When the outside air temperature is at or below that required off the cooling coil, the refrigeration load is zero and the refrigeration plant can be shut down.
This technique also saves boiler energy since the cooling coil is then preheating the air. The arrange- ment and control of dry fan coil systems are described in Figures 1. After sunset there is generally a marked drop in outside air temperature giving scope for precooling the building struc- ture and contents by operating the fans at low speed with the dampers on full fresh air.
This operation requires monitoring controls which permit the fans to operate only when: a The building is likely to require cooling the following day. Assume that the mechanical cooling plant requires an elec- trical power input of 1 W to provide 2 W of cooling when operating during the occupied period 1.
Electrical power to operate cooling plant to remove 1. It is likely that outside air temperature will fall at a higher rate than the internal MRT of the building so that economic operation of the extract fan will be sustained through the night until the temperature rise dropped below 1.
The actual setting of the controls for the night cooling regime should be based on this measured per- formance of the building and system. Any system of heat pumping requires electrical energy to operate the refrigeration compressor; e.
However, this load is often dealt with independently by point-of-use electric or gas water heaters and not from the main space heating boiler plant. This occurs only during the period of plant operation. In some applications this control is omitted and the pump switched off above limiting outside air temperatures. The boiler power should be reduced by the heat transferred through the run-around system at design conditions. This gives a small reduction in the required mechanical refrigeration capacity.
Various types of extended surface are used, some of which are hygroscopic and transfer moisture as well as heat from the exhaust air to the intake air. Heat transfer capacity is controlled by varying the speed of rotation. Even so, the extended surface requires periodic cleaning Figure 1. Plate heat exchangers are limited in the sizes available. This is also inherent in boiler design and maintenance. They can be minimized by selecting fewer, larger units as these present less surface area for the loss of heat than do a greater number of small boilers.
Better insulation also reduces this loss. As a boiler cools down its temperature and its heat loss reduce. These standing losses may be minimized by the measures described for reducing boiler casing convec- tion losses and also by the boiler burner design.
Reducing the water and thermal capacity of the boiler heat exchanger also reduces the standing loss that occurs at each shutdown. The increased temperature difference between combustion gas and water enables more heat to be extracted from the combustion gas, particularly at the back end of the boiler where the gas is at a lower temperature.
Condensing boilers are the ultimate example of exploiting this fact and their application is described in Chapter 2. In practice, boilers operate for most of the time in a partly fouled condition at part load and often intermittently. This can only be fully exploited by using microprocessor- controlled electronic expansion valves.
These give a marked increase in COP under most operating con- ditions. Chillers only operate at full load for very short periods of the year. The part load COP is therefore more important. This is achieved by increasing the water capacity of the system.
In the case of chiller with positive displacement compressors, the more steps of control the better; this is achieved by multiple compressors or sometimes by incremental speed changes. The choice of air supply temperature needs careful consideration as it affects the system energy con- sumption in many ways, e. This effect can be offset by improved insulation, but needs evaluating.
Remember that while the mains loss only reduces slightly with increasing ambient temperature, the mean annual rate of heat delivered is probably less than half of the design maximum.
For example, a system with a design mains loss of 20 per cent of the design heat load may have a seasonal mains loss of about 50 per cent of the heat delivered. Mains losses can be reduced by: i Designing water distribution systems with as high a water temperature differential as is economic, having regard to a practical selection of equipment, particu- larly control and regulating valves.
Chilled water temperatures are usually determined by cooling coil selection to meet psychrometric requirements and are dealt with in Chapter 8. The incidence of excessive air leakage causing problems on recently erected buildings is quite high, in some cases affecting occupant comfort, and resulting in a failure of the heating system to provide design internal temperatures under windy condi- tions.
Assume the following: Floor to ceiling height is 2. Depth from outside wall is 6 m. A window 1 m wide and 1. External wall 1 m wide and 1. The steady-state fabric heat loss would be Wall: 1. The main factors appear to be air temperature, mean radiant temperature i. Relative humidity is a less important factor and many occupants are unable to detect quite large variations in humidity.
