Energy efficient ventilation systems

Transcription

1 Energy efficient ventilation systems Gyöngyike Timár, Mechanical engineer Corresponding SUMMARY Changes of actual load often lead to trouble with the operation of the ventilation system. The design data are no longer correct if, for any reason, the actual load has permanently changed during the usage. The proposed ventilating system maintains the most commonly developed harmful gaseous contaminant content below the permissible level in spaces with forced ventilation. Sensors are mounted in every room, in a given height. By following the demand, the supplied fresh air volume matches the momentary load. Thus, possible health hazards can be mitigated and acceptable life- and working conditions can be maintained in these spaces. The requested indoor air quality is assured in any ventilated space without disturbing the air supply to other spaces. One of the main advantages of this system is that no energy is wasted for the handling of any excess air when the system load is below the design load. 1. INTRODUCTION 1.1 Background When the air exchange rate in a room is maintained at the nominal design value during periods of excess load, the concentration of air contaminants is likely to exceed the permissible limits. Some of these contaminants can be identified easily, but there might be some that can not be perceived with senses. Continuous inhalation of the air of such compounds can cause harmful physiological effects 1. Proper indoor air quality can be maintained if the fresh air supply rate corresponds to the load imposed by the number of people, by their activity in the room, and by the altered technology s requirements. Of course, if the air supply rate is high enough to match the highest load, the excess circulation wastes energy. A ventilating system is usually selected for a given, constant scenario, for the demand expected at the time of the design. These data are no longer correct if for any reason, the actual load has changed during the usage. Experience shows that the differences between the design and the real load of ventilation systems can be significant. Without listing all the possible reasons, they are the following: - The customer is not aware of the exact load, the number of people, and the equipment heat load in the spaces at the time of the design. - The load, the number of people or the equipment heat load changes considerably during operation of the ventilation system. - The layout of the space is altered resulting in changes of the volume and the air flow or the function of the space is changed. Usually, it is not simple to predict these variations, and it is not possible to mitigate the effect of the load distribution changes during operation. Thus the whole ventilation system should be developed in a way that it would be capable of furnishing sufficient fresh air according to the varying load. Applying adequate control and suitably modified air duct layout, the gas contaminants concentration can be kept below the health hazard limit.

2 1.2 Theoretically demanded air flow Requirements of indoor air quality comprise the physical characteristics of the air, and its composition. Because of the natural usage, harmful gaseous contaminants, most commonly carbon dioxide, carbon monoxide, and different nitrogen based mixtures develop in spaces that have no windows or doors opening directly to the fresh air. The concentrations of these gases might reach critical values 2 during natural usage of any space if it is not compensated with sufficient fresh air. People exhale l/h carbon dioxide 3 depending on their activity. In practice, the average of this range of carbon dioxide is considered to be the determinant of fresh air needed per person. According to the customary proportional control formula the fresh air demand due to respiration is: V s = K/(p er - k out ) [m 3 /h] /1/ Where V s the supplied fresh air quantity [m 3 /h]; K - the generated air contamination quantity [g/h]; k per - the permitted level of the contaminant in the room [g/m 3 ]; k out - the average content of air contaminant in the outside air [g/m 3 ] 4. There are professional recommendations proposing the fresh air supply as a function of the activity between m 3 /h per person 5 for office work. 1.3 The task to solve The load often changes during operation of a ventilating system. The load might diminish or rise depending on the number of people in the room, on the time they spend there, on their activity level and on the technology s requirements. In the first case, energy will be wasted because of the excess air supplied. In the latter case, it is vital to provide fresh air, to prevent the contamination's possible harmful effect on the health of the occupants. There is need for a general method or system that can operate in a reliable way in every air conditioning or ventilating system to suit the altered demand. There is need for such a system that assures the appropriate indoor air quality in order to protect people from the mentioned harmful effects. The ventilating system should follow the load between certain limits, neither less than necessary to maintain acceptable working conditions, nor more, to avoid wasting energy. The reason of application of contaminant controlled ventilation is twofold. On the one hand, the supplied fresh air quantity can be controlled independently from varying human load. On the other hand, energy efficient operation should be assured. 2. METHODS 2.1 Motivation In addition to the requirements on the thermodynamic properties of the indoor air, for example temperature and humidity, there are requirements on its chemical composition. Up until now, no regular attempt were made to monitor the variation of the inside air s carbon dioxide content in ventilated spaces during the operation of the ventilation system. During normal usage, different types of gaseous contaminants enter the air that are hazardous to health. One of these contaminants is carbon dioxide. It is used as the most common indicator of the indoor air quality, is not poisonous but in continued excess concentration might lead to oxygen deprivation. Since this contaminant gas is colorless and odorless, the occupants are not aware of its presence. It affects the occupants to varying degrees, depending on their individual susceptibility, particularly their alertness and effectiveness. The equation in 2.2 would only be valid in reality if the level of the inside air's carbon dioxide content /k per / remained below the design limit. In reality, this value differs from the

