Following is a description of the cooling cycle in a direct-fired, two-stage, absorption liquid chiller, powered by natural gas. Absorption cooling relies on three principles: 1) Liquids, such as water, boil (evaporate) at a lower temperature when under low pressure; 2) Lithium bromide has a strong affinity for water and readily absorbs it, and; 3) in a sealed (hermetic) system a vacuum is created when water in the system is absorbed by lithium bromide. While the description of the process is broken down into stages to make the process easier to understand, in actuality the process is continuous.
1. Solution Pump/Heat Exchangers
A dilute solution of lithium bromide and water descends from the Absorber to the Solution Pump which moves the dilute solution to a Heat Exchanger (there are actually two Heat Exchangers, but for simplicity, only one is shown). The flow of dilute solution is split into two streams. One stream is pumped to the First-Stage Generator, the other is pumped to the Second-Stage Generator.
2. First-Stage Generator
The dilute solution in the First-Stage Generator is heated by a gas burner. Heating the dilute solution boils off much of the water, leaving a very concentrated lithium bromide solution. The concentrated solution is returned to the Heat Exchanger, while the hot water vapour goes to the Second-Stage Generator.
3. Second-Stage Generator
The dilute solution in the Second-Stage Generator is heated by the hot water vapour from the First-Stage Generator. As the water vapour gives up its heat it condenses to liquid. As in the First-Stage Generator, heating the dilute solution produces hot water vapour and concentrated solution. The concentrated solution goes to the heat exchanger while both the water vapour and the liquid water go to the Condenser.
4. Condenser
As it enters the Condenser, the liquid water vapourizes due to the low pressure inside the condenser. This water vapour combines with the water vapour coming from the Second-Stage Generator. The combined water vapour condenses to liquid as it is cooled by the condenser water. The liquid then flows down to the Evaporator.
5. Evaporator
The Evaporator is where the system water used for cooling the facility's supply air is chilled. The liquid water coming from the Condenser is sprayed over the Evaporator tubes which carry the system chilled water. Due to the extreme vacuum in the Evaporator some of the water vapourizes. As the water vapourizes it draws heat from the returning system water. The chilled water is then resupplied to the system.
6. Absorber
The vacuum needed to vapourize the water in both the Condenser and the Evaporator is created by the absorption of water by the lithium bromide inside the Absorber. As the water liquid/vapour mix descends to the Absorber from the Evaporator, concentrated lithium bromide solution from the First and Second-Stage Generators is sprayed into the flow of the descending liquid/vapour mix and the water is absorbed by the lithium bromide. As the water is absorbed a vacuum is created. As the water also gives off heat as it is absorbed, which is removed by the condenser water. The resulting dilute lithium bromide solution collects in the Absorber where it flows down to the Solution Pump to begin the cycle again.
Gas Engine Chillers
A gas engine chiller, as shown in the schematic above, mates a natural gas internal combustion engine to a compressor-driven chiller.
The Natural Gas Engine
Besides providing the energy needed to drive the compressor, the gas engine also generates heat which, if recovered, can be used for other purposes, such as water heating. The heat recovery feature is partly what makes gas engine chillers cost less to operate over electric chillers.
The Vapour Compression Cycle
The underlying process of any compressor-driven chiller is the vapour compression cycle. The vapour compression cycle relies on two principles.
The first principle is that a fluid absorbs heat when changing from a liquid to a gas and releases heat when changing from a gas to a liquid. During this change in state, the temperature of the fluid does not change until all of the fluid is evaporated or condensed. The energy needed to change the state of a fluid is called latent energy or latent heat. A proportionately larger quantity of heat is needed to change state than to raise temperature. This is because when a fluid evaporates the molecular bonds that hold the fluid in a liquid state must be broken, and breaking molecular bonds requires a lot of energy.
The second principle is that the boiling point of a liquid changes with pressure. The lower the pressure, the lower the boiling point. In fact, it is possible to boil a liquid without adding heat, but simply by sufficiently lowering the pressure it is under. Following is an explanation of the vapour compression cycle.
1. The Compressor
Refrigerant enters the compressor as a gas. The compressor applies pressure to the gas, reducing its volume and raising its temperature. Compressing the refrigerant also raises the temperature at which the refrigerant will condense to liquid. After leaving the compressor the hot, compressed refrigerant gas travels to the condenser.
2. The Condenser
In the condenser, cold water is used to cool the refrigerant gas and condense it to liquid. While the temperature of the refrigerant changes, the pressure of the refrigerant is the same as it was when it left the compressor.
3. The Expansion Valve
After leaving the condenser, the high-pressure liquid refrigerant passes through an expansion valve that restricts the flow of refrigerant. The flow restriction reduces the pressure of the refrigerant. By reducing the pressure, the boiling point is lowered making the refrigerant more efficient at removing heat. As the pressure is reduced the refrigerant begins to expand and its temperature drops.
4. The Evaporator
After flowing through the expansion valve the refrigerant enters the evaporator. Due to the lower pressure in the evaporator, some of the refrigerant immediately evaporates and absorbs heat from the system water, thus chilling it. As the refrigerant flows through the evaporator, more and more of it evaporates as heat is absorbed. By the time the refrigerant leaves the evaporator it is entirely gaseous. It is then piped to the compressor where the cycle begins again.
