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Building services
The InternutconalJoccrnal
of Museum Muna~encent and Curatorship
(1984), 3, 337-342
Building Services ALFRED READING
For the keepers of collections, building services are usually just nuisances that they wish they could do without. But these services are needed to maintain the environment that they demand and their collections need. Ideal services would produce perfect conditions in absolute silence, and they would take up no space, not even that bit of the basement which could store some of the reserve collection. They would be invisible so that they did not spoil the aesthetics. Unfortunately, life is not like that and this paper is about the decisions necessary to get the correct balance of effectiveness, aesthetics and cost in the provision of lighting, heating, ventilation and, if needed, air-conditioning of museum and gallery buildings. Few of the buildings with which we are concerned in the museum world were designed and built to match modern standards, with full provision for these services, and even the more modern ones were designed in an era of cheap energy. Many are old, some are adapted from other uses, some are listed buildings and are exhibits in their own right. There is therefore no one balance that can be struck between conflicting needs in our approach to the ideal environment. Indeed, there is quite a range of opinions as to what constitutes that ideal, though stability of conditions is a universal demand. Conservators of the metalwork artefacts want low humidity, but higher values of 50 per cent are general for many items; art galleries prefer 55 per cent, and paper may well need it a little higher still. Care must be taken in these higher ranges that local variations do not aid mould growth. Some conservators have the opinion that a modest fluctuation of temperature is a good thing, but most want as much stability as possible. Thus it is appropriate to deal with the building as a protective envelope which separates the collection from outside conditions. Consider the dangers. Rain brings high humidities, the sun brings temperature rises, bright light and especially ultra-violet. Since our principal concern in these papers is with the United Kingdom, wind usually means colder weather, but the greatest danger is probably people themselves. Their activities, especially traffic, produce vibration, dust and pollution. Then, of course, they visit the museums and galleries, and bring in with them dust, heat and humidity. Finally, the lighting services themselves add yet more heat. Though opinions vary on the ideal environment I take 2OC, 50 per cent RH and lighting levels of 200 lux as a compromise basis for discussion. Additionally, the oxides of sulphur and nitrogen, together with dust and other pollutants, must be removed from the air which is supplied. Of course, the visitors want to breathe, so we need fresh-air supplies. All this implies full air-conditioning and a building construction which is suitable for its implementation. Air-conditioning is very expensive to install and to operate, so many have to be content with simpler equipment. How to choose the right level of installation for your circumstances is discussed later in this paper, but first let us see what air-conditioning involves. For those who have never been in an air-conditioning plantroom a brief review follows of a typical system and the effects the designer has to consider. #CCrown Copyright 1984. Reproducedwith permission of the Controller of HMSO.
338
Building Services
The air-handling plant at the centre of a system treats the air, some of it fresh but the majority recirculated from the conditioned space. In the best systems fresh air is prefiltered to remove the worst of the dust and polluting gases, with a pre-heater in the intake to prevent freezing in winter. This fresh air is mixed with the recirculated air and enters the main plant. Here a filter removes dust from the air stream, then a further filter of activated carbon removes gaseous pollution like oxides of sulphur and nitrogen. In many plants a spray chamber adds moisture to the air before the chiller. This, in addition to its washing effect, means that there is always an excess of moisture in the air and the extra moisture is removed by passing the air through a chiller coil whose temperature is controlled to reduce the air to the correct dew point. This is by far the most common way to regulate relative humidity. The air is then reheated to the required supply temperature. A further filter at this point removes the finer particles which passed through the earlier filters or which have been generated in the plant. Finally, there are two fans to move the air-one for the supply and the other for the return air. Chilled water comes from a refrigeration plant with either a cooling tower or an air blast condenser to get rid of the heat. In small plants these are often combined as one unit. A boiler is required to produce hot water to reheat the air to the supply temperature even in summer. This is because the dew point to which the air is cooled to remove the moisture is below an acceptable temperature for air blown into the conditioned space. With so much equipment involved in an air-conditioning system there will be times when some has to be taken out of service for maintenance. To maintain the conditions continuously, therefore, standby plant becomes necessary with further cost and space implications. However, returning to the air-handling plant, we can consider various alternatives and compromises. Firstly, by changing to steam injection to adjust the moisture content when it is too low the