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Experimental study of heat transfer additive influence on the absorption chiller performance
International Journal of Refrigeration 25 (2002) 538–545 www.elsevier.com/locate/ijrefrig
Experimental study of heat transfer additive influence on the absorption chiller performance Dmitrey Glebov*,1, Fredrik Setterwall Department of Chemical Engineering and Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 22 December 2000; received in revised form 28 March 2001; accepted 3 April 2001
Abstract The influence of 2-methyl-1-pentanol (2MP) on the cooling effect of pilot absorption chiller has been studied experimentally. In one experimental series the additive was injected into LiBr solution. The enhancement ratio up to 20% was observed at the optimum additive concentration. In the second experimental series the additive was injected into the refrigerant. The enhancement ratio became 32% at higher additive concentration. Different additive concentrations have been tested in both series. These experimental results clearly showed that the presence of the additive in the vapour phase, even in very small amounts, favours the enhancement more than the additive in the LiBr solution. Also, it has been noticed, that the additive travels around the absorption cycle during long-term operation. # 2002 Elsevier Science Ltd and IIR. All rights reserved. Keywords: Absorption system; Water/lithium bromide; Solution; Surface tension; Additive; Peformance
Etude expe´rimentale sur l’influence d’un additif sur le transfert de chaleur sur la performance d’un refroidisseur d’eau a` absorption Re´sume´ Les auteurs ont e´tudie´ de fac¸on expe´rimentale l’influence de 2-me´thyle-1-pentanol (2MP) sur l’efficacite´ d’un refroidisseur d’eau a` absorption expe´rimental. Une augmentation de jusqu’a` 20% de la performance a e´te´ obtenue a` l’aide d’une concentration optimale d’additif. Dans une deuxie`me se´rie, lorsqu’on a injecte´ l’additif directement dans le frigorige`ne, une augmentation de la performance de 32% a e´te´ obtenue graˆce a` la concentration d’additif plus e´leve´e. Diverses concentrations d’additif ont e´te´ teste´es dans ces deux se´ries. Les re´sultats expe´rimentaux montrent que la pre´sence de l’additif dans la phase vapeur, meˆme en petites quantite´s, favorise la performance accrue de fac¸on plus marque´e que l’additif dans la solution de bromure de lithium. Les auteurs ont e´galement remarque´ que l’additif se de´place dans le cycle a` absorption au cours du fonctionnement de longue dure´e. # 2002 Elsevier Science Ltd and IIR. All rights reserved. Mots cle´s : Syste`me a` absorption ; Eau/bromure de lithium ; Solution ; Tension superficielle ; Additif ; Peformance
* Corresponding author. Tel.: +46-8-790-6719; fax: +46-8-21-27-47. E-mail address: [email protected] (D. Glebov). 1 Member of in IIR. 0140-7007/02/$22.00 # 2002 Elsevier Science Ltd and IIR. All rights reserved. PII: S0140-7007(01)00042-1
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1. Introduction With respect to energy savings and environmental protection, thermally activated systems are of a major importance in the areas of refrigeration and air conditioning. Absorption chillers, utilising lithium bromidewater solutions as a working pair are widely used in cold water production. In the absorption chiller, heat is delivered to the generator where the weak lithium bromide solution comes in. Some of the volatile refrigerant is evaporated resulting in a strong solution. The vapour generated is then condensed in the condenser at a lower temperature. The condensate is fed to the evaporator via an expansion valve and is evaporated at low pressure and, hence, at low temperature producing cooling energy. The vapour formed is then absorbed in the strong lithium bromide solution that is pumped from the generator to the absorber. Water vapour absorption into an aqueous LiBr solution is a key stage in the functioning of a conventional absorption chiller. Coupled heat and mass transfer takes place during the process. Enhancement of the absorption rate has for a long time been considered an important issue to improve the efficiency and reduce the transfer area. The most cost-effective means of enhancing the absorption of water vapour to LiBr/H2O solution are additives. Saturated chain and branch alcohols with 6–8 carbons in the chain, e.g. n-octanol, and 2ethyl-1-hexanol are used as additives in commercial absorption machines [1]. The presence of these alcohols at the vapour–liquid interface was found to induce agitation at the surface when absorption takes place. Local changes of temperature and concentration at the interface during the absorption process can result in surface tension gradients. Surface tension gradients can become large enough to induce convection from the interface to the bulk liquid enhancing heat and mass transfer. This phenomenon is well known as Marangoni convection. Much experimental work, as well as theoretical, has been reported. There are two different approaches, in terms of unit design and flow configuration in studying additives effect on heat and mass transfer during the absorption: stagnant pools [2, 6, 11, 16] and falling films [3–5, 7–14]. The results of vertical falling film experiments on the steam absorption into aqueous LiBr solutions with 2ethyl-1-hexanol have been presented by Kim [3]. The system operated as a batch system. The absorber unit had two concentric tubes, an inner stainless steel tube and an outer Pyrex-tube. The aqueous LiBr solution flowed down the outside of the inner stainless steel tube. The upward flow of cooling water inside the inner tube removed the heat of absorption. The heat transfer additive concentrations were used up to 200 ppm. The overall heat transfer coefficients were obtained from the energy balance. The absorption rate was calculated from a species balance. It was noticed that heat transfer
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started to increase at approximately 3–6 ppm and reached a maximum value near 30 ppm. The total heat transfer rate was accelerated by a factor of as much as 3 compared to the case without heat transfer additives. The various effects of operating parameters including the absorber pressure, the solution flow rate and the solution temperature on heat transfer were investigated. Study in falling film hydrodynamics with and without additives was presented by Setterwall and Nordgren [7]. The experiments were made with a mixture of water and glycerol. 1-Octanol was used as a surface-active agent. The falling film column was a 6 m stainless steel tube. It was reported that addition of surfactant into a falling liquid film would stabilise the film and decrease both heat and mass transfer, on condition that there is neither evaporation nor absorption. Experimental investigation of heat and mass transfer in a horizontal-tube falling film absorber with aqueous solution of LiBr with and without surfactants was presented by Hoffmann et al. [12]. The absorber consisted of a row of 24 horizontal tubes, arranged one above the other. Two different types of tubes were tested, one with a plain, the other with a knurled surface. The influence of two surfactants, 1-octanol and 2-ethyl-1- hexanol, on the absorption process was examined quantitatively. The heat transfer coefficients were calculated from the total heat flux data of cooling water and values of mean log driving force. The heat transfer coefficients with those additives were in the range of 800–1400 Wm2 K1 for 56 wt.