Medical Gases Pty Ltd ABN 61 121 276 079201033134627

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Medical Gases Pty Ltd ABN 61 121 276 079201033134627 Powered By Docstoc
					            Medical Gases Pty Ltd                      ABN 61 121 276 079

      Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

Selection of Medical and Laboratory Air Plant
Part 2
There are a considerable number of types of compressors in use in the
production of compressed air for Medical and Laboratory purposes in a number of
styles and types. Should anybody wish to investigate the existing Hospitals and
Laboratories throughout NSW alone it would provide some interesting statistics,
there is no doubt that some of the older Hospitals will have plant that is so
outdated that spare parts are probably made to order as the models no longer
Installations of recent times will consist of a variety of manufacturers as well as
different types of compressors that will include reciprocating, screw, scroll and
centrifugal compressors, in Australia the only thing that these units will have in
common is that they are all Oil Free. At this time there does not appear to be any
data that offers definitive information about what types of units to use under what
We offer the following guidelines used by this company to attempt to select the
correct compressor type for the particular installation based on the operating
criteria for different types of compressors to suit a variety of situations.

Types of Compressors used in Medical applications
As a starting point let’s establish what the different types of compressor are and
what are their strong points and their weaknesses.
For Medical Air applications in Australia in particular Compressors are only
selected from one style, i.e. Oil Free, this encompasses Oil Less Compressors,
and Oil Free Compressors.
Oil less compressors can be described as compressors that do not include a
sump containing lubricating oils, these types include reciprocating units that use
sealed bearings and are generally low flow rate compressors through to Scroll,
water injected screw and Centrifugal.

Oil Free compressors include a wide range of compressor types from small
diaphragm type low pressure units through to very large screw compressors that
are used in high flow applications, mid range types include reciprocating oil
flooded sump types, and hook style compressors.

Types of Compressors used in Laboratory applications
Compressors for Laboratory applications are generally similar to the medical
range, there are however some applications that will require pressure and
cleanliness levels not commonly found in medical applications. There is a
common misconception that Medical Gases are particularly clean when
compared to other gases. This is true when compared to gases purchased from
your local gas supplier who provides industrial purity gases, Medical applications
require a much higher level of cleanliness than these gases however Laboratory
Gases in general have an even higher level of purity again.
             Medical Gases Pty Ltd                       ABN 61 121 276 079

        Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

Purity levels of Medical Air is dealt with further on in this document

Compressor Types
The various manufacturers of the compressors provide similar styles and types of
units so we shall look at them as types rather than as manufacturers models,
each type has characteristics that meet a certain supply requirement, these
requirements vary widely but can be broken into some basic types. The driving
forces that will promote the purchase of one type over another include pressure,
price, longevity, locality and atmospheric conditions, local support, system
design, flow requirements and quality.

The pressure requirements for any compressor plant is of primary importance,
however in the majority of Hospitals the Medical Air pipeline pressure is regulated
to 410 kPa which is well below the operating pressure of most standard off the
shelf type compressors, the majority of these operate around the range of 600 to
1,000 kPaG. In Europe it is common practice to reticulate the system at the
compressors operating pressure and then to regulate it down to an operating
pressure of 410 kPaG at local regulating stations, in Australia we regulate the
system pressure at the source in the plantroom. The advantage of the higher
pressure system is that it allows the use of smaller pipelines due to the ability of
the pipes to carry higher flows in smaller pipes and allows for greater storage of
compressed air in the pipeline than would otherwise be available. The
disadvantage is that there are considerably more items of equipment that are
susceptible to wear and tear and the initial installation costs are higher.