spray chamber is not needed and for some of the time the chiller coil and the reheat coil are used thus using less energy and reducing costs. only to adjust the supply temperature, Unfortunately, there are always periods when the mixed air is too moist and the chiller must bring its temperature down to dew point, then reheat is needed to bring it up to a suitable supply temperature. Other savings can be made by reducing filtration standards, eliminating the fresh-air filtration and depending on the main plant filters to provide the requirements. Energy can be saved by having a lower mean temperature in the conditioned space in the winter, say 19 “C, and increasing it through spring to a value of 22 ‘C in summer. This has the effect of reducing the peak loads to be handled and also the size and cost of the plant. Higher mean relative humidities of, say, 5455 per cent reduce the amount of cooling required to reach dew point in summer and may help to reduce plant size. Air-conditioning, however desirable, is not practicable if the building is unsuitable so let me say what makes a building suitable. First, it must have minimal leakage. By that I do not mean rain through the roof, though unfortunately that problem is always with us. I mean air leakage into the room through the building structure, through doorways, the gaps around windows, through the eaves in some buildings, or the rooflights and even the pores of the walls themselves. In even a good building this infiltration can amount to a quarter of the air volume each and every hour. In many buildings it is much worse. Unconditioned air entering the conditioned space makes control difficult and increases costs. We need to seal all possible gaps and entry points for unconditioned air. At least two sets of doors between the entrance and the conditioned space are essential to maintain control. The next consideration is solar gain. The sun heats up the walls and they act as radiators putting heat into the rooms in summer when we need it least. Skylights and windows let it in, directly adding to the problem. Heavy-weight walls, with no windows or skylights and no doors, would be best, but people have to get in and many prefer daylight for viewing exhibits. Thus, we must
ALFRED READING
339
compromise and let a limited amount in, but preferably keep most of the heat out with external louvres. All glazing should be treated to minimize ultra-violet radiation. Engineers always prefer fluorescent lighting because it is economical in power consumption and produces less heat, while exhibition designers prefer tungsten spot lamps for modelling and sparkle. The penalty in heat gain from tungsten lighting is severe and SO restraint in display lighting not only reduces your lighting bills but also the capital and running costs of air-conditioning. No matter how good the building may be regarding insulation, solar gain and suchlike, it cannot be air-conditioned without ducts to take the air to and from the rooms. This is not a simple exercise since low velocity ducts to achieve low noise levels are essentially large and awkward to install. Hiding them for aesthetic reasons adds to the difficulties. Safety is also important since the ducts will go through fire walls and one needs to install fire dampers in the ductwork. Where these ducts go through walls has to be considered carefully as the structure may be seriously weakened if a bad decision is made. Structural considerations also apply to the choice of plant location. Chiller plant in particular is heavy and creates vibration, so it must be mounted on a strong structure with isolation for noise and vibration. Maintenance must also be considered at the earliest design stage of any installation, whether of air-conditioning or simpler equipment. Plant requires regular attention to keep it in good operating condition and this means that thought must be given to access. I find that architects seem to believe that engineers are slim 1 metre high contortionists with extendable arms and legs. If you make equipment inaccessible you can guarantee that it will not be maintained as it should. Building services do not last the life of the building and they must be replaced. Minor repairs and adjustments are a continuing need but it must be remembered that the working life of major items of plant is usually 20 to 25 years so they will be taken out and replaced four or five times in a century. This means there must be room to take them apart and to put in replacements. Often forgotten is the fact that plant does not magically appear in the plantroom. A route must be available to get it there. It must be large enough and strong enough to take the biggest element of plant throughout its length. I have deliberately chosen to dwell on air-conditioning because all I have said about the building in relation to air-conditioning is equally applicable to places where air-conditioning is impracticable. In other words the first step in designing any services for a building is to minimize the need for them. This must be done with thought and care or the wrong answer may result; for instance, where a building is well sealed to keep out the external hazards but only radiators or convection heaters are being installed, the building will get stuffy, windows will be opened, and all the benefits of good sealing lost. If fresh air is introduced through a fan and filter then most of the benefits are retained. An improvement on this is to have a heat exchanger so that exhaust air can temper the condition of the fresh air to eliminate cold draughts in winter. So by steps one can approach the ideal until a limit is reached which may be imposed by the building