For some purposes, it is convenient to classify boilers according to operating pressure. Most classification of this type are approximate rather than exact. Header-type boilers and some older drum-type boilers are often called 400-PSI BOILERS even though their operating pressures range from about 435 psi to 700 psi.

The term high-pressure boiler is at present used rather loosely to identify any boiler that operates at a substantially higher pressure than the socalled 600-PSI BOILERS. In general, we will consider any boiler that operates at 751 psi or above as a high-pressure boiler. Many boilers in naval ships operate at about 1200 psi. These boilers are referred to as 1200-PSI BOILERS.

As you can see, classifying boilers by operating pressure is not very precise since actual operating pressure may vary widely within any one group. Also, any classification based on operating pressure may easily become obsolete. What is called a high-pressure boiler today may well be called a low-pressure boiler tomorrow.

BOILER COMPONENTS

Boilers used onboard naval ships have essentially the same components: steam and water drums, generating and circulating tubes, superheaters, economizers, and accessories and fittings for controlling steam pressure and temperature and other aspects of boiler control and operation.

Figure 4-1shows a cutaway view of a D-type boiler. You should refer to this figure as a guide to the arrangement of the boiler components. As we discuss the boiler and its components, imagine that you are assembling a similar boiler. As you

Figure 4-1.-Cutaway view of a D-type boiler.

add each part to your boiler, follow the line drawings introduced in the following paragraphs that describe the position of each component.

STEAM DRUM

The steam drum is a cylinder located at the top of the boiler. It runs lengthwise fig 4-1 from the front to the back of the boiler. The steam drum provides a space for the saturated steam generated in the tubes and for the separation of moisture from the steam. (Remember, saturated steam is steam that has not been heated above the temperature of the water from which it was generated). The steam drum also serves as a storage space for boiler water, which is distributed from the steam drum to the downcomer tubes. During normal operation, the steam drum is kept about half full of water. The steam drum either contains or is connected to many of the important controls and fittings required for the operation of the boiler.

At the bottom right side of the boiler you will find the water drum, and on the bottom left side

Figure 4-2.-Steam drum, water drum, and header.

is the sidewall header fig 4-2. Notice the header is smaller than the water drum. Most boilers have more than one header. They are identified by their location. For example, a header at the back of the boiler is called a rear wall header. A header on a screen wall is called a screen wall header.