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ITC/ITF Series
inverter driven screw chillers
R134a refrigerant
Introduction.
In these last 10 years awareness in
energy saving has continuously increased and, in both
industrial and HVAC applications, we strive to regulate
the electrical consumption of cooling equipment using
the most effective energy efficient techniques. In the
refrigeration and heating pump sectors
increasingly stringent European standards and directives
are now applied to the appropriate and proper use of
materials, innovation and the application of new
technologies to improve refrigeration system
efficiencies.
In the refrigeration and air conditioning
sector continuous attempts are made to reduce the energy
consumption of all systems, by improving the management
of cooling power, optimizing the use of water-glycol
flow and providing greater temperature accuracy.
HVAC design for Data Centre - total load 2,6Mw
Recent research has developed the use of
environmentally friendly refrigerants with lower
ambient impact and excellent thermal performances.
However, large refrigerant equipment
still has very high power requirements, especially those
demanded from very large air or water cooled chillers
with screw compressors. The utilization of screw
compressors is very high amongst large capacity chillers
(>300 kW) and therefore the optimization of partial load
performance, which is a condition present in almost all
refrigeration plants at various stages throughout the
year, is the goal prefixed by Hitema.
A modern and intelligent technology is to
control the large power demands of screw compressors
with the frequency control network, using inverter
electronic devices.
What are the energy advantages offered by
Hitema with the inverter technology?
An inverter (VFD) is an electrical device
acting on the variation of voltage and frequency. The
inverter uses the line alternate voltage (a.c.) to
produce a direct voltage (diode bridge – d.c.). From
this direct voltage an alternate voltage is regenerated
(PWM technique) with a frequency f between 0 and fmax
(maximum frequency) and voltage V < Vnet (electric net
voltage).
Inverters are widely used in
film-polyester capacitor configuration, which is a
similar technique used on the photovoltaic plants. The
absence of the electrolytic material avoids the early
aging due to temperature, currents and stocking periods.
The current distortion THDI is much lower than in
electrolytic capacitors since the equivalent electrical
capacity is lower. Moreover the compactness and
longevity of inverters above film capacitors is also a
consequence of the superior effectiveness of cooling
directly with liquid refrigerant line. By increasing the
frequency the number of compressor revolutions
increases linearly. As frequency increases compressors
r.p.m. increases and as frequency decreases compressor
r.p.m. decreases. The range of application typically
used is between 30 Hz and 70 Hz.
Main advantages are:
·
The starting current is effectively equal
to 0, as current is directly proportional to frequency,
the inverter starts the screw compressor with void
frequency and it causes an absorption equal to zero.
·
Cooling power increases until to 20%
above the optimal cooling power referred to at 50Hz,
this is because the screw compressor can rotate with
higher gears reaching higher frequencies up to 70 Hz.
·
Reduced electrical consumption at partial
load between 30 and 50 Hz compared to a standard screw
compressor with a slide valve capacity control. This
results in a measured absorbed power of up to 15%.
·
Superior control of water outlet
temperature exhibiting less fluctuation around the set
point temperature. Typically tolerances of +/- 0.5°C
around the set point are possible.
·
Reduced mechanical compressors wear, as
the screws will rotate for most of year with reduced RPM
(higher MTBF).
·
Inverter technology with screw compressor
variable Vi, that is the rapport between aspiration
volume and discharge volume of gas as a function of
condensation temperature. Therefore at every load and at
every ambient temperature, compressor efficiency is
always maximised.
Performance of a Hitema chiller with
compressors driven by inverter.
Fig. 1 illustrates the first important
point showing lower absorbed power of the screw
compressor when installed with an inverter, compared
against a standard screw compressor with slide valve.
 Fig.1
With a slide valve the gas flow control
is less accurate than the inverter controlled
counterpart. With standard compressor the capacity steps
are static and prefixed (e.g. 100%, 75%, 50% and 25%).
With the inverter solution, the screw revolution
decreases proportionally and the gas flow is modulated
in a linearly.
Fig. 2 shows the real data performance
for air cooled chiller. The COP is ratio between the
chiller cooling capacity and the required compressor
power input. The EER is the
relationship between the cooling capacity
of chiller
and the total power consumption of the
refrigeration unit (compressors + fans).
 Fig.2
If
the cooling load decreases whilst the ambient
temperature decreases, then the absorbed power of
chiller decreases much more rapidly than the reduction
in cooling capacity in a non-linear relationship.
These COP and EER values are very
competitive with other available technologies (e.g.
centrifugal compressors) and COP values with inverter
screw compressors can attain > 8 and EER > 7. This chart
above refers to an air cooled chiller with R134a
refrigerant, with an inlet water temperature of 12°C
and an outlet water temperature of 7°C. The graph
illustrates the massively beneficial effect on the
efficiency of the chiller unit as the ambient air
temperature and the load on the chiller reduce from 100%
load in a 35°C ambient (worse case), to a situation
whereby the load on the chiller is 50% and ambient
temperature is 15°C. As can be clearly observed, the EER
for the unit at 50% load in a 15°C ambient is over
double (~7) the value compared to when at 100% load in a
35°C ambient temperature (~3). The thermal performances
indicated demonstrate that this unit easily qualifies
for Class A efficiency categorisation.
