Optimisation of low temperature driven absorption chiller (h1) - infodag 2002

Dmitri Glebov, Tekn.DrInst. för kemiteknik, KTH, Stockholm, [email protected]Viktoria Martin, Tekn.DrInst. för kemiteknik, KTH, Stockholm, [email protected]Optimisation of Low Temperature Driven Absorption Chiller SUMMARY Optimisation of low temperature driven absorption chiller is carried out by means of a tailor-made simulation program. This program consist of two main modules: heat transfer coefficients calculations for each unit and mass and heat balances for the whole cycle. In order to complete this simulation program number of experiments and analysis were undertaken utilising equipment of different scale. BACKGROUND With respect to energy savings and environmental
power plant may instead of district heat produce a
protection, thermally activated systems are of a major
importance in the areas of refrigeration and airconditioning. Absorption chillers utilising lithium
The design of an absorption chiller for low
bromide-water solutions as a working pair are widely
temperature applications started in Klimat 21.
used in cold water production. In the absorption
According to the initial plan, the full-scale absorption
chiller, heat is delivered to the generator where the
chiller should work in the cooling season of the year
weak lithium bromide solution arrives. Here, some of
2000. This meant that the manufacturing had to start
the volatile refrigerant (water in this case) is
by the end of year 1999 and all design values had to be
evaporated resulting in a strong salt solution. The
ready in good time before that. In order to fulfil this
water vapour generated is then condensed in the
goal experiments were performed in two different
condenser at a lower temperature. The condensate is
units of different sizes; 1kW and 30 kW cooling effect
fed to the evaporator via an expansion valve and is
evaporated at low pressure and, hence, at lowtemperature thus generating a cooling effect. The
The result of Klimat 21 (project 1999-2000) a1,15
vapour formed is then absorbed in the strong lithium
MW absorption chiller utilising down to 70 oC hot
bromide solution that is pumped from the generator to
water from the district heating system has been installed
the absorber. Water vapour absorption into an aqueous
and is operating at Chalmers Technical University in
LiBr solution is a key stage in functioning of
Gothenburg, Sweden. The COP however is only in the
conventional absorption chiller. Coupled heat and
order of 0,6, which is lower than what is achieved with
mass transfer takes place during this process.
other types of absorption chillers [3]. The reason for thelow COP is the relatively high rate of circulation of
In a sustainable energy system, it would be interesting
lithium bromide solution between the absorber and the
to produce cold by operating absorption chillers on
generator. This high rate of circulation also leads to a
low grade heat such as waste heat or heat from a
high consumption of electricity for pumping. Therefore,
combined heat and power plant supplying heat to a
the solution heat exchanger in the pilot unit is divided
district heating system. The temperature in district
into two units. In one unit the lean and the strong
heating systems ranges from 70 – 120 oC. The lower
solution exchanges heat between each other whereas in
temperature range is used in summertime when
the second, the strong solution is cooled down further
heating of tap water is the main heat sink. However,
by the cooling water. This was done in order to utilise
absorption chillers available on the market are usually
the absorber for absorption only, not for cooling down
designed for a driving temperature of 120 oC or more.
the strong solution to the absorption temperature. It is
Since the design has to be based on the generator, the
our belief that by lowering the LiBr solution flow rate,
choice of such an absorption chiller for low
and by modifying the heat exchanger layout, the COP
temperature applications leads to a too large absorber
will become in the same range as for conventional
and evaporator. Hence, in the summertime when the
district heating demand is low, a combined heat and
Dmitri Glebov, Tekn.DrViktoria Martin, Tekn.Dr
EXPERIMENTAL RESULTS Optimisation of the low temperature driven absorption
chiller is carried out by means of a tailor-made
simulation program. This program consists of two
main modules: heat transfer calculations (heat transfer
dynamic) and heat and mass balances for the wholecycle. The second part of the simulation program was
completed during the first project phase: Klimat 21. In
order to complete the first part of the simulation
program, experimental research has been undertaken
utilising equipment of different scale. District heating temperature and LiBr solution flowrate effect on the chiller capacity and COPA 1,15 MW pilot chiller was installed at ChalmersTechnological Institute in May year 2000 and started
Figure 1. District heating temperature influence on the
to operate in June [3]. The main features of the chiller
that the chiller is flexible with respect to thegeometrical configuration hence being adaptable
the surfaces for heat and mass transfer are of
lamella type instead of the commonly used tubularheat exchangers;
new heat transfer additive is used instead of 2-
extra preheating heat exchanger is installed before
the generator in order to increase energy
extra subcooling heat exchanger is installed before
the absorber in order to decrease LiBr solution’s
temperature and utilise the absorber heat transfersurface for the absorption purpose only.
