The coolers described in this specification are intended for use on board Navy surface ships and submarines for cooling lubricating oil , hydraulic oil, and fresh water. No one cooler is intended to cool more than one of these three categories of fluid in any one application. The coolers are divided into two types according to their design and to whether the fluid to be cooled is the inner fluid or outer (see 1.2). The coolers are also divided into five classes. according to whether they are for use on submarines or surfaces ships, whether they use seawater or fresh water as cooling medium, and whether they are to be subjected to seawater pressure at depths greater than 200 feet (see 1.2). coolers of Type I may be of any class;coolers of type II may be of any class except class 1.