Physical Properties

Anhydrous ammonia is a clear liquid that boils at a temperature of -28°F. In refrigeration systems, the liquid is stored in closed containers under pressure. When the pressure is released, the liquid evaporates rapidly, generally forming an invisible vapor or gas. The rapid evaporation causes the temperature of the liquid to drop until it reaches the normal boiling point of -28°F, a similar effect occurs when water evaporates off the skin, thus cooling it. This is why ammonia is used in refrigeration systems.

Liquid anhydrous ammonia weighs less than water. About eight gallons of ammonia weighs the same as five gallons of water.

Liquid and gas ammonia expand and contract with changes in pressure and temperature. For example, if liquid anhydrous ammonia is in a partially filled, closed container it is heated from 0°F to 68°F, the volume of the liquid will increase by about 10 percent. If the tank is 90 percent full at 0°F, it will become 99 percent full at 68°F. At the same time, the pressure in the container will increase from 16 pounds per square inch (psi) to 110 psi.

Liquid ammonia will expand by 850 times when evaporating:

• Anhydrous ammonia gas is considerably lighter than air and will rise in dry air. However, because of ammonias tremendous affinity for water, it reacts immediately with the humidity in the air and may remain close to the ground.
• The odor threshold for ammonia is between 5 - 50 parts per million (ppm) of air. The permissible exposure limit (PEL) is 50 ppm averaged over an 8 hour shift. It is recommended that if an employee can smell it they ought to back off and determine if they need to be using respiratory protection.

To learn more about Ammonia Refrigeration visit our Ammonia Refrigeration Safety Online Training web page.