Using High Pressure Compressors (1,200 KpaG)

The design of Medical Air plants in Australia is generally such that the storage
provided by the air receiver is all that is available for the system to draw upon
while in operation due to the pipelines being regulated to a constant 400 kPaG
pressure. If it is the intention of the system designer to use the 10 starts per hour
rule as the basis for the size of the air receiver the increase in pressure will only
provide an additional storage volume of 7,920 litres (based on a 2,000 litre vessel
operating between 600 kPaG and 800 kPaG versus a 2,000 litre vessel operating
between 600 kPaG and 1,200 kPaG), this will add approximately 2.6 minutes
running time to a 50 l/sec flow rate compressor. To provide the additional
pressure the compressor will require additional power for the extra compression
required, additional heat will be generated and will need to be removed from the
plant room and it’s environs, more expensive storage vessels will be necessary
as the extra pressure comes at additional manufacturers cost.

The price of any compressor is always a major concern however if the unit is
being installed in harsh climatic conditions it may well be worth spending extra
money on the unit to provide additional protection, better quality and serviceability
            Medical Gases Pty Ltd                       ABN 61 121 276 079

      Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

for the future. Unfortunately in many instances budgetary constraints will over ride
the above with future cost blow outs for repairs and maintenance.
The selection of the correct compressor for the function should be based on the
long term operating costs and the maximising of features that provide real
advantages in future repairs and maintenance.

The lifetime expectancy of a compressor will depend on a number of factors,
quality of the unit, locality of the installation, maintenance provided and it’s
availability, expected workload and running time. The better the quality the longer
the unit will stand up to the rigours of hard use, the other side to that is that
installing the best quality compressor in a situation where it will operate minimally
under good operating conditions with regular maintenance and services is
unnecessary, a reasonable quality compressor will adequately provide this

Locality and atmospheric conditions
The site conditions will vary considerably from installation to installation, a plant
being installed for example in Darwin in an open plantroom will be subjected to
the extremes of humidity and temperatures that an equivalent compressor in
Melbourne could not expect. Installation in an air conditioned plantroom may be
necessary if the compressor plant and equipment selected has maximum
operating temperature limitations. Water cooled plant is also an option that will
need special consideration as water treatment will be necessary as well as the
additional costs and safety concerns that go along with it.

Local support
The ability of local service providers to maintain and service the plant will play a
significant role in the selection of the manufacturer, spare parts that need to be
purchased from warehouses that are distant from the installation are
unacceptable especially if the installation is for Medical use, remember that
Medical Air is a drug and is classified as such so patients and hospital staff rely
on the ongoing supply being readily available, extended shutdowns while waiting
for spare parts should be avoided at all costs.

System Design and Flow requirements
The system designer must take all of the above into consideration when
specifying any equipment, flow rates are specified for Medical installations in AS
2896-1998, pipeline pressures are also nominated as 400 kPa. The biggest
hurdle in designing a Medical Air plant will be the selection of the compressor
type for the installation, considerations such as noise transmission to the building,
vibration, heat rejection, power availability and space are just some of the
variables that will also need to be considered. In some instances it may be
necessary to install additional drying for the compressed air due to local ambient
conditions, if desiccant driers are used it will be necessary to increase the plant
              Medical Gases Pty Ltd                     ABN 61 121 276 079

         Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

design flow rate to compensate for the additional consumption used in the drying

Compressor Types

Reciprocating compressors are now relatively uncommon in medical use, there
are still a number of older installations using Broomwade, Ingersoll Rand and
Ross units but they are being phased out in favour of more modern technology.
These compressors have issues with vibration (both noise and mechanical), heat
rejection, many were water cooled with the ongoing problems with water
treatment and associated concerns.
There are compressors available that are able to provide oil free air at 1,400 kPa
for use as a driving force for surgical tools. These compressors may be
expensive especially when the cost of desiccant dryers are added, if there is
likely to be a continued demand for air at this pressure it is worth considering, if
the demand is inconsistent then a cylinder manifold may be the way to go. The
alternative to these compressors is to use a Medical Air cylinder system which
will be dramatically cheaper but will have the ongoing costs for rental and
replacement of cylinders. The long term costs should be investigated prior to
making a decision of the selection of the supply source.


