Boiler design and construction

The key elements of a boiler include the burner, combustion chamber, heat exchanger, exhaust stack, and controls. Boiler accessories including the flue gas economizer are also commonly used as an effective method to recover heat from a boiler.

Figure 2.1 Boiler design

The combustion chamber, usually made of cast iron or steel, houses the burners and combustion process. Temperatures inside the combustion chamber can reach several hundred degrees very quickly.

Heat exchangers may be made from cast iron, steel tube bundles, or, in the case of some smaller boilers, copper or copper-clad steel.

The exhaust stack or flue is the piping that conveys the hot combustion gasses away from the boiler to the outside. Typically, this piping is made of steel, but in the case of condensing boilers it needs to be constructed of stainless steel to handle the corrosive condensate. Another consideration is whether the exhaust stack will be under a positive or negative pressure. This can determine how the joints of the exhaust stack must be sealed.

Boiler controls help produce hot water or steam in a regulated, efficient, and safe manner. Combustion and operating controls regulate the rate of fuel use to meet the demand. The main operating control monitors hot water temperature or steam pressure and sends a signal to control the firing rate, the rate at which fuel and air enters the burner. Common burner firing sequences include on/off, high/low/off and modulating.

Boiler safety controls include high pressure and temperature, high and low gas/oil pressure, and high and low water level and flame safeguard controls. These controls are considered safeties or limits that break the electrical circuit to prevent firing of the boiler. Flame safeguard controls are programmed to operate the burner and cycle it through the stages of operation.

All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water.

A drum must be provided where steam and water can separate. The steam drum is also used for the following:

1. To mix the saturated water that remains after steam separation with the incoming feedwater

2. To mix the chemicals that are put into the drum for the purpose of corrosion control and water treatment

3. To purify the steam by removing contaminants and residual moisture

4. To provide the source for a blowdown system where a portion of the water is rejected as a means of controlling the boiler water chemistry and reducing the solids content

5. To provide a storage of water to accommodate any rapid changes in the boiler load.

All of the tubing surrounding the furnace is called heat transfer surface.

There must also be a variety of fittings and controls to ensure that fuel oil, air and feedwater supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler.

The steam is produced in a steam drum and may be drawn off for use from here. It is known as 'wet' or saturated steam in this condition because it will contain small quantities of water. Alternatively, the steam may pass to a superheater which is located within the boiler. Here steam is further heated and 'dried', i.e. all traces of water are converted into steam. This superheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. The hot gases produced in the furnace are used to heat the feedwater to produce steam and also to superheat the steam from the boiler drum. The gases then pass over an economiser through which the feedwater passes before it enters the boiler. The exhaust gases may also pass over an air heater which warms the combustion air before it enters the furnace. In this way a large proportion of the heat energy from the hot gases is used before they are exhausted from the funnel.

VOCABULARY

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EXERCISES

I. Translate into Russian.

The combustion chamber, cast iron, the burners, heat exchanger, copper, copper-clad steel, stainless steel, to handle the corrosive condensate, the joints of the exhaust stack, to be sealed, in a safe manner, to meet the demand, the main operating control, the firing rate, flame safeguard controls, to break the electrical circuit, to prevent firing of the boiler, in the event, the pressure limit setting, the fuel valve, switch contacts for low water cutoff, high limits, air proving switches, redundant safety and operating controls, and flame detectors.