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Air Conditioning - Refrigeration
Chillers
Therefore check, who will be eternally bound
special part
Lars Keller, Munich,
Ralf Beleth, Karlsruhe
This comparison of various chiller units
and associated heat exchangers should
provide help in determining on a
project specific basis the optimal
machine. The general rule is that all
relevant factors be taken into account
for the application, and to carry out a
weighting and evaluation.
Rating the Efficiency
To assess the efficiency of chillers, the
valuation by Eurovent and ARI are
widespread. The ratio of nominal
cooling capacity to electrical power
consumption of the compressors and
the power consumption of all control
and security measures, is according to
Eurovent defined as EER (ENERGY
EFFICIENCY RATIO) and is the basis of
classification from the most efficient
"Class A" extending to the most inefficient
"Class F". The ESEER (EUROPEAN
SEASONAL ENERGY EFFICIENCY RATIO)
according to Eurovent and IPLV
(INTEGRATED PART LOAD VALUE)
according to ARI Standard 550/590-
2003 makes a statement about the
efficiency of chillers at full and partial
load with decreasing coolant temperature.
Autoren
Dipl. Ing. (FH) Lars Keller
as Head of Sales / Marketing at
Kältetechnik aircool GmbH and author
of numerous technical publications and
the book Guide for Ventilation and Air
Conditioning.
This is true, not only in life, but also in the purchase of a new chiller.
Water-cooled chillers with capacities ranging from approximately 2
MW are currently available in various configurations. Compressors are
of the semi-hermetic screw or turbo type, the refrigerant R-134a and
R410A are prevalent, and shell and tube heat exchangers will be
installed. In the cooling system, the operator is faced with the decision
to use dry coolers, Hybrid coolers or cooling towers.
But to get a reliable statement concerning
the anticipated cost of operation,
the expected load characteristics have
to be determined and calculated. This
must be done for the entire system
(chiller, pumps and heat exchangers),
as a one-sided optimization of the
chiller does not necessarily minimize
the total cost of operation.
Here, the following criteria are to be
observed:
• The lower the cooling water outlet
temperature at the condenser of the
refrigeration unit, the more efficiently
it is operating and it provides more
capacity. (Vendor Specific limits are to
be observed, as a minimum pressure
ratio pc/ po is necessary)
• The smaller the difference between
the cooling water outlet temperature
of the cooling tower and the wet bulb
temperature, the higher are investment
costs and space requirements of
the cooling tower.
• The greater the cooling water temperature
spread, the lower the flow rate,
pump capacity and pipe dimensions,
practice has shown that 5 to 8 K to be
an economic size for the entire plant.
Dipl.-Ing. (FH) Ralf Beleth
is a sales representative at the
KTK Kühlturm Karlsruhe GmbH
Which is the correct
refrigerant
For years, the mono-chlorine-free
refrigerant R134a has established itself
due to its favorable thermodynamic
properties. The advantage is seen in
the low differential pressure pc-po and
the high critical temperature, which is
not achieved during operation. Turbo
chillers are always operated with
R134a. The non-azeotropic refrigerant
R410A is a mixture of 50% R125 and
R32, with a negligible boiling range at a
phase change of less than 0.2 K. The
disadvantage is the high pressure
needed, which places enormous
demands on the screw compressor
technology. Due to the high volumetric
cooling capacity, less Refrigerant
charge is needed and smaller compressors
are necessary than for R134a. To
illustrate, here an example: A screw
compressor with R134a as refrigerant
provides 100 kW cooling capacity, by
using R410A it will provide about 230
kW! This results in compact equipment
dimensions and low cost. The ODP
(Ozone Depletion Potential) makes a
statement about the destructive effect
of ozone, the refrigerant and data are
based on the CFC R11 with ODP = 1
The GWP (Greenhouse Warming Potential
/ Global Warming Potential) represents
the global warming potential of a
substance as opposed to CO2. The
GWP-factor makes a statement about
how many times stronger in comparison
to CO2, the direct contribution to
the greenhouse effect is over a specified
time horizon of 100 years. For
example, if 1 kg of refrigerant R134a is
emitted into the atmosphere, this is
equivalent to an emission of 1300 kg
C02.
