Fans operating at non-ambient temperatures
Figure 8.15 Labyrinth seal
Figure 8.16 Labyrinth seal with annular springs
Figure 8.17 Labyrinth seal with floating bushing
Better tightness can be achieved with a floating bushing. The carbon rings are made in two or three parts which are kept closely to the shaft with annular springs (Figure 8.16). Afloating bushing as shown in Figure 8.17 can also be used.
If the fan operates at a high pressure, ordinary packing may be unsatisfactory. Some form of mechanical seal must then be employed. Atypical example is shown in Figure 8.18.
In this design a collar is attached to the shaft by a setscrew. The position of the collar causes the compression springs to exert a
Calculation of the duty requirement
Whilst not exactly a special feature it is convenient at this point to say something about the calculation of the required fan performance.
When fans handle air or some other gas, which has a density differing from the standard 1.2 kg/m3 then performance will vary in accordance with the Fan Laws (see Chapter 4). Thus at a constant volumetric flowrate, the pressure developed, the weight flowrate and the power absorbed will all vary directly with the density of the air or gas being handled. Fan efficiency remains unchanged.
Afan being essentially a “constant-volume” machine, it is necessary to know how the duty requirement has been calculated.
A) Fan flowrate must always be converted to the actual conditions at the fan inlet. Does the customer require the same volume or weight flow?
B) It is important to know under what conditions the fan pressure has been calculated. How will this vary with temperature?
C) Will the fan be required to start on cold air? Is there a need for dampers to assist?
D) Find out the maximum temperature reached during operation — there may be a heat build-up.
An understanding of these rules is important for correct fan selection, determining the correct operating speed where this is variable and also to determining the power consumption over the duty cycle.
Mechanical fitness at high temperature
The strength of metals and plastics varies according to their temperature. When handling air or gas at conditions other than ambient the materials of construction of the fan will therefore also vary from the values normally given in textbooks.
It is important to remember that all elements of the fan must be satisfactory:
Elements within the air or gas stream are likely to take up the same temperature, but elements outside may take up a temperature somewhere between that of the gas stream and the ambient air around the fan.
For detailed methods of calculation to determine material suitability refer to Chapter 7.
Maintaining the effectiveness of the fan bearings
It is important that the “temper" of the balls or rollers is maintained. Normal greases are likely to break down at temperatures above about 90°C. For these two reasons it is essential to reduce the amount of heat which is transmitted from the gas stream, along the shaft to the first bearing. There are a number of ways in which this objective may be achieved.
A) The first and most important method is to add an auxiliary cooling disc to the shaft between the casing and inner
Figure 8.19 Belt driven centrifugal fan with air cooled bearings
Figure 8.20 Fabricated plug type fan with internal shrouded copper cooling impeller
Figure 8.21 Plug fan for the glass industry
Bearing. With a simple aluminium bolt-on construction having six open radial blades this extends the operating gas temperature from 75 °C to a maximum of 350 °C as heat is dissipated from the shaft and the temperature at the bearing reduced to less than 90 °C, see Figure 8.19. A more sophisticated shrouded copper impeller has been used with d) below for gas temperatures up to 650 °C. This is just visible through the mesh in Figure 8.20.
B) At higher temperatures water-cooled sleeve bearings may be used. The water ensures that the oil lubricant does not become too thin and also that the white metal babbit does not melt. (See Chapter 10.)
C) Spacer couplings which make a heat “break” in the shaft may also be used above 400 °C. Shaft slots have also been used.
D) Insulated “plugs” on the drive side are typically used above 500 °C to minimise problems from radiated heat, (see Figure 8.21).
Bearings are manufactured with various grades of clearance between the rotating elements and the raceways, the normal clearance being designated CN. Table 8.2 gives typical details of the grades available, it being noted that C3, C4 and C5 have clearances greater than normal. Whilst C3 bearings are commonly used where the product of bearing size in mm and rotational speed in rev/min exceeds 175 000 to dissipate frictional heat, C4 or C5 may be necessary with fans handling gases at up to 650 °C.
These may require the ability to withstand loads externally applied at high temperatures due to the expansion of the customer’s ducting. A preferable alternative is to provide high temperature flexible connections on the fan inlet and outlet and to ensure that clients separately support their ducting.
The casing itself will expand, growing up from its feet. As the pedestal will be cooler, this may destroy the clearances between inlet cone and impeller eye or shaft and shaft entry point. The growth is a function of temperature and size. Clearances of inlet cones and at shaft entry may then need to be increased above about 350°C. At temperatures above about 450°C it is common to support the fan casing near its centreline so that growth of all parts is radially outwards and clearances are not affected.
Table 8.2 Typical radial internal clearance of deep groove ball bearings
Where oxygen is present in the gases, “scaling” of a mild steel case will take place above 400°C at increasing rates to 500 °C where it becomes catastrophic. COR-TEN® steel and other
proprietary grades, which have a copper content, scale at a slower rate. Information is available from the manufacturer on the rate for these and many other steels.
As an alternative, the casing may be “aluminised”, which effectively eliminates the problem. Above about 570 °C stainless steel casings are usually necessary from scaling, strength and stability considerations.
It should be noted that scaling will not occur if the gases are inert e. g. nitrogen. Flue gases may be inert under conditions of perfect combustion, i. e. do not contain oxygen in its free form.
European legislation now covers the maximum safe temperature for surfaces which may come into contact with the hands or other parts of the human body. It may also be desirable for efficiency reasons to limit the amount of heat which may be dissipated from the casing. In these cases, lagging cleats should be added to assist in the anchoring of insulating materials.
Mechanical fitness at low temperature
There are no real problems with gas temperatures down to about -30 °C but allowance must be made for the power increase due to the higher air density. Below -40°C mild steel becomes increasingly brittle. It may be necessary to use an aluminium impeller or steel with high nickel content. Shafting should also be of nickel steel whilst bearing plummer blocks must be cast steel (not cast iron). Grease lubricants should be checked for suitability — they must not solidify or separate.
Posted in Fans Ventilation A Practical Guide