structure, aesthetic considerations, or in many cases finance. If you can add summer cooling the temperature range can be reduced. Steam injection in a ducted air system, or humidifiers in the rooms, can help control humidity in winter. But these improvements must be introduced with skill. Too much humidification in the depths of winter will lead to condensation problems, wood rot and other disasters. Too much summer cooling on humid days and you could have more of the same as well as cold draughts. The degree to which any of these improvements can be implemented depends primarily on the amount of protection the building structure gives the exhibition space. For example, a well-sealed building, double-glazed with a good air lock between the display areas and the outside world, will allow one to maintain 50 per cent relative humidity under virtually all circumstances, If there is
340
Building Srrvices
only single-glazing or other weaknesses in insulation then 45 per cent RH is the maximum safe value for much of the winter, and it would need to be reduced to 40 per cent RH in severe weather. Lowering the temperature in winter aids the maintenance of better RH values, but warders will complain with justification if the standards of the Shops, Offices and Railway Premises Act are not met. I mentioned lighting earlier as a source of heat which puts up the electricity bill directly, and also as an additional air-conditioning load. For the museum and gallery exhibits it also has equally direct effects. Firstly, it can induce chemical change and fading in delicate materials, especially if ultra-violet is present. But what is not so well known or acknowledged is the thermal stress which can be induced by the radiant heat from tungsten lamps. A surface temperature rise of several degrees is not unusual and no amount of air-conditioning will prevent that. It may in fact make it worse by maintaining a constant environmental temperature so that the front of the object is trying to expand while the back is held constant. It must also put a physical strain on the material as lamps are turned on and off. An extreme case of local heating was found at the British Museum where during alterations a scaffold board was near a 150 W spotlight for a few hours and had a hole burned in it. The temperature rise imposed on the museum artefacts themselves is not as great but it is happening every day for years. The keeper of a collection must give careful consideration to what he or she must have in the way of environmental conditions. It is easy to read Garry Thomson’s Museum Environment and quote the best conditions that are advocated, but only full air-conditioning for 24 hours a day, seven days a week, will maintain them. Most cannot afford that and many buildings cannot be adapted for it. The realist must then take a clear look at the building and consider which improvements are practicable and economically viable. Firstly, what can be done to keep more of the outside out. The simple things that any householder should do such as draught-proofing the doors and windows, putting down mats to collect some of the dirt, and keeping valuable and delicate objects further from the front door are all helpful. If there are two good doors between the outside and the collection, so much the better. Where solar gain is a problem consider louvres-external if at all possible-to keep excess heat and light outside the building, and the next step is to put good control on the heating, with thermostatic controls in each room. Too many systems have an outside temperature sensor which sets the boiler temperature, with the usually forlorn hope that the flows through the radiators have been so balanced that the room temperatures will be right. If a system is in perfect balance in cloudy weather, with the wind from the south-west, the balance will be upset by winter sunshine or north-easterly winds. It costs a little more, but always insist that sensors are provided to measure what you want controlled and to control it. Do not accept indirect control based on assumptions of system performance which may not be accurate even initially, and certainly will not stay accurate for the life of the plant. This applies to all the systems I have mentioned earlier. No system, however simple or complex, can operate successfully without controls of suitable design and good quality. A simple heating system with good quality controls will maintain a better environment than any more elaborate system whose controls are inadequate. Never try to save money on controls. They are so important that every pressure should be put on designers to ensure the best possible control system is installed and, equally importantly, that those who will maintain the system are made fully aware of how it operates. The most difficult control is for relative humidity. Most standard air-conditioning systems do not measure humidity at all. What happens is that assumptions are made about the amount of humidity which will be contributed by the occupants, air infiltration and other sources of moisture, and a dew-point temperature for the supply air is chosen which, if these
ALFREDREADING
341
