Refrigeration compressors are essentially a pump for refrigerant gas. Ethane can be used as a refrigerant in cryogenic refrigeration systems. It was originally developed as a refrigerant to replace Freon. Combining an adsorbent with a refrigerant, adsorption chillers use heat to provide a cooling effect. This spurred on the refrigeration industry, which long used ammonia as its refrigerant. One of the most widespread refrigerant is "Freon". That warm air evaporates the liquid part of the cold refrigerant mixture. Phosgene may also be produced during testing for leaks of older-style refrigerant gases. By the time the refrigerant is discharged, it is fully pressurized. The cooling tower set point used depends upon the refrigerant being used. It is commonly used as a propellant and refrigerant. Because of ammonia's vaporization properties, it is a useful refrigerant. Tetrafluoromethane is a useful refrigerant but also a potent greenhouse gas. It was replaced with R-134a refrigerant, which has no ozone depletion potential. The single-effect absorption cycle uses water as the refrigerant and lithium bromide as the absorbent.