2. Description of the Conventional Art Absorption chillers do not use Freon (CFC) as a refrigerant, the use of which is restricted due to the depletion of ozone layer. In addition, absorption chillers use a relatively cheap natural gas instead of electric power as the main energy source. Thus, absorption chillers reduce electric power demand during summers. Because of these advantages, the demand for absorption chillers has increased as a cooling system in a large-sized building.

Generally, an absorption chiller comprises shell-and-tube heat exchangers such as an evaporator, an absorber, a condenser, and high and low temperature regenerators. Improvements in heat and mass transfer of the heat exchangers contribute to the enhancement in the performance of the absorption chiller. In addition, the improvements can reduce the size and

weight of the absorption chiller and, thereby, contribute to the reduction in the size of required space for installation. Therefore, studies are being carried out on heat transfer enhancement techniques in the heat exchangers.

Conventionally, methods for enhancing the heat transfer include a method of increasing the surface area of the heat transfer tubes by deforming the shape of the tube by a mechanical machining, and a method of coating the surface with a surfactant agent by submerging the tube into a chemical solution mixed at certain ratio. However, the former method incurs a high processing cost and a new capital investment and thus, it has a low economic efficiency. Meanwhile, the latter method has a drawback in that the coated solution dissolves in water and the heat transfer performance thereby degrades.

Therefore, a new technique for resolving various problems in the conventional method and enhancing the heat transfer is required.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to enhance heat and mass transfer by providing a hydrophilic surface to heat transfer tubes of a condenser, an evaporator, an absorber, etc. which are main components of an absorption chiller, by modifying the surface with plasma polymerization.

A surface modification method used in the present invention is directed to semi-permanently maintain the heat transfer performance of a heat exchanger by forming a hydrophilic polymerized film on a metal surface using plasma which is generated by a DC discharge or RF discharge.

Therefore, various problems encountered in the conventional method can be greatly resolved.

The present invention provides an absorption chiller having excellent heat transfer performance by simple treatment or modification of the surface of the heat transfer tubes and thereby make it possible to obtain a maximized energy efficiency and a smaller and more lightweight absorption system than the conventional absorption system. The hydrophilic surface modification by plasma polymerization can be performed on heat transfer tubes of a heat exchanger installed in a conventional absorption system to enhance the heat transfer performance thereof. Also, the hydrophilic surface modification by plasma polymerization can increase heat transfer characteristics of not only bare tubes but also mechanically machined tubes.

To achieve the above objects, the present invention provides a heat transferring part, for example, a copper heat transfer tube having a hydrophilic surface modified by plasma polymerization to allow a heat exchanger to have excellent heat and mass transfer performance compared to the conventional heat exchanger of an absorption system.

According to the preferred embodiment of the present invention, a plasma polymerized film with a receding contact angle of not more than 30° is formed on the surface of the heat transfer tubes. The hydrophilic plasma polymerization according to the present invention is applicable to a mechanically machined tube, as well as a bare tube, of a condenser, an evaporator, and an absorber which are main components of an absorption chillers. In addition, the present invention is also applicable to a general heat exchanger, especially a general shell-and-tube heat exchanger for condensation or evaporation.

Additional advantages, objects and features of the invention will

become more apparent from the description which follows.

BRIEF DESCRIPTION OF THE INVENTION.

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration, not limitation of the present invention, wherein : Figure 1 is a block diagram of a conventional absorption chiller; Figure 2A is a view illustrating a thick water film condensation on an outer wall of a conventional horizontal tube; Figure 2B is a view illustrating a thin water film condensation on an outer wall of a hydrophilic surface modified horizontal tube according to the present invention; Figure 3A is a view illustrating a thick water film evaporation on an outer wall of a conventional horizontal tube; Figure 3B is a view illustrating a thin water film evaporation on an outer wall of a hydrophilic surface modified horizontal tube according to the present invention ; Figure 4A is a view illustrating water sprayed on a bare tube before a hydrophilic surface modification; Figure 4B is a view illustrating water sprayed on a bare tube after a hydrophilic surface modification according to the present invention; Figure 5 is a schematic view of an apparatus for testing a condenser of an absorption chiller ; Figure 6 is a graph illustrating condensation heat transfer characteristics

