Friday, April 03, 2009

Absorption Chillers: Think Large Air Conditioner

Absorption chillers use thermal energy sources to produce chilled water. Water has a very high specific heat and latent heat of vaporization, which makes it a great refrigerant. How is water boiling at 212°F going to create chilled water at 44°? First, the boiling temperature of water is a direct function of pressure and at a pressure of 1 atmosphere (29.92 Hg), water boils at 212°. When the pressure on the water is decreased, the water boiling temperature is lowered. Absorption chillers have substantially reduced internal pressures to take advantage of the lower water boiling temperatures. Absorption chiller internal pressures can range from 0.1 atmosphere (atm) to below 0.01 atm. The Capitol Power Plant uses an absorption chiller to produce chilled water to provide air conditioning to Congress (below).

There are a number of absorption chillers available, including single-effect indirect-fired (steam, hot water); double-effect indirect-fired; and double-effect direct-fired (gas and/or oil burner). Single-effect absorption chillers have a single generator/concentrator and condense all vaporized refrigerant in a single condenser. Double-effect absorption chillers have two generator/concentrators and the vaporized refrigerant from the high temperature generator/concentrator is the thermal source for the low temperature generator/concentrator, reducing the cooling requirement for the vaporized refrigerant (See Figure 2).

The single-effect indirect-fired absorption chiller has five main steps (Figure 1): 1) condensing (condenser), 2) expansion (expansion pipe), 3) evaporation (evaporator), 4) absorption (absorber), and 5) generator/concentrator.

1) Condenser. In the condenser, the cooling water absorbs the heat of condensation from the vaporized refrigerant, changing the refrigerant into a liquid.

2) Expansion. The liquid refrigerant (water) travels from the condenser through expansion piping to the evaporator during which the liquid refrigerant experiences a drop in pressure and temperature.

3) Evaporator. The liquid refrigerant (water) is pumped to the chilled water tube bundle top and sprayed on the tube bundle. At the low evaporator pressure, the liquid refrigerant vaporizes at approximately 38°, removing energy from the chilled water. Liquid refrigerant that is vaporized travels from the evaporator to the absorber.

4) Absorber. The vaporized refrigerant enters a liquid lithium-bromide solution spray within the absorber. The lithium bromide solution absorbs the vaporized refrigerant and the cooling water absorbs the heat of vapor absorption. After the absorption, the liquid lithium-bromide solution takes one of two paths. One path has the liquid bromide solution mixing with a concentrated lithium bromide solution and being pumped to the absorber spray nozzles. The other path has the liquid bromide solution being heated and pumped to the generator/concentrator.

5) Generator/concentrator. The lithium-bromide solution enters the generator/concentrator and is heated by steam or hot water, raising the lithium bromide solution to a temperature where the liquid refrigerant (water) vaporizes and travels to the condenser, completing the refrigerant cycle. The concentrated lithium bromide solution flows down to the absorber, completing the absorber cycle.

By Vincent A. Sakraida

For Engineered Systems

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