% LiBr solution, equivalent to an enhancement of 60–150%. The improvement achieved by the application of knurled tubes was almost three times smaller. Other falling-film absorption experiments with and without additives were presented by Kim et al. [9]. The falling-film facility consisted of a single-tube falling film absorber, a generator, a solution loop and a cooling water circuit. The steam was generated in the generator, flowed to the absorber where the strong LiBr solution was introduced. The system operated in a continuous mode. All tests were conducted with an aqueous LiBr solution with 59.6% mass fraction. It was reported that the heat transfer coefficient was increased by 72% when the additive (2-ethyl-1-hexanol) concentration increased from 10 to 50 ppm. The film heat transfer coefficients were calculated from the absorber heat flux, the inlet and outlet temperatures of the cooling water, solution temperatures and their flow rates. The heat transfer coefficient appeared to flatten out after the additive concentration reached 50 ppm. Several criteria for the additive selection have also been suggested. Authors claimed that the candidate additive’s vapor pressure must be as small as possible. Results have been presented by Hoffman and Ziegler [13] that are similar to [3,12]. They have reported experimental data of heat and mass transfer in film absorption on horizontal and vertical tubes without
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additives. The experimental facility was similar to [3,12]. The primary measure of performance was the absorbed mass flux, which was calculated from the measured flow rates and concentrations. The effects of cooling water flow, concentration of the strong solution, solution’s flow rate and dispenser height were studied experimentally. Experimental results showed that stronger LiBr solutions resulted in lower pressure and weaker absorption. Higher flow rates either for cooling water or LiBr solution resulted in stronger absorption. The comparison between smooth and structured horizontal tubes in terms of film heat transfer coefficient revealed that the heat transfer coefficient is smaller for the structured tubes if the actual enlargement of the outside surface is taken into consideration. Miller has tested the joint effect of additive and advanced surfaces in a horizontal-tube absorber [15]. Absorber load was used to estimate the absorption efficiency for different cases and the additive was injected into the vapour phase. Sheehan et al. [6] proposed an explanation of enhancement, called the diffusion theory, which considers the time taken for the diffusion of the additive to the solution-vapour interface as a key factor. According to this theory, the ability of an additive to enhance mass transfer may be related not so much to its activity at the interface, but to its ability to diffuse and absorb at the interface. This theory seemed to be supported by the fact that additives were more effective when presented in the vapour phase, rather than in the liquid [8, 10, 15]. Since diffusion in vapours is larger compared to that in liquids, it is possible that the additive will be transported faster to the interface from the vapour, rather than from the liquid phase. The kinetic controlled adsorption model was presented by Ziegler et al. [17]. This model considers the surface tension relaxation times as a key parameter for calculations of surface tension induced heat and mass transfer. Based on absorption experiments on a vertical tube, water condensation and surface tension measurements, Herold et al. have advanced another theory [10], called vapour surfactant theory. This theory retains Marangoni convection as the ultimate mechanism of enhancement. The additives are surfactants that concentrate at the surface of the liquid and reduce the surface tension. When there are gradients in the surface concentration of the additive, the resulting surface tension gradients cause the surface flows. The new aspect of the theory is the realisation that the primary transport mechanism for the additive is via bulk flow of the vapour toward the surface. Thus, when absorption occurs, the additive is delivered to the surface. This mechanism causes highly unstable flow patterns during absorption because, as the surfactant arrives at the surface, it causes a flow which sweeps itself away exposing
a strong absorbent which further encourages absorption at that point. Thus, the vapour pressure of the additive is an important factor that determines its effectiveness in producing enhancement. 2-Ethyl-1-hexanol was used as an additive in amounts between 10 and 100 ppm. Authors estimated that the additive content in the vapour phase is approximately 1 wt.%. The estimate was based on the partial pressure of additive vapour in the vapour entering the absorber/condenser. Gustafsson [18,21] presented a stability analysis for a vertical falling film system with aqueous LiBr solution and additives. The analysis showed that the rate of the surface tension decrease is more important than the surface tension value itself. The additive diffusivity appeared to be an important value for surface tension relaxation time and subsequently for the absorption rate. Based on stability analysis, Gustafsson also suggested candidate additives in his study. Some of them are 3methyl-1-bytanol, 2,2,3,3,4,4,5,5-octafluoro-1-pentanol and 2-methyl-1-pentanol. Experimental values for surface tension data with the last two additives for 50 and 60 wt.% LiBr solutions at different temperatures have also been presented in [18]. The objective of this study was to establish 2-methyl1-pentanol effect directly on the cooling capacity of the absorption chiller utilising low-temperature heating agent in the generator. This low-temperature agent is associated with waste heat or low-temperature district heating that can be used in commercial machines.
2. Experimental facility and conditions A mini absorption chiller was used in order to determine the additive influence on the whole absorption cycle performance. The experimental set-up consists of an absorber/evaporator unit and a condenser/generator one, a cooling water system, a cold-carrier system, heating agent system, a vacuum generating system and measuring equipment. The absorber consists of 5 vertical lamellas arranged in parallel with a full heat-transfer area of 1.1 m2. Each lamella has its own distribution device. The evaporator consists of 4 lamellas with full heat transfer area of 0.95 m2, the generator and condenser of 3 each and with transfer areas of 0.55 m2 each. From the distributors, the solution is introduced as a film on the lamellas. However, a lot of effort was put into solving the distribution problem. The formation of dry patches may still be a problem to consider. The lamellas in the absorber and evaporator sections were made to different sizes in order to fit the tubular shell (see Fig. 1). The aqueous LiBr solution flowed in the absorber on the outside of the surface as a falling film and the cooling water flowed inside the lamellas through the absorber and condenser sections in a counter current flow. An electric heater generating hot water was used as a driving
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Fig. 1. Experimental set-up. Fig. 1. Sche´ma expe´rimental.
