to 87% boiler efficiency, gross heat input ranges from 20,000 to 23,000 Btu/ton-h (1.67 to 1.92 kWhh/kWhr) on an HHV basis, reducing COP to between 0.60 and 0.52.

Figure 38-4 is a cutaway illustration of a single-effect absorption chiller. Figure 38-5 shows a single-shell design and Figure 38-6 shows a two-shell design in which the upper shell houses the generator and condenser and the lower shell houses the absorber and evaporator. Representative part-load performance data is provided in Figure 38-7.

1. Purge system to expel non-condensable gases from units external purge chamber

2. Service access to controls and service points

3. Stablizer control to limit excessive cycling due to rapid reduction in load or condenser water temperature

4. Unloader control to maintain refrigerant level for proper pump operation during low-capacity and condenser-water conditions

5. Hermetic pumps

6. Decrystallization system to automatically correct minor crystallization

7. Double-walled evaporator

8. Water and generator pass heads

1. Purge system to expel non-condensable gases from units external purge chamber

2. Service access to controls and service points

3. Stablizer control to limit excessive cycling due to rapid reduction in load or condenser water temperature

4. Unloader control to maintain refrigerant level for proper pump operation during low-capacity and condenser-water conditions

5. Hermetic pumps

6. Decrystallization system to automatically correct minor crystallization

7. Double-walled evaporator

8. Water and generator pass heads

Power requirements for internal auxiliary components such as solution, refrigerant, and purge pumps and controls typically range from 0.01 to 0.04 kW/ton (0.003 to 0.01 kWe/kWT), with a minimum of 0.004 kW/ton (0.001 kWe/kW for some smaller machines. Heat rejection, which is close to 30,000 Btu/ton-h (2.5 kWhh/kWhr), requires cooling water flow rates of between 3.6 and 6.4 gpm/ton (3.9 to 6.9 lpm/kW).

Double Effect LiBr-^O Cycle Absorption Systems

Double-effect, or two-stage, absorption systems are shown schematically in Figure 38-8. The double-effect cycle uses a second generator, condenser, and heat exchanger that operate at higher temperature. Refrigerant vapor is recovered from the first-stage generator in the high-temperature condenser. The heat from this condensing process is used

Fig. 38-5 Single-Effect LiBr Absorption Chiller with Single-Shell Design. Source: The Trane Company

Fig. 38-4 Cutaway Illustration of Single-Effect LiBr Absorption Chiller. Source: York International

Fig. 38-6 Single-Effect Absorption Chiller with Two-Shell Design. Source: Carrier Corp.
Fig. 38-7 Energy Input vs. Load at Various Cooling Water Temperatures for Single-Effect Absorption Chiller. Source: The Trane Company

to boil additional water from the low-temperature, second-stage generator. In addition, a recuperative heat exchanger is used to recover heat from solution leaving the low-temperature generator. As a result of this double-effect, a thermal efficiency improvement of about 70% is achieved versus the single-stage cycle.

Operating temperature for double-effect units is about 370°F (188°C), corresponding to a saturated steam pressure of about 115 psig (8.9 bar). Typical full-load steam rates, under ARI standard conditions, range from 9.7 to 10 lbm/ton-h (1.25 to 1.29 kg/kWhr), with corresponding COPs of 1.22 to 1.19. At 80 to 87% boiler efficiency, gross heat input ranges from 11,300 to 12,600 Btu/ton-h (0.94 to 1.05 kWhh/kWhr), reducing the range of COPs to 1.08 to 0.96. Direct-fired units typically consume between 11,800 and 13,400 Btu/ton-h (0.99 to 1.11 kWhh/kWhr), on an HHV basis, with COPs of 1.02 to 0.90. Figure 38-9 shows a double-effect absorption machine.

Double-effect design options include series flow, parallel flow, and reverse flow machines. With the series flow design, shown in Figure 38-10, a weak solution is pumped through low-temperature and high-temperature heat exchangers in series before entering the primary generator. In the parallel flow design, the solution stream leaving the low-temperature heat exchanger is split between the high-temperature heat exchanger and a secondary low-temperature generator. In the reverse flow design, weak solution is preheated by hot vapor from the first-stage generator before passing into the second-stage generator.

As with single-effect systems, capacity of double-

High Temp. Generator

Heat Input

(Refrig. Vapor)

Second State Generator

Heat Input

Heat Exchanger

(Refrig. Vapor)

Condenser

Heat Exchanger

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