Ventilation, natural

As part of a Process Hazards Analysis (PHA). I was required to check a naturally ventilated building containing electrical equipment and a fuel gas supply, for adequate air flow due to thermal forces (stack effect). API RP 500 has a method that they recommend for buildings of l,000fP or less. The building in question was much larger, because  

The building had exhaust fans so the calculation was only to check for natural ventilation when the fans were off. 

Perhaps the calculation would show far more natural ventilation than required. 

it was verified that the building ventilation geometry was to API Standards (no significant internal resistance and inlet and outlet openings vertically separated and on opposite walls). The building had many windows on both sides and 3-ft wide louvers at the roof peak. So the inlets were on both sides and the outlet at the top middle. This arrangement was judged adequate. 

H = Vertical distance (center to center) between Ai and A2, ft. This was weight averaged for various w indows and open doors. 

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Siting equipment which is hable to leak outdoors so that leaks of flammable gases and vapors are dispersed by natural ventilation. 

As part of PHA the author checked the natural ventilation system for an auxiliary electrical generator building that contained gas engine generators. The procedure to perform this API recommended practice can be found in Section 19—Safety-Natural Ventilation. 

Avoid other materials stacked near restricting natural ventilation. 

Controlled Airflow through an Envelope Principles of Natural Ventilation 587... 

Forced convection or supply air jets introduced into the room by mechanical or natural ventilation systems, or their combination (Section 7.4) ... 

The p coefficient value depends upon the supply air method (e.g., p = 0 with displacement and natural ventilation, P = 1 with convective plume dissipating within the occupied zone due to interaction with supply jets, airflows created by moving objects, etc.). 

Untreated outside air supplied by the mechanical or natural ventilation system... 

FIGURE 7.9 Influence of exhausced airflow on airflow pattern in the naturally ventilated room fo> airflow in the convective plume smaller than exhausted airflow (b) airflow in the convective pluime equal to the exhausted airflow (c) airflow In the thermal plume at the stratification level equal to the exhausted airflow (t, air temperature along the room height, t, - average room temperaoire)... 

This section will describe general features of airflow patterns and then present information on the dimensions and locations of recirculating (stagnant) zones around the building envelope, which determine wind pressures and contaminant dilution. This knowledge allows one to select the locations of stacks and air intakes and to calculate infiltration and natural ventilation rates. 

One of the effects of airflow or wind around buildings is the exertion of wind pressure forces on rhe surface of the building, which contributes to natural ventilation of the building and infiltration of outside air into the building. As discussed above, pressures tend to be positive (into rhe building) on upwind surfaces and negative (suction) on lateral, downwind, and roof surfaces. 

A detailed method of determining pressure coefficients is to perform experiments with a wind tunnel facility. Cochran and Cermak compared wind tunnel pressure coefficient measurements with field measures on a test building and found excellent results, with the exception of small areas beneath the vortices near the upwind roof corner for winds approaching at 45 . For infiltration and natural ventilation designs, wind tunnel results should be sufficiently accurate. 

Natural ventilation is the controlled flow of air through doors, windows, vents, and other purposely provided openings caused by stack effect and wind pressure. Natural ventilation is used in spaces with a significant heat release, when process and hygienic requirements for indoor air quality allow outdoor air supply without filtration and treatment. Natural ventilation cannot be used when incoming outdoor air causes mist or condensation. Natural ventilation allows significant air change rates (20 to 50 ach) for heat removal with ntinimal operation costs. 

Natural ventilation design allows one to size the inlets, and outlets, / p based on their pressure loss characteristics, Cp, and on the airflow rate, G , required to maintain the occupied zone within desired limits. The reverse design procedure is commonly used to evaluate the airflow rate through the building given the sizes, characteristics, and locations of inlets and outlets and the heat load and characteristics of heat sources. 

The use of a natural ventilation system assumes temperature stratification throughout the room height. Air close to heat sources is heated and rises as a thermal plume (Fig. 7.105). Part of this heated air is evacuated through air outlets in the upper zone, and part of it remains in the upper zone, in the so-called heat cushion. The separation level between the upper and lower zones is defined in terms of the equality of and G, which are the airflow rate in thermal plumes above heat sources and the airflow supplied to the occupied zone, respectively. It is assumed that the air temperature in the lower zone is equal to that in the occupied zone, and that the air temperature in the upper zone is equal to that of the evacuated air,... 

FIGURE 7.105 Schematics of naturally ventilated space /, air inlet 2, exhaust vents. 

Another factor influencing contaminant and heat transfer from dirty to clean zones against the stable airflow is a turbulent exchange between these zones. This process should be considered in the design of displacement or natural ventilation systems and evaluation of the emission rate of contaminants from the encapsulated process equipment (Fig. 7.111a). 

B.S. 5925. 1980. Code of Practice for Design of Buildings Ventilation Principles and Designing for Natural Ventilation. British Standards Institution, London. 

Swann, M. V., and S. Chandra. 1988. Correlations for pressure distribution on building.s and calculation of natural-ventilation airflow. ASHRAE Transactions, vol. 94, no. 1, pp. 243-266. 

Etheridge, D. W. 1977. Crack flow equations and. scale effect. Build. Environ, vol. 12. de Gids, W. E 1978. Calculation method for the natural ventilation of buildings. Verwarming Vent, no. 7, pp. 552-564. 

Utilization of natural ventilation for air conditioning will normally interact with the architectural design, and it should be considered early in the design process. 

P. Cooper and P. F. Linden, Natural ventilation of an enclosure containing two buoyancy sources, Journal of Fluid Mechanics, 1996, 311, 153-176. 

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. 

The term natural ventilation relates to the airflow in a building that is caused by three natural factors ... 

At least three factors must be considered when using Eq. (10.132). The first is that the room air is seldom well mixed the second is that the emission rate is usually unknown and the third is that measurement of is often very difficult, especially in spaces where a mixture of mechanical and natural ventilation is used. If the room air is poorly mixed, a sampling strategy is needed in order to determine the room air concentration (C,). There exist different strategies for this, and two are as follows ... 

Air temperature in rooms imposed by building dynamics and ventilation, nighttime cooling, cooling by natural ventilation... 

In an actual design, thermal modeling (Section 11.3) for diffetent seasoii-s will come fitst to. set tempetatute boundary conditions. Multizone aitflow simulation (Section 11.4) will follow to define ventilation needs in each zone. For large enclosed space.s, for natural ventilation, and for a variety of other special problems, CFD (Section 11.2) and integrated modeling (Section 11..S) are applied. 

Airflow through openings between rooms or between inside and outside (e.g., natural ventilation or flow through process-related openings to outside)... 

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