energy source for the cycle where the hot water was passed on the inside of the lamellas. A recuperative solution heat exchanger was installed between the absorber and generator as shown in Fig. 1. The test facility is mainly constructed from a steel alloy — SS2343. However, some couplings and valves were fitted with Swage Lock equipment made from Teflon. A schematic of the experimental set-up is shown in Fig. 1. The experiments were carefully run under controlled and equal conditions with and without the additive so that the enhancement effects could be isolated. Lamella heat exchangers have been shown to perform very well in earlier experiments. For liquid–liquid heat exchange, it was shown that the overall heat transfer coefficient in a lamella heat exchanger was about twice as high as in a tubular heat exchanger [18,20]. Furthermore, the spot welded construction of the lamella makes an excellent turbulent promoter on both the inside and the outside of the surface. The system was run continuously as a whole cycle during all tests. During the experiments, the refrigerant was evaporated in the evaporator by extracting heat from the cold carrier. The vapor was absorbed in the LiBr solution flow distributed on the outside of the lamella surfaces in the absorber. The solution was distributed on the lamella surfaces by using a specially designed distributor. The distributor is basically a small tube fitted on the top of each lamella with a small gap between the distributor and the lamella surface, of approximately 0.5 mm. Different gap widths were tested but the best wetting was obtained for 0.5 mm. Furthermore, when the additive was present in the solution and the distribution was tested at atmospheric pressure, the wetting was some-
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what enhanced with the addition of a surfactant. Weak LiBr solution was pumped continuously from the bottom of the absorber in such a way that part of it returned to the absorber and another part was transported to the generator via a recuperative heat exchanger. LiBr solution was heated in the generator by means of hot water supplied from the electric heater and some amount of refrigerant was boiled off. Strong LiBr solution was pumped from the generator to absorber. The refrigerant from the condenser was throttled to the evaporator through an expansion valve. In the evaporator, the refrigerant was sucked from the bottom of the vessel and circulated over the lamellas by means of a circulation pump (see Fig. 1). The total volume of aqueous LiBr solution in the absorber was carefully checked for each test run (with and without additive) and amounted to 10 l. LiBr solution did not contain corrosion inhibitor. In order to determine the base-line, the first experiments were carried out without the additive. After that, two experimental series have been conducted. In one experimental series the additive was injected into LiBr solution gradually from 100 to 2000 ppm. 2-Methyl-1pentanol was used as an additive during all tests. The surfactant was added to the absorber solution before each test run and was circulated for approximately 30 min to achieve an even distribution in the solution. After the circulation, the system was evacuated and the test conditions had been set (i.e. temperatures and flow rates). After that the system was running 15–20 min in order to reach steady-state. When the steady state was established experimental data were scanned. The following parameters were kept constant during all measurements. Cold carrier flow rate in the evaporator was 310–315 l/h with the temperature at the inlet at 19 0.1 C. Cool water flow rate in the absorber/condenser line was 450.0 2.0 l/h with the inlet temperature at 10.0 0.1 C. Hot water flow rate in the generator was 950.0 5.0 l/h with the inlet temperature at 85.0 1.0 C. LiBr solution flow rate from the generator was 140.0 3.5 l/h. After external conditions were set, the equilibrium LiBr solution composition in the absorber became 55.0 0.5 wt.% which was carefully checked by measuring density of the sample. The equilibrium pressure in the generator was 80.0 5.0 mbar and in the evaporator — 11.0 1.0 mbar. During additive tests the equilibrium pressure in the absorber/evaporator part established at somewhat higher level and became — 11.4 1.0 mbar. All temperatures were measured by Almemo copper-constantan thermocouples ZA 9020FSK mounted according to Fig. 1. The thermocouples were connected to a computer via a signal converter in order to collect test data during the experiments. The pressures were measured by two dial pressure gauges mounted on the two shells. The flow of cooling water, cold carrier and hot water were monitored by water
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calibrated rotameters. Tailor-made and calibrated rotameters were used for LiBr solution. The cooling effect — Qcool — was then directly determined for all cases from the definition Qcool ¼ G Cp ðTev:in Tev:out Þ
ð1Þ
Where G is the cold carrier flow rate, kg/s, Cp is the specific heat capacity, J kg1 C1, Tev.in=19 0.1 C, Tev.out is the cold carrier temperature at the outlet, C. All tests were run at steady-state conditions as could be established by monitoring temperatures, flow rates and pressures. Data acquisition was partly carried out by a data logger and a workstation. Temperature data was scanned and stored every 20 s. The pressure gauges and flow meters were monitored manually during the whole experimental run. The whole cycle was initially tested and energy balances were established to check the overall performance. Furthermore, the equipment was tested for leaks and this was a serious problem that was managed at last. The equipment was finally tested by applying the low pressure and then monitored the pressure as a function of time until we reached satisfactory results for the required time to perform the test runs. In the second experimental series the additive was injected into refrigerant. The amount of additive was taken according to 1 mg of 2MP per 1 kg of LiBr solution charged to the facility. The amount of additive varied between 300 and 2000 mg/kg (300–2000 ppm). The following experimental procedures were conducted before each experiment: 1. Before each test run, the density of the solution was carefully measured in order to determine the composition. The equilibrium LiBr solution concentration corresponding to the external conditions was 55 wt.%. After each test run, the inlet solution concentration was adjusted to this value by concentrating the solution or absorbing water. The surfactant was added to the solution and the absorber solution was allowed to circulate for approximately 30 min. 2. The desired external conditions were set. The desired temperatures for the cold carrier and cooling water were established by using heating devices installed in the test rig. A steady-state was obtained when the flow rates and temperatures reached their required values. The flow rates were slightly changed depending on the net load and were manually adjusted to a desired value. 3. The system was evacuated by means of a vacuum pump to the equilibrium level, which corresponds to the external conditions. The vacuum trap was installed between the pump and the unit in order to trap the steam and protect the working part of the pump. 4. The data logger was switched on and temperature
data were logged for about 15 min while the flow rates and pressure were monitored visually. The cooling effect was then calculated from the average temperatures of the ingoing and outgoing water from the evaporator, the specific heat and the mass flow rate. 5. Before changing to a new experimental series, the absorber/generator part of the system was cleaned three times by means of pure and diluted LiBr solution, which was circulating between the absorber and generator. The hot water was supplied to the generator in order to concentrate this solution. The condensate was then used to clean the condenser and evaporator.