3 theoretically proposed one. This content is determined by the load. That is, its quantity depends on how many people are staying in a room and for how long or how much fresh air is demanded by the altered technology. While the physical characteristics of the indoor air are directly measurable, its quality is harder to monitor. For example, it can be measured indirectly by following the change of the CO 2 content. With suitable instrumentation and appropriate controls, its concentration can be reliably measured, and the required indoor air quality can be maintained in the ventilated space. There is another issue that requires special concern. The ventilation system seems to be the only among building services that might cause direct harm on the surrounding, on the occupants health when it performs poorly. It is quite likely, that people working inside a building, spent about 90 percent of their lives inside. During this time they might be exposed to the mentioned harmful effect. Offices for example can be regarded as continuous operation, where people stay mostly at the same location. One can assume that the individual s performance depends on the surrounding conditions as well as on their own capability. The office environment along with the suitable indoor air quality is an important part of these surroundings. That is why the proper ventilation is so important especially where the forced ventilation is the single source of supplying fresh air. That is in spaces having no doors or windows directly to the fresh air. Because of the nature of public building, continuously moving and varying number of people can be expected. The consequence is the change in ventilation load. Either people sojourning there change their place, leave or arrive, or visitors arrive or leave in unknown number and time frame. Between certain limits this change should be predictable for most of the systems at the time of the design based on the building s function. The building s owner, the user, and the operator should be aware of the design limits. Recent professional studies have examined the main advantages of CO 2 based control applied in ventilation systems. According to the published articles, CO 2 based control assures adequate indoor environment. The symptoms reported because of inappropriate inside environment reduced or disappeared. There are also indications that buildings installed with CO 2 controlled ventilation show reduced demand under part load conditions RESULTS 3.1 Proposed arrangement The arrangement on Figure 1. is one among the numerous recommendations with the purpose to assure the required indoor air quality. There are two prospects considering the most economic operation of the proposed ventilating system. Energy might be saved when the transitory surplus air heating- or cooling energy can be prevented, and also when the fan is working at its highest efficiency. The control system is tracking the actual load, and supplies the necessary supplemental fresh air. By monitoring the instantaneous load, the fresh air supplied to the space is adjusted. Consequently, the air contaminant's quantity is maintained below the critical value as the supplied fresh air quantity suits the actual load, the number of people the activity in the room or the technology requirement. There are two prerequisites concerning this system; individual spaces should be separated from each other regarding the ventilation, and the heat load generated by people and the heat loss due to transmission is not considered. Sensors are mounted in every room at an appropriate height in the breathing zone. Following the sensors' signals, the room is supplied with fresh air until the concentration of the contaminants decreases below the threshold limit. The control system gives alarm signals,

4 when the monitored gaseous contaminant's concentration constantly exceeds the harmful limit. These alarms may be digital, acoustic or visual. This control system does not replace the control system based on the air physical characteristics. It is proposed to ensure the air supply within V s = 0,2-1,2 V nom intervals referring to the whole system. There is need for P Av P Av P Figure 1. System layout supplemental air when the load in the space exceeds the design conditions. The high limit of the air flow has been chosen at 1,2 V nominal regarding the whole system. During periods of low loads, through the third duct the excess air recirculates to the fan inlet, or the surplus air is flowing ahead the fan into the system through a central by-pass of the supply duct. The discussion entirely disregards the control in the air-heating and air-cooling process. Room temperature supposed to be constant. One of the main purpose of this arrangement is the stable operation of the fan on the designed or on the nearby operating point and with the possible favorable efficiency. The smaller fans indicated on Figure 1. are provided for extended periods, potentially several days, of low load conditions. Then the controlled data, pressures, air volumes determines when to shift fans. All the proposed limits are arbitrarily chosen. Their values might vary depending on the size of the room, and the building, on the type or function of the building. In any case, it has to be considered during the design stage. It has to be mentioned that the load tracking control system does not substitute the control system based on the indoor air's temperature. The suitable air quality in each supplied space can be assured solely by the sufficiently harmonized operation of both control systems. 3.2 Proposed steps of sizing The proper operation of the ventilation system can not be achieved or maintained in the ventilated spaces without adequate control. The determination of the exact or the near exact quantity of fresh air is important in order to avoid any probable health s risks on occupants and also to provide the energy efficiency of the ventilation system. The basis of calculation is

