Furthermore, water's pressure ratio is very high, imposing additional challenges for compressors that must operate in a deep vacuum. One such system provides cooling to the lego factory in Denmark, another to a mine in South Africa. In both cases, open systems are employed, and the need for heat exchangers is eliminated by using direct-contact heat exchange. Thus, the chilled water that circulates through the facility is also used as the refrigerant. Although various research projects are underway worldwide, a demanding compressor technology, the need for vacuum pumps, and degassing remain great challenges in terms of cost and design.
Ammonia is also a very good refrigerant and is used to a significant extent in large warehouses.
Ammonia is toxic and, under certain limited conditions, flammable and even explosive. However, with its intense, pungent odor, it is a self-alarming refrigerant. Ammonia has emerged as a refrigerant for water chillers in Europe. These units are entirely self-contained, including a gastight cabinet that houses the entire unit and a water tank to dissolve any ammonia in case of a leak. These measures, to be sure, increase costs considerably.