assumptions are correct and if the controls are accurate, will give the correct relative humidity if the room temperature is as predicted. That is an awful lot of ‘ifs’, and though it is good enough in most offices with a wide range of acceptable relative humidity, it is not adequate for museums. The better systems have humidistats in the return air duct to correct the dew-point setting if the humidity rises above a preset limit. All systems relying on dew-point control are extravagant in energy as the air always has to be cooled to dew point and reheated to a suitable supply temperature. In the main, temperature sensors are accurate and consistent, but with few exceptions humidity sensors suffer from a variety of ills. Some have long-term drift due to changing Some are temperature sensitive. Many have characteristics of the sensing medium. hysteresis, that is they give a different reading at a given humidity depending on whether the humidity is rising or falling. I know of only one type of humidity sensor which is reliable in the long term. This is based on a special electrolytic resistor and is expensive. There are other sensors based on thin film capacitors which are very good and somewhat cheaper, which we are installing on a major project. I expect that they will give comparable performance though they have not yet been tested for the length of time needed to give absolute certainty. These two types are certainly better than most of the others which depend on hair, hygroscopic plastic film, cotton strip and the like as their measuring element. Control theory is a complex subject not amenable to discussion here, but I will say that modern electronic control systems are now reliable. Two elements are essential to success in control. First, the designer must consider all the parameters, and ensure that the sensors are placed correctly to measure the parameters to be controlled. Too often what is measured is not what is to be controlled but another parameter related to it which is more convenient. An assumed relationship is not guaranteed under all circumstances. For instance, it is common practice to monitor temperature and humidity in the return air duct as representative of the room. If there is return at high level, there can be significant gains or losses as the air rises, so the conditions at picture level in a gallery could be different from the return air conditions. Secondly, enough time and effort must be put into a properly thought-out commissioning of the system to ensure that all elements of the plant and its controls are operating to specification. In air-conditioning systems this is always a difficult process which cannot be hurried, so enough time must be allowed in the programme and sufficient money in the tender. In simpler heating and ventilating systems the problems may be less but the importance of commissioning is just as great.
Other
Services
Other building services and their application to museums deserve consideration here. These are security, fire and small power. Security is a specialized subject and we are fortunate in the PSA to have experts to advise us. No security system will stop thieves, just as no fire alarm will stop a fire. Before considering systems, the building’s physical security and fire resistance should be brought to as high a state as possible. It is then that the best decisions can be made on the remaining risks. The essential property of fire and security systems is that they buy time. In fire systems it is early detection which leads to prompt response, and this is true of security systems where, in addition, there is also a need to evaluate the type of response in these days of armed robbery where hostages might be taken. No response time will be quick enough if the building has no resistance, so I make no apology for repeating that the building must be considered first, not the system. Since systems must be tailored to suit the building there is little point in expounding details here. Small power, by which I mean mainly the 13 amp sockets of the kind you need all round
342
Building Seroices
the place for cleaners, staff kettles and sometimes working exhibits, is normally a simple reliable service which if properly installed gives no trouble for many years. It therefore tends to be neglected. The life of vulcanized india rubber insulation, called VIR, in a warm dry atmosphere is about 25 years. It is probably 30 years since the last of it was installed, yet there are many buildings still mainly wired in VIR. Plans should be made immediately for its progressive replacement before it becomes a fire hazard. Leave it and it becomes an expensive project which must be done all at once whether convenient or not. Every installation should be checked every five years, more frequently if they are VIR. Conclusion
To sum up, the building services are a necessary part of maintaining any collection of art treasures. The quality of these services may be limited by the quality of the building envelope, the aesthetic considerations of the installation, or just plain lack of funds, so a balancing act is essential. The first and most important step is to make the building as good a protective envelope as possible and thus limit the load on the services. Next, assess the quality of environment which is attainable within that shell: it may be plain heating and lighting; perhaps mechanical ventilation can be added with filtration; or better still, the controlled air can be heated and cooled with partial control of humidity. Best of-all is a full air-conditioning system to maintain stable conditions at all times. Which of these is the right answer in any individual case will require a careful assessment of all the parameters. I must end on a warning note. Do not try to get better conditions than can be afforded or than the building will allow, and remember conditions can be maintained only by well-engineered plant working within its capacity. Plant working to its utmost limit, or bought to meet a cost limit rather than a performance standard, will be trouble for all its working life, which is liable to be short. Equally, plant that is difficult to get at and maintain will give poor results and fail soon.