The range of adjustment of a ther- mostat by an occupant should, however, be restricted to avoid overheating or overcooling the structure beyond the possibility of quick recovery. Noise is an important factor the mechanics of which are dealt with in Chapter 3. It is important to remember that it is possible to have rooms too quiet for comfort so that the occasional intermittent noises that occur in most buildings become disturbing and speech privacy is not obtained.
It is usual to rely on the air conditioning or ventilation system to provide this level of background noise. Alternatively, the noise may be elec- tronically generated and transmitted over the PA system. Other factors affecting comfort are colour and lighting levels which are outside the scope of this book. A useful measure of the adequacy of fresh air introduction in an occupied building is the measure of CO2 concentration, which generally should not exceed about 0.
How Fresh air introduced naturally through cracks around win- dows and doors used to be entirely adequate for heated-only buildings of traditional construction and proportions.
However, present construction methods often use relatively impervious external cladding panels with sealed windows. Amount The minimum amount of fresh air required is about 8 to 12 l s 1 per person or 1. Generally, intakes at low level, near busy roads should be avoided and intakes at the top of buildings are preferred, but care is necessary to mini- mize the risk of picking up contaminants and odours from other roof level exhausts, possibly from adjacent buildings.
The use of any form of cowl to prevent the ingress of rainwater must be avoided and rainwater should be drained at the base of the discharge duct. However, more thought needs to be given to the other ther- mal characteristics of buildings such as the mass, the time constant and the admittance factors of the internal surfaces, as these can have a considerable affect on energy consump- tion, summertime temperatures, the need for mechanical cooling and external design criteria.
For example, buildings with a high mass and consequently a high thermal capacity on the room side of the insulation will change temperature at a slow rate for a given energy gain or loss. A similarly sized building with light-weight construction on the room side of the insulation will be subject to much larger temperature changes under the same conditions.
Such a building will, for the same reason, recover temperature at a higher rate when preheating and, because of its lower heat loss during the off period, less heat energy is required when subject to intermittent heating. For continuously heated buildings the time constant makes little difference to the heat energy required see Figure 1. Figure 1. The cooling capacity or cooling energy stored will reduce the mechanical cooling energy required the following day, or alternatively reduce the room temperature if not mechani- cally cooled.
This method of exploiting the thermal storage of buildings has not been fully appreciated until recently. Under some circumstances it may increase them; 4 increase time constants. Higher building mass inside insulation 1 increases heating energy requirements of intermit- tently heated buildings; 2 reduces cooling energy requirements, especially where controlled night cooling ventilation is used.
This is usually low level, low velocity supply, coupled with a high level extract. This system is discussed in greater detail in Chapter 3. Maximum air to slab contact can be arranged by having supply airways through the slab and by exposing the underside of the slab to the room without the insulating effect of a suspended ceiling.
For modes of operation see Figure 1. Fans at low speed. Maximum outside Supply fan controlled from air damper Supply air flow rate opening of supply diffusers Outside enthalpy Filter Cooling Minimum exhaust coil damper Concrete slab with airways Recirculation Filter Heat Filter damper Minimum outside exchanger with Extract condensation drain enthalpy VAV supply air air damper diffusers Ducted extract Extract Maximum fan controlled Extract exhaust to match grill damper supply air flow NOTES: 1 The lower mean radiant room temperature produced by the cool slab permits higher room air temperatures without discomfort 2 The size and routing of the slab airways should be arranged to give the lowest air flow and air resistance that will give the required heat transfer from slab to air 3 For VAV control description see Section 5.
Fans stop when extract air temperature indicates that the slabs are cool enough to provide the next day's cooling requirements. Dampers on maximum outside air if the outside air enthalpy is lower than the extract air. Dampers on full recirculation when the outside air enthalpy is higher than the extract air.
Mechanical cooling operating with off-peak electricity. Fans and mechanical cooling stop when the extract temperature indicates that the slabs are cool enough to provide the next day's cooling requirements.
See Section 5. Outside air intake and exhaust dampers shut tight. Perimeter heating under control of room low temperature limit ther- mostats. Slabs store heat during the day to offset night heat loss.
Perimeter heating would normally be off. Perim- eter heating maintains building at minimum acceptable temperature. Lack of maintenance is also a frequent cause of problems, poor environmental conditions, high energy costs and sometimes illness.
