Ventilation mechanical

Figure 6.6 and Tables 6.4-6.6 give ranges for local thermal discomfort parameters for the three categories listed in Table 6.3. The acceptable mean air velocity is a function of local air temperature and turbulence intensity. 7 he turbulence intensity may vary between 30% and 60% in spaces with mixed flow air distribution. In spaces with displacement ventilation or without mechanical ventilation, the turbulence intensity may be lower. 

Chow, W. K., and W. Y. Fung. 1997. Experimental studies on the airflow characreri.stics of spaces with mechanical ventilation. ASHRAE Transactions, p. 1. 

Combined Effect of Gravity Forces, Wind, and Mechanical Ventilation... 

The airflow rate infiltrating and exfiltrating through each air leakage pass, Q , due to the combined effect of wind, stack, and mechanical ventilation system perfotmance can be calculated ftom the mass balance equation... 

The airflow rate Q, for each air leakage path is expressed with Eqs. (7.237), (7.242), and (7.243) using the infotmation on effective leakage area, CjA, and a pressure difference across the path. The total pressure acting on an opening from the outside is the sum of the pressure due to wind, gravity forces, and mechanical ventilation performance, and the static pressure inside the building results from Eq. (7.244). 

The combined effect of gravity forces, wind, and mechanical ventilation on the pressure across the opening is illustrated in Fig. 7.102. 

Assume the use of mechanical ventilation with full mixing ventilation in the room, and calculate a rough estimate of ventilation airflow found by assuming dilution of contaminants released to the room air to one-third of the TLV level given for the substance in question. 

M. Pozin, Mathematical modelling of heat and air processes in mechanically ventilated spaces, in Roomvent 92, vol. 1, Aalborg, Denmark, 1992. 

This is the usual method of ventilation in domestic dwellings and many small office buildings and workshops. New standards, however, require buildings to have set ventilation rates, which require mechanical ventilation systems. However, as covered later, use is made of natural ventilation to control the air-change rate, regardless of the external conditions. This approach is not practical for industrial applications. 

A fan is a rotodynamic device and is the driving part of all mechanical ventilating systems. The energy of rotation applied to the fan shaft is converted into a pressure difference, causing the air, gas, or particulate matter to flow through the ductwork or discharge into a free space. 

When air flows at a certain rate through the space, energy is transported in relation to the difference between supply and extract air temperature. Such airflow can be induced by natural or mechanical ventilation. See Section 11.5 on the interaction between naturally induced airflows and the thermal behav ior of the room. 

Sha w, C. Y. Methods for estimating air change rates and sizing mechanical ventilation systems for houses. Ottawa NRC, 1987. 

Extract ventilation The mechanical ventilation arrangement to extract polluted air away from a space, either directly or by means of ductwork. 

Hybrid (mixed mode) ventilation A system that makes use of a mix of natural and mechanical ventilation. May be further subdivided into seasonal hybrid, e.g., natural ventilation in summer and mechanical ventilation in winter or spatial hybrid, e.g., mechanical ventilation in core areas and natural ventilation at the perimeter. 

Induced leakage Leakage into an enclosure to equalize a pressure difference due to natural or mechanical ventilation. 

Mechanical ventilation Ventilation created by fans or other air-moving devices within a building, which can be divided into the following classifications ... 

Natural ventilation Ventilation achieved by means of wind forces or density differences or a combination of the two, as opposed to mechanical ventilation, which depends on a rotodynamic device. 

Replacement air Air supplied to a space to replace the air removed by a combustion process or by natural or mechanical ventilation. 

Thermal load The heating or cooling requirements of a space due to structural gains or losses and the air infiltration and mechanical ventilation. The load produced by a process in a working environment. 

Ventilation thermal load The heating or cooling load required to compensate for thermal losses resulting from natural or mechanical ventilation. 

Buildings are ventilated mechanically with the ITVAC systems where it is a controlled process, as well as via air infiltration and through the openable windows and doors where it is largely an uncontrolled process. However, as discussed earlier, mechanical ventilation is one of the most energy-intensive methods of reducing indoor pollutant concentrations primarily because of the need to thermally condition air before it can be circulated inside the occupied spaces. It is estimated that the... 

Drafts, condensation on windows, ice damming, excessive noise from outdoors or equipment operation, and rooms that are cold in winter and hot in summer will diminish comfort in a home. Air-sealed construction, improved insulation, high-performance windows, right-sized, efficient hcating/cooling distribution systems, and mechanical ventilation commonly found in energy-efficient homes all work together to effectively eliminate these problems. 

However, simple-case type solutions (which are often good enough to establish whether mechanical ventilation is required or excessive air infiltration problems may exist) can be identified. These simple solutions are based on knowledge of ... 

Mechanical ventilation See Powered ventilation. Natural ventilation A ventilation system in which air movement is produced through purpose-designed openings by natural forces (wind and thermal buoyancy). Occupational Exposure Standards OES) Limits of concentration of airborne toxic contaminants, listed by the Health and Safety Executive which are regarded as safe for prolonged exposure for 8 hours per day. 

Existing mechanical ventilation will affect the flow adversely ... 

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