Since the district heating temperature and solution’s
Figure 2. LiBr solution flow rate influence on the
flow rate are important parameters, a number of tests
varying these have been carried out [2]. The districtheating temperature was varied between 70 and 90 0C. Effect of the district heating temperature on thecooling capacity of the chiller is shown on Figure 1.
District heating temperature has significant effect on
the chiller efficiency. Under higher district heatingtemperature stronger LiBr solution can be delivered
from the generator which subsequently will result in
better absorption, lower pressure and bigger
temperature difference in the evaporator.
Effect of the LiBr solution’s flow rate on the cooling
capacity and COP (coefficient of performance) is
shown on Figures 2 and 3 respectively. As can be seenfrom Figure 2, higher flow rates resulted in higher
capacity of the chiller. Also, coefficients of
performance values tend to slightly increase and they
Figure 3. Solution flow rate influence on the COP.Dmitri Glebov, Tekn.DrViktoria Martin, Tekn.Dr
appear to reach maximum at 18,2±0,2 kg/s: At this
transfer surface is of lamella-type. Lamella is a thin
flow rate, further increase does not seem to improve
plate with spot-welded construction that serves as a
the COP. This phenomenon can be explained by
considering the following: higher solution flow rateresults in intensive heat transfer in the absorber as well
The experimental facility consisted of a hot water
as in the generator thus resulting in higher capacity.
cycle, a solution one, five lamellas (0.5m x 0,5m with
However, the heat consumption in the generator also
total area of 2,5 m2), a hot water generator, two
increases. This means that for a certain design
pumps, solution collector and measuring equipment.
configuration and external conditions there is an
The system operates as a batch system. Hot water
optimum LiBr solution flow rate in terms of the COP
generator produces 75±0,3 oC water, which is
value as shown in Figure 3. Also of importance is that
transported by means of pump via flow meter
higher solution flows result in weaker LiBr
concentration in the absorbent because of shorterresidence times. Finally, although the cycle COP did
Because of the high corrosivity of LiBr solution, a
vary with solution flow rate, COP was not
62,6 wt% glycerol solution was used as a modelling
considerably sensitive to the solution flow rates: a 30 -
liquid. In this way, 60-65 wt% glycerol/water solution
40% change in flow rate resulted in 5-7% difference in
at 60-70 oC possesses of almost the same viscosity as a
LiBr solution under conditions in the generator in lowtemperature applications [2].
Under similar conditions two tests have beenperformed – with and without subcooler. Subcooler is
Glycerol solution is transported from the collector by
an auxiliary heat exchanger for cooling down LiBr
means of a pump via rotameter into the box distributor
solution before it enters the absorber. The subcooler
through three inlet tubes. The slit width (distance
has 400 – 450 kW capacity. During test run without
between the distributor and lamella) is 0,5 mm.
subcooler the absorber capacity increased almost by
Glycerol solution is flowing over lamellas as a falling
the same value as the subcooler capacity but the
film. Glycerol flow was changing from 32,0±1,0 l/min
cooling effect decreased rather significantly. The
up to 65,0±1,0 l/min. Warm water flow remained
decrease of the cooling capacity of the chiller can be
constant in all measurements and made up 6,6 m3/h.
attributed to the fact that part of the absorber surface
Heat transfer took place between the warm water and
started to act as a heat exchanger without absorption. It
the viscous solution through the lamella wall. The
is our belief that it is more beneficial to utilise the
solutions temperature was increasing during
absorber surface for absorption process only and not
recirculation. But the warm water inlet temperature
for cooling down the solution which can be realised by
remained constant (by means of hot water generator).
external (to the absorber) heat exchanger. The COP
Glycerol and water temperatures were measured by
value was lower for the case without subcooler due to
means of Almemo thermocouples connected to the
computer via a signal converter and acquired afterevery six seconds. Experimental investigation of viscous liquiddistribution along heat transfer surface of lamella-
As the glycerol flow was changing, the ”behaviour” of
the falling film was changing also. The summary of
Achieving an even distribution of a liquid along heat
the visual observation is presented in Table 1.