It is therefore important that HVAC systems are designed to minimize the amount of maintenance required, and especially maintenance in the occupied space.
Adequate access must be provided to all equipment requiring regular maintenance. The drain trays and drain pipes can become a source of odours and possible infection, unless regularly cleaned. Ductwork should have access openings for regular inspection and cleaning. The control of steam injection always ensures that the mix does not approach saturation. These might produce local internal surface temperatures close to the dew point of the air.
The combination of internal duct deposits and moisture will inevitably lead to mould and bacteria growths. Indeed, for many comfort applications the need for any at all is questionable. See Figures 5. This is obviously a very wasteful control method but unfortunately it is still very commonplace in many air conditioning systems and even some heating systems.
These are probably the most wasteful. The system supplies the sum of the maximum cooling requirements of every zone which is cancelled as required under partial load conditions by reheat locally at the zone. Suggested improvements 1 Convert to VAV. Any reheat control valve reaching a fully closed position initiates a progressive decrease in supply air temperature until the valve starts to reopen.
All reheat control valves opening more than say 10 per cent of their full movement initiate a progressive increase in supply air temperature until any valve backs off. This type of control reduces reheat to a minimum. Heating is provided by primary air at constant volume and a variable temperature that is adjusted automatically with changes in external tem- perature, according to a predetermined schedule. This invariably means cancelling some of the heating capacity with cooling by the terminal cooling coil under its local control.
Suggested improvements 1 Control primary air temperature to suit the requirements of the least favourable terminal con- trol by removing the outside compensator winter control resetting the supply air temperature and substituting control from the status of the terminal control valves Any single valve, or group of valves, reaching a fully closed position initiates a progressive increase in supply air temperature until it starts to reopen.
All valves opening more than say 10 per cent of their full movement initiate a progressive reduction of supply air temperature until any valve backs off.
This type of control ensures that the primary air tem- perature is continually adjusted to the lowest accept- able temperature. Each zone controlled as described above.
This may be achieved automatically by a control system which resets the supply air temperature within suitable limits from the status of terminal diffuser throttling devices. Upon further reduction of cooling load, the air supply is reheated at constant volume. Suggested solutions As for improvement 2, given in Example 3. Terminal control is essential to avoid overheating and thermal overlap caused by occupants opening windows.
This usually takes the form of ther- mostatic radiator valves. References 1. Ventilation air heat load Outside air has to be introduced into the building to provide oxygen for occupants, and to dilute, disperse and remove contaminants and odours. Heat is required to raise the temperature of the water and to change its state from liquid to vapour.
The HWS load to be dealt with by the boilers can be reduced by the use of low capacity, fast recovery hotwater storage heat exchangers combined with a control system which sheds part of the space heating load to give priority to HWS recovery.
Buildings which are unheated for long periods of time will have lower internal surface temperatures and will absorb more heat during the building heat input period. CIBSE Guide A gives an indication of allowances to be added to normally calculated heat losses for different room heights and heating systems. CIBSE gives data to make a judgement of the outside tem- peratures to be used.
CIBSE Guide J is usually used and in conjunction with the usual assessment of air change rates gives satisfac- tory results for tight buildings. Experience has shown that this data is safe to use for well-constructed buildings and is probably exces- sive for some types of external sealed cladding now com- monly used for commercial buildings.
Window and cladding manu- facturers often publish leakage performance data obtained under laboratory test conditions. These can vary considerably depending on the quality of building workmanship and the detailing.
These factors are not known at the design stage. The whole building heat load should be used for assessing boiler power not the sum of the module heat loads or terminal outputs. Figure 2. It would be possible to extend this boost into the occupied period thereby reducing the maximum heat load but at the penalty of higher internal temperatures during the occupied period.
This requires no increase in the required heating capacity. Although this operation is viable, the energy savings com- pared with continuous operation are very small. Older buildings with low insulation standards lose temperature at a higher rate and will give larger savings.
Intermittent heating is worth while provided it does not involve increasing the heating capacity required to deal with the peak design winter condition.
