The industry that processes carcasses and offal - the rendering industry - ensures that these animal remains are processed in a proper and harmless way that prevents diseases. These remains are unsuitable to be reprocessed into anything else but animal feed. The rendering process starts when carcasses and offal arrive. First, all parts are grounded and left to boil for twenty minutes at 130°C, then the 'slurry' is transferred to a flash tank.
The broth of meat is dried using the heat in disk dryers until it has reached a water content of 5%. Drying means boiling off the water content of the broth. The water vapor or steam that is formed as a result of this boiling has a recognizable smell and must therefore be condensed.
There are two ways to condensate the steam, the first way is at a pressure just above ambient, the second is at a vacuum. In an Air-Cooled Rendering Condenser at atmospheric pressure, the steam is condensed, and the condensate is cooled down to a maximum of 10°C above ambient. This is necessary as biological wastewater purification processes the condensate and active bacteria in the purification will die if temperatures reach more than 40°C.
The heat in the atmospheric condensers is transferred to the atmosphere and is lost. The other way is to boil down the vapor in a Shell and Tube heat exchanger. Part of the condensing heat can then be used in other parts of the process. The rest of the vapor must subsequently be condensed at a lower pressure, which also means a lower temperature, resulting in bigger condensers. For total plant efficiency this is of course the better solution.