or (and) mass transfer surface is a key factor in heatand mass transfer improvement. Even distribution of a
liquid results in that the total interface is involved in
the transfer process and decreases the local deviations
in falling film thickness. One way to improve the
distribution quality is an additive. The additive
changes the surface tension of a system and improves
the wetting capability of the liquid [1]. Another way is
tailor-made distributors. Special geometry and
construction can arrange even distribution of a liquid
phase along the heat (mass) transfer surface. Liquid
phase flow rate is also an important parameter that
From the practical point of view, an important
The objective of this research was to experimentally
conclusion was drawn from these observations. When
study the flow rate effect on the distribution quality,
the box distributor is completely filled with the liquid
falling film hydrodynamic and falling film heat
phase then the heat transfer surface is completely
transfer coefficient. As previously mentioned, the heat
Dmitri Glebov, Tekn.DrViktoria Martin, Tekn.Dr
wetted. The falling film was smooth at relatively smallflows. As the flow increases, waves were observed on
From the practical point of view, it is important to
the surface and the film itself became somewhat
have a definition in order to determine heat transfer
thicker. Entering the distributor flow was suggested to
coefficient for different conditions. Usually, these
divide in three parts utilising three inlet tubes in order
experimental definitions present Nusselt number as a
to spread out the liquid evenly inside the box
function of other numbers like Reynolds number,
distributor. It is necessary to mention that this design
of the box distributor provided rather even distribution
of the liquid under relatively small flow rates.
The overall heat transfer coefficients, the K-values,were calculated using experimental data. In order to
measurements heat transfer coefficient values have
been smoothened using ”5 points linear smoothing”
Heat transfer coefficient inside lamellas is considered
to be constant as well as wall resistance. The first one
is calculated from the following equation [4]
Figure 4 shows how the K-values depend on the mean
The wall resistance was r=1/10667 m2 oC/W according
log ∆T for different solution flow rates. For any
to Berglunds Rostfria AB data. Film-side heat transfer
presented flow, the K-value is very sensitive to the
coefficient can be found as follows.
temperature difference in the region up to 25 degrees.
When the temperature difference is fairly small (5
degrees and smaller) the K-values generally do not
depend on the flow rate. This trend can be
Table 2 presents film-side heat transfer coefficients
explained by the fact that at small ∆T, the solution’stemperature is quite close to the boiling point so that
Table 2. Falling film characteristics.
the heat transfer coefficient is mainly governed by the
temperature gradient in the film rather than the falling
film hydrodynamic. Mean log temperature difference
(X-axis) is determined from definition (12). In
commercial absorption chillers driving potential in the
absorber and generator usually does not exceed 10-11
HTC dependence on dT
attributed to 10 degrees mean log ∆T, Reynolds
numbers, film thickness and film Nusselt numbers.
The relationship between Nusselt number and other
numbers has been suggested to be as follows [4]:
Experimental coefficients C and m were calculated
from data presented in Table 2 utilising ”least squares
method” which resulted in the following values
Mean log dT, oC Figure 4. Temperature difference effect on K-values
In order to verify this equation and study distribution
on the big scale equipment some experiments has also
● - 32 l/min, ■ - 41 l/min, ▲- 46 l/min,
been carried out at Chalmers. Two experimental series
x – 47,5 l/min, □ - 57,5 l/min, ○ - 60 l/min.
were carried out with 13,1 and 15,2 kg/s inlet LiBr
degrees for low temperature applications [2]. Dmitri Glebov, Tekn.DrViktoria Martin, Tekn.Dr
solution flow. Cooling water flow was constant and
and some of the surface is dry and not involved in the
equal to 103,0±0,2 l/s and inlet temperature to
heat transfer process. Thus, the main conclusion is
24,0±0,1 oC during first experimental series. Cooling
that, if this assumption is correct, 30-40% of the
water temperature during second experimental series
surface is not involved and reconstruction of the
was somewhat higher to 24,5±0,1 oC because of
distribution box is recommended in order to use all
cooling tower performance. District heating flow in
the generator was rather small – 2,2 l/s in order to keepoutgoing solution at the boiling point but without
NOTATION
stable boiling. Auxiliary subcooler was shut off in
order to keep higher delta T in the absorber. The
chiller was running for 30 min in order to have all
values stable as much as possible for the commercial
machine. Temperatures and flows were scanned and
C – constant in equation (8)K – overall heat transfer coefficient, kW/m2 oC
Absorber capacity was calculated as an average
capacity between cooling water side and LiBr solution
side. There is 4-5% difference between those values
y – experimentally determined K-value, W/m2 oC
because of the time delay in thermo couple and flow
Mean log temperature difference was calculated as
− T ) − (T − T )
α – falling film heat transfer coefficient, W/m2 oC
Roughly assuming that all surface is evenly wetted
with the solution, overall heat transfer coefficient can
λ – specific heat conductivity, W/m oC
µ – dynamic viscosity, Pa*s µw – dynamic viscosity at wall temperature, Pa*s
Lamella-space heat transfer coefficient was calculated
according to equation (6). Film-side heat transfer
coefficient was calculated according to the equation
(8). Wall conductivity was taken as 1/r=8000 W/m2 K.