of Museum Muna~encent and Curatorship
(1984), 3, 337-342
Building Services ALFRED READING
For the keepers of collections, building services are usually just nuisances that they wish they could do without. But these services are needed to maintain the environment that they demand and their collections need. Ideal services would produce perfect conditions in absolute silence, and they would take up no space, not even that bit of the basement which could store some of the reserve collection. They would be invisible so that they did not spoil the aesthetics. Unfortunately, life is not like that and this paper is about the decisions necessary to get the correct balance of effectiveness, aesthetics and cost in the provision of lighting, heating, ventilation and, if needed, air-conditioning of museum and gallery buildings. Few of the buildings with which we are concerned in the museum world were designed and built to match modern standards, with full provision for these services, and even the more modern ones were designed in an era of cheap energy. Many are old, some are adapted from other uses, some are listed buildings and are exhibits in their own right. There is therefore no one balance that can be struck between conflicting needs in our approach to the ideal environment. Indeed, there is quite a range of opinions as to what constitutes that ideal, though stability of conditions is a universal demand. Conservators of the metalwork artefacts want low humidity, but higher values of 50 per cent are general for many items; art galleries prefer 55 per cent, and paper may well need it a little higher still. Care must be taken in these higher ranges that local variations do not aid mould growth. Some conservators have the opinion that a modest fluctuation of temperature is a good thing, but most want as much stability as possible. Thus it is appropriate to deal with the building as a protective envelope which separates the collection from outside conditions. Consider the dangers. Rain brings high humidities, the sun brings temperature rises, bright light and especially ultra-violet. Since our principal concern in these papers is with the United Kingdom, wind usually means colder weather, but the greatest danger is probably people themselves. Their activities, especially traffic, produce vibration, dust and pollution. Then, of course, they visit the museums and galleries, and bring in with them dust, heat and humidity. Finally, the lighting services themselves add yet more heat. Though opinions vary on the ideal environment I take 2OC, 50 per cent RH and lighting levels of 200 lux as a compromise basis for discussion. Additionally, the oxides of sulphur and nitrogen, together with dust and other pollutants, must be removed from the air which is supplied. Of course, the visitors want to breathe, so we need fresh-air supplies. All this implies full air-conditioning and a building construction which is suitable for its implementation. Air-conditioning is very expensive to install and to operate, so many have to be content with simpler equipment. How to choose the right level of installation for your circumstances is discussed later in this paper, but first let us see what air-conditioning involves. For those who have never been in an air-conditioning plantroom a brief review follows of a typical system and the effects the designer has to consider. #CCrown Copyright 1984. Reproducedwith permission of the Controller of HMSO.
338
Building Services
The air-handling plant at the centre of a system treats the air, some of it fresh but the majority recirculated from the conditioned space. In the best systems fresh air is prefiltered to remove the worst of the dust and polluting gases, with a pre-heater in the intake to prevent freezing in winter. This fresh air is mixed with the recirculated air and enters the main plant. Here a filter removes dust from the air stream, then a further filter of activated carbon removes gaseous pollution like oxides of sulphur and nitrogen. In many plants a spray chamber adds moisture to the air before the chiller. This, in addition to its washing effect, means that there is always an excess of moisture in the air and the extra moisture is removed by passing the air through a chiller coil whose temperature is controlled to reduce the air to the correct dew point. This is by far the most common way to regulate relative humidity. The air is then reheated to the required supply temperature. A further filter at this point removes the finer particles which passed through the earlier filters or which have been generated in the plant. Finally, there are two fans to move the air-one for the supply and the other for the return air. Chilled water comes from a refrigeration plant with either a cooling tower or an air blast condenser to get rid of the heat. In small plants these are often combined as one unit. A boiler is required to produce hot water to reheat the air to the supply temperature even in summer. This is because the dew point to which the air is cooled to remove the moisture is below an acceptable temperature for air blown into the conditioned space. With so much equipment involved in an air-conditioning system there will be times when some has to be taken out of service for maintenance. To maintain the conditions continuously, therefore, standby plant becomes necessary with further cost and space implications. However, returning to the air-handling plant, we can consider various alternatives and compromises. Firstly, by changing to steam injection to adjust the moisture content when it is too low the spray chamber is not needed and for some of the time the chiller coil and the reheat