Control of temperature and humidity surrounding the stone chamber of Takamatsuzuka tumulus during its dismantlement

Transcription

1 Control of temperature and humidity surrounding the stone chamber of Takamatsuzuka tumulus during its dismantlement Daisuke Ogura 1, Masahide Inuzuka 2, Shuichi Hokoi 1, Takeshi Ishizaki 2, Hiroyuki Kitahara 3 and Jiro Tarama 1 Introduction The Takamatsuzuka tumulus was discovered on 21 st of March, 1972, in Asuka village in Nara prefecture. A stone chamber, which is 1.03 m wide, 1.13 m high, and 2.65 m deep, is buried within the mound of soil. Paintings on the lime wallplaster inside the chamber are considered to have been painted between the end of the seventh century and the beginning of the eighth century. The mural paintings were designated a national treasure in It was decided that the mural paintings should be preserved on-site to be kept under the same condition as when found. For this purpose, a conservation facility, which consists of three antechambers, was constructed in In the spring of 2001, mould was found at the entrance of the stone chamber after the soil had been consolidated there. In the autumn of the same year, the mould was also found inside the stone chamber. In 2005, the Committee for the Conservation of the National Treasure, the Takamatsuzuka tumulus, decided to dismantle the stone chamber in 2007 for restoration. However, the mould activity was already an urgent problem in In order to suppress the mould growth, in July cooling pipes were installed on the surface of the mound and underneath the stone chamber and the cooling process was started in September The temperature in the stone chamber was decreased to 10 degrees Celsius as had previously been decided. In October, 2006, the excavation started and the dismantling of the stone chamber commenced in April During the excavation and dismantling, the stone chamber would be uncovered and the mural paintings exposed. Therefore, it was necessary to control the temperature and humidity surrounding the stone chamber in order to prevent exfoliation of the plaster layer and biological damage. For this purpose, a thermally insulated facility was constructed around the stone chamber to control the temperature (T) and relative humidity (RH) of the air. It was important to prevent quick drying, which would increase the risk of exfoliation of the plaster layer. However, if the humidity was too high, such as 100% RH, condensation would occur, leading to the growth of mould and microbes. Thus, we decided to maintain the temperature and relative humidity inside the stone chamber at 10 degrees Celsius and 85-95% RH, respectively. In addition to the condition of the paintings, the health of the people working there needed to be considered. During excavation and dismantling, the temperature and humidity were controlled successfully, avoiding dryness and dew condensation. In the following sections, we show the full scale experiments, which were conducted in Kamo city in Kyoto prefecture in advance of the dismantling, a simulation of an air-conditioning system for the thermally insulated facility, and explain the results of the air conditioning and counter-measures required to prevent deterioration of the mural paintings on-site. The thermally insulated facility and air-conditioning system The floor plan and cross-section of the thermally insulated facility are shown in Fig.1 and 2, respectively *1. The facility has a steel frame which supports the crane used to lift the stones, and two rooms, room A and room B, as shown in Fig. 1 and 2. A piece of the stone chamber was to be picked up in room A, rotated, and then packed in a metallic frame in room B. The thermal insulation envelope consisted 1 Kyoto University, 2 National Research Institute for Cultural Properties, Tokyo, 3 Total System Laboratory 75

