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Heating Systems Guide - Industrial / Commercial Boilers
By Jackie LoBuglio - JD Supply Boiler & Hydronic Heating Outlet

Heating Systems, Cooling Systems, and Heating, Ventilating, & Air Conditioning (HVAC) Systems, have been around for many years. In this HVAC / Heating Systems Guide we describe the several types of Residential Heating Systems and Commercial & Industrial Heating Systems that are commonly available in North America. You may also wish to refer to the section on Combined Heating & Cooling Systems.

1.2.4. Industrial Boilers

Industrial Boilers may be used to continuously reheat a flow of Hot Water or Steam, for distribution to a range of heat emitting units. Boilers are generally rated on input and/or output in British thermal units per hour (Btu/h), or in kilowatts (1 kW = 3,412 Btu/h). Boiler output should be adequate to offset building design heat loss, piping losses (if they run through unheated space), plus any additional heating needs. There are several Industrial Boiler types and configurations:

• Cast Iron Sectional Boilers
• Steel Sectional Boilers
• Electrode Boilers
• Steam Generators
• Vertical Shell Boilers
• Waste Heat Boilers
• Fluid-Bed Boilers
• Auxiliary Boilers

Cast Iron Sectional Boilers
Cast Iron Sectional Boilers are used for hot water services with a maximum operating pressure of 5 bar and a maximum output in the order of 1500KW.

Site assembly of the unit (consisting of a bank of cast iron sections, with internal waterways) is necessary. The boiler sections are assembled with screwed or tapered nipples at top and bottom for water circulation, and sealing between the sections contains the products of combustion. Tie rods compress the sections together.

An assembly of standard boiler sections may be used to give a range of outputs, dependent on the number of sections used. After assembly of the sections, the mountings, insulation and combustion appliances are fitted. This flexible system makes Sectional Boilers suitable for locations where it is impractical to deliver a fully assembled Packaged Unit (eg.basements where inadequate access is available, or rooftop plant rooms where sections may be taken up using the elevator shafts).

Models are available to use liquid, gaseous or solid fuel.

Steel Sectional Boilers
Steel Sectional Boilers are similar in rated outputs to Cast Iron Sectional Boilers. Construction is of rolled steel annular drums for the pressure vessel. These units may be of either Vertical or Horizontal configuration, depending upon the manufacturer. In their vertical pattern they may be supplied for steam rising.

Electrode Boilers
Electrode Boilers are available for steam rising up to 3600kg/h and 2 design choices are available:

Smaller "Element Boilers" offer evaporation less than 500kg/h. In these, an immersed electric element heats the water and a set of water-level probes positioned above the element controls the water level being interconnected to the feed water pump and the element electrical supply.

Larger units are true "Electrode Boilers". Normal working pressure would be 10bar, but higher pressures are available. Construction is a vertical pattern pressure shell containing the electrodes. The lengths of the electrodes control the maximum and minimum water level. The electrical resistance of the water allows a current to flow through the water, which in turn, boils and releases steam. Since water has to be present within the electrode system for it to heat, lack of water cannot burn out the boiler. The main advantage with these units is that they may be located at the point where steam is required and, as no combustion fumes are produced, no chimney is required. Steam may also be raised relatively quickly, as there is little thermal stressing to consider.

Steam Generators
Steam Generators are coil-type boilers that work in the evaporative range up to 3600 kg/h of steam. With steam pressure being contained by a tubular coil, pressures of 35bar and above are available, although the majority of these units are supplied to operate only up to 10 bar.

Steam generators are suitable for firing with liquid or gaseous fuels, although the use of heavy fuel oil is unusual. The coiled tube is contained within a pressurized combustion chamber and receives both radiant and convected heat. A control system matches the burner-firing rate proportional to the steam demand, and feed water is pumped through the coil and partially flashed to steam in a separator.

The remaining water is recirculated to a feed water heat exchanger, before being run to waste. Because there is no stored water in this type of unit they are relatively lightweight, and therefore suitable for sitting on mezzanine or upper floors adjacent to the plant requiring steam.

Additionally, as the water content is minimal, steam rising can be achieved very quickly and can respond to fluctuating demand within the capacity of the generator. It must be noted that close control of suitable water treatment is essential to protect the coil against any build-up of deposits.

Vertical Shell Boilers
Vertical Shell Boilers are cylindrical boilers, with the shell axis vertical to the firing floor. The original Vertical Shell Boiler contained a combustion appliance in a chamber at the lower end of the shell. Hot combustion exhaust gases exited the chamber to rise vertically through a flue surrounded by water. Large diameter (100mm) cross tubes were fitted across this flue to help extract heat from the gases, which then proceeded to the chimney.

Later versions replaced the vertical flue with one or two banks of small-bore tubes running horizontally before the gases discharged to the chimney. The steam was contained in a hemispherical chamber forming the top of the shell.

The latest version of the Vertical Shell Boiler is generally used to recover exhaust gas heat from power generation or marine applications. The gases pass through small-bore vertical tube banks. The same shell may also contain an independently fired section to produce steam when there is insufficient or no exhaust gas available.

Waste Heat Boilers
Waste Heat Boilers may be horizontal or vertical shell boilers, or water tube boilers. They are designed to suit individual applications, working with hot waste gases from furnaces, incinerators, gas turbines and diesel exhausts. The prime requirement is that the waste gases must contain sufficient usable heat to produce steam or hot water at the condition required. Supplementary firing equipment may also be included, if a standby heat load is to be met, and the waste gas source is intermittent.