This means that at near design conditions the system offsetting the fabric loss should operate continuously. The parameters for this decision are complex, and simple measurements of internal temperatures are not necessarily appropriate. The room low temperature limit control is equally important as this must ensure that the internal air temperature in the most exposed rooms does not fall below a value from which a recovery to comfort temperature can be achieved before occupancy.
In Example 2. In practice due to the complexity and lack of information about the thermal behaviour of a building, the settings of optimum start and low limit controls should be found experimentally from thermographs in the most exposed rooms under cold weather conditions.
The fabric heating system with local terminals con- trolled from room temperature should be sized to have capacity to deal with the peak design fabric heat loss plus reheating the ventilation air up to room temperature. In this way some limited cooling is available from the ventilation air in mild weather when the internal gains exceed the fabric heat loss.
The weather tape Figure 2. Care must be taken to correctly assess the minimum internal gain that is likely to occur this will usually be from the lights and also to ensure that they are switched on early enough for their gain to become effective before starting the ventilation plant. The minimum internal gain can be credited to the required boiler power but not to the heat emitters some rooms may not have internal gains. The assessment of boiler power should take into account the effect of one boiler unit failing although the risk of this happening on the rare occa- sion of design full load is remote.
These units do not use a heat exchanger, the combustion products mainly water vapour and CO2 mix with the incoming fresh air.
These units are only suitable for makeup fresh air applications in large buildings such as fac- tories, warehouses and shopping centres. They cannot be used in a recirculation mode because of the build-up of CO2 and humidity, but some units are designed to deliver rela- tively small amounts of high temperature, high pressure air via specially designed high induction nozzles which entrain and mix large volumes of secondary room air.
They must be combined and interlocked with a matching extract system. Indirect air heaters have a wider application as they can be used with air recirculation.
The main drawback with warm air heating compared with radiant heating in high rooms is the higher temperature gradient and heat loss.
Unless the supply air mixes thoroughly with the room air before it loses momentum, its buoyancy will take over and it will rise, increasing the temperature gradient and failing to achieve comfort temperatures at occupant level. This phe- nomenon is a common cause of problems with warm air systems, and may be caused by a combination of the fol- lowing: a Leaving air temperature too high. If the air outlet velocity is too high discomfort many result from excessive air movement.
The units are usually mounted a high level and used for industrial type applications. High radiation intensity on the head of occupants can lead to discomfort depending on duration of occupancy. Most manufacturers give guidance on spacing and mounting height. It is also necessary to avoid mounting close to external walls as the radiant heat transfer will increase the wall heat loss by raising the internal surface temperature of the wall or window.
However, the capital cost of radiant heating is usually higher than warm air heating and the fans in warm air heaters can, in some cases, be used in the summer to improve comfort by increasing air movement. A diffusion barrier coating on the tube prevents the passage of oxygen and other gases through the pipe wall, thus avoiding potential corrosion problems elsewhere in the system. The main disadvantage of embedded panel heating is its high thermal capacity and hence slow response to deal with sudden load changes.
Their output is mainly radiant but some convection does occur. They are useful for providing local comfort in unheated areas. Other applications are for buildings such as churches and community halls which are used intermittently. In these cases it is often not possible or economic to warm the building structure to any extent and the heaters need to shine directly at the occupants to compensate for low air temperatures and building cold surfaces.
When noise problems occur they are usually due to free air in the water or cavitation at the control valve due to excessive pressure drop. Radiators can be unobtrusive if carefully planned and selected. They can be arranged as separate units or in a continuous line wall to wall. Their output is mainly convective.
Unfortunately the air side control does not stop the heat output completely. The damper is rarely a tight shut off and the steel casing temperature increases with the damper closed resulting in waste and overheating see Figure 2. Water heaters would be a more correct term unless they generate steam.
Other heat sources are heat recovery from exhaust air, condenser heat rejection from refrigeration plant, heat pumps using outside air or water as the low grade heat source, direct electric resistance heating, combined heat and power where heat is used from the exhaust gas and jacket of the engine driving the generator for electrical power. The construction of the main boiler convective heat transfer surface is similar to conventional boilers described above but the products of combustion are then passed through a sepa- rate additional heat exchanger constructed from corrosion resistant material such as stainless steel or glass-lined cast iron.