Theoretical value of the overall heat transfercoefficient can be found as follows
Table 3. Comparison between K and Kth.
Comparison between experimental and theoretical
values of K for two solution flow rates is presented inTable 3.
As the solution flow increases, the heat transfercoefficient increases and the wetting becomes better,
CONCLUSION
as can be seen from the Table 1. There is quite large
Utilisation of low temperature (70 – 90 0C) district
difference between calculated and experimental
heating during warm season for comfort cooling seems
values. Two things can explain this difference.
to be very attractive and the interest is growing.
Coefficients in equation (8) were derived for a
Commercially available absorption chillers are
laboratory test rig and scale-up effect can influence on
designed for 100 – 120 0C driving energy and
the results. And second is uneven distribution. Most
therefore cannot be applied without obvious capacity
likely that uneven distribution contributes much to this
losses. The objective of our project is to design an
difference. This is partly supported by visual
optimum absorption chiller specifically for low
observations. Assuming that the distribution is uneven
Dmitri Glebov, Tekn.DrViktoria Martin, Tekn.Dr
In proceedings: 16. Nordiske Kølemøde, København
Experimental tests and analysis have been performed
utilising equipment of different scale. A number of
3. Fredrik Setterwall, Dmitrey Glebov, Bo Wikensten,
tests showed that the district heating temperature has
Low Temperature Driven Absorption Chiller, In: The
significant effect on the cooling capacity of the chiller.
2nd International Heat Powered Cycles Conference,
LiBr solution flow rate is also an important factor. It
Paris, France, September 2001, pp 349-353
seems that for a certain design configuration and
4. David Azbel, Fundamentals of Heat Transfer for
external conditions there is an optimum LiBr solution
Test runs with and without subcooler revealed that itwas more beneficial for the cooling capacity of thechiller to cool down LiBr solution in an external heatexchanger and utilise the absorber surface for steamabsorption.
The flow rate effect on the distribution quality andfalling film heat transfer coefficient were studiedexperimentally. Visual observations revealed thatwhen the distribution box is filled with the liquidphase, the heat transfer surface is fully wetted. Whenthe box distributor is partly filled with the liquid phasethen dry areas appear on the lamella surface.
The overall heat transfer coefficient calculated fromexperimental measurements appeared to depend on thetemperature difference between two phases as well ason the flow rate. In the commercial low temperaturedriven absorption chiller mean log temperaturedifference in the absorber and generator is about of 10
oC. Falling film heat transfer coefficients werecalculated on the basis of experimental data andattributed to 10 oC mean log ∆T. A correlation
between film-side heat transfer coefficient in the formof (Nu) Nusselt number and the flow rate (Reynoldsnumber) was derived.
Experimental data about heat transfer in the absorberon a 1,15 MW chiller revealed that dry surface (up to40 % depends on the flow) occur during operation. The reconstruction of the distributor will lead to thecapacity increase.
Results from these experiments and analysis will beincorporated in the absorption chiller simulationprogram. Hence, it will be possible to complete themodeling tool for optimisation of the chiller. REFERENCES
1. Glebov Dmitrey, Setterwall Fredrik, ExperimentalStudy of Heat Transfer additive Influence on theAbsorption Chiller Performance, Journal ofInternational Institute of Refrigeration (accepted),20002. Glebov Dmitrey & Setterwall Fredrik, ExperimentalStudy of Low Temperature Driven Absorption ChillerUtilising Vertical Lamellas As Heat Transfer Surfaces.

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