A process engineer of a refinery wants to purchase a new reactor for the process. He wants to replace the old reactor with the similar type of reactor available in the market. In order to purchase fruitfully he wants to calculate the depreciation of the reactor. He has three methods of calculation with him. These are declining balance, double declining balance and straight line method. As a process engineer decide which method is more convenient, realistic and matched with the real data. Establish your statement with one example. 

Also he knows that the piping and building cost is depends on the type of processing plant. As a contractor of the processing unit explain how and why these costs are depending on the type of processing plant?