Partial optimisation
The two major costs for many fan plants are the costs of ducting on the investment side and the energy costs on the operational side. The investment costs increase with increased duct diameter whilst at the same time operational costs decrease, see Figure 19.7.
The total costs curve represents a minimum for a certain duct diameter. As indicated, the shape of the total costs curve is
F. 2 Aqv |
Ef = |
1 + 3 |
Pfm-t |
3fLp 2 Pl _ _5j5 — clv ® |
32ti d |
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Figure 19.8 Annual costs for ventilation plant using various duct diameters |
Figure 19.7 Effects on costs of duct diameter |
Equ 19.23 |
Q»i = Where Qvi K D TlO 3f P F T Ke |
Such that it rises more rapidly with reduced diameter, to the left of the economic diameter, than in cases of increased diameter, to the right of the optimum value. In cases of doubt choose the one having the largest duct diameter from the feasible alternatives. By calculating the total costs for a number of different duct diameters it is possible to establish the most economic duct diameter. The economic diameter varies for different situations. Short operating periods and costly duct sections, for example, stainless steel, tend to reduce the economic duct diameter and to increase the economic flow velocity. By carrying out a partial optimisation, i. e. if it is assumed that all costs are independent of the duct diameter except for the costs of the ducting and energy, the following relationship is obtained: |
The mean flow for which duct diameters d, and d2 give the same annual cost (m3/s) Cost/m of ducting (currency/m) Duct diameter (m) Fan system overall efficiency factor (decimal) Annuity factor (decimal) Liquid density (kg/m3) Pipeline loss coefficient Operating time per year(hour/annum) Cost of energy (currency/kWh) |
K2 — k, ri0-7t ■ aF — 10 1 d? 1 d”’32 • p • f • t • ke |
Pim Where |
T-k. F+K,, |
«P2mtkeF+Kl2 |
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Where
K = damper cost including connectors for duct diameter d.
It should be noted that an improved fan efficiency factor tends to reduce the economic duct diameter. (See Figure 19.8.)
The efficiency of other components can be tested in the same way. Conversion to electrical power used must always be made, however. The closer the power consumption lies to the useful air power in the conversion chain, the greater the energy saving and the greater the motivation for additional investment. This is illustrated typically in Figure 19.10.
E P* >i O C O O E © W S O S |
Energy el |
Ficient |
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Standard |
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4′ |
80 |
60 |
40 |
20 |
0.9 |
0.8 |
0.7 |
1/4L 1/2L 3/4L Fraction of full load |
FL |
Figure 19.10 Typical efficiency and power factor values for a 37 kW 4 pole motor
In the case of existing plant there are certain limitations to energy saving possibilities. Changes to the layout and ducts are often difficult to make. Possible improvements are replacements or the addition of supplementary components which do not require too great a disturbance to the plant.
All reductions of the air power, by means of improvements which reduce the system resistance in the case of a regulated plant, only result in a greater pressure loss across the damper and are thus worthless from an energy conservation point of view. To realise a saving of energy some form of improvement must also be carried out on the fan side For speed-regulated plant all pressure reductions are automatically used.
When planning a project at the time of determining fan specifications, it is often the case that not all of the plant details are known. It is not therefore unnatural to choose fan sizes with a certain safety margin “to be on the safe side”. For this reason many fans are oversized and result, if adjustments are not made, in unnecessarily high energy costs.
In the case of basic oversizing, the following alterations to the fan may be considered:
For centrifugal fans:
• a new impeller with reduced diameter
• a new impeller with reduced width
• a new impeller with reduce blade angles For axial flow fans:
• a new or existing impeller with reduced pitch angle
• a newor existing impellerwith a reduced number of blades
• fitting a slower running motor For indirect drive fans:
• fitting a smaller motor
• fitting a slower vee rope drive
• changing the gear ratio of a gearbox
It may also be advantageous to review the reasons for the ventilation system and what it is supposed to do. In a drying plant, for example, it may be possible to change operating procedures to reduce the demand for flow variation.
Attention should always be given to energy efficient methods of control such as speed regulation for all types of fan, variable pitch in motion for axial flow fans and radial inlet vane control for centrifugal fans. The advantages and disadvantages of each are given in Chapter 6.
Posted in Fans Ventilation A Practical Guide