Occupied zone

The main controlled zone is normally a large area, which is often the same as the occupied zone. 

The operative temperature at all locations within the occupied zone of a space should at all times be within the permissible range. This means that the permissible range should cover both spatial and temporary variations, including fluctuations caused by the control system. 

The total radiant component of the heat load introduced by each source into the space can be divided between the upper and the lower (occupied) zones of this space ... 

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.). 

Current mixing-type air distribution methods typically consider ventilation of the occupied zone with jets intercepting its upper boundary (e.g.. Fig. 7.6l2, c). Also, the occupied zone can be ventilated by the reverse flow produced as the supply jet degrades above the occupied zone level (Fig. 7.6h]. Mixing-type air distribution methods include air supply with jets projected vertically downward, inclined jets, jets directed vertically upward, and horizontal jets along room surfaces. 

FIGURE 7.7 Schematics of air supply (o) with inclined jets toward the occupied zone (b) with horizontal jets and occupied zone ventilated by reverse flow (c) with vertical jets. Shaded areas show the effect of buoyant forces on airflow pattern when supply air is excessively heated over the room air" ... 

One of the unidirectional flow system modifications is air supply through diffusers located above the occupietl zone. The supply air temperature is lower than the desired room air temperature in the occupied zone, and air velocity is lower compared to a mixing-type air supply, bur higher than for a thennal displacement ventilation. Polluted air of the occupied zone is suppressed by an tiverlying air cushion that displaces the contaminated air toward floor-level exhausts (Fig. ". 12). 

FIGURE 7.12 Unidirectional flow system with air supply through diffusers located above the occupied zone. ... 

The influence of room transverse cross-section configuration on airflow patterns created by air jets supplied through round nozzles in proximity to the ceiling was studied by Baharev and Troyanovsky and Nielsen (see Fig. 7.37). Based on experimental data, they concluded that when the room width B is less than 3.5H, the jet attaches to the ceiling and spreads, filling the whole width of the room in the manner of a linear jet. The reverse flow develops under the jet. When B > 4H, the reverse flow also develops along the jet sides. Baharev and Troyanovsky indicated that air temperature and velocity distribution in the occupied zone is more uniform when the jet develops in the upper zone and the occupied zone is ventilated by the reverse flow. Thus, they proposed limiting room width to 3-3.5H,. 

To avoid high velocities in the occupied zone due to direct effect from the supply air jet, and to increase the length of the effectively ventilated zone for a single jet in rooms with height from 4 m to 10 m, Baharev and Troyanovskyi proposed supplying air from the height bg = 0.6-0.7H,. ... 

If the width of the jet (calculated for free conditions) at the point of its intercept with the occupied zone exceeds the room width, side walls transform the jet as if it was formed by the linear jet impingement on a floor. 

The latter information is important in evaluating the size of the rxrcupied zone that can be effectively ventilated by inclined jets. It was proposed that the occupied zone of rooms is well ventilated by inclined jets (particularly in industrial rooms with contaminant release) if air velocity in the occupied zone exceeds 0.1 m/s. 

Air supplied in confined space by downward vertical jets creates a similar flow pattern as in the case of air supply by horizontal nonattached jets. With vertical air supply, the occupied zone is ventilated directly by air jets. Grimitlyn suggests that the area of occupied zone ventilated by one jet be sized based on the jet s cross-sectional area at the point it enters the occupied zone. The jet cross-sectional area and configuration depend upon the height of the air supply, the type of air jet, and diffuser characteristics ( K, and K, ). 

Joint solution of Eqs. (7.170) and (7.178) allows one to calculate the maximum amount of heat supplied by a directing jet with the assumption that the jet reaches the occupied zone > 0-1 m/s) and (fgp -fgP)/Tgp is less than 0.2 at the point where it enters the occupied zone. The maximum initial temperature difference of the air supplied by vertical directing jet is... 

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,... 

The air exchange rate, G , required for temperature control in the occupied zone can be calculated from the room heat balance equation ... 

