Lennart Vamling

Replacement of R22 in tube-and-shell condensers: experiments and simulations

An investigation of the change in condenser overall heat transfer coefficient when replacing R22 with one of the three mixtures R407C, R404A and R410B was made, both experimentally and theoretically. Measurements have been carried out on a full-scale test plant consisting of a horizontal shell-side condenser. According to the measurements the decrease in overall heat transfer coefficient for the non-azeotropic mixture R407C was very large, up to 70% compared to R22, while for the near-azeotropic mixture R404A the decrease was less than 15%. Simulations of the condenser were done with a comprehensive computer program, calculating the condensation heat transfer with an approximate method including a correction for mass resistance. The calculation model was not able to predict this large degradation for the non-azeotropic mixture, while the predictions agreed rather well with the measurements for the pure fluid and the near-azeotropic mixtures.

Calculation methods for comparing the performance of pure and mixed working fluids in heat pump applications

Three methods for comparing cycle performance of working fluids, pure as well as non-azeotropic mixtures, are investigated for two applications and for two mixture pairs, HCFC22-CFC114 and HCFC22-HCFC142b, and their pure components. The methods differ in the way of calculating the heat exchange processes. They assume, respectively, equal minimum approach temperatures, equal mean temperature differences and equal heat transfer areas. Changes of coefficient of performance (COP) with composition are explained for all methods. It is shown that transport properties must be taken into account when making rigorous comparisons between working fluids. To predict the relations between fluids with high accuracy, one must use the method with equal heat transfer areas. By the method with equal mean temperature differences, the COP can be estimated with the same accuracy for mixtures as for pure fluids, and can be used for rough estimations of the COP level with different fluids. The method of equal minimum approach temperatures should be avoided for non-azeotropic mixtures.

The effect of temperature on the catalytic conversion of Kraft lignin using near-critical water

The catalytic conversion of suspended LignoBoost Kraft lignin was performed in near-critical water using ZrO2/K2CO3 as the catalytic system and phenol as the co-solvent and char suppressing agent. The reaction temperature was varied from 290 to 370°C and its effect on the process was investigated in a continuous flow (1kg/h). The yields of water-soluble organics (WSO), bio-oil and char (dry lignin basis) were in the ranges of 5-11%, 69-87% and 16-22%, respectively. The bio-oil, being partially deoxygenated, exhibited higher carbon content and heat value, but lower sulphur content than lignin. The main 1-ring aromatics (in WSO and diethylether-soluble bio-oil) were anisoles, alkylphenols, catechols and guaiacols. The results show that increasing temperature increases the yield of 1-ring aromatics remarkably, while it increases the formation of char moderately. An increase in the yields of anisoles, alkylphenols and catechols, together with a decrease in the yield of guaiacols, was also observed.

Development of models for prediction of solubility for HFC working fluids in pentaerythritol ester compressor oils

This study aims to further develop a model of the group contribution type that can predict the solubility of mixtures of HFC working fluids and pentaerythritol ester compressor oils. The investigation is based on solubility data for 20 systems of five different HFCs in four different pentaerythritol esters. First, five different activity factor-based thermodynamic models were investigated to determine to what extent they can describe the solubility data. Two promising models were selected and extended with group contribution parameters to be able to predict solubility data. The first model, a modified Flory–Huggins based predictive model (termed MFH), needs only 15 general parameters to describe the data with a relative deviation of less than 3.4%. The second model, a predictive Unifac model with an extended temperature dependence, needs 33 general parameters to describe the solubility data for the 20 systems with a relative deviation of less than 3.5%.This study aims to further develop a model of the group contribution type that can predict the solubility of mixtures of HFC working fluids and pentaerythritol ester compressor oils. The investigation is based on solubility data for 20 systems of five different HFCs in four different pentaerythritol esters. First, five different activity factor-based thermodynamic models were investigated to determine to what extent they can describe the solubility data. Two promising models were selected and extended with group contribution parameters to be able to predict solubility data. The first model, a modified Flory–Huggins based predictive model (termed MFH), needs only 15 general parameters to describe the data with a relative deviation of less than 3.4%. The second model, a predictive Unifac model with an extended temperature dependence, needs 33 general parameters to describe the solubility data for the 20 systems with a relative deviation of less than 3.5%.

Heat transfer and pressure drop in a plate-type evaporator

The purpose of the work reported here was to determine heat transfer and pressure drop in a plate-type evaporator both experimentally and theoretically. An experimental fa-cility was built to provide the means for detailed measure-ments at different conditions. Subsequently, a calculation method was developed to simultaneously calculate heat transfer and pressure drop in a variable number of steps along the evaporator. Results from measurements and calcu-lations are compared and discussed. In the investigated range the calculation method overestimates heat transfer by between 1-8% and underestimates pressure drop by between 7% and, at very high outlet vapour fractions, 30%.