of a bare tube with respect to various conditions of the hydrophilic surface modification, wherein a heat transfer quantity Q in condensation is 2902W for a bare tube before a hydrophilic surface modification; Figure 7 is a graph illustrating condensation heat transfer characteristics of a bare tube with respect to various inlet temperatures of a cooling water into a condenser of an absorption chiller; Figure 8 is a view illustrating condensation heat transfer characteristics of a bare tube with respect to a time lapse; Figure 9 is a schematic view of an apparatus for testing an evaporator of an absorption chiller; Figure 10 is a graph illustrating evaporation heat transfer characteristics of a bare tube with respect to various inlet temperatures of a hot water into an evaporator of an absorption chiller, wherein an evaporating pressure was 31.8 torr; Figure 11 is a schematic view of an apparatus for testing a lithium bromide absorber ; Figure 12 is a graph illustrating absorption heat transfer characteristics of a bare tube with respect to various flow rates of a lithium bromide through a lithium bromide absorber; Figure 13 is a graph illustrating absorption mass transfer characteristics of a bare tube with respect to various flow rates of a lithium bromide through an lithium bromide absorber ; Figure 14 is a graph illustrating absorption heat transfer characteristics of a bare tube with respect to various flow rates of a cooling water through an lithium bromide absorber; and

Figure 15 is a graph illustrating absorption heat transfer characteristics of a bare tube with respect to various flow rates of a cooling water through a lithium bromide absorber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 illustrates the construction of a conventional double effect absorption chiller, which is the most commonly used absorption chiller. The absorption chiller comprises a plurality of basic heat exchangers: an evaporator 1 for refrigeration operation, an absorber 2 for absorbing refrigerant, a condenser 3 for condensing the refrigerant, a high temperature regenerator 4 for regenerating the refrigerant by boiling the same, and a low temperature regenerator 5. The double effect absorption chiller further includes, as auxiliary units, a solution heat exchangers 6 and 7 for enhancing the thermal efficiency, a solution circulator 8, and a refrigerant circulator 9, a steam extraction unit 10 for maintaining vacuum, etc..

In the evaporator 1, vacuum is maintained in the evaporator 1 and the refrigerant is uniformly sprayed onto an outer surface of a heat transfer tube by means of the refrigerant circulator 9. In the above state, the sprayed refrigerant is evaporated by taking the latent heat of vaporization from the cold water flowing through the heat transfer tube and the cold water is further cooled. In this process, the refrigerant removes heat from the cold water.

The absorber 2 serves to prevent an increase in the evaporation temperature due to the increase in the partial pressure of water vapor

generated when the refrigerant in the evaporator is evaporated. A thin solution film is formed by spraying an absorption solution (LiBr) on an outer surface of the heat transfer tube in the absorber. The water vapor is absorbed into the solution film and then, absorption heat is generated. The absorption heat is transferred to the cooling water flowing through the heat transfer tube. Since the quantity of absorbed water vapor is dependent on the concentration of the absorption solution, a concentrated solution supplied from the generator is used as the absorption solution. The method for spraying the absorption solution in the absorber 2 is the same as that for spraying the refrigerant in the evaporator 1.

In the condenser 3, the refrigerant which was evaporated by the low temperature regenerator 5 and then was delivered through an eliminator 12 is condensed by cooling water flowing through the heat transfer tube in the condenser 3. The condenser 3 serves to return the condensed refrigerant, along with the high temperature refrigerant condensed by the low temperature regenerator 5, through the lower section of the condenser 3 to the evaporator 1.

Generally, the above-mentioned evaporator 1, absorber 2, and condenser 3 have a shell-and-tube structure, and bare tubes or mechanically machined tubes such as a low fin tube is used as a heat transfer tube.

Figures 2A through 3B are views illustrating condensation or evaporation with a thick water film or a thin water film formed on an outer wall of a bare tube (Figures 2A and 3A) and a hydrophilic surface modified tube (Figures 2B and 3B) respectively. As shown in Figure 2A, in the case of condensation on the bare tube, irregular waterdrops are created and grow and thereby a relatively thick water film is formed on the bare tube. As shown in

Figure 3A, in the case of evaporation, sprayed refrigerant forms a thick water film on the outer surface of the bare tube. On the contrary, in the case of a tube having hydrophilic surface modified by plasma polymerization according to the present invention, a thin water film is formed on the outer surface of the tube, and thereby the performance of heat transfer is enhanced due to the increase of the heat-transfer area and the decrease of the thermal resistance.