40 HLH Bd. 60 (2009) Nr. 10 - October

Air Conditioning - Refrigeration
R134a R410A
Volumetric cooling capacity 0°/40° kJ/m 3 2.050 4.781
Evaporation heat kJ/kg 216 268
Critical temperature °C 101 71
Absolute pressure at 0 ° C(p o ) bar 2,93 8,06
Absolute pressure at 40 ° C (p c ) bar 10,17 24,36
Differential pressure p c - p o bar 7,24 16,3
Pressure ratio p c / p o - 3,47 3,02
Temperature glide K 0 < 0,2
ODP - 0 0
GWP (relative to CO2, 100a) - 1.300 1.720
OEL (occupational exposure limit) kg/m 3 0,25 0,44
The TEWI (Total Equivalent Warming
Impact) takes into account the sum of
direct (GWP contribution of refrigerant
leaks during installation and disposal)
and indirect (contribution of CO2
emissions resulting from energy
consumption to operate the plant)
emissions of greenhouse gases. If the
refrigerant loss is minimized, then the
influence of the GWP in the TEWI is very
low. A comparison of the main physical
data is shown in Table 1.
Turbo or Screw
The semi-hermetical screw compressors
are in the power range up to 2200
kW, depending on the refrigerant, the
operating conditions and number of
compressors. They show a wide range
of operation, they can be used with the
default configuration; varying flow
temperatures of -8 ° C to 15 ° C with
cooling water outlet temperatures of
up to 55 ° C. The performance adjustment
should be continuously adjustable
from 25 to 100% of the standard
cooling capacity, a flow temperature
control is to be considered as prior art.
Compressors are started via the
star-delta, to reduce the starting
current a soft start can be used, more
recently, frequency converters are
used. McQuay semi-hermetic-turbo
compressors are used in the power
range from 900 to 4500 kW. Since a
maximum of two compressors can be
used in a refrigeration circuit, a capacity
of up to 9 MW can be achieved. Each
turbo chiller is based on a projectspecific
configuration; the selection of
the main components such as motors,
compressors, gears and gear ratios,
impeller and heat exchangers is dependent
on the operating conditions. For
this reason, as well as flooded evaporation,
which requires an exact Refrigerant
charge, varying operating points
Table 1
are not quite as arbitrary as with screw
chillers and must be verified for the
configuration. The adjustable power
control from 10 to 100% is best carried
out using the inlet guide vane setting
via oil pressure. The turbine can be
started via star / delta or soft-start, by
mounting an optional VFD (Variable
Frequency Drive, frequency converter)
corresponding to the starting current,
respective to the operating current
(Figure 1). Again, the VFD will significantly
contribute to an increase in the
ESEER / IPLV, through the loss dissipation
of the frequency converter; however
the 100% EER decreases by about
2.5%. In Table 2 a comparison of
different Chillers at different operating
points and cooling systems can be
seen.
Cooling Towers
When it comes to cooling towers everyone
has probably one of those big
steaming power generation plant
cooling towers in mind. For a large
portion of the cooling tower processes,
standard cooling towers are implemented.
However, they perform their job in
Physical Data for the refrigerants
R134a und R410A
most instances well concealed and can
only be seen when taking a closer look
at the roofs. Often, cooling tower
issues of are placed in the background,
even with respect to: The relatively
small cost compared to the cooling
system, although the choice of an
appropriate process, system, tuning of
the refrigeration unit and integration
into the overall system has a significant
impact on the efficiency of the overall
system.