5 not a specific air volume any more, but an interval of air volume. There are upper and lower limits upon sizing and selection should be based. Figure 2. System flow chart The basic data of sizing is similar to that of any other calculation, geometric and building data, tracing of air duct, spaces available and the nominal loads. New types of data are the predicted surcharges by rooms and the predicted lowest loads. The predicted surcharges comprise the maximal number of occupants by ventilated spaces, and the duration they can sojourn there without being exposed to any harmful effect. As the calculation, it is essential to be aware of the permissible surcharge of the ventilating system. When its exact value is not known at the time of the design, the proposed limits of the supplied air volume are 0,2 V nominal 1,2 V nominal. The main steps of sizing are shown in Figure 2. Accordingly, after reading the INPUT data, succeeds the preliminary duct sizing based on the usual limits, as the velocity limit in the air duct. After completing this cycle the next is the air duct sizing, the pressure condition of the by-pass or that of the 'third duct'. Afterwards the proposed sizing made by using the predicted load limits of every room, and the controlled values succeed. The next is the determination of simultaneity loads for the whole system, the

6 lowest and permissible highest loads by rooms, and also the calculation of the preset value for each room s damper. The process continues with the fans' selection for the most disadvantageous loads along with the estimation of their operational costs. This is the time for revision and entering the revised values. Re-controlling is based on the revised data is the next step. Then the selection of the fan of smaller capacity comes after if it is necessary, when the load is below 0,6 V nominal. The result, the OUTPUT of the calculation is the air ducts' sizes, the permissible extreme load limits, the sets of air dampers for 100% and for the extreme loads, the selection of sensors insertion points in the duct, and that of the measuring devices, the operating characteristics of the fans, and forecast for energy-saving operation. It is also highly recommended to be marked on the design the maximal permissible load per ventilated spaces, the supplied surplus air volumes along with other values. The floor plan and the scheme should contain the maximum number of occupants per spaces, the insertion points of all the control and measuring devices. Besides checking the system and the definition of the room's permissible load it calculates, how much energy can be saved by operating this increased valued ventilating system. 3.3 Example for theoretically possible energy savings Figure 3. Theoretically possible energy saving of air heating

7 Due to variable load the difference between the design and the actual fresh air need resulted also a difference in energy use of the operation of the ventilating system. This is the difference at low load conditions that can be saved by using the contaminant controlled ventilation system. There are two prospects considering the most economic operation of the proposed ventilating system. Energy might be saved when the transitory surplus air heatingor cooling energy can be prevented, and also when the fan is working on its highest efficiency. The next example will discuss the theoretically possible energy savings under varying air supply. The energy need is directly proportional to the transported air volume, to the pressure difference consumed by the air distribution system itself. Concerning energy savings, the example is studying the change of energy demand in winter season. The data do not contain any heat gain generated by people, or heat loss through the building structure. It refers solely for the heating of the supplied fresh air. Otherwise, the energy need for cooling is disregarded in this discussion. Conditions are the next: space inside temperature t i = +20 o C; outside temperature t o = - 15 o C; system operation: 5 days a week; duration of daily operation: 10 hours; fan1 supplied fresh air quantity V 1 =1500 m3/h; fan 2: V 2 =3000 m3/h; fan 3: V 3 =4500 m3/h. On Figure 3. are indicated the yearly energy needs for fan No.1, No.2, and No.3 transporting the different air volumes, from Vs= 0,2 V nominal, up to Vs= V nominal. There are indicated the yearly heating energy needs. The yearly fresh air heating energy demand is determined according to 7 : Q a = G a x V x c x 10-6 [ GJ/year ] /2/ where Q a heat load of the transported fresh air [ GJ/year ]; G a yearly heat gap of ventilation [kh/y]; V transported fresh air [m3/h ]; c specific heat [kj/kgk]. Since the proposed ventilating system is capable to supply varying quantity of air from the lowest load 0,2 V nominal, the theoretically possible energy saving during heating season might reach 80%. Although this study does not contain operating energy savings possibilities, beyond this probable energy savings, the fan s operation should be mentioned too. There are estimations showing that fan operating costs takes almost half of the energy consumptions of a building. It means that the fan s energy demand might influence considerably the building energy consumption. While energy saving in cooling season is not comprised in this paper, it has to be mentioned that heating is seasonal operation, while the fans are working continuously, almost year round. This is one of the features that do augment the achievable energy savings. The values in the example are theoretical that in reality might vary slightly because of different constructing, operating or simultaneity conditions. 4. DISCUSSION 4.1 Differences from traditional ventilating systems The proposed arrangement is based on the traditional ventilating system s element, as e.g. air handling unit, the air distributing duct system, the supply and exhaust grids, exhaust ducts and exhaust fans. There are a few differences from the traditional ventilating systems: - V supply =1,2 V nominal, oversized system /grids, duct, AHU/ - Air volume controlling devices at every room supply connection - Third duct to reveal actual surplus air V supply = 0,8 V nominal

