The two-fluid absorption refrigeration system succeeded in replacing a compressor which requires a large amount of shaft work by a liquid pump with a negligible energy requirement compared to the refrigeration effect. By addition of a third fluid, the pump is removed, completely eliminating all moving parts. This system is also called the von Platen-Munters system after its Swedish inventors. This type of system is shown in Figure 3.48. The most commonly used fluids are ammonia (as refrigerant), water (as absorbent), and hydrogen, a neutral gas used to support a portion of the total pressure in part of the system. Hydrogen is called the carrier gas. The unit consists of four main parts: the boiler, condenser, evaporator and absorber. In gas units, heat is supplied by a burner, and when the unit operates on electricity the heat is supplied by a heating element. The unit charge consists of a quantity of ammonia, water and hydrogen at a sufficient pressure to condense ammonia at the room temperature for which the unit is designed. This method of absorption refrigeration is presently used in domestic systems where the COP is less important than quiet trouble-free operation. In the system shown in Figure 3.48, the cold ammonia vapor with hydrogen is circulated by natural convection through a gas-gas heat exchanger to the absorber, where ammonia vapor comes in contact with the weak solution from the separator. At the low temperature of the ammonia and hydrogen, absorption of the ammonia occurs and hence hydrogen alone rises through the heat exchanger to the evaporator, while the strong solution flows down by gravity to the generator.