Simple Vertical Boiler: Construction, Merits, Demerits, Applications

A simple vertical boiler is a type of fire-tube boiler. It is the most simple version of a vertical boiler used on an industrial level for small to moderate-scale applications. It was built and designed in the late eighteenth century and was subjected to industrial use by then.

Though it is obsolete today on account of its obsolete design, it is studied as a case boiler for building a sound foundation and authority in boiler engineering.

The following section elaborates on the construction of the simple vertical boiler first and then its working principle or operation.

Construction and Operation

1. Shell

It is a type of fire tube boiler with an upright cylindrical shell and an internal firebox arrangement. The diameter of the nearly cylindrical fire-box is considerably less than that of the shell. Likewise, the height of the fire-box is also less compared with the length of the shell.

2. Fire-Box

The round fire-box tube is made to run from the fire-box crown through the top of the shell to exit gases.

3. Fire-Hole

A fire hole is provided at the bottom of the shell for feeding fuel through the firebox walls.

4. Grate

The grate is provided at the bottom of the fire-box for the retention of fuel.

5. Uptake

The uptake tube is made to pass from the crown of the fire-box to that of the shell.

Simple Vertical Boiler with labeling

6. Chminey

At the top of the uptake tube is a chimney constructed to exit flue gases from the boiler.

7. Cross-tubes

The fire-box is fitted with two slightly inclined large cross tubes. These tubes are shown in the cross box in the figure above. They are water-filled, and their purpose is to enhance the circulation of water and to break up the flue gases, thereby absorbing their heat.

8. Shell Top

The top of the combustion chamber is rarely flat. In most cases, it is slightly dished and, therefore needs to be stayed (in place) to the top of the outer shell.

9. Shell Bottom

The bottom of the boiler is rigidified by the double thickness of the combustion chamber and the shell. It sits on an angle ring, which provides it with added structural stability and strength.

10. Hand-Holes

There are drilled hand holes for the purpose of cleaning and maintaining the cross-tubes.

11. Fusible Plug

It’s an accessory of the simple vertical boiler whose purpose is to protect the boiler from explosive damage due to overheating of water caused by low water levels.

12. Feed Check Valve

It’s a key valve: its purpose is to allow the unidirectional flow of water from the feedwater pump to the boiler and prevent the blow flow.

13. Steam Stop Valve

Its purpose is to regulate the flow of steam from the steam space to the steam line.

14. Safety Valve

The boiler safety valve is a pressure relief valve whose purpose is to release steam pressure when it reaches the boiler’s maximum sustainable or design pressure.

15. Man-Hole

A man-hole is designed to clean and inspect the boiler with quite ease. Typically, they are used where space is a top-notch concern. It can reach up to 10 bar as maximum working pressure.

In short, a simple vertical boiler is the second name of a cross-tube vertical boiler in which fire tubes are fitted in a criss-cross arrangement with the fire-box compared with straight tube boilers of vertical type. It is a simple riveted construction.

The modern version of the vertical cross-tube boiler is shown in the figure below. It contains 16 such tubes fashioned in a zig-zag manner. Its working pressure is 7 bar.

multitube modern simple vertical boiler


The merits of using a simple vertical boiler include the following:

  1. Concise and compact physical footprint in comparison to its elaborate counterparts that occupy large land areas.
  2. Easy and rapid installation and commissioning.
  3. Responsive towards fluctuating loads due to its ability to instantly change its evaporative capacity on account of its small design.
  4. Effective in applications such as heating buildings etc. where low to moderate pressure steam is sufficient.
  5. Straightforward maintenance due to its rather simple internal components.
  6. As a lab apparatus for study and research purposes.


Although there are many industrial benefits of the simple vertical boiler, there are some demerits that limit its application. They include:

  1. Relatively low evaporative capacity or steam output per hour due to its smaller yet compact engineering design.
  2. Sub-optimal thermal efficiency due to restricted heating surface areas as well as compact combustion chamber geometry.
  3. Moderate-pressure steam is not required at the time of large-scale industrial needs.
  4. Limited available space for storing fuel and water, thereby necessitating the need to feed the boiler continuously.
  5. An unaligned center of gravity makes it vulnerable to higher stresses.
  6. Comparatively challenging maintenance due to the internality of the boiler parts.
  7. Less fuel is diverse.


Simple vertical boilers are used where the operational requirement is low to moderate pressure steam to be used on an industrial level. For example:

  1. In the pharmaceutical, textile, and food industries, operations such as sterilization, drying, and cooking, respectively, among many others.
  2. For internal heating of apartments and residential buildings when the weather feels cold outside.
  3. In steam locomotives and marine vessels.
  4. For ordinary operations such as ironing and pressing clothes.
  5. In the agricultural sector, it is used to create a conducive environment for plants and saplings in cold weather.
  6. As a lab apparatus for study and research.
  7. He is a stand-by steam generator due to his quick responsiveness and agility.

Boiler Specification 

Just like any other boiler, a simple vertical boiler is rated in KW. The following expression gives it:

KW Rating = Steam Generation (kg/hr) / Overall station steam rate (KWH)

Steam generating capacity is also expressed in terms of Equivalent Evaporation (EE). It is defined as the heat required by the boiler to evaporate a specific amount of feedwater from 100C wet steam to dry saturated steam at 100C.