The Complete Steam Guide

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Comparison of Coil Type and Shell Type Boilers


Coil Type Boiler

Smoke Tube Boiler

Overall Steam : Fuel Efficiency

Low, typically 75% as found by actual trials

As high as 89% on NCV

Steam space

A coil type boiler is water tube design, the water travels through the coil and is surrounded with flue gases. The steam generated is directly fed to the process. There is no scope for steam storage as these are once through type boiler.

Shell and tube type boilers have adequate steam space above the normal water level of the shell. The available steam space caters to the fluctuation of the steam load in the plant.

Load fluctuation

As the steam is continuously fed to the process and there is no steam accumulation or buffer, pressure control is hard to achieve. At higher loads even, the rate of steam generation falls below the water pumping rate. This causes water carry over, higher moisture content and thus steam starvation in the process. The coil type boiler has poor performance under fluctuating load.

Whenever the steam load increases higher amount of steam is drawn from the boiler steam space and results in drop of boiler pressure due to which again burner is fired, hence the pressure control in the Smoke Tube boiler is better. This also avoids water carry over, and thus dry steam in the process.

Water holding capacity

Cannot exceed water holding capacity of 22.5 Kg per coil

No such limitation.

Dryness Fraction

Low. Coil type boilers claim to generate steam with dryness fraction of about 80% but actual dryness fractions is around 65-75%. This means the moisture content in the steam is 30%. This results in extended batch timings and lower productivity and product quality.

High. Shell and tube boilers have dryness fraction up to 98%. This means the moisture content in the steam is 2%. This results in higher heat content of steam thus shorter batch timings and higher productivity and product quality.

Condensate recovery

Not possible

Possible and thus a better steam to fuel ratio

Susceptibility to poor water

Water tube design requires strict water quality treatment

Smoke tube design can do with medium treatment

Susceptibility to scaling

Coils get scaled and are prone to failure due to TDS and pH of the feed water

In case of smoke tube design, inner surface of tubes can be cleaned easily

Maintenance Expenses

Recurring cost of replacing the coils frequently

Periodic tube cleaning is sufficient

On-line Blowdown to maintain TDS

Not possible

Adequate water holding capacity enables on-line blowdown & hence TDS control

Build Quality of manufacture

No universal standards depends on manufacturer, no stage inspection during manufacture, poor testing

Stage wise inspection at each stage of manufacture and exhaustive testing ensures build standard