ASHRAE Journal - July 2011 - 46

fan. The flow through these openings is the same and creates a velocity in each of these openings. The motor, usually an electromotor, delivers energy to the fan impeller and the impeller transforms the energy into the energy in the air handled by the fan. If we define control surfaces at the inlet and outlet openings, the energy in the air delivered by the fan is the difference between the energy of the air in the outlet opening minus the energy in the air in the inlet opening of the fan. This energy has two components, static and dynamic. Since the flow is equal for both inlet and outlet, we may define the energy addition by the change in pressure between the two openings. The fan total pressure is then defined as the difference of the total pressures in the fan outlet and inlet openings. The relations among these pressures are depicted in Figure 1, Page 45. These pressures are defined in ISO 5801 and ANSI/AMCA Standard 210-07/ANSI/ASHRAE 5107, Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating. Any other terms such as total static pressure, external static pressure and external total static pressure are not defined and, therefore, should not be used. The fan static pressure as it is defined generally cannot be measured by fan testing except for a fan having the same air velocity in its inlet and outlet openings. In such case, the fan static pressure is equal to the static pressure rise across the fan.

4.0 3.5 3.0 FTP FSP (in. w.g.) , 2.5 2.0 1.5 1.0 0.5 0.0 7 13,625 8 9 10 11 12 13 14 15 16 Flow (cfm in thousands) FTP System Curve FSP 1,035 rpm 6.22 6.05 Power

8 7 6 5 4 3 2 1 0 Fan Power (bhp) Fan Power (bhp)

Figure 2: Fan selection made by matching the pressure drop across the system with fan static pressure.
4.0 989 rpm 3.5 3.0 FTP FSP (in. w.g.) , 5.29 2.5 Power 2.0 1.5 1.0 0.5 FTP System Curve 4 3 2 1 5 7 6 8

Using Fan Static Pressure for Fan Selection

First, we must stress that the fan performance is FSP 0 0.0 usually presented in terms of FTP vs. flow or FSP vs. 7 8 9 10 11 12 13 14 15 16 flow. In Figures 2 and 3 both presentations are used Flow (cfm in thousands) simultaneously for a typical utility centrifugal fan size 24 in. (0.6 m). The requested operating point of the Figure 3: Fan selection made by matching the pressure drop across the system is 13,000 cfm (6135 L/s) at 1.5 in. w.g. (374 system with fan total pressure. Pa) pressure at standard air density. In Figure 2 the system pressure drop has been matched with FSP, values of flow, static pressure, speed and power. Conversion of and it was found that the fan speed would be 1,035 rpm, and the fan these data to total pressure is simple (the velocity pressure is would need a power of 6.22 bhp (4.64 kW). But, in reality, the fan calculated from the flow and outlet area), but it is assumed that at that speed would operate at a flow of 13,625 cfm (6430 L/s) at a the connection of the fan to the system is such that the conversystem pressure drop of 1.65 in. w.g. (411 Pa) The flow in the sys- sion of velocity pressure to static pressure at the outlet of the tem is about 5% higher, and the power need is 6.05 bhp (4.51 kW). fan is identical to the conversion experienced in the test set-up In Figure 3 the system pressure drop has been matched with from which the data were derived. The use of total pressure FTP, and it was found that the fan speed is 989 rpm, and the makes this assumption obvious, while the use of static pressure fan would need a power of 5.29 bhp (3.95 kW). The flow now for selection, while matching the flow to the required value, matches the requirement, the fan speed is down by almost 5%, does not call into question the validity of the selection. and the power is down by 12.5%. The fan total pressure is simply the difference between the The benefits of the correct match are lower fan speed, lower total pressure at the outlet and the total pressure at the inlet. sound and significantly lower power. Fan static pressure is defined to be fan total pressure minus We can see that the selection of fans must be done by the outlet velocity pressure. Equating the fan static pressure to matching the fan performance (total pressure vs. flow) with the “total static pressure” will not result in a proper selection. the system requirement. Many fan manufacturers present perAs an example, consider a blow-through system using a pair formance data in the form of multi-rating tables, which include of axial fans whose outlets are connected to a plenum contain46 ASHRAE Journal ashrae.org July 2011
ASHRAE Journal - July 2011

Table of Contents for the Digital Edition of ASHRAE Journal - July 2011
