- Education and Science
Shell and Tube Heat Exchanger
Shell and tube heat exchangers are normally used for lubricating oil and jacket water cooling systems. Seawater flows through the tubes and jacket water or lube oil is passed around the tubes (shell).
Alternatively in central cooling system, seawater is replaced by central cooling fresh water, which circulates as a closed system, there by protecting the contact surfaces from corrosion. Baffles are extensions in the shell side which direct the medium to be cooled across the tubes as it flows through the cooler, which ensures maximum surface contact time with the tube surface hence ensuring better cooling efficiency. The baffles have also a function as tube supporter.
Material for the tubes are aluminium-brass (76% copper, 22% zinc, 2% aluminium) which is corrosion and erosion resistant. Commonly used ferrous metals or simply brass cannot be used. It was observed that not only the presence of aluminium brass but also the formation of a protective film formed throughout the tube surface by the deterioration of iron, helps to prolong the tube life. It should be kept in mind that this iron coating is coming from the ferrous parts of the cooler such as water boxes. Here less noble ferrous metal is corroded there by protecting more noble aluminium brass. Considering the heat exchanger as a whole this results in increased maintenance costs by frequent renewal of water boxes. To avoid these difficulties as a remedy sacrificial soft iron or mild steel anodes are fitted to the water boxes which introduces ferrous ions to protect the system. Alternatively iron can be fed directly to the system as 'ferrous sulphate' in a prescribed dose.
Tubes are normally expanded to the tube plate by expanders. Hence there is only metal to metal contact between tubes and tube plate. Alternatively after expanding tubes may be soldered to the plates.
In coolers with aluminium brass tubes, normally naval brass tube plates are used. Alternatively aluminium bronze, special alloys, gun metal, etc. also can be used. As a cooler is concerned medium to be cooled and cooling water are at different temperatures. Also cooler construction consist of using different materials as seen above. This cause the tubes to get expanded and contracted during working. For accommodating these expansions different construction methods are followed. One common construction technique is to fix one end of the tube plate shell body and the other end loosely connected to the body with proper sealings to allow axial movement of the tubes.
Water Boxes & Covers
Water boxes are usually made from cast iron or mild steel. They can be easily detached from the cooler facilitating easy cleaning of the tubes. Water boxes may coated with rubber lining or special paints for their protection. Aluminium brass tubes, coated water boxes fitted with sacrificial anodes runs for a long time without giving any maintenance problems. Only sacrificial anodes to be checked and replaced if necessary while cleaning the tubes.
Flow rates through the tubes to be carefully designed since higher flow velocity through the tubes cause erosion and deterioration of the cooler parts. The combination of coated cast iron water box, sacrificial anodes, naval brass tube plates and aluminium brass tubes found to be the best in long run.
The shell houses tube stack and tube plates. Medium to be cooled flows through the shell. If oil is to be cooled, it offers to corrosion problems to the shell. In case of jacket water chemical inhibitors are added to prevent scale formation and corrosion. While maintenance no serious material deterioration is observed since shell is not in contact with seawater.