2 a 4QQO # Return 1-3 4QQO $ 5VQPG %JCODGT +PUWNCVGF &QQT 1-4 5JWVVGT a Fig.1 Positions compared by CFD analysis Plan of the thermally insulated facility Figure 1 Plan of the thermally insulated facility Crane 4 Positions compared by CFD analysis 1-3 Return 1-3 Stone Chamber Fig.2 Cross-section of the thermally insulated facility of a 10 cm-thick insulation board. Windows set into Figure Cross-section of illumination the thermally insulated the2wall and ceiling for were designedfacility to be double-glazed so that the insulation efficiency was enhanced*2. In order to reduce air leaks to the outside, an adjustable shutter between room A and room B and an insulated door between Room B and the outside were set up. Figure 3 shows a schematic diagram of the airfan Coil (Temperature Scrubber (Humidity Room Surrounding Front Room the Stone Room B Chamber (Room A) Fan Coil (Temperature Electric heater 䋨Temperature Control䋩 Chiller (Temperature Ground under the Stone Chamber Air flow 䋨Duct䋩 Water Flow 䋨Cooling Pipe䋩 Fig.3 Schematic diagram of air-conditioning system Figure 3 Schematic diagram of air-conditioning system conditioning system of the thermally insulated facility. The temperature and humidity in room A were controlled and the temperature in room B was controlled as a buffer zone between room A and the outside. A scrubber was used to control the humidity within a high, narrow range, and fan coil units and ground cooling pipes underneath the stone chamber were used to control the temperature. The number of supply and suction openings in room A were 4 and 3, respectively, as shown in Fig 1. Full-scale experiment in advance of the dismantling Purposes of the experiment The purpose of the experiment was to verify the performance of the air-conditioning system. To avoid condensation within the facility, we set appropriate air flow and direction, and investigated measures to counter condensation (Ogura et al. 2007). The experiment was carried out from September 26 to October 13 in 2006 in Kamo city in Kyoto prefecture. The investigated items were as follows: Verification of the performance of control of temperature and humidity The target temperature and relative humidity were 20 degrees Celsius and 85 95% RH, respectively. The on-site target temperature during the dismantling differed from it by about 10 degrees Celsius because the ground temperature was not controlled and was about 20 degrees Celsius at that period. Also, the airconditioning of room B had not yet been installed, and there was no thermally insulated door at the entrance of room B. Hence, the cooling capacity was small and both the air-tightness and the insulation performance of the facility were poor. Investigation of appropriate airflow and wind direction In order to prevent condensation and drying inside the stone chamber, the air velocity surrounding it needed to be maintained at less than 0.1 m/s. Detection of condensation droplets and measure for them We needed to prevent condensation both in the stone chamber and at other places where water might *1 Fig.1 and 2 are the plan and a cross-section of the test site, and are different from the detailed on-site plan. *2 Windows on the test site were single-glazed, though they were supposed to be double-glazed. They were, however, kept closed on site. 76

3 drop in and on the stone chamber. We checked the places where condensation might take place and investigated the measures of prevention. Ceiling Summary of experimental results Summary of experimental results are shown as follows. Temperature and humidity behavior The temperature and relative humidity surrounding the stone chamber were maintained at degrees Celsius and 95-98% RH, respectively. However, the air temperature of the upper zone of the stone chamber increased in accordance with the outside temperature because of the lack of cooling capacity of the air conditioning. Therefore, in order to sufficiently control the temperature and humidity of all the zones in the facility, we decided to add a chiller to the air conditioning system, and installed a thermally insulated outside door in room B in order to control the temperature in room B. Airflow behavior There was no direct airflow to the stone chamber. The air velocity surrounding the stone chamber was less than 0.1 m/s. The supply airflow and wind direction well suited the requirements. Condensation behavior The places where condensation most easily occurred were the skylights at night because the skylights cooled due to the decrease of the outdoor temperature. Measures to avoid this condensation included improved thermal insulation performance of the skylight, by using a double-glazed window, and covering the stone chamber with a waterproof sheet to prevent drops of condensation water from reaching it. All the windows were, however, closed to reduce the risk of condensation on site. Analysis of air flow and temperature distribution inside the experimental thermally insulated facility Fig.4 Shutter Wall Analysis object for CFD simulation Return3 Return2 Return1 Analysis method and conditions The object to be analyzed was the room surrounding the stone chamber shown in Fig.4. The model for CFD was a standard k-ε model. The general-purpose CFD program, STAR-CD made by CD-adapco Japan, was adopted for the calculation. Boundary conditions are shown in Table 1. Table 1 Boundary conditions for CFD simulation Openings Return Openings Wall Air Velocity : Uniform Distribution of Measurement Average Temperature: Measurement Average Natural discharge Air Velocity: No Slip Temperature: Measurement Convective heat transfer coefficient: Yurges s Equation. Results and discussion Comparisons between experimental and calculated results for air velocity and temperature are shown in Fig. 5 and 6, respectively. In air velocity, the calculated results agreed with experimental results Calculation Purpose In this section, air flow and temperature distribution in the experimental thermally insulated facility are recreated by computational fluid dynamics (CFD) simulation (Ogura et al. 2008). Height [cm] Experiment Air Velocity [m/s] Fig.5 Comparison between experimental and calculation results for air velocity 77

