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Anhydrous Ammonia Vapor Withdrawal

 

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Ammonia is shipped and stored under pressure in liquid form, but it is most often used in its vapor state.  A specific amount of heat called "latent heat of vaporization" is necessary to convert each pound of liquid ammonia into a gas (vapor).  

 

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Approximately 500 BTU's per pound is the required heat necessary to provide the latent heat of vaporization to maintain a specific vapor withdrawal rate.  If this heat is not available from the ambient air surrounding the tank, the heat will be taken from the liquid ammonia remaining in the tank, causing the liquid temperature to drop.

 

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When the liquid temperature drops it causes the vapor pressure to decrease.  If the vapor pressure drops too low, it can reach a point where there is an insufficient amount of pressure to operate the system.  

 

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The situation is further compounded when the temperature of the ambient air surrounding the tank drops and the moisture in the air freezes on the tank surface.  The ice acts as an insulation, further preventing heat from entering the tank.  

 

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A vaporizer is used in situations where the vapor withdrawal rates are so high that ambient heat is not sufficient to sustain the vapor flow.  As a general rule, a vaporizer should be utilized when the vapor withdrawal rate of a 1000 gallon tank exceeds 500 SCFH.

 

Anhydrous Ammonia Vaporizers

 

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Vaporization takes place when liquid ammonia vaporizes and replaces the vapor that is being withdrawn from the top of the tank.  A vaporizer is needed if the heat from the air surrounding the tank is not sufficient or cannot flow through the walls of the tank fast enough to maintain the vaporizing rate.  If sufficient heat is not available, the pressure in the tank will drop causing a system shut-down.  

 

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A vaporizer will close the pressure switch electrical contacts and turn on the heaters inside the vaporizer shell when the pressure drops below the pressure switch setting.  Liquid will flow out of the bottom of the tank and into the vaporizer shell where it is heated and converted to vapor.  The vapor then flows to the top of the tank where it can be withdrawn to maintain the vaporizing rate of the system.

 

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The requirement or need for this supplemental heat is dependant on several factors:  the ambient temperature of air surrounding the tank, the air movement around the tank, the volume of liquid within the tank and the vapor withdrawal rate.  

 

Ammonia Vaporizer Skid

 

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Vaporizer packages are available in two (2) configurations:  

               Option 1:     Steam ammonia vaporizer with welded steel mounting brackets

               Option 2:    Electric heating elements with ASME code vaporizer shell

 

Ammonia Pump Skid

 

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Pump packages include:  Two (2) NH3 pumps with strainers, by-pass relief, isolation valves and pressure gauges.

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Each pump can deliver from 15 to 200 gallons per minute of liquid ammonia @ 75 psi differential, depending on size and flow requirements.

 

Typical Anhydrous Ammonia Vaporizer Skid

 

                             

 

 

 

Copyright [2003] [R.M. Technologies Inc.]

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