3. Experimental results and discussion In order to determine the base-line, the first experiments were carried out without the additive. The additive enhancement ratio was used as a parameter to establish the additive effectiveness. After the base-line effect had been established the additive enhancement ratio was derived as follows E¼ cooling effect with additive cooling effect without additive 100% cooling effect without additive ð2Þ
The cooling effect values were calculated according to (1) using experimental values. During the first experimental series, 2-methyl-1-pentanol was injected into LiBr solution in amounts from 100 to 2000 ppm. Fig. 2 shows the additive concentration effect on the enhancement ratio. Testing with 2-methyl-1-pentanol causes the cooling effect to increase compared to the additive free case. The enhancement ratio reaches a maximum value to approximately 20% at 500–700 ppm and then flattens out at a somewhat lower value. This additive effect on the absorber performance has also been reported in other works [3,9,13,10,18]. The effect of the additive is at least twofold. Lower surface tension results in better wetting and better solution distribution along lamellas. The presence of the additive at the vapor/liquid interface during steam absorption resulted in high surface tension gradients that in turn trigger the mixing across the falling film. This mixing, called Marangoni convection, increases the falling film heat transfer coefficient. This in turn results in higher absorber load and higher cooling effect. At 100 ppm the enhancement ratio is rather small and made up only 1.5%. At 300 ppm the enhancement ratio became 10.5% and maximum enhancement was achieved at 500–700 ppm. It is necessary to notice that the additive concentration in the system is somewhat lower because of three reasons. Firstly, some additive can stick in
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Fig. 2. 2-Methyl-1-pentanol effect on the enhancement ratio (additive-in-solution case). Fig. 2. L’effet du 2-me´thyl-1-pentanol sur la performance (cas de l’additif dans la solution).
stagnant zones in the machine and reside there for a long time. Secondly, some additive can stick to surfaces in the chiller during operation and finally some additive can be pumped away during the purging process. The second experimental series were conducted under the same conditions, but the additive was injected into the refrigerant side (evaporator) in order to find out whether or not its better to have the additive in the vapor phase. Presence of the additive in the vapor phase definitely facilitates the additive transportation to the interface. The LiBr solution was absolutely additive free. 2Methyl-1-pentanol was gradually injected to the evaporator. Fig. 3 shows the surfactant concentration effect on the enhancement ratio when added to the refrigerant. The increase of additive concentration in the refrigerant side increased the cooling effect and subsequently the enhancement ratio. The qualitative trend of behavior is the same as for the previous case but the enhancement ratio value is substantially higher at the optimum, which corresponds to 1300–1500 ppm. LiBr solution subcooling in the absorber (difference between inlet and outlet temperatures) reduced with the increased additive concentration either in the solution or in the refrigerant. The same effect was observed and described by Miller [15]. Reduced subcooling in the absorber resulted in somewhat higher COP (coefficient of performance) values. For example, for the optimum additive concentration (additive-in-refrigerant case) COP was 16.7% higher than for the additive free case. One reason is a reduced subcooling in the absorber and another reason is an increased cooling effect. The maximum enhancement ratio was 32.3% compared to the case without additive. LiBr solution samples were carefully examined after each measurement in order to determine their composition. A few water samples have also been taken out from the evaporator. Observations revealed the presence of the additive islands on the LiBr solution surface during the first experimental series. However, it does not necessary
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Fig. 3. 2-Methyl-1-pentanol effect on the enhancement ratio (additive-in-refrigerant case). Fig. 3. L’effet du 2-me´thyl-1-pentanol sur la performance (cas de l’additif dans le frigorige`ne).
mean that 2MP was present in the form of droplets on the surface during the absorption process. Water samples from the evaporator also revealed the presence of the additive. This can be explained by the fact that the additive is partly boiled off in the generator and arrives at the evaporator with the condensate. LiBr solution samples from the second experimental series also revealed the presence of the additive. But neither surfactant islands were present on the surface. 2Methyl-1-pentanol existed in the solution in amounts smaller than the solubility limit [18]. All these facts support the suggestion that the additive is a volatile component that travels around the cycle and hence is present in the vapour phase. Rough estimation of the vapour composition in the absorber/evaporator unit resulted in a value of 0.05–0.1 wt.%. The presence of the additive in the vapour phase in very small amounts promotes heat and mass transfer in the absorber more efficiently compared to the additive-in-solution case. Experimental results and observations seem to support theories advanced by Herold [10] and Setterwall [19]. Finally, this new additive — 2MP — was used in a commercial absorption chiller in Sweden. Calculated amount of additive was injected into evaporator. The cooling effect was then increased. Estimated enhancement ratio made up 30–35% that is in an excellent agreement with experimental data. Cooling effect stabilised at a higher level and lasted during 50 h until the chiller was shut down. As long as the additive is present in the vapour phase as well as in the liquid, it can be pumped out from the machine during purging of noncondensables and after a while the additive effect reduces and the cooling effect decreases.
4. Conclusions Number of experimental and theoretical studies have been dedicated to the question about additive effect on
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the absorption cycle performance or on the absorber performance in particular. Experimental research was carried out, generally, with absorbers utilising vertical tubes or a bundle of horizontal tubes. Falling film heat transfer coefficient or absorption rate were used as parameters in order to establish the enhancement caused by the additive. 2-Ethyl-1-hexanol and 1-octanol are generally used in the experiments. In the present study we have chosen 2-methyl-1-pentanol. The choice of this additive was based on the stability analysis fulfilled by Gustafsson [18, 21]. The objective of this study was to establish experimentally 2MP effect on the absorption chiller performance. A mini absorption chiller utilising lamella surfaces was used in the experiments. Cooling effect in the evaporator was measured experimentally and used as a parameter in order to estimate the enhancement ratio. Two experimental series have been carried out. In one experimental series the additive was injected into the LiBr solution. The enhancement ratio up to 20% was observed at the optimum additive concentration. In the second experimental series the additive was injected into the refrigerant. The amount of additive for this case was taken according to 1 mg of 2MP per 1 kg of LiBr solution charged to the facility. The enhancement ratio became 32%, which is substantially better. Different additive concentrations have been tested in both series. LiBr solution samples as well as refrigerant samples have been examined during the experiments. Examination revealed that 2MP performed maximum enhancement at the concentration over the solubility limit when it was injected into LiBr solution. LiBr solution samples also contained additive when it was injected into evaporator. This can be explained by the fact that the additive as a volatile component transported to the vapour phase and then absorbed in the absorber. The additive content in the LiBr solution samples was smaller than the solubility limit when 2MP was injected into evaporator. Rough estimation of the vapour composition in the absorber/evaporator unit revealed a value of 0.05– 0.1 wt.%. The presence of additive in the vapour phase in very small amounts promotes heat and mass transfer in the absorber more efficiently compared to the additivein-solution case. Experimental results and observations seem to support theories advanced by Herold [10] and Setterwall [19]. This new additive was subsequently used to increase the capacity of the commercial absorption chiller. The capacity has been increased by a factor of 30–35% that is in a very good agreement with the experimental data.
Acknowledgements This research was carried out within the framework of the Klimat 21 program supported by the Swedish
National Energy Authority. Special thanks to Dr. Magnus Gustafsson for useful discussions.