It is clearly important to know the
ambient temperature, the water temperature and the
maximum load during the year (month by month) in order
to assess the real operating efficiency of a chiller
unit.
Fig. 3 shows the EER value trend of a
chiller without free cooling when operating with a water
outlet temperature of 5°C.
. Fig.
3
This chiller is designed for 680kW at
full load. During the warmer months of the season
(June-August) the required load is 100%, whilst during
the colder period (November-March), when ambient
temperature is much lower, the chiller load is estimated
between 50 and 60%. It is interesting to observe that
the EER in the hotter months, during the worst ambient
conditions, has minimum value of 4, whilst when the
chiller load is around 60%, the EER values are
typically between 6 and 7, much higher than a standard
chiller. All these values refer to the maximum ambient
temperature for each month so these EER are considered
as a minimum.
 Fig.
4
Fig. 4 shows the outlet water temperature
trend. Inverter control achieves much greater water
accuracy than is possible control with a standard screw
compressor. It is evident that the water temperature
fluctuation is only +/- 0.5°C deviation from the set
point. Furthermore the set point value is established
much rapidly than on a standard chiller.
In the European market the adopted index
to classify chiller performance is called E.S.E.E.R and
this is in accordance with E.E.C.C.A.C. proposal (Energy
Efficiency and Certification of Central Air
Conditioner). The formula used to calculate this is:
ESEER = 0.03 x EER (100%) + 0.33 x EER
(75%) + 0.41 x EER (50%) + 0.23 x EER (25%)
The EER value (%) is the efficiency of
the chiller at 100%, 75%, 50%, 25% of load under the
various conditions in accordance with the table below:
For chillers with screw compressor
installed with inverter control and variable Vi, the
increase achieved in the ESEER is 15%.
Axial Fans with inverter
Hitema also offer the use of inverters
applicable to standard axial fans. To control the volume
of air circulated through the condensers in air cooled
chillers simple fan speed regulators to cut the phase
applied to the fan motor are currently widely used. With
this method it is possible to decrease the rotation of
the motor by intervening directly on the supply voltage.
However, with the use of inverters, which modulate the
frequency from 20 Hz to 50 Hz it is possible to steadily
reduce the air flow and achieve improved condensing
control.
The benefits observed from the use of
frequency variation with respect to voltage variation
are:
·
Reduced noise levels,
which is a key point when using axial fans for
refrigeration in the air conditioning sector, as air
cooled chillers are widely used in residential, external
applications; when installed with fans operating from a
variable frequency drive, significant noise reductions
up to 6 dBA for the same chiller unit are possible
(ISO3744).
·
Lower energy consumption.
For low-medium speed (rpm) the frequency variation
allows a reduced power consumption. However the motor
efficiency is completely utilized with all cooling
load. A cut phase adjustable fan-motor has an
efficiency ratio between 72 -74%, whereas the same motor
with frequency driver has a performance ratio of 80%.
Centrifugal process pump (2-4 poles)
with inverter.
Hitema propose the application of the
inverter control on one or more centrifugal pumps, in
order to obtain a non-dissipative regulation of power
with the pump speed variation, depending on the heat
load required.
Hence we have obtained significant
results in energy saving as you trace the real load
energy requirements without any additional loss or
consumption being incurred by the process.
To understand how the non-dissipative
adjustment is able to act in this method, we can
consider the operating curve of a centrifugal pump
below. (fig.5)
 Fig.
5
The intersection of the characteristic
operating range for a centrifugal pump with a typical
flow-pressure curve, can be used to identify the
required point of working regime (point A = 100% design
flow). If the load in the system requires a flow of 75%
of the maximum design flow by the regulation by the
classic choke valve installed after the pump then an
additional pressure drop is artificially introduced and
the system must overcome a higher pressure drop (kPa or
m.c.a.) than is actually required by the load (point B).
Furthermore by moving the operating flow point, the pump
efficiency is also changed, which then introduces a
further efficiency loss.
 Fig.
6
By adjusting inverter frequency instead,
it follows the real load demand by altering the
pump curve (fig.6). Varying the speed of the pump varies
its actual operating curve, which will move vertically
downwards and thus we reach into the new operating point
(point C) without any artificial valve introduction.
This results in a real energy saving of 30%.
The process pumps when installed with an
inverter can effectively have zero starting current if
the water flow can be gradually increased up to the
maximum flow, which again avoids potentially damaging
water hammer. The correct management of the inverter
location completes the full system optimization.
Conclusions.
Today there are many applications that
require effective and innovative solutions to reduce the
absorbed powers requirements of refrigeration hardware
in process cooling industries, commercial air
conditioning and data centres facilities.
The optimum operation of refrigeration
equipment at partial loads is especially significant in
conditions where the medium annual ambient air
temperatures
are between +5°C and +20°C, typical for
the vast majority of European conditions.
For even lower ambient temperatures the
combination of inverter technology coupled with that of
free-cooling, whereby chilled water can be produced
using only fans energy, can be effectively used to
produce chiller units with even greater efficiencies
than previously considered possible.
HITEMA makes sure its Customers' business is constantly up and running, thereby helping to increase their competitive advantage.
The creation of highly customized solutions semplifies handling in the working place
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