Medical Air Plant
a. General
Medical air shall be provided by air plant capable of providing a net flow rate of
………. L/min @ 10 bar with one compressor not running (after dryer losses). The
plant shall consist of two air compressors and one air receiver, duplex filter/dryer
module and pressure regulation group.
b. Standards
The medical air plant shall be designed to fully conform to the requirements of
UK National Health Service-Model Engineering Specification C11 and Health
Technical Memorandum No. 2022 (HTM2022). The air compressor shall be of proven
reliability and shall be driven by electric motors to class F insulation and noise. Levels
tested to BS 4999. The receiver vessel(s) shall be constructed and tested to EN 286
or BS 5169/1975 Class 3 Grade E. The control panels and electric installations shall
conform to the latest IEE wiring regulations and the starters are to BS 4941.
The standard range of Medical Air Plant is “CE” marked under the Medical
Devices Directive 93/42/EEC. With approval notified body number. Under this
directive, the specified products are as Class IIb Medical Devices.
c. Air Compressors
A total of two identical rotary screw type air compressors shall be provided
each directly driven by an electric motor and incorporating an air inlet filter and
silencer. Each compressor shall be provided with internal after coolers and
auto/manual drains. The compressor electric motors shall be class F type suitable for
415 Volt 50 Hz supply.
d. Receiver Vessel(s)
One No. vertically mounted welded steel receiver vessel (s) shall be provided
with access capacity will be ……. litres. Receiver shall comply with BS 5169:1992
Class 3. Receiver shall be protected by a pressure relief valve and fusible plug and
shall be provided with an automatic drain.
e. Filter Dryer Assembly
Air delivery is via duplex sets of filters, pre-filter, dust filter, activated carbon
filter and bacterial filter. Each filter shall be fitted with differential pressure gauges to
monitor filter performance. Filters shall comply with BS 7226:1989 and be either dry
medium filters or grade CA paper element filters. The air at the outlet shall contain no
more than:
Water
95 mg/m 3
Dry particulate
0.01mg/m 3
Oil mist
0.01mg/m 3
Oil vapour
0.03mg/m 3
Carbon monoxide
5.0 mg/m 3
5 ppm v/v
Carbon dioxide
450 mg/m 3
500 ppm v/v
Duplex desiccant dryers shall be used and controlled by a separate control
panel and incorporate a dew point alarm system.
f. Control Panels
A dedicated control panel shall be provided, for air compressor, to house the
isolating switch, starter, MCB, ammeter, hours run meter, lead compressor selector
switch, pressure switch, alarm and BMS contacts. Panel shall contain master alarm
unit, dryer control unit and compressor control unit separately. The plant control unit
shall have a separate supply for each compressor, controlled by a separate subcircuit. The panel shall allow either manual selection of duty/standby for each of the
compressor or have an automatic sequence selection with a means for manual
override. Under normal conditions the capacity of the plant shall be 100% system
design flow with one compressor not running. A pressure transducer shall sense the
compressor on or off, maintaining the design pressure. A digital readout of vessel
pressure shall be located on the central control panel door capable of being switched
to show pressure in bar. During periods of peak demand the lag compressor shall
automatically be activated to operate in addition to the lead compressor.
There shall be individual starter units for each compressor. Each starter unit
shall contain an isolator interlocked with the covers, suitable circuit breaker, an
industrial grade ammeter to BS 89, a total hours counter and a green ‘cut in’ and a
red ‘cut out’ indicator.
The compressor(s) automatically shut down once pressure levels have been
restored. In the event of a lead compressor failure, the lag compressor shall take
over and the first stage of the remote alarm system shall be activated by the control
panel. The control panel shall be equipped with an emergency stop button against
emergency conditions.
The air plant shall have four stages of alarm conditions inputing to the alarm
system and follows:
1. Plant faults caused by:
Control circuit failure.
Compressor temperature high
Activation of any other safety device.
Failure of a selected compressor to run up on time.
2. Plant Emergency caused by:
Low receiver pressure fault (0.5 bar below cut-in pressure of the lag
compressor(s) or 0.5 bar above cut-out pressure of the lead compressor.)
Dew point above -26 C at atmospheric pressure.
3. Reserve Fault caused by:
Reserve manifold pressure less than 50%.
4. Pressure fault caused by:
High or low pipeline pressure.
Building Management System contacts shall be:
(Normally Open or Normally Closed contacts)
Plant fault,
Plant Emergency,
Reserve Fault,
Pipeline Pressure Fault.
The dryer control unit shall contain the following
a. Dryer selector switch for automatic and manual operating
b. Dew point meter and a pressure sensor
c. Individually fused, separate cycling systems for each dryer
d. A system to control regeneration of the dryers
e. Green and red function indicators for each cylinder of dryer sub assembly
to indicate
I. Drying (green)
II. Regenerating (red)
Master Alarm unit shall comprise of two sections, one for plant status
indicators and the other one for dryer status. Plant status section shall have an LCD
display which shows pipeline pressure, output air temperature, humidity and dew
point value. Panel shall have test, mute and programming facility in order to set cut in
and cut out pressure level of each compressor. Plant status section shall indicate
following conditions;
1. Green coloured LED to indicate “Cut in” for each compressor
2. Yellow coloured LED to indicate “Cut out” for each compressor
3. Red coloured LED to indicate “Defective” for each compressor
4. Yellow coloured LED to indicate “Plant Emergency”
5. Yellow coloured LED to indicate “Plant Fault”
6. Red coloured LED to indicate “Line Pressure Fault”
7. Yellow coloured LED to indicate “Reserve Low”
8. Green coloured LED to indicate “Main Power Supply On”
Dryer status section shall indicate following conditions;
1. Green coloured LED to indicate “Dryer on” for each dryer
2. Yellow coloured LED to indicate “Dryer Failure” for each dryer
3. Yellow coloured LED to indicate “Pressure Fault”
4. Red coloured LED to indicate “Dew Point Alarm”
5. Yellow coloured LED to indicate “Filter Dirty”
6. Yellow coloured LED to indicate “High Water Level”