Therefore, when a tube which has hydrophilic surface modified by plasma polymerization according to the present invention is used as a heat transfer tube, excellent heat and mass transfer can be obtained. In addition, when a mechanically machined tube such as a low fin tube, a corrugated tube, a spiral tube, and a thermoexcel fin tube, as well as the bare tube, is hydrophilic surface modified by plasma polymerization, the performance of heat and mass transfer is also improved.

In a method of hydrophilic surface modification by plasma polymerization, an object metal material (tube) to be hydrophilic surface modified is provided as an anode electrode in a vacuum tank, and a cathode electrode is positioned therein at a certain distance, and a relatively low vacuum (not more than 103mmHg) is maintained by means of a vacuum pump.

A reactive gas (acetylene and nitrogen which are used in the preferred embodiment of the present application ; and other polymerization gases suitable for giving rise to a functional surface modification) and a hydrocarbon gas with a certain pressure are mixed at a certain ratio and then are injected into the vacuum tank in which vacuum is maintained. After the gas injection finishes then a voltage is applied to both metal electrodes to perform a DC discharge, a plasma including positive ions, negative ions,

radicals, electrons, etc. is generated. Methods for generating plasma on an metallic object material include a RF discharge method and a DC discharge method. The present invention is also applicable to a metal treated by means of the RF discharge method. A hydrophilic polymerized film is formed on a metal surface by means of plasma generated by a DC discharge or a RF discharge, and thereby the metal maintains a functional surface which is physically and chemically more stable compared to other surface modification methods.

A plasma polymer layer formed by means of the plasma polymerization process has an excellent hydrophilic property of a receding contact angle not more than 30° when measuring a dynamic contact angle. Plasma generated from a mixed gas of monomer and a nonpolymerizable gas forms a polymer layer with an excellent hydrophilic property on an object material. For instance, when a plasma polymerization is performed using C2H2 as a monomer gas and N2 as a nonpolymerizable gas, the ratio of composite gases (C2H : N2) being changed from 9: 1 to 1: 9, all the plasma polymer layers have the value of a receding contact angle not more than 30°. The above-mentioned hydrophilic properties of a polymer layer are directly related to the chemical properties of a polymer surface. It can be considered that the polymer surface layer formed under a plasma polymerization of the mixed gas has a high surface energy which allows water to diffuse well. The high surface bringing about the hydrophilic properties is due to hydrophilic functional groups of the surface layer. A polymer formed with a change of the mixed gas ratio is comprised of 65-50% C, 5-20% O, and 0-15% N, except for H. As a result of an analysis with XPS (x-ray photoelectron spectroscopy) and FT IR (Fourier

transformation infrared spectroscopy), it was verified that hydrophilic functional groups of C-O, C O, (C=O) O, C-N, N-H are formed in the plasma polymer. These hydrophilic functional groups provide the polymer with excellent hydrophilic properties.

The method for forming the hydrophilic functional groups by plasma polymerization is not limited to the method by using the mixed gas of C2H2 and N2. The present invention may have various embodiments. For example, a plasma hydrophilic polymerization may be performed using hydrocarbon as a substitute for C2H2 as a monomer and using N202, etc. as a nonpolymerizable gas.

[Table 1] Kind of I C2H2: N2 Plasma treatment Heat treatment sample (Composition ratio) conditions conditions HPT 1 1: 9 Initial Annealing HPT 2 3 : 7 pressure: 103mmHg treatment at 200°C HPT 3 5 : 5 for 20 minutes. HPT 4 7: 3 Operating 9:1HPT5 pressure:0.3mmHg 7:3HPT6 HPT6 and HPT 7 HPT 7 5: 5 Current: 800mA are not heat- Process time: 90sec treated.

Table 1 shows the composition ratios and heat treatment conditions in the case that acetylene and nitrogen are used as the reactive gas for the hydrophilic surface modification. It is noted that the conditions described herein, which were selected for comparing performances related to the hydrophilic treatment of the present invention, is no more than a part of many conditions suitable for the functional surface modification. Therefore, the

present invention is applicable even in the case that other composite gases, composition ratios, and process conditions are used.

Figures 4A and 4B are views comparing the shape of waterdrops formed on outer wall of a bare tube when the tube was hydrophilic-treated or not treated. As shown in Figure 4B, a thin water film is widespread on the outer wall of the bare tube being hydrophilic treated, and thereby the contact property is improved compared to the non-treated bare tube.