Distinction between the
methods of heat transfer
to the outside
Basically, two methods are used to
dissipate the excess heat to the
environment. Dry cooling transfers the
heat to the surrounding air by the
temperature gradient of the coolant to
ambient air temperature. Accordingly,
the high temperature of the cooling
medium of approx. 45/40 ° C is
assumed when designing for the
Picture 1
2.5 MW turbo chiller WSC 100 with
free-standing frequency converter of
protection IP54 to improve part-load
efficiency at decreasing coolant temperature.
HLH Bd. 60 (2009) Nr. 10 - October
41

Air Conditioning - Refrigeration
Chiller Specifications
Unit
Proximus Evolution 596.2 XE S T
WSC
100
WDC
079
Cooling capacity
kW 2.268 2.196 1.933 2.200
2.200
2.200 2.200 2.200 2.200
Power consumption at the terminal box kW
434
461
553
310
352
509
314
352
523
Condenser capacity
kW 2.702 2.657 2.486 2.510
2.552
2.709 2.514 2.552 2.723
Cold water temperatures °C
12 / 7 12/ 7 12
/ 7
Cooling water temperatures °C 27 / 32 30 / 35 40 / 45 27 / 32
30 / 35
40
/ 45 27 / 32 30 / 35 40 / 45
Medium Water 34% Glycol 34% Glycol Water 34% Glycol
34% Glycol Water 34% Glycol 34% Glycol
Performance figures - 5,2
4,8
3,5
7,1
6,3
4,3
7,0
6,3
4, 2
special part
ESEER-Value - - 5,38 - -
6,21
-
-
7,48 -
Evaporation
dry
flooded
flooded
Number / type of compressors (Units) Quantity 2 / semi-hermetic screw compressor 1 / semi-hermetic turbo compressor
2 / semi-hermetic turbo compressor
Capacity control at constant cooling
water temperature
% 12.5 - 100
10 - 100
10 - 100
20
- 100 5 - 100 5 - 100 20 - 100
Length x Width x Hight
mm
4.800 x 1.350 x 2 .550
4.300
x 2.100 x 2 .550
5.600
x 1.800 x 2.550
Refrigerant circuits
Quantity
2 1
1
Refrigerant
R
410A
134a
134a
Refrigerant charge amount
kg
2 x 130
636
670
670
884
920
878
Cooling
Circuit
Technical Data heat exchangers
Design temperature
Unit
° C
Evaporation cooler for an open
circuit with centrifugal fans
2 x KD 2/18-28-S2
Evaporation cooler for a
closed circuit with
centrifugal fans
2 x KI 3/12-28-12
Dual cooling system / hybrid
coolers for closed circuit
with axial fans
3 x KA VH-09-2x6
Chiller for a closed circuit
with centrifugal fans
3 x KA VL-09-2x6
Evaporative c ooling
Evaporative
cooling / Dry c ooling
Dry
cooling
open c ircuit
closed
circuit
closed circuit
wet bulb 21 °C wet bulb 21 °C
wet bulb and ambient air
temperature 21°C / 18 °C
ambient air temperature 32°C
Cooling agent
Water
Water-glycol mixture 34%
Water-glycol mixture 34%
Cooling water temperature °C
32 / 27
35
/ 30
45
/ 40
Cooling c apacity
kW
2.700
- 2.500 2.650 - 2.550 2.650 - 2.550
2.700 - 2.500
Ventilator shaft capacity max. kW 20,8 48,44
86,4
90,00
Type of water q uality
Basis
VDI 3803 Basis VDI 3803
Osmotic water
-
Additional water conditioning necessary
- hardness stabilization, corrosion
yes yes
no
-
protection, organic load
Evaporation water quantity max.