coil are used thus using less energy and reducing costs. only to adjust the supply temperature, Unfortunately, there are always periods when the mixed air is too moist and the chiller must bring its temperature down to dew point, then reheat is needed to bring it up to a suitable supply temperature. Other savings can be made by reducing filtration standards, eliminating the fresh-air filtration and depending on the main plant filters to provide the requirements. Energy can be saved by having a lower mean temperature in the conditioned space in the winter, say 19 “C, and increasing it through spring to a value of 22 ‘C in summer. This has the effect of reducing the peak loads to be handled and also the size and cost of the plant. Higher mean relative humidities of, say, 5455 per cent reduce the amount of cooling required to reach dew point in summer and may help to reduce plant size. Air-conditioning, however desirable, is not practicable if the building is unsuitable so let me say what makes a building suitable. First, it must have minimal leakage. By that I do not mean rain through the roof, though unfortunately that problem is always with us. I mean air leakage into the room through the building structure, through doorways, the gaps around windows, through the eaves in some buildings, or the rooflights and even the pores of the walls themselves. In even a good building this infiltration can amount to a quarter of the air volume each and every hour. In many buildings it is much worse. Unconditioned air entering the conditioned space makes control difficult and increases costs. We need to seal all possible gaps and entry points for unconditioned air. At least two sets of doors between the entrance and the conditioned space are essential to maintain control. The next consideration is solar gain. The sun heats up the walls and they act as radiators putting heat into the rooms in summer when we need it least. Skylights and windows let it in, directly adding to the problem. Heavy-weight walls, with no windows or skylights and no doors, would be best, but people have to get in and many prefer daylight for viewing exhibits. Thus, we must
ALFRED READING
339
compromise and let a limited amount in, but preferably keep most of the heat out with external louvres. All glazing should be treated to minimize ultra-violet radiation. Engineers always prefer fluorescent lighting because it is economical in power consumption and produces less heat, while exhibition designers prefer tungsten spot lamps for modelling and sparkle. The penalty in heat gain from tungsten lighting is severe and SO restraint in display lighting not only reduces your lighting bills but also the capital and running costs of air-conditioning. No matter how good the building may be regarding insulation, solar gain and suchlike, it cannot be air-conditioned without ducts to take the air to and from the rooms. This is not a simple exercise since low velocity ducts to achieve low noise levels are essentially large and awkward to install. Hiding them for aesthetic reasons adds to the difficulties. Safety is also important since the ducts will go through fire walls and one needs to install fire dampers in the ductwork. Where these ducts go through walls has to be considered carefully as the structure may be seriously weakened if a bad decision is made. Structural considerations also apply to the choice of plant location. Chiller plant in particular is heavy and creates vibration, so it must be mounted on a strong structure with isolation for noise and vibration. Maintenance must also be considered at the earliest design stage of any installation, whether of air-conditioning or simpler equipment. Plant requires regular attention to keep it in good operating condition and this means that thought must be given to access. I find that architects seem to believe that engineers are slim 1 metre high contortionists with extendable arms and legs. If you make equipment inaccessible you can guarantee that it will not be maintained as it should. Building services do not last the life of the building and they must be replaced. Minor repairs and adjustments are a continuing need but it must be remembered that the working life of major items of plant is usually 20 to 25 years so they will be taken out and replaced four or five times in a century. This means there must be room to take them apart and to put in replacements. Often forgotten is the fact that plant does not magically appear in the plantroom. A route must be available to get it there. It must be large enough and strong enough to take the biggest element of plant throughout its length. I have deliberately chosen to dwell on air-conditioning because all I have said about the building in relation to air-conditioning is equally applicable to places where air-conditioning is impracticable. In other words the first step in designing any services for a building is to minimize the need for them. This must be done with thought and care or the wrong answer may result; for instance, where a building is well sealed to keep out the external hazards but only radiators or convection heaters are being installed, the building will get stuffy, windows will be opened, and all the benefits of good sealing lost. If fresh air is introduced through a fan and filter then most of the benefits are retained. An improvement on this is to have a heat exchanger so that exhaust air can temper the condition of the fresh air to eliminate cold draughts in winter. So by steps one can approach the ideal until a limit is reached which may be imposed by the building structure, aesthetic considerations, or in many cases finance. If you can add summer cooling the temperature range can be reduced. Steam injection in a ducted air system, or humidifiers in the rooms, can help control humidity in winter. But these improvements must be introduced with skill. Too much humidification in the depths of winter will lead to condensation problems, wood rot and other disasters. Too much summer cooling on humid days and you could have more of the same as well as cold draughts. The degree to which any of these improvements can be implemented depends primarily on the amount of protection the building structure gives the exhibition space. For example, a well-sealed building, double-glazed with a good air lock between the display areas and the outside world, will allow one to maintain 50 per cent relative humidity under virtually all circumstances, If there is