[Table 2] Cooling water Inlet temperature (°C) 15 0.1 Flow rate (l/min) 3.9 0.1 Electric heater Heater capacity (kW) 3.5 Area of heat transfer 3 x 3 tube bank 0.1077 surface Figure 5 is a schematic view of an apparatus for testing a condenser of an absorption chiller. Table 2 shows the test conditions for a condenser of an absorption system. The test for a condenser is performed under an atmospheric environment in order to measure and evaluate the performance of condensation by a supply of vapor.

The test apparatus includes a condenser 22 being a test volume, a cooling water circulator 23 for supplying cooling water of a certain temperature to the test volume, a duct 24 and thermostatic bath 25 for supplying vapor uniformly onto the entire surface of the tube in the condenser 22, an inlet 26 and outlet 27 of the condenser 22, a condensate water

temperature measuring apparatus 28, and a flow rate measuring apparatus 29.

A vapor supplying system is comprised of a thermostatic bath (40 liter) 25 in which an electric heater (3.5kW) 30 is provided, and a duct 24 for transferring a generated vapor. After the water of about 30 liter is supplied into the thermostatic bath 25, an electric power supplied to the electric heater 30 is connected with a voltage regulator (0-240V) 30 to maintain a constant voltage (208V), and thereby a vapor of a constant flow rate is generated. A saturated vapor is supplied to the test volume through the adiabatic-treated duct.

The cooling water circulating system is comprised of a cooler (3RT) 31 which is a supply source of cooling water, a thermostatic bath (80 liter) 32 for maintaining a constant inlet temperature of the cooling water, a circulating pump 23 for supplying the cooling water into a heat exchanger being a test volume, and a flow meter 29 for measuring the flow rate of the cooling water. The cooling water flown into the tube is always maintained at a constant temperature by an agitator in the thermostatic bath 32. The cooling water flown out from the thermostatic bath 32 is supplied into the heat exchanger 22 through the flow meter 29. The cooling water heated by heat exchange with a vapor supplied through the duct 24 is flown into the cooler 31 again for re-circulation.

The flow rate per minute of condensed water generated in the heat exchanger is measured by means of an electronic scale (degree of precision: 0.01g) 33. The temperatures of the cooling water at the inlet and outlet of the heat exchanger 22 are measured by a T-type thermocouple (a relative temperature error at the inlet and the outlet: 0.1 °C) respectively.

Figure 6 is a graph comparing condensation heat transfer quantities (Qw) of a bare tube with respect to various hydrophilic treatment conditions.

Under all hydrophilic treatment conditions, the heat transfer performance is about 8-12% higher than that of a conventional non-treated bare tube (Q in condensation for a non-treated bare tube is 2902W).

Figure 7 is a graph comparing condensation heat transfer quantities Q with respect to various inlet temperatures of a cooling water into a condenser of an absorption chiller, wherein the range of an inlet temperature is between 10°C-30°C and tests are performed at a 5°C interval.

HPT5 (C2H2: N2=7: 3) which had the best performance in the test result of the heat transfer quantity Q and the conventional bare tube were compared with each other. A hydrophilic-treated tube had a high performance of heat transfer compared to the conventional bare tube. As the inlet temperature of the cooling water increased, the heat transfer quantity Q tended to decrease.

Figure 8 illustrates a degradation in the heat transfer performance of a plasma hydrophilic treated bare tube with respect to the lapse of time. As shown in figure 8, assuming that a test is performed with the tube fully submerged for 20 minutes and then dried per one cycle, the heat treatment performance was continuously maintained even after the 100 cycle was performed.

Figure 9 is a schematic view of an apparatus for testing an evaporator of an absorption chiller.

Table 3 shows the test conditions for the evaporator.

[Table3] Cooling Water Inlet Temperature (°C) 10 0.5 Circulation Flow 3.9 0.1 rate (l/min) Area of Heat Transfer 0.2154 Surface (m2) High Temperature Inlet Temperature (°C) 35-60 0.5 Water Circulation Flow 3.9 0.1 rate (l/min) Area of Heat Transfer 0.1077 Surface (m2) Nozzle Spray Temperature (°C) 15 0.5 Spray Flow rate gamin 240 Spray Angle (°) 60

In the test for the evaporator, the performance of evaporation was evaluated by spraying distilled water onto the surface of a heat transfer tube.

