Economic optimisation

Present and impending legislation, as well as a greener ap­proach from us all, dictate that fans and their systems shall be­come more efficient. This Chapter is intended to remind fan en­gineers and their customers of the fundamentals. Without consideration for the fan system and the way in which the flowrate must be controlled (see Chapter 6), it is possible to se­lect fans, transmission motors and controls which will not give the expected power savings. The matching of all these terms is detailed and warrants more than the cursory attention often given.

Despite extensive research alternative fuels and refrigerants we have to agree that global warming is now largely accepted as a probable future threat to mankind. Not all nations (e. g. the USA) or scientists (e. g. in The Skeptical Environmentalist) are convinced of the validity of the case being made, but a “safety first” policy suggests that we should all adopt the Kyoto Proto­col.

The six strategies outlined in the introduction to Chapter 6, Sec­tion 6.1, are of the utmost importance and should be followed.

The efficiency factor

An important characteristic of a fan system, from an economic point of view, is its efficiency factor, i. e. the efficiency with which it converts the absorbed energy, usually electrical energy, into air power. This is especially important, since efficiency factors normally have extremely low values, much lower than usually quoted. Reasons for low efficiency factors are discussed and suggestions for improvement are presented.

The secondary costs for the main process, of which the fan unit is a part, can be considerable if factors such as process adapta­tion and available capacity are not considered when designing the plant. The most precious form of energy in ventilation is air energy, which warrants attention because of the parameters which not only determine the air power requirements but also the energy consumption.

The basic requirement of every fan installation is to transport air or some other gas. The object of economic optimisation is to enable the air conveyance to be carried out at the lowest possi­ble cost. The actual liquid transportation, however, often con­stitutes a small part of a much larger process. An evaluation of the air transport costs must, therefore, also include the costs of adaptation to the main process, or if adaptation is not carried out, the resultant costs of a less effective main process.

Since the required air movement cannot be accomplished with­out the contribution of all components in the ventilation plant, the complete system should be considered when carrying out economic optimisation. Only when the remaining costs are not affected, can partial optimisations give completely accurate re­sults. Partial optimisations do, however, have the advantage that they are easier to evaluate and are therefore applied exten­sively, despite the risk of certain errors.

The cost which should be minimised is the plant whole-life cost, i. e. the summation of all the costs which occur during the total economic life span of the plant. This begins with planning and drafting and ends when the plant is written off or otherwise dis­posed of. Whole-life costs are traditionally divided into invest­ment and operating costs.

The investment costs consists of the sum of the following:

• Planning

• Design and draughting

• Equipment

• Installation and commissioning

• Process adaptation

• Staff training

• Disposal, writing off

Operating costs consist of the sum of:

• Maintenance, parts and labour

• Energy consumed.

If the fan and ventilation plant is critical to a process and space is not available for stand-by capacity, the cost of lost production can be considered as a running cost.

The investment costs are basically fixed price in character, i. e. in principle they are independent of the extent to which the plant is used, whereas the operating costs increase with the number of hours of the operation. It should, however, be noted that the investment on a production plant will be very much affected by the size of the “run” and the ability to amortize the cost over a large number.

Economic optimisation

Figure 19.1 Graphical representation of whole-life costs

The two alternatives in Figure 19.1 show the influence which the number of operating hours has in respect of plant whole-life costs. Plant I, which is characterised by its simple construction with low investment costs, gives the lowest total cost despite its higher operating costs, low system efficiency, when the total number of operating hours is low.

For longer operating times, however, the relatively expensive plant II, with its high efficiency, shows the lowest whole-life cost. In practice, the operating costs will not be linear. As the plant ages, efficiency may reduce and maintenance costs and spares usage will increase.

New and existing plant

The design and layout of new plant offers a greater freedom of choice for economic optimisation than is the case for existing plant. For new plant the conditions are more favourable for total optimisation, since all the plant components can be chosen freely.

Normally, economic optimisation is carried out for a number of technically feasible alternatives. The costs for the various items are determined and summate as in Section 19.1.2. The alter­native selected is that which fulfils the gas transportation re­quirements whilst incurring the lowest whole-life costs. The op­portunities presented by a new plant should be seized upon to achieve good adaptation to the process. Since the major costs are often associated with the main process, of which gas trans­portation is only a part, considerable cost reductions can be achieved by suitably designing the ventilation plant.

The following examples are given to explain what is meant by the term “process adaptation”.

• The ventilation of hazardous gases; careful selection of the fan size and design of the shaft seals reduces the costs of environmental protection.

• The gentle pneumatic conveyance of food products; re­duces product damage and costs.

• The correct choice of low speed fans for abrasive gas-solid mixtures; reduces wear, costs of spare parts and labour costs.

• The scheduled adjustment of fan configuration, adding a stage, changing impeller diameter, changing impeller pitch angle; matches flow/pressure requirements to system changes reducing energy consumption.

Optimisation of existing plant should also consider the costs of process adaptation. Here, however, many parameters are al­ready established which imposes limitations. Optimisation of existing plant assumes therefore, a partial optimisation func­tion. The question then, is whether an extra investment can re­duce the operating costs sufficiently to make the improvement profitable.

Examples of modifications proposed for existing plant are:

• Replacement of fan, replacement where possible of impel­ler with larger diameter, reduce impeller diameter, change pitch angle.

• Change method of regulation or control.

• Ductwork or system modifications to reduce pressure losses.

• The introduction of improved shaft sealing arrangements.

All modifications are introduced with the intention of reducing future maintenance and energy costs. Another type of optimi­sation occurs when the air movement requirements can be per­formed by alternative existing fan units.

Such situations appear, for example, in large duct networks, which are supplied by several fan stations. Here, since the fixed costs for the plant cannot be changed, the rule is to choose the fan unit which results in the lowest operational costs. The oper­ational costs in these instances are complicated by differing fan efficiencies and varying system pressure losses due to dis­tance and duct size.

Posted in Fans Ventilation A Practical Guide


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