The refrigerant used absorbent in the absorption unit must have certain characteristics in order for the device to work effectively. The following list higjiliglits some important qualities.
Currently, there are three refrigerant-absorbent in the combination of common use. The oldest in terms of the use of ammonia and water.
In these systems, ammonia refrigerant and water absorbent. Another combination of water and lithium bromide. In these systems the water is the refrigerant and lithium bromide, hygroscopic salts, absorbent. Both combinations are still used. Ammonia-water and waterlithium bromide is used in large systems, cooling water, and ammonia-water-hydrogen mixtures are used in small domestic refrigerator applications. The relative advantages and disadvantages of these systems are discussed in the following sections.
- Both fluids must be safe, stable and noncorro-sive, both individually and in solution.
- Absorbent should have a strong affinity for the refrigerant vapor.
- Tlie spacer two fluids should be mutually soluble in the desired range of operating conditions.
- Ideally, the youth, must be low volatility, so that the refrigerant vapor leaving the generator will carry little or no absorbent.
- The refrigerant must have a high latent heat value to maintain reasonable refrigerant flow rates.
- Working fluid pressure, must be low enough, preferably close to atmospheric pressure. This minimizes equipment nested structure and leakage in the system.
Ammonia - Water systems Am monia-water systems, widely used in commercial and industrial refrigeration systems, where the temperature of the evaporator below 32F (0C). Ammonia refrigerant is high latent heat, reducing the required refrigerant charge and size of equipment. Water absorbent has a very strong affinity for ammonia vapors produced in the evaporator. These two mutually soluble substances in a wide range of operating conditions, are highly stable and compatible with most materials found in refrigeration systems. One notable exception is copper and its alloys. These materials are not suitable for use in any system incorporating ammonia. Ammonia refrigerant high working pressure and slightly toxic, which limits the possibility of their use in smaller charge and industrial applications.
One of the main drawbacks of an ammonia-water system is that absorbent material (water) is a fairly volatile in this application. Therefore, refrigerant (ammonia) pair leaving tlie spacer generator, usually contain a large amount of water vapor. If this ability pair is permitted to pass from the condenser to the evaporator, it will raise the temperature of the evaporator, reducing the cooling effect. Water vapor is also cariy liquid refrigerant from the evaporator, further reducing capacity. For these reasons, the effectiveness of ammonia-water system improvements, adding analyzer and rectifier (reflux) to the system.