Ventilation Parameters that Influence the Building Construction and Process Design
Many parameters and variables in a ventilation system may influence the re quirements on the building (and sometimes also the equipment inside the building) and the process. This makes it necessary to take into account the decided target levels when designing the building and sometimes also when choosing the process. It is impossible to assign detailed numbers to effects, so only a description of different parameters and their importance will be given here.
The main target levels inside a building are normally set for temperature, humidity, air velocity, airflow rate (and air distribution), and contaminant concentration.1’2
The temperature limits inside a building are mostly within the variations of the temperature outside, and the heat resistance requirements on the building materials inside are the same as the requirements on materials used outside. There could be some additional requirements on outside materials depending on rain, snow, wind, sunshine, etc. When the temperatures inside, whether higher or lower than outside, will be used because of the process, the building materials must be chosen with these requirements taken into account, especially when heat radiation is a factor. This should not be confused with demands on temperature and humidity insulation.
To maintain the target level for temperature, a specific amount of insulation may be needed, since too little insulation makes it impossible to keep the temperature levels. For each building it is necessary to make a detailed cost — benefit calculation of insulation and heating/cooling costs. The same discussion is applicable to temperature variation requirements, both for the rate of change and the period lengths (see Chapter 16).
The requirements on the building materials for resistance against humidity and moisture are connected to the requirements for temperature, since the vapor pressure could have large variations in a small temperature range. Moreover, if there is a risk of condensation (e. g., when the process generates water vapor), it is necessary to have surfaces that do not adsorb moisture. There is also the possibility of rust with high humidity. Normally, a relative humidity below 40-50% is assumed sufficient to prevent rust. If there are some contaminants present, this value can change.3 There is also some evidence that higher humidity provides greater exposure to contaminants,4 High humidity may promote the growth of some biocontaminants, such as molds, but generally humidity levels below 70% prevent the formation of mold. On the other hand, there are a few types of biocontaminants (bacteria) that grow better in low humidity. Depending on the process and likely biocontaminants, it is necessary to choose both the humidity level and the quality of surface material. This is especially important for those processes where changes in temperature and humidity level are frequent.
The requirements on building materials due to air velocities inside the building are generally negligible. However, sometimes the allowed contaminant concentrations can be of such magnitude that moving air may affect surfaces. In such cases it is necessary to use materials with sustainable surfaces. Normally this demand is valid only for the transport of dust-laden air in
Ducts. For the process the air velocity may have a major influence, both on the possibilities of using local ventilation (see Chapter 10) and on many processes in which a high air velocity disturbs the process and changes the result (product). Examples are weaving, pharmaceutical manufacture, and steel casting.
The airflow rate does not interfere with the building or the building materials, and usually not with the process. The air distribution may vary a lot, however, if the building and process layout and sometimes the building materials are not chosen with great care. Lamps, ladders, window shutters, cabins, process equipment, etc. are often placed where the ventilation designer did not expect any obstructions. Thus continuous cooperation between the different designers is necessary to prevent mistakes and to facilitate common solutions to this type of problem (see Chapter 8).
Normally there is some connection between the airflow rate and noise and vibration generation. This could modify the building construction either to prevent spreading or to diminish the levels of noise and vibrations from the air-handling units. This naturally includes all parts of these units, i. e., fans, pumps, and valves (see Chapters 5 and 9). The demands on noise insulation also include the noise and vibrations from the process equipment, which often has a higher level of noise and vibration than the ventilation system.
Contaminants nearly always need some air humidity to cause damage to the building and building materials. This is mainly an environmental problem. Since the outside air is often used inside without being cleaned of gases and some outside air can enter through other ways than the ventilation system, this may be important for the materials used inside. The contaminants that do most damage are liquid acids and particles consisting of acid compounds. If these types of contaminants are generated in a process inside the building, all materials must be chosen very carefully for both the building and the process.3,5 For particles the primary effect on materials is surface dirtying. In many cases this can be prevented by proper cleaning procedures. The damage on internal surfaces by air pollution is usually of little significance, and the cost for material of a higher grade must be compared with the building’s lifetime and the cost for cleaning.6
One other important factor that determines the building construction is the mechanical load on the building by the process equipment and by different parts of the ventilation system. These can either be surface loads or concentrated loads. For example, it is necessary to design for a high load on a roof when fans and other air treatment units are to be placed there. Since cranes are often fastened to walls or roofs, the loads on these parts must be carefully analyzed. For all placings of ventilation systems there are demands on availability, accident prevention, and emergency exits that may change the building construction and process design. There should be no problems with necessary space for ducts and other equipment inside or outside buildings with industrial ventilation. Sometimes, however, it may be necessary to make sure all ventilation equipment gets enough space on the floor or the roof. This naturally includes proper space for spare parts and maintenance.
Sometimes the mechanical load can be in the form of air pressure on the surfaces. If the ventilation system has a pressure difference to the outside or between rooms, the building must withstand the pressure differences. It must also have a structural tightness that permits the ventilation system to maintain these differences.7 The sensitivity of the process to air pressures and air pressure changes must also be checked.
In addition to the building parameters, there are often some requirements from suppliers of processes and equipment to make sure their parts function properly. This may mean that the requirements for the equipment decide the target levels (e. g., in pharmaceutical and electronics industries). In other cases there are restrictions on deviations from the target levels (e. g., on temperature for machine control system or on humidity and vibration for optical measurement systems).7
When there are requirements from the manufacturers of equipment regarding the environment and building, it is necessary to check these values—first, to decide if the limits given by the manufacturer are appropriate, and second, to make a comparison with the target levels. If the target levels are more stringent than the demands for the equipment, there will be no problem. If one or more of the demands is more stringent than the corresponding target level, it is necessary to change the target levels or in some cases to reevaluate the requirements.
Whatever the case, the levels necessary for the processes and the machines and their influence on the target levels must be quantified, and the designer of the ventilation system must take appropriate action.
A summary of the different influences on the building and the process is given in Table 6.21. “Large” means that the (ventilation) parameter usually has a large influence on the construction variables. “Variable” indicates that the parameter’s influence usually varies depending on other ventilation parameters or construction variables. “Small” means that the parameter’s influence usually has little influence. The numbers describe the required target levels according to Section 6.4. For example, if the target level is II (good industrial level), the temperature limits could have a large influence on the building’s construction and choice of material. On the other hand, the target levels III to V usually have a limited influence on how the building is constructed and materials chosen.
TABLE 6.21 Simplified Table Showing How Parameters, Mainly of the Ventilation System, Influence the Building and Process
Numbers refer to the target level classes presented in Fig, 6.10.
The distribution given in the table should be used only as a guide, not for designing. There are many cases where low demands on target level for some ventilation parameter have a great influence on the building system.
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