340
Building Srrvices
only single-glazing or other weaknesses in insulation then 45 per cent RH is the maximum safe value for much of the winter, and it would need to be reduced to 40 per cent RH in severe weather. Lowering the temperature in winter aids the maintenance of better RH values, but warders will complain with justification if the standards of the Shops, Offices and Railway Premises Act are not met. I mentioned lighting earlier as a source of heat which puts up the electricity bill directly, and also as an additional air-conditioning load. For the museum and gallery exhibits it also has equally direct effects. Firstly, it can induce chemical change and fading in delicate materials, especially if ultra-violet is present. But what is not so well known or acknowledged is the thermal stress which can be induced by the radiant heat from tungsten lamps. A surface temperature rise of several degrees is not unusual and no amount of air-conditioning will prevent that. It may in fact make it worse by maintaining a constant environmental temperature so that the front of the object is trying to expand while the back is held constant. It must also put a physical strain on the material as lamps are turned on and off. An extreme case of local heating was found at the British Museum where during alterations a scaffold board was near a 150 W spotlight for a few hours and had a hole burned in it. The temperature rise imposed on the museum artefacts themselves is not as great but it is happening every day for years. The keeper of a collection must give careful consideration to what he or she must have in the way of environmental conditions. It is easy to read Garry Thomson’s Museum Environment and quote the best conditions that are advocated, but only full air-conditioning for 24 hours a day, seven days a week, will maintain them. Most cannot afford that and many buildings cannot be adapted for it. The realist must then take a clear look at the building and consider which improvements are practicable and economically viable. Firstly, what can be done to keep more of the outside out. The simple things that any householder should do such as draught-proofing the doors and windows, putting down mats to collect some of the dirt, and keeping valuable and delicate objects further from the front door are all helpful. If there are two good doors between the outside and the collection, so much the better. Where solar gain is a problem consider louvres-external if at all possible-to keep excess heat and light outside the building, and the next step is to put good control on the heating, with thermostatic controls in each room. Too many systems have an outside temperature sensor which sets the boiler temperature, with the usually forlorn hope that the flows through the radiators have been so balanced that the room temperatures will be right. If a system is in perfect balance in cloudy weather, with the wind from the south-west, the balance will be upset by winter sunshine or north-easterly winds. It costs a little more, but always insist that sensors are provided to measure what you want controlled and to control it. Do not accept indirect control based on assumptions of system performance which may not be accurate even initially, and certainly will not stay accurate for the life of the plant. This applies to all the systems I have mentioned earlier. No system, however simple or complex, can operate successfully without controls of suitable design and good quality. A simple heating system with good quality controls will maintain a better environment than any more elaborate system whose controls are inadequate. Never try to save money on controls. They are so important that every pressure should be put on designers to ensure the best possible control system is installed and, equally importantly, that those who will maintain the system are made fully aware of how it operates. The most difficult control is for relative humidity. Most standard air-conditioning systems do not measure humidity at all. What happens is that assumptions are made about the amount of humidity which will be contributed by the occupants, air infiltration and other sources of moisture, and a dew-point temperature for the supply air is chosen which, if these
ALFREDREADING
341
