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.