The test apparatus included an evaporator 36 in which hot water of a certain temperature is supplied through the heat transfer tube, and distilled water sprayed from a nozzle is evaporated at an outer wall of the tube; a condenser 37 for condensing a generated vapor by supplying a cooling water of a certain temperature through a tube of the condenser 37; a spray device 38 for spraying distilled water uniformly onto the surface of the evaporator; a vacuum pump 39 for maintaining a test volume under a constant pressure; thermostatic bathes 48 and 52 for supplying hot water and cooling water of a constant temperature; a test volume pressure measuring apparatus 55; an inlet 41 and an outlet 42 of the cooling water, an inlet 43 and an outlet 44 of the hot water; a nozzle inlet temperature measuring apparatus 45; and a flow rate measuring apparatus 46. The test volume was provided with a window through which the inside of the test volume could be observed. The test was performed with the pressure in the test volume under vacuum.

The evaporating system includes a thermostatic bath 48 in which the temperature is maintained constant by means of an electric heater (IkW) 47, a circulating pump 49 for supplying hot water into the evaporator 36, and a flow meter (0.18-0. 96m3/h) 50 for measuring the flow rate of the hot water.

The inlet temperature of the hot water in the evaporator 36 is maintained constant by an agitator installed in the thermostatic bath 48. The hot water passes through the flow meter and flows into the heat exchanger and is cooled by distilled water sprayed through a nozzle and then flows into the thermostatic bath 48 again for re-circulation.

The condensing system includes a cooler (3RT) 51, a thermostatic bath (80 liter) 52 for keeping the cooling water inlet temperature constant, a circulating pump 54 for supplying cooling water into the condensation heat exchanger, and a flow meter 46. The temperature of the cooling water flown into the tube of the condenser 46 is maintained constant by means of the thermostatic bath 52. The cooling water passes through the flow meter 46, flown into the heat exchanger 37, and is heated by a heat exchange with a vapor generated by the evaporator 36, and then is flown into the cooler 51 for re-circulation.

An apparatus for keeping the test volume in vacuum includes a vacuum pump 39, a vacuum gauge 55, and a vacuum valve 40.

Considering the length (400mm) of the heat exchanger, a spray device 38 includes three spray nozzles at an interval of 120mm. A high pressure pump 56 is used as a driving source of the spray device 38. The temperatures of the high temperature water and the cooling water are measured at the inlet and the outlet of the heat exchanger by means of a T-type thermocouple

(relative temperature error at the inlet and the outlet: 0.1 °C). The temperature at the nozzle inlet is also measured by means of the T-type thermocouple.

Figure 10 is a graph illustrating heat transfer quantities with respect to various inlet temperatures of the evaporator, wherein the pressure in a test volume is 31.8mmHg. As shown in Figure 10, in the case that the hot water is circulated at a temperature higher than 30°C which corresponds to evaporating temperature at the pressure of 31.8mmHg, the amount of heat of vaporization increases almost linearly according to an increase of the temperature of the circulating hot water. In addition, a heat transfer performance of a hydrophilic-treated tube is improved by about 25-44% compared to the conventional bare tube. This is because that a thin film spreads widely on the outer wall of the hydrophilic-treated bare tube with the tube maintaining a contact angle smaller than that of the conventional bare tube, and thereby the thermal resistance decreases and the area of heat exchange surface increases.

Figure 11 is a schematic view of an apparatus for testing a lithium bromide absorber, and Table 4 shows the test conditions for the lithium bromide absorber.

[Table4] Item Test Parameter Test Condition Refrigerant Evaporating 8 1 Temperature(°C) LiBr solution Inlet Concentration (wt%) 60 0.5 Inlet Temperature (°C) 45 0.5 Circulation Flow 2.6-6.8 rate(kg/min) Cooling Water Inlet Concentration (°C) 28 0.5 Circulation Flow 50-83 rate (kg/min)

The absorber test apparatus includes an absorber 61, an evaporator 62, a condenser 63, a regenerator 64, concentrated solution and dilute solution tanks 65 and 66 respectively, a refrigerant tank 67, a hydrostatic head tank 69, a cooling water tank 70, a cooling tower 71, a heater 72, a hot water tank 73, and pipes for connecting the above-mentioned elements. In order to maintain vacuum in the system, a vacuum pump is installed, and in order to measure the pressure of the apparatus, a vacuum mercury manometer is installed. In addition, a pump is installed for the absorber and the evaporator in order to circulate the cooling water and the cold water, respectively. In order to measure the inlet and outlet temperatures of pipes of the absorber 61, evaporator 62, regenerator 64, condenser 63, and tanks, T-type thermocouples (C-C) are installed. In order to measure the flow rate of the cooling water, cold water, and solution, a water flow meter and a solution flow meter are installed.