4 Fig.6 Height [cm] Calculation Experiment Temperature [] Comparison between experimental and calculation results for temperature from top to bottom, especially those surrounding the stone chamber and the height of the supply openings. The temperature distribution of the calculation agreed with that of the experiment as a whole. Thus, the CFD model was validated in the analysis of the thermal environment of the facility. Analysis of the problems expected with the climatic conditions inside the thermally insulated facility during the excavation and the dismantling period We investigated the problematic climatic conditions which were expected during the excavation and the dismantling using the model validated in the preceding section. The problematic conditions included a cold draft on the building envelope and heat generation due to operations of the excavation and the dismantlement. From the viewpoint of preventing deterioration of the mural paintings, it was considered desirable to have a uniform temperature surrounding the stone chamber since unequal temperature distribution would cause either condensation or dryness on the inside surface of the stone chamber, and to maintain a low air temperature of the air surrounding the stone chamber in order to prevent the growth of microbes. and the stone chamber were uniformly set at 10 degrees Celsius. Case 2 In the case of covering the stone chamber with an insulation sheet We investigated the effect of placing an insulation sheet above the stone chamber in order to contain the descending cold draft. The stone chamber was covered with a thermally insulated sheet 30 cm above the top of the stone. Case 3 In the case of heat generation by operations We evaluated the impact of the operators as a heat source since a heat source would affect the surrounding temperature and air velocity. A rectangular solid was set up next to the stone chamber as a heat source, and the surface was always kept at 35 degrees Celsius. Results and discussion Effect of the insulation sheet The temperature distribution in cases 1 and 2 are shown in Fig.7. In case 1, a cold draft from the skylight descended along the wall downwards, impacting on the stone chamber. In case 2, most of the cold draft is stopped by the insulation sheet. Since the overall temperature surrounding the stone chamber was about 10 degrees Celsius, the insulation sheet worked. However, if the cold draft is directed onto the sheet, there is a risk of condensation because the humidity of the air beneath the sheet is about 90% RH. We concluded that we needed to place the insulation sheet as close as possible to the stone chamber in order to avoid drops of condensation. During the excavation period, with cooperation from the excavators, the insulation sheet was placed on top of the stone chamber during night hours. Temperature[] Analysis of conditions Case 1 In the case of a descending cold draft The excavation and dismantlement was planned to begin during a cold period of the year, when a cold draft could be expected to descend on the stone chamber. The outdoor air temperature was assumed to be 0 degrees Celsius and the envelope including the insulation material was treated as a thermal resistance. The surface temperatures of the ground Fig.7 Without Insulation Sheet Insulation Sheet With Insulation Sheet Temperature distribution in the facility without and with insulation sheet Effect of heat sources A comparison of temperature distribution with and without operators as a heat source in the facility 78

5 Temperature[] Air velocity [m/s] Fig.8 Temperature Distribution Human (Heat source Air Velocity Distribution Temperature and air velocity distribution in the facility with a heat source is shown in Fig.8. The high-temperature domain above the heat source forms a kind of plume rising up to the ceiling. This impact would affect the temperature of the stones. As a result of the heat source, the temperature surrounding the stone chamber would rise. The supply of cooled air cannot reach the stone chamber directly to counter the temperature rise. Therefore, as countermeasures, we should limit the maximum temperature surrounding the stone chamber and the number of workers. At the same time, we needed to limit the use of equipment which generated heat. While work was being carried out, very careful attention was paid, with cooperation from the excavators, to comply with all these requirements. Climatic control for the actual work Schedule and climatic control during the works Before the start of excavation in October 2006, the upper ground cooling pipe system was removed. Thus, only the lower ground cooling pipe remained and the upper part of the mound was exposed directly to the outside air. As the outdoor temperature decreased and the mound layer on top of the stone Fig.10 The thermally insulated facility chamber became thinner, the ceiling area temperature inside the stone chamber declined. Since those conditions carried the risk of condensation on the ceiling, insulation boards were installed on the ground above the stone chamber to prevent the decrease of temperature (Fig.9). As a result, we were able to suppress the amount of temperature decrease inside the stone chamber. After the construction of the thermally insulated facility was completed (Fig.10), the insulation boards were removed. Air conditioning in the facility started operation from the end of January During the dismantling period, our target temperature and humidity were 10 degrees Celsius and 90% RH, until all the stones with paintings were removed. Also, the windows of the facility were closed for the prevention of condensation. From April to August, stones were removed, and the air conditioning was stopped in September Because of the increasing risk of condensation on the ground and inside the stone chamber with the increase of outside temperature, we stopped the operation of cooling pipes in June. After the removal of all the stones with mural paintings, we considered the working conditions for the operators, and the targets were adjusted. Fig.9 Insulation boards installed on the ground above the stone chamber While the protection of the mural paintings was very important, at the same time we had to ensure the safety and health of the operators. In order to prevent damage to the mural paintings, the operators wore clean clothes, clean masks, and special work shoes. At the same time, this gear protected the workers from the inhalation of mold and other contaminants inside the facility. The workers were obliged to wear 79