assumptions are correct and if the controls are accurate, will give the correct relative humidity if the room temperature is as predicted. That is an awful lot of ‘ifs’, and though it is good enough in most offices with a wide range of acceptable relative humidity, it is not adequate for museums. The better systems have humidistats in the return air duct to correct the dew-point setting if the humidity rises above a preset limit. All systems relying on dew-point control are extravagant in energy as the air always has to be cooled to dew point and reheated to a suitable supply temperature. In the main, temperature sensors are accurate and consistent, but with few exceptions humidity sensors suffer from a variety of ills. Some have long-term drift due to changing Some are temperature sensitive. Many have characteristics of the sensing medium. hysteresis, that is they give a different reading at a given humidity depending on whether the humidity is rising or falling. I know of only one type of humidity sensor which is reliable in the long term. This is based on a special electrolytic resistor and is expensive. There are other sensors based on thin film capacitors which are very good and somewhat cheaper, which we are installing on a major project. I expect that they will give comparable performance though they have not yet been tested for the length of time needed to give absolute certainty. These two types are certainly better than most of the others which depend on hair, hygroscopic plastic film, cotton strip and the like as their measuring element. Control theory is a complex subject not amenable to discussion here, but I will say that modern electronic control systems are now reliable. Two elements are essential to success in control. First, the designer must consider all the parameters, and ensure that the sensors are placed correctly to measure the parameters to be controlled. Too often what is measured is not what is to be controlled but another parameter related to it which is more convenient. An assumed relationship is not guaranteed under all circumstances. For instance, it is common practice to monitor temperature and humidity in the return air duct as representative of the room. If there is return at high level, there can be significant gains or losses as the air rises, so the conditions at picture level in a gallery could be different from the return air conditions. Secondly, enough time and effort must be put into a properly thought-out commissioning of the system to ensure that all elements of the plant and its controls are operating to specification. In air-conditioning systems this is always a difficult process which cannot be hurried, so enough time must be allowed in the programme and sufficient money in the tender. In simpler heating and ventilating systems the problems may be less but the importance of commissioning is just as great.
Other
Services
Other building services and their application to museums deserve consideration here. These are security, fire and small power. Security is a specialized subject and we are fortunate in the PSA to have experts to advise us. No security system will stop thieves, just as no fire alarm will stop a fire. Before considering systems, the building’s physical security and fire resistance should be brought to as high a state as possible. It is then that the best decisions can be made on the remaining risks. The essential property of fire and security systems is that they buy time. In fire systems it is early detection which leads to prompt response, and this is true of security systems where, in addition, there is also a need to evaluate the type of response in these days of armed robbery where hostages might be taken. No response time will be quick enough if the building has no resistance, so I make no apology for repeating that the building must be considered first, not the system. Since systems must be tailored to suit the building there is little point in expounding details here. Small power, by which I mean mainly the 13 amp sockets of the kind you need all round
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Building Seroices
the place for cleaners, staff kettles and sometimes working exhibits, is normally a simple reliable service which if properly installed gives no trouble for many years. It therefore tends to be neglected. The life of vulcanized india rubber insulation, called VIR, in a warm dry atmosphere is about 25 years. It is probably 30 years since the last of it was installed, yet there are many buildings still mainly wired in VIR. Plans should be made immediately for its progressive replacement before it becomes a fire hazard. Leave it and it becomes an expensive project which must be done all at once whether convenient or not. Every installation should be checked every five years, more frequently if they are VIR. Conclusion
To sum up, the building services are a necessary part of maintaining any collection of art treasures. The quality of these services may be limited by the quality of the building envelope, the aesthetic considerations of the installation, or just plain lack of funds, so a balancing act is essential. The first and most important step is to make the building as good a protective envelope as possible and thus limit the load on the services. Next, assess the quality of environment which is attainable within that shell: it may be plain heating and lighting; perhaps mechanical ventilation can be added with filtration; or better still, the controlled air can be heated and cooled with partial control of humidity. Best of-all is a full air-conditioning system to maintain stable conditions at all times. Which of these is the right answer in any individual case will require a careful assessment of all the parameters. I must end on a warning note. Do not try to get better conditions than can be afforded or than the building will allow, and remember conditions can be maintained only by well-engineered plant working within its capacity. Plant working to its utmost limit, or bought to meet a cost limit rather than a performance standard, will be trouble for all its working life, which is liable to be short. Equally, plant that is difficult to get at and maintain will give poor results and fail soon.