Note that the normal bypass three-port terminal control in- creases the return water with falling load. The evaporator cools a low grade heat source such as outside air or water and the condenser becomes the heat source. Some are equipped with refrigerant reversing valves which reverse the roles of evaporator and condenser when the unit is required to cool instead of heat. The use of heat pumping is most economical when heating and cooling are required at the same time which is often the case with air conditioned buildings during mid-season.
The heat is stored either by heating water in pressurized cylinders or by melting eutectic salts or by heating blocks of masonry. The heat is recovered from the store when needed either by water circulation or by direct heat exchange to air.
High grade heat from the exhaust gas may be used to power absorption water chillers. Supplementary boiler plant and heat rejection equipments are therefore required. These complications lead a complex arrangement of plant and controls that is generally better suited to large scale applica- tions see Figure 2.
This is often achieved by an optimum start device which varies the start time dependent on the building tem- perature. In winter the ventila- tion system should normally be stopped when occupants leave. High building insulation standards and consequent small heating capacities mean that the heating system will recover afallin the temperature ofthe buildingfabriconlyataveryslow rate see Figure 1.
Under near full design external temperatures the heating should operate continuously. Control functions to give these protection are: a Automatic tight sealing of fresh air inlet and exhaust air dampers when the air handling unit fans stop in winter.
This may be caused by control malfunction or heat source failure. With large water content systems it is necessary to arrange for the boiler water circulation to bypass the system so that on start-up the boiler water temperature reaches normal operating levels as quickly as possible and the boiler circulates water from and to the system only when it achieves normal operating temperatures.
Boilers need to have the water circulation maintained for a short period after burner shutdown to remove residual heat, this is usually achieved by a time delay controlled pump run on. This is only viable on a large scale usually in conjunction with steam power generation and district heating. Steel oil storage tanks require a bund oil-proof wall to contain the contents of the oil tank in the event of leakage.
Electricity can be used to generate heat by direct electric resistance and by driving heat pumps. Mechanical ventilation must be balanced to ensure a slight positive air pressure in the boiler house and some low level ventilation is necessary to prevent possible build up of CO2, which is heavier than air. This treatment entails storage of chemicals, vessels for mixing, drainage and emergency showers if acids or alkalis are involved. Chemical dosing of closed circuits is usually required for corrosion inhibiting, pH correction, and sometimes biocides are required for control of organisms.
Figs 2. Calculation of U Values. Table 1. Equations 5. External Temperatures. Average External Temperature. A simple load diagram can be drawn Figure 3. The transmission gain depends on the difference between the outside and room air temperatures and the load line is easily established: point 1 transmission gain for the summer design outside temperature is joined to point 6 heat loss for the winter design outside temperature.
Solar gain through windows is not related to outside air temperature, but maximum solar gains occur in summer, when the outside temperature is high, and minimum gains in winter, when the outside temperature is low. Hence, for the case of a west-facing window, the design solar heat gain is associated with the outside design temperature in July to establish the point 5 and the winter design outside temperature linked with the solar gain in January to identify the point For windows facing south the maximum solar gain is at noon in the spring or autumn and it is reasonable to link the midday outside temperature in March or September with the solar heat gain.
This gives a cranked load line shown broken through the points 5 0 , 11 and 10 0 , in Figure 3. East-facing rooms have a peak solar gain at about h or h, suntime, in June or July, when the outside air tem- perature is lower than the summer design value. A similar procedure can be adopted. There is likely to be a net sensible gain, requiring cooling for the conditioned room, over much of the year. Population will change and there will be variation in the use of lights and machines.
Hence load diagrams show the maximum heat gains likely, but average gains will be less and, for some of the time in winter, there will be no heat gains at all and the load will be entirely heating, particularly at start-up in the morning. Sensible gains must not be included when calculating design heating loads and boiler powers. The average moisture content of the air tends to remain fairly constant in a particular month and hence the humidity rises as the temperature falls after h, with the possible formation of dew if the temperature drop is enough.
Meteorological data are available for many places in the UK and throughout the world and they may be analysed to yield a suitable design condition for h suntime in July. It is customary to record maximum daily air temperatures and relative humidities, usually at the same time in the afternoon. Over a given month the maximum temperatures on each day are noted and this is repeated for several years.
