General operating conditions

Everything about a fan application is important. Potential pur­chasers should not make value judgments on what information is, or is not important. It may well be that the missing data will devalue the manufacturer’s warranty. Guarantees are given for specified operating conditions. If there are operating conditions which have not been included in the specification, then the warranty does not cover them.

Whilst the majority of fans handle “clean” ambient air, others will be subject to hazards which may even be experienced when the fan is stationary—the fan can be a “heat sink” or may still be subject to corrosion.

Engineers completing or filling in fan data sheets, where these are part of a manufacturer’s questionnaire, may be over cau­tious. They may think that they could be “caught out” and thus read too much into the questions. Data sheets often request the “discharge pressure”; the data sheet does not say “all pres­sures up to and including”. The range of operating variables is critically important. Concentrating on maxima is a mistake; the full range of values is important.

Air/gas properties and operating conditions

To consider any type of fan for an application, the following in­formation must be given:

• flowrate

• temperature

• barometric pressure

• fan pressure

• air/gas composition

• solids or other contaminants.

For many applications, such as ventilation or comfort air condi­tioning, the fan may be handling normal ambient air with a mini­mum of foreign matter—in which case say so! Fans completely suitable for such clean air applications may last less than 20% of the time predicted if abrasive or corrosive elements are present.

The properties of the air/gas are of critical importance for se­lecting the correct type of fan. The density of the air/gas varies with temperature and barometric pressure, whilst the pressure developed by the fan and the power absorbed is directly propor­tional to this density. Sometimes the required fan pressure has been calculated under “standard air conditions” which do not exist. It is necessary to be specific and to know whether a con­stant mass flowrate of a constant volumetric flowrate is required.

The corrosive or erosive properties of the mixture being han­dled will be important for determining the correct type of fan. If the most suitable material for the manufacture of the fan cannot be cast or welded, then this may restrict the choice of fan types.

Any variations in the operating conditions must be quantified. For any specific variation, the other conditions must be stated. The duration of expected running should also be specified. Changes in operating conditions can take place relatively slowly or very rapidly. It is important to know which is the case. If very slow then instability or surging is less likely to occur. Rapid temperature changes can create distortion or cracking. It is possible to crack even thick casings due to thermal shock. Any rapid changes in operating conditions must therefore be identified.

The duty cycle

For fans running continuously, it is desirable to specify the Mean Time Between Failures, MTBF.

Chemical processes, oil refineries, offshore platforms, power utilities and even many comfort air conditioning applications, all have fans which are required to operate for 25 or 50 weeks with­out a shut-down. Other fans, however, may only be required to operate for shorter periods.

The following operating descriptions can then be useful:

— Continuous — over 8 hours running in any 24-hour period

— Light — 3 to 8 hours running in any 24-hour period

— Intermittent—up to 3 hours running in any 24-hour period

— Irregular—the fan operates for differing times with various periods of extended rest between operations

— Cyclic — the fan operates with a set pattern of rest or un­loaded operation followed by a period on-load

The definitions given above are based on experience. They are not standardised but do appear from time to time in specifica­tions, especially in the oil industry. The important factor is whether the equipment warms up fully. This is critical for fan and motor bearings and for motor windings. Where parts “grow” un­der the influence of gas temperature, this may also be impor­tant. If equipment runs long enough for all temperatures to sta­bilise then it is considered to run “continuously”. Small equipment can warm up continuously but large equipment may take much longer. In extreme cases even 8 hours running may be insufficient for all temperatures and dimensions to stabilise. More costly, slower equipment can, through higher availability, reliability and maintainability (the well-known “ARM” in military terminology) pay for itself very quickly from increased plant output.

Flow variations

Variations in the required flowrate determine whether the appli­cation should be shared by several fans. The system curve, (see Chapter 5) of flow versus pressure then determines whetherfans in series or parallel are preferable. Whilst identical fans are usually desirable in the latter case, it may indicate where unequal-sized units are possible.

Variations in flowrate demand and the consequential change or otherwise, of the system pressure, determine the type of flow regulation to be used (see Chapter 6). In this context, the work­ing time of the fan is of considerable importance. The efficiency of the fan and its driver etc., at all the operating conditions, must be used to calculate the total energy requirement kWh/annum. As stated many times in this book, the energy costs of running a fan can be as great as its purchase price after only a few months of operation. The cost of the energy supply must, therefore, be closely calculated. It may be that alternative energy sources are indicated.

Flow should be considered from both long term and short term points of view. Different regulation methods, with different initial costs and running efficiencies can be applied, depending on the frequency of the changes.

The effects of system pressure changes on the fan flow may be important. A fan with a steep pressure/flowrate characteristic (such as a narrow backward bladed centrifugal fan) may have only a small variation in flowrate over a wide range of pres­sures. This can simplify overall flow control methods.

Operating conditions can alter as the installation wears, cor­rodes or fouls. Changes to the fan unit, from “as new” to “pro­gressive ageing” may be used to optimise performance at dif­ferent duty points.

The size, nature and concentration of any particles are also of course, important factors. Hard abrasive solids will generally have a much more serious effect on fan life and efficiency than soft deformable solids such as wax. The abrasive properties of hard solids can be quantified by testing such as the Miller Number Test.

Fans handling solids

When handling solids either as dusts or larger pieces, the differ­ence between abrasive and non-abrasive materials, should be recognised. The Miller Number Test, developed initially for re­ciprocating pumps, has sometimes been used, for classifica­tion purposes, of the dusts handled by fans. Stauffer of Escher Wyss conducted modified tests using a sand/water mixture with various metals, The relative abrasion resistance, taking a case hardened carbon steel as unity, were as found in Table 20.1

Material

Relative abrasion resistance

Cast iron

0.09

Ni Al bronze

0.12

Al bronze

0.13

Carbon steel 195 BHN

0.22

316 L stainless steel

0.26

13Cr steel 441 BHN

0.32

27 Cr cast iron

1.00

Ni-Hard

0.89 to 1.11

Stellite 6

0.83 to 3.31

Tungsten carbide

1.39 to 4.12

Hard chrome plate

2.06 to 2.28

Table 20.1 Relative abrasion resistance to a sand/water mixture

Materials which can be considered as abrasive when trans­ported uy air or some other gas are:

• Al203 alundum

• coal dust (pulverised fuel)

• copper concentrate

• fly ash

• Fe208 limonite

• microsphorite

• magnetile

• phosphate

• FeS2, CuFeS2 pyrites

• sand

• serpentine (magnesium silicates)

• sintered materials

• tailings

• tar sand

The particle size and concentration is of course of great impor­tance in determining whether a particular fan will be suitable for a given application. In a homologous series of fans it is possible to build up a database of impeller lives. By the use of the for­mula given in Equation 20.1, it is then possible to predict the life of another fan at another speed. For worthwhile results, many fans, many applications and many speeds are essential to have confidence in the predicted lives.

K x103

Impeller life hours =—-— Equ20.1

Where:

K = 50

D = gm dust / kg gas

V =25

U = impeller peripheral velocity (m/s)

Where 100 > u > 40

N = an index dependent on the type of dust

E. g:

N = 1.0 for a non abrasive soft dust such as

Zinc oxide

N = 1.75 for limestone and cement

N = 2.5 for sinter dust

By a series of experiments it is possible for the manufacturer to arrive at similar equations (other values for k, v, etc.) for his own particular product range. The safe life would typically be set at 75% of the fan hours to catastrophic failure.

Posted in Fans Ventilation A Practical Guide


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