Usually only used on high flow systems for industrial applications, not really
suited for Medical due to size constraints. The compressors use screw
technology and non lubricated air ends to provide compressed air and have
minimum capacities in the 30 kW plus range depending on manufacturer.


This type of compressors is becoming very popular, the units meet the demand
requirements of the majority of hospitals that are in the 50 to 300 bed range. They
compressors are quiet and vibration free, provided as self contained units that are
available in package units with matching refrigerated driers built in, they take up
minimal floor space but may need the cooling air exhausted from the plantroom.
They are usually supplied as multi element units with highly sophisticated control
systems that will provide a variable air supply by operating enough of the
elements to meet the system demand.


There is a range of oil free compressors that use a hook and claw style
compression chamber, they are becoming more prevalent in the hospitals that
have in excess of 300 beds or have unusually high air demands. They are also
relatively quiet and vibration free self contained units that are available in
package units with matching refrigerated driers built in, they take up minimal floor
space but may need the waste cooling air exhausted from the plantroom. They
              Medical Gases Pty Ltd                  ABN 61 121 276 079

      Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

are available in a variable speed format and all units may be easily interfaced
with any BMS. They are sold by Atlas Copco in their ZT range


These units are preferred for use in constant demand situations, they are
predominantly used in high flow applications where demand does not fluctuate a
great deal. They produce a constant flow under normal operating conditions and
as they operate continuously they vent unused air when demand falls.

Purity Levels of Compressed Air
Medical Air is supplied with specifications that allow maximum levels of various
contaminants as set down in Australian Standards and the European
Pharmacopeia, if we look at the levels of common contaminants such as
hydrocarbons, moisture, Carbon Dioxide and Carbon Monoxide for example we
find that cylinders of Medical Air as supplied in Australia they shall contain a
maximum content of the following as per AS 2568
Carbon Dioxide       500 ml/m3
Carbon Monoxide 5 ml/m3
Hydrocarbons         27.2 ml/m3(Total Hydrocarbons)
Nitrogen Dioxide     0.5 ml/m3
Nitrous Oxide        5 ml/m3
Flourides            1 mg/m3
Sulphur Dioxide      1 ml/m3
Moisture             max dew point of minus 20 C or less than the minimum
                     recorded temperature at pipeline pressure. (minus 20 C is
                     the equivalent of approx 1,000 PPM at ambient conditions)
Argon                0.9%
Oxygen               21% + or – 1%
Nitrogen             79%
Oil                  0.5 mg/m3

If we compare these to Instrument Grade Air the standard levels supplied by the
manufacturers are
Hydrocarbons       Not referenced
Moisture           Less than 25 PPM
Carbon Dioxide     300 PPM
Methane            Less than 5 PPM
Argon              0.9%
Oxygen             21%
Nitrogen           78%

If we look at Zero Grade Air the levels are
Hydrocarbons         Less than 1 PPM
Moisture             Less than 25 PPM
Oxides of Nitrogen Less than 1 PPM
Oxides of Sulphur Less than 1 PPM
Argon                0.9%
            Medical Gases Pty Ltd                       ABN 61 121 276 079

      Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

Oxygen               21% +- 1%
Nitrogen             Balance

You can see from the above that Laboratory Gases have far more stringent
requirements than Medical Grade Gases and specific plant requirements are
necessary to meet these specifications.

The selection of the plant we provide is therefore of critical importance, the levels
of hydrocarbons should be nil as we are using oil free or oil less compressors.
Having said that when the system is tested during the commissioning period an
oil level of below 0.5 mg/m3 would be a pass for the plant, the test results should
be nil so even a very low oil concentration would indicate a major plant fault or an
air intake problem.
Levels of Carbon Dioxide and Carbon Monoxide are within the commonly found
levels of atmospheric air, increased levels could be found if the air compressor
intakes were located near exhaust systems, in carparks, industrial facilities etc or
may also indicate a compressor fault, some gases can be produced as a by
product of compression when other contaminants are being introduced into the
compressed air stream.