(full load)
m³/h
4,0
3,9
3,9
-
Water discharge reference value
m³/h
1, 3
1,3
0,6
-
Sound output level with out / with
additional sound elimination measures
dB(A)
99 / 78 105 / 84 108 / -
99 / -
Dimensions Length x Width x Height mm 2 x 5.000 x 3.700 x 2.400 2 x 5.000 x 3.700 x 2.900 3 x 9.300 x 2.250 x 2.400
3 x 9.300 x 2.250 x 2.370
Floor space approx. (without
maintenance area)
m²
37
37
63
63
Operational weight cooling towers
kg
12.800
29.000
24.600
12.900
Table 2
Technical Data of various refrigeration
units and heat exchangers
typical maximum surrounding air
temperatures in the summer of about
32 to 34 ° C in Germany. In wet or
evaporative cooling, one takes advantage
of the physical effect that for a
corresponding change in the
aggregate state, vaporization energy is
required. Thus evaporative cooling
towers have a considerably greater
efficiency of heat dissipation because
of latent heat. Compared to dry
cooling, significantly lower the temperatures
of the cooling medium can be
achieved, since the ambient air temperature
is not critical, but the wet bulb
temperature is. Thus cooling media
temperatures of e.g. 32/27 ° C can be
achieved in an implementation for a
wet bulb temperature of 21 to 22 ° (Fig.
2). Dry or glycol coolers are often given
preference over evaporative cooling,
because it avoids the hassle and cost of
water consumption and water
treatment. For decentralized and
relatively small plants dry cooling may
be the choice, but in large and centralized
systems, evaporative cooling is to
be preferred.
42 HLH Bd. 60 (2009) Nr. 10 - October

Air Conditioning - Refrigeration
A comparison of space and power
consumption can be seen in Table 2.
However, in any case the choice
between dry / evaporative cooling, the
impact on the chiller should also be
taken into account. The example of the
turbo chiller WSC 100, the clamp power
consumption when using a dry cooler
is 509 kW, when using an evaporative
cooling tower, however, the power
consumption is only 310 kW: Thus just
by the choice of implementing another
cooling process the electrical energy
used by the chiller can be reduced by
40%!
Distinction based on the cooling
medium system
Here one can also distinguish two basic
systems. Certainly the oldest and best
known method is that heated water is
exposed directly to the ambient air,
and by way of evaporation of part of
the water, cooling of the same is achieved.
Cooling towers where the water is
directly exposed to the ambient air are
designated as open cooling towers
(open cooling water circuit). The
earliest cooling towers emerged at the
beginning of industrialization in the
form of natural draft cooling towers
which were relatively simple wooden
structures. Graduation houses are in
use today for another purpose but
operate on the same principle. The
most efficient heat transfer is achieved
by way of direct contact of water with
the ambient air. However, the water is
exposed to contamination in the air
and adds to the impurities such as
water dissolved minerals are thereby
concentrated, since only pure water in
its vapor phase may leave the circuit,
this should be taken into account in
plant design, e.g. through a impurity
factor in the chiller. If this qualitative
change in the water can not be
accepted, protection may be achieved
by separation of the circuits to be
cooled machine / system from the
influence of the polluted water. If it is
realized directly in the cooling tower
loop separation, usually by the installation
of a tube bundle heat exchanger),
it is known as a "closed cooling tower"
(closed cooling water circuit).
Which is the cooling tower
that you would like to have
To begin with, this question can not be
answered here. If the end-user / operator
wants to have an efficiently operating
system, the cooling tower manufacturers
will be happy to provide a
detailed consultation. In recent
decades, a wide variety of different
types of cooling towers for various
applications have been developed,
which are described below.
Open cooling towers with
centrifugal fans
They are well suited by their compact
design for use in a small space, are of
low weight and move large amounts of
heat with as little energy as possible.
The centrifugal fans offer the ability to
mount additional silencers, so that
even demanding noise requirements
can be met. Because of the mode of
operation it is obviously not possible to
use dry cooling, which means that over
the entire operating life one must
expect correspondingly high water
consumption. However, open cooling
towers may be used at sufficiently low
temperatures for "free cooling" (i.e.
cooling without refrigeration machine
operation). The minimum cooling water
temperature usable with free cooling,
however, is limited due to the risk of ice
formation at below10 degrees C.
