- Ventilation and Coastal Corrosion
Introduction
When you hear the phrase “coastal protection” what images does it conjure up? Robust piles
of large stones carefully constructed to protect the shoreline from eroding? Wooden groynes
stretching out to sea to prevent the sand on the beach from disappearing?
The United Kingdom is made up of a number of islands. Coton in the Elms a village and
parish in the English county of Derbyshire is 70 miles from the coast which makes it the
furthest place in the United Kingdom from coastal waters and is equidistant from Fosdyke
Wash in Lincolnshire, White Sands between Neston in Cheshire and Flint in Wales and
Westbury-in-Severn in Gloucestershire. The UK coastline as measured by the ordnance
survey is about 11,073 miles long and if the larger islands are added, the coastline as
measured by the standard method at the Mean High Water Mark rises to about 19,491
miles. Thus inevitably many hospitals find themselves in close proximity to the sea which
has consequences for the fabric of the building and associated plant due to the increased
concentration of salt in the atmosphere
However what happens when sea air is drawn into the fabric of a hospital? Is this also
capable of causing problems and of so how can we, as hospital engineers and designers,
mitigate the effect? This article deals with the specific case of Air Handling Units (AHU’s) in
hospitals which are deemed to be in coastal areas. Interestingly the design advice “Health
Building Note 00-01: General design guidance for healthcare buildings” currently does not
include any specific advice on design of healthcare premises which are near the sea.
In my field as an Authorising Engineer – Ventilation it refers to the necessary protection of
Air Handling Units that are sited within up to 5 miles from the sea or tidal rivers. This article
will explain how the environment in coastal areas affects the integrity of AHU plant, which
parts of an AHU are mostly affected, how environmental conditions can accelerate the
rusting process, what the appearance of rust means to the efficient and safe operation of the
AHU unit and then goes on to describe measures which can be taken to protect the plant.
Coastal corrosion of AHU plant
Coastal corrosion can affect any part of a hospital building. A well publicized case involved
the new Isle of Wight hospital, St Mary’s, where 18 months after the building was opened in
1992 the stainless steel cladding was beginning to show signs of corrosion particularly at the
junctions of the cladding where trapped water was causing the metal sheets to corrode. The
defects were investigated and it was discovered that the salt-laden sea air which blows over
the Isle of Wight was having a deleterious effect on the outside of the hospital.
Metal corrosion accounts for 40% of the premature failure of AHU plant and equipment.
Heating, Ventilation and Air Conditioning (HVAC) plant located in outdoor environments up
to 8 miles from the coast or tidal estuaries are constantly exposed to moist salt laden air.
The combination of water, salt and untreated metal surfaces provides the ideal conditions for
the formation of ferrous oxide which is commonly known as rust.
Air Handling Units draw in outside air to provide filtered interior air to hospital buildings.
Therefore the quality of the outside air is vitally important to the ultimate quality of the interior
air. In buildings which have been designed to have natural ventilation systems – cooling
stacks and opening windows there is no need to use ventilation plant. However healthcare
premises where clinical procedures are carried out by necessity have air handling systems
which are categorised as “Specialised ventilation units” and their design and operation is
specified in the standard “HTM03-01: specialised ventilation for healthcare premises – Part
A”.
Wind conditions, humidity, prevalence of foggy conditions temperature and proximity to the
source of salt water are all factors which may result in premature failure of plant and
equipment. In the case of Air Handling Units the intake section must be sited to allow the
intake of fresh air. This often means the plant itself is sited outdoors. Even where the
majority of the plant is housed inside a building the intake itself must be open to the outside
air. See schematic of a typical AHU unit used in NHS Hospitals.
Diagram of a typical AHU unit used in hospitals
The corrosion associated with air laden with salt and water primarily affects the supply
ductwork and the HVAC heat exchanger known as a “Frost coil” or “Fog coil” (see Diagram
1). The AHU itself – see diagram – draws in air form the atmosphere immediately outside the
hospital. The air intakes of the AHU are often on the roof of buildings or at a high level in a
plant room through a louvre with a vermin grille protecting the extract duct. A critical
ventilation AHU contains a series filters, typically four pre-filter panel filters and four F7 bag
filters. The denomination of filters is associated with the size of particle that gets trapped in
the filter. A G4 filter traps 80% of air borne particles larger than 10 micron size which is the
dimension of flower pollen. The F7 bag filters are designed to remove smaller particles of
contaminants from the air which has already been through the F7 bag filter.