Moisture Removal
Moisture is another variable that has two acceptable criteria, Medical Air from
cylinders will have an approximate moisture content of 50 PPM (equivalent of an
atmospheric dew point of minus 48 degrees C), Medical Air from a compressor
Plant is required to be dehydrated to a pressure dew point of 2-3 degrees C at 7
Bar pressure (equivalent of an atmospheric dew point of minus 20 degrees C or
1,000 PPM). Two completely different levels of contamination, 50 versus 1,000
both of which pass the requirements of AS 2896-1998 and AS 2568-

The removal of moisture will directly effect the air stream and as noted above for
Medical applications this is not of great concern, in a Laboratory however it may
cause problems if the equipment using the Air system is sensitive to minor
volumes of condensation or moisture in the air stream.
A Refrigerant Dryer uses a standard refrigeration type compressor to cool the air
stream, this system cools the incoming air to a temperature of between 2 and 3
degrees Celsius causing the moisture to condense for removal and then reheats
the air stream by passing it over the incoming air supply. This temperature range
must not go below zero otherwise freezing of the condensate and pipe blockage
will occur.
Desiccant Driers are one method of removing this moisture in Laboratory Air
systems if necessary, these dryers are available in various temperature ranges
that will give varying levels of residual moisture, standard ranges are dew points
of minus 40 and minus 70 degs Celsius. This type of dryer uses a portion of the
pre-dried air stream to dehydrate the incoming air stream. The more recent
models are available with a variable air demand that reduces the drying air flow to
suit the demand so the unit will almost shut down when the demand stops. Each
dryer of this type will use approximately 15% of the air it produces to assist in the
drying process, this will need to be taken into consideration when calculating the
capacity of the compressor plant.
            Medical Gases Pty Ltd                       ABN 61 121 276 079

      Tel: 61 2 9714 2166 Fax: 61 2 9714 2199

Particulate Removal
Entrained particles and vapours can be removed by using standard filters that are
available from the majority of the filter suppliers in the compressed air equipment
suppliers market place. Under normal circumstances this is carried out in three
stages, Pre Filtration, Activated Carbon Filtration and Final Filtration.
Pre Filters are installed prior to the Refrigerated Dryers and remove particulates
down to 5 micron size, this protects the drier’s drain system from becoming fouled
with dust particles that have passed through the compressor air intake (nominally
a 20 micron filter) the filters are provided with a differential pressure gauge and
an integrated automatic drain to monitor the filter elements condition under
normal operating conditions.
Activated Carbon filters use a molecular sieve to trap odours and any oil
vapours that may have entered the pipeline at the compressor intake. They will
also be fitted with a differential pressure gauge and an integrated automatic drain,
the drain should not be necessary as any moisture will be removed by the dryer
prior to the air coming into the drain.
Final Filters are installed after all other procedures have been carried out, they
remove particulates down to 0.01 microns with an efficiency of 99.9995%, this
filtration size is capable of removing almost all known Bacteria and some of the
larger Virus’s. When removing and or replacing these filter elements it should be
standard procedure to wear protective clothing suitable for use with bio
hazardous materials.
Carbon Monoxide Removal
In some installations it may be necessary to install filtration to remove Carbon
Monoxide, to do this there are filtration units that will convert the Carbon
Monoxide to Carbon Dioxide by the use of a catalytic converter. The process
used inside the converter requires very dry air and it is absolutely necessary to
install desiccant dryers prior to the unit. Additional filtration may also be
necessary downstream of the dryer depending on the manufacturers

Monitoring of the Air Stream
In some installations it will be necessary to monitor the air stream for purity,
monitors that measure the dew point, carbon monoxide and carbon dioxide levels
are readily available. This may be required if the air supply is of such a nature as
to include high levels of these contaminants from the local air, causes such as
large motorways, local industrial plants for various chemicals etc need to be
ascertained during the design stage and tests may even be recommended prior
to the installation proceeding.


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