Mass flow rate, fans

If the volume or mass flow rates are required in a round or rectangular ducts then one must take measurements at various locations across the duct. It is important that the Pitot traverse be conducted at least 8 duct diameters downstream or 2 diameters upstream of an obstruction such as fans, elbows, dampers, expansions, and so on. Figure 4.28 shows the locations for a Pitot tube traverse in a round or rectangular duct, based on centroids of equal area, for determining the volume or mass flow rate [28]. 

Thermal performance of a cooling tower depends on a specific mass fiow rate of air through the fill (pounds of dry air per minute), whereas the fan does its job purely in terms of volume (cubic feet per minute). Since the specific voliune of air (cubic feet per pound) increases with temperature, it can be seen that a larger volume of air leaves the tower than enters it. The actual cfin handled by the fan is the product of mass fiow rate times the specific volume of dry edr corresponding to the temperature at which the air leaves the tower. This volumetric flow rate is the Q used in the following formulas, and it must be sufficient to produce the correct mass flow rate or the tower will be short of thermal capacity. 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

The mass flows include fuel (F), oxygen (O2), liquid water (1), evaporated water vapor (g) and forced flows (fan). The chemical energy or firepower is designated as Q and all of the heat loss rates by q. While Figure 12.4 does not necessarily represent a fire in a room, the heat loss formulations of Chapter 11 apply. From Equation (3.48), the functional form of the energy equation is... 

The blast furnace operates continuously although the individual particles see a batch mode of reaction. The actual reaction conditions must be based on the batch reactor sequence for the particles since complete conversion is desired. This requires control of the mass throughput in the furnace, but primarily it requires accurate temperature control. Control of the solids is maintained at the bottom discharge port. Gas flow rate is controlled by blowers or by a stack discharge fan. 

The needed rate of mass flow of drying air increases when its temperature decreases, but the needed heating power increases less. According to Fig. tS, the needed massflow of drying air increases by 140%, when its temperature decreases from 90 to 40 °C. That is because of the ability of air to contain water vapour decreases. At the same time the needed heating effect of air decreases 8%. The increasing rate of mass flow of air increases the power input of fans. 

With a slow-drying impermeable species a fan speed as low as 1.5-2.0 m s may be sufficient as the moisture eontent at the surface drops quickly below the fibre saturation point - no mass flow - and there is no point in installing overly powerful fans just to strip off surface moisture for the first few minutes of a long kiln schedule (>14 days) thereafter the slow rate of transfer of moisture from the centres of the boards to their surfaces becomes more important than the rate of evaporation. 

The flow rate can be determined by means of an orifice meter. Prior to beginning the measurement, the fan is started and the desired airflow adjusted by a throttle valve the tan-perature and moisture content of the air are controlled by means of an electric heater and a steam valve. In the next section the development of an adequate value of flow rate may be ensured by means of screens. The time-varying mass of the sample to be dried is measured continuously and recorded. 

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