The test for an absorber is carried out by three processes: a test condition setting process, a performance measurement process, and a solution regeneration process. In the test condition setting process, the inside of the system is sufficiently vacuumed, and then the solution in the concentrated solution tank 65 is circulated by means of a solution pump. At this time, the solution is controlled to have a desired temperature by means of a heat exchanger through which a hot water and cooling water of an adequate temperature flows, and then is stored in the concentrated solution tank 65. The

temperature of the hot water is controlled by means of a thermostat in the hot water heater 72, and the temperature of the cooling water is controlled by means of a cooling tower 71 and a cooling water tank 70. The hot water and cooling water adjusted to have adequate temperature are transferred to each heat exchanger by the pump.

In the performance measurement process, the concentrated solution in the concentrated solution tank 65 is transferred to the absorber 61 by means of the solution pump. Thereafter, the concentrated solution is flown down from the upper side of the absorber 61 to a test heat transfer tube, and then flown over the tube in the form of a liquid film. At this time, the solution flow rate at the absorber inlet and the solution temperature at the absorber inlet and outlet are measured. When transferred to the absorber 61, the concentrated solution is flown at a constant flow rate by means of a hydrostatic head tank.

An excess absorption solution is returned to the concentrated solution tank 65 through a by-pass pipe. The refrigerant stored in the refrigerant tank 67 is flown to the evaporator 62 by means of a refrigerant pump, and then is evaporated therein. The evaporated refrigerant is absorbed into the solution flowing down to the test heat transfer tube in the absorber, and the non- evaporated refrigerant is flown back into the refrigerant tank 67. The solution diluted by absorbing refrigerant is stored in the dilute solution tank 66, and the concentration of the solution is measured through a dilute solution bled at the absorber outlet by a sampling valve. The cooling water in the cooling water tank 70 is transferred into the evaporator 62 and the absorber 61 by the pump. In the absorber 61, the cooling water in the tube flows parallel with the absorption solution. The cooling water which obtains heat in the absorber 61

and the cold water from which heat is taken away in the evaporator 62 is gathered again at the outlet, and then cooled to an adequate temperature in the cooling tower 71. The flow rates at the inlets of the evaporator 62 and the absorber 61 are measured respectively, and the cooling water temperatures at the absorber inlet and outlet and each path are measured.

In the solution regeneration process, firstly, the dilute solution gathered in the dilute solution tank 66 is transferred to the concentrated solution tank 65 by means of a pump. When all the solution is transferred to the concentrated solution tank 65, the dilute solution collected in the concentrated solution tank 65 is circulated by means of the concentrated solution pump to be heated by the hot water flowing through the heat transfer tube of a high temperature regenerator. The refrigerant is separated from the heated dilute solution and thereby the dilute solution becomes concentrated. The separated refrigerant is cooled and condensed by the cooling water in the condenser 63, and then is stored in the refrigerant tank 67. The concentrated solution is bled by means of the sampling valve and heated by the high temperature regenerator until an adequate concentration is obtained. The cooling water flowing through the condenser 63 is circulated in the apparatus by means of a cooling water pump, and cooled again in the cooling tower. The hot water is heated by a hot water heater 72 and the temperature thereof is maintained constant by a thermostat in the same manner as in the test condition setting process.

Figure 12 is a graph illustrating heat transfer coefficients with respect to various solution flow rates in a lithium bromide absorber. An analytical method for calculating the heat transfer coefficient is based on a logarithmic

mean temperature difference Dola, and an overall heat transmission coefficient K. A logarithmic mean temperature difference ATlm of a heat exchanger is obtained by Formula 1, and an overall heat transmission coefficient K is obtained by Formula 2.