The average of these temperatures is established and termed the mean daily maximum dry-bulb for the period of years considered. The greatest temperature in each month is also noted and the average calculated over the same period of years.
This is called the mean monthly maximum dry-bulb temperature. Thus mean daily maximum temperatures can be regarded as referring to typical weather in the month while mean monthly maxima refer to spells of warm weather in the month. Since the moisture content of the air does not change very much in a particular month it is determined from the mean daily maximum dry-bulb in the month , expressed at the same time of the day as the measurement of relative humid- ity.
Knowing the moisture content and the mean monthly maximum dry-bulb temperature for the month allows the Figure 3. See Section 3. If the meteorological station from which the data are obtained is in a rural district the design value of dry-bulb temperature, obtained as above, may be used directly.
This accounts for the solar radiation absorbed by the building surfaces during the morning and later convected into the air, increasing its temperature. Table 3. Analysis Frequency of occurrence Exceeded for Coincident with Common usage Solar gains through windows play a dominant part in heat gain calculations and another approach is to choose an out- side design state that is linked with periods of high solar intensity.
The CIBSE Guide [1] tabulates dry-bulb tempera- tures for the summer months coincident with solar radiation intensities exceeded for 2.
Assuming an air conditioning system life of 25 years it is suggested that A study of human comfort is therefore relevant [3]. Dry-bulb temperature is the most important of these factors and is under the direct control of the air conditioning system. The duration of occupancy is relevant in all coun- tries. Air velocity is next in importance and its value should gen- erally not exceed about 0.
The part of the body on which the air movement is directed is relevant. Although the air movement is not automatically controlled by the system to give com- fortable conditions, a proper selection of the supply air dis- tribution terminals and system for the treated space must be made to ensure comfort. Mean radiant temperature is not under the control of the air conditioning system except for systems using chilled ceilings but high intensity solar radiation through windows must be excluded.
This is done by the provision of suitable solar control methods for windows that can be exposed to direct solar radiation. It usually takes the form of internal Venetian blinds for commercial buildings but the use of external shading is best although not always practical in the UK. Research [3] shows that if account is taken of all the variables involved, namely, the metabolic rate related to the activity, body surface area, the clothing worn, air dry-bulb tempera- ture, air velocity, mean radiant temperature and relative humidity, satisfying more than 95 per cent of a mixed population is impossible.
Several synthetic scales of comfort have been developed over the years, with mixed success. In Europe and the UK dry resultant temperature tres is often taken as an index of comfort. In equation 3. In summary, the fol- lowing conditions are desirable for human comfort in a room: 1 The dry-bulb should exceed the mean radiant tem- perature in summer but be less than it in winter. There is a reasonable view that the design dry-bulb could be Stack effect In summer, the air outside an air conditioned building is warmer and less dense than the air within.
Consequently air tends to enter openings in the upper part of the building fabric and to leave through openings in the lower parts. A practical equation can then be developed by assuming an air density of 1. In the UK a value of 0.
The intensity of direct solar radiation reaching a place on the surface of the earth depends on its path length through the atmosphere and is related to the position of the sun in the sky.
In turn, this depends on the month of the year, the latitude of the place on the surface of the earth and the time of the day.
Figure 3. The position of the sun in the sky is expressed by two coor- dinates: solar altitude, a, and solar azimuth, z. Values of these angles are tabulated [6]. Numerical values of such intensity are given in reference [6].
The angle of incidence of direct radiation on an actual wall, roof, or window varies with the position of the sun in the sky and it is customary to resolve the direct radiation in a direc- tion at right angles to the actual receiving surface. Diffuse radiation. In its passage through the upper atmo- sphere the total solar radiation is scattered by the molecules of nitrogen, oxygen and water vapour.
Some of the radiation is also absorbed, mostly by molecules of carbon dioxide, ozone and water vapour, which re-radiate thermal energy in all directions. Part of this scattered and re-radiated energy reaches the surface of the earth and is termed diffuse, scattered or sky radiation.
