Pressure coefficients

The pressure coefficients can be related to the volume coefficients by reverting the series and one finds that... 

Proton magnetrogyric ratio Yp Relative pressure coefficient p... 

Factory Mutual provides loss prevention data sheets that explain how to protect buildings from wind damage. Pressure coefficients that define increased uplift at corners and edges adjust the calculated uplift pressures. A laboratory uplift pressure test rates roofing assemblies. An uplift pressure of 2.9 kPa (0.42 psi) must be withstood under FM conditions to meet the Class 1-60 requirements. The FM approval guide is revised aimuaHy (37). 

The pressure coefficient is the pressure or pressure rise expressed in dimensionless form... 

Flow coefficients and pressure coefficients can be used to determine various off-design characteristics. Reynolds number affects the flow calculations for skin friction and velocity distribution. 

Round off the number of stages to the nearest whole number, then recalculate the pressure coefficient, X, using... 

If the pressure coefficient just calculated is within 5 % of the original value of 0.29, then proceed using the calculated pressure coefficient and the assumed value of mean blade velocity as the final value. Continue to the speed calculation. If the pressure coefficient is higher than 5 %, add an additional stage to the compressor and again calculate the pressure coefficient using Equation 6.16. 

If the pressure coefficient is now, or was in an earlier step, 5% uiutei the 0.29 value, calculate a new mean blade velocity using the rounded off number of stages and the original pressure coefficient, 0.29. Use the calculated blade velocity in the subsequent step for compressor speed Calculate the speed. 

Then using Equation 6.12 and the pressure coefficient, p =. 29 and 720 fps given in the chapter. 

Pressure at a given location on the building surface is usually expressed as a pressure coefficient times a reference wind pressure at the building height without the building in place ... 

Determination of the pressure coefficient, Cp, for the particular location on the building... 

The second part of computing building pressures involves the pressure coefficient for a particular spot on the building. The surface pressure coefficient, Cp, indicates the share of the wind kinetic energy that is transferred to the static pressure ... 

For a building with sharp corners, Cp is almost independent of the wind speed (i.e., Reynolds number) because the flow separation points normally occur at the sharp edges. This may not be the case for round buildings, w here the position of the separation point can be affected by the wind speed. For the most common case of the building with a rectangular shape, Cp values are normally between 0.6 and 0.8 for the upwind wall, and for the leeward wall 0,6 < C, < —0.4. Figure 7.99 and Table 7.32 show an example of the distribution of surface pressure coefficient values on the typical industrial building envelope. 

Values of Cp for simple building geometries may be obtained from the British Standards Institution or from Liddament. The following relationship between wind incident angle a, building side ratio, and average surface pressure coefficient is based on the database developed by Swami and Chandra ... 

FIGURE 7.99 Example of surface pressure coefficient values for a typical industrial building envelope. 

TABLE 7.32 Approximate Surface Pressure Coefficient Values for a Building with a Rooftop Vent... 

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. 

Principles of wind speed and surface pressure coefficient evaluation were covered earlier. 

Pressure coefficients over the building envelope for the wind directions under consideration... 

The airflow network (Fig. 11.41 is composed of nodes, interconnected by links, representing individual airflow paths. Internal nodes represent the individual zones of the building, and external nodes represent faqade locations, related to a specific set of wind pressure coefficients. Each link represents a specific airflow conductance type. 

An external node represents a boundary node, i.e., a fixed pressure value or a location on the building facade which is linked to a specific set of vvind pressure coefficients for this location (a set of values for different wind directions Q ). The pressure at such a location e is then given by wind velocity at reference level p, air density) ... 

Airflows are determined basically by a steady-state calculation for each time step. At each time step, first, pressures at external nodes are calculated on the basis of the wind pressure coefficients and the actual wind speed and direction. Then, for all conductances, the local pressures at each side of the link are calculated. At internal links, this pressure is dependent on the (unknown) zone pressure p and the aerostatic pressure variation due to the height of the link with respect to the zone reference height. At external links, this pressure is dependent on the external node pressure and the aerostatic pressure variation due to the height of the link with respect to the stack reference height. For the aerostatic pressure, the air density is determined considering the temperature, the humidity, and (if relevant) the contaminant concentrations in the zone or in the outside air, respectively. From this, the pressure differences across each conductance can be calculated, and from this the mass airflow tor each conductance /. 

Wind pressure distribution Set of wind pressure coefficient data for each external node... 

Data are available only for simple building geometries. In Allard," a tool for the calculation of wind pressure coefficients for simple geometries is made available, and another tool is described in Knoll et al. Existing wind pressure data have to be examined carefully, because many data represent peak pressure values needed for static building analysis. Real cases with obstructions and buildings in the close surroundings are difficult to handle. Wind-tunnel tests on scale models or CFD analysis will be required. 

Nevertheless, in many cases, mean wind velocities can be assumed. In ventilation-system reliability studies, e.g., where minimum ventilation rates are to be determined, a calm situation with little wind must be assumed anyhow, and the need for accurate wind pressure coefficient data is not so obvious. 

The factory is modeled as a two-zone network with door, horiztmtally pivoted windows, and roof shed windows as airflow elements. The extract tan and the duct and hood are modeled as additional airflow elements. Wind pressure coefficient data are taken from literature for a simple rectangular buihl-ing shape surrounded by buildings of equal height. 

Figure 12-461, Part 3. Stage performance of a compressor is usually represented in a pressure coefficient, n, or M, and efficiency, t), versus Q/N (capacity vs. speed). A given impeller stage design will have a different characteristic depending on the relationship of its operating speed to the inlet sonic velocity of the gas. For higher ratios of speed to sonic velocity, N/A , the head or pressure coefficient curve will be steeper at flows higher than the design. (Used by permission Bui. 423, 1992. Dresser-Rand Company.)...

Stage performance of a compressor is usually represented In a pressure coefficient (Mu) and efficiency (Eta) vs. Q/N (Capacity vs. Speed)... 

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