For the low pressure cylinder the oil is fed into the receiver, and usually at a point near the slide valves.
Large marine cylinders are usually constructed with a separate lining, which may be replaced when worn or otherwise required.
A surface condenser consists of a cast iron shell or chamber forming the back of the engine frame. At each end of this chamber is a short partition, so that the condenser is divided lengthways into what may be called three compartments, of which the middle one is the longest and contains a number of thin brass tubes about 5⁄8 or 3⁄4 inch in diameter, the ends of these tubes being held in the plates or tube sheets forming the partitions. The object of providing tubes of small diameter is to obtain a large area of cooling surface.
The exhaust steam from the engine generally passes into the shell or body of the condenser, filling the middle partition and surrounding the tubes.
The condensing or circulating water passes through the tubes, and by keeping them cool condenses the steam and forms a vacuum or partial vacuum in the condenser, which, having open communication with the low pressure cylinder, therefore gives a corresponding degree of vacuum on the exhaust side of the low pressure piston.
In some designs, however, the steam passes through the tubes and the circulating water fills the middle compartment of the condenser. As, however, there is no pressure to counterbalance the weight of the water, it is preferable to have the water inside the tubes, so that they are subjected to a bursting pressure, in which case they may, for a given strength, be made thinner, because the strength of the tube to resist bursting is greater than its strength to resist collapsing, hence the circulating water usually passes through the tubes. The chamber at the ends of the condenser permits the water to distribute through all the tubes.
In some cases the chamber at one end is divided horizontally into two compartments, so that the water is compelled to pass through one half and return through the other half of the tubes.
The water of condensation falls to the bottom of the condenser, from which it is removed by the air pump, which delivers it to the hot well.
The hot well is situated on the side of, and extends above, the pump, whose upper end it covers, thus water sealing the top of the air pump and preventing air from passing into it through a leaky valve or bucket.
The top of the hot well is provided with a vapor pipe, which permits the air and gases to pass overboard. This pipe emerges through the side of the ship above the water line, and as there is no valve between the hot well and the sea, no pressure can possibly accumulate in the hot well.