In very approx- imate terms about 10 per cent of the solar radiation reaching the surface of the earth on a clear day is diffuse, the remainder being direct. Dif- fuse radiation is more intense when coming from the part of the sky in the vicinity of the sun and is stronger for higher solar altitudes but is not strong enough to cast a shadow. References 6 and 7 provide numerical values.
Increases in height above sea level give a reduction in the strength of the scattered radiation but the direct radiation is correspond- ingly greater. Ground radiation. Shading and solar control glass Direct solar radiation must be prevented from passing through a window into an air conditioned room.
Motorized external shades are excellent but not a practical proposition unless there is adequate access for maintenance. This is seldom the case in the UK. Fixed external shades are of little use in the UK and in northern, high latitudes, because the sun is low in the sky for most of the day and year, allowing direct radiation to pene- trate into the depth of the room for much of the time.
Double glazing is unnecessary for air conditioning unless relative humidity is provided at a value that would give condensation on single glass in winter. A thermal break must be provided in metal frames when used with double glazing.
A calculation should always be carried out to establish the inner surface tem- perature of the double glass, in relation to the room dew- point and the coldest expected outside condition. However, there may be a case for hospitals and the like, occupied continuously. The only reason for double glazing other than dealing with condensation in special circumstances , is acoustic. The mean attenuation provided by a single glazed, openable window without weather stripping, is about 20 dB over the range of frequencies from to Hz.
If an openable, double glazed window is provided having a mm air gap, the mean attenuation rises to 40 dB. The effect of building mass Solar radiation entering a room termed the instantaneous gain does not immediately cause the air temperature to rise and provide a load for the air conditioning system.
Some heat is convected from the warm blinds over the sunlit windows and this causes an immediate rise in the air temperature. The radiation is absorbed by the upper thickness of the solid surfaces on which it is incident and warms the material. Time is taken for this to occur and some of the incident energy is stored in the mass of the material but is eventually convected into the room to provide a load on the air con- ditioning system.
Hence the building mass plays an important part in the calculation of solar heat gain through windows. The response factor is unsuitable for expressing the weight of a building when calculating solar heat gains through glazing. Referring to equation 3. In fact, a building with many partitions is regarded as a lightweight building when calculating solar heat gain through glazing.
The practical calculation of solar heat gain The CIBSE tables published in [10], based on a theo- retical analysis [11], accounted for all the relevant variables and gave the solar heat gain to the room as a load on the air conditioning system. They were easy to use and in reasonable agreement with well-established, other methods, proved in use. The tables were usable for all the UK. Plant operation was taken as 10 h daily but no correction was given for other hours of use. The tables assumed that the air conditioning system would maintain a constant dry resultant temperature within the conditioned space and solar gain data were provided for both lightweight and heavyweight buildings.
This makes it an essential tool for engineers involved in the design and day-to-day running of mechanical services in buildings, and a valuable reference for managers, students and engineers in related fields. This pocket reference gives the reader access to the knowledge and knowhow of the team of professional engineers who wrote the sixteen chapters that cover all aspects of mechanical building services.
Topic coverage includes heating systems, ventilation, air conditioning, refrigeration, fans, ductwork, pipework and plumbing, drainage, and fire protection.
The result is a comprehensive guide covering the selection of HVAC systems, and the design process from initial drafts through to implementation. Electricians and technicians will find this a useful reference during training and a helpful memory aid at work. This is a highly illustrated guide, designed for ready use.
The contents are presented in pictures and checklists. Each page has a series of 'how-to' instructions and illustrations. In this way the subject is covered in a. In the almost sixty years since the publication of the first edition of HVAC Engineer's Handbook, it has become widely known as a highly useful and definitive reference for HVAC engineers and technicians alike, and those working on domestic hot and cold water services, gas supply and steam services.
The 11th edition continues in the tradition of previous editions, being easily transportable and therefore an integral part of the HVAC engineer or technician's daily tools. Newly updated data on natural. With coverage including the key principles of electrical engineering and the design and operation of electrical equipment, the book uses clear descriptions and logical presentation of data to explain electrical power and its applications.
Each chapter is written by leading professionals and academics, and many sections conclude. Author : M. Author : S. Author : F. Author : E. Reeves,Martin J. Author : D.
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