Figure 1: The adsorption process


Initial state Ambient Temperature Low pressure


High concentration 40° C


40° C 0° C


170° C


Process 1


Carbon bed is heated, ammonia is driven off and pressure increases until…


170° C


Pressure


40° C Flow


40° C 0° C


Heat Input


Process 2 starts


The saturation pressure is reached and ammonia condenses in the right hand vessel at ambient temperature.


170° C


Pressure


Process 2 continues


More ammonia is driven out from the carbon and condensed in the right hand vessel.


40° C Flow


40° C 0° C


170° C


Pressure


40° C Flow


40° C 0° C


Heat output


Heat Input


Heat input


Process 3


The carbon is cooled, the concentration increases and the pressure drops.


170° C


Pressure


Process 4


The carbon is cooled towards ambient and the concentration increases.


40° C Flow Heat rejected


40° C 0° C


Ammonia boils in the right hand vessel giving the refrigerating effect.


170° C


Pressure


40° C Flow Heat rejected


40° C 0° C


Cooling load


End of Process 4


The system is returned to the starting condition


170° C


Pressure


40° C


40° C 0° C


continuous. The adsorbent (the active carbon) is heated to drive off the working fluid (ammonia) and then subsequently allowed to cool again to readsorb the ammonia. For commercial systems the adsorber is split into a number of ‘beds’ and arranged so that while one or more beds are adsorbing the ammonia, others are being heated (‘desorbed’ or ‘regenerated’) to drive off the ammonia so that the vapour is made available again for the cooling process. The adsorbent beds then switch their function (using changeover valves to redirect the ammonia refrigerant) so that those now saturated with ammonia can be


30 CIBSE Journal August 2011 www.cibsejournal.com


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