8 - By pass in the main supply duct with possible air transport of V supply = 0,6 V nominal - A supplemental fan of smaller capacity in the supply and exhaust main duct - Carbon dioxide and/or other sensors installed in the ventilated spaces - Pressure and air volume sensors in the air duct 4.2 Advantages The advantages of the proposed ventilating system are: proper life- and working conditions can be maintained in the ventilated space; appropriate indoor air quality can be assured in any distinct room with varying load, without disturbing other space's ventilation; the possible health hazard effect of gaseous contaminants to people is prevented; demand controlled ventilation can be assured in every spaces; beyond the nominal ventilating fresh air, between certain limit, sufficient amount of fresh air can be supplied to any ventilated room; none of the parts or element of the ventilating system is operating unnecessarily; the cooling, heating or transporting energy of the momentary superfluous air can be saved; occupants are continuously informed of the momentary air quality; occupants and operator are warned when indoor air quality is unacceptable in any space; This arrangement requires slightly larger duct, grids or possible air handling unit, and an additional control. All of these augment the value of the ventilation system. What is more, these are one time expences that might recover soon not only in energy savings but also in the enhanced working ability the occupants. Depending on architectural conditions, it can be installed in existing ventilating systems too. Although the control system can be developed according to the description, its efficiency can only be proved after successful experimental testing, by an installed and reliable working system. REFERENCES 1 - Bánhidi László, Kajtár László: Komfortelmélet /p. 222./ Műegyetemi Kiadó 2000, Budapest, Hungary 2. - Épületgépészet 2000 Alapismeretek, Table 13.4, /p.344./ Épületgépészeti Kiadó Kft., 2000 Budapest, Hungary 3 - proposed CO 2 maximum 1000 ppm, ANSI/ASHRAE , Ventilation for Acceptable Indoor Air Quality, Quantitative Evaluation /p.13./, proposed CO 2 concentration, Category C: 1190 ppm CEN CR 1752 Ventilation for Buildings Design Criteria for the indoor Environment, A /p.23/ 4 - Recknagel, Sprenger, Schramek: Fűtés- és Klímatechnika /p. 75/ Dialóg Campus, Pécs, 2000 Hungary 5-25 m 3 /h, person, Recknagel-Sprenger-Hönmann, Taschenbuch für Heizung + Klimatechnik 90/91 /p. 68./ - 30 m 3 /h, person, MSZ , /Hungarian Norm/, Labour Safety Requirements of Heating and Ventilation of Workplaces - 36 m 3 /h, p, ANSI/ASHRAE , Ventilation for Acceptable Indoor Air Quality 1990 Table 2. /P. 8./ - Up to average 50 m 3 /h, person, when 20% of occupants are smoker in the room, CEN CR -1752:1998 Ventilation for Building-Design Criteria for Indoor Environment, Table 2, /P 11/ m 3 /h, p is recommended in spaces with no specific requirement regarding indoor air quality. Bánhidi-Kajtár Komfortelmélet /p. 272/ Műegyetemi kiadó 2000, Budapest 6 Strategies for improving IAQ James E. Megerson, C. Torline ASHRAE Journal May 2006 /pp / 7 Recknagel-Sprenger-Hönmann, Taschenbuch für Heizung + Klimatechnik 90/91 /p.1678./