Waste heat boilers may be designed to use either radiant or convected heat sources. In some cases, due consideration must be taken to avoid problems arising due to the source of waste heat. Examples include plastic content in waste being burned in incinerators, carry-over from some type of furnaces causing strongly bonded deposits, and carbon from heavy oil fired engines. Some issues may be dealt with by maintaining gas exit temperatures at a predetermined level to prevent a dew point being reached, and others by soot blowing.

Currently, there is a strong interest in small Combined Heat and Power (CHP) stations, which normally incorporate a waste heat boiler.

Fluid-Bed Boilers
Fluid-Bed Boilers derive their name from the fluid bed produced by a mixture of silica sand and ash through which air is blown to maintain the particles in suspension. There are 3 categories:

• Shallow Bed Fluid Bed Boilers
• Deep Bed Fluid Bed Boilers
• Recirculating Fluid Bed Boilers

Shallow Bed Fluid Bed Boilers - are the most common type, being about 150-250mm in depth in their slumped condition and around twice that when fluidized. Heat, from auxiliary oil or gas burners, is applied to the bed to raise its temperature to around 600^oC. At this temperature coal and/or waste is fed into the bed, which is then controlled to operate at 800-900^oC. Water-cooling surfaces incorporated into the bed are connected to the water system of the boiler.

Several applications of the shallow bed system are available for industrial boilers; the two most used being the open-bottom shell boiler and the composite boiler:
Open Bottom Shell Shallow Bed Boilers - have the combustor sited below the shell and the gases then pass through two banks of horizontal tubes.
Composite Shallow Bed Boilers - have the combustion space and water tube chamber directly connected to a single-pass shell boiler which contains the fluid bed. In order to fluidize the bed, the fan power required is greater than that with other forms of firing equipment. To its advantage, the fluid bed may utilize fuels with higher ash contents than other systems will tolerate. It is also possible to control acid emissions with chemical additions to the bed during combustion. These boilers are also less selective in fuels, and can cope with a wide range of solid-fuel characteristics.

Deep Bed Fluid Bed Boilers - are similar to shallow bed boilers, but they are larger, with beds up to 3m deep in the fluidized state.

Recirculating Fluid Bed Boilers - are only applicable for large water tube boiler applications.

Auxiliary Boilers
Auxiliary Boilers are primarily used on Naval Ships. There are 2 basic types:
Fire-Tube Auxiliary Boilers and Water-Tube, Natural-Circulation Auxiliary Boilers.

Fire-Tube Auxiliary Boilers - also often called Scotch Marine Boilers, were originally developed for use on steamships, and have since been used in all types of boiler applications, with a number of fire-tube type auxiliary boilers in current diesel-driven ship application. Scotch Marine Boilers, in which hot combustion gases pass through tubes that are surrounded by water, are available in 2 basic configurations -
(a)Dry Back Scotch Boilers and (b) Wet Back Scotch Boilers:

Water-Tube, Natural-Circulation Auxiliary Boilers - consist basically of a steam drum and a water drum connected by a bank of generating tubes. The two drums are also connected by a row of water tubes, which forms a water-cooled sidewall opposite the tube bank. The water-wall tubes pass beneath the refractory furnace floor before they enter the water drum.

In natural-circulation boilers, the steam and water drums are connected by several tubes of larger diameter, called Downcomers or Water Tubes (not shown). These tubes are positioned away from the flow of hot combustion gases. Refractory is also used to protect these Downcomers from contact with the combustion gases.

The operating principle of a natural-circulation boiler is quite simple. It relies on the difference in density between the cooler (denser) water in the downcomers and the hot (less dense) water in the steam-generating tubes. This is the force that causes the hot water and steam mixture to rise from the water drum, through the generating bank tubes, to the steam drum. Steam then separates from the water and rises to the top of the steam drum.

The flow of water up the tubes of the steam-generating bank must be maintained; otherwise, the tubes would quickly melt. A constant flow of water and steam up the tubes is required to carry away heat at the proper rate. If the flow from natural circulation is allowed to stop, such as when the water level in the steam drum falls below the openings of the bank of tubes for the water wall, the tubes of the generating bank will be severely damaged and the boiler will need major repairs. (Replacing boiler tubes is very expensive operation!)

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Cast Iron
Sectional Boilers

Smith Cast Iron Boiler

Steam Generators

Steam Generators are suitable for firing with liquid or gaseous fuels

Vertical Shell Boilers

Vertical Shell Boilers are cylindrical
boilers, with the shell axis vertical
to the firing floor

Waste Heat Boilers

Waste Heat Boilers may be Horizontal
or Vertical Shell Boilers,
or Water Tube Boilers

Fluid Bed Boilers

Schematic Fluid Bed Boiler

Fire-Tube Boilers

Fire-Tube Auxiliary Boilers are also known as Scotch Marine Boilers

Dry Back
Scotch Boilers

The Dry Back Scotch Boiler
offers easy maintenance

Wet Back
Scotch Boilers

The Wet Back Scotch Boiler
offers less refractory maintenance

Water-Tube
īNatural-Circulationī
Boilers

Water-Tube Natural-Circulation Boiler