[Formula 1] <BR> <BR> <BR> <BR> <BR> TAcoo)-(TAso-TAcoi)][(TAsi- <BR> #Tlm = (1)<BR> ln[(TAsi - TAcoi)]- [Formula 2] wherein Tas ils the absorption solution temperature at the inlet of the absorber 61 measured in the test; Tas ils the outlet temperature ; TACoi and TACOO are the inlet temperature and outlet temperature of cooling water, respectively; do is the outer diameter of a tube; and L is the length of a heat transfer tube. In order to obtain a heat transfer coefficient hi of the cooling water in the tube, "Dittus-Boelter"Formula 3 representing the Nusselt number is used as a relational expression of the heat transfer at a bare tube in which the cooling water is in the state of the turbulent flow.

[Formula 3] Nu =hi # di/#(3)0.023R30.8#Pr0.4 wherein di is the inner diameter of the tube, and X is the heat conductivity of the inside and outside of the tube. The heat transfer coefficient hi of the absorption solution is obtained by Formula 4, and the thermal resistance on the heat transfer tube walls are ignored.

[Formula 4] <BR> <BR> <BR> <BR> <BR> <BR> 1<BR> <BR> <BR> A=--------(4) wherein d, is the inner diameter of the tube, and do is the outer diameter of the tube, and hi is the heat transfer coefficient of the inside of the tube. Reynolds number of a film (Ref) and the flow rate of a liquid film per width (yS) are defined as Formulas 5 and 6, respectively.

[Formula 5] Ref (5) vs [Formula 6] <BR> <BR> <BR> <BR> <BR> <BR> G<BR> <BR> <BR> --(6) wherein G, represents the mass flow of the absorption solution.

Figure 12 illustrates a linear increase of the heat transfer coefficient of the outside of the tube according to an increase of the solution flow rate. As shown in Figure 12, a heat transfer performance of a hydrophilic-treated tube is improved compared to that of a conventional bare tube.

Figure 13 illustrates mass transfer coefficients with respect to various solution flow rates in a lithium bromide absorber. In the calculation of the mass transfer coefficient, only the mass transfer between a gas-liquid interface and an absorption solution is taken into consideration, while the refrigerant mass transfer resistance between a refrigerant vapor and an absorption solution, and the resistance between a vapor space and a gas-liquid

interface are ignored. In addition, assuming that the pressure at the gas-liquid interface of a down-flow liquid film is equal to the gas pressure, a logarithmic mean temperature difference Z mbetween the equilibrium concentration #* of the gas-liquid interface and the concentration of the down-flow liquid film # is defined as Formula 7.

[Formula 7] Using the above properties, the mass transfer coefficient is obtained as shown in Formula 8.

[Formula 8] wherein GR is the absorbed amount of a refrigerant vapor, and Pm is the average density of a solution which is obtained by Formula 9.

[Formula 9] (#As@+#Asi)/2(9)#m= wherein pas ils the solution density at the absorber inlet, and PA,,, is the solution density at the absorber outlet. A change of the mass transfer coefficient according to a change of the flow rate of lithium bromide solution shown in Figure 13 is small compared to the change of the heat transfer coefficient according to the change of the flow rate of lithium bromide solution shown in Figure 12. However, it is shown that the mass transfer coefficient of a hydrophilic-treated tube is higher than that of a conventional bare tube.

Figure 14 illustrates the change of the heat transfer coefficient according to the change of the flow rate of cooling water in a lithium bromide absorber. When the flow rate of cooling water is 3m3/h, the heat transfer coefficient of a hydrophilic-treated tube is about twice that of the conventional bare tube.

Figure 15 illustrates mass transfer coefficients with respect to the flow rates of cooling water in a lithium bromide absorber, which coefficient is constant to a certain degree, irrespective of the change of the flow rate. When the flow rate of cooling water is 3m3/h, the mass transfer coefficient of a hydrophilic-treated tube is about 1.5 times that of the conventional bare tube.

According to the present invention above-described, many problems presented in the conventional hydrophilic coating method using a surfactant agent or a heat transfer enhancement method by a mechanical machining can be overcome, and thereby the performance of heat and mass transfer is improved. In addition, the present invention allows absorption chiller, absorption chiller-heater unit, absorption heat pumps, etc. to be smaller in size, more lightweight and cheaper in the manufacturing cost. In addition, the present invention is also applicable to a general shell and tube heat exchanger, and therefore it can be expected that the present invention has a powerful economic potential.