Ammonia synthesis reactors differ in the method used to recover the reaction heat and control the temperature of the reactants. There are two methods used; the quenching and the indirect cooling methods. 

In the quenching method (direct cooling), a part of the recycle gas enters the first catalyst bed at about 400^oC and exits at 500^oC. The gas temperature is then lowered (quenched) prior to its entrance into the following catalyst beds, by the insertion of a cooler (150-200^oC) recycle gas (cold shot) (UNIDO, 1979 p.173). The introduction of the cooling gas lowers the ammonia concentrations and decreases the temperatures at the exit of the reactor. 

In the indirect cooling method, heat exchangers are used to cool the gas and recover heat out of each catalyst bed. The heat recovered can then be used to produce high-pressure steam, preheat the boiler feed-water and even heat the incoming syngas to the desired inlet temperature. Reactors employing the indirect cooling method have lower energy requirements than reactors employing the quenching method (ECCJ, 1999, p.149)

Other possible benefits from adopting the indirect cooling method are higher conversion rate per pass and lower requirements in catalyst volumes (IPTS/EC, 2007 p.84).

This measure is applicable for all feedstocks and for both retrofit and new plants.