In practical ARSs the utilization of one or two heat exchangers is very common. Figure 3.47 represents a practical absorption refrigeration system using a working fluid of ammonia as the refrigerant and water as the absorbent, with two exchangers. As can be seen from the figure, in addition to two heat exchangers, this system employs an analyzer and a rectifier. These devices are used to remove the water vapor that may have formed in the generator, so that only ammonia vapor goes to the condenser.
The system shown in Figure 3.47 utilizes the inherent ability of water to absorb and release ammonia as the refrigerant. The amount of ammonia vapor which can be absorbed and held in a water solution increases with rising pressure and decreases with rising temperature. Its operation is same as the system given in Figure 3.46, except for the analyzer, rectifier, and heat exchangers. In the absorber, the water absorbs the ammonia at the condenser temperature supplied by circulating water or air, and hence a strong solution (about 38% ammonia concentration) occurs.
Because of physical limitations, sometimes complete equilibrium saturation may not be reached in the absorber, and the strong solution leaving the absorber may not be as fully saturated with water as its pressure and temperature would require. This strong solution from the absorber enters the solution pump (the only moving part of the system), which raises its pressure and delivers the solution into the generator through the heat exchanger. Pumped strong solution passes into generator via heat exchanger where strong solution is preheated before being discharged into ammonia generator. Note that the pumping energy required is only a few percent of the entire refrigeration energy requirement. The generator, which is heated by an energy source (saturated steam or other heat source via heating coils or tube bundles), raises the temperature of the strong solution, causing the ammonia to separate from it. The remaining weak solution (about 24% ammonia concentration) absorbs some of the water vapor coming from the analyzer/rectifier combination and flows down to the expansion valve through the heat exchanger. It is then throttled into the absorber for further cooling as it picks up a new charge of the ammonia vapor, thus becoming a strong solution. The hot ammonia in the vapor phase from the generator is driven out of solution and rises through the rectifier for possible separation of the remaining water vapor. Then it enters the condenser and is released to the liquid phase. Liquid ammonia enters the second heat exchanger and loses some heat to the cool ammonia vapor. The pressure of liquid ammonia significantly drops in the throttling valve before it enters the evaporator. The cycle is completed when the desired cooling load is achieved in the evaporator. Cool ammonia vapor obtained from the evaporator passes into the absorber and is absorbed there. This absorption activity lowers the pressure in the absorber and causes the vapor to be taken off from the evaporator. When the vapor goes into liquid solution it releases both its latent heat and a heat of dilution. This energy release has to be continuously dissipated by the cooling water or air.
The heat introduced into the absorption system in the generator (from steam heat) and the evaporator (from actual refrigeration operation) has to be rejected to outside. One heat ejection occurs in the ammonia condenser and other heat ejection occurs in the ammonia absorber. Reabsorpion of ammonia into weak solution generates heat and unfortunately this heat has to be rejected so the absorption process can function. Aqua ammonia consists of water and ammonia. Water can easily absorb ammonia and stay in solution under normal temperature, hence the absorber has to be cooled with cooling water or air. Evaporated ammonia in the generator is passed through the distilling column where the ammonia is concentrated into nearly pure ammonia vapor before going into the condenser. Once ammonia is turned into liquid it is let down into the evaporator, low pressure side, where ammonia is again turned into vapor, by evaporation, while picking up heat from the confined refrigerated space. Ammonia vapor is then absorbed in the absorber to complete the cycle.
For ammonia-water ARSs, the most suitable absorber is the film-type absorber for the following reasons :
* high heat and mass transfer rates,
* good overall performance, and
* large concentration rates.