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Experimental study of heat transfer additive influence on the absorption chiller performance Dmitrey Glebov*,1, Fredrik Setterwall Department of Chemical Engineering and Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 22 December 2000; received in revised form 28 March 2001; accepted 3 April 2001
Abstract The influence of 2-methyl-1-pentanol (2MP) on the cooling effect of pilot absorption chiller has been studied experimentally. In one experimental series the additive was injected into LiBr solution. The enhancement ratio up to 20% was observed at the optimum additive concentration. In the second experimental series the additive was injected into the refrigerant. The enhancement ratio became 32% at higher additive concentration. Different additive concentrations have been tested in both series. These experimental results clearly showed that the presence of the additive in the vapour phase, even in very small amounts, favours the enhancement more than the additive in the LiBr solution. Also, it has been noticed, that the additive travels around the absorption cycle during long-term operation. # 2002 Elsevier Science Ltd and IIR. All rights reserved. Keywords: Absorption system; Water/lithium bromide; Solution; Surface tension; Additive; Peformance
Etude expe´rimentale sur l’influence d’un additif sur le transfert de chaleur sur la performance d’un refroidisseur d’eau a` absorption Re´sume´ Les auteurs ont e´tudie´ de fac¸on expe´rimentale l’influence de 2-me´thyle-1-pentanol (2MP) sur l’efficacite´ d’un refroidisseur d’eau a` absorption expe´rimental. Une augmentation de jusqu’a` 20% de la performance a e´te´ obtenue a` l’aide d’une concentration optimale d’additif. Dans une deuxie`me se´rie, lorsqu’on a injecte´ l’additif directement dans le frigorige`ne, une augmentation de la performance de 32% a e´te´ obtenue graˆce a` la concentration d’additif plus e´leve´e. Diverses concentrations d’additif ont e´te´ teste´es dans ces deux se´ries. Les re´sultats expe´rimentaux montrent que la pre´sence de l’additif dans la phase vapeur, meˆme en petites quantite´s, favorise la performance accrue de fac¸on plus marque´e que l’additif dans la solution de bromure de lithium. Les auteurs ont e´galement remarque´ que l’additif se de´place dans le cycle a` absorption au cours du fonctionnement de longue dure´e. # 2002 Elsevier Science Ltd and IIR. All rights reserved. Mots cle´s : Syste`me a` absorption ; Eau/bromure de lithium ; Solution ; Tension superficielle ; Additif ; Peformance
* Corresponding author. Tel.: +46-8-790-6719; fax: +46-8-21-27-47. E-mail address: [email protected] (D. Glebov). 1 Member of in IIR. 0140-7007/02/$22.00 # 2002 Elsevier Science Ltd and IIR. All rights reserved. PII: S0140-7007(01)00042-1
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1. Introduction With respect to energy savings and environmental protection, thermally activated systems are of a major importance in the areas of refrigeration and air conditioning. Absorption chillers, utilising lithium bromidewater solutions as a working pair are widely used in cold water production. In the absorption chiller, heat is delivered to the generator where the weak lithium bromide solution comes in. Some of the volatile refrigerant is evaporated resulting in a strong solution. The vapour generated is then condensed in the condenser at a lower temperature. The condensate is fed to the evaporator via an expansion valve and is evaporated at low pressure and, hence, at low temperature producing cooling energy. The vapour formed is then absorbed in the strong lithium bromide solution that is pumped from the generator to the absorber. Water vapour absorption into an aqueous LiBr solution is a key stage in the functioning of a conventional absorption chiller. Coupled heat and mass transfer takes place during the process. Enhancement of the absorption rate has for a long time been considered an important issue to improve the efficiency and reduce the transfer area. The most cost-effective means of enhancing the absorption of water vapour to LiBr/H2O solution are additives. Saturated chain and branch alcohols with 6–8 carbons in the chain, e.g. n-octanol, and 2ethyl-1-hexanol are used as additives in commercial absorption machines [1]. The presence of these alcohols at the vapour–liquid interface was found to induce agitation at the surface when absorption takes place. Local changes of temperature and concentration at the interface during the absorption process can result in surface tension gradients. Surface tension gradients can become large enough to induce convection from the interface to the bulk liquid enhancing heat and mass transfer. This phenomenon is well known as Marangoni convection. Much experimental work, as well as theoretical, has been reported. There are two different approaches, in terms of unit design and flow configuration in studying additives effect on heat and mass transfer during the absorption: stagnant pools [2, 6, 11, 16] and falling films [3–5, 7–14]. The results of vertical falling film experiments on the steam absorption into aqueous LiBr solutions with 2ethyl-1-hexanol have been presented by Kim [3]. The system operated as a batch system. The absorber unit had two concentric tubes, an inner stainless steel tube and an outer Pyrex-tube. The aqueous LiBr solution flowed down the outside of the inner stainless steel tube. The upward flow of cooling water inside the inner tube removed the heat of absorption. The heat transfer additive concentrations were used up to 200 ppm. The overall heat transfer coefficients were obtained from the energy balance. The absorption rate was calculated from a species balance. It was noticed that heat transfer
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started to increase at approximately 3–6 ppm and reached a maximum value near 30 ppm. The total heat transfer rate was accelerated by a factor of as much as 3 compared to the case without heat transfer additives. The various effects of operating parameters including the absorber pressure, the solution flow rate and the solution temperature on heat transfer were investigated. Study in falling film hydrodynamics with and without additives was presented by Setterwall and Nordgren [7]. The experiments were made with a mixture of water and glycerol. 1-Octanol was used as a surface-active agent. The falling film column was a 6 m stainless steel tube. It was reported that addition of surfactant into a falling liquid film would stabilise the film and decrease both heat and mass transfer, on condition that there is neither evaporation nor absorption. Experimental investigation of heat and mass transfer in a horizontal-tube falling film absorber with aqueous solution of LiBr with and without surfactants was presented by Hoffmann et al. [12]. The absorber consisted of a row of 24 horizontal tubes, arranged one above the other. Two different types of tubes were tested, one with a plain, the other with a knurled surface. The influence of two surfactants, 1-octanol and 2-ethyl-1- hexanol, on the absorption process was examined quantitatively. The heat transfer coefficients were calculated from the total heat flux data of cooling water and values of mean log driving force. The heat transfer coefficients with those additives were in the range of 800–1400 Wm2 K1 for 56 wt.