In the current review, the term effectiveness of air distribution will be used to describe the ratio of the occupied zone area (where thermal comfort and contaminant concentration are within ranges required by standards and codes) to the total occupied zone area. This hygienic criterion allows one to judge how well the HVAC system fulfills its main task—creating thermal comfort conditions and controlling contaminants in the occupied zone. 

In the stratification strategy the supply air is used to substitute the outgoing air from the ventilated (in most cases occupied) zone, thus preventing circulation patterns between the zones. The supply air has to be distributed in such a way that the buoyancy flows are not disturbed. Exhaust air openings are to be located downstream in order to avoid reverse currents within the room. The location of the contaminant sources and the heat sources causing density differences must be the same in order to carry out the contaminants with equal or higher density than air. 

During summer. Fig. S. 6a, there is a need for cooling in the occupied zone (area up to 2 m from the floor level) rhus it is desirable to apply the stratification strategy with vertical temperature and contaminant stratification in the hall in order ro save cooling energy costs. This can be done, for example, by using a low-impulse air supply with the devices at the floor level. 

It the main reason for the stratification strategy is contaminant control in [he occupied zone, the same strategy should be applied in winter conditions, too. Thus, the selected hearing method has to fulfill two requirements to siip-pttrt the creation of the vertical stratification and not to create disturbing airflows into the hall. In this case one option would be the floor heating method see Fig. 8.16c. Additionally, one should consider the prevention of boundary layer flows along the outer walls using, for example, passive methods. -... 

All outlets must be positioned at such a height that they do not create drafts in occupied zones. 

Air iniakes positioned to ensure draft-free conditions in the occupied zone FIGURE 9.26 Upward ventilation, large fiall. 

It is essential that the velocity envelope in the occupied zone be in the range of 0.2-0.25 m s" to avoid drafts. However, in hot industrial environments, these velcKities are frequently exceeded in order to provide adequate body cooling. 

One of the commonly used ventilation parameters is ventilation effectiveness, and it shows how certain regions in the room are influenced by contaminant sources introduced into the room. Three definitions of ventilation effectiveness are often used, namely, the ventilation effectiveness in the occupied zone the local ventilation index and the mean ventilation effectiveness They are defined as... 

C) and Cgm denote the mean concentration in the occupied zone, concentration at a given point P, the mean concentration in the room, and the concentration at the outlet, respectively. To numerically simulate these parameters, the velocity field is first computed. Then a contaminant source is introduced at a cell (or cells) of a region to be studied, and the transport equation for contaminant C is solved. The transport equation for C is... 

Outside air entering the space through openings near the ground spreads over the floor and absorbs energy from the floor surface. The resulting air temperature increase leads to buoyancy and forces the air up into the upper hall zone. This results in a temperature stratification in the hall. Due to this vertical temperature gradient, the air in the occupied zone does not reach the exhaust air temperature (see Fig. 11.37). 

The relationships between air exchange rate and temperature difference were determined using COMB (Fig. 11.51) and then integrated as the ventilation model in the thermal model. The rhermai behavior is modeled with the TRNSYS multizone type, considering the hall and the room below the thick concrete test floor slab. For the hall, a room model with two air temperature nodes (one for the occupied zone and one for the rest of the hall) and geometrically detailed radiation exchange is used. 

The thertnal model consists of two zones (Fig. 11,54). The hall is modeled w ith an air node for the occupied zone and another air node for the rest of the hall. The main heat source in the hall is the insnlatiou through the glazed roof. Additional internal hear sources are rather marginal and therefore are nor considered in the simuiation. 

The thermal comfort was evaluated with hourly mean values of the air temperature in the occupied zone, plotted against the maximum I h mean outdoor temperature value of the day. Only the period from April 1 to October 30 and only working hours (7 a.m. to 6 p.m. are considered. 7 his evaluation method is based on the Swiss standard SIA V382/2. The minimum and maximum allowable comfort temperatures are adapted to the usual activity and clothing levels of the workers in the hall (see Figs. 11.55 and 11.56). 

The draft risk due to cold air pillows under the roof glazing dropping into the occupied zone was determined by transient CFD calculations. As can be seen from Fig. 11.57, velocities do not exceed 0.2 m/s. Therefore, the draft risk was assumed to be marginal. 

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