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CPD Programme
is a more efficient heat exchanger than
the D-X type, but refrigerant level has to
be controlled, often by critical refrigerant
charge. Centrifugal chillers have always used
flooded evaporators, but oil management was
Impellers always a disadvantage. Oil-free centrifugal
Front Motor Rear Axial compressors eliminate this problem and
Radial Radial Bearing
Bearing Bearing
result in further increased efficiency. Features
of the flooded evaporator incorporated into
Sensor Ring Sensor Ring
these chillers include:
Figure 1 Components of a Magnetic Bearing System • Being designed and optimised specifically
for R134A with specially finned tubes;
> position. Permanent magnetic bearings do efficiency is reduced. Magnetic bearings • A refrigerant level sensor, which makes it
the primary work, while digitally controlled eliminate the need for oil lubrication and possible to take full advantage of the total heat
electromagnets provide the fine positioning. with it the need for oil pumps, sumps, heaters, exchange surface area. This helps to create
Four positioning sensors per bearing hold coolers and oil separators, plus the associated higher saturated suction gas temperatures
the levitated assembly within a tolerance of maintenance requirements. Figure 2 shows and higher EER values while lowering energy
0.0005 mm, so that when the shaft moves the components of the latest centrifugal consumption; and
from the centre point the electromagnet’s compressor. • An added heat exchanger further increasing
intensity is adjusted to correct the position, the overall energy performance levels by
six million times per minute. When the interchanging heat between the saturated
compressor is shut down the shaft rests on suction gas and condensed liquid refrigerant,
graphite lined, radially located bearings. The raising the temperature of the suction gas
compressor uses capacitors to smooth ripples while sub cooling the liquid, improving the
in the DC link to the motor drive and, in the cooling efficiency and EER values.
event of a power failure, the motor becomes
a generator, using its momentum to produce Variable speed compressor
electricity, keeping the capacitors charged for The centrifugal, oil-free compressor
the brief “stopping” period and providing incorporates an inverter-driven, permanent
enough power to maintain the shaft’s magnet, brushless motor. Typically,
levitation. The use of magnetic bearings also traditional induction motors of this size are in
incurs more than 500 times fewer frictional the 92 per cent efficiency range, but the new
losses than conventional bearings. This
Figure 2 The latest development in centrifugal
compressor’s permanent magnet DC motor
results in a reduction in the starting current
compressors
provides an efficiency of between 96 and 97
to just 2A, removing the transient starting per cent. The variable speed drive provides a
spikes frequently associated with screw and Floating head pressure speed range from 18,000 to 48,000 rpm.
reciprocating compressors and the need for An important aspect of the control philosophy
expensive soft start arrangements, as well as for the chiller is that the system is designed Micro-channel aluminium
eliminating the risk of penalty charges for to operate with a floating head pressure, condensers
exceeding maximum power demands. providing opportunities for savings not The micro-channel, parallel flow, air-cooled
available to conventional designs, where condenser coils are designed to increase
Oil free compressor head or condenser pressure is maintained at performance by up to 45 per cent, making
Until the development of the magnetic or near design condition to provide system it possible to significantly reduce condenser
shaft bearing, all compressors required oil and expansion valve stability during variable dimensions for a given capacity. Constructed
lubrication by varying degree, which meant load periods. Floating head pressure allows entirely of aluminium, the coils’ weight
the oil mixing with the refrigerant as it was the condensing pressure to fall during is also reduced by as much as 45 per cent
compressed. In this scenario, some oil leaves low ambient conditions, which increases while the manufacturing method reduces
the compressor and enters the refrigerant the efficiency of the chiller. Matching the the potential for refrigerant leakage, which
system. Highly efficient oil separating vessels components of the chiller is important makes it easier for end-users to comply
can be fitted to the compressor discharge and to maximise the potential of this new with the F-Gas regulation. Other benefits
return the oil to the compressor, but a small compressor. Integration of the controls and of micro-channel condenser coils include
amount of oil will still enter the system and managing the floating head pressures is vital a reduction in coil depth, which lowers air
travel in or with the refrigerant, through the to optimise chiller performance, particularly pressure drop, reducing fan power required
condenser and into the evaporator. Many when managing the liquid pressure to create the design air flow rate. The smaller
air conditioning packaged chillers with D-X amplification for free cooling (see later). coils also result in a 30 per cent reduction
evaporators have no oil separator and rely in the refrigerant volume and a reduction
on refrigerant velocity to carry oil back to the Flooded evaporators in refrigerant pressure drop across the coil
compressor. Oil adversely affects the heat A flooded shell and tube evaporator has the of 65 per cent, enhancing energy efficiency
transfer rate and pressure drop, particularly refrigerant in the shell covering the tubes still further. The condenser fans feature low
in the evaporator, so cooling capacity and through which the chilled water flows. It noise sickle blades that are arranged so that,
50 CIBSE Journal July 2009 www.cibsejournal.com
CIBSEjul09 pp49-52 cpd.indd 50 25/6/09 16:09:07
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