The abdoul-rauzy-pénéloux group contribution equation of state extended to CFC-containing mixtures

This study contributes to the development of new thermodynamic models, useful for the prediction of refrigerant-mixture properties. Mixtures are considered to be potential substitutes for the environmentally harmful chlorofluorocarbons. Here the Abdoul—Rauzy—Peneloux (ARP) equation of state (EOs) is used, which combines a group contribution excess Helmholz energy model with traditional cubic EOSs. This EOS, initially developed for different petroleum mixtures, has been extended to cover also certain compounds containing chlorine and fluorine. A predictive method for PVT modelling of mixtures containing CFCs, HCFCs, HFCs, FCs and hydrocarbons has thus been developed. From experimental data for 31 different binary systems, 19 general group parameters were estimated. the error on the calculated vapor pressure for these systems does not exceed 5.1% for any of the considered mixtures. This demonstrates the effectiveness of the method since the traditional mixing rules would have demanded 31 specific interaction parameters to describe the same number of systems, without the possibility of extending their validity to other mixtures. In order to verify the predicting capacity of the method, it has also been applied to a ternary and an azeotropic system, that have not been included in the estimation procedure of the parameters. The results from these applications are deviations on the calculated pressure of 0.7 and 26% for the ternary and the azeotropic system, respectively.

Drop-in replacement of R22 in heat pumps used for district heating – influence of equipment and property limitations

In Swedish district heating systems several large (25 MW) turbo-compressor driven heat pumps using R22 are installed. The only commercially available alternative is R134a, but its use could decrease the heating capacity by 35%. In this paper a method for finding the best working fluid for a specific heat pump plant is presented, and applied to a district heating plant. First, a screening is made among almost 2000 mixtures, using criteria such as condenser pressure, Mach number and temperature glide. Simulations of the plant are then made to investigate the change in heating capacity and COP when using a mixture instead of R134a. The results show that there are mixtures that can offer a substantially higher heating capacity than R134a, but there is a decrease in COP. The importance of considering the limiting parameters of the heat pump, such as maximum volume flow to each compressor stage and minimum evaporator pressure, is shown.

Changes in optimal design of a dry-expansion evaporation when replacing r22 with r407c

An investigation was made on whether the optimal number of tubes in a dry-expansion evaporator, for a given area or a given cost, will change when R22 is replaced with the non-azeotropic mixture R407C. A comprehensive computer program was used to simulate the tube-and-shell evaporator, under several sets of conditions. The simulations show that there are only small differences between R22 and R407C in the optimal number of tubes. However, the results indicate that one can allow a somewhat higher pressure drop for R407C, and that the number of tubes for a given area therefore can be smaller when R407C is used. For both fluids, the evaporator performance is more sensitive to the optimization for a given cost than for a given area.

Scale formation and growth when evaporating black liquor with high carbonate to sulphate ratio

Scaling in the recovery cycle of kraft pulp mills has long been recognised as a problem, especially in black liquor evaporators. Most problems are connected with the precipitation of sodium salts. In the black liquor evaporation, as the black liquor becomes concentrated, sodium carbonate and sodium sulphate will saturate and precipitate from the solution. When precipitation occurs the salts may crystallise on the heat transfer surface or adhere to the surface as well as they can crystallise in the bulk. If crystals remain on the heat transfer surface an insulating layer of scales can quickly build up, causing the heat flux to drop and eventually forcing the evaporator to be shut down for cleaning. In this research the scaling rate, or the reduction in heat transfer rate over time, has been analysed. For the research a pilot evaporator has been used, built in cooperation with Metso Power (former Kvaerner Power). The falling film type evaporator is a 4.5 m tube with the black liquor on the outside. One of the objectives of the thesis was to investigate whether there are local scaling variations at different positions on the heat transfer area by monitoring the fouling growth rate at different local positions in the pilot evaporator. The scaling was found to depend on the vertical position along the heat transfer surface. The scales start to form on the bottom of the evaporator tube where the concentration is highest, and then expand upwards. A clean surface starts to foul only when the area beneath it has fouled. Furthermore the research in this thesis focused primarily on scaling for black liquors with high ratio of carbonate to sulphate, a condition shown to be important in industrial black liquor evaporators (Frederick et al., 2004). The aim was to find favourable operating conditions decreasing or eliminating scaling. The examined parameters were different heat flux, circulation flow rate, internal residence time and black liquor feed concentration. For the investigated operating conditions, the main parameters affecting scaling were the circulation flow rate and heat flux, whereas the scaling rate was seen to be independent of the internal residence time.

Changes in optimal distribution of heat exchanger area between the evaporator and suction gas heat exchanger when replacing R22 with R407C

An investigation was made on whether the optimal distribution of heat exchanger area between the evaporator and the suction gas heat exchanger, at a given cost, will change when R22 is replaced with the non-azeotropic mixture R407C. A comprehensive computer program was used to simulate a refrigeration plant, originally designed for R22, where the tube-andshell evaporator is of dry-expansion type. The simulations show that for R22, the area distribution in the original design is optimal for every set of conditions simulated. For R407C it is, in most of the cases simulated, favourable to move some heat exchanger area from the evaporator to the suction gas heat exchanger. The improvement in COP can be up to 3.4%. Further, it was observed that the optimal area distribution for R407C corresponds to the maximum use of the glide in the evaporator, and for R22 to the minimum pressure drop in the evaporator. The superheat at the evaporator exit is an important parameter, especially for the non-azeotropic mixture. q1998 Elsevier Science Ltd and IIR. All rights reserved.