Steam raising: You have a choice

Chris Horsley, MD of Babcock Wanson UK, suggests the move away from traditional fire tube boilers to coil type steam generators as an efficient means of industrial steam raising

Both fire tube boilers and steam generators have the same goal: to produce 'dry' saturated steam to maximise the delivery of heat to the user.

Both types of boiler operate along the same basic principles in that water is confined in a restricted space and is heated by burning a fuel source. The energy of combustion is transferred from the flame to the water by radiation and conduction, heating the water and ultimately raising steam. However, the means of heating the water to the point of 'dry' steam are markedly different.

In a traditional fire tube boiler hot flue gases from the combustion process pass through one or more tubes running through a large sealed tank of pressurised water. The large volume of pressurised stored water is used to resolve the standard potential variables that affect all boiler type systems, including variations in the feed water, fuel and combustion quality and temperatures and importantly steam load variation.

With modern coil tube steam generators a precisely controlled burner creates an accurately defined heat input to a multi coil of steel tube through which a precise quantity of feed water is pumped. Only a small amount of excess water is fed into the coil to ensure control of steam quality. A separator is normally incorporated at the coil outlet to ensure best conditions for steam output and to return excess water back to the hotwell.

Both types of steam generation systems are in widespread use today, but it's the coil tube steam generator that continues to grow in popularity. Why? The immediately obvious benefit of a coil steam generator over a fire tube boiler is size; steam generators are around half the footprint of comparable fire tube boilers as they have no large drums of pressurised water; instead, the tube is coiled concentrically within a compact unit.

Of course, the coiled nature of the tube also maximises the surface area to be heated thereby ensuring minimal loss of efficiency from the boiler surface.

Another fundamentally important aspect is that of safety. The chance of a pressurised water explosion is virtually eliminated with a coil type steam generator as the steam generation process takes place inside the tube with its relatively small volume of water under pressure.

Precision control of energy usage and all aspects of operation can also be better achieved with a coil steam generator. From a cold start a full head of steam is available in as little as three minutes in many cases.

Furthermore, the very low overall thermal inertia means rapid response to changing load conditions. When combined with modern control equipment, fuel and water flow control can be matched to changing conditions. This, along with the very low surface heat losses, can provide an immediate substantial fuel saving over the alternatives.

A coil steam generator is also more flexible in many respects than its counterpart. For example, it can accommodate a wide quality of feed water, including supplies with naturally high levels of total dissolved solids (TDS), which would lead to excessive blowdown and greater risk of carry-over from a fire tube boiler.

With all these benefits to be had, you might expect a coil steam generator to be a complex piece of engineering with a price tag to match. In fact, a coil steam generator is really very simple to operate. What's more, all steam generator functions can be remotely monitored and simple, safe sequence control of multi-unit installations is also possible, removing one further operative action. As for the price, they are comparable to a traditional fire tube boiler and generally have a low whole life cost.

Fire tube boilers aren't about to disappear from our radar any time soon and they often remain an important solution for very large steam outputs. But for many users with more general steam requirements, coil type steam generators are an option worth considering.