Closed cooling towers with
centrifugal fans
The tube bundle heat exchangers of the
cooling water circuit is protected from
pollution, but the required space,
weight and energy requirements are
higher than for an open cooling tower.
Silencers can be mounted, as well as
the operation for free cooling. Additionally,
it is possible to run the cooling
tower in dry mode. In observance of the
cooling water temperatures and the
ambient air temperature, the tube
bundle heat exchanger in dry operation,
can dissipate approximately 10 to
20% of the designated capacity of the
chiller in operation. An application
example is the air conditioning of a
building, which in the transitional
period and in the winter only one server
room needs to be cooled.
HLH Bd. 60 (2009) Nr. 10 - October
43

Air Conditioning - Refrigeration
Dual heat exchangers or
hybrid coolers
special part
The aim of these units is to combine
the benefits of evaporative and dry
cooling in one device. Thus it is possible
on the one hand, to reach the low
temperatures of an evaporative
cooling, and on the other hand
through the rational selection of the
switching point between wet and dry
operation of approximately 15 to 18 ° C
to achieve considerable savings of
water. Lower set points worsen the
water savings and thus the cost
savings. The devices are designed so
that even at the switchover point, the
full cooling tower capacity is available.
These systems with intelligent and
effectively controlled equipment also
have their price.
Dry coolers or "glycol"
These devices have a relatively simple
structure, consisting of a tube bundle
with slats designed to achieve the
greatest possible heat transfer surface
area. Fans provide the necessary
exchange of air. Of course you do not
need any extra water, but usually have
higher electric power consumption
than an evaporative cooler and, as
already mentioned, the higher coolant
temperatures have a negative impact
on the performance numbers of the
chiller. Advantageous is the ease of
installation, the closed cooling circuit
and the high potential for free cooling
capacity.
Water preparation and treatment
for evaporative cooling towers
Evaporative cooling towers can fulfill
their purpose, only when the water
quality is good. The best cooling tower
will fail, if insufficient attention is paid
to the manufacturer's guidelines for
water quality. The currently available
equipment e.g. for water softening,
desalination are relatively easy to
handle. When designing the system,
specialized firms will provide appropriate
guidance.
Energy-efficient operation
of cooling towers
Because a cooling tower is usually
designed for the highest summer
temperatures (e.g. wet bulb temperature
of 21 ° C) it is at all lower temperatures
actually oversized. The resulting
Picture 2
h-x diagram, a representation of
the process of dry and wet
cooling. In the initial state at the
entry point of the air is the same.
Through the various processes
involved the corresponding line
results.
"extra" power is not really needed. The
performance adjustment can be
effected by regulation of the airflow
through the cooling tower. For a long
time, the fans were operated with
motors having 2 or sometimes even 3
switchable speeds. Thanks to the fact
that the frequency converter which is
only slightly more expensive than the
contactor control circuit for a speed
switch, they are frequently implemented
and for energy-efficient operation
they are the best choice. In connection
with the control of the chiller by use of
a frequency converter, a continuously
variable cooling water temperature
can easily be achieved.
Conclusion
A blanket statement, which system
version should be implemented is not
possible, the project relevant factors
such as capital and operating costs,
efficiency, redundancy, reliability,
setup time and situation, plant layout,
ambient temperatures, and noise
standards, as well as (to name but a
few) must be taken into account. To
find the ideal chiller / cooling tower
combination on the basis of investment
and operating costs, in preparation
a load profile must be created,
which also provides information on the
required partial load performance of
each unit. Frequency converters provide
low starting currents and high
ESEER / IPLV values, and at constant
cooling water temperatures the operating
cost savings are not so great.
Basically, it is necessary to determine
whether heat recovery and / or free
cooling can be implemented, the
performance numbers (benefits / costs)
which can thus be realized, cannot be
achieved by any of the most efficient
compression refrigeration machines!
44 HLH Bd. 60 (2009) Nr. 10 - October