Steam boilers are frequently employed in various industrial and domestic applications where saturated or superheated steam is required. Steam generated via steam boilers is put into use for heating, cooking and reboiling purposes, mainly because of its heat energy. On the other hand, the pressure energy associated with steam primarily finds its use in reciprocating steam engines and steam turbines.
Steam boilers can produce steam by utilizing heat from variety of sources such as burning of fuel, nuclear reactions, solar energy and heat dissipated from several other processes. A steam boiler should be designed in such a way that it can handle exceedingly high pressure steam generated at immensely high temperatures. In order to achieve this, special care must be taken regarding steam boiler construction. Following factors must be specifically considered while designing a steam boiler:
- Building materials
- Construction methods
- Mechanisms like consistent heat-up and cool-down
In residential applications, one-pipe steam systems are usually employed. In these systems, a temperature controlling device called thermostat is used which basically generates a signal of heat requirement. Upon receiving this signal, the steam boiler commences its operation and steam starts to spread out in the whole system. “As the steam expands, it displaces air contained in the pipes and radiators, and that air must be allowed to escape through air vents typically located at the top of each steam radiator.”1
As soon as the steam arrives at the radiator, these air vents close up while the steam is locked inside. The heat energy from the steam is then transferred via radiators. After this, the steam gets condensed and returned back in the form of water towards the boiler. The water flows back due to gravity action via the same pipe, hence it is known as single pipe steam system. A typical single pipe steam heating system is shown in the figure2 below.
Two-pipe steam systems consisting of a separate condensate-return pipe are also available but they are rarely put into use for residential purposes.
Types of Steam Boilers
In early days, the boiler configurations were very simple which included only iron tanks placed over a wood fire. However, nowadays, advanced boiler designs have been developed with following two major configurations:
Both configurations mentioned above consist of a fire box inside which burning of fuel takes place via continuous introduction of fuel and air. The hot exhaust gases released from the fire box are then utilized for heating the water and converting it into steam. The steam generated is then passed via radiators to provide necessary heat.
Following are the major components incorporated inside a steam boiler system:
- Steam valves: These valves are mounted upon radiators to allow the expulsion of air from the pipes and keeping the steam inside. However, in cases where steam starts escaping from the system, a white calcium layer is formed around the steam valves. Hence, this situation can be easily detected and corrected.
- Gauge glass: This device is basically used to indicate the level of water inside the boiler. Typically, the average water level is in the center of the gauge glass. But once the burner starts operating, the level of water changes to small extent.
- Low-water cutoff: According to code requirements, this device is generally used to stop the burner in situations where the level of water drops below the required water level limits.
- Automatic water feeder: This is an automatic device which is used for the purpose of feeding water into the boiler when the level of water decreases. Use of this device is optional and entirely depends upon the preference of the homeowner. However, a homeowner must always keep a periodic check on the water level in boiler even if an automatic water feed is employed.
- Pressure relief valve: This valve is used for safety purpose since it protects the boiler system from getting over pressurized.
- Steam limit control: This device is also referred to as pressuretrol. Its sole purpose is to cut off the burner as soon as the adequate steam pressure is build up.
A steam boiler is capable of generating superheated steam having temperature more than 212 degrees F. However, this temperature can go beyond unsafe limits and result in fire breakout, if adequate safety controls are not incorporated within the boiler system. Hence, for safe and sound operation of steam boilers, following safety controls must always be provided:
- Limit switches: These are the safety switches which are basically included in the system to detect the increase of temperature beyond a certain limit. As soon as the temperature inside the boiler crosses the defined limits, a limit switch operates resulting in the shut down of the whole system. It is usually mounted near the draft diverters. It proves to be a useful device in preventing fire breakouts and plugging up of chimney or flue gases. It helps in avoiding carbon monoxide poisoning or any other dangerous situation related to temperature rise.
- Low water cut-off: This safety feature should always be incorporated in the boiler system for keeping a check on the level of water in the boiler. The normal level of water is in the half-way in the gauge/ sight glass which should always be maintained. For carrying out a regular visual inspection of water level, the sight glass indicating the level must be kept clean and free from dirt. Lower than average water level in the boiler may lead to undesirable results.
- Automatic water feeder: This device is generally employed for automatically feeding water into the boiler system whenever the water level drops below a certain point. Appropriate water level will result in generation of high quality steam heat. However, small adjustments are needed in the system to maintain correct water level i.e. neither too much high nor too much low. Since excess water will not heat up the system whereas scarcity of water may cause hazardous situation.
- Pressure cut-off control: Various devices such as pressure relief valves and blow-down valves etc. are employed in the boiler system for exercising control over excess steam pressures.
Following are the major problems associated with the use of steam boilers:
- Water Hammer or Banging Pipes: This is a very frequent problem which mainly takes place due to the mixing of condensed water with the steam. Owing to the cold nature of water and hot nature of steam, the pipes in the system tend to expand and contract very quickly. Because of this pipe action, a sound resembling to pipe beating with a hammer somewhere down in the basement is produced, hence it is named water hammer problem. The problem can be raised in the system when the condensed water flows back via the Hartford loop towards the boiler. “It could mean a strainer or trap is plugged with sediment and limiting the return of condensate water returning to the boiler.”3
The automatic water feed employed in the system will continuously work to maintain the proper water level in the boiler. However, the water level in the system will become too high that the mains will overflow. Hence, to avoid this problem, it is necessary to maintain the efficiency of return water piping system. It is very difficult to totally eliminate this problem but it can be reduced to a great extent with the help of a professionally qualified HVAC consultant and use of appropriate devices.
- Leakage of Water via Steam Vents: In case of single pipe steam systems, steam radiators are usually not kept on level. Hence, when the floor tends to settle, the radiator results in leakage of water from the steam valve. To avoid this, a shim can be positioned beneath the radiator legs opposed to the pipe which will enable the condensed water discharged from the radiator to flow back towards the boiler via the return piping system. If the problem persists even after employing a shim in the system, an HVAC professional must be consulted.
- Radiators not heating: There can be various reasons lying behind this problem based upon the type of radiator employed in the system i.e. whether it is two-pipe or single-pipe radiator. To detect the cause of this problem, first of all one should verify that the valve is in open position at the bottom of radiator except in cases where a thermostatically controlled valve is used. In case of single pipe steam systems, the thermostatically controlled valve is located at the steam vent. On the other hand, in case of two-pipe systems, this valve is usually mounted upon the supply pipe approaching towards the radiator. If the thermostat is completely turned down the valve will close whereas if the thermostat is altogether turned up, the valve will accordingly open up. Even if after applying all these techniques, nothing solves the problem then HVAC professional needs to be consulted for advice.
- Total Dissolved Solids (TDS): This problem usually occurs in the boiler when a proper and regular blow-down of the boiler is not being performed. Blow-down always needs to be done at regular intervals and a pre-defined frequency. In a boiler system, when water converts into steam, the dissolved solids are left behind in the boilers. If these solids are not timely discharged, then they tend to build up and result in various problems. Besides, they affect the efficiency and safety of the boiler system. “This is something usually done by the homeowner or maintenance person (if it is a business) but should be done according to the advice of an HVAC professional as far as blow down frequency is concerned.”4
- “The build-up of carbonic acids can eat through hydronic piping and needs to be addressed by either venting oxygen and CO2 or having water treatment.”5