In a coastal area where sea salt is in the air pre-heating the incoming air by using the frost
coil as a fogging coil to reduce the humidity will effectively remove the effects of the airborne
salt in the first section of the AHU. However this means that the salt laden air has travelled
through the air intake and vermin grille, through the attenuator, over the motorised damper
and past the fog coil before it hits the first panel filter. Thus these parts of the AHU will
inevitably be exposed to salt laden air and are liable to increased corrosion when located in
a coastal area, particularly the ducting of the AHU and the casing of the fog coil.
This corrosion can be predicted in a coastal setting and therefore design of AHU’s in coastal
areas needs to take it into account. Measures that can be taken include ensuring that the
vermin screen, which protects the fresh air inlet form ingress of outside objects for example
leaves and also prevents the AHU being invaded by small mammals and birds like pigeons,
is made of stainless steel. In addition the fresh air intake ducting needs to have a rust
prevention coating. Following the path of the salt laden air it also makes sense to provide the
fog coil with a casing made of stainless steel.
These are all measures which add cost to a project. I have seen a case where the coastal
protection was “value engineered” out of the hospital building project. Now the air handling
plant at this particular coastal site is failing due to excessive corrosion after only 10 years
when ordinarily the type of AHU plant and ductwork installed in hospitals has a working life
of over 30 years.
Environmental factors which influence rapid corrosion
The rate of corrosion of unprotected metal components sited in proximity to the coast is
accelerated by a number of factors. Increased in humidity will have the effect of hastening
the production of rust as the water acts as an electrolyte in the chemical reaction between
iron and water which produces ferrous oxide. Chemical reactions are generally temperature
sensitive. Increases in temperature will cause a faster reaction rate. As the global
temperature rises caused by global warming this type of corrosion enhancement will become more important.
The air intake components of a typical AHU are not under tremendous physical forces so it is
reasonable to ask why does it matter if these parts of the plant show a surface covering of
rust? The rust may not penetrate the steel ductwork for many years so why is it considered
such a serious problem?
The answer lies in the chemical structure of the rust itself. The oxygen contained in the
ferrous oxide compound is known to feed certain bacterial species. These species of
bacteria metabolically utilize solid iron oxides as a terminal electron acceptor thereby
reducing Fe(III) oxides (FeO3) to Fe(II) (FEO2) containing oxides. The bacteria capable of
existing on a rusty surface include Shewanella oneidensis, Geobacter sulfurreducens and
Geobacter metallireducens.
As the main function of a hospital Air Handling Unit, especially those supplying air to critical
areas such as operating theatres and isolation rooms is to supply very clean air it is not
desirable to have bacterial growth within the plant that is delivering the air.
Measures that can be taken to protect AHU plant (Coastal Protection)
As mentioned earlier the intake section of the plant – fresh air inlet, supply duct, dampers
and frost coil – can be originally installed made of stainless steel. In the event that the
original installation is made of untreated steel and has started to rapidly corrode there are a
number of things that can be done to stop the rust.
Vulnerable areas of the AHU can be given a protective coating. Various types exist including
polyurethane coatings, epoxy resins and fluro polymers. Another alternative to is give the
exposed metal parts an electro-coating which will provide a corrosion resistant layer on the
surface of the plant. If the rust has progressed to the point where the supply duct is
perforated then there is no option but to replace the damaged section or the entire plant.
Conclusion
The environment for a building situated in a coastal area, which is defined as anywhere
within 8 miles of the coast or a tidal river, includes the risk of rapidly accelerating corrosion of
external unprotected metal components of AHU plant. The coastal conditions of humid, salt
laden air with mild temperature conditions provide an ideal environment for the chemical
reaction between iron and oxygen which produces rust.
Rust within an Air Handling Unit can result in the growth of bacteria within the plant itself
which is less than ideal for a building service component which is primarily used to supply
clean air particularly to Operating theatres, Critical Care Units, isolation and treatment
rooms.
Hospitals situated near the coast can design their Air Handling Units to include coastal
protection design features such as components exposed to external salt laden air being
manufactured out of stainless steel and BMS control strategies to pre-heat the air using the
frost/fog coil to reduce the humidity of the incoming air.
If the plant has already started to show signs of accelerated rust the affected areas can be
treated with a variety of coatings or in very badly damaged plant the entire replacement of the AHU.
David Livingstone
Authorising Engineer and corrosion specialist
DRLC Ltd www.drlc.uk