6 all of the gear throughout their work. We kept the relative humidity in the space at 90% using a humidifier, but we needed to prevent the contamination of water used in the humidifier from dangerous bacteria such as Legionella. So, we disinfected the humidifier using chlorine in order to keep the water clean. For monitoring and countermeasures, we carried out online remote monitoring of climatic control that included the temperature and relative humidity inside the stone chamber and the facility, and the airconditioning system. When there was a risk of condensation or drying inside the stone chamber in terms of the data observed by remote monitoring, the situation was quickly examined and adjustments were made on the spot. Reducing the number of workers inside the facility and shortening the use of heat-generating equipment were implemented as needed. As we removed each stone, naturally an opening was left in the stone chamber. In order to maintain the climatic condition inside it, we took measures to cover this opening using insulation boards. However, when the humidity inside the chamber became too high, we partially uncovered the opening. Fig.11 shows the opening covered with insulation boards after the removal of the northern wall. Insulation Boards Fig.11 Covered opening with insulation boards after removing the northern wall Results and discussion of control work Figure 12 shows temperature fluctuations of the return air, outdoor air and the target value inside the facility. The temperature of the return air is regarded as the representative indoor temperature of room A in the facility, and changes in compliance with the target value although its temperature increases during the daytime because of a rise of the outdoor temperature and the heat generated by the operators. Fig.12 Temperature variations in the facility Figure 13 shows the relative humidity fluctuations of the return air, outdoor air and the target value inside the facility. Figure For 12 Temperature humidity, the variations target was in the 90% facility and return air was maintained within 10% variation. Indoor relative humidity was maintained around the target value. Fig.13 Relative humidity variations in the facility Figure 13 Relative humidity variations in the facility Figure 14 shows the temperature and relative humidity variations inside the stone chamber. Removal of the stones with mural paintings took place between April and July. A temperature of 10 degrees Celsius was maintained until the removal work started. Subsequently there were some Fig.14 Temperature Figure 14 and Temperature humidity and variations humidity variations in the stone in the stone chamber chamber 80

7 fluctuations because the stone chamber was opened and the thermohygrometer was removed during the daytime when the operators worked in the stone chamber. As for humidity, when the first stone was removed, the humidity dropped from 98 % to 90 %, then rose again. In the latter half of April to May, the humidity reached 100 % for a while. This might have been because of moisture discharged from the stones. In order to decrease the humidity in the stone chamber, a part of the insulation covering the opening was removed so that the humidity could decrease to around the 90 % mark. From then on we were able to maintain about 90% humidity inside the chamber while all the stones were raised and taken out. As a result of our efforts to control the temperature and humidity inside the stone chamber at the time of excavation and dismantling, we were able to maintain the desired values of temperature and humidity, avoid condensation, and suppress dryness. The countermeasures mentioned above were probably the main reasons why the environment in the stone chamber could be safely maintained. Conservation 46, (in Japanese) 1-10 (2007) Ogura, D., Inuzuka, M., Hokoi, S., Ishizaki, T., Kitahara, H. and Tarama, J., Control of Temperature and Humidity Surrounding the Stone Chamber of Takamatsuzuka Tumulus during Its Dismantlement, Science for Conservation 47, (in Japanese) 1-8 (2008) Conclusions In this report, the control of temperature and humidity inside and surrounding the stone chamber during dismantlement of the Takamatsuzuka tumulus are described. First, we investigated the results of the preceding experiment and numerical analysis of the thermal environment inside the thermally insulated facility using an air-conditioning system. Next, we explained the results of these controls and on-site countermeasures. As a result of our efforts to control the temperature and humidity inside the stone chamber at the time of excavation and dismantlement, we were able to maintain the objective value of temperature and humidity, avoid condensation, and suppress dryness of the mural paintings. References Ogura, D., Ishizaki, T., Hokoi, S., Kitahara, H., Inuzuka, M., Tarama, J. and Kinoshita, M., Investigation of Air Conditioning Method for the Conservation of the Mural Painting during the Dismantlement of the Stone Chamber of Takamatsuzuka.Tumulus, Science for 81