% LiBr solution, equivalent to an enhancement of 60–150%. The improvement achieved by the application of knurled tubes was almost three times smaller. Other falling-film absorption experiments with and without additives were presented by Kim et al. [9]. The falling-film facility consisted of a single-tube falling film absorber, a generator, a solution loop and a cooling water circuit. The steam was generated in the generator, flowed to the absorber where the strong LiBr solution was introduced. The system operated in a continuous mode. All tests were conducted with an aqueous LiBr solution with 59.6% mass fraction. It was reported that the heat transfer coefficient was increased by 72% when the additive (2-ethyl-1-hexanol) concentration increased from 10 to 50 ppm. The film heat transfer coefficients were calculated from the absorber heat flux, the inlet and outlet temperatures of the cooling water, solution temperatures and their flow rates. The heat transfer coefficient appeared to flatten out after the additive concentration reached 50 ppm. Several criteria for the additive selection have also been suggested. Authors claimed that the candidate additive’s vapor pressure must be as small as possible. Results have been presented by Hoffman and Ziegler [13] that are similar to [3,12]. They have reported experimental data of heat and mass transfer in film absorption on horizontal and vertical tubes without
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additives. The experimental facility was similar to [3,12]. The primary measure of performance was the absorbed mass flux, which was calculated from the measured flow rates and concentrations. The effects of cooling water flow, concentration of the strong solution, solution’s flow rate and dispenser height were studied experimentally. Experimental results showed that stronger LiBr solutions resulted in lower pressure and weaker absorption. Higher flow rates either for cooling water or LiBr solution resulted in stronger absorption. The comparison between smooth and structured horizontal tubes in terms of film heat transfer coefficient revealed that the heat transfer coefficient is smaller for the structured tubes if the actual enlargement of the outside surface is taken into consideration. Miller has tested the joint effect of additive and advanced surfaces in a horizontal-tube absorber [15]. Absorber load was used to estimate the absorption efficiency for different cases and the additive was injected into the vapour phase. Sheehan et al. [6] proposed an explanation of enhancement, called the diffusion theory, which considers the time taken for the diffusion of the additive to the solution-vapour interface as a key factor. According to this theory, the ability of an additive to enhance mass transfer may be related not so much to its activity at the interface, but to its ability to diffuse and absorb at the interface. This theory seemed to be supported by the fact that additives were more effective when presented in the vapour phase, rather than in the liquid [8, 10, 15]. Since diffusion in vapours is larger compared to that in liquids, it is possible that the additive will be transported faster to the interface from the vapour, rather than from the liquid phase. The kinetic controlled adsorption model was presented by Ziegler et al. [17]. This model considers the surface tension relaxation times as a key parameter for calculations of surface tension induced heat and mass transfer. Based on absorption experiments on a vertical tube, water condensation and surface tension measurements, Herold et al. have advanced another theory [10], called vapour surfactant theory. This theory retains Marangoni convection as the ultimate mechanism of enhancement. The additives are surfactants that concentrate at the surface of the liquid and reduce the surface tension. When there are gradients in the surface concentration of the additive, the resulting surface tension gradients cause the surface flows. The new aspect of the theory is the realisation that the primary transport mechanism for the additive is via bulk flow of the vapour toward the surface. Thus, when absorption occurs, the additive is delivered to the surface. This mechanism causes highly unstable flow patterns during absorption because, as the surfactant arrives at the surface, it causes a flow which sweeps itself away exposing
a strong absorbent which further encourages absorption at that point. Thus, the vapour pressure of the additive is an important factor that determines its effectiveness in producing enhancement. 2-Ethyl-1-hexanol was used as an additive in amounts between 10 and 100 ppm. Authors estimated that the additive content in the vapour phase is approximately 1 wt.%. The estimate was based on the partial pressure of additive vapour in the vapour entering the absorber/condenser. Gustafsson [18,21] presented a stability analysis for a vertical falling film system with aqueous LiBr solution and additives. The analysis showed that the rate of the surface tension decrease is more important than the surface tension value itself. The additive diffusivity appeared to be an important value for surface tension relaxation time and subsequently for the absorption rate. Based on stability analysis, Gustafsson also suggested candidate additives in his study. Some of them are 3methyl-1-bytanol, 2,2,3,3,4,4,5,5-octafluoro-1-pentanol and 2-methyl-1-pentanol. Experimental values for surface tension data with the last two additives for 50 and 60 wt.% LiBr solutions at different temperatures have also been presented in [18]. The objective of this study was to establish 2-methyl1-pentanol effect directly on the cooling capacity of the absorption chiller utilising low-temperature heating agent in the generator. This low-temperature agent is associated with waste heat or low-temperature district heating that can be used in commercial machines.
2. Experimental facility and conditions A mini absorption chiller was used in order to determine the additive influence on the whole absorption cycle performance. The experimental set-up consists of an absorber/evaporator unit and a condenser/generator one, a cooling water system, a cold-carrier system, heating agent system, a vacuum generating system and measuring equipment. The absorber consists of 5 vertical lamellas arranged in parallel with a full heat-transfer area of 1.1 m2. Each lamella has its own distribution device. The evaporator consists of 4 lamellas with full heat transfer area of 0.95 m2, the generator and condenser of 3 each and with transfer areas of 0.55 m2 each. From the distributors, the solution is introduced as a film on the lamellas. However, a lot of effort was put into solving the distribution problem. The formation of dry patches may still be a problem to consider. The lamellas in the absorber and evaporator sections were made to different sizes in order to fit the tubular shell (see Fig. 1). The aqueous LiBr solution flowed in the absorber on the outside of the surface as a falling film and the cooling water flowed inside the lamellas through the absorber and condenser sections in a counter current flow. An electric heater generating hot water was used as a driving
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Fig. 1. Experimental set-up. Fig. 1. Sche´ma expe´rimental.
energy source for the cycle where the hot water was passed on the inside of the lamellas. A recuperative solution heat exchanger was installed between the absorber and generator as shown in Fig. 1. The test facility is mainly constructed from a steel alloy — SS2343. However, some couplings and valves were fitted with Swage Lock equipment made from Teflon. A schematic of the experimental set-up is shown in Fig. 1. The experiments were carefully run under controlled and equal conditions with and without the additive so that the enhancement effects could be isolated. Lamella heat exchangers have been shown to perform very well in earlier experiments. For liquid–liquid heat exchange, it was shown that the overall heat transfer coefficient in a lamella heat exchanger was about twice as high as in a tubular heat exchanger [18,20]. Furthermore, the spot welded construction of the lamella makes an excellent turbulent promoter on both the inside and the outside of the surface. The system was run continuously as a whole cycle during all tests. During the experiments, the refrigerant was evaporated in the evaporator by extracting heat from the cold carrier. The vapor was absorbed in the LiBr solution flow distributed on the outside of the lamella surfaces in the absorber. The solution was distributed on the lamella surfaces by using a specially designed distributor. The distributor is basically a small tube fitted on the top of each lamella with a small gap between the distributor and the lamella surface, of approximately 0.5 mm. Different gap widths were tested but the best wetting was obtained for 0.5 mm. Furthermore, when the additive was present in the solution and the distribution was tested at atmospheric pressure, the wetting was some-
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what enhanced with the addition of a surfactant. Weak LiBr solution was pumped continuously from the bottom of the absorber in such a way that part of it returned to the absorber and another part was transported to the generator via a recuperative heat exchanger. LiBr solution was heated in the generator by means of hot water supplied from the electric heater and some amount of refrigerant was boiled off. Strong LiBr solution was pumped from the generator to absorber. The refrigerant from the condenser was throttled to the evaporator through an expansion valve. In the evaporator, the refrigerant was sucked from the bottom of the vessel and circulated over the lamellas by means of a circulation pump (see Fig. 1). The total volume of aqueous LiBr solution in the absorber was carefully checked for each test run (with and without additive) and amounted to 10 l. LiBr solution did not contain corrosion inhibitor. In order to determine the base-line, the first experiments were carried out without the additive. After that, two experimental series have been conducted. In one experimental series the additive was injected into LiBr solution gradually from 100 to 2000 ppm. 2-Methyl-1pentanol was used as an additive during all tests. The surfactant was added to the absorber solution before each test run and was circulated for approximately 30 min to achieve an even distribution in the solution. After the circulation, the system was evacuated and the test conditions had been set (i.e. temperatures and flow rates). After that the system was running 15–20 min in order to reach steady-state. When the steady state was established experimental data were scanned. The following parameters were kept constant during all measurements. Cold carrier flow rate in the evaporator was 310–315 l/h with the temperature at the inlet at 19 0.1 C. Cool water flow rate in the absorber/condenser line was 450.0 2.0 l/h with the inlet temperature at 10.0 0.1 C. Hot water flow rate in the generator was 950.0 5.0 l/h with the inlet temperature at 85.0 1.0 C. LiBr solution flow rate from the generator was 140.0 3.5 l/h. After external conditions were set, the equilibrium LiBr solution composition in the absorber became 55.0 0.5 wt.% which was carefully checked by measuring density of the sample. The equilibrium pressure in the generator was 80.0 5.0 mbar and in the evaporator — 11.0 1.0 mbar. During additive tests the equilibrium pressure in the absorber/evaporator part established at somewhat higher level and became — 11.4 1.0 mbar. All temperatures were measured by Almemo copper-constantan thermocouples ZA 9020FSK mounted according to Fig. 1. The thermocouples were connected to a computer via a signal converter in order to collect test data during the experiments. The pressures were measured by two dial pressure gauges mounted on the two shells. The flow of cooling water, cold carrier and hot water were monitored by water
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calibrated rotameters. Tailor-made and calibrated rotameters were used for LiBr solution. The cooling effect — Qcool — was then directly determined for all cases from the definition Qcool ¼ G Cp ðTev:in Tev:out Þ
ð1Þ
Where G is the cold carrier flow rate, kg/s, Cp is the specific heat capacity, J kg1 C1, Tev.in=19 0.1 C, Tev.out is the cold carrier temperature at the outlet, C. All tests were run at steady-state conditions as could be established by monitoring temperatures, flow rates and pressures. Data acquisition was partly carried out by a data logger and a workstation. Temperature data was scanned and stored every 20 s. The pressure gauges and flow meters were monitored manually during the whole experimental run. The whole cycle was initially tested and energy balances were established to check the overall performance. Furthermore, the equipment was tested for leaks and this was a serious problem that was managed at last. The equipment was finally tested by applying the low pressure and then monitored the pressure as a function of time until we reached satisfactory results for the required time to perform the test runs. In the second experimental series the additive was injected into refrigerant. The amount of additive was taken according to 1 mg of 2MP per 1 kg of LiBr solution charged to the facility. The amount of additive varied between 300 and 2000 mg/kg (300–2000 ppm). The following experimental procedures were conducted before each experiment: 1. Before each test run, the density of the solution was carefully measured in order to determine the composition. The equilibrium LiBr solution concentration corresponding to the external conditions was 55 wt.%. After each test run, the inlet solution concentration was adjusted to this value by concentrating the solution or absorbing water. The surfactant was added to the solution and the absorber solution was allowed to circulate for approximately 30 min. 2. The desired external conditions were set. The desired temperatures for the cold carrier and cooling water were established by using heating devices installed in the test rig. A steady-state was obtained when the flow rates and temperatures reached their required values. The flow rates were slightly changed depending on the net load and were manually adjusted to a desired value. 3. The system was evacuated by means of a vacuum pump to the equilibrium level, which corresponds to the external conditions. The vacuum trap was installed between the pump and the unit in order to trap the steam and protect the working part of the pump. 4. The data logger was switched on and temperature
data were logged for about 15 min while the flow rates and pressure were monitored visually. The cooling effect was then calculated from the average temperatures of the ingoing and outgoing water from the evaporator, the specific heat and the mass flow rate. 5. Before changing to a new experimental series, the absorber/generator part of the system was cleaned three times by means of pure and diluted LiBr solution, which was circulating between the absorber and generator. The hot water was supplied to the generator in order to concentrate this solution. The condensate was then used to clean the condenser and evaporator.
