Normal quantities and units used in fan technology

The fan and air movement industries within the English speak­ing world have for many years used a number of specialised units. These have been based on the imperial system, adapted as necessary. Often an arbitrary choice was made, for example pressure measured in inches water gauge.

Table 22.11 gives a number of conversion factors designed to assist those who are unfamiliar with the magnitude of the SI units. They will also be useful in converting values from earlier textbooks, catalogues, and other data.

Quantity	Imperial unit	SI unit	Conversion factors
Volume flowrate	Cfm (ft3/min)	Cubic metres per second (m3/s)	4.7195 x 10-4
Cfm (ft3/min)	Litres per second (I/s)	4.7195 x 10-1
Cu sec (ft3/s)	Cubic metres per second (m3/s)	2.8316 x10 2
Pressure	Inches w. g.	Pascal (Pa or N/m2)	2.4909 x 102
Inches w. g.	KiloPascal (kPa)	2.4909 ХІ0’1
Inches w. g.	Millibar (mbar)	2.4909
Inches H. g.	KiloPascal (kPa)	3.3864
Power	Hp (bhp or ahp)	Watt (W or J/s)	7.4570 x 102
Hp	KiloWatt (kW)	7.4570 x 10-1
Torque (5)	Ibf-in	Newton metre (Nm)	1.1298 x 10-1
Ibf-ft	Newton metre (Nm)	1.3558
Density	Lb/ft3	Kilogram per cubic metre (kg/m3)	1.6018 x 10

Quantity	Imperial unit	SI unit	Conversion factors
Tip speed Outlet velocity or Duct velocity	Fpm (ft/min)	Metres per second (m/s)	5.0800 x 10’3
Fps (ft/sec)	Metres per second (m/s)	3.0480 x 10’1
Mph (miles/hr)	Metres per second (m/s)	4.4704 x 10-’
Rotational speed (2)	Rpm (rev/min)	Revolutions per second (rev/s)	1.6667 x 10*2
Dimensions	Inches	Millimetres (mm)	2.5400 x 10
Feet	Metre (m)	3.0480 x 10’1
Thou (mil) = 0.001 in	Micrometre ( m)	2.5400×10
Moment of inertia (6)	Lb-ft2	Kilogram metre squared (kg m2)	4.2140 x 10-2
Slug-ft2	Kilogram metre squared (kg m2)	1.3558
Stress (5)	Ibf-ft2	Pascal (Pa or N/m2)	6.8948 x 103
Ton f-in2	MegaPascal (MPa)	1.5444 x 10
Energy (work or heat equivalent)	Therm	MegaJoule (MJ)	1.0551 x 10-2
Hp hr (horsepower hour)	MegaJoule (MJ)	2.6845
Btu (British thermal unit)	KiloJoule (kJ)	1.0551
Ft-lbf	Joule (J)	1.3558
KW hr	MegaJoule (MJ)	3.6000
Temperature (3)	DF	Kelvin	(DF+459.67) 1.8

Table 22.11 Metric and Imperial conversion factors

Notes to Table 22.11:

1. The choice of the appropriate multiple or sub-multiple of an SI unit is governed by convenience. The multiple cho­sen for a particular application should be the one which will lead to numerical values within a practical range (ie kilo­Pascal for pressure, kilowatts for power, and megaPascal for stress).

2. The second is the SI base unit of time, although outside SI the minute has been recognised by CIPM as necessary to retain for use because of its practical importance. The use of rev/min for rotational speed is still therefore continued.

3. The Kelvin is the SI base unit of thermodynamic tempera­ture and is preferred for most scientific and technological purposes. The degree Celsius (°C) is acceptable for prac­tical applications.

4. Multiply Imperial unit by this factor to obtain SI Standard, except the Kelvin temperature.

5. Great care must be taken in the conversion of these units. In the Imperial system the pound force or weight Ibf (mass x acceleration due to gravity) was often loosely referred to as ‘lb’.

6. For reasons as in (5) above inertia was often given as

— k2 ie slug/ft2.

9

Multiples:

Name	Symbol	Factor
Micro		10-6
Milli	M	10-3
Kilo	K	103
Mega	M	106

Examples:

A 5712 c. f.m. = 5712 x 4.7195 x 10-4

= 2.6958 m3/s

20.6 x 4.7198 x 10‘1 9.7228 I/s 1.35 x 2.4909 x 102 336.27 Pa 40.6 x 2.4909 x 10’1 10.11 kPa

B 20.6 c. f.m. c 1.35 in. w.g. d 40.6 in. w.g. and so on.

The USA continues to use such units based on the Imperial sys­tem. In view of their dominant position in the air conditioning market, such units will have widespread currency for many years to come. It is essential, therefore, for engineers to be con­

	Versant with the conversion factors used for translating be­tween the two systems.

[1] Radial misalignment, where the shafts are parallel although not lying on a common centre line.

[2] +j Lo tan klc

It will be noted that the sound power in the discharge duct is a function not only of the outlet duct length and outlet terminating load, but also of the inlet duct length and inlet terminating load.

Bolton and Margetts have also looked at the influence of chang­ing duct configurations on the noise generated and concluded that, there was no way of estimating the inlet or outlet sound power for one particular installation category from tests carried out on another. Tests are, therefore, necessary in all four cate­gories from which it may be possible to identify those fan de­signs that are installation sensitive.

It will also be noted that it should be possible in a fully ducted sit­uation (Installation category D) to position the fan for the mini­mum noise at a desired observer location. Figure 14.20 shows the differences for a bifurcated for the same aerodynamic duty

Posted in Fans Ventilation A Practical Guide