3. Experimental results and discussion In order to determine the base-line, the first experiments were carried out without the additive. The additive enhancement ratio was used as a parameter to establish the additive effectiveness. After the base-line effect had been established the additive enhancement ratio was derived as follows E¼ cooling effect with additive cooling effect without additive 100% cooling effect without additive ð2Þ
The cooling effect values were calculated according to (1) using experimental values. During the first experimental series, 2-methyl-1-pentanol was injected into LiBr solution in amounts from 100 to 2000 ppm. Fig. 2 shows the additive concentration effect on the enhancement ratio. Testing with 2-methyl-1-pentanol causes the cooling effect to increase compared to the additive free case. The enhancement ratio reaches a maximum value to approximately 20% at 500–700 ppm and then flattens out at a somewhat lower value. This additive effect on the absorber performance has also been reported in other works [3,9,13,10,18]. The effect of the additive is at least twofold. Lower surface tension results in better wetting and better solution distribution along lamellas. The presence of the additive at the vapor/liquid interface during steam absorption resulted in high surface tension gradients that in turn trigger the mixing across the falling film. This mixing, called Marangoni convection, increases the falling film heat transfer coefficient. This in turn results in higher absorber load and higher cooling effect. At 100 ppm the enhancement ratio is rather small and made up only 1.5%. At 300 ppm the enhancement ratio became 10.5% and maximum enhancement was achieved at 500–700 ppm. It is necessary to notice that the additive concentration in the system is somewhat lower because of three reasons. Firstly, some additive can stick in
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Fig. 2. 2-Methyl-1-pentanol effect on the enhancement ratio (additive-in-solution case). Fig. 2. L’effet du 2-me´thyl-1-pentanol sur la performance (cas de l’additif dans la solution).
stagnant zones in the machine and reside there for a long time. Secondly, some additive can stick to surfaces in the chiller during operation and finally some additive can be pumped away during the purging process. The second experimental series were conducted under the same conditions, but the additive was injected into the refrigerant side (evaporator) in order to find out whether or not its better to have the additive in the vapor phase. Presence of the additive in the vapor phase definitely facilitates the additive transportation to the interface. The LiBr solution was absolutely additive free. 2Methyl-1-pentanol was gradually injected to the evaporator. Fig. 3 shows the surfactant concentration effect on the enhancement ratio when added to the refrigerant. The increase of additive concentration in the refrigerant side increased the cooling effect and subsequently the enhancement ratio. The qualitative trend of behavior is the same as for the previous case but the enhancement ratio value is substantially higher at the optimum, which corresponds to 1300–1500 ppm. LiBr solution subcooling in the absorber (difference between inlet and outlet temperatures) reduced with the increased additive concentration either in the solution or in the refrigerant. The same effect was observed and described by Miller [15]. Reduced subcooling in the absorber resulted in somewhat higher COP (coefficient of performance) values. For example, for the optimum additive concentration (additive-in-refrigerant case) COP was 16.7% higher than for the additive free case. One reason is a reduced subcooling in the absorber and another reason is an increased cooling effect. The maximum enhancement ratio was 32.3% compared to the case without additive. LiBr solution samples were carefully examined after each measurement in order to determine their composition. A few water samples have also been taken out from the evaporator. Observations revealed the presence of the additive islands on the LiBr solution surface during the first experimental series. However, it does not necessary
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Fig. 3. 2-Methyl-1-pentanol effect on the enhancement ratio (additive-in-refrigerant case). Fig. 3. L’effet du 2-me´thyl-1-pentanol sur la performance (cas de l’additif dans le frigorige`ne).
mean that 2MP was present in the form of droplets on the surface during the absorption process. Water samples from the evaporator also revealed the presence of the additive. This can be explained by the fact that the additive is partly boiled off in the generator and arrives at the evaporator with the condensate. LiBr solution samples from the second experimental series also revealed the presence of the additive. But neither surfactant islands were present on the surface. 2Methyl-1-pentanol existed in the solution in amounts smaller than the solubility limit [18]. All these facts support the suggestion that the additive is a volatile component that travels around the cycle and hence is present in the vapour phase. Rough estimation of the vapour composition in the absorber/evaporator unit resulted in a value of 0.05–0.1 wt.%. The presence of the additive in the vapour phase in very small amounts promotes heat and mass transfer in the absorber more efficiently compared to the additive-in-solution case. Experimental results and observations seem to support theories advanced by Herold [10] and Setterwall [19]. Finally, this new additive — 2MP — was used in a commercial absorption chiller in Sweden. Calculated amount of additive was injected into evaporator. The cooling effect was then increased. Estimated enhancement ratio made up 30–35% that is in an excellent agreement with experimental data. Cooling effect stabilised at a higher level and lasted during 50 h until the chiller was shut down. As long as the additive is present in the vapour phase as well as in the liquid, it can be pumped out from the machine during purging of noncondensables and after a while the additive effect reduces and the cooling effect decreases.
4. Conclusions Number of experimental and theoretical studies have been dedicated to the question about additive effect on
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the absorption cycle performance or on the absorber performance in particular. Experimental research was carried out, generally, with absorbers utilising vertical tubes or a bundle of horizontal tubes. Falling film heat transfer coefficient or absorption rate were used as parameters in order to establish the enhancement caused by the additive. 2-Ethyl-1-hexanol and 1-octanol are generally used in the experiments. In the present study we have chosen 2-methyl-1-pentanol. The choice of this additive was based on the stability analysis fulfilled by Gustafsson [18, 21]. The objective of this study was to establish experimentally 2MP effect on the absorption chiller performance. A mini absorption chiller utilising lamella surfaces was used in the experiments. Cooling effect in the evaporator was measured experimentally and used as a parameter in order to estimate the enhancement ratio. Two experimental series have been carried out. In one experimental series the additive was injected into the LiBr solution. The enhancement ratio up to 20% was observed at the optimum additive concentration. In the second experimental series the additive was injected into the refrigerant. The amount of additive for this case was taken according to 1 mg of 2MP per 1 kg of LiBr solution charged to the facility. The enhancement ratio became 32%, which is substantially better. Different additive concentrations have been tested in both series. LiBr solution samples as well as refrigerant samples have been examined during the experiments. Examination revealed that 2MP performed maximum enhancement at the concentration over the solubility limit when it was injected into LiBr solution. LiBr solution samples also contained additive when it was injected into evaporator. This can be explained by the fact that the additive as a volatile component transported to the vapour phase and then absorbed in the absorber. The additive content in the LiBr solution samples was smaller than the solubility limit when 2MP was injected into evaporator. Rough estimation of the vapour composition in the absorber/evaporator unit revealed a value of 0.05– 0.1 wt.%. The presence of additive in the vapour phase in very small amounts promotes heat and mass transfer in the absorber more efficiently compared to the additivein-solution case. Experimental results and observations seem to support theories advanced by Herold [10] and Setterwall [19]. This new additive was subsequently used to increase the capacity of the commercial absorption chiller. The capacity has been increased by a factor of 30–35% that is in a very good agreement with the experimental data.
Acknowledgements This research was carried out within the framework of the Klimat 21 program supported by the Swedish
National Energy Authority. Special